Genotype-structure-phenotype relationships diverge in paralogs ATP1A1, ATP1A2, and ATP1A3

Objective

We tested the assumption that closely related genes should have similar pathogenic variants by analyzing >200 pathogenic variants in a gene family with high neurologic impact and high sequence identity, the Na,K-ATPases ATP1A1, ATP1A2, and ATP1A3.

Methods

Data sets of disease-associated variants were compared. Their equivalent positions in protein crystal structures were used for insights into pathogenicity and correlated with the phenotype and conservation of homology.

Results

Relatively few mutations affected the corresponding amino acids in 2 genes. In the membrane domain of ATP1A3 (primarily expressed in neurons), variants producing milder neurologic phenotypes had different structural positions than variants producing severe phenotypes. In ATP1A2 (primarily expressed in astrocytes), membrane domain variants characteristic of severe phenotypes in ATP1A3 were absent from patient data. The known variants in ATP1A1 fell into 2 distinct groups. Sequence conservation was an imperfect indicator: it varied among structural domains, and some variants with demonstrated pathogenicity were in low conservation sites.

Conclusions

Pathogenic variants varied between genes despite high sequence identity, and there is a genotype-structure-phenotype relationship in ATP1A3 that correlates with neurologic outcomes. The absence of "severe" pathogenic variants in ATP1A2 patients predicts that they will manifest either in a different tissue or by death in utero and that new ATP1A1 variants will produce additional phenotypes. It is important that some variants in poorly conserved amino acids are nonetheless pathogenic and could be incorrectly predicted to be benign.

Widespread distribution of calcium pump in calcium-transporting tissues. Immunohistochemical studies

We examined the distribution of the plasma membrane calcium pump in several calcium-transporting tissues. Epitopes of the pump were found in the human kidney, specifically in the basolateral membrane of the distal tubule. The 28-kD calcium-binding protein co-localized with the calcium pump in cells of the distal tubule. Other human tissues that contain calcium pump epitopes include the basal surface of the syncytiotrophoblast of the placenta, human osteoblast-like cells and human choroid plexus. In the rat, calcium pump epitopes were found in the distal tubule of the kidney, the duodenum, the ileum, jejunum, placenta and in the choroid plexus. The pump is widely distributed in various calcium-transporting tissues and probably plays an important role in the translocation of calcium.

Apart from its known function, the plasma membrane Ca²⁺ATPase can regulate Ca²⁺ signaling by controlling phosphatidylinositol 4,5-bisphosphate levels

Plasma membrane Ca(2+) ATPases (PMCAs, also known as ATP2B1-ATP2B4) are known targets of phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P₂], but if and how they control the PtdIns(4,5)P₂ pool has not been considered. We demonstrate here that PMCAs protect PtdIns(4,5)P₂ in the plasma membrane from hydrolysis by phospholipase C (PLC). Comparison of active and inactive PMCAs indicates that the protection operates by two mechanisms; one requiring active PMCAs, the other not. It appears that the mechanism requiring activity is the removal of the Ca(2+) required for sustained PLC activity, whereas the mechanism not requiring activity is PtdIns(4,5)P₂ binding. We show that in PMCA overexpressing cells, PtdIns(4,5)P₂ binding can lead to less inositol 1,4,5-triphosphate (InsP₃) and diminished Ca(2+) release from intracellular Ca(2+) pools. Inspection of a homology model of PMCA suggests that PMCAs have a conserved cluster of basic residues forming a 'blue collar' at the interface between the membrane core and the cytoplasmic domains. By molecular dynamics simulation, we found that the blue collar forms four binding pockets for the phosphorylated inositol head group of PtdIns(4,5)P₂; these pockets bind PtdIns(4,5)P₂ strongly and frequently. Our studies suggest that by having the ability to bind PtdIns(4,5)P₂, PMCAs can control the accessibility of PtdIns(4,5)P₂ for PLC and other PtdIns(4,5)P₂-mediated processes.

Alternative pathways for association and dissociation of the calmodulin-binding domain of plasma membrane Ca(2+)-ATPase isoform 4b (PMCA4b)

The calmodulin (CaM)-binding domain of isoform 4b of the plasma membrane Ca(2+) -ATPase (PMCA) pump is represented by peptide C28. CaM binds to either PMCA or C28 by a mechanism in which the primary anchor residue Trp-1093 binds to the C-terminal lobe of the extended CaM molecule, followed by collapse of CaM with the N-terminal lobe binding to the secondary anchor Phe-1110 (Juranic, N., Atanasova, E., Filoteo, A. G., Macura, S., Prendergast, F. G., Penniston, J. T., and Strehler, E. E. (2010) J. Biol. Chem. 285, 4015-4024). This is a relatively rapid reaction, with an apparent half-time of ~1 s. The dissociation of CaM from PMCA4b or C28 is much slower, with an overall half-time of ~10 min. Using targeted molecular dynamics, we now show that dissociation of Ca(2+)-CaM from C28 may occur by a pathway in which Trp-1093, although deeply embedded in a pocket in the C-terminal lobe of CaM, leaves first. The dissociation begins by relatively rapid release of Trp-1093, followed by very slow release of Phe-1110, removal of C28, and return of CaM to its conformation in the free state. Fluorescence measurements and molecular dynamics calculations concur in showing that this alternative path of release of the PMCA4b CaM-binding domain is quite different from that of binding. The intermediate of dissociation with exposed Trp-1093 has a long lifetime (minutes) and may keep the PMCA primed for activation.

