From the Division of Cell Biology and Biophysics,
School of Biological Sciences, University of Missouri-Kansas City,
Kansas City, Missouri 64110-2499 and the
Department of
Biochemistry and Molecular Biology, University of Kansas Medical
Center, Kansas City, Kansas 66160-7421
Received for publication, November 7, 2000, and in revised form, November 27, 2000
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ABSTRACT |
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Eukaryotic phosphatidylinositol transfer protein
is a ubiquitous multifunctional protein that transports phospholipids
between membrane surfaces and participates in cellular phospholipid
metabolism during signal transduction and vesicular trafficking. The
three-dimensional structure of the Phosphatidylinositol transfer proteins
(PITPs)1 constitute a highly
conserved family of proteins in multicellular animals, with members in
species ranging from Homo sapiens to Dictyostelium discoideum (reviewed in Refs. 1-2, see also Refs. 3-5). PITPs catalyze an exchange of phosphatidylinositol (PtdIns) or
phosphatidylcholine (PtdCho) molecules between the surfaces of
biological or artificial membranes. The protein appears to be
multifunctional, with cellular functions ranging from intermembrane
phospholipid transport to possible substrate presentation to
lipid-modifying enzymes. Exogenous PITP is able to restore
hormone-stimulated G protein-coupled activation of
PtdIns(4,5)P2 hydrolysis in human promyelocytic cells (6) and to reconstitute Ca2+-stimulated,
ATP-dependent secretion of catecholamines in rat pheochromocytoma cells (7). A variety of cultured mammalian cells
respond to peptide agonists, epidermal growth factor, and antigens for
the IgE receptor with a PITP-dependent stimulation of
PtdIns 4-kinase and phospholipase C (8-9). PITP and PtdIns 3-kinase
together exhibit a capacity to phosphorylate PtdIns to PtdIns(3)P (10).
In cell-free systems the budding of vesicles from the
trans-Golgi network and vesicle transport between
cis and medial Golgi compartments are both
stimulated by PITP (11-12). Existence of PITP-containing multiprotein
complexes has been proposed, poised to respond to extracellular stimuli
and to channel PtdIns efficiently from PITP to kinases and
phospholipases for the generation of signal transducing molecules and
regulation of vesicular trafficking (13-14).
Multiple isoforms of cytosolic, 32-kDa PITPs are common and ubiquitous
(15-16). Mammalian PITP Protein Preparation and Crystallization--
There were two
criteria that guided all crystallization experiments: use of unmodified
protein and maintenance of a protein-bound ligand of physiological
significance. Considerable biochemical data have accumulated indicating
that the C-terminal residues of PITP exhibit a conformational change
upon binding to membranes. We therefore avoided the use of truncated or
tagged proteins, particularly at the C-terminus. To improve the
likelihood that a bound phospholipid would be observed in the x-ray
data, no detergents of any kind were employed in any step of
purification or crystallization. Cells of Escherichia coli
B834(DE3), a methionine auxotroph containing plasmids for expression of
groELS and rat PITP Structure Determination and Analysis--
For x-ray data
collection, crystals were mounted from the crystallization drop in a
fiber loop and frozen directly in a cold nitrogen stream using an ADSC
cryostat. Frozen crystals were sent to NSLS beamline X12C for "Fed-Ex
Data Collection" on a Brandeis B4 CCD detector. The crystal structure
of the PITP The protein features a large concave -isoform of rat
phosphatidylinositol transfer protein complexed with one molecule of
phosphatidylcholine, one of its physiological ligands, has been
determined to 2.2 Å resolution by x-ray diffraction techniques. A
single
-sheet and several long
-helices define an enclosed
internal cavity in which a single molecule of the phospholipid is
accommodated with its polar head group in the center of the protein and
fatty acyl chains projected toward the surface. Other structural
features suggest mechanisms by which cytosolic phosphatidylinositol
transfer protein interacts with membranes for lipid exchange and
associates with a variety of lipid and protein kinases.
