1 Departament de Bioquimica i Biologia Molecular, Facultat de Biologia, Universitat de Barcelona, Barcelona E-08028; and 2 Protein Design Group, Centro Nacional de Biotecnología-Consejo Superior de Investigaciones Científicas, Madrid E-28049, Spain
![]() |
ABSTRACT |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
The heteromeric amino acid transporters (HATs) are
composed of two polypeptides: a heavy subunit (HSHAT) and a light
subunit (LSHAT) linked by a disulfide bridge. HSHATs are
N-glycosylated type II membrane glycoproteins, whereas
LSHATs are nonglycosylated polytopic membrane proteins. The HSHATs
have been known since 1992, and the LSHATs have been described in the
last three years. HATs represent several of the classic mammalian amino
acid transport systems (e.g., L isoforms, y+L isoforms,
asc, x
light and heavy subunits; cystinuria; lysinuric protein intolerance; CD98 (4F2) complex and integrins; rBAT
![]() |
INTRODUCTION |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
SIX FAMILIES OF AMINO ACID
transporters for the cell plasma membrane have been described in
mammals (reviewed in Refs. 85, 121,
123), one of which has a heteromeric structure. These
heteromeric amino acid transporters (HATs) are composed of a heavy
subunit (rBAT or 4F2hc) and the corresponding light subunit, linked by a disulfide bridge (Table 1 and Fig.
1). These transporters were identified
after the cloning of rBAT (also named NBAT and D2) from kidney by their
functional expression in oocytes (9, 171, 186). Amino acid
transport in oocytes was also induced by the expression of the
rBAT-homologous protein, the heavy chain of the surface antigen 4F2
(4F2hc; 4F2 is also referred to as CD98) (8, 187). These
two heavy subunits of HAT (HSHATs) are type II membrane glycoproteins
with a single transmembrane (TM) domain, an intracellular
NH2 terminus, and an extracellular domain that shows
significant homology with bacterial -glucosidases. rBAT is mainly
expressed in the epithelial cells of the kidney proximal tubule and of
the small intestine, where it is located in the brush border. In
contrast, 4F2hc is ubiquitous, with a basolateral location in
epithelial cells. The first studies of rBAT and 4F2hc have been
reviewed extensively (121).
|
|
Studies of covalent inactivation by mercurial agents demonstrated that
4F2hc needs an accompanying subunit(s) to express transport activity
(47; reviewed in Ref. 122). The first light subunits of
HAT (LSHATs) were identified in 1998 (LAT-1, y+LAT-1,
and y+LAT-2) (79, 101, 128, 133, 175). Since
then, four more mammalian LSHAT members have been cloned xCT, LAT-2,
asc-1 and b0,+AT (BAT1 in Ref. 27) (7, 18, 27, 52, 47a, 112, 127, 131, 137, 138, 141, 149, 150, 154). The LSHAT
members have recently been reviewed (40, 118, 178, 179).
Their general characteristics are the following. First, LSHATs are not
N-glycosylated and are highly hydrophobic, with 12 putative
TMs (Fig. 1). The lack of N-glycosylation of LAT-1 after
translation in vitro has been demonstrated (79). The
highly hydrophobic character of LSHATs (molecular mass ~50
kDa) results in an anomalous mobility in SDS-PAGE, compatible with an apparent molecular mass of 35-40 kDa. Second, LSHATs are linked to the corresponding HSHAT by a disulfide bridge. For this reason, HATs are also named glycoprotein-associated amino
acid transporters (gpaATs) (178). The intervening cysteine
residues are located in the putative extracellular loop II of LSHATs
and a few residues toward the COOH terminus from the single putative TM
of HSHATs (Fig. 1). Evidence for the formation of these heterodimers (~125 kDa) in heterologous expression systems has been obtained for
4F2hc (~85 kDa) with LAT-1, LAT-2, or y+LAT-1 (101,
110, 128, 129, 141, 175) and for rBAT (~94 kDa) with
b0,+AT (Fernández E, Chillarón J, and
Palacín M, unpublished observations), and by
coimmunoprecipitation of LAT-1 and 4F2hc (100) and
b0,+AT and rBAT (Fernández E, Chillarón J, and
Palacín M, unpublished observations) from naturally occurring
tissues. Third, LSHAT members need coexpression with the corresponding
HSHAT to reach the plasma membrane in heterologous expression systems
[LAT-1, LAT-2, asc-1, y+LAT-1, and xCT with 4F2hc
(7, 101, 109, 110, 129, 131), and b0,+AT with
rBAT (47a)]. Fourth, LSHATs confer specific amino acid transport
activity to the heteromeric complex (Table 1). Coexpression of 4F2hc
with LAT-1 induces a variant of system L (sodium-dependent transport of
neutral amino acids with a large side chain) (79, 101),
with LAT-2 induces another variant of system L for neutral amino acids
of any size (131, 141, 154), with asc-1 induces system asc
(sodium-independent transport for neutral amino acids of small side
chain) (52, 112), with xCT induces system
x
![]() |
THE MECHANISM OF EXCHANGE OF HATs |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
The amino acid exchange activity of HATs was evidenced before the identification of LSHATs (reviewed in Ref. 124). The seminal observations were described simultaneously and independently by two groups (21, 34), who reported outward positive currents associated with the heteroexchange of neutral (efflux) and dibasic amino acids (influx) via system b0,+ in whole or cut-open oocytes expressing rBAT. Further studies demonstrated that system b0,+/rBAT acts as a tertiary active transporter, mediating the electrogenic exchange of dibasic amino acids (influx) for neutral amino acids (efflux) with a stoichiometry of 1:1 (30). This exchange has also been demonstrated in the apical plasma membrane of the proximal tubular cell model of opossum kidney (OK) cells in rBAT-antisense experiments (105) and in chicken brush-border jejunum (174).
System y+L induced by 4F2hc in oocytes also behaves as an
electroneutral and asymmetric amino acid exchanger (30):
it mediates the efflux of dibasic amino acids and the influx of neutral
amino acids plus sodium. The transport of sodium via
4F2hc/y+LAT-1-induced system y+L has recently
been demonstrated in oocytes (78). Similarly, the amino
acid transport activity x
A functional model for rBAT-induced system b0,+ exchange activity was proposed by Coady et al. (33). These authors observed in rabbit rBAT-expressing, cut-open oocytes that aminoisobutyric acid (AIB) induced amino acid currents across system b0,+ without being transported itself, thus suggesting variable stoichiometry of exchange. To explain these results, a "double-gated" pore model with a binding site accessible at each side of the membrane was proposed.
Very recently, Torras-Llort et al. (174) studied system b0,+ in chicken brush-border jejunum. In these vesicles, accessibility to both sides of the plasma membrane allowed kinetic and simulation analysis of the system b0,+ amino acid exchanger. The results were compatible with a sequential mechanism, which implies the formation of a ternary complex (i.e., the transporter bound to a substrate at each side of the membrane). In contrast, the results ruled out a "ping-pong" mechanism (i.e., binding of a substrate on one side of the membrane and then translocation and release of the substrate on the other). The study did not distinguish between ordered and random binding of substrates to the transporter in the sequential mechanism, but the estimated dissociation constants for extracellular (extravesicular or external) or intracellular (intravesicular or internal) substrates suggest that the binding affinity for the extracellular amino acid is higher than for the intracellular substrate. An ordered mechanism, in which the free transporter binds first to the external amino acid and then to the internal one, may account for these results. However, because the binding affinity for the internal amino acid is high (in the micromolar range), the results could also be explained by a random mechanism with a preferential route (i.e., preference for the binding of the external amino acid first). Such preferential behavior might be due to the negative membrane potential, which would favor the binding of cationic amino acids to the transporter from the external side rather than from the internal one. The "double-gated" pore model was not supported in the chicken brush-border jejunum studies because interaction of AIB with system b0,+ was not substantiated (174).
The results in chicken brush-border jejunum are compatible with a
double-exchange pathway with alternating access (Fig.
2). A similar model was proposed by
Dierks et al. (42) for the mitochondrial aspartate/glutamate antiporter, which includes two functional "subunits" or pathways with binding sites alternating at each membrane domain in which the translocation step is under membrane potential control. The functional oligomeric structure of
rBAT/b0,+AT system b0,+ is unknown, as it is
for the other HATs. Interestingly, Western blot analysis revealed a
high-molecular-mass complex (~250 kDa), in addition to the ~125-kDa
heterodimer, for rBAT (120, 181) and for
b0,+AT (Fernández E, Chillarón J, and
Palacín M, unpublished observations) in kidney brush-border
preparations under nonreducing conditions. This suggests that system
b0,+ might be a heterotetramer comprising two heterodimers
of rBAT and b0,+AT linked by disulfide bridges. In this
scenario, each heterodimer would represent a single pathway of
transport with alternating accessibility, and the two heterodimers
together would represent the double-exchange pathway. Structural and
functional (i.e., with dominant negative mutants) studies to define the
functional structural unit of the heteromeric amino acid transporters
are presently in progress.
|
![]() |
INHERITED AMINOACIDURIAS |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
The transport characteristics of two of the LSHAT-associated transport systems are relevant to inherited aminoacidurias cystinuria and lysinuric protein intolerance (LPI; see Transepithelial transport of amino acids). First, system b0,+ (induced by rBAT and b0,+AT) acts as a tertiary active mechanism of renal reabsorption and intestinal absorption of dibasic amino acids and cystine; it mediates the electrogenic exchange of dibasic amino acids (influx) for neutral amino acids (efflux). Second, system y+L (induced by 4F2hc and y+LAT-1) mediates the electroneutral exchange of dibasic amino acids (lysine, arginine, and ornithine; efflux) for neutral amino acids plus sodium (30, 40, 78). It is assumed that this transport system allows the efflux of dibasic amino acids against the membrane potential in many cell types, particularly in the basolateral domain of epithelial cells. The role of rBAT, b0,+AT, and y+LAT-1 in cystinuria and LPI has recently been reviewed (119).
Cystinuria
Cystinuria (MIM 220100) is an autosomal-recessive disorder, with an average prevalence of 1 in 7,000 births (153). The disease is caused by the defective transport of cystine and dibasic amino acids across the apical membranes of proximal renal tubular and jejunal epithelial cells. Because of the low solubility of cystine, it precipitates to form kidney calculi that produce obstruction, infection, and, ultimately, renal insufficiency. Cystinuria represents 1-2% of overall renal lithiasis and 6-8% of renal lithiasis in pediatric patients.Presently, we classify two types of cystinuria types: type I (MIM 220100) and non-type I (MIM 600918) (124). Type I heterozygotes are silent, whereas in non-type I heterozygotes there is a variable degree of urinary hyperexcretion of cystine and dibasic amino acids. Patients with a mixed type, inheriting type I and non-type I alleles from either parent, have also been described (57). Type I cystinuria represents >60% of the cases of the disease.
The amino acid transport activity associated with rBAT (system b0,+) and the expression of rBAT in the brush border of the renal epithelial cells of the proximal tubule and of the small intestine pointed to the rBAT gene (SLC3A1) as a candidate for cystinuria. Mutational and linkage studies demonstrated that mutations in SLC3A1 cause type I cystinuria (23, 24, 54). Over 60 distinct rBAT mutations have been described, including nonsense, missense, splice site, and frameshift mutations, as well as long deletions; mutation M467T is the main type I cystinuria allele found worldwide in 38 nonrelated chromosomes (reviewed in Ref. 124). Cystinuria resembling type I, due to mutations in canine SLC3A1, has been reported in Newfoundland dogs (61).
The gene causing non-type I cystinuria was assigned by linkage analysis to the 19q12-13.1 region (11, 166, 184). In 1999, the non-type I cystinuria gene was identified as SLC7A9 (47a). SLC7A9 was a positional candidate gene for non-type I cystinuria because it has the proper chromosomal location, rBAT-associated amino acid transport activity (system b0,+), and tissue expression (mainly in kidney and small intestine, but also in pancreas and liver). The protein product encoded by SLC7A9 was termed b0,+AT (for b0,+ amino acid transporter).
SLC7A9 is the main, if not the only, non-type I cystinuria gene. In fact, after an exhaustive screening of the open reading frame of SLC7A9 by the International Cystinuria Consortium (49a), 35 distinct mutations were found, accounting for 79% of the carrier chromosomes in 61 non-type I patients, mutation G105R being the main non-type I cystinuria allele (25%). The unexplained alleles might be due to mutations outside the open reading frame of SLC3A1, although other gene(s) might be involved in non-type I cystinuria.
All the data discussed so far strongly indicate that rBAT and b0,+AT are subunits of the amino acid transporter b0,+. However, this view is challenged by the finding that rBAT has a gradient of expression along the kidney proximal tubule: segment S3 > S2 > S1 (27, 53, 80, 127, 130, 136), whereas b0,+AT has the opposite gradient of expression: S1 > S2 > S3 (27, 104, 127, 136) [S1 being the small initial part of the proximal convoluted tubule (PCT), S2 the rest of PCT plus the cortical proximal straight tubule (PST), and S3 the terminal part of the PST located in the outer stripe of the outer medulla]. Then, an additional LSHAT for rBAT or HSHAT for b0,+AT might be available. Identification of these proteins might help us to understand the molecular bases of cystinuria and why mutations in SLC3A1 are completely recessive, whereas mutations in SLC7A9 are incompletely recessive (for a detailed discussion, see Refs. 47a and 49a).
LPI
LPI (MIM 222700) is a rare autosomal-recessive disease caused by the defective transport of dibasic amino acids at the basolateral membranes of epithelial cells in the renal tubules and small intestine (159). LPI is more prevalent in Finland, but clusters of LPI families are also known in souhern Italy and Japan. The disease is characterized by reduced intestinal absorption of dibasic amino acids, increased renal excretion, and low plasma concentrations of dibasic amino acids, orotic aciduria, and dysfunction of the urea cycle, leading to hyperammonemia. Major clinical symptoms include vomiting, diarrhea, failure to thrive, hepatosplenomegaly, episodes of hyperammonemic coma, and osteoporosis. Life-threatening alveolar proteinosis in the lungs and severe renal involvement were also reported (145, 159). The pathogenic mechanism of several clinical complications of LPI, such as alveolar proteinosis and urea cycle dysfunction, are still unclear.The LPI locus was assigned to chromosome 14q11.2 by linkage analysis (92, 93). The LPI gene was later identified by candidate positional cloning (14, 176) after identification of y+LAT-1 (SLC7A7 gene) (175). y+LAT-1 is expressed in target tissues of LPI such as kidney, lung, and small intestine, among others, and induces system y+L transport activity when coexpressed with 4F2hc (128, 175). The defective system y+L transport in LPI is restricted to intestine, kidney, and probably liver and lung. In fact, system y+L is not altered in LPI erythrocytes or fibroblasts (15, 38), most probably because of the expression of the y+L transporter isoform y+LAT-2 in these cells.
Twenty-five distinct LPI-associated mutations spread along the entire SLC7A7 gene have been identified in 96 LPI patients; only 3 alleles remain to be explained (reviewed in Ref. 119). All Finnish LPI patients share the same founder mutation: a splice site mutation (IVS6-2 AT) that creates a frameshift after Val298 within putative extracellular loop 4 and a premature stop-codon 9 amino acid residues thereafter (109, 176). A genotype-to-phenotype correlation cannot be established in LPI because of extensive clinical variability associated with the same genotype (reviewed in Ref. 119). Then, other factors in addition to mutations of SLC7A7might have a role in the pathogenesis and clinical manifestations of LPI.