Calmodulin wraps around its binding domain in the plasma membrane Ca2+ pump anchored by a novel 18-1 motif

Using solution NMR spectroscopy, we obtained the structure of Ca(2+)-calmodulin (holoCaM) in complex with peptide C28 from the binding domain of the plasma membrane Ca(2+)-ATPase (PMCA) pump isoform 4b. This provides the first atomic resolution insight into the binding mode of holoCaM to the full-length binding domain of PMCA. Structural comparison of the previously determined holoCaM.C20 complex with this holoCaM.C28 complex supports the idea that the initial binding step is represented by (holoCaM.C20) and the final bound complex by (holoCaM.C28). This affirms the existing multi-step kinetic model of PMCA4b activation by CaM. The complex exhibits a new binding motif in which holoCaM is wrapped around helical C28 peptide using two anchoring residues from the peptide at relative positions 18 and 1. The anchors correspond to Phe-1110 and Trp-1093, respectively, in full-length PMCA4b, and the peptide and CaM are oriented in an anti-parallel manner. This is a greater sequence distance between anchors than in any of the known holoCaM complexes with a helical peptide. Analysis of the geometry of holoCaM-peptide binding for the cases where the target peptide adopts an alpha(D)-helix with its anchors buried in the main hydrophobic pockets of the two CaM lobes establishes that only relative sequential positions of 10, 14, 17, and 18 are allowed for the second anchor.

Plasma-membrane Ca(2+) pumps: structural diversity as the basis for functional versatility

Plasma-membrane calcium pumps [PMCAs (plasma-membrane Ca(2+)-ATPases)] expel Ca(2+) from eukaryotic cells to maintain overall Ca(2+) homoeostasis and to provide local control of intracellular Ca(2+) signalling. Recent work indicates functional versatility among PMCA isoforms, with specific pumps being essential for cochlear hair cell function, sperm motility, feedback signalling in the heart and pre- and post-synaptic Ca(2+) regulation in neurons. The functional versatility of PMCAs is due to differences in their regulation by CaM (calmodulin), kinases and other signalling proteins, as well as to their differential targeting and retention in defined plasma membrane domains. The basis for this is the structural diversity of PMCAs. In mammals, four genes encode PMCA isoforms 1-4, and each of these has multiple variants generated by alternative RNA splicing. The alternatively spliced regions are intimately involved in the regulatory interactions and differential membrane localization of the pumps. The alternatively spliced C-terminal tail acts as an autoinhibitory domain by interacting with the catalytic core of the pump. The degree of inhibition and the kinetics of interaction with the major activator CaM differ between PMCA variants. This translates into functional differences in how PMCAs handle Ca(2+) signals of different magnitude and frequency. Accumulating evidence thus demonstrates how structural diversity provides functional versatility in the PMCAs.

The plasma membrane Ca2+ pump isoform 4a differs from isoform 4b in the mechanism of calmodulin binding and activation kinetics: implications for Ca2+ signaling

The inhibition by the regulatory domain and the interaction with calmodulin (CaM) vary among plasma membrane calcium pump (PMCA) isoforms. To explore these differences, the kinetics of CaM effects on PMCA4a were investigated and compared with those of PMCA4b. The maximal apparent rate constant for CaM activation of PMCA4a was almost twice that for PMCA4b, whereas the rates of activation for both isoforms showed similar dependence on Ca2+. The inactivation of PMCA4a by CaM removal was also faster than for PMCA4b, and Ca2+ showed a much smaller effect (2- versus 30-fold modification). The rate constants of the individual steps that determine the overall rates were obtained from stopped-flow experiments in which binding of TA-CaM was observed by changes in its fluorescence. TA-CaM binds to two conformations of PMCA4a, an "open" conformation with high activity, and a "closed" one with lower activity. Compared with PMCA4b (Penheiter, A. R., Bajzer, Z., Filoteo, A. G., Thorogate, R., Török, K., and Caride, A. J. (2003) Biochemistry 41, 12115-12124), the model for PMCA4a predicts less inhibition in the closed form and a much faster equilibrium between the open and closed forms. Based on the available kinetic parameters, we determined the constants to fit the shape of a Ca2+ signal in PMCA4b-overexpressing Chinese hamster ovary cells. Using the constants for PMCA4a, and allowing small variations in parameters of other systems contributing to a Ca2+ signal, we then simulated the effect of PMCA4a on the shape of a Ca2+ signal in Chinese hamster ovary cells. The results reproduce the published data (Brini, M., Coletto, L., Pierobon, N., Kraev, N., Guerini, D., and Carafoli, E. (2003) J. Biol. Chem. 278, 24500-24508), and thereby demonstrate the importance of altered regulatory kinetics for the different functional properties of PMCA isoforms.

Plasma membrane Ca2+ ATPases as dynamic regulators of cellular calcium handling

Plasma membrane Ca2+ ATPases (PMCAs) are essential components of the cellular toolkit to regulate and fine-tune cytosolic Ca2+ concentrations. Historically, the PMCAs have been assigned a housekeeping role in the maintenance of intracellular Ca2+ homeostasis. More recent work has revealed a perplexing multitude of PMCA isoforms and alternative splice variants, raising questions about their specific role in Ca2+ handling under conditions of varying Ca2+ loads. Studies on the kinetics of individual isoforms, combined with expression and localization studies suggest that PMCAs are optimized to function in Ca2+ regulation according to tissue- and cell-specific demands. Different PMCA isoforms help control slow, tonic Ca2+ signals in some cells and rapid, efficient Ca2+ extrusion in others. Localized Ca2+ handling requires targeting of the pumps to specialized cellular locales, such as the apical membrane of cochlear hair cells or the basolateral membrane of kidney epithelial cells. Recent studies suggest that alternatively spliced regions in the PMCAs are responsible for their unique targeting, membrane localization, and signaling cross-talk. The regulated deployment and retrieval of PMCAs from specific membranes provide a dynamic system for a cell to respond to changing needs of Ca2+ regulation.