INTRODUCTION
TOP
ABSTRACT
INTRODUCTION
EXPERIMENTAL PROCEDURES
RESULTS
DISCUSSION
REFERENCES
and PITP
share nearly 80% sequence
identity; both isoforms are capable of forming noncovalent and
stoichiometric complexes with PtdIns or PtdCho and transporting these
lipid substrates through an aqueous environment. Sequence identity is
more than 98% among mammalian PITP
isoforms (Fig. 1). PITP
and PITP
represent unique
gene products (17p13.3 and 22q12.1, respectively, in humans) and they
exhibit differential tissue expression. The N terminus of the 140-kDa
membrane-associated retinal degeneration B proteins (rdgBs) contains a
region homologous to the soluble PITPs (17-20). Significantly, the
rdgB-PITP region, comprising 266 amino acids, can be expressed as a
soluble protein that exhibits both PtdIns and PtdCho transfer
activities. We have solved the structure of rat PITP
bound to
phosphatidylcholine using x-ray diffraction techniques. The results
provide a framework for understanding the interactions of PITPs with
lipid molecules and membrane surfaces in lipid transport processes and
with other cellular proteins during signaling and trafficking
events.
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Fig. 1.
Sequence alignment of the PITP and rdgB
proteins. Amino acids showing absolute conservation among all PITP
and rdgB proteins are highlighted in red. Amino acids
exhibiting 80-99% conserved identities are highlighted in
yellow. Species nomenclature is as follows:
RN, Rattus norvegicus; HS, H. sapien; OC, Oryctolagus cuniculus;
MM, Mus musculus; DM,
Drosophilia melanogaster; DD, D. discoideum; CE, C. elegans.
EXPERIMENTAL PROCEDURES
TOP
ABSTRACT
INTRODUCTION
EXPERIMENTAL PROCEDURES
RESULTS
DISCUSSION
REFERENCES
(21), were grown in chemically defined media at
20 °C with selenomethionine in place of methionine (22). The
selenomethionyl-substituted protein was purified by established
procedures for the wild-type protein and the bound bacterial
phosphatidylglycerol (PtdGro) was replaced with
sn-1,2-dioleoyl-PtdCho (21). The protein was crystallized under similar conditions as the wild-type protein when supplemented with seeding (23).
·PtdCho complex was solved by multiwavelength
anomalous diffraction (MAD) methods (24). A three-wavelength MAD x-ray
data set was collected, with one wavelength at the selenium absorption
edge, one at the absorption peak, and one at a low energy remote (Table
I). Five of the eight selenium sites were
identified by SOLVE (25) and refined by SHARP (26). The resulting
electron density map was modified by solvent flipping as implemented in
SOLOMON (27). From the resulting electron density map, the dominant
secondary structure elements were traced. The final model was built
through several cycles of manual model building with O (28), followed
by refinement with CNS (29). The program DALI (30) was used to
determine structurally related proteins. The lipid cavity was defined
and volumes calculated using the CAST (31) web-server.
X-ray data collection and refinement statistics
RESULTS
TOP
ABSTRACT
INTRODUCTION
EXPERIMENTAL PROCEDURES
RESULTS
DISCUSSION
REFERENCES
-sheet and several long
-helices (Fig. 2). The eight-stranded
-sheet, strands 1 to 8, has a connectivity of 71865432. The
-sheet is antiparallel, except for the orientation of strands 1 and
7, which are parallel. There are three long
-helices, composed of 5 to 7 turns (A, F, and G), and four shorter
-helices, each with less
than two complete turns (B, C, D, and E). Helices A and F are oriented
on the interior side of the
-sheet, with helix A aligned coincident
with strand 2 and helix F oriented diagonally across the
-sheet. The
phospholipid is clearly visible in the electron density map (Fig.
3), with the binding site between the
interior side of the
-sheet and
-helices A and F.
