From the Laboratory of Physiology, K.U. Leuven, Campus Gasthuisberg, Herestraat 49, B-3000 Leuven Belgium
Received for publication, November 22, 2000
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ABSTRACT |
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In recent years, it has been well established
that the Ca2+ concentration in the lumen of
intracellular organelles is a key determinant of cell function. Despite
the fact that essential functions of the Golgi apparatus depend on the
Ca2+ and Mn2+ concentration in its lumen,
little is known on the transport system responsible for ion
accumulation. The Golgi ion pump PMR1 has been functionally studied
only in yeast. In humans, mutations in the orthologous gene
ATP2C1 cause Hailey-Hailey disease. We report here the
identification of the PMR1 homologue in the model organism
Caenorhabditis elegans and after ectopic expression the direct study of its ion transport in permeabilized COS-1 cells. The
C. elegans genome is predicted to contain a single
PMR1 orthologue on chromosome I. We found evidence for
alternative splicing in the 5'-untranslated region, but no indication
for the generation of different protein isoforms. C. elegans PMR1 overexpressed in COS-1 cells transports
Ca2+ and Mn2+ with high affinity into the Golgi
apparatus in a thapsigargin-insensitive manner. Part of the accumulated
Ca2+ can be released by inositol 1,4,5-trisphosphate, in
agreement with the idea that the Golgi apparatus is an inositol
1,4,5-trisphosphate-sensitive Ca2+ store.
In eukaryotic cells, cytosolic Ca2+ acts as a second
messenger in a large variety of cell functions. The increase of the
cytosolic free Ca2+ concentration in response to a stimulus
results from the opening of Ca2+ channels present in the
plasma membrane and in the membranes of intracellular Ca2+
stores, mainly the endoplasmic reticulum
(ER).1 It has more recently
also become clear that the Ca2+ stored in the lumen of many
intracellular compartments not only serves as a reservoir of releasable
Ca2+, but also plays a regulatory role in several important
cell biological functions. Luminal Ca2+ in the ER as well
as in the Golgi or other components of the secretory pathway is
required for the proper translation, translocation, folding, and
processing of secreted proteins (for review, see Ref. 1). A
sufficiently high level of intraorganellar Ca2+ has been
implicated also in intra-Golgi membrane transport (2), transport
between the Golgi and the ER (3), and in endosome fusion (4). Besides
these constitutive actions, luminal Ca2+ controls movements
of the Ca2+ ion itself: its release from the intracellular
stores, its permeability through the nuclear pore complexes, and
capacitative Ca2+ entry at the plasma membrane (for review,
see Ref. 5).
The pivotal role of the ER as a Ca2+ store is well
established. There is a growing consensus that also the Golgi apparatus
can function as an agonist-releasable Ca2+ store (6), the
importance of which may, however, greatly differ among the different
cell types and ranges from relatively unimportant in Drosophila
melanogaster S2 cells (7) to very important in renal
LLC-PK1 cells (8). The storage capacity for
Ca2+ in the Golgi is greatly increased by specific proteins
like Calnuc/nucleobindin (9, 10) and members of the CREC family of
Ca2+-binding proteins (reviewed in Ref. 11). In contrast to
the growing unanimity on the role of Golgi as a Ca2+ store,
there remains some confusion as to the type of
Ca2+-accumulation system responsible to replenish the
store. Ca2+ pumps belonging to the class of P-type
ion-motive ATPases have been proposed to take up this task: SERCA2 (6,
10), the plasma-membrane Ca2+ pump PMCA en route to the
plasma membrane (12) and PMR1 (6, 13). The SERCA pumps, which are
encoded by three different genes in vertebrates (human gene names:
ATP2A1-3), but apparently only by a single gene in
invertebrates like C. elegans, are the best characterized
members of this class and are responsible for the accumulation of
Ca2+ into the ER or into the sarcoplasmic reticulum. The
PMCA Ca2+ pumps (corresponding human gene names:
ATP2B1-4) responsible for the extrusion of Ca2+
out of the cell, also belong to the same class but are themselves not
involved in Ca2+ transport into the stores, with as a
possible exception the above mentioned Golgi-based PMCA pumps which are
on their way to the plasma membrane. The PMR1-type of
Ca2+-transport ATPases was first identified in the yeast
Saccharomyces cerevisiae (14) and localized to the Golgi or
one of its subcompartments (15). Genes homologous to the S. cerevisiae PMR1 have been reported for a number of other fungi
(see Ref. 16 and references therein). The PMR1 ion-motive ATPase
supplies the secretory pathway with Ca2+ and
Mn2+ ions required for glycosylation, sorting, and
ER-associated protein degradation (17, 18). A recent study has
demonstrated capacitative Ca2+ entry in S. cerevisiae, a mechanism that in higher eukaryotes is thought to be
initiated by depletion of intracellular stores that are filled by the
SERCA Ca2+ pump (19). The process was stimulated in
pmr1 mutants, indicating that in yeast capacitative
Ca2+ entry in combination with PMR1 activity supplies the
secretory pathway with Ca2+. Yeast pmr1 mutants
do not grow on a medium containing submicromolar concentrations of
Ca2+ and show defects in the maturation of secretory
proteins which are suppressed by supplying millimolar Ca2+
or micromolar Mn2+ to the growth medium. Mutations in
PMR1 were also reported to rescue yeast mutants, which as a
result of the lack of superoxide dismutase, show impaired growth in
aerobic conditions (20). This effect is ascribed to increased cytosolic
levels of Mn2+ resulting from a lack of accumulation of the
ion in the Golgi compartment. Mn2+ is known for its
capacity to scavenge superoxide ions. Mandal et al. (21)
pointed recently to the critical role of transmembrane segment M6 in
yeast PMR1 for defining the cation-binding sites in general and in
particular of residue Gln783 in this segment for the
Mn2+ selectivity.