Isoform-specific up-regulation of plasma membrane Ca2+ATPase expression during colon and gastric cancer cell differentiation

In this work we demonstrate a differentiation-induced up-regulation of the expression of plasma membrane Ca2+ATPase (PMCA) isoforms being present in various gastric/colon cancer cell types. We found PMCA1b as the major isoform in non-differentiated cancer cell lines, whereas the expression level of PMCA4b was significantly lower. Cell differentiation initiated with short chain fatty acids (SCFAs) and trichostatin A, or spontaneous differentiation of post-confluent cell cultures resulted in a marked induction of PMCA4b expression, while only moderately increased PMCA1b levels. Up-regulation of PMCA4b expression was demonstrated both at the protein and mRNA levels, and closely correlated with the induction of established differentiation markers. In contrast, the expression level of the Na+/K+-ATPase or that of the sarco/endoplasmic reticulum Ca2+ATPase 2 protein did not change significantly under these conditions. In membrane vesicles obtained from SCFA-treated gastric/colon cancer cells a marked increase in the PMCA-dependent Ca2+ transport activity was observed, indicating a general increase of PMCA function during the differentiation of these cancer cells. Because various PMCA isoforms display distinct functional characteristics, we suggest that up-regulated PMCA expression, together with a major switch in PMCA isoform pattern may significantly contribute to the differentiation of gastric/colon cancer cells. The analysis of PMCA expression may provide a new diagnostic tool for monitoring the tumor phenotype.

Cleavage of the Plasma Membrane Ca+ATPase during Apoptosis

Maintenance of Ca2+ homeostasis is essential for normal cellular function and survival. Recent evidences suggest that Ca2+ is also an important player of apoptosis. We demonstrated that the plasma membrane Ca2+ ATPase (PMCA) isoform 4b, a key element of cellular Ca2+ homeostasis, was cleaved by caspase-3 during the course of apoptosis. This cleavage of PMCA removed the entire regulatory region from the C terminus, leaving behind a 120-kDa catalytic fragment. Since loss of PMCA activity could lead to intracellular Ca2+ overload and consequently necrotic cell death, an important question is whether the apoptotic fragment of PMCA retains full activity or it is inactivated. To address this question, we constructed a C-terminally truncated mutant that corresponded to the caspase-3 fragment of PMCA4b and showed that it was fully and constitutively active. This mutant was targeted properly to the plasma membrane when it was expressed stably or transiently in several different cell lines. We followed truncation of PMCA during apoptosis induced by mitochondrial or receptor-mediated pathways and found that a similar fragment of 120 kDa was formed and remained intact for several hours after treatment. We have also demonstrated that the caspase-3 cleavage site is an important structural element of PMCA and found that the accessibility of the caspase-3 site depended strongly on the conformational state of the protein.

Modification of human hearing loss by plasma-membrane calcium pump PMCA2

Five adult siblings presented with autosomal recessive sensorineural hearing loss: two had high-frequency loss, whereas the other three had severe-to-profound loss affecting all frequencies. Genetic evaluation revealed that a homozygous mutation in CDH23 (which encodes cadherin 23) caused the hearing loss in all five siblings and that a heterozygous, hypofunctional variant (V586M) in plasma-membrane calcium pump PMCA2, which is encoded by ATP2B2, was associated with increased loss in the three severely affected siblings. V586M was detected in two unrelated persons with increased sensorineural hearing loss, in the other caused by a mutation in MYO6 (which encodes myosin VI) in one and by noise exposure, suggesting that this variant may modify the severity of sensorineural hearing loss caused by a variety of factors.

Kinetic analysis of the calmodulin-binding region of the plasma membrane calcium pump isoform 4b

The sequence L(1086)RRGQILWFRGLNRIQTQIKVVKAFHSS(1113) (peptide C28) is responsible for calmodulin binding to PMCA4b. In this work, peptides following the above sequence were progressively shortened either at the N-terminus (C28NDelta3, C28NDelta5, or C28NDelta6) or at the C-terminus (C20, C22, C23, and C25). Competitive inhibition of PMCA activity was used to measure apparent dissociation constants of the complexes between calmodulin and C28 or progressively shortened peptides. Additionally, equilibrium titrations were used to measure the apparent dissociation constants of the various peptides with TA-calmodulin by changes in TA-calmodulin fluorescence and Trp fluorescence of the peptides. At the N-terminus, deletion of five residues did not change calmodulin affinity, but deletion of six residues resulted in a 5-fold decrease in affinity. There were no major differences in the time course of TA-CaM binding, but C28NDelta6 exhibited a different time course of Trp fluorescence change. At the C-terminus, deletion of five residues (C23) or more resulted in a net increase in fluorescence of TA-CaM upon binding, while longer peptides (C25 and C28) produced both a transient increase and a net decrease in the fluorescence of TA-CaM. Global regression analysis revealed that binding of TA-CaM to the C23 peptide could be fit by a two-step model, while longer peptides required three-step models for adequate fitting. TA-calmodulin dissociated rapidly from C23, C22, and C20, resulting in a marked increase in apparent K(d). Thus, the sequence I(1091)LWFRGLNRIQTQIKVVKAF(1110) (C25NDelta5) is required to reproduce the calmodulin-binding properties of C28. When F(1110) was replaced by A, the TA-calmodulin association and dissociation kinetics resembled C23 kinetics, but changing V(1107) to A produced a smaller effect, suggesting that F(1110), rather than V(1107), is the main anchor for the N-terminal lobe of calmodulin in PMCA4b.