![]() |
HSHATs |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Six sequences are available for mammalian rBAT (human, rat, mouse,
rabbit, dog, and a partial sequence for the American opossum), sharing
69-89% amino acid identity and five sequences for vertebrate 4F2hc (human, rat, mouse, Chinese hamster, and zebrafish), sharing 41-89% amino acid identity (see GenBank accession nos. in the legend for Table 1). The rBAT protein (685 amino acid residues for the
human counterpart) is longer than the 4F2hc protein (529 for the human
counterpart), and they share ~25% amino acid sequence identity (Fig.
3). N-glycosylation was shown
for both proteins (reviewed in Ref. 121).
|
Membrane topology algorithms predicted that both proteins would be type
II membrane glycoproteins, with an NH2 terminus inside the
cell, a single TM, and a bulky COOH terminus located outside the cell
(reviewed in Ref. 121). The cysteine residue participating in the disulfide bridge with the LSHATs is four to five amino acids
away from the TM, toward the COOH terminus (Fig. 3). In contrast to
this view, Mosckovitz and co-workers (107), on the basis
of accessibility studies with various antibodies, proposed that rBAT
contains at least four TMs, with NH2 and COOH termini located intracellularly. Recently, however, Fenczik and co-workers (49) showed with HA-tag constructs that the
NH2 terminus of 4F2hc is intracellular whereas the COOH
terminus is extracellular (49), as expected for a type II
membrane glycoprotein. A further argument in favor of this structure is
the homology of the HSHAT bulky COOH-terminal domain with -amylases
(9, 134, 171, 172, 186). Indeed, this HSHAT domain shows
some homology only with insect maltase (
-glucosidase) and
maltase-like precursors (35-40% amino acid identity) and with
bacterial
-glucosidases (~30% amino acid identity).
This -amylase family consists of a large group of starch hydrolases
and related enzymes, comprising ~20 different enzyme specificities,
and is presently known as glycosyl hydrolase family 13 (75). The members have a similar architecture, with a
catalytic (
/
)8-barrel or TIM-barrel (domain A),
interrupted by a small calcium-binding subdomain (domain B)
protruding between the third
-strand (A
3) and the third
-helix
(A
3), and a COOH-terminal domain (domain C) with an
antiparallel
-barrel structure. Major differences in amino acid
sequence among the
-amylase family members occur within domain
B. Janecek et al. (75) clustered the
-amylase
members in five groups with >50% sequence identity for domain
B and suggested that it varies with enzyme specificity. The group
defined by Bacillus cereus oligo1,6-glucosidase (O1,6G) also
includes the rBAT proteins. Domain B of O1,6G has a complex topology not shared by other structurally solved
-amylases, which, in turn, have a common
-
architecture known as a two-layer
sandwich (CATH; protein structure classification:
http://www. biochem.ucl.ac.uk/bsm/cath_new/).
The three-dimensional structure of O1,6G has been refined at
2.0-Å resolution (185). The secondary structural elements
of this enzyme are indicated in Fig. 3, together with an alignment with
human rBAT and 4F2hc, one putative HSHAT from Drosophila melanogaster and two from Caenorhabditis elegans.
Because of the different levels of sequence similarity for each domain
(A, B, and C) between HSHATs and
O1,6G, a global alignment has first been performed for the
corresponding glycosyl hydrolase family, and, finally, ambiguous
regions have been refined by combining secondary structure predictions
and threading techniques (see legend for Fig. 3 for details). Sequence
homology between HSHATs and O1,6G starts with two contiguous tryptophan
residues a few amino acid residues away from the cysteine residue
involved in the formation of the disulfide bridge with LSHATs.
Domain A of O1,6G is highly conserved in HSHATs. Indeed, the
secondary structure elements of the (/
)8 barrel, i.e., A
1,
A
1, A
2, A
2, A
3, A
3, A
4, A
8, and A
8, are easily
identified by sequence homology in human rBAT and 4F2hc and in the
putative HSHATs from D. melanogaster and C. elegans (Fig. 4). In contrast,
sequence homology with O1,6G is poor for HASATs in the region between
4 and
7. Then, threading and secondary structure prediction have
been used to assign the secondary structure elements, i.e., A
4,
A
5, A
5, A
6, A
7, and A
7, of the (
/
)8 barrel in the
protein sequence of most of the vertebrate and invertebrate HSHATs
(Fig. 3). This strongly suggests that the structure of the bulky
COOH-terminal domain of HSHATs corresponds to a TIM-barrel.
|
Three catalytic residues (D199, E255, and D329 in O1,6G) and two
residues for substrate binding (H103 and H328) within the TIM-barrel
constitute the active site of family 13 glycosyl hydrolases (reviewed
in Ref. 185). All these residues are in the COOH-terminal face of the TIM-barrel and, together with the protruding domain B, constitute the active site cleft. Of these TIM-barrel residues of O1,6G, only D199 in the A4 region, D329 between A
7 and A
7', and H105 before domain B can be identified unambiguously in
the mammalian rBAT proteins, but none of them in the vertebrate 4F2hc proteins (Fig. 3). Similarly, none of the three invertebrate putative HSHAT proteins share all the active site residues of O1,6G (Fig. 3). In
agreement with this, Wells and co-workers (187) did not observe
-glucosidase activity for 4F2hc after expression in
Xenopus laevis oocytes.
Janecek and co-workers (75) reported that domain B, including the COOH-terminal motif QPDLN (residues 167-171 in O1,6G), is conserved in the rBAT proteins but not in the 4F2hc proteins. Indeed, there is complete absence of domain B for 4F2hc and the putative HSHAT O45298 from C. elegans (Fig. 3). In contrast, rBAT and Q9XVU3 from C. elegans and Q9VHX9 from D. melanogaster contain a domain B with the structural features of this domain in O1,6G, including a complete or partial conservation of the COOH-terminal motif. This suggests that O45298 is the C. elegans ortholog of 4F2hc and that Q9XVU3 and Q9VHX9 are the C. elegans and D. melanogaster orthologs of rBAT, respectively. A search through the entire human genome and the genome of D. melanogaster and C. elegans revealed no other putative HSHAT proteins.
The COOH-terminal domain (domain C) of -amylases
corresponds to a
-barrel structure of eight antiparallel
-strands
folded in double Greek key motifs, which is distorted in the sixth
strand C
6 (185). Sequence alignment, threading, and
secondary structure prediction fit the entire O1,6G domain C
into the COOH-terminal region of the vertebrate and invertebrate
HSHATs, with the exception of C
6, which is not clearly predicted in
these proteins (Fig. 3). The function of domain C, located
far from the active site of O1,6G, is unknown. Finally, the last 30 amino acid residues of the rBAT proteins do not align with the
-amylases.
Structure-Function Relationship Studies
Three studies have dealt with COOH-terminal deletions of HSHATs (19, 39, 102). The first two studies were performed before the identification of LSHATs and therefore rely on the expression of HSHATs in oocytes that, together with endogenous LSHATs, elicited transport of amino acids. Miyamoto et al. (102) showed that a COOH-terminal deletion (Deora and co-workers (39) studied a series of
COOH-terminal deletions of rat rBAT. Surprisingly, expression of these
truncated proteins in oocytes yielded an unusual bimodal pattern of the induction of amino acid transport activity. Thus minimal COOH-terminal truncations (658-683, which eliminates the COOH-terminal tail, and
615-683, which eliminates the last four
-strands of the domain B and the COOH-terminal tail; Fig. 3) abolished
transport activity. The next mutants in the series (
588-683,
elimination from the last 6
-strands of domain B; Fig. 3)
induced amino acid transport almost like that of the complete rBAT and
with the characteristics of rBAT/system b0,+. Further
deletions (
566-683, elimination from all of domain C, and
508-683, elimination from the last
-helix of the
TIM-barrel; Fig. 3) abolished amino acid transport induction. There is
no obvious reason for the discrepancy of the amino acid transport induction by
508-683 rat rBAT (39) and
511-685 human rBAT (102). Deora and co-workers
(39) studied further the transport-active deletion
588-683. The cysteine residue at position 111 in rBAT forms
part of the disulfide bridge with the corresponding LSHAT. A mutation
to the serine of this residue (C111S) renders a protein that induces
70% of the amino acid transport induced by wild-type rBAT. A notable
difference is that the C111S mutant in
588-683 rBAT completely
abolished its transport activity. This suggests the following. First,
the formation of the disulfide bridge with the corresponding LSHAT is
not necessary for the functional association with rBAT. This has also
been demonstrated for 4F2hc alone and 4F2hc/y+LAT-1- or
LAT-1-induced transport (47, 129, 175). Thus other interactions beside the disulfide bridge keep the functional transport complex intact. Second, lacking C111 (rat rBAT numbering), the
588-683/C111S mutant cannot form a stable complex with the
endogenous LSHAT. Thus, in the absence of the disulfide bridge the
integrity of domain B and the COOH-terminal tail is
necessary for a functional transport complex. Third, the COOH terminus
holds rBAT in an active conformation, perhaps by providing the sites
for interaction with other rBAT regions or with the light subunit.
The above-mentioned experiments with truncated versions of rBAT could
also be interpreted as the result of interactions, depending on the
different COOH-terminal deletions, with different oocyte LSHATs.
Indeed, Peter and co-workers (126) analyzed the mutations of the three conserved cysteine residues located in the COOH-terminal tail of rBAT (C664, C671, and C683 in rat rBAT; Fig. 4): replacement of
C664 by alanine eliminates the functional interaction of rBAT with the
putative endogenous b0,+AT-type subunit and keeps (or
causes) the functional interaction with a putative endogenous
y+LAT-type subunit. Bröer and co-workers
(19) studied COOH-terminal deletions of 4F2hc coexpressed
in oocytes with LAT-1, LAT-2, or y+LAT-2. Surprisingly,
association of these LSHATs requires different domains. Thus
trafficking to the plasma membrane and induction of LAT-1/system L
transport activity require only the NH2-terminal tail, the
TM domain of 4F2hc, and 30 extracellular amino acid residues, including
the disulfide bridge-forming cysteine residue and the first -strand
of domain B (Fig. 3) (the longest deletion studied in this
work). In contrast, functional recognition of LAT-2 and
y+LAT-2 needs the complete extracellular domain of 4F2hc.
This suggests that the 4F2hc protein has different interaction sites
for its associated light chains. In this study, all truncated versions of 4F2hc delayed the trafficking of LAT-1 to the plasma membrane. Moreover, this trafficking was more severely affected by truncations involving part of the extracellular glucosidase-like domain of 4F2hc
than by those that eliminate it almost completely. Some short
COOH-terminal truncations resulted in large aggregates that might be
responsible for the severe trafficking defect. Finally, the more severe
defects in LAT-1 recognition occurred when the last 70 amino acids were
removed (i.e., the shortest deletion studied, which eliminates the last
6
-strands of domain B and the COOH-terminal tail; Fig.
3). This is reminiscent of the study by Deora et al. (39)
of truncated versions of rBAT: the activity lost when only small parts
of the COOH-terminal domain of the HSHAT are removed can be regained by
larger COOH-terminal deletions. This suggests that the COOH-terminal
tail of HSHATs plays a role in the proper folding of rBAT and/or in its
interaction with the corresponding LSHAT.
In addition to the amino acid transport function of HSHATs, 4F2hc has been related to integrin function (see CELL PHYSIOLOGY OF THE CD98 COMPLEX). Fenczik and co-workers (49) have examined which domains of 4F2hc play a role in amino acid transport and in regulation of integrins function. By constructing chimeras with 4F2hc and the type II TM protein CD69, the authors showed that the NH2 terminal and the TM domain of 4F2hc are required for its effects on integrin function, whereas the extracellular glucosidase-like domain is required for the stimulation of LAT-1 amino acid transport. This study together with that mentioned above on truncated versions of 4F2hc (19) point to multiple interactions, both at the NH2-terminal tail and the TM domain, and at the extracellular glucosidase-like domain, between 4F2hc and LAT-1. Thus the NH2-terminal tail and the TM domain are sufficient for the functional interaction of 4F2hc with LAT-1, but replacement of the extracellular domain by another prevents this functional interaction.
All but one of the rBAT missense mutations described in type I cystinuria (Fig. 3) are located in the extracellular glucosidase-like domain, which is consistent with its proposed role in amino acid transport. The most obvious role of HSHATs in amino acid transport is to help the trafficking of LSHATs to the plasma membrane. In agreement with this, for several cystinuria-specific rBAT mutations a trafficking defect to the plasma membrane has been substantiated (M467T, M467K) or suggested (T216M, S217R) (31, 143). On the other hand, some cystinuria-specific rBAT mutations affecting transport properties of system b0,+ are presently under study, suggesting a participation of rBAT in the transport mechanism of the holotransporter. Reconstitution studies of HATs will be needed to demonstrate whether isolated LSHATs display amino acid transport activity and to identify the role of HSHATs in the mechanism of transport.
Another unsolved question is why the extracellular domain of HSHATs
resembles that of glucosidases but without apparent catalytic activity. One could envisage that the noncatalytic glucosidase-like domain of HSHATs might hold extracellular glucidic structures to
locate HATs properly in the plasma membrane. This question should be
solved by purification and testing of glucid binding to the
extracellular domain of HSHATs. Homology modeling of HSHATs with
-glucosidases is in progress, but it could be hampered by the low
identity of parts of domain A with crystallized
-amylases and the lack of domain B in 4F2hc proteins. Purification and
crystallization may be needed to establish the structure of the bulky
extracellular region of HSHATs.
![]() |
LSHATs |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
There are 23 reported LSHATs with ascribed amino acid transport function: 22 vertebrate sequences that correspond to 7 LSHAT paralogs and the Schistosoma mansonii SPRM1 protein (see the beginning of this study and legend for Table 1 for details). Amino acid sequence identity among these LSHATs ranges between 39 and 70% for different paralogs and between 85 and 98% for different mammalian orthologs. In addition, two other orphan LSHAT cDNAs have been cloned in mice (LSHAT-8 and -9) and humans (LSHAT-8) (Bassi MT, Gasol E, Zorzano A, Palacín M, and Borsani G, unpublished observations). Identity of the mammalian orphan LSHATs with the LSHATs with known transport functions drops to 23-29%.
LSHATs belong to the large superfamily of APC transporters (>175 transporters; for amino acids, polyamines, and organocations). Jack and co-workers (74) clustered this superfamily into 10 families. One of these is the LAT family (TC 2.A.3.8), which received its name from the first LSHAT identified (LAT-1; Refs. 79 and 101) and clusters the above-mentioned vertebrate LSHAT members and SPRM1, the yeast high-affinity methionine permease MUP1 (U40316) and the hypothetical yeast protein MUP3 (protein GenBank accession no. P38734) (72), and several hypothetical proteins from C. elegans and D. melanogaster. Jack et al. (74) also identified a signature sequence specific to the LAT family, G[WFY][DNFS]X[LIV][NH][FYT][LIVAGS][TALIV] [EGPS]E[LIVM]X[NDE]PX[RK][NT][LIVM][PK], where X represents any residue. This signature is located in the third putative intracellular loop of the topology model shown in Fig. 4 (see below for discussion of this model).