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Fig. 2.
A ribbon diagram of the folding topology of
PITP ·PtdCho. The lipid-binding core of
the protein is indicated in blue, the regulatory loop in
green, and the C-terminal region in red. The
phospholipid is represented in space-filled spheres using
CPK coloring.
-sheets are labeled 1-8 and
-helices A-G. The
figure was generated with MOLSCRIPT (54) and RASTER3D (55).
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Fig. 3.
A stereo diagram of the electron density map
in the vicinity of the choline group in the lipid-binding core. An
omit map was calculated in which all phospholipid and water atoms were
removed and one cycle of simulated annealing was run prior to the
2Fo-Fc electron density calculation. Electron density surrounding the
PtdCho molecule is green, around the water molecules is
light blue, and around the protein amino acids is
brown. All maps are contoured at 1.0 . Protein atoms are
represented as thick sticks using CPK coloring and water
molecules are represented by small red
spheres.
Several regions of the electron density map were difficult to model. These areas consisted of residues 239-242 connecting helix F to G (containing one of the undetected selenium sites at Met241) and residues 263-271 at the C terminus. The solvent-exposed side chain density of residues on helix G is somewhat weak, although the interior, buried sidechain density is unambiguous. The solvent-exposed Met247 has very weak side chain density and is one of the undetected selenium sites from the MAD data. The last undetected selenium site is residue 1, for which no electron density is observed.
Three Functional Regions of PITP--
We designate the dominant
region of the PITP structure as the lipid-binding core. This region, a
cavity in which the phospholipid molecule binds, is formed by
-helices A and F and the
-sheet. Limited segments of helices A
and F and strands 4 and 5 are amphipathic and provide excellent
hydrophobic interaction with the phospolipid molecule (Fig.
4). The lipid-binding core comprises
residues 1-118 and 191-232. Examination of the PITP structure reveals
that the intervening residues 119-190 form an extensive loop region
between strands 7 and 8. Postulating that this loop is likely required for the association of PITP with a wide variety of lipid- and protein-modifying enzymes, we refer to this region as the regulatory loop. The recent identification of Ser166 as a site for
limited protein kinase C-dependent phosphorylation (32)
supports this hypothesis. All PITPs and rdgBs studied to date contain a
regulatory loop region; only Caenorhabditis elegans PITP
lacks the equivalent Ser166 phosphorylation site. The
remaining residues in the structure comprise the C-terminal residues
233-271. Consisting of the long
-helix G as the principal secondary
structural element and an additional 11 residues that limit access to
the lipid-binding cavity, we call this region the C-terminal
region and suggest its critical participation in membrane
association and dissociation. The C-terminal region is the least
conserved region among the cytosolic PITPs and is replaced in the rdgB
proteins by up to 1000 additional amino acids, some of which appear to
be transmembrane.
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Our functional region designation correlates reasonably well to the location of conserved amino acid residues in the PITP family (Fig. 1). The majority (nearly 75%) of the absolutely conserved amino acids are located in the lipid-binding core. Most of the others are part of the regulatory loop. Consistent with their proposed functions, both the regulatory loop and the C-terminal region were predicted to exhibit significant polypeptide backbone flexibility (33), a structural feature corroborated by specific B factors from the x-ray data.
Characterization of the Lipid-binding Cavity--
The residues in
close proximity to the PtdCho head group are shown in Figs. 3 and
4. The majority of these interacting
residues are highly conserved in all PITP and rdgB proteins. Nine water molecules are within 3.2 Å of the phospholipid or an amino acid residue lining the phospholipid cavity. The phosphate oxygen O1P forms
hydrogen bonds with the NZ atom of the conserved Lys195 and
a water molecule, and oxygen O2P hydrogen bonds with the OG1 atom of the conserved Thr97 and another water molecule.