Low endolymph calcium concentrations in deafwaddler2J mice suggest that PMCA2 contributes to endolymph calcium maintenance

In vertebrates, transduction of sound into an electrochemical signal is carried out by hair cells that rely on calcium to perform specialized functions. The apical surfaces of hair cells are surrounded by endolymphatic fluid containing calcium at concentrations that must be maintained by active transport. The mechanism of this transport is unknown, but an ATP-dependent pump is believed to participate. Mutation of the Atp2b2 gene that encodes plasma membrane calcium ATPase type 2 (PMCA2) produces the deaf, ataxic mouse: deafwaddler2J (dfw2J). We hypothesized that PMCA2 might transport calcium into the endolymph and that dfw2J mice would have low endolymph calcium concentrations, possibly contributing to their deafness and ataxia. First, using immunocytochemistry, we demonstrated that PMCA2 is present in control mice inner and outer hair cell stereocilia where it could pump calcium into the endolymph and that PMCA2 is absent in dfw2J stereocilia. Second, using an aspirating microelectrode and calcium-sensitive fluorescent dye, we found that dfw2J mice endolymph calcium concentrations are significantly lower than those of control mice. These findings suggest that PMCA2, located in hair cell stereocilia, contributes significantly to endolymph calcium maintenance.

Mutations in the Na+/K+ -ATPase alpha3 gene ATP1A3 are associated with rapid-onset dystonia parkinsonism

Rapid-onset dystonia-parkinsonism (RDP, DYT12) is a distinctive autosomal-dominant movement disorder with variable expressivity and reduced penetrance characterized by abrupt onset of dystonia, usually accompanied by signs of parkinsonism. The sudden onset of symptoms over hours to a few weeks, often associated with physical or emotional stress, suggests a trigger initiating a nervous system insult resulting in permanent neurologic disability. We report the finding of six missense mutations in the gene for the Na+/K+ -ATPase alpha3 subunit (ATP1A3) in seven unrelated families with RDP. Functional studies and structural analysis of the protein suggest that these mutations impair enzyme activity or stability. This finding implicates the Na+/K+ pump, a crucial protein responsible for the electrochemical gradient across the cell membrane, in dystonia and parkinsonism.

Intramolecular interactions of the regulatory region with the catalytic core in the plasma membrane calcium pump

The access of three proteases to their sites of cleavage was used as a measure of regulatory interactions in the plasma membrane Ca2+ pump isoform 4b (PMCA4b). When the proteases could not cut at their sites in the C-terminal regulatory region, the interaction was judged to be tight. This was the case in the absence of Ca2+, when chymotrypsin and caspase cut PMCA only very slowly. Ca2+ accelerated the fragmentation, but the digestion remained incomplete. In the presence of Ca2+ plus calmodulin, the digestion became nearly complete in all cases, indicating a more flexible conformation of the carboxyl terminus in the fully activated state. The acceleration of proteolysis by Ca2+ or Ca2+ plus calmodulin occurred equally at the caspase site upstream of the calmodulin-binding domain and the chymotrypsin and calpain sites downstream of that domain. Replacing Trp1093 (a key residue within the calmodulin-binding domain) with alanine had a much more specific effect, because it exposed only proteolytic sites within the calmodulin-binding domain that had previously been shielded in the native protein. At these sites, both calpain and chymotrypsin cut the Trp1093 --> Ala mutant in the absence of calmodulin. These data indicate that, in the auto-inhibited conformation, the calmodulin-binding/auto-inhibitory sequence and the regions both upstream and downstream are in close contact with the catalytic core. Trp1093 plays an essential role not only in stabilizing the Ca2+-calmodulin/calmodulin-binding domain complex but also in the formation or stability of the inhibitory conformation of that domain when it interacts with the catalytic core of PMCA4b.

Asp1080 upstream of the calmodulin-binding domain is critical for autoinhibition of hPMCA4b

The role of the plasma membrane Ca(2+) pump (PMCA) is to remove excess Ca(2+) from the cytosol to maintain low intracellular Ca(2+) levels. Asp(1080) lies within an acidic sequence between the C-terminal inhibitory region and the catalytic core of PMCAs and is part of the caspase-3 recognition site of isoform 4b. Caspase-3 cuts immediately after this residue and activates the pump by removing the inhibitory region (Pászty, K., Verma, A. K., Padányi, R., Filoteo, A. G., Penniston, J. T., and Enyedi, A. (2002) J. Biol. Chem. 277, 6822-6829). Asp(1080) had not been believed to have any other role, but here we show that it also plays a critical role in the autoinhibition and calmodulin activation of PMCA4b. Site-specific mutation of Asp(1080) to Asn, Ala, or Lys in PMCA4b resulted in a substantial increase in the basal activity in the absence of calmodulin. All Asp(1080) mutants exhibited an increased affinity for calmodulin because of an increase in the rate of activation by calmodulin. This rate was higher when the inhibition was weaker, showing that a strong inhibitory interaction slows the activation rate. In contrast, mutating the nearby Asp(1077) had no effect on basal activity or calmodulin activation. We propose that the conserved Asp(1080), even though it is neither in the regulatory domain nor in the catalytic core, plays an essential role in inhibition by stabilizing the inhibited state of the enzyme.