Relatively less is known on the PMR1 homologues in animal cells. The
cDNA of the putative rat form of the yeast PMR1 was already cloned
in 1992 with a SERCA-derived probe (13), but the authors failed in
their efforts to show that the corresponding protein, upon its
expression in COS cells, was able to catalyze the uptake of
Ca2+ into vesicles consisting of fragmented membranes. In
the meanwhile homologous cDNAs or genes were reported for D. melanogaster,2 Bos
taurus,3 and C. elegans. But again until now no direct indication that any of
these was involved in Ca2+ uptake has been provided.
However, indirect evidence comes from two recent reports which show
that Hailey-Hailey disease (MIM 16960), which is manifested by the
impaired intercellular adhesion of epidermal keratinocytes, results
from mutations in one of the alleles of a human orthologue of yeast
PMR1, the ATP2C1 gene (22, 23). The symptoms of
Hailey-Hailey disease strongly resemble those of Darier-White disease
(MIM 124200) which is due to a mutation in one of the alleles of the
SERCA2 gene ATP2A2. This, together with the observation that
expression of the mammalian SERCA1a prevented the lethality of the
pmr1-pmc1 double mutations in yeast (24), strongly suggests
that the human PMR1 pump can act as a Ca2+ pump.
We now show for the first time that an animal PMR1 homologue can
transport Ca2+ or Mn2+ into the Golgi apparatus
of COS-1 cells with high affinity and in a thapsigargin-insensitive
manner. The accumulated Ca2+ can be released by
IP3, in line with the view that the Golgi apparatus is an
IP3-sensitive Ca2+ store.
Materials--
[ Constructs for Expression--
The EST clones yk218a11 and
yk334d5 were obtained from Yuji Kohara's database, and
sequenced. Both clones contained the complete open reading frame of
C. elegans PMR1. It should, however, be remarked that the
open reading frame is 63 bp smaller than predicted in the database
annotation (see "Results"). Clone yk334d5 was used to make an
expression construct. The 3.2-kb PMR1-encoding EcoRI/XhoI
fragment of yk334d5 was ligated into the dephosphorylated pcDNA3
expression vector (Invitrogen Co., British Biotechnology Products Ltd.,
Abingdon, United Kingdom) cut with the same restriction enzymes. The
pig SERCA2a expression vector has been described by Verboomen et
al. (25). The full-length rabbit SERCA1a was cloned in the pMT2
expression vector (26).
Cell Culture and DNA Transfection--
For microsome
preparation, COS-1 cells were seeded in 100-mm culture dishes at a
density of 2.5 × 106 cells per plate. For
immunocytochemistry 2.0 × 104 cells were seeded on
gelatin (1%)-coated coverslips. For 45Ca2+ or
54Mn2+ fluxes 3.0 × 104 cells
were seeded in gelatin-coated 12-well plates. For microsome preparation
and immunocytochemistry, transfection was performed the day after
seeding. For isotope fluxes the period between seeding and transfection
was extended to 5 days to allow better attachment of the cells to the
plates. After transfection, the cells were incubated for 60 h at
37 °C and 5% CO2.
Preparation of Antiserum to PMR1 Protein--
The immunogen was
a recombinant protein corresponding to the putative large cytoplasmic
loop between transmembrane segments 4 and 5 of C. elegans
PMR1. The protein was expressed in Escherichia coli using
the QIAexpress Type IV System (Qiagen, Hilden, Germany). In a first
step the cDNA corresponding to the loop region was amplified by PCR
with primers PMR1CYTF (5'-CAAGCATGCCGTGAAGAAGATGCCAGCAG-3') and
PMR1CYTR (5'-CTAGTCGACTTTTCCCTCCTCAATCGCC-3') containing, respectively,
a SphI and SalI restriction site at their 5' end. The PMR1CYTF primer corresponds to nucleotides 26812-26831 of cosmid
CECC4 (accession number Z81490) and PMR1CYTR primer to the inverse
complement of nucleotides 136-154 of cosmid CEZK256 (accession number
Z82088). PCR was carried out for 20 cycles using the ExpandTM Long
Template PCR System from Roche Molecular Diagnostics. Each cycle
consisted of 10 s of denaturation at 94 °C, 30 s of
annealing at 55 °C, and 2 min of elongation at 68 °C. The
SphI/SalI-cut PCR fragment was ligated in the
SphI/SalI sites of the pQE-31 bacterial
expression vector (Qiagen), containing a 6xHis tag coding sequence 5'
to the cloning region. The expression of the recombinant protein in
E. coli M15 cells was induced by 1 mM
isopropyl-1-thio-
Rabbits were immunized with 0.1 mg of recombinant protein in 0.5 ml of
phosphate-buffered saline emulsified with 0.5 ml of complete Freund's
adjuvant. Booster injections of the same immunogen with incomplete
Freund's adjuvant were given at 4-week intervals. Preimmune serum and
serum obtained after 4 boosters were used. The antiserum is designated
as Celpmrloop.
Reverse Transcriptase-PCR Analysis--
Total RNA was
prepared from C. elegans with TriPureTM Isolation Reagent
(Roche Molecular Diagnostics) according to the manufacturer's instructions. First strand cDNA synthesis was performed with the ThermoscriptTM RT-PCR System (Life Technologies NV, Merelbeke, Belgium) using the modified oligo(dT) primer
5'-CCAGTGAGCAGAGTGACGAGGACTCGAGCTCAAGC(dT)10-3' (27).