The solvent-accessible volume of the phospholipid-binding cavity
(calculated with a 1.4 Å probe) is 670 Å3, whereas the
molecular-surface volume is 2725 Å3. The van der Waals
volume of the PtdCho molecule is 671 Å3. Of the 271 residues in the protein, 72 residues (27%) have atoms contributing to
the lipid cavity.
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DISCUSSION |
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Characteristics of the interactions between PITP and the hydrophobic region of PtdIns and PtdCho molecules have been described (34-35). Using lipids with defined fatty acyl species at their sn-1 and sn-2 positions, combined with one or more fluorescent acyl derivatives (pyrenylacyl, parinaroyl), both binding to PITP and protein-catalyzed transfer from a vesicle surface were measured. For the sn-1 group a broad range of structures could be accommodated, with preference for 14- and 16-carbon saturated or monounsaturated fatty acids; nevertheless, 18-carbon polyunsaturated fatty acids were still bound and transferred. In contrast, the sn-2 group exhibits a more restricted range, preferring 16- and 18-carbon mono- and polyunsaturated fatty acids whose double bonds were toward the methyl-end of the lipids. PtdCho molecules containing arachidonate (20:4n-6) or docosahexaenoate (22:6n-3) in the sn-2 position bind weakly to PITP. It was suggested that the sn-2 binding site in PITP was "bottle-shaped," with the neck toward the ester linkage. Little difference was observed for acyl chain preferences between the PtdIns and PtdCho class of phospholipids.
In the PITP structure reported here, the PtdCho ligand contains two
oleate (18:1n9) acyl groups. The conformation of the sn-2
acyl chain is more extended than the sn-1 chain, such that the sn-2 group extends almost to the surface of the protein.
The lipid cavity follows the contour of the sn-2 acyl chain
closely, but the cavity around the sn-1 chain is less
confining. These features may explain the poor binding of phospholipid
molecules with sn-2 acyl moieties longer than 18 carbons and
the less restrictive range of groups accommodated in the
sn-1 acyl region of the cavity.
PITP preferentially binds PtdIns and PtdCho. Whereas PtdCho was
used in the experimental protocol, inositol can be readily modeled into
the electron density for the choline moiety (except for the sugar
hydroxyls). There are numerous charged or polar amino acid residues in
the region that may be potential hydrogen bonding partners to the
inositol hydroxyls (Fig. 5).
Nevertheless, the tertiary structure of the model does not completely
explain polar head group specificity of PITP, for which further studies are needed.
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Conformation, Stability, and Function of the C-terminal Region-- To transfer a phospholipid, PITP must first bind to the membrane, exchange its bound lipid for a membrane lipid, and then dissociate from the membrane. Previous studies have shown that there are significant conformational changes in the C-terminal residues when PITP binds to lipid membranes (21). Although cytosolic PITP is highly resistant to trypsin digestion, protein bound to membranes is rapidly cleaved at Arg253 and Arg259. In the crystal structure these two sites are part of helix G in the C-terminal region. Studies have shown that generally sites that are subject to limited proteolytic cleavage by trypsin assume a loop-like structure with a conformation similar to that of trypsin inhibitors (36). Thus, this helix must "melt" prior to cleavage by trypsin, a process facilitated by interaction with a membrane surface. The relative instability of portions of this helix is supported by the high B factors calculated for residues 243-250.
Residues 260-271 also appear to be important for maintaining a tight compact structure. PITPs truncated at residues 253 or 259 have more relaxed conformations than full-length PITP, defined by increased quenching of Trp fluorescence by acrylamide, increased Tyr exposure, increased reactivity of Cys, and increased anilino-naphthalene sulfonate binding (37). Inspection of the native structure shows that such changes cannot be caused by removal and exposure of residues without a conformational change in the remainder of the protein. Several studies have shown that removal of one or a few amino acids from the N- or C-terminus of a protein may have a significant effect on protein stability and/or dynamics without producing major changes in conformation (38-39). Our data on the conformation and biological activity of truncated PITPs are consistent with a critical role of the C-terminus in the complete packing of the hydrophobic core and folding and stabilization of the native structure.