Reincorporated plasma membrane Ca2+-ATPase can mediate B-Type Ca2+ channels observed in native membrane of human red blood cells

Recently, we reported indirect evidence that plasma membrane Ca2+-ATPase (PMCA) can mediate B-type Ca2+ channels of cardiac myocytes. In the present study, in order to bring more direct evidence, purified PMCA from human red blood cells (RBC) was reconstituted into giant azolectin liposomes amenable to the patch-clamp technique. Purified RBC PMCA was used because it is available pure in larger quantity than cardiac PMCA. The presence of B-type Ca2+ channels was first investigated in native membranes of human RBC. They were detected and share the characteristics of cardiac myocytes. They spontaneously appeared in scarce short bursts of activity, they were activated by chlorpromazine (CPZ) with an EC50 of 149 mmole/l or 1 mmole/l vanadate, and then switched off by 10 mmole/l eosin or dose-dependently blocked by 1-5 mmole/l ATP. Independent of membrane potential, the channel gating exhibited complex patterns of many conductance levels, with three most often observed conductance levels of 22, 47 and 80 pS. The activation by vanadate suggests that these channels could play a role in the influx of extracellular Ca2+ involved in the vanadate-induced Gardos effect. In PMCA-reconstituted proteoliposomes, nearly half of the ATPase activity was retained and clear "channel-like" openings of Ba2+- or Ca2+-conducting channels were detected. Channel activity could be spontaneously present, lasting the patch lifetime or, when previously quiescent, activity could be induced by application of 50 mmole/l CPZ only in presence of 25 U/ml calmodulin (CaM), or by application of 1 mmole/l vanadate alone. Eosin (10 mmole/l) and ATP (5 mmole/l) significantly reduced spontaneous activity. Channel gating characteristics were similar to those of RBC, with main conductance levels of 21, 40 and 72 pS. The lack of direct activation by CPZ alone might be attributed to a purification-induced modification or absence of unidentified regulatory component(s) of PMCA. Despite a few differences in results between RBC and reincorporated PMCA, most probably attributable to the decrease in ATPase activity following the procedure of reincorporation, the present experimental conditions appear to reveal a channel-mode of the PMCA that shares many similarities with the B-type Ca2+ channel.

Tryptophan 1093 is largely responsible for the slow off rate of calmodulin from plasma membrane Ca2+ pump 4b

Tryptophan 1093 resides in the 28-residue calmodulin-binding/autoinhibitory domain of the plasma membrane Ca(2+) pump (PMCA). Previous studies with the isolated calmodulin-binding/autoinhibitory peptide from PMCA have shown that mutations of the tryptophan residue decrease the affinity of the peptide for calmodulin and its affinity as an inhibitor of proteolytically activated pump. In this study, the PMCA mutation in which tryptophan 1093 is converted to alanine (W1093A) was constructed in the full-length PMCA isoform 4b. The mutant pump was expressed in COS cells, and its steady state and pre-steady state kinetic properties were examined. The W1093A pump exhibited an increased basal activity in the absence of calmodulin, so the activation was approximately 2-fold (it is 10-fold in the wild type). The W1093A mutation also lowered the steady state affinity for calmodulin from K(0.5) of 9 nm for wild type to 144 nm (assayed at 700 nm free Ca(2+)). Pre-steady state measurements of the rate of activation by Ca(2+)-calmodulin revealed that the W1093A mutant responded 2.5-fold faster to calmodulin. In contrast to these relatively modest effects, the half-time of inactivation of the mutant was reduced by more than 2 orders of magnitude from 41 min to 7 s. We conclude that tryptophan 1093 does not play a substantial role in Ca(2+)-calmodulin recognition; rather it functions primarily to slow the inactivation of the calmodulin-activated pump.

Plasma membrane Ca2+ATPase isoform 4b is cleaved and activated by caspase-3 during the early phase of apoptosis

The plasma membrane Ca(2+) pump (PMCA) is an essential element in the complex of mechanisms that maintain low intracellular Ca(2+) concentration in the living cell. This pump is tightly regulated by calmodulin through binding to a high affinity calmodulin-binding domain at the C terminus that also serves as an autoinhibitor of the enzyme. Inspection of the C terminus of hPMCA4b, the most widely distributed form of PMCA, revealed a caspase-3 consensus sequence ((1077)DEID(1080)) just a few residues upstream of the calmodulin-binding domain. We demonstrate here that, in the early phase of apoptosis, hPMCA4b is cleaved at aspartic acid Asp(1080) in hPMCA4b-transfected COS-7 cells or in HeLa cells that naturally express this protein. This cleavage of hPMCA4b produces a single 120-kDa fragment that is fully active in the absence of calmodulin, because the whole inhibitory region downstream of the (1077)DEID(1080) sequence is removed. Our experiments show that caspase-3 or a caspase-3-like protease is responsible for the formation of the constitutively active 120-kDa PMCA4b fragment: 1) Pretreatment of the cells with the caspase-3 inhibitor Z-DEVD-FMK (benzyloxycarbonyl-Asp(OMe)-Glu(OMe)-Val-Asp(OMe)-fluoromethyl ketone) was able to block the production of the 120-kDa fragment. 2) In vitro treatment of hPMCA4b with recombinant caspase-3 also generated a 120-kDa cleavage product, consistent with that seen in cells undergoing apoptosis. 3) Mutants in which the caspase-3 consensus sequence was altered ((1077)AEID(1080), (1077)DEIA(1080), and (1077)AEIA(1080) mutants) were resistant to proteolysis. Based on these data, we conclude that hPMCA4b is a newly identified, natural caspase-3 substrate. We suggest that a constitutively active form of this protein, responding much faster to an increase in Ca(2+) concentration than the autoinhibited form, may have an important role in regulating intracellular Ca(2+) concentration in the apoptotic cell.