Transcription of C. elegans Pmr1 was checked by 5' and 3'
rapid amplification of cDNA ends PCR using the AmpliTaq GoldTM PCR
system (PerkinElmer Life Sciences, Gouda, The Netherlands) and primer
CePMR1For (5'-CCGACAATAAAGTAACACCACG-3'), corresponding to nucleotides
1955-1976 of cosmid CEZK256, primer 468 (5'-GAGGACTCGAGCTCAAGC-3') which partially contains the sequence of the modified oligo-(dT) primer, primer CePMR1Rev (5'-TGCTTCGGCTCAGTTTCTCC-3')
corresponding to the inverse complement of nucleotides 25104-25123 of
cosmid CECC4, and SL1 primer (5'-GGTTTAATTACCCAAGTTTGAG-3') and SL2
primer (5'-GGTTTTAACCCAGTTACTCAAG-3'), which contain the sequence of splice leaders SL1 and SL2, respectively. PCR amplifications were carried out for 30-35 cycles of 1 min at 94 °C, 1 min at
50-58 °C, and 2 min at 72 °C. PCR fragments were gel-purified
(QIAquick Gel Purification Kit, Qiagen) and subcloned into pGEM-T Easy
vector (Promega, Madison, WI). Several individual clones were sequenced.
Membrane Preparations and Immunoblotting
Analysis--
Microsomes were isolated from COS-1 cells as described
by Verboomen et al. (25). Membranes from C. elegans were isolated as described by Baylis et al.
(28). Protein concentrations were determined by the bicinchoninic acid
method (Pierce, Rockford, IL). Denaturing gel electrophoresis and
Western blotting were done as described earlier (29).
45Ca2+/54Mn2+
Fluxes--
COS-1 cells were grown on 12-well plates. Loading with
45Ca2+ and efflux were done essentially as
described earlier (30). Cells were treated for 10 min with 20 µg/ml
saponin at 25 °C and loaded for the indicated lengths of time in 120 mM KCl, 30 mM imidazole-HCl (pH 6.8), 5 mM ATP, 10 mM NaN3, 0.44 mM (for Ca2+ fluxes) or 0.1 mM (for
Mn2+ fluxes) EGTA. MgCl2, CaCl2,
and MnCl2 were added to obtain a calculated free
Mg2+ concentration of 0.5 mM and the indicated
concentrations of free Ca2+ and Mn2+. Free
concentrations of Ca2+ and Mn2+ were calculated
based on the stability constants for EGTA and ATP given by Fabiato and
Fabiato (31) (for Ca2+) and by Martell and Smith (32) (for
Mn2+). Thapsigargin (2 µM) was added if
inhibition of pumping by the SERCA Ca2+ pump was needed.
Efflux was performed in 120 mM KCl, 30 mM
imidazole (pH 6.8), and 1 mM EGTA. All experiments on
Mn2+ transporting activity or Mn2+ effects on
Ca2+ uptake activity were performed with Chelex 100 (Bio-Rad, Eke, Belgium)-treated solutions.
32P-Phosphoenzyme Formation and Electrophoresis in
Acid SDS Gels--
The phosphorylation reaction was carried out on ice
in 100 µl of solution containing 15 µg of microsomal protein, 80 mM KCl, 17 mM K-Hepes (pH 7.0), 5 mM NaN3, 1 mM dithiothreitol, and
the indicated concentrations of total EGTA and free Mg2+,
Ca2+, and Mn2+. Thapsigargin was used at 0.1 µM. The reaction was started by adding 5 µl of
[ Immunocytochemistry--
Cells grown on coverslips were fixed in
4% paraformaldehyde in phosphate-buffered saline for 10 min at room
temperature, washed in phosphate-buffered saline, and permeabilized in
0.2% Triton X-100. Primary and secondary antibodies were diluted in
phosphate-buffered saline containing 3% bovine serum albumin at the
indicated dilutions.
Gene Structure of C. elegans PMR1 and Alternative Transcript
Processing--
The C. elegans genome is predicted to
contain a single PMR1 orthologue on chromosome I, which is
annotated ZK256.1 by the C. elegans sequencing consortium
(34). The genomic sequence can be found on the two overlapping cosmids
CECC4 (accession number Z81490) and CEZK256 (accession number Z82088).
Two EST clones from Yuji Kohara's database, yk218a11 and yk334d5,
representing cDNA clones of the C. elegans PMR1 gene,
were completely sequenced. The deduced C. elegans PMR1
protein would contain 901 amino acids instead of the 922 predicted in
the database annotation (protein identification number CAB04015.1).
This is because a sequence of 63 bp is wrongly assigned by the
Genefinder program to the putative exon 8 (303 bp). We found this
sequence to be absent from both cDNA clones (see Fig.
1A) and hence the actual
protein should correspondingly be 21 amino acids shorter. Furthermore, it was clear from clone yk334d5 that the 5' UTR of C. elegans PMR1 is at least 257 bp long and contains, besides exon 1, two extra exons, comprising only untranslated sequences and designated exon
A comparison of the exon/intron layout of CePMR1 to that of
the D. melanogaster (accession number AC014929) and human
orthologues (gene ATP2C1, Refs. 22 and 23) shows that the
human gene contains the most elaborate exon/intron layout, D. melanogaster the simplest, whereas C. elegans takes an
intermediate position (Fig. 1B). Apparently C. elegans and to a larger degree D. melanogaster have
lost most of the introns during evolution. The worm and human have in
total 5 conserved exon boundaries, the worm and the fly only one. The localization of the exon 5/6 boundary in CePMR1 corresponds
to the exon 1/2 boundary in D. melanogaster and to the exon
18/19 junction in the human gene. There are four extra conserved
exon/intron positions between C. elegans and the human
ATP2C1 gene (asterisk in Fig. 1B)
while there is no other exon/intron junction at homologous positions
between the C. elegans and D. melanogaster
PMR1 genes. One extra exon boundary is conserved between exons 2 and 3 in D. melanogaster and between exons 21 and 22 in the
human ATP2C1 gene. Among the exon borders of the PMR1
family, only one (exon 1/2 in CePMR1, exon 5/6 in the human
ATP2C1) is conserved in the mammalian SERCA genes
(exon 4/5 in human ATP2A1-3). Compared with the mammalian
PMCA family, none of the exon/intron borders is conserved.