There is a key structural feature of LSHAT members, the conserved cysteine residue in the putative extracellular loop 2 that participates in the disulfide bridge with the corresponding HSHAT (Fig. 4), which was first functionally identified by Pfeiffer and co-workers (129). This cysteine residue is present in 34 LSHATs: 25 vertebrate sequences and SPRM1 (see above), D. melanogaster minidisks (AF139834), and 4 D. melanogaster (GenBank accession nos. CG1617, CG12317, CG6070, and CG9413) and 3 C. elegans (protein GenBank accession nos. T21445, T32479, and T28818) hypothetical proteins. In contrast, this residue is not conserved in the other LAT family members: yeast MUP1 and MUP3 and in 5 C. elegans (T15226, T16854, T24837, T32821, T31554) hypothetical proteins. We propose to cluster the former group of LAT family members (i.e., those with the conserved cysteine residue) in the LSHAT subfamily. Sequence analysis (173) revealed a specific signature sequence for this subfamily (34 transporters) located between transmembrane (TM) domain I and IL1 (see Fig. 4): [IVFL] G[SAT]GIF[VI][STA]P(X26)[GS][AST][LYVIF][CSAV] [YFSN][AS]E[LI][GSA](X5)SG[GA]X[YW]X[YF], where X represents any residue.
Phylogenetic analysis (173) of the LSHAT subfamily revealed a nodal relationship of CG12317 and CG9413 from D. melanogaster with LAT-1 and b0,+AT transporters, respectively, suggesting that these hypothetical proteins correspond to the orthologs of these transporters. In contrast, the rest of the hypothetical proteins from C. elegans and D. melanogaster cluster in two nodes (T32479 and T21445 from C. elegans and CG1607 from D. melanogaster; T28818 from C. elegans and minidisks and CG6070 from D. melanogaster) together with transporters for neutral amino acids of the LSHAT subfamily (i.e., LAT-1, LAT-2, and asc-1). This sequence analysis is therefore not enough to ascribe the amino acid transport function to these C. elegans and D. melanogaster LSHAT members.
Membrane topology predictions revealed that the transporters of the APC family display 10, 12, or 14 TMs (74). Most of the APC families (8 families and the bacterial transporters of the CAT family) display 12 TMs. Figure 4 shows the 12-TM model for human b0,+AT. This model is based on TM-HMM algorithms (164) with the multialignment of the 34 transporters of the LSHAT subfamily. To our knowledge, four studies deal with the experimental determination of the topology of APC transporters: LysP [amino acid transporter (AAT) family, specific for bacteria (46)]; PotE [basic amino acid/ polyamine antiporter (APA) family, specific for bacteria] (83); AroP (AAT family) (35); and the B subunit of glyceraldehyde-3-phosphate dehydrogenase (GABP; AAT family) (63). All of these studies used the strategy of transporter-monitor enzyme hybrids as topological sensors. The four studies obtained results compatible with the 12-TM model with the NH2 and COOH termini located intracellularly. There is agreement on the location of the amino acids tested in these transporters and the membrane topology model for LSHATs shown in Fig. 4 for b0,+AT. In contrast, Isnard et al. (72) proposed a 13-TM model for MUP1 and MUP3. Sequence alignment between these two yeast transporters and LSHAT transporters could be traced in their whole sequences, and TM VIII in the 13-TM model does not show enough hydrophobicity in any of the sequences. This strongly supports the 12-TM model for the LSHAT transporters, but direct experimental evidence is lacking.
Structure-Function Relationship Studies
The structure-function studies available in LSHATs are those related to the impact of missense mutations in y+LAT-1 and b0,+AT that cause LPI and non-type I cystinuria, respectively. Of the seven missense mutations described in LPI (M1L, M50K, G54V, T188I, L334R, G338D and S386R) (118), two have been analyzed for transport function in oocytes: G54V (within TM I) and L334R (within intracellular loop 4). These two mutated proteins reach the oocyte plasma membrane but do not induce amino acid transport activity (109) (Table 2). This demonstrates that G54V and L334R inactivate the transport function of y+LAT-1. Functional analysis of these mutations in mammalian transfected cells has not yet been reported, and therefore it is unknown whether these mutations affect trafficking to the plasma membrane of y+LAT-1 in such an expression system.
|
The location of the 22 cystinuria-specific mutations affecting single amino acid residues of b0,+AT is shown in Fig. 3. Seventeen mutations involve amino acid residues within the putative TM domains of the protein and the rest within the putative intracellular loops. Thus none of the mutations is located within the putative extracellular loops of b0,+AT. This is also the case for the seven missense LPI mutations mentioned above. Similarly, all the transport activity-relevant residues identified in PotE are located in the loops and TM facing the cytoplasmic site of the transporter (82, 83). It has also been shown that the main functional amino acids of the lactose/H+ symporter (76, 132) and the metal tetracycline/H+ antiporter (192) are located on the cytoplasmic side of the protein. Kashiwagi and co-workers (82) interpreted this asymmetry as the structural basis to provide a quick response of the transporter to any change in cellular substrate concentration.
Six missense b0,+AT mutations (A70V, V170M, A182T, A354T, G105R, and R333W) have been tested for function after coexpression with rBAT in HeLa cells (49a). Some of these mutations (V170M, A354T, G105R, and R333W) cause a complete or almost complete loss of function (<10% residual transport activity), whereas the others (A70V and A182T) have only a partial effect (>50% residual activity) (Table 2). It remains to be determined whether these b0,+AT mutations affect trafficking to the plasma membrane or whether they inactivate the transporter.
Urinary excretion of cystine and the three dibasic amino acids (arginine, lysine, and ornithine) in heterozygotes bearing the major missense SLC7A9 mutations (G105R, V170M, A182T, R333W) showed mutation-specific severity (49a). Thus A182T heterozygotes are associated with a mild phenotype (urine cystine and dibasic amino acid levels within 5 times the average of the urinary excretion values in controls). In contrast, heterozygotes bearing the other common mutations show higher urinary excretion values (8-18 times higher than controls). This parallels the degree of the transport defect associated with these mutations (Table 2). If this correlation is extended to uncommon cystinuria-specific b0,+AT mutations (Table 2), several implications may be extracted (49a). First, b0,+AT and y+LAT-1 mutations affecting highly conserved amino acid residues within the LSHAT subfamily result in severe amino acid transport defects and/or severe urinary phenotypes in heterozygotes, whereas mutations in nonconserved residues are associated with mild phenotypes. The only exception is the mild urinary excretion phenotype found in two of the three G63R heterozygotes analyzed; individual variability might explain the behavior of these heterozygotes (49a). Second, four b0,+AT or y+LAT-1 mutations are located within putative TM and involve residues with a small side chain (Gly, Ala, or Ser) in all the LSHAT transporters. These four mutations are associated with a severe phenotype. In contrast, mutations A70V, A126T, and A182T, which involve residues in putative TM with different side chain sizes in other LSHAT transporters, are associated with mild phenotypes.
The relevance of small-side chain residues (Gly, Ala, or Ser) in
contact regions of -helix TMs has recently been highlighted. Thus
highly conserved residues with a small side chain (Gly or Ala) are
present in the contact regions of the TM
-helices of aquaporin-1
(AQP1) (108). Moreover, the GlyxxxGly motif (where x
represents any residue and Gly can be replaced by Ser) has been reported to be a framework for the TM helix-helix association (142), and in this sense, helixes 3 and
6 of AQP1 contain a GlyxxxGly motif (where Gly can be
replaced by Ala). Similarly, SmxxxSm motifs (where Sm stands for
residues with a small side chain: Gly, Ala, or Ser) are present in the
LSHAT subfamily in TM I, VI, VII, and VII (Fig. 4). Mutation G259R in
b0,+AT and mutation G54V in y+LAT-1 involve
SmxxxSm helix-helix association motifs in TM VII and I, respectively.
G259R is associated with a severe urinary phenotype in heterozygotes,
and G54V is associated with a dramatic loss of transport function (49a,
109). This suggests that residues with small side chains, which are
conserved in TMs of LSHAT transporters, may participate in their TM
helix-helix associations.
![]() |
CELL PHYSIOLOGY OF THE CD98 COMPLEX |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
In this section, we will use the name CD98 to refer to the 4F2 complex and CD98hc for the 4F2hc, to maintain the nomenclature as used in the papers cited. The light chains are termed LSHATs, as before.
As stated above, LSHATs are believed to mediate amino acid transport itself due to their polytopic structure, whereas the type II protein CD98hc seems to act as a guidance molecule for the light chains on their way to the plasma membrane (110). However, localization and maturation of CD98hc in the absence of light chains are not well studied. At least in oocytes and L cells, CD98hc is expressed at the plasma membrane without a light chain (101, 172). Overexpression of CD98hc leads to its expression, apparently as a monomer, at the plasma membrane (47, 49). However, no study has measured the amount of plasma membrane free CD98hc in the normal in vivo situation (for instance, in activated T cells, proximal tubule cells, intestinal epithelial cells, etc.), which is important to an understanding of some of the data that will be discussed below.
CD98 was identified in the 1980s as an early activation antigen of T and B cells (60, 99, 134). Since those studies, many different functions besides amino acid transport have been ascribed to the complex, from cellular activation and division (41, 167, 191) to cell adhesion, fusion, and differentiation (114, 117, 170, 182). A recent careful review from Devés and Boyd (40) offered the first systematic overview of the huge amount of data dealing with these apparently disconnected functions. In this section, we will focus on the recent work performed in the fields of integrin activation, viral and cell fusion and differentiation, T cell activation, and oncogenesis.
Integrin Activation
Integrins are heterodimers of specific combinations of
|
The same laboratory went further in the characterization of
CD98hc-mediated CODS (194). They found that solubilized
CD98 binds in vitro to 1A- and
3-integrin
tails (but not to
1D or
7). This binding
correlated with the lack of CD98hc-mediated CODS caused by
1D- or
7-integrin cytoplasmic tails.
1A-Cytoplasmic tail deletion mutants that still bound
CD98hc were no longer able to mediate dominant suppression
(194), suggesting that the mechanism of dominant
suppression by integrin cytoplasmic tails is due to mechanisms other
than titration of endogenous CD98hc molecules. The role of the
different domains of the CD98hc molecule in CODS, integrin cytoplasmic
tail binding, and amino acid transport induction was also investigated
by using chimeric constructs (see HSHATS) (49). The cytoplasmics tail and TM and extracellular
domains of CD98hc and CD69 (another type II membrane protein) were
exchanged. The cytoplasmic and the TM of CD98hc were necessary and
sufficient for CODS or integrin binding, whereas the extracellular
domain was dispensable for these functions but essential for amino acid transport. Overexpressed wild-type and cysteine mutants of CD98hc (including the one that partially impairs amino acid transport and
light chain association) were localized in monomeric form at the plasma
membrane and mediated CODS and bound integrin cytoplasmic tails.
Interestingly, the authors pointed out that coclustering of CD98
molecules and integrins might also help to localize amino acid
transport in defined regions of the plasma membrane. For instance,
1A-integrin is basolateral in epithelial cells
(193), where CD98 is also found.
Together, these studies showed that CD98hc binds some, but not all,
-integrin tails and can perform CODS independently of amino acid
transport and LSHAT. It should be noted, however, that CODS depends
on the overexpression of CD98hc molecules that most likely act by
titration of excess
-integrin tails, exerting the dominant
suppressive effect. Whether integrin activation per se (not CODS or
-integrin tail binding) requires association between CD98hc and
LSHAT proteins is a matter of speculation (see Oncogenic Potential of CD98).
Viral and Cell Fusion and Differentiation
Membrane fusion leading to multinucleated cell formation is a physiological process specially relevant for osteoclastogenesis and myogenesis. Moreover, many enveloped viruses, such as paramyxoviruses, induce syncitium cell formation; others, like the human immunodeficiency virus, require fusion mechanisms to enter their cellular hosts.In the early 1990s, Ito and colleagues (73) isolated
monoclonal antibodies (MAbs) that enhanced cell fusion of the Newcastle disease virus. These MAbs immunoprecipitated either CD98hc or the 3-integrin subunit (114), named fusion
regulatory protein-1 and -2, respectively. Since then, the
aforementioned authors have extended their studies to
paramyxovirus-induced syncitium cell formation and human
immunodeficiency virus entry into monocyte/macrophage cells (115,
117), isolating new anti-CD98hc MAbs that are able to either
induce or inhibit those processes. The fact that different anti-CD98hc
MAbs have opposing effects on fusion events suggests that the MAbs can
induce or fix conformations of the extracellular domain of CD98hc,
competent or not to transduce cell fusion signals into the cell.
Moreover, in regard to the work of Ginsberg and co-workers (48,
49, 194), it is worth mentioning that fusion induced by
anti CD98hc mAbs was blocked by fibronectin and
anti-
1-integrin antibodies, indicating functional and/or
physical interactions between the integrin system and CD98hc.
The role of CD98hc was further investigated by the generation of stable
cell lines expressing a chimera in which the cytoplasmic domain of
human CD98hc was replaced by the cytoplasmic domain of the
hemaglutinin-neuraminidase from the human parainfluenza virus type 2 and the mutant C330S [which does not impair either disulfide
linking to the LSHATs or amino acid transport (129)]. Both mutants suppressed the cell fusion-enhancing activity of anti-CD98hc MAbs (117). The mechanisms of these
dominant-negative effects are not known. If LSHATs are required for
cell fusion, the mutant CD98hc may titrate them in a way not competent
for fusion. The conformation of the CD98hc extracellular domain may also change to that of a fusion-incompetent molecule. The binding to
the MAbs, however, was not affected. Titration of -integrin cytoplasmic tails (194) by the C330S mutant could also
explain the dominant-negative effect, but this mechanism cannot be
invoked for the chimera.
More recently, Ito's group (62) has concentrated its efforts on the osteoclast differentiation pathway. Anti-CD98hc MAbs induced homotypic cell aggregation and multinucleated giant cell formation of monocytes without any other fusogen (62). These polykaryocytes displayed several (but not all) of the exclusive features of the osteoclasts. Two recent reports have begun to delineate monocyte-to-osteoclast signaling pathways elicited by the anti-CD98hc MAbs. The first (103) highlights the importance of induction of c-src expression and activation by the MAbs. This protein kinase is widely believed to play a role in cell differentiation (17). Targeted disruption of the c-src gene causes a form of osteopetrosis whereby osteoclasts are present but inactive (165), indicating that c-src is involved in osteoclastogenesis. Transcription of c-src is dependent on Sp1, which is also upregulated by anti-CD98hc MAbs. The use of a panel of inhibitors suggested the involvement of a tyrosine kinase-Ras-Map kinase-Sp1 pathway in the anti-CD98hc MAb induction of c-src in monocytes. As expected, an anti-CD98hc MAb that inhibited polykaryocyte formation (in the presence of an anti-CD98hc active MAb) also suppressed Sp1 and c-src expression.
The second report links the two known routes of osteoclastogenesis, the CD98-mediated pathway and that mediated by the osteoclast differentiation factor (ODF; a member of the cytokine family) (106) by showing that the latter is suppressed by an inhibitory anti-CD98hc MAb and that the former is inhibited by osteoclast inhibitory factor (a secreted member of the tumor necrosis factor receptor family), the classic inhibitor of the ODF-mediated pathway. The expression of the ODF receptor increases on incubation with the active anti-CD98hc MAbs.