Expression and cellular distribution pattern of plasma membrane calcium pump isoforms in rat pancreatic islets

This work is aimed at identifying the presence and cellular distribution pattern of plasma membrane calcium pump (PMCA) isoforms in normal rat pancreatic islet. Microsomal fractions of isolated islets and exocrine tissue were analyzed to detect different PMCA isoforms. The cellular distribution pattern of these PMCAs in the islets was also studied in fixed pancreas sections incubated with antibodies against PMCAs and insulin. Antibody 5F10, which reacts with all PMCA variants, showed multiple bands in the blots in the 127-134 kDa region, indicating the presence of several isoforms. Microsomes also reacted positively with specific antibodies for individual PMCA isoforms, generating a band of the expected size. Antibody 5F10 immunocytochemically labeled the plasma cell membrane of both b- and non-b-cells, but predominantly the former. All islet cells were also labeled with antibodies against isoforms 1 and 4, while the antibody reacting with isoform 3 labeled exclusively b-cells. A few b- and non-b-cells were positively labeled with the antibody reacting with PMCA b variant. Negative results were obtained with the antibody against isoform 2. Further studies, together with previous reports on the modulatory effect of insulin secretagogues and blockers upon PMCA activity, may provide evidence of the importance of this particular PMCA expression for islet function under normal and pathological conditions.

Characterization of the deafwaddler mutant of the rat plasma membrane calcium-ATPase 2

The deafwaddler mutant in mice was the first spontaneous mutant discovered in the plasma membrane Ca(2+) pump (PMCA) [Street, V.A. et al., 1998, Nat. Genet. 19, 390-394]. A nucleotide substitution in deafwaddler results in a Gly to Ser transition at amino acid 283 in the small cytoplasmic loop of PMCA isoform 2 (PMCA2). PMCA2 is abundant in the stereocilia of auditory and vestibular hair cells, neurons of the spiral ganglion, and participates in inner ear development. Mice that are homozygous for deafwaddler are deaf and have poor balance. However, the balance and hearing disorders of the deafwaddler mice appear to be less severe than homozygotes for a functionally null frameshift mutant or homozygous PMCA2 knockout mice, suggesting that deafwaddler PMCA2 retains some biological activity. To examine the enzymic effects of the deafwaddler mutant, PMCA2 wild-type and deafwaddler were produced by transient expression in COS cells as well as baculovirus-mediated expression in Sf9 insect cells. Membrane preparations were assayed for calcium transport and ATPase activity. No significant differences in the regulation by calmodulin of the wild-type and deafwaddler PMCA2b were found. Steady-state transport assays and pre-steady-state ATPase assays of these two proteins revealed that the K(0.5) for Ca(2+), K(0.5) for calmodulin, degree of activation by calmodulin and rate of activation by Ca-calmodulin were nearly identical. However, calcium transport of the deafwaddler pump was reduced to 30% of the wild-type activity. Although calcium transport activity was reduced in the deafwaddler pump, total phosphoenzyme formation from ATP was slightly higher for deafwaddler than for wild-type. 50 microM LaCl3 (which blocks the E(1)P to E(2)P conformational transition) increased the steady-state level of phosphoenzyme 3-fold for the wild-type but had no effect on the deafwaddler. Taken together, the kinetic data suggest that the deafwaddler mutation affects PMCA2 by slowing the E(1)P to E(2)P transition, resulting in approximately 70% reduction in the PMCA2-mediated Ca(2+) export.

The plasma membrane calcium pump displays memory of past calcium spikes. Differences between isoforms 2b and 4b

To understand how the plasma membrane Ca(2+) pump (PMCA) behaves under changing Ca(2+) concentrations, it is necessary to obtain information about the Ca(2+) dependence of the rate constants for calmodulin activation (k(act)) and for inactivation by calmodulin removal (k(inact)). Here we studied these constants for isoforms 2b and 4b. We measured the ATPase activity of these isoforms expressed in Sf9 cells. For both PMCA4b and 2b, k(act) increased with Ca(2+) along a sigmoidal curve. At all Ca(2+) concentrations, 2b showed a faster reaction with calmodulin than 4b but a slower off rate. On the basis of the measured rate constants, we simulated mathematically the behavior of these pumps upon repetitive changes in Ca(2+) concentration and also tested these simulations experimentally; PMCA was activated by 500 nm Ca(2+) and then exposed to 50 nm Ca(2+) for 10 to 150 s, and then Ca(2+) was increased again to 500 nm. During the second exposure to 500 nm Ca(2+), the activity reached steady state faster than during the first exposure at 500 nm Ca(2+). This memory effect is longer for PMCA2b than for 4b. In a separate experiment, a calmodulin-binding peptide from myosin light chain kinase, which has no direct interaction with the pump, was added during the second exposure to 500 nm Ca(2+). The peptide inhibited the activity of PMCA2b when the exposure to 50 nm Ca(2+) was 150 s but had little or no effect when this exposure was only 15 s. This suggests that the memory effect is due to calmodulin remaining bound to the enzyme during the period at low Ca(2+). The memory effect observed in PMCA2b and 4b will allow cells expressing either of them to remove Ca(2+) more quickly in subsequent spikes after an initial activating spike.