Because in C. elegans the majority of the mRNAs are
trans-spliced, we have investigated whether this is also the
case for transcripts of CePMR1 in the worm. 5' Rapid
amplification of cDNA ends experiments were performed on whole worm
RNA. PCR reactions with primer pairs SL1/CePMR1Rev and SL2/CePMR1Rev
resulted in the amplification of an 824- and 818-bp fragment,
respectively. The 818-bp PCR product corresponded to a cDNA in
which splice leader SL2 was trans-spliced to exon
We also performed 3' rapid amplification of cDNA ends with primer
pair CePMR1For/468 to check for the possibility of 3' alternative processing at the 3' end of the gene's transcript. PCR amplification gave a product of about 600 bp. By subcloning and sequencing of several
individual clones, it became clear that the gel band actually consisted
of a mixture of three products with small differences in length and
corresponding to three polyadenylation isoforms. The isoforms are the
result of the use of three polyadenylation sites (pA1,
pA2, and pA3) in the 3' UTR of the gene (Fig.
1B). Both yk218a11 and yk334d5 apparently used
pA2. However, no indication was found for 3' alternative
splicing, neither in our 3' rapid amplification of cDNA ends
experiments nor by database searching.
In summary, the C. elegans PMR1 gene consists of 12 exons
( The C. elegans PMR1 Protein--
Fig.
2 shows the predicted amino acid sequence
of C. elegans PMR1 and its major domains together with an
alignment with the corresponding sequences of three distant species,
Homo sapiens, D. melanogaster, and S. cerevisiae. In the sequences of all species 10 hydrophobic
segments can be identified, which like in the SERCA Ca2+
pumps, presumably form the transmembrane domain. The highest degree of
sequence similarity occurs around the phosphorylation site, in regions
demonstrated in SERCA to contribute to the ATP-binding site or to form
structurally important loops, and in transmembrane segments M4, M6, and
M8, which have been documented in SERCA to form the binding sites for
Ca2+ (35, 36). The overall amino acid sequence is 37%
identical to rat SERCA2a. The percentage identity with the PMR1
sequences of human, D. melanogaster, and S. cerevisiae is, respectively, 59, 57, and 49%. The amino acids
that form the binding site for one of the transported Ca2+
ions in SERCA, more specifically the site II, are conserved in the PMR1
homologues in all species. The amino acids belonging to site I are not
conserved in the PMR1 protein. In S. cerevisiae, Gln783 has been demonstrated to define the Mn2+
selectivity of the PMR1 ion pump (21). Also this residue is conserved
in all species.
The N-terminal region upstream of the first transmembrane domain of
C. elegans PMR1 has the same length as in the human PMR1 sequence reported by Sudbrak et al. (23), whereas the human sequence predicted by Hu et al. (22) is 16 residues longer. The length of the C-terminal part is more similar to the product of the
human splice variant ATP2C1a than to the shorter ATP2C1b described by
Hu et al. (22). As mentioned above, there is no evidence for
the generation of C-terminal PMR1 protein variants in C. elegans. The C-terminal sequence does not contain an eleventh hydrophobic region as in the SERCA2b splice variant. The EF hand-like domain near the N terminus of the S. cerevisiae sequence has
been shown to play a role in modulating ion transport (37). Because its
primary structure is poorly conserved, it remains to be demonstrated whether a similar function occurs in other species.
Characterization and Functional Analysis of C. elegans PMR1
Protein--
The Celpmrloop antibody, raised against the large
cytosolic loop between transmembrane segments 4 and 5 of PMR1 of
C. elegans, clearly demonstrated the expression of the
protein in COS-1 cells transfected with the corresponding cDNA,
both by Western blot analysis and immunocytochemistry (Fig.
3). On Western blots, the immunoreactive
band migrated slightly below the predicted theoretical Mr value of 98,505. A strong immunoreaction was
also seen on blots of fragmented membranes prepared from whole worms.
Hence, it is clear that our antibody is able to recognize the PMR1
protein both in C. elegans and after its ectopic expression
in COS-1 cells. The vertebrate PMR1 homologue found in untransfected
COS-1 cells appears not to react with the antiserum as shown by the
controls of untransfected COS-1 cells. Furthermore, immunocytochemistry of PMR1-overexpressing and control cells reveals the correct targeting of PMR1 to the Golgi compartment of the COS-1 cells (Fig.
3B). In conclusion, by using our polyclonal anti-PMR1
antiserum it became clear that PMR1 is expressed in the worms, that it
can be overexpressed in COS-1 cells and that it contains all the
information needed to target the PMR1 protein to the Golgi
membranes.
Because the fraction of Golgi-derived membranes in microsomes of COS-1
cells is relatively small compared with that of ER, we could not rely
on conventional techniques used to measure Ca2+ transport,
like those for the ER-based SERCA transport ATPases (25). Instead we
took advantage of the 45Ca2+-flux system
utilizing detergent-permeabilized cells (30).
In a first series of experiments we tested PMR1 of C. elegans for its ability to transport Ca2+. Control
COS-1 cells, cells overexpressing rabbit SERCA1a (as a positive control
for Ca2+ pumping), and cells overexpressing PMR1 were
permeabilized with saponin in a medium mimicking a cytosolic
composition and loaded with 45Ca2+ for 45 min
in the presence of NaN3 to prevent mitochondrial
Ca2+ uptake. Ca2+ transport via SERCA-type
Ca2+-ATPases was determined by comparing the uptake in the
presence and absence of 2 µM thapsigargin. The loading of
the cells was followed by an efflux for 20 min in a
Ca2+-free medium. IP3 (10 µM) was
administered after 10 min of efflux. Fig.