The authors did not investigate the role of the integrin system in
osteoclast differentiation. One might speculate that similar results to
those seen in viral fusion would have been observed, suggesting the
intimate relationship between CD98 and integrins. In this sense, Suga
and colleagues (168) reported recently that crosslinking
of CD98 by MAbs mediated cell aggregation and adhesion of lymphocytes,
most likely by increasing the avidity of the
L
2- integrin for intercellular adhesion
molecule. The activation was dependent on phosphatidylinositol
(PI)3-kinase and on the persistent activation of the Ras-related small
GTPase Rap1. Moreover, in vitro fertilization of murine eggs (which
express CD98 on the surface) is inhibited by anti-CD98hc antibodies.
Fertilization is also inhibited by the recombinant soluble disintegrin
domain of A disintegrin and metalloprotease-3 protein (10,
195), which seems to interact with
1-integrins on
the egg surface. This interaction is also inhibited by anti-CD98
antibodies (170).
It becomes evident from the above results that the anti-CD98hc MAbs
could somehow mimic natural ligands for this protein, involved in cell
fusion and adhesion processes. CD98hc is a possible receptor for
galectin-3 (44), a 26-kDa -galactoside binding protein
of the galectin family (65). This protein is secreted by
monocytes/macrophages (151) and epithelial cells
(96) and may have a role in cell-cycle control, prevention
of T cell apoptosis, activation of several cell types,
including lymphocytes and monocytes/macrophages, and as a mediator of
cell-cell and cell-extracellular matrix adhesion (67, 86,
97). Very recently, galectin-3 has been shown to be a
chemoattractant for monocytes and macrophages (144) and to
induce uptake of extracellular calcium in T cells (43).
More studies are needed to define the roles of galectin-3-CD98hc
interactions and to identify other possible ligands of the CD98 complex.
T Cell Activation
Activation of T lymphocytes depends on two signals. One is mediated by the CD3 complex after interaction between the T cell receptor and the mysoin heavy chain-peptide complex (90). The second signal is independent of antigen. This less-characterized signal can be mediated by different T cell membrane proteins and/or cell adhesion molecules and ligands, including antibodies against integrins (177, 189), together with anti CD3 antibodies. CD98 is involved in this costimulatory signal, because some anti-CD98 antibodies can costimulate T lymphocytes, whereas others can inhibit them (41, 111). Little is known, however, about the mechanisms of these effects. Recently, Warren et al. (183) screened antibodies for their ability to costimulate T cells together with anti-CD3 antibodies and found a new CD98-specific antibody. This antibody alone induced EDTA-sensitive aggregation of T cells, a typical feature of cellular adhesion. More importantly, anti-integrin antibodies that inhibit costimulation mediated by integrins, but not by nonintegrins, were able to inhibit anti-CD98 antibody-mediated costimulation, again indicating a functional interaction between CD98 and integrins.Oncogenic Potential of CD98
Besides its expression in epithelial cells, activated B and T cells, and monocytes/macrophages, CD98hc is highly expressed in proliferative normal tissues and also in almost all tumor cells (125, 191). Overexpression of human and rat CD98hc has transforming activity in NIH3T3 cells (156). More importantly, this requires tight association with the LSHATs, because the rat CD98hc C103S mutant, which partially impairs association with light chain and amino acid transport, showed much lower reduced anchorage-independent growth than wild-type and C325S. The latter showed greater transforming activity than the wild-type control. Moreover, tumorigenicity of C103S transfectant Balb3T3 clones in nude mice was negligible compared with wild-type and C325S mutant. Amino acid transport activity and the proliferation rate of all transfectants remained the same, suggesting they are both dispensable for the tumorigenic effect. The enhanced effect of the C325S mutant might be caused by a conformational change caused by the mutation on the extracellular domain of CD98hc. The authors also reported (58) a positive correlation between progressive COOH-terminal deletions of the CD98hc extracellular domain and tumorigenic potential. Anti-CD98hc MAbs inhibited anchorage-independent growth of the mutants and wild-type (only until the epitopes recognized by the MAbs were lost). At this point, it is pertinent to remember the dominant-negative behavior of the human C330S mutant (homologous to the rat C32S mutant) on the anti-CD98hc-mediated enhancing effect on paramyxovirus syncitium cell formation (117). Whether this is related to its enhanced tumorigenic effect remains to be studied.The question arises as to whether a similar mechanism of CD98 action underlies all pf these different functions. A key CD98-integrin interaction may well be essential for cell fusion/differentiation, oncogenesis, and T cell activation. Cytoplasmic and TM of CD98hc seem to play a role in inside-out integrin signaling, and the extracellular domain is important for amino acid transport (48, 49, 194), although this may be light-subunit specific (19). However, it is not clear whether association with the light chain is necessary for integrin activation mediated by CD98hc. The disulfide bond between heavy and light chains of CD98, but not amino acid transport, is important for the oncogenic potential of the protein, perhaps via some steric restrictions imposed by this link. An analogous uncoupling between amino acid transport activity and disulfide bond formation between heavy and light subunits has been reported by Wagner et al. (180): the human CD98hc-LAT-1 complex induced the expression of a nonselective cation channel in oocytes; the human CD98hc C109S mutant only slightly affected amino acid transport but totally inactivated the expression of the associated channel. Further experiments may elucidate the possible role of the integrin system in CD98-mediated osteoclastogenesis and tumorigenesis.
![]() |
CELL PHYSIOLOGY OF THE LSHAT PROTEINS |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
LAT-1: Role in Normal and Tumorigenic Growth and Proliferation
LAT-1 protein was cloned as a truncated form (E16/TA1), rapidly induced and degraded on stimulation of primary lymphocytes, a pattern of expression indicative of an early-activation antigen (55). It was later shown to be highly expressed in many tumor cell lines, in proliferating tissues, and in primary human tumors but at barely detectable levels in adult tissues except brain, ovary, and placenta. It was proposed as a possible tumor marker (188), and recently its message has been found to be differentially expressed in the hepatoblastoma cell line HepG2 after exposure to the teratogenic agent 2,3,7,8-tetrachlorodibenzo-LAT-1 mRNA and system L respond to the amino acid availability in
primary rat hepatoma cells, suggesting adaptive regulation (157). A decrease in arginine, but not glutamine, levels
leads to the upregulation of LAT-1 message and system L transport, but not of 4F2hc message. Neither other amino acids nor the changes in
expression of other amino acid transporters (specifically other 4F2hc
partners) were studied (25). Arginine is not a substrate of LAT-1, perhaps indicating a complex interplay between amino acid
concentrations at either side of the membrane and the expression of the different amino acid transporters. Interestingly, arginine deprivation-induced upregulation was lost in tumor cells and in -glutamyl-transpeptidase (GGT)-positive transformed cells, where constitutively high levels of LAT-1 mRNA and system L activity were
found. These findings were extended in a recent paper in which the
levels of LAT-1 and system L were compared in nontransformed mouse
hepatocytes vs. NIH3T3 fibroblasts (26). On a cellular basis, LAT-1 and system L activity were lower in mouse hepatocytes, where system L activity was increased solely by the transfection of
LAT-1. This indicates either that 4F2hc was not limiting in that system
or that LAT-1 could displace other light chain binding to 4F2hc. The
first possibility seems more likely because cotransfection of 4F2hc and
LAT-1 did not increase the transport levels. LAT-1-transfected hepatocytes had a growth advantage in low-arginine medium compared with
nontransfected controls. This growth advantage was not due to changes
in the cell cycle and could be ascribed to different proliferation
rates, saturation densities, adhesion, or cell survival, although no
further experiments were performed to address these issues. The
fibroblasts displayed higher levels of LAT-1 and system L, which were
increased only by the cotransfection of both 4F2hc and LAT-1. The
authors speculated that LAT-1 upregulation in low-arginine media and
the constitutive high levels of system L in tumor cells may be
adaptations (and a positively selected mutation in the tumors) to
limited nutrient media, such as those in the tumor microenvironment,
where amino acid availability is low (68). This is
analogous to the role of GGT, which is upregulated early during hepatic
carcinogenesis. In this microenvironment, levels of cystine are low and
glutathione acts as a source of cysteine via GGT.
Masuko's group (58, 156) reported the oncogenic capacity of overexpressed wild-type and mutant forms of 4F2hc in NIH3T3 cells. This ability required a disulfide link with a light chain (Refs. 58 and 156 and see Oncogenic Potential of CD98), the identity of which was not investigated. A good candidate for this light chain could be LAT-1, given the high levels of system L in this system and the possibility that increases in system L give a growth advantage to the cells. However, no increase in amino acid transport was obtained on 4F2hc overexpression. Very recently, Bröer and co-workers (19) have shown that truncated 4F2hc forms retaining the cysteine important for heterodimer formation are still fully able to bind LAT-1 (but not other isoforms, like LAT-2 and y+LAT-2) and induce system L transport in oocytes.
The possible link between 4F2hc tumorigenic potential and the growth advantage through LAT-1 overexpression needs further study. The different 4F2 complexes expressed at the plasma membrane should be quantified and correlated with amino acid transport activities. Heteromeric and monomeric (if it exists) 4F2hc protein levels should be compared on the surface and in intracellular membranes of normal and tumor cell lines and tissues. Similar considerations apply to adaptive regulation: many other proteins are also regulated by amino acid deprivation, including the CAT1 cationic amino acid transporter (2), aspartate synthase (56), ornithine decarboxylase (29), system A (16), c-jun and c-myc (66), etc. This is consistent, for instance, with the general upregulation of amino acid transport activities and protein synthesis in hepatic tumors (13) and with the positive effects of amino acid transport on tumor invasiveness and proliferation (160). The availability of transcriptomics and proteomics technology may help in delineating general responses to individual amino acid availability and in mapping changes in transporter expression in proliferating cells and tumors and during development. Such studies are needed to obtain a general picture of amino acid transport regulation, which will help in the rational design of experiments to uncover the signaling pathways for these effects (e.g., the intriguing upregulation of LAT-1 expression by low arginine levels).
Recently, a general response to amino acid deprivation has been found in yeast (98). In mammalian cells there appears to be a similar signaling pathway, with the PI3-kinase-related mammalian target of rapamycin (mTOR) as its key point (84). mTOR directly or indirectly regulates two translational regulators, eukaryotic initiation factor 4E binding protein (eIF-4E BP1) and p70 S6 kinase (see Ref. 140 for review). In general, amino acid deprivation deactivates p70 S6 kinase and dephosphorylates eIF-4E BP1 (both events need mTOR activity), leading to a decrease in the rate of translation (59). In most studies, the authors found that the amino acids having a major individual impact in mimicking the effect of general amino acid deprivation were branched amino acids, specially L-leucine (50, 190). To date, the only branched amino acid transporters are ATB0+, a sodium- and chloride-dependent amino acid transporter, and some LSHAT proteins, such as LAT-1 and 2 and y+LAT-1 and -2 (19, 79, 131, 162, 175). This prompted the idea that some 4F2 complexes might act as sensors of amino acid availability, although this speculation is not supported by evidence. Thus the pathway from amino acid deprivation to mTOR remains a mystery. Regulation of protein synthesis is also important at the whole body level. Insulin increases general translation in peripheral tissues after a meal (87). Amino acids, especially leucine and arginine, are required for this effect, which is again mediated by an mTOR-dependent pathway. Actually, amino acids alone can produce the same effect as insulin. This could have physiological significance especially after a high-protein meal, when amino acid levels in blood are increased and can substantially contribute to the increase in protein synthesis, besides the insulin effect (169). Moreover, the positive effect of branched-chain amino acids (specially leucine) on protein turnover in muscle is well known (22).
Finally, it is worth mentioning that an increase in protein synthesis is essential for entry to mitosis (94). It will be interesting to investigate whether this could relate to the effect of LAT-1 expression and activity on growth and proliferation.
xCT: Protection Against Oxidative Stress
In most mammalian cell lines (4, 5), xSato and colleagues (149) took advantage of this latter
property for the expression cloning of the xCT protein that mediates x
Nitric oxide (NO)-mediated upregulation of xCT has also been reported
in other cell models (95). However, in LPS-activated macrophages, NO production is not induced, indicating that it is not
involved in LPS activation of xB and
electrophil-response element (EpRE) binding sites, but their
participation in LPS activation of xCT was excluded. On the other hand,
diethylmaleate (an electrophil agent) induction of xCT transport is
mediated, in part, by EpRE (71).
xCT in the brain is highly expressed in astrocytes, which protect
neurons from damage (91), and coculture experiments have shown that neuronal glutathione is maintained by astrocytes
(45). Moreover, embryonic neuronal/glial cultures depend
on x
Transepithelial Transport of Amino Acids
The reader is referred to the excellent review by Verrey and co-workers (179) on the role of HSHAT and LSHAT protein families in transepithelial transport. Therein, the role of the exchange mechanism and the asymmetry of transport by these proteins in an understanding of transepithelial transport is highlighted.Most of our knowledge about transepithelial transport of amino acids comes from early experiments in vivo (using microperfused tubules, kidney cortex slices, membrane vesicles, etc.) (158). The results were interpreted in the light of experiments with heterologous expression of amino acid transporters mainly in the X. laevis oocyte system, immunolocalization and in situ hybridization studies in tissues, and genetic data from the study of cystinuria and lysinuria (see INHERITED AMINOACIDURIAS) (118, 121). However, there is still a gap between former physiological studies and the molecular biological approaches. For instance, despite the discovery of two cystinuria genes, reabsorption of cystine is still unsolved in molecular terms: high-affinity transport of cystine in the S3 segment of the nephron is mediated by rBAT, but the identity of its partner LSHAT is controversial because b0,+AT is scarce in this segment (see Cystinuria). On the other hand, the protein responsible for the low-affinity transport of cystine at the S1 segment is not known. A good candidate is b0,+AT, because it is expressed at the appropiate site; however, it mediates high-affinity transport when heterologously expressed with rBAT (which, in turn, is barely detectable in the S1 segment!) (see Ref. 47a for review). Cystine transport is ensured because of its rapid reduction to cysteine in the cytosol. To complete reabsorption, cysteine is transported, in part, to the blood through a basolateral transport system(s), which is unknown. Moreover, although cationic amino acids can be reabsorbed by apical b0,+-like system in S3, a common apical influx of cystine and cationic amino acids is not demonstrated in S1 or S2 (see Ref. 124 for review), opening the possibility of a different apical cationic transporter in these segments.
Surprisingly, no polarized cell model has been used in a systematic way to study transepithelial transport in molecular terms. The well-known Madin-Darby canine kidney cell model appears as one possibility: Verrey's group has successfully stably expressed system b0,+AT in these cells (127). A complementary approach is to study a polarized cell model in which these transporters are already present. Our group reported the expression of rBAT in the proximal tubule-like OK cells. The b0,+-like transport activity was demonstrated in these cells, and it was shown to be dependent on rBAT expression using an antisense strategy (105). This demonstrates the usefulness of the model in the study of amino acid transport using conventional molecular biology techniques. Murer's group (81) has also used these cells successfully for transient transfection of phosphate transporters to study its polarized transport. So far, we have found y+L and L activities on the basolateral side of OK cells that matched y+LAT-1- and LAT-2-induced activities in oocytes, respectively, and we have cloned the opossum cDNAs for those proteins by homology (Fernández E and Palacín M, unpublished observations). We are presently performing antisense experiments to elucidate the role of these transporters in transepithelial transport of amino acids in OK cells. Although the function of y+LAT-1 is obvious due to its identification as the LPI gene (see LPI), the function of LAT-2 is still unknown.