Plasma membrane Ca2+-ATPase isoform 2a is the PMCA of hair bundles

Mechanoelectrical transduction channels of hair cells allow for the entry of appreciable amounts of Ca(2+), which regulates adaptation and triggers the mechanical activity of hair bundles. Most Ca(2+) that enters transduction channels is extruded by the plasma membrane Ca(2+)-ATPase (PMCA), a Ca(2+) pump that is highly concentrated in hair bundles and may be essential for normal hair cell function. Because PMCA isozymes and splice forms are regulated differentially and have distinct biochemical properties, we determined the identity of hair bundle PMCA in frog and rat hair cells. By screening a bullfrog saccular cDNA library, we identified abundant PMCA1b and PMCA2a clones as well as rare PMCA2b and PMCA2c clones. Using immunocytochemistry and immunoprecipitation experiments, we showed in bullfrog sacculus that PMCA1b is the major isozyme of hair cell and supporting cell basolateral membranes and that PMCA2a is the only PMCA present in hair bundles. This complete segregation of PMCA1 and PMCA2 isozymes holds for rat auditory and vestibular hair cells; PMCA2a is the only PMCA isoform in hair bundles of outer hair cells and vestibular hair cells and is the predominant PMCA of hair bundles of inner hair cells. Our data suggest that hair cells control plasma membrane Ca(2+)-pumping activity by targeting specific PMCA isozymes to distinct subcellular locations. Because PMCA2a is the only Ca(2+) pump present at appreciable levels in hair bundles, the biochemical properties of this pump must account fully for the physiological features of transmembrane Ca(2+) pumping in bundles.

Delayed activation of the plasma membrane calcium pump by a sudden increase in Ca2+: fast pumps reside in fast cells

There are four genes encoding isoforms of the plasma membrane Ca(2+) pump (PMCA). PMCA variability is increased by the presence of two splicing sites. Functional differences between the variants of PMCA have been described, but little is known about the adaptive advantages of this great diversity of pumps. In this paper we studied how the different isoforms respond to a sudden increase in Ca(2+) concentration. We found that different PMCAs are activated by Ca(2+) at different rates, PMCA 3f and 2a being the fastest, and 4b the slowest. The rate of activation by Ca(2+) depends both on the rate of calmodulin binding and the magnitude of the activation by calmodulin. We found that 2a is located in heart and the stereocilia of inner ear hair cells, 3f in skeletal muscle and 4b was identified in Jurkat cells. Both cardiac and skeletal muscle, and stereocilia recover very rapidly after a cytoplasmic Ca(2+)peak, while in Jurkat cells the recovery takes up to a minute. In stereocilia, 2a is the only method for export of Ca(2+), making the analysis of them unusually straightforward. This indicates that these rates of PMCA activation by Ca(2+) are correlated with the speed of Ca(2+) concentration decay after a Ca2 spike in the cells in which these variants of PMCA are expressed. The results suggest that the type of PMCA expressed will correspond with the speed of Ca(2+) signals in the cell.

Chimaeras reveal the role of the catalytic core in the activation of the plasma membrane Ca2+ pump

Isoform 2b of the plasma membrane calcium pump differs from the ubiquitous isoform 4b in the following: (a) higher basal activity in the absence of calmodulin; (b) higher affinity for calmodulin; and (c) higher affinity for Ca(2+) in the presence of calmodulin [Elwess, Filoteo, Enyedi and Penniston (1997) J. Biol. Chem. 272, 17981-17986]. To investigate which parts of the molecule determine these kinetic differences, we made four chimaeric constructs in which portions of isoform 2b were grafted into isoform 4b: chimaera I contains only the C-terminal regulatory region of isoform 2b; chimaera II contains the N-terminal moiety of isoform 2b, including both cytoplasmic loops; chimaera III contains the sequence of isoform 2b starting from the N-terminus to after the end of the first (small) cytoplasmic loop; and chimaera IV contains only the second (large) cytoplasmic loop. Surprisingly, chimaera I showed low basal activity in the absence of calmodulin and low affinity for calmodulin, unlike isoform 2b. In contrast, the chimaera containing both loops showed high basal activity, and Ca(2+) activation curves (both in the absence and in the presence of calmodulin) similar to those of isoform 2b. The rates of activation by calmodulin and of inactivation by calmodulin removal were measured, and the apparent K(d) for calmodulin was calculated from the ratio between these rate constants. The order of affinity was: 2b=II>4b=IV>III=I. From these results it is clear that the construct that most closely resembles isoform 2b is chimaera II. This shows that, in order to obtain an enzyme with properties similar to those of isoform 2b, both cytoplasmic loops are needed.