4A shows that control and
SERCA1a-transfected cells exhibit a Ca2+ uptake, which is
blocked by 2 µM thapsigargin. In control COS-1 cells and
in PMR1-expressing cells, the difference in Ca2+ content
with thapsigargin and without thapsigargin represents the
Ca2+ pump activity of the endogenous SERCA2b. In
SERCA1a-transfected cells, it represents that of endogenous SERCA2b
plus overexpressed SERCA1a. In contrast, COS-1 cells overexpressing
PMR1 show an additional Ca2+ pump activity even in the
presence of thapsigargin. This suggests that PMR1 is a
Ca2+-transporting protein residing in the Golgi and that it
is insensitive to thapsigargin. Fig. 4A also shows that
Golgi membranes contain IP3 receptors, since 10 µM IP3 induced a more rapid decrease in the
store Ca2+ content.
Fig. 4B shows the time course of Ca2+ uptake by
PMR1-expressing cells. Transfected cells were loaded with
45Ca2+ for different time intervals in the
presence of thapsigargin to block Ca2+ uptake by endogenous
SERCA2b. Ca2+ pump activity reaches almost a plateau after
20 min of Ca2+ loading. Subsequent experiments were
performed after 10 min of loading with Ca2+. Fig.
4C shows the Ca2+ dependence of Ca2+
uptake by PMR1. The Ca2+ concentration needed for
half-maximal activation was 0.25 µM Ca2+.
In a second series of experiments we explored the possibility of PMR1
of C. elegans to function as a Mn2+ pump, since
previous reports based on the activation of ATP hydrolysis by
Mn2+ and on the inhibition of Ca2+ transport by
Mn2+ suggested that yeast PMR1 could act as a
Mn2+ pump. Control cells and PMR1-expressing cells were
loaded with radioactive Mn2+ for 10 min in the presence of
thapsigargin. The efflux was followed for 10 min (Fig.
5A). Fig. 5, A and
C, provide direct evidence that PMR1 can indeed act as a
Mn2+-transporting protein. PMR1-overexpressing cells show
an enhanced uptake of Mn2+. The accumulated
Mn2+ was released by the ionophore A23187 (10 µM), demonstrating that the Mn2+ has been
transported into a membrane-delineated compartment. However, the
addition of IP3 did not have a significant effect on the
rate of efflux. The Mn2+ uptake was inhibited by
Ca2+ (Fig. 5C), and conversely the
Ca2+ uptake by PMR1 was inhibited by Mn2+ (Fig.
5B). It is clear from Fig. 5B that at higher
Ca2+ concentrations more Mn2+ was needed to
inhibit the transport of Ca2+. Half-maximal inhibition was
observed at 1, 0.5, and 0.25 µM Mn2+ for
loading at, respectively, 1.0, 0.32, and 0.1 µM
Ca2+.
Formation of the Phosphoenzyme Intermediate--
A determining
characteristic of all P-type ion-transport ATPases is the transient
transfer of the The analysis of the C. elegans genome has resulted in
the previous identification of several members of the superfamily of P-type Ca2+-transport ATPases. A single gene encoding a
member of the SERCA-type subfamily is found on chromosome III (cosmid
K9D11) and three genes (mca1-3) encoding members of the
PMCA subfamily reside on chromosome IV (38). In this work, another gene
(designated CePMR1) is identified encoding a P-type
Ca2+-transport ATPase that is located on chromosome I. The
conservation of some intron positions, the overall sequence similarity
of the encoded protein, and the conservation of major domains and
critical motifs of the primary sequence unequivocally place it in the
PMR1 subfamily of P-type Ca2+-transport ATPases. Besides
the CePMR1 genomic sequence, we have in this work also
characterized its transcripts and protein product.
With respect to the number of exons, the CePMR1 gene takes
an intermediate position between the human and D. melanogaster orthologues. Only some of the exon/intron borders are
conserved between these different species. All splice sites follow the
GT ... AG rule. Surprisingly, with the exception of the intron
between exon 2 and 3, all introns in this gene are relatively long
considering the fact that most of the introns in C. elegans
genes have a length of only about 50 nucleotides (39). A particularly
long intron (~7 kb) is that between exons At the protein level, the major domains described in other P-type
transport ATPases can be recognized in C. elegans PMR1. Also
sequence motifs demonstrated to be critical for function in other
P-type transport ATPases are conserved in the C. elegans sequence (Fig. 2). On the basis of these comparisons, it can be firmly
concluded that the coding sequence identified in the C. elegans genome and whose protein product has been investigated in
this study is a member of the family of PMR1 ion-transport ATPases.
This conclusion is further substantiated by the transport and
phosphorylation studies on the protein expressed in COS-1 cells.
Functional data on the PMR1 transporter are up till now available only
for the yeast S. cerevisae (21, 37, 40). In the present work
the first characterization of the ion transporting activity of an
animal PMR1 enzyme is presented. The C. elegans PMR1 protein
overexpressed in COS-1 cells showed a predominantly Golgi-like
distribution as shown by immunocytochemistry. ATP-dependent uptake of 45Ca and 54Mn was demonstrated in
cells permeabilized with saponin. Cells overexpressing PMR1 accumulated
more Ca2+ and Mn2+ than control cells, and this
additional uptake was not diminished by the SERCA-specific inhibitor
thapsigargin, demonstrating that PMR1 is not only able to transport
Ca2+ but also Mn2+. Part of this
thapsigargin-insensitive Ca2+ pool was released by
IP3, which is compatible with existing evidence that the
Golgi apparatus is an IP3-sensitive Ca2+ store
(6). The rate of 45Ca2+ uptake as determined in
our experimental system was half-maximal at 0.25 µM
Ca2+, which is slightly higher than the value determined
from the ATPase activity of the purified PMR1 protein of S. cerevisiae (21). The 45Ca2+ uptake was
progressively inhibited by increasing concentrations of
Mn2+. Half-maximal inhibition occurred at a
Mn2+ concentration that is about the same as the
half-maximal Ca2+ concentration for stimulating the uptake,
indicating that the affinity of the C. elegans PMR1
transport ATPase for Ca2+ and Mn2+ is
approximately the same. A similar competition between Ca2+
and Mn2+ has been observed for the ATPase activity of
S. cerevisiae PMR1, but the yeast showed a somewhat higher
affinity for Mn2+ than for Ca2+ (40).