In Fig. 6, a model for OK transepithelial
amino acid transport is shown. The cloned opossum transporters are
indicated. As pointed out in Verrey's review (179), at
least a sodium-dependent, neutral amino acid transporter on the apical
side and a net efflux transporter on the basolateral have to be invoked
to fully explain neutral and basic (and cystine) transepithelial
transport. There are some candidates for the first (162)
but no clues for the efflux transporter. The model should apply to the
kidney and most likely also to the intestine. The information available
for other epithelial tissues, such as liver, placenta, and the
blood-brain barrier, is scarce, although this situation is beginning to
change (3, 89).
|
Unexpected Substrates for LSHAT Proteins
The substrate specificity of LSHAT proteins has been extended in intriguing ways. Transport of T3 and T4 hormones by oocytes coinjected with 4F2hc and LAT-1 is saturable, with a Michaelis-Menten coefficient in the high-affinity range (<10 µM), and is inhibited by prototypic system L substrates, tryptophan, and the amino acid analog 2-amino-2 norbornane-carboxylic acid (BCH) (139). T3 and T4 are transported by other pathways [organic anion transporters Oatp (1) and Ntcp (51), and the recently cloned system T (85)], so the physiological relevance of the LAT-1 pathway is unclear. It should be remembered (see LAT-1: Role in Normal and Tumorigenic Growth and Proliferation) that LAT-1 and 4F2hc are highly expressed in proliferating cells and the key role of thyroid hormones in development is well known.L-DOPA, a precursor of dopamine, seems to be transported by 4F2hc-LAT-1 and also by rBAT in different cellular models (77, 163). Especially relevant could be the transport of L-DOPA via LAT-1 at the blood-brain barrier, where LAT-1 is highly expressed (12). In a cell model of the blood-brain barrier (MBEC4 cells), L-DOPA transport was strongly inhibited by Phe, Leu, and BCH, all LAT-1 substrates. In the kidney, dopamine is thought to be an autocrine-paracrine signal stimulating cAMP accumulation in tubule cells expressing L-amino acid decarboxylase (which converts L-DOPA to dopamine). BCH inhibited apical sodium-independent transport of L-DOPA in LLC-PK1 cells, indicating the presence of LAT-1or -2 at the apical border of these L-amino acid decarboxylase-positive cells. This finding is at odds with the known basolateral localization of 4F2hc, the partner of LAT-1 and -2, in tubule cells (135).
![]() |
ACKNOWLEDGEMENTS |
---|
The authors thank R. Rycroft for editorial help.
![]() |
FOOTNOTES |
---|
This work was supported by Grants PM96/0060 and PM99/0172 from the Dirección General de Investigación Científica y Técnica (Spain), Grant BIOMED2 CT98-BMH4-3514 from the European Commission, Grant 981930 from Fundació La Marató-TV3 (Catalonia, Spain), and the Comissionat per a Universitats I Recerca de la Generalitat de Catalunya (Spain; to M. Palacín).
Address for reprint requests and other correspondence: M. Palacín, Departament de Bioquimica i Biologia Molecular, Facultat de Biologia, Universitat de Barcelona, Avda. Diagonal 645, Barcelona E-08028, Spain. (E-mail: mpalacin{at}bio.ub.es).
![]() |
REFERENCES |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
1.
Abe, T,
Kakyo M,
Kagami H,
Tokui T,
Nishio T,
Tanemoto M,
Nomura H,
Hebert SC,
Matsuno S,
Kondo H,
and
Yawo H.
Molecular characterization and tissue distribution of a new organic anion transporter subtype (oatp3) that transports thyroid hormones and taurocholate and comparison with oatp2.
J Biol Chem
273:
22395-22401,
1998
2.
Aulak, KS,
Mishra R,
Zhou L,
Hyatt SL,
de Jonge W,
Lamers W,
Snider M,
and
Hatzoglou M.
Post-transcriptional regulation of the arginine transporter Cat-1 by amino acid availability.
J Biol Chem
274:
30424-30432,
1999
3.
Ayuk, PT,
Sibley CP,
Donnai P,
D'Souza S,
and
Glazier JD.
Development and polarization of cationic amino acid transporters and regulators in the human placenta.
Am J Physiol Cell Physiol
278:
C1162-C1171,
2000
4.
Bannai, S.
Exchange of cystine and glutamate across plasma membrane of human fibroblasts.
J Biol Chem
26:
2256-2263,
1986.
5.
Bannai, S,
Christensen HN,
Vadgama JV,
Ellory JC,
Englesberg E,
Guidotti GG,
Gazzola GC,
Kilberg MS,
Lajtha A,
Sacktor B,
Sepúlveda FV,
Yoing JD,
Yudilevich D,
and
Mann G.
Amino acid transport systems (Abstract).
Nature
311:
308,
1984[ISI][Medline].
6.
Bannai, S,
and
Ishii T.
Transport of cystine and cysteine and cell growth in cultured human diploid fibroblasts: effect of glutamate and homocysteate.
J Cell Physiol
112:
265-272,
1982[ISI][Medline].
7.
Bassi, MT,
Gasol E,
Manzoni M,
Pineda M,
Riboni M,
Martín R,
Zorzano A,
Borsani G,
and
Palacín M.
Identification and characterization of human xCT, which coexpresses with 4F2hc the amino acid transport system x
8.
Bertran, J,
Magagnin S,
Werner A,
Markovich D,
Biber J,
Testar X,
Zorzano A,
Kuhn LC,
Palacín M,
and
Murer H.
Stimulation of system y(+)-like amino acid transport by the heavy chain of human 4F2 surface antigen in Xenopus laevis oocytes.
Proc Natl Acad Sci USA
89:
5606-5610,
1992[Abstract].
9.
Bertran, J,
Werner A,
Moore ML,
Stange G,
Markovich D,
Biber J,
Testar X,
Zorzano A,
Palacín M,
and
Murer H.
Expression cloning of a cDNA from rabbit kidney cortex that induces a single transport system for cystine and dibasic and neutral amino acids.
Proc Natl Acad Sci USA
89:
5601-5605,
1992[Abstract].
10.
Bigler, D,
Takahashi Y,
Chen MS,
Almeida EA,
Osbourne L,
and
White JM.
Sequence-specific interaction between the disintegrin domain of mouse ADAM 2 (fertilin beta) and murine eggs. Role of the 6 integrin subunit.
J Biol Chem
275:
11576-11584,
2000
11.
Bisceglia, L,
Calonge MJ,
Totaro A,
Feliubadalo L,
Melchionda S,
Garcia J,
Testar X,
Gallucci M,
Ponzone A,
Zelante L,
Zorzano A,
Estivill X,
Gasparini P,
Nunes V,
and
Palacín M.
Localization, by linkage analysis, of the cystinuria type III gene to chromosome 19q13.1.
Am J Hum Genet
60:
611-616,
1997[ISI][Medline].
12.
Boado, RJ,
Li JY,
Nagaya M,
Zhang C,
and
Pardridge WM.
Selective expression of the large neutral amino acid transporter at the blood-brain barrier.
Proc Natl Acad Sci USA
96:
12079-12084,
1999
13.
Bode, BP,
and
Souba WW.
Modulation of cellular proliferation alters glutamine transport and metabolism in human hepatoma cells.
Ann Surg
220:
411-444,
1994[ISI][Medline].
14.
Borsani, G,
Bassi MT,
Sperandeo MP,
De Grandi A,
Buoninconti A,
Riboni M,
Manzoni M,
Incerti B,
Pepe A,
Andria G,
Ballabio A,
and
Sebastio G.
SLC7A7, encoding a putative permease-related protein, is mutated in patients with lysinuric protein intolerance.
Nature Genet
21:
297-301,
1999[ISI][Medline].
15.
Boyd, CA,
Devés R,
Laynes R,
Kudo Y,
and
Sebastio G.
Cationic amino acid transport through system y+L in erythrocytes of patients with lysinuric protein intolerance.
Pflügers Arch
439:
513-516,
2000[ISI][Medline].
16.
Bracy, DS,
Handlogten ME,
Barber EF,
Han HP,
and
Kilberg MS.
Cis-inhibition, trans-inhibition, and repression of hepatic amino acid transport mediated by System A. Substrate specificity and other properties.
J Biol Chem
261:
1514-1520,
1986
17.
Brickell, PM.
The p60c-src family of protein-tyrosine kinases: structure, regulation, and function.
Crit Rev Oncog
3:
401-446,
1992[Medline].
18.
Bridges, CC,
Kekuda R,
Wang H,
Prasad PD,
Mehta P,
Huang W,
Smith SB,
and
Ganapathy V.
Structure, function, and regulation of human cystine/glutamate transporter in retinal pigment epithelial cells.
Invest Ophthalmol Vis Sci
42:
47-54,
2001
19.
Bröer, A,
Friedrich B,
Wagner CA,
Fillon S,
Ganapathy V,
Lang F,
and
Bröer S.
Association of 4F2hc with light chains LAT1, LAT2 or y+LAT2 requires different domains.
Biochem J
355:
725-731,
2001[ISI][Medline].
20.
Bröer, A,
Wagner CA,
Lang F,
and
Bröer S.
The heterodimeric amino acid transporter 4F2hc/y+LAT2 mediates arginine efflux in exchange with glutamine.
Biochem J
349:
787-795,
2000[ISI][Medline].
21.
Busch, AE,
Herzer T,
Waldegger S,
Schmidt F,
Palacín M,
Biber J,
Markovich D,
Murer H,
and
Lang F.
Opposite directed currents induced by the transport of dibasic and neutral amino acids in Xenopus oocytes expressing the protein rBAT.
J Biol Chem
269:
25581-25586,
1994
22.
Buse, MG,
and
Reid Leucine SS.
A possible regulator of protein turnover in muscle.
J Clin Invest
56:
1250-1261,
1975[ISI][Medline].
23.
Calonge, MJ,
Gasparini P,
Chillarón J,
Chillon M,
Gallucci M,
Rousaud F,
Zelante L,
Testar X,
Dallapiccola B,
Di Silverio F,
Barceló P,
Estivill X,
Zorzano A,
Nunes V,
and
Palacín M.
Cystinuria caused by mutations in rBAT, a gene involved in the transport of cystine.
Nature Genet
6:
420-425,
1994[ISI][Medline].
24.
Calonge, MJ,
Volpini V,
Bisceglia L,
Rousaud F,
de Sanctis L,
Beccia E,
Zelante L,
Testar X,
Zorzano A,
Estivill X,
Gasparini P,
Nunes V,
and
Palacín M.
Genetic heterogeneity in cystinuria: the rBAT gene is linked to type I but not to type III cystinuria.
Proc Natl Acad Sci USA
92:
9667-9671,
1995[Abstract].
25.
Campbell, WA,
Sah DE,
Medina MM,
Albina JE,
Coleman WB,
and
Thompson NL.
TA1/LAT-1/CD98 light chain and system L activity, but not 4F2/CD98 heavy chain, respond to arginine availability in rat hepatic cells. Loss of response in tumor cells.
J Biol Chem
275:
5347-5354,
2000
26.
Campbell, WA,
and
Thompson NL.
Overexpression of LAT1/CD98 light chain is sufficient to increase system L-amino acid transport activity in mouse hepatocytes but not fibroblasts.
J Biol Chem
276:
16877-16884,
2001
27.
Chairoungdua, A,
Segawa H,
Kim JY,
Miyamoto K,
Haga H,
Fukui Y,
Mizoguchi K,
Ito H,
Takeda E,
Endou H,
and
Kanai Y.
Identification of an amino acid transporter associated with the cystinuria-related type II membrane glycoprotein.
J Biol Chem
274:
28845-28848,
1999
28.
Chen, YP,
O'Toole TE,
Shipley T,
Forsyth J,
LaFlamme SE,
Yamada KM,
Shattil SJ,
and
Ginsberg MH.
"Inside-out" signal transduction inhibited by isolated integrin cytoplasmic domains.
J Biol Chem
269:
18307-18310,
1994
29.
Chen, ZP,
and
Chen KY.
Mechanism of regulation of ornithine decarboxylase gene expression by asparagine in a variant mouse neuroblastoma cell line.
J Biol Chem
267:
6946-6951,
1992
30.
Chillarón, J,
Estévez R,
Mora C,
Wagner CA,
Suessbrich H,
Lang F,
Gelpi JL,
Testar X,
Busch AE,
Zorzano A,
and
Palacín M.
Obligatory amino acid exchange via systems b0,+-like and y+L-like. A tertiary active transport mechanism for renal reabsorption of cystine and dibasic amino acids.
J Biol Chem
271:
17761-17770,
1996
31.
Chillarón, J,
Estévez R,
Samarzija I,
Waldegger S,
Testar X,
Lang F,
Zorzano A,
Busch A,
and
Palacín M.
An intracellular trafficking defect in type I cystinuria rBAT mutants M467T and M467K.
J Biol Chem
272:
9543-9549,
1997
32.
Clark, EA,
and
Brugge JS.
Integrins and signal transduction pathways: the road taken.
Science
268:
233-239,
1995[ISI][Medline].
33.
Coady, MJ,
Chen XZ,
and
Lapointe JY.
rBAT is an amino acid exchanger with variable stoichiometry.
J Membr Biol
149:
1-8,
1996[ISI][Medline].
34.
Coady, MJ,
Jalal F,
Chen X,
Lemay G,
Berteloot A,
and
Lapointe JY.
Electrogenic amino acid exchange via the rBAT transporter.
FEBS Lett
356:
174-178,
1994[ISI][Medline].
35.
Cosgriff, AJ,
and
Pittard AJ.
A topological model for the general aromatic amino acid permease, AroP, of Escherichia coli.
J Bacteriol
179:
3317-3323,
1997[Abstract].
36.
Crowe, DT,
Chiu H,
Fong S,
and
Weissman IL.
Regulation of the avidity of integrin 4
7 by the
7 cytoplasmic domain.
J Biol Chem
269:
14411-14418,
1994
37.
Cuff, JA,
and
Barton GJ.
Evaluation and improvement of multiple sequence methods for protein secondary structure prediction.
Proteins
34:
508-519,
1999[ISI][Medline].
38.
Dall'Asta, V,
Bussolati O,
Sala R,
Franchi-Gazzola R,
Visigalli R,
Dall'Asta V,
Sala R,
Woo SK,
Kwon HM,
Gazzola GC,
and
Bussolati O.
Arginine transport through system y(+)L in cultured human fibroblasts: normal phenotype of cells from LPI subjects.
Am J Physiol Cell Physiol
279:
C1829-C1837,
2000
39.
Deora, AB,
Ghosh RN,
and
Tate SS.
Progressive C-terminal deletions of the renal cystine transporter, NBAT, reveal a novel bimodal pattern of functional expression.
J Biol Chem
273:
32980-32987,
1998
40.
Devés, R,
and
Boyd CA.
Surface antigen CD98(4F2): not a single membrane protein, but a family of proteins with multiple functions.
J Membr Biol
173:
165-177,
2000[ISI][Medline].
41.