The ability of PITP to bind phospholipid substrates is necessary but not sufficient for subsequent phospholipid transfer. Removal of up to 20 C-terminal residues greatly reduces phospholipid transfer, yet these truncated proteins produced in E. coli contain PtdGro, or less commonly, phosphatidylethanolamine (PtdEtn) with a near normal 1:1 stoichiometry (37, 40). Hence, residues 254-271 are dispensable for lipid binding, although they are important in conferring selectivity for PtdIns and for efficient transfer of phospholipids. It is not immediately obvious from the PITP structure why phospholipids such as PtdEtn and phosphatidic acid (PtdOH) are bound and/or transferred so poorly.
We had earlier reported that residues 260-271 could be eliminated with variable effects on transfer activity that reflected the composition of the membranes employed in the assay system (41). With membranes containing only PtdCho, the truncated PITP (1) has transfer activity identical to the full-length protein. However, with as little as 2% PtdOH in the donor and/or acceptor membranes, transfer activity decreases to 0-20% relative to that observed with PtdCho membranes. Such concentrations have minimal effect on transfer by native PITP. We demonstrated that this and the more extensively truncated PITP (1) have greatly enhanced affinities for membrane binding, which would result in inefficient lipid transfer activity. We suggest that the C-terminal region of PITP acts to maintain a low affinity of the full-length protein for membranes, especially those containing acidic phospholipids. Structural perturbations within the C-terminal region lead to enhanced electrostatic interactions and increased membrane affinity (41).
Structure-Function Correlation of Point Mutations--
Knowledge
of the tertiary structure of PITP provides an opportunity to evaluate
studies describing and characterizing the random mutagenesis of the
cDNA encoding rat PITP (42). Mutations of amino acids
Ser25, Thr59, Pro78, and
Glu248 were, in general, observed to abolish PtdIns
transfer, whereas PtdCho transfer was not affected. With the exception
of Ser25, all are absolutely conserved among PITPs and
rdgBs. Our data support a role of the Thr59 hydroxyl group
in a putative hydrogen bonding interaction with the inositol moiety
(Fig. 5) and place Ser25 in the vicinity of the fatty
acid-glycerol ester linkages. Glu248 makes a salt bridge
with the conserved Lys61 in a region close to the choline
and inositol polar head group moieties. Pro78 is near the
end of the portion of the cavity that accommodates the sn-2
acyl chain. Why mutations of these residues impact selectively and
variably on PtdIns binding and transfer remains unresolved.
Mutations to Glu6 and Pro12, two absolutely conserved residues, yield poor expression of recombinant mammalian PITP in a yeast host (42). Based on our structure, these residues likely stabilize the association between the lipid-binding core and the regulatory loop. Glu6 participates in salt bridges with the conserved Arg8 and with His127; similarly, Pro12 packs tightly against the conserved Leu174.
Membrane Binding and Mechanism of Phospholipid Transfer-- Analysis of the exterior features of PITP, such as charge distribution and hydrophobicity, provided little insight into what region might constitute the membrane-binding surface. We presently favor a model in which the C-terminal region is a part of this surface. In the interaction between protein and membrane, most if not all of the C-terminal region must be displaced from helix A in the lipid-binding core and acquire increased backbone flexibility sufficient to melt a portion of helix G and become sensitive to proteolytic digestion. We further envision that the refolding and reassociation of the C-terminal region with the bulk protein provides a driving force for eventual dissociation of PITP from the membrane surface.