Conversely, the uptake of Mn2+ was inhibited by
Ca2+, further confirming the competition between
Ca2+ and Mn2+ for binding to the transport sites.
The phosphorylation experiments further validated the conclusion that
the high-affinity transport sites can be activated either by
Ca2+ or Mn2+. Phosphoprotein formation was
maximal at submicromolar concentrations of Ca2+ or
Mn2+, confirming the high affinity for both cations derived
from the competition between Ca2+ and Mn2+ for
transport. The phosphoprotein formation was strongly inhibited by
La3+. This effect is in the same direction but more
pronounced than that observed with the SERCA-type Ca2+
pumps, whereas the phosphoprotein level of PMCA-type
Ca2+-transport ATPases is increased by La3+
(41).
In conclusion, we have identified and characterized the C. elegans homologue of the PMR1 ion-transport ATPase previously
characterized only in yeast. We have shown that the PMR1 protein
overexpressed in COS-1 cells shows a predominantly Golgi-like
distribution and that its ion transport activity can be measured
following permeabilization of the plasma membrane. These experiments
directly demonstrate that C. elegans PMR1 is
able to accumulate Ca2+ and Mn2+ with high
affinity into the Golgi membranes. Part of the accumulated Ca2+ can be released by IP3, confirming the
observation of Pinton et al. (6) that the Golgi apparatus is
an IP3-sensitive Ca2+ store.
INTRODUCTION
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ABSTRACT
INTRODUCTION
EXPERIMENTAL PROCEDURES
RESULTS
DISCUSSION
REFERENCES
EXPERIMENTAL PROCEDURES
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ABSTRACT
INTRODUCTION
EXPERIMENTAL PROCEDURES
RESULTS
DISCUSSION
REFERENCES
-32P]ATP and 45Ca
were obtained from Amersham Pharmacia Biotech (Roosendaal, The
Netherlands), 54Mn from PerkinElmer Life Sciences (Boston,
MA). Sequencing was done with the AutoReadTM 200 Sequencing Kit from
Amersham Pharmacia Biotech. COS-1 cells were transiently transfected
with FuGENETM 6 transfection reagent (Roche Molecular Diagnostics,
Brussels, Belgium) according to the manufacturer's instructions.
Fluorescein isothiocyanate-labeled goat anti-rabbit antibodies
were obtained from Sigma (Sigma/Aldrich NV, Bornem, Belgium) and
horseradish peroxidase-labeled swine anti-rabbit antibodies from Dako
(Dako A/S, Glostrup, Denmark). The full-length rabbit SERCA1a clone was
kindly provided by J. P. Andersen and B. Vilsen (University of
Aarhus, Denmark).
-D-galactopyranoside and purification
of the 6xHis-tagged protein was achieved by a Ni-NTA agarose-based method as described in the manufacturer's instructions. The
recombinant protein migrated with an apparent molecular mass of about
42 kDa in 12% SDS-polyacrylamide gels and reacted with the monoclonal anti-polyhistidine antibody (clone HIS-1, from Sigma, dilution 1:1000)
on Western blots. In a last step the recombinant cytoplasmic loop was
concentrated by CentriconTM Plus-20 centrifugal filtering
(Millipore, Bedford, MA).
-32P]ATP of 2 µCi/µl. After 20 s the
reaction was stopped by adding 0.4 ml of ice-cold stop solution (6%
trichloroacetic acid, 10 mM phosphoric acid, 1 mM ATP). The mixture was left on ice for 0.5 h and
centrifuged in the cold to precipitate the protein. The pellet was
washed two more times in stop solution and once in 0.2 M
acetic acid-NaOH (pH 5.3). To test the sensitivity of the
phosphoprotein to hydroxylamine, the samples were additionally incubated for 20 min at room temperature in 0.2 M acetic
acid-NaOH with or without 0.2 M hydroxylamine. The samples
were then dissolved in a modified SDS loading buffer and subjected to
SDS-gel electrophoresis in acid gels as described by Sarkadi et
al. (33). The gels were dried between gel drying sheets (Promega)
and exposed to screens for quantification of the radioactive bands on a
Storm840TM scanner in combination with the ImageQuantTM
software (Molecular Dynamics, Sunnyvale, CA).
RESULTS
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ABSTRACT
INTRODUCTION
EXPERIMENTAL PROCEDURES
RESULTS
DISCUSSION
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1 and exon
3 for reasons discussed below (Fig. 1B, second line). Exon 1 (399 bp) starts at position
87 relative to the ATG
start codon, which corresponds to nucleotide 23653 on cosmid CECC4.
Exon
1 (105 bp) and exon
3 (65 bp) correspond, respectively, to
nucleotides 23111-23215 and 15957-16021. This implies that exon
3
is located more than 7 kb upstream from exon
1. EST clone yk218a11
does obviously not contain the entire 5' UTR, as only the last part of
exon 1 is represented in this clone.
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Fig. 1.
The exon/intron layout of the
CePMR1 gene and its comparison to other
Pmr1 genes. A, shows the structure
of the CePMR1 gene. The thin horizontal line
represents the introns. Exons are represented by boxes.
Wide boxes depict the open reading frame. ATG and TAA
indicate the start and stop translation codons, respectively.
pA1-pA3 represent the alternative
polyadenylation signals. The two alternative splicings are shown on the
second and third lines. SL1 and SL2
are splice leaders added after transcription by
trans-splicing. The distance between exon 3 and exon
1
amounts to about >7 kb. Note that exon 8 is 63 bp shorter at its 3'
end than predicted by the Genefinder program. B, comparison
of exon layout between cDNAs from C. elegans PMR1 (this
study, upper two lines), and the orthologues from D. melanogaster (DmPMR1) and H. sapiens (HsPMR1; Refs. 22
and 23). Black boxes represent untranslated sequences.