Diaz, LA, Jr,
Friedman AW,
He X,
Kuick RD,
Hanash SM,
and
Fox DA.
Monocyte-dependent regulation of T lymphocyte activation through CD98.
Int Immunol
9:
1221-1231,
1997[Abstract].
42.
Dierks, T,
Riemer E,
and
Kramer R.
Reaction mechanism of the reconstituted aspartate/glutamate carrier from bovine heart mitochondria.
Biochim Biophys Acta
943:
231-244,
1988[ISI][Medline].
43.
Dong, S,
and
Hughes RC.
Galectin-3 stimulates uptake of extracellular Ca2+ in human Jurkat T-cells.
FEBS Lett
395:
165-169,
1996[ISI][Medline].
44.
Dong, S,
and
Hughes RC.
Macrophage surface glycoproteins binding to galectin-3 (Mac-2-antigen).
Glycoconj J
14:
267-274,
1997[ISI][Medline].
45.
Dringen, R,
Pfeiffer B,
and
Hamprecht B.
Synthesis of the antioxidant glutathione in neurons: supply by astrocytes of CysGly as precursor for neuronal glutathione.
J Neurosci
19:
562-569,
1999
46.
Ellis, J,
Carlin A,
Steffes C,
Wu J,
Liu J,
and
Rosen BP.
Topological analysis of the lysine-specific permease of Escherichia coli.
Microbiology
141:
1927-1935,
1995[Abstract].
47.
Estévez, R,
Camps M,
Rojas AM,
Testar X,
Devés R,
Hediger MA,
Zorzano A,
and
Palacín M.
The amino acid transport system y+L/4F2hc is a heteromultimeric complex.
FASEB J
12:
1319-1329,
1998
47a.
Feliubadalo, L,
Font M,
Purroy J,
Rousaud F,
Estivill X,
Nunes V,
Golomb E,
Centola M,
Aksentijevich I,
Kreiss Y,
Goldman B,
Pras M,
Kastner DL,
Pras E,
Gasparini P,
Bisceglia L,
Beccia E,
Gallucci M,
de Sanctis L,
Ponzone A,
Rizzoni GF,
Zelante L,
Bassi MT,
George AL, Jr,
Manzoni M,
De Grandi A,
Riboni M,
Endsley JK,
Ballabio A,
Borsani G,
Reig N,
Fernández E,
Estévez R,
Pineda M,
Torrents D,
Camps M,
Lloberas J,
Zorzano Z,
and
Palacín M.
Non-type I cystinuria caused by mutations in SLC7A9, encoding a subunit (b0,+AT) of rBAT.
Nature Genet
23:
52-57,
1999[ISI][Medline].
48.
Fenczik, CA,
Sethi T,
Ramos JW,
Hughes PE,
and
Ginsberg MH.
Complementation of dominant suppression implicates CD98 in integrin activation.
Nature
390:
81-85,
1997[ISI][Medline].
49.
Fenczik, CA,
Zent R,
Dellos M,
Calderwood DA,
Satriano J,
Kelly C,
and
Ginsberg MH.
Distinct domains of CD98hc regulate integrins and amino acid transport.
J Biol Chem
276:
8746-8752,
2001
49a.
Font, MA,
Feliubadalo L,
Estivill X,
Nunes V,
Golomb E,
Kreiss Y,
Pras E,
Bisceglia L,
d'Adamo AP,
Zelante L,
Gasparini P,
Bassi MT,
George AL, Jr,
Manzoni M,
Riboni M,
Ballabio A,
Borsani G,
Reig N,
Fernandez E,
Zorzano A,
Bertran J,
and
Palacín M.
Functional analysis of mutations in SLC7A9, and genotype/phenotype correlation in non-type I cystinuria.
Hum Mol Genet
10:
305-316,
2001
50.
Fox, HL,
Pham PT,
Kimball SR,
Jefferson LS,
and
Lynch CJ.
Amino acid effects on translational repressor 4E-BP1 are mediated primarily by L-leucine in isolated adipocytes.
Am J Physiol Cell Physiol
275:
C1232-C1238,
1998
51.
Friesema, ECH,
Docter R,
Moerings EPCM,
Stieger B,
Hagenbuch B,
Meier PJ,
Krenning EP,
Hennemann G,
and
Visser TJ.
Identification of thyroid hormone transporters.
Biochem Biophys Res Commun
254:
497-501,
1999[ISI][Medline].
52.
Fukasawa, Y,
Segawa H,
Kim JY,
Chairoungdua A,
Kim DK,
Matsuo H,
Cha SH,
Endou H,
and
Kanai Y.
Identification and characterization of a Na+-independent neutral amino acid transporter that associates with the 4F2 heavy chain and exhibits substrate selectivity for small neutral D- and L-amino acids.
J Biol Chem
275:
9690-9698,
2000
53.
Furriols, M,
Chillarón J,
Mora C,
Castello A,
Bertran J,
Camps M,
Testar X,
Vilaro S,
Zorzano A,
and
Palacín M.
rBAT, related to L-cysteine transport, is localized to the microvilli of proximal straight tubules, and its expression is regulated in kidney by development.
J Biol Chem
268:
27060-27068,
1993
54.
Gasparini, P,
Calonge MJ,
Bisceglia L,
Purroy J,
de Sanctis L,
Notarangelo A,
Rousaud F,
Gallucci M,
Testar X,
Ponzone A,
Estivill X,
Zorzano A,
Palacín M,
Nunes V,
and
Zelante L.
Molecular genetics of cystinuria: identification of 4 new mutations, 7 polymorphisms, and evidence for heterogeneity.
Am J Hum Genet
57:
781-788,
1995[ISI][Medline].
55.
Gaugitsch, HW,
Prieschl EE,
Kalthoff F,
Huber NE,
and
Baumruker T.
A novel transiently expressed, integral membrane protein linked to cell activation. Molecular cloning via the rapid degradation signal AUUUA.
J Biol Chem
267:
11267-11273,
1992
56.
Gong, SS,
Guerrini L,
and
Basilico C.
Regulation of asparagine synthetase gene expression by amino acid starvation.
Mol Cell Biol
11:
6059-6066,
1991[ISI][Medline].
57.
Goodyer, PR,
Clow C,
Reade T,
and
Girardin C.
Prospective analysis and classification of patients with cystinuria identified in a newborn screening program.
J Pediatr
122:
568-572,
1993[ISI][Medline].
58.
Hara, K,
Kudoh H,
Enomoto T,
Hashimoto Y,
and
Masuko T.
Enhanced tumorigenicity caused by truncation of the extracellular domain of GP125/CD98 heavy chain.
Oncogene
19:
6209-6215,
2000[ISI][Medline].
59.
Hara, K,
Yonezawa K,
Weng QP,
Kozlowski MT,
Belham C,
and
Avruch J.
Amino acid sufficiency and mTOR regulate p70 S6 kinase and eIF-4E BP1 through a common effector mechanism.
J Biol Chem
273:
14484-14494,
1998
60.
Haynes, BF,
Hemler ME,
Mann DL,
Eisenbarth GS,
Shelhamer J,
Mostowski HS,
Thomas CA,
Strominger JL,
and
Fauci AS.
Characterization of a monoclonal antibody (4F2) that binds to human monocytes and to a subset of activated lymphocytes.
J Immunol
126:
1409-1414,
1981
61.
Henthorn, PS,
Liu J,
Gidalevich T,
Fang J,
Casal ML,
Patterson DF,
and
Giger U.
Canine cystinuria: polymorphism in the canine SLC3A1 gene and identification of a nonsense mutation in cystinuric Newfoundland dogs.
Hum Genet
107:
295-303,
2000[ISI][Medline].
62.
Higuchi, S,
Tabata N,
Tajima M,
Ito M,
Tsurudome M,
Sudo A,
Uchida A,
and
Ito Y.
Induction of human osteoclast-like cells by treatment of blood monocytes with anti-fusion regulatory protein-1/CD98 monoclonal antibodies.
J Bone Miner Res
13:
44-49,
1998[ISI][Medline].
63.
Hu, LA,
and
King SC.
Membrane topology of the Escherichia coli gamma-aminobutyrate transporter: implications on the topography and mechanism of prokaryotic and eukaryotic transporters from the APC superfamily.
Biochem J
336:
69-76,
1998[ISI][Medline].
64.
Hughes, PE,
and
Pfaff M.
Integrin affinity modulation.
Trends Cell Biol
8:
359-364,
1998[ISI][Medline].
65.
Hughes, RC.
The galectin family of mammalian carbohydrate-binding molecules.
Biochem Soc Trans
25:
1194-1198,
1997[ISI][Medline].
66.
Hyatt, SL,
Aulak KS,
Malandro M,
Kilberg MS,
and
Hatzoglou M.
Adaptive regulation of the cationic amino acid transporter-1 (Cat-1) in Fao cells.
J Biol Chem
272:
19951-19957,
1997
67.
Inohara, H,
Akahani S,
Koths K,
and
Raz A.
Interactions between galectin-3 and Mac-2-binding protein mediate cell-cell adhesion.
Cancer Res
56:
4530-4534,
1996[Abstract].
68.
Inoue, Y,
Bode BP,
Copeland EM,
and
Souba WW.
Enhanced hepatic amino acid transport in tumor-bearing rats is partially blocked by antibody to tumor necrosis factor.
Cancer Res
55:
3525-3530,
1995[Abstract].
71.
Ishii, T,
Itoh K,
Takahashi S,
Sato H,
Yanagawa T,
Katoh Y,
Bannai S,
and
Yamamoto M.
Transcription factor Nrf2 coordinately regulates a group of oxidative stress-inducible genes in macrophages.
J Biol Chem
275:
16023-16029,
2000
72.
Isnard, AD,
Thomas D,
and
Surdin-Kerjan Y.
The study of methionine uptake in Saccharomyces cerevisiae reveals a new family of amino acid permeases.
J Mol Biol
262:
473-484,
1996[ISI][Medline].
73.
Ito, Y,
Komada H,
Kusagawa S,
Tsurudome M,
Matsumura H,
Kawano M,
Ohta H,
and
Nishio M.
Fusion regulation proteins on the cell surface: isolation and characterization of monoclonal antibodies which enhance giant polykaryocyte formation in Newcastle disease virus-infected cell lines of human origin.
J Virol
66:
5999-6007,
1992[Abstract].
74.
Jack, DL,
Paulsen IT,
and
Saier MH.
The amino acid/polyamine/organocation (APC) superfamily of transporters specific for amino acids, polyamines and organocations.
Microbiology
146:
1797-1814,
2000
75.
Janecek, S,
Svensson B,
and
Henrissat B.
Domain evolution in the alpha-amylase family.
J Mol Evol
45:
322-331,
1997[ISI][Medline].
76.
Kaback, HR.
Use of site-directed mutagenesis to study the mechanism of a membrane transport protein.
Biochemistry
26:
2071-2076,
1987[ISI][Medline].
77.
Kageyama, T,
Nakamura M,
Matsuo A,
Yamasaki Y,
Takakura Y,
Hashida M,
Kanai Y,
Naito M,
Tsuruo T,
Minato N,
and
Shimohama S.
The 4F2hc/LAT1 complex transports L-DOPA across the blood-brain barrier.
Brain Res
879:
115-121,
2000[ISI][Medline].
78.
Kanai, Y,
Fukasawa Y,
Cha SH,
Segawa H,
Chairoungdua A,
Kim DK,
Matsuo H,
Kim JY,
Miyamoto K,
Takeda E,
and
Endou H.
Transport properties of a system y+L neutral and basic amino acid transporter. Insights into the mechanisms of substrate recognition.
J Biol Chem
275:
20787-20793,
2000
79.
Kanai, Y,
Segawa H,
Miyamoto Ki,
Uchino H,
Takeda E,
and
Endou H.
Expression cloning and characterization of a transporter for large neutral amino acids activated by the heavy chain of 4F2 antigen (CD98).
J Biol Chem
273:
23629-23632,
1998
80.
Kanai, Y,
Stelzner MG,
Lee WS,
Wells RG,
Brown D,
and
Hediger MA.
Expression of mRNA (D2) encoding a protein involved in amino acid transport in S3 proximal tubule.
Am J Physiol Renal Fluid Electrolyte Physiol
263:
F1087-F1093,
1992
81.
Karim-Jimenez, Z,
Hernando N,
Biber J,
and
Murer H.
A dibasic motif involved in parathyroid hormone-induced down-regulation of the type IIa NaPi cotransporter.
Proc Natl Acad Sci USA
97:
12896-128901,
2000
82.
Kashiwagi, K,
Kuraishi A,
Tomitori H,
Igarashi A,
Nishimura K,
Shirahata A,
and
Igarashi K.
Identification of the putrescine recognition site on polyamine transport protein PotE.
J Biol Chem
275:
36007-36012,
2000
83.
Kashiwagi, K,
Shibuya S,
Tomitori H,
Kuraishi A,
and
Igarashi K.
Excretion and uptake of putrescine by the PotE protein in Escherichia coli.
J Biol Chem
272:
6318-6323,
1997
84.
Kilberg, MS,
Hutson RG,
and
Laine RO.
Amino acid-regulated gene expression in eukaryotic cells.
FASEB J
8:
13-19,
1994
85.
Kim, DK,
Kanai Y,
Chairoungdua A,
Matsuo H,
Cha SH,
and
Endou H.
Expression cloning of a Na+-independent aromatic amino acid transporter with structural similarity to H+/monocarboxylate transporters.
J Biol Chem
276:
17221-17228,
2001
86.
Kim, HR,
Lin HM,
Biliran H,
and
Raz A.
Cell cycle arrest and inhibition of anoikis by galectin-in human breast epithelial cells.
Cancer Res
59:
4148-41454,
1999
87.
Kimball, SR,
Vary TC,
and
Jefferson LS.
Regulation of protein synthesis by insulin.
Annu Rev Physiol
56:
321-348,
1994[ISI][Medline].
88.
Koyama, Y,
Kimura Y,
Hashimoto H,
Matsuda T,
and
Baba A.
L-lactate inhibits L-cystine/L-glutamate exchange transport and decreases glutathione content in rat cultured astrocytes.
J Neurosci Res
59:
685-691,
2000[ISI][Medline].
89.
Kudo, Y,
and
Boyd CA.
Characterisation of L-tryptophan transporters in human placenta: a comparison of brush border and basal membrane vesicles.
J Physiol (Lond)
531:
405-416,
2001
90.
Lanzavecchia, A,
Lezzi G,
and
Viola A.
From TCR engagement to T cell activation: a kinetic view of T cell behavior.
Cell
96:
1-4,
1999[ISI][Medline].
91.
Largo, C,
Cuevas P,
Somjen GG,
Martin del Rio R,
and
Herreras O.
The effect of depressing glial function in rat brain in situ on ion homeostasis, synaptic transmission, and neuron survival.
J Neurosci
16:
1219-1229,
1996[Abstract].
92.
Lauteala, T,
Mykkanen J,
Sperandeo MP,
Gasparini P,
Savontaus ML,
Simell O,
Andria G,
Sebastio G,
and
Aula P.
Genetic homogeneity of lysinuric protein intolerance.
Eur J Hum Genet
6:
612-615,
1998[ISI][Medline].
93.
Lauteala, T,
Sistonen P,
Savontaus ML,
Mykkanen J,
Simell J,
Lukkarinen M,
Simell O,
and
Aula P.