The detailed mechanism by which PITP releases its bound lipid, exchanges it for another present in the membrane surface and dissociates from the membrane remains to be elucidated. A plausible model is one in which the C-terminal region of the protein binds to the membrane and the lipid-binding core opens, somewhat like a clam shell, exposing its bound lipid to the membrane surface. The protein-bound lipid is absorbed into the membrane and replaced by a membrane lipid. Subsequently, the C-terminal region refolds and reassociates with the body of the protein, i.e. the clam shell closes, and the protein dissociates from the membrane surface. The affinity for the membrane surface and the affinity for a specific phospholipid must not be too great, otherwise dissociation and transfer would be too slow and inefficient. As suggested above, the decreased rate of membrane dissociation, at least in part, makes C-terminal-truncated PITP derivatives weak catalysts of lipid exchange with membranes containing acidic phospholipids.
To account for other biological activities of PITP, we suggest that the
protein-bound phospholipid becomes accessible to phosphorylation following interaction of a lipid kinase with a complementary site on
PITP, perhaps on the regulatory loop. One-third of the residues within
this 72-amino acid segment are identical or >90% conserved among all
PITP and rdgB proteins (Fig. 1). Protein-protein interactions are
proposed to alter the conformation of PITP at the head group region of
the lipid-binding core (Fig. 6). This
could lead to either the presentation of PtdIns to the active site of a
membrane-bound PtdIns 4-kinase or the exposure of the inositol moiety
of a PITP-bound PtdIns to the action of a soluble PtdIns 3-kinase or
4-kinase (43-44). Evidence supporting such a function of PITP in
substrate presentation to a PtdIns 3-kinase has been reported (10).
Following modification of the inositol 3- or 4-hydroxyl function, the
affinity of the product for PITP would likely decrease so significantly that it would remain with the kinase or return to the lipid surface. Consistent with this mechanism is the inability of PITP to bind and
transfer PtdIns (4)P (45).
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Comparison with other Lipid-binding Proteins--
Using the DALI
algorithm (30) to search for proteins structurally similar to PITP, the
recently described START (StAR-related lipid-transfer) domain (46) was
identified as sharing significant tertiary structure. MLN64 (PDB:1EM2),
a protein associated with some human breast carcinomas, has served as a
prototype for structure determination and mechanistic studies of the
START domain (47). The MLN64 START domain contains a hydrophobic tunnel
that extends the length of the protein with small openings on both
ends. A nine-stranded antiparallel -sheet and two
-helices form
an interior hydrophobic chamber that is believed to be just large
enough to accommodate a single molecule of cholesterol. Cholesterol was not included in the crystallization solutions, but docked into the
finished protein model. START domain proteins are proposed to bind
lipids and interact with membranes, properties also common to PITPs.
Steroidogenic acute regulatory (StAR) protein, whose function is to
deliver cholesterol to the inner mitochondrial membrane and cytochrome
P450scc, contains a START domain near its C-terminus. Nearly the entire
sequence of PtdCho transfer protein (PCTP), another cytosolic lipid
transfer protein, constitutes a single START domain. In contrast to
PITP, PCTP has a marked specificity for PtdCho and highly restricted
tissue expression in mammals (2). Although not identified by the DALI
search, the recently reported structure of sterol carrier protein-2
(PDB 1QND, Ref. 48) is another example of a protein with a
lipid-binding core similar to that observed in MLN64 and PITP.
Despite reasonable similarity at the secondary and tertiary levels of
structure (Fig. 7), there is no primary
structure homology between members of the PITP family and proteins with
a START domain. It should be noted that the lipid cavity in PITP is
larger than the cavity in MLN64. Upon superposition, one of the
-helices comprising the putative lipid-binding cavity of MLN64
clashes with the sn-1 acyl chain of the bound PtdCho in the
PITP structure. This is not surprising in that PITP binds diacyl
phospholipids, whereas MLN64 binds the more compact cholesterol
molecule. The START and PITP protein families represent an excellent
example of functional evolutionary convergence to generate comparable domain architecture and related biological function from apparently unrelated genetic information.