Open boxes indicate the open reading frame and the
gray boxes at the left indicate the splice leader
sequences SL1 and SL2. The arrow indicates the exon boundary
conserved between all three animal species. Asterisks
indicate the exon boundaries conserved between C. elegans
and H. sapiens. Triangles denote the exon
boundary conserved between D. melanogaster and H. sapiens.
3 and
exon
3 was in turn spliced to exon
1, i.e. an exon
layout as found in EST clone yk334d5. By sequencing the SL1/CePMR1Rev
fragment of 824 bp, we found that SL1 was spliced to a novel exon (
2)
in the 5' UTR of the gene, which corresponds to nucleotides
22950-23020 on cosmid CECC4 (Fig. 1B, upper line). Thus it
appears that the C. elegans transcripts are both
alternatively spliced and trans-spliced at their 5' end.
Both types of splicing are coupled: a mRNA transcript containing
exon
1 and exon
2 is trans-spliced to SL1, a mRNA containing exon
1 and exon
3 is trans-spliced to SL2.
3 to 9) of moderate length and spans a region of more than 19 kb of
genomic DNA. The transcripts are both alternatively and trans-spliced at their 5' end. At their 3' end, however, no
alternative splicing could be documented but alternative
polyadenylation occurs.
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Fig. 2.
Alignment of the predicted amino acid
sequences of PMR1 homologues of four distant species. Amino acids
conserved in all four species are in bold. The function of
some critical residues demonstrated in SERCA and conserved in PMR1 are
indicated above the aligned sequences: the phosphate
accepting Asp (Asp336 in C. elegans), residues
involved in binding of ATP and fluorescein isothiocyanate, and parts of
structurally important loops. The conserved DPPR motif is part of the
loop (NP connection) between two major cytosolic domains, the
nucleotide-binding domain and the phosphorylation domain (36). The 10 hydrophobic stretches that presumably form the transmembrane domain are
underlined and labeled M1-M10. Near the N terminus, an EF
hand-like stretch in S. cerevisiae that has been shown to
bind metals (37) is indicated below the alignment. The
asterisks (*) indicate conserved amino acids in
transmembrane domains M4 and M6 that form in SERCA the site II
Ca2+-binding site. Site I residues are not conserved in
PMR1. Mn indicates Gln783 in S. cerevisiae that has been shown to contribute to the selectivity
for Ca2+ and Mn2+ (40). Dmelanog,
D. melanogaster (see text); Hsapiens, H. sapiens, the longer ATP2C1a alternative transcript (22);
Celegans, C. elegans (see text);
Scerevis, S. cerevisiae, SwissProt P13586
(14).
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Fig. 3.
C. elegans PMR1 expression in
C. elegans microsomes and in COS-1 cells.
A, Western blot of a 7.5% SDS-polyacrylamide gel
immunostained with 1:1000 diluted preimmune serum (lane 3)
or with polyclonal anti-Pmr1 antibody (lanes 1 and
2; dilution 1:10000; lane 4, dilution 1:1000).
Secondary antibodies were used at 1:1000 dilution. Lanes 1 and 2, microsomal protein (10 µg), isolated from COS-1
cells transiently transfected with, respectively, empty vector or
C. elegans Pmr1 expression vector. Lanes 3 and 4, membrane fraction (20 µg) prepared from whole
nematodes. B, immunocytochemical staining of COS-1 cells
transfected with empty vector (control, panel 1) or
C. elegans PMR1 expression vector (panel 2).
Primary incubation was with anti-PMR1 antiserum (1:1000) and secondary
antibody was fluorescein isothiocyanate-conjugated goat anti-rabbit
(dilution 1:160).
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Fig. 4.
Ca2+ transport activity of
C. elegans PMR1. A, Ca2+
loading of COS-1 cells transfected with empty vector ( ,
), rabbit
SERCA1a (
,
), and C. elegans PMR1 (
,
) in the
presence (filled symbols) or absence (open
symbols) of 2 µM thapsigargin. Cells were loaded at
240 nM free Ca2+ for 45 min and
Ca2+ efflux was followed for 20 min. After 10 min of efflux
10 µM IP3 was administered
(arrow). B, time dependent Ca2+
loading of COS-1 cells transfected with C. elegans PMR1.
Cells were incubated with loading medium containing 240 nM
free Ca2+ for 0.5, 1, 2, 4, 10, 20, or 40 min. The
Ca2+ content of the stores is plotted as a function of the
duration of loading. The results are presented as mean values ± S.D. of four experiments. C, Ca2+ dependence of
loading of C. elegans PMR1. Ca2+ uptake (10 min)
into COS-1 cells transfected with empty vector (control) and C. elegans PMR1 was measured at different free Ca2+
concentrations. The figure displays the Ca2+ content of the
stores, plotted as a function of the Ca2+ concentration in
the loading medium. The data points are the difference in
Ca2+ uptake between cells transfected with C. elegans PMR1 and control and represent the mean ± S.E. of
eight separate experiments.
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Fig. 5.
Mn2+ transport by C. elegans PMR1 and competition between Ca2+ and
Mn2+ for transport. A, Mn2+
loading of COS-1 cells transfected with native vector (control, ) or
C. elegans PMR1 (
) in the presence of 2 µM
thapsigargin. Cells were loaded at 10 µM free
Mn2+ for 10 min, washed three times with efflux medium, and
subjected to a passive efflux for 10 min. After 4 min of efflux 10 µM A23187 was added. B, inhibition of
Ca2+ uptake of C. elegans PMR1 by
Mn2+. Control cells and C. elegans
PMR1-transfected COS-1 cells were incubated with loading medium
containing 1 µM free Ca2+ (
), 0.316 µM (
) or 0.1 µM (
) free
Ca2+ and different free Mn2+ concentrations.