Lysinuric protein intolerance (LPI) gene maps to the long arm of chromosome 14.
Am J Hum Genet
60:
1479-1486,
1997[ISI][Medline].
94.
Lawrence, JC, Jr,
and
Abraham RT.
PHAS/4E-BPs as regulators of mRNA translation and cell proliferation.
Trends Biochem Sci
22:
345-349,
1997[ISI][Medline].
95.
Li, H,
Marshall ZM,
and
Whorton AR.
Stimulation of cystine uptake by nitric oxide: regulation of endothelial cell glutathione levels.
Am J Physiol Cell Physiol
276:
C803-C811,
1999
96.
Lindstedt, R,
Apodaca G,
Barondes SH,
Mostov KE,
and
Leffler H.
Apical secretion of a cytosolic protein by Madin-Darby canine kidney cells. Evidence for polarized release of an endogenous lectin by a nonclassical secretory pathway.
J Biol Chem
268:
11750-11757,
1993
97.
Liu, FT,
Hsu DK,
Zuberi RI,
Kuwabara I,
Chi EY,
and
Henderson WR, Jr.
Expression and function of galectin-3, a -galactoside-binding lectin, in human monocytes and macrophages.
Am J Pathol
147:
1016-1028,
1995[Abstract].
98.
Lorenz, MC,
and
Heitman J.
The MEP2 ammonium permease regulates pseudohyphal differentiation in Saccharomyces cerevisiae.
EMBO J
17:
1236-1247,
1998
99.
Luscher, B,
Rousseaux M,
Lees R,
MacDonald HR,
and
Bron C.
Cell surface glycoproteins involved in the stimulation of interleukin 1-dependent interleukin 2 production by a subline of EL4 thymoma cells. II. Structure, biosynthesis, and maturation.
J Immunol
135:
3951-3957,
1985
100.
Mannion, BA,
Kolesnikova TV,
Lin SH,
Wang S,
Thompson NL,
and
Hemler ME.
The light chain of CD98 is identified as E16/TA1 protein.
J Biol Chem
273:
33127-33129,
1998
101.
Mastroberardino, L,
Spindler B,
Pfeiffer R,
Skelly PJ,
Loffing J,
Shoemaker CB,
and
Verrey F.
Amino-acid transport by heterodimers of 4F2hc/CD98 and members of a permease family.
Nature
395:
288-291,
1998[ISI][Medline].
102.
Miyamoto, K,
Segawa H,
Tatsumi S,
Katai K,
Yamamoto H,
Taketani Y,
Haga H,
Morita K,
and
Takeda E.
Effects of truncation of the COOH-terminal region of a Na+-independent neutral and basic amino acid transporter on amino acid transport in Xenopus oocytes.
J Biol Chem
271:
16758-16763,
1996
103.
Miyamoto, N,
Higuchi Y,
Tsurudome M,
Ito M,
Nishio M,
Kawano M,
Sudo A,
Kato K,
Uchida A,
and
Ito Y.
Induction of c-Src in human blood monocytes by anti-CD98/FRP-1 mAb in an Sp1-dependent fashion.
Cell Immunol
204:
105-113,
2000[ISI][Medline].
104.
Mizoguchi, K,
Cha SH,
Chairoungdua A,
Kim DK,
Shigeta Y,
Matsuo H,
Fukushima JI,
Awa Y,
Akakura K,
Goya T,
Ito H,
Endou H,
and
Kanai Y.
Human cystinuria-related transporter: localization and functional characterization.
Kidney Int
59:
1821-1833,
2001[ISI][Medline].
105.
Mora, C,
Chillarón J,
Calonge MJ,
Forgo J,
Testar X,
Nunes V,
Murer H,
Zorzano A,
and
Palacín M.
The rBAT gene is responsible for L-cystine uptake via the b0,(+)-like amino acid transport system in a "renal proximal tubular" cell line (OK cells).
J Biol Chem
271:
10569-10576,
1996
106.
Mori, K,
Miyamoto N,
Higuchi Y,
Nanba K,
Ito M,
Tsurudome M,
Nishio M,
Kawano M,
Uchida A,
and
Ito Y.
Cross-talk between RANKL and FRP-1/CD98 Systems: RANKL-mediated osteoclastogenesis is suppressed by an inhibitory anti-CD98 heavy chain mAb and CD98-mediated osteoclastogenesis is suppressed by osteoclastogenesis inhibitory factor.
Cell Immunol
207:
118-126,
2001[ISI][Medline].
107.
Mosckovitz, R,
Udenfriend S,
Felix A,
Heimer E,
and
Tate SS.
Membrane topology of the rat kidney neutral and basic amino acid transporter.
FASEB J
8:
1069-1074,
1994
108.
Murata, K,
Mitsuoka K,
Hirai T,
Walz T,
Agre P,
Bernard Heymann J,
Engel A,
and
Fujiyoshi Y.
Structural determinants of water permeation through aquaporin-1.
Nature
407:
599-605,
2000[ISI][Medline].
109.
Mykkanen, J,
Torrents D,
Pineda M,
Camps M,
Yoldi ME,
Horelli-Kuitunen N,
Huoponen K,
Heinonen M,
Oksanen J,
Simell O,
Savontaus ML,
Zorzano A,
Palacín M,
and
Aula P.
Functional analysis of novel mutations in y(+)LAT-1 amino acid transporter gene causing lysinuric protein intolerance (LPI).
Hum Mol Genet
9:
431-438,
2000
110.
Nakamura, E,
Sato M,
Yang H,
Miyagawa F,
Harasaki M,
Tomita K,
Matsuoka S,
Noma A,
Iwai K,
and
Minato N.
4F2 (CD98) heavy chain is associated covalently with an amino acid transporter and controls intracellular trafficking and membrane topology of 4F2 heterodimer.
J Biol Chem
274:
3009-3016,
1999
111.
Nakao, M,
Kubo K,
Hara A,
Hirohashi N,
Futagami E,
Shichijo S,
Sagawa K,
and
Itoh K.
A monoclonal antibody (H227) recognizing a new epitope of 4F2 molecular complex associated with T cell activation.
Cell Immunol
152:
226-233,
1993[ISI][Medline].
112.
Nakauchi, J,
Matsuo H,
Kim DK,
Goto A,
Chairoungdua A,
Cha SH,
Inatomi J,
Shiokawa Y,
Yamaguchi K,
Saito I,
Endou H,
and
Kanai Y.
Cloning and characterization of a human brain Na+-independent transporter for small neutral amino acids that transports D-serine with high affinity.
Neurosci Lett
287:
231-235,
2000[ISI][Medline].
113.
Notredame, C,
Higgins D,
and
Heringa J.
T-coffee: a novel method for multiple sequence alignments.
J Mol Biol
302:
205-217,
2000[ISI][Medline].
114.
Ohgimoto, S,
Tabata N,
Suga S,
Nishio M,
Ohta H,
Tsurudome M,
Komada H,
Kawano M,
Watanabe N,
and
Ito Y.
Molecular characterization of fusion regulatory protein-1 (FRP-1) that induces multinucleated giant cell formation of monocytes and HIV gp160-mediated cell fusion. FRP-1 and 4F2/CD98 are identical molecules.
J Immunol
155:
3585-3592,
1995[Abstract].
115.
Ohta, H,
Tsurudome M,
Matsumura H,
Koga Y,
Morikawa S,
Kawano M,
Kusugawa S,
Komada H,
Nishio M,
and
Ito Y.
Molecular and biological characterization of fusion regulatory proteins (FRPs): anti-FRP mAbs induced HIV-mediated cell fusion via an integrin system.
EMBO J
13:
2044-2055,
1994[Abstract].
116.
Oka, A,
Belliveau MJ,
Rosenberg PA,
and
Volpe JJ.
Vulnerability of oligodendroglia to glutamate: pharmacology, mechanisms, and prevention.
J Neurosci
13:
1441-1453,
1993[Abstract].
117.
Okamoto, K,
Ohgimoto S,
Nishio M,
Tsurudome M,
Kawano M,
Komada H,
Ito M,
Sakakura Y,
and
Ito Y.
Paramyxovirus-induced syncytium cell formation is suppressed by a dominant negative fusion regulatory protein-1 (FRP-1)/CD98 mutated construct: an important role of FRP-1 in virus-induced cell fusion.
J Gen Virol
78:
775-783,
1997[Abstract].
118.
Palacín, M,
Bertran J,
and
Zorzano A.
Heteromeric amino acid transporters explain inherited aminoacidurias.
Curr Opin Nephrol Hypert
9:
547-553,
2000[ISI][Medline].
119.
Palacín, M,
Borsani G,
and
Sebastio G.
The molecular bases of cystinuria and lysinuric protein intolerance.
Curr Opin Genet Develop
11:
328-335,
2001[ISI][Medline].
120.
Palacín, M,
Chillarón J,
and
Mora C.
Role of the b(0,+)-like amino acid-transport system in the renal reabsorption of cystine and dibasic amino acids.
Biochem Soc Trans
24:
856-863,
1996[ISI][Medline].
121.
Palacín, M,
Estévez R,
Bertran J,
and
Zorzano A.
Molecular biology of mammalian plasma membrane amino acid transporters.
Physiol Rev
78:
969-1054,
1998
122.
Palacín, M,
Estévez R,
and
Zorzano A.
Cystinuria calls for heteromultimeric amino acid transporters.
Curr Opin Cell Biol
10:
455-461,
1998[ISI][Medline].
123.
Palacín, M,
Fernández E,
Chillarón J,
and
Zorzano A.
The amino acid transport system b0,+ and cystinuria.
Mol Membr Biol
18:
21-26,
2001[ISI][Medline].
124.
Palacín, M,
Goodyer P,
Nunes V,
and
Gasparini P.
Cystinuria.
In: Metabolic and Molecular Bases of Inherited Diseases (8th ed.), edited by Scriver CR,
Beaudet AL,
Sly SW,
and Valle D.. New York: McGraw-Hill, 2001, p. 4909-4932.
125.
Parmacek, MS,
Karpinski BA,
Gottesdiener KM,
Thompson CB,
and
Leiden JM.
Structure, expression and regulation of the murine 4F2 heavy chain.
Nucleic Acids Res
17:
1915-1931,
1989[Abstract].
126.
Peter, GJ,
Panova TB,
Christie GR,
and
Taylor PM.
Cysteine residues in the C-terminus of the neutral- and basic-amino-acid transporter heavy-chain subunit contribute to functional properties of the system b(0,+)-type amino acid transporter.
Biochem J
351:
677-682,
2000[ISI][Medline].
127.
Pfeiffer, R,
Loffing J,
Rossier G,
Bauch C,
Meier C,
Eggermann T,
Loffing-Cueni D,
Kuhn LC,
and
Verrey F.
Luminal heterodimeric amino acid transporter defective in cystinuria.
Mol Biol Cell
10:
4135-4147,
1999
128.
Pfeiffer, R,
Rossier G,
Spindler B,
Meier C,
Kuhn L,
and
Verrey F.
Amino acid transport of y(+)L-type by heterodimers of 4F2hc/CD98 and members of the glycoprotein-associated amino acid transporter family.
EMBO J
18:
49-57,
1999
129.
Pfeiffer, R,
Spindler B,
Loffing J,
Skelly PJ,
Shoemaker CB,
and
Verrey F.
Functional heterodimeric amino acid transporters lacking cystine residues involved in disulfide bond.
FEBS Lett
439:
157-162,
1998[ISI][Medline].
130.
Pickel, VM,
Nirenberg MJ,
Chan J,
Mosckovitz R,
Udenfriend S,
and
Tate SS.
Ultrastructural localization of a neutral and basic amino acid transporter in rat kidney and intestine.
Proc Natl Acad Sci USA
90:
7779-7783,
1993
131.
Pineda, M,
Fernandez E,
Torrents D,
Estévez R,
Lopez C,
Camps M,
Lloberas J,
Zorzano A,
and
Palacín M.
Identification of a membrane protein, LAT-2, that co-expresses with 4F2 heavy chain, an L-type amino acid transport activity with broad specificity for small and large zwitterionic amino acids.
J Biol Chem
274:
19738-19744,
1999
132.
Poolman, B,
Modderman R,
and
Reizer J.
Lactose transport system of Streptococcus thermophilus. The role of histidine residues.
J Biol Chem
267:
9150-9157,
1992
133.
Prasad, PD,
Wang H,
Huang W,
Kekuda R,
Rajan DP,
Leibach FH,
and
Ganapathy V.
Human LAT1, a subunit of system L amino acid transporter: molecular cloning and transport function.
Biochem Biophys Res Commun
255:
283-288,
1999[ISI][Medline].
134.
Quackenbush, E,
Clabby M,
Gottesdiener KM,
Barbosa J,
Jones NH,
Strominger JL,
Speck S,
and
Leiden JM.
Molecular cloning of complementary DNAs encoding the heavy chain of the human 4F2 cell-surface antigen: a type II membrane glycoprotein involved in normal and neoplastic cell growth.
Proc Natl Acad Sci USA
84:
6526-6530,
1987[Abstract].
135.
Quackenbush, EJ,
Gougos A,
Baumal R,
and
Letarte M.
Differential localization within human kidney of five membrane proteins expressed on acutelymphoblastic leukemia cells.
J Immunol
136:
118-124,
1986
136.
Rajan, DP,
Huang W,
Kekuda R,
George RL,
Wang J,
Conway SJ,
Devoe LD,
Leibach FH,
Prasad PD,
and
Ganapathy V.
Differential influence of the 4F2 heavy chain and the protein related to b(0,+) amino acid transport on substrate affinity of the heteromeric b(0,+) amino acid transporter.
J Biol Chem
275:
14331-14335,
2000
137.
Rajan, DP,
Kekuda R,
Huang W,
Devoe LD,
Leibach FH,
Prasad PD,
and
Ganapathy V.
Cloning and functional characterization of a Na(+)-independent, broad-specific neutral amino acid transporter from mammalian intestine.
Biochim Biophys Acta
1463:
6-14,
2000[ISI][Medline].
138.
Rajan, DP,
Kekuda R,
Huang W,
Wang H,
Devoe LD,
Leibach FH,
Prasad PD,
and
Ganapathy V.
Cloning and expression of a b(0,+)-like amino acid transporter functioning as a heterodimer with 4F2hc instead of rBAT. A new candidate gene for cystinuria.
J Biol Chem
274:
29005-29010,
1999
139.
Ritchie, JW,
Peter GJ,
Shi YB,
and
Taylor PM.
Thyroid hormone transport by 4F2hc-IU12 heterodimers expressed in Xenopus oocytes.
J Endocrinol
163:
R5-R9,
1999[Abstract].
140.
Rohde, J,
Heitman J,
and
Cardenas ME.
The TOR kinases link nutrient sensing to cell growth.
J Biol Chem
276:
9583-9586,
2001
141.
Rossier, G,
Meier C,
Bauch C,
Summa V,
Sordat B,
Verrey F,
and
Kuhn LC.
LAT2, a new basolateral 4F2hc/CD98-associated amino acid transporter of kidney and intestine.
J Biol Chem
274:
34948-34954,
1999
142.
Russ, WP,
and
Engelman DM.
The GxxxG motif: a framework for transmembrane helix-helix association.
J Mol Biol
296:
911-919,
2000[ISI][Medline].
143.