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Fungi and plants contain Sec14p, a cytosolic protein that transports
PtdIns and PtdCho (13, 49-52) and is required for protein export and
cell viability. Recently, the three-dimensional structure of
Saccharomyces cerevisiae Sec14p, one member of this
conserved family, was reported (53). PITP and Sec14p are similar in
size, exhibit similar in vitro lipid-binding specificity and
transport abilities and display significant and highly complementary
interspecies activity. However, the two proteins have little structural
similarity. In contrast to the PITP·PtdCho structure, Sec14p
structure has no bound phospholipid molecule.
Conclusions--
Elucidation of the PITP·PtdCho structure
reveals a complex amphipathic lipid completely enclosed within a folded
polypeptide cavity. The three functional regions of the protein
provides a molecular framework for understanding the details of
protein-lipid interactions on several levels. Within its lipid-binding
core, PITP has been shown to sequester phospholipids from the aqueous environment. Lipid ligand specificity, whereas not completely explained
by the tertiary structure, is dictated by extensive noncovalent
interactions. The C-terminal region of PITP is proposed to promote the
transient association to and dissociation from lipid surfaces. Through
its regulatory loop, PITP is thought to present lipid substrates to
kinases for subsequent phosphorylation. Further mutagenic and
structural studies of PITP and other members of the PITP-rdgB family
are required to test these and other hypotheses. Detailed analyses of
PITP interactions with membranes and kinases are needed to gain a more
meaningful understanding of the role of PITP in cellular
phospholipid metabolism.
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ACKNOWLEDGEMENTS |
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The atomic coordinates for the crystal
structure of PITP·PtdCho have been deposited in the Research
Collaboratory for Structural Bioinformatics Protein Data Bank with
accession code 1FVZ. We thank Chuong Doan and Sienna Sifuentes for
technical assistance. Diffraction data for this study were collected at
Brookhaven National Laboratory in the Biology Department single-crystal
diffraction facility at beamline X12-C in the National Synchrotron
Light Source. This facility is supported by the United States
Department of Energy Offices of Health and Environmental Research and
of Basic Energy Sciences under prime contract DE-AC02-98CH10886, by the National Science Foundation and by National Institutes of Health Grant
1P41 RR12408-01A1. We are grateful for the assistance of Michael Becker
and Robert Sweet with "Fed-Ex Data Collection" at X12-C.
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FOOTNOTES |
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* This work was supported by National Institutes of Health Grants GM24035 (to G. M. H. and L. R. Y.) and GM59162 (to G. M. H., L. R. Y., and M. D. Y.), the American Heart Association (to L. R. Y.), and the University of Kansas Medical Center Research Institute (to G. M. H.).The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
The atomic coordinates and the structure factors (code 1FVZ) have been deposited in the Protein Data Bank, Research Collaboratory for Structural Bioinformatics, Rutgers University, New Brunswick, NJ (http://www.rcsb.org/).
§ To whom correspondence should be addressed: Division of Cell Biology and Biophysics, School of Biological Sciences, University of Missouri-Kansas City, 5100 Rockhill Rd., Kansas City, MO 64110-2499. Tel.: 816-235-1986; Fax: 816-235-1503; E-mail: yoderm@umkc.edu.
¶ Present address: Howard Hughes Medical Inst., Division of Biology, California Inst. of Technology, Pasadena, CA 91125.
** Present address: Zymogenetics, 1201 Eastlake Ave. E., Seattle, WA 98102.
These authors contributed equally to this work.
Published, JBC Papers in Press, December 4, 2000, DOI 10.1074/jbc.M010131200
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ABBREVIATIONS |
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The abbreviations used are: PITP, phosphatidylinositol transfer protein; PtdIns, phosphatidylinositol; PtdCho, phosphatidylcholine; rdgB, retinol degeneration B protein; PtdGro, phosphatidylglycerol; MAD, multiwavelength anomalous diffraction; PtdEtn, phosphatidylethanolamine; PtdOH, phosphatidic acid; START, StAR-related lipid-transfer; StAR, steroidogenic acute regulatory; PCTP, phosphatidylcholine transfer protein.
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