The Ca2+ content of the stores is plotted as a function of
the Mn2+ concentration and is represented as the mean ± S.E. of four independent experiments. C, inhibition of
Mn2+ uptake of C. elegans PMR1 by
Ca2+. Control COS-1 cells and C. elegans
PMR1-expressing cells were loaded with 54Mn2+
at 1 µM in the presence of 2 µM
thapsigargin and at different free Ca2+ concentrations
(
, nominal Ca2+ free;
, 2 µM
Ca2+;
, 10 µM Ca2+). Cells
were fluxed for 10 min following three quick washes with efflux medium
to reduce passive binding of Mn2+. 10 µM
IP3 was added after 4 min of efflux. The values represent
the difference between C. elegans PMR1-transfected and
control COS-1 cells.
-phosphate of ATP to the protein, forming a covalent
bond with the carboxyl group of a conserved aspartic acid residue in
the large cytosolic domain. The radioactively labeled phosphoprotein
can be preserved during SDS-gel electrophoresis by quenching the
reaction in acid and maintaining acid conditions throughout
electrophoresis. Fig. 6 shows the
radioactively labeled phosphointermediate in an SDS gel of microsomes
from COS-1 cells overexpressing PMR1. The labeling was completely
removed by treatment with hydroxylamine, demonstrating that the
phosphate was bound to a carboxyl and not to a hydroxyl group (data not
shown). As for the protein detected on Western blots, the
phosphoprotein migrated slightly faster relative to the markers than
expected from the predicted Mr of 98,505. The anomalous migration is probably due to the gel system because a similar
shift is also observed for the SERCA2a Ca2+-transport
ATPase, which has a predicted Mr of 109,720 (Fig. 6A). The phosphointermediate formation was stimulated
by Ca2+ and Mn2+. The maximum levels were
observed below 1 µM for both Ca2+ and
Mn2+ and these maximum levels were not significantly
different (data not shown). There remains a small residual amount of
phosphoprotein also in the presence of EGTA without added
Ca2+ or Mn2+. At present we do not have a
straightforward explanation for this background labeling. Possibly, a
small fraction of the transport-protein molecules in the COS-1 cell
membranes is able to reach a conformational state that allows
phosphorylation without occupation of the transport sites. The
Ca2+- or Mn2+-dependent
phosphoprotein formation was strongly inhibited by 50 µM
La3+ (Fig. 6B). The phosphorylation experiments
thus confirm the high affinity of the C. elegans PMR1
transporter for both Ca2+ and Mn2+.
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Fig. 6.
Autoradiogram of the phosphoprotein
intermediate of C. elegans PMR1 expressed in COS-1
cells. A, the phosphorylation reaction was carried out
on microsomes from PMR1- or SERCA2a-transfected COS-1 cells. The
phosphorylation medium contained 0.5 mM total EGTA, a
calculated free Mg2+ concentration of 0.5 mM,
in the absence or presence of 1 µM free Ca2+
or Mn2+. In the case of PMR1-overexpressing microsomes, 1 µM thapsigargin was included. The PMR1 phosphoenzyme
could not be seen in SERCA-transfected cells (left lanes)
nor in nontransfected cells (data not shown). B, the effect
of 50 µM La3+ on the phosphoprotein formation
of C. elegans PMR1 in the presence of 1 µM
free Ca2+ or Mn2+.
DISCUSSION
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ABSTRACT
INTRODUCTION
EXPERIMENTAL PROCEDURES
RESULTS
DISCUSSION
REFERENCES
3 and
2, both located
in the 5' UTR. Interestingly, we observed the possibility of
alternative splicing in the 5' UTR, which was coupled to
trans-splicing to SL1 or SL2. The meaning of such an
alternative trans-splicing which only affects the 5' UTR
remains unknown. It should be noted that for the human PMR1
orthologue (gene ATP2C1), 5'-end alternative splicing has
also been suggested but here it affects the open reading frame (23),
whereas in C. elegans it does not result in the formation of
distinct protein isoforms. At the 3' end of the CePMR1 gene
three polyadenylation sites were predicted and also experimentally
detected by PCR analysis. However, there was neither any predicted nor
any experimental indication for alternative splicing. Both for the
corresponding human gene and for the rat gene alternative splicing and
different protein tails have been suggested. However, the alternative
splice sites in human and rat appear not to be conserved.
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ACKNOWLEDGEMENTS |
---|
We thank Dr. Yuji Kohara (National Institute of Genetics, Mishima Japan) for the gift of yk218a11 and yk334d5 C. elegans cDNA clones.
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FOOTNOTES |
---|
* This work was supported by Fonds voor Wetenschappelijk Onderzoek Vlaanderen (FWO) Grant G.0137.00 and Inter-University Poles of Attraction Program P4/23 of the Belgian State, Prime Minister's Office for Scientific, Technical and Cultural Affairs.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 nucleotide sequence(s) reported in this paper has been submitted to the GenBankTM/EMBL Data Bank with accession number(s) AJ303081 and AJ303082.
To whom correspondence should be addressed. Tel.: 32-16-345834;
Fax: 32-16-345991; E-mail: Kurt.Vanbaelen@med.kuleuven.ac.be.
Published, JBC Papers in Press, December 29, 2000, DOI 10.1074/jbc.M010553200
2 M. Adams and J. C. Venter, accession number AC014929.
3 T. A. Reinhardt and S. Prapong, accession number AF230532.
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ABBREVIATIONS |
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The abbreviations used are: ER, endoplasmic reticulum; SERCA, sarco(endo)plasmic reticulum calcium adenosine triphosphatase; PMCA, plasma membrane calcium adenosine triphosphatase; EST, expressed sequence tag; PCR, polymerase chain reaction; CePMR1, C. elegans orthologous gene of the PMR1 gene of S. cerevisae; IP3, inositol 1,4,5-trisphosphate; UTR, untranslated region; bp, base pair(s); kb, kilobase pair(s).
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