Saadi, I,
Chen XZ,
Hediger M,
Ong P,
Pereira P,
Goodyer P,
and
Rozen R.
Molecular genetics of cystinuria: mutation analysis of SLC3A1 and evidence for another gene in type I (silent) phenotype.
Kidney Int
54:
48-55,
1998[ISI][Medline].
144.
Sano, H,
Hsu DK,
Yu L,
Apgar JR,
Kuwabara I,
Yamanaka T,
Hirashima M,
and
Liu FT.
Human galectin-3 is a novel chemoattractant for monocytes and macrophages.
J Immunol
165:
2156-2164,
2000
145.
Santamaria, F,
Parenti G,
Guidi G,
Rotondo A,
Grillo G,
Larocca MR,
Celentano L,
Strisciuglio P,
Sebastio G,
and
Andria G.
Early detection of lung involvement in lysinuric protein intolerance: role of high resolution computed tomography and radioisotopic methods.
Am J Respir Crit Care Med
153:
731-735,
1996[Abstract].
146.
Sarkar, D,
Kambe F,
Hirata A,
Iseki A,
Ohmori S,
and
Seo H.
Expression of E16/CD98LC/hLAT1 is responsive to 2,3,7,8-tetrachlorodibenzo-dioxin.
FEBS Lett
462:
430-434,
1999[ISI][Medline].
147.
Sato, H,
Fujiwara K,
Sagara J,
and
Bannai S.
Induction of cystine transport activity in mouse peritoneal macrophages by bacterial lipopolysaccharide.
Biochem J
310:
547-551,
1995[ISI][Medline].
148.
Sato, H,
Kuriyama-Matsumura K,
Hashimoto T,
Sasaki H,
Wang H,
Ishii T,
Mann GE,
and
Bannai S.
Effect of oxygen on induction of the cystine transporter by bacterial lipopolysaccharide in mouse peritoneal macrophages.
J Biol Chem
276:
10407-10412,
2001
149.
Sato, H,
Tamba M,
Ishii T,
and
Bannai S.
Cloning and expression of a plasma membrane cystine/glutamate exchange transporter composed of two distinct proteins.
J Biol Chem
274:
11455-11448,
1999
150.
Sato, H,
Tamba M,
Kuriyama-Matsumura K,
Okuno S,
and
Bannai S.
Molecular cloning and expression of human xCT, the light chain of amino acid transport system x
151.
Sato, S,
and
Hughes RC.
Regulation of secretion and surface expression of Mac-2, a galactoside-binding protein of macrophages.
J Biol Chem
269:
4424-4430,
1994
152.
Schlaepfer, DD,
and
Hunter T.
Integrin signalling and tyrosine phosphorylation: just the FAKs?
Trends Cell Biol
8:
151-157,
1998[ISI][Medline].
153.
Segal, S,
and
Thier SO.
Cystinuria.
In: Metabolic and Molecular Bases of Inherited Diseases, edited by Scriver CH,
Beaudet AL,
Sly WS,
and Valle D.. New York: McGraw-Hill, 1995, p. 2479-2496.
154.
Segawa, H,
Fukasawa Y,
Miyamoto K,
Takeda E,
Endou H,
and
Kanai Y.
Identification and functional characterization of a Na+-independent neutral amino acid transporter with broad substrate selectivity.
J Biol Chem
274:
19745-19751,
1999
155.
Shi J, Blundell TL, and Mizuguchi K. FUGUE: sequence-structure
homology recognition using enviorement-specific substitution tables and
structure-dependent gap penalties. J Mol Biol. In press.
156.
Shishido, T,
Uno S,
Kamohara M,
Tsuneoka-Suzuki T,
Hashimoto Y,
Enomoto T,
and
Masuko T.
Transformation of BALB3T3 cells caused by over-expression of rat CD98 heavy chain (HC) requires its association with light chain: mis-sense mutation in a cysteine residue of CD98HC eliminates its transforming activity.
Int J Cancer
87:
311-316,
2000[ISI][Medline].
157.
Shultz, VD,
Campbell W,
Karr S,
Hixson DC,
and
Thompson NL.
TA1 oncofetal rat liver cDNA and putative amino acid permease: temporal correlation with c-myc during acute CCl4 liver injury and variation of RNA levels in response to amino acids in hepatocyte cultures.
Toxicol Appl Pharmacol
154:
84-96,
1999[ISI][Medline].
158.
Silbernagl, S.
The renal handling of amino acids and oligopeptides.
Physiol Rev
68:
911-1007,
1988
159.
Simell, O.
Lysinuric protein intolerance and other cationic aminoacidurias.
In: Metabolic and Molecular Bases of Inherited Diseases (8th ed.), edited by Scriver CR,
Beaudet AL,
Sly SW,
and Valle D.. New York: McGraw-Hill, 2001, p. 4933-4956.
160.
Singh, RK,
Rinehart CA,
Kim JP,
Tolleson-Rinehart S,
Lawing LF,
Kaufman DG,
and
Siegal GP.
Tumor cell invasion of basement membrane in vitro is regulated by amino acids.
Cancer Invest
14:
6-18,
1996[ISI][Medline].
161.
Skelly, PJ,
Pfeiffer R,
Verrey F,
and
Shoemaker CB.
SPRM1lc, a heterodimeric amino acid permease light chain of the human parasitic platyhelminth, Schistosoma mansoni.
Parasitology
119:
569-576,
1999[ISI][Medline].
162.
Sloan, JL,
and
Mager S.
Cloning and functional expression of a human Na(+) and Cl()-dependent neutral and cationic amino acid transporter B(0+).
J Biol Chem
274:
23740-23745,
1999
163.
Soares-Da-Silva, P,
and
Serrao MP.
Molecular modulation of inward and outward apical transporters of L-dopa in LLC-PK1 cells.
Am J Physiol Renal Physiol
279:
F736-F746,
2000
164.
Sonnhammer, ELL,
von Heijne G,
and
Krogh A.
A hidden Markov model for predicting transmembrane helices in protein sequences.
In: Proceedings of Sixth International Conference on Intelligent Systems for Molecular Biology, edited by Glasgow J,
Littlejohn T,
Major F,
Lathrop R,
Sankoff D,
and Sensen C.. Menlo Park, CA: AAAI, 1998, p. 175-182.
165.
Soriano, P,
Montgomery C,
Geske R,
and
Bradley A.
Targeted disruption of the c-src proto-oncogene leads to osteopetrosis in mice.
Cell
64:
693-702,
1991[ISI][Medline].
166.
Stoller, ML,
Bruce JE,
Bruce CA,
Foroud T,
Kirkwood SC,
and
Stambrook PJ.
Linkage of type II and type III cystinuria to 19q13.1: codominant inheritance of two cystinuric alleles at 19q13.1 produces an extreme stone-forming phenotype.
Am J Med Genet
86:
134-139,
1999[ISI][Medline].
167.
Stonehouse, TJ,
Woodhead VE,
Herridge PS,
Ashrafian H,
George M,
Chain BM,
and
Katz DR.
Molecular characterization of U937-dependent T-cell co-stimulation.
Immunology
96:
35-47,
1999[ISI][Medline].
168.
Suga, K,
Katagiri K,
Kinashi T,
Harazaki M,
Iizuka T,
Hattori M,
and
Minato N.
CD98 induces LFA-1-mediated cell adhesion in lymphoid cells via activation of Rap1.
FEBS Lett
489:
249-253,
2001[ISI][Medline].
169.
Svanberg, E,
Jefferson LS,
Lundholm K,
and
Kimball SR.
Postprandial stimulation of muscle protein synthesis is independent of changes in insulin.
Am J Physiol Endocrinol Metab
272:
E841-E847,
1997
170.
Takahashi, Y,
Bigler D,
Ito Y,
and
White JM.
Sequence-specific interaction between the disintegrin domain of mouse ADAM 3 and murine eggs: role of 1 integrin-associated proteins CD9, CD81, and CD98.
Mol Biol Cell
12:
809-820,
2001
171.
Tate, SS,
Yan N,
and
Udenfriend S.
Expression cloning of a Na(+)-independent neutral amino acid transporter from rat kidney.
Proc Natl Acad Sci USA
89:
1-5,
1992[Abstract].
172.
Teixeira, S,
Di Grandi S,
and
Kuhn LC.
Primary structure of the human 4F2 antigen heavy chain predicts a transmembrane protein with a cytoplasmic NH2 terminus.
J Biol Chem
262:
9574-9580,
1987
173.
Thompson, JD,
Gibson TJ,
Plewniak F,
Jeanmougin F,
and
Higgins DG.
The ClustalX windows interface: flexible strategies for multiple sequence alignment aided by quality analysis tools.
Nucl Acid Res
24:
4876-4882,
1997.
174.
Torras-Llort, M,
Torrents D,
Soriano-Garcia JF,
Gelpi JL,
Estévez R,
Ferrer R,
Palacín M,
and
Moreto M.
Sequential amino acid exchange across b(0,+)-like system in chicken brush border jejunum.
J Membr Biol
180:
213-220,
2001[ISI][Medline].
175.
Torrents, D,
Estévez R,
Pineda M,
Fernandez E,
Lloberas J,
Shi YB,
Zorzano A,
and
Palacín M.
Identification and characterization of a membrane protein (y+L amino acid transporter-1) that associates with 4F2hc to encode the amino acid transport activity y+L. A candidate gene for lysinuric protein intolerance.
J Biol Chem
273:
32437-32445,
1998
176.
Torrents, D,
Mykkanen J,
Pineda M,
Feliubadalo L,
Estévez R,
de Cid R,
Sanjurjo P,
Zorzano A,
Nunes V,
Huoponen K,
Reinikainen A,
Simell O,
Savontaus ML,
Aula P,
and
Palacín M.
Identification of SLC7A7, encoding y+LAT-1, as the lysinuric protein intolerance gene.
Nature Genet
21:
293-296,
1999[ISI][Medline].
177.
Van Seventer, GA,
Shimizu Y,
Horgan KJ,
and
Shaw S.
The LFA-1 ligand ICAM-1 provides an important costimulatory signal for T cell receptor-mediated activation of resting T cells.
J Immunol
144:
4579-4586,
1990
178.
Verrey, F,
Jack DL,
Paulsen IT,
Saier MH, Jr,
and
Pfeiffer R.
New glycoprotein-associated amino acid transporters.
J Membr Biol
172:
181-192,
1999[ISI][Medline].
179.
Verrey, F,
Meier C,
Rossier G,
and
Kuhn LC.
Glycoprotein-associated amino acid exchangers: broadening the range of transport specificity.
Pflügers Arch
440:
503-512,
2000[ISI][Medline].
180.
Wagner, CA,
Bröer A,
Albers A,
Gamper N,
Lang F,
and
Bröer S.
The heterodimeric amino acid transporter 4F2hc/LAT1 is associated in Xenopus oocytes with a non-selective cation channel that is regulated by the serine/threonine kinase sgk-1.
J Physiol (Lond)
526:
35-46,
2000
181.
Wang, Y,
and
Tate SS.
Oligomeric structure of a renal cystine transporter: implications in cystinuria.
FEBS Lett
368:
389-392,
1995[ISI][Medline].
182.
Warren, AP,
Patel K,
McConkey DJ,
and
Palacios R.
CD98: a type II transmembrane glycoprotein expressed from the beginning of primitive and definitive hematopoiesis may play a critical role in the development of hematopoietic cells.
Blood
87:
3676-3687,
1996
183.
Warren, AP,
Patel K,
Miyamoto Y,
Wygant JN,
Woodside DG,
and
McIntyre BW.
Convergence between CD98 and integrin-mediated T-lymphocyte co-stimulation.
Immunology
99:
62-68,
2000[ISI][Medline].
184.
Wartenfeld, R,
Golomb E,
Katz G,
Bale SJ,
Goldman B,
Pras M,
Kastner DL,
and
Pras E.
Molecular analysis of cystinuria in Libyan Jews: exclusion of the SLC3A1 gene and mapping of a new locus on 19q.
Am J Hum Genet
60:
617-624,
1997[ISI][Medline].
185.
Watanabe, K,
Hata Y,
Kizaki H,
Katsube Y,
and
Suzuki Y.
The refined crystal structure of Bacillus cereus oligo-1,6-glucosidase at 2.0. A resolution: structural characterization of proline-substitution sites for protein thermostabilization.
J Mol Biol
269:
142-153,
1997[ISI][Medline].
186.
Wells, RG,
and
Hediger MA.
Cloning of a rat kidney cDNA that stimulates dibasic and neutral amino acid transport and has sequence similarity to glucosidases.
Proc Natl Acad Sci USA
89:
5596-5600,
1992[Abstract].
187.
Wells, RG,
Lee WS,
Kanai Y,
Leiden JM,
and
Hediger MA.
The 4F2 antigen heavy chain induces uptake of neutral and dibasic amino acids in Xenopus oocytes.
J Biol Chem
267:
15285-15288,
1992
188.
Wolf, DA,
Wang S,
Panzica MA,
Bassily NH,
and
Thompson NL.
Expression of a highly conserved oncofetal gene, TA1/E16, in human colon carcinoma and other primary cancers: homology to Schistosoma mansoni amino acid permease and Caenorhabditis elegans gene products.
Cancer Res
56:
5012-5022,
1996[Abstract].
189.
Woodside, DG,
Teague TK,
and
McIntyre BW.
Specific inhibition of T lymphocyte coactivation by triggering integrin 1 reveals convergence of
1,
2, and
7 signaling pathways.
J Immunol
157:
700-706,
1996[Abstract].
190.
Xu, G,
Kwon G,
Marshall CA,
Lin TA,
Lawrence JC, Jr,
and
McDaniel ML.
Branched-chain amino acids are essential in the regulation of PHAS-I and p70 S6 kinase by pancreatic -cells. A possible role in protein translation and mitogenic signaling.
J Biol Chem
273:
28178-28184,
1998
191.
Yagita, H,
Masuko T,
and
Hashimoto Y.
Inhibition of tumor cell growth in vitro by murine monoclonal antibodies that recognize a proliferation-associated cell surface antigen system in rats and humans.
Cancer Res
46:
1478-1484,
1986[Abstract].
192.
Yamaguchi, A,
Nakatani M,
and
Sawai T.
Aspartic acid-66 is the only essential negatively charged residue in the putative hydrophilic loop region of the metal-tetracycline/H+ antiporter encoded by transposon Tn10 of Escherichia coli.
Biochemistry
31:
8344-8348,
1992[ISI][Medline].
193.
Zambruno, G,
Marchisio PC,
Marconi A,
Vaschieri C,
Melchiori A,
Giannetti A,
and
De Luca M.
Transforming growth factor-1 modulates
1 and
5 integrin receptors and induces the de novo expression of the
v
6 heterodimer in normal human keratinocytes: implications for wound healing.
J Cell Biol
129:
853-865,
1995[Abstract].
194.
Zent, R,
Fenczik CA,
Calderwood DA,
Liu S,
Dellos M,
and
Ginsberg MH.
Class- and splice variant-specific association of CD98 with integrin cytoplasmic domains.
J Biol Chem
275:
5059-5064,
2000
195.
Zhu, X,
Bansal NP,
and
Evans JP.
Identification of key functional amino acids of the mouse fertilin (ADAM2) disintegrin loop for cell-cell adhesion during fertilization.
J Biol Chem
275:
7677-7683,
2000