Ca(2+)-ATPase protein expression in mammary tissue

Protein expression of plasma membrane Ca(2+)-ATPases (PMCAs) and the putative Golgi secretory pathway Ca(2+)-ATPase (SPCA) was examined in rat mammary tissue. As lactation started, PMCA protein expression increased dramatically, and this increased expression paralleled milk production. Mammary PMCA was primarily PMCA2b but was approximately 4,000 daltons larger than expected. RT-PCR showed that the primary mammary PMCA2b transcript was alternatively spliced, at splice site A, to include an additional 135 bp, resulting in the insertion of 45 amino acids. This splice form is designated 2bw. PMCA2bw is secreted into milk, associated with the milk fat globule membrane. Therefore, PMCA2bw is located on the apical membrane of the secretory cell. Smaller amounts of PMCA1b and 4b protein were found in mammary tissue. PMCA4b was the major PMCA expressed in developing tissue, and its level declined as lactation started. PMCA1b expression increased moderately during lactation. SPCA protein expression increased 1 wk before parturition and increased further as lactation proceeded. The abundance and cell location of PMCA2b suggest that it is important for macro-Ca(2+) homeostasis in lactating tissue. The pattern of expression and abundance of SPCA suggest that it is a candidate for the Golgi Ca(2+)-ATPase.

mRNAs coding for the calcium-sensing receptor along the rat nephron: effect of a low-phosphate diet

We investigated the localization of mRNA encoding the calcium-sensing receptor (CaSR) along the rat nephron. For this purpose, we combined microdissection of nephron segments and RT-PCR techniques. The results indicate that mRNA encoding rat CaSR is present in rat glomeruli and distal segments (medullary thick ascending limb, cortical thick ascending limb, distal convoluted tubule and cortical collecting duct), whereas it was not detected in proximal convoluted tubules or proximal straight tubules. We also studied whether the CaSR transcription in kidney cortex was modified in response to low dietary phosphate. No significant changes were detected. Given the fact that a low-phosphate diet increased Ca2+ excretion by more than 50-fold, the results suggest that if the CaSR regulates Ca2+ reabsorption, it does so through receptor occupancy by Ca2+ rather than by changes in receptor expression.

Plasma membrane Ca(2+) pump isoform 3f is weakly stimulated by calmodulin

Isoform 3f of the plasma membrane Ca(2+) pump is a major isoform of this pump in rat skeletal muscle. It has an unusual structure, with a short carboxyl-terminal regulatory region of only 33 residues when compared with the 77 to 124 residues found in the other isoforms. Also, whereas the regulatory regions of the other isoforms, downstream of the alternative splice, consist of two homologous groups, the sequence of 3f is not related to either group. A synthetic peptide representing the calmodulin binding domain of isoform 3f had a much lower calmodulin affinity (with a K(d) of 15 nM) than the corresponding peptide of isoform 2b (K(d) value was 0.2 nM). The characteristics of this domain were further studied by making chimeras of the 3f regulatory region with the catalytic core of isoform 4 and by making the full-length isoform 3f. Both constructs bound to calmodulin-Sepharose. The chimera was fully active without calmodulin, showing no stimulation of activity when calmodulin was added. The full-length isoform 3f was slightly activated by calmodulin. These data show that the regulatory region of isoform 3f is only a weak autoinhibitor of the enzyme, in contrast to the properties of all the other isoforms studied so far. Rather, this isoform is a special-purpose, constitutively active form of the enzyme, expressed primarily in skeletal muscle and as a minor isoform in brain.

The rate of activation by calmodulin of isoform 4 of the plasma membrane Ca(2+) pump is slow and is changed by alternative splicing

A reconstitution system allowed us to measure the ATPase activity of specific isoforms of the plasma membrane Ca(2+) pump continuously, and to measure the effects of adding or removing calmodulin. The rate of activation by calmodulin of isoform 4b was found to be very slow, with a half-time (at 235 nM calmodulin and 0.5 microM free Ca(2+)) of about 1 min. The rate of inactivation of isoform 4b when calmodulin was removed was even slower, with a half-time of about 20 min. Isoform 4a has a lower apparent affinity for calmodulin than 4b, but its activation rate was surprisingly faster (half time about 20 s). This was coupled with a much faster inactivation rate, consistent with its low affinity. A truncated mutant of isoform 4b also had a more rapid activation rate, indicating that the downstream inhibitory region of full-length 4b contributed to its slow activation. The results indicate that the slow activation is due to occlusion of the calmodulin-binding domain of 4b, caused by its strong interaction with the catalytic core. Since the activation of 4b occurs on a time scale comparable to that of many Ca(2+) spikes, this phenomenon is important to the function of the pump in living cells. The slow response of 4b indicates that this isoform may be the appropriate one for cells which respond slowly to Ca(2+) signals.

Selective decrease of mRNAs encoding plasma membrane calcium pump isoforms 2 and 3 in rat kidney

BACKGROUND

Although the existence of multiple isoforms of plasma membrane calcium ATPase (PMCA) is now well documented, their biological functions are not yet known. In this study, we set out to investigate the potential role of PMCA isoforms, previously identified in renal cortical tissue, in tubular reabsorption of calcium (Ca2+).

METHODS

With use of reverse transcription-polymerase chain reaction analysis, we determined levels of mRNAs encoding isoforms of PMCA1 through PMCA4 in renal cortex, liver, and brain of rats with hypercalciuria induced by feeding with a low-phosphate diet (LPD) as compared with Ca2+-retaining rats that were fed a high-phosphate diet (HPD).

RESULTS

We observed that in hypercalciuric LPD-fed rats, the mRNAs encoding isoforms PMCA2b and PMCA3(a + c) are significantly lower (Delta approximately-50%) than in HPD-fed hypocalciuric rats, whereas no changes in mRNAs encoding isoforms PMCA1b and PMCA4 were observed, and mRNA encoding calbindin 28 kDa was increased. On the other hand, the content of mRNAs encoding PMCA2b and PMCA3(a + c) in liver and brain, respectively, was not changed.

CONCLUSION

These findings are evidence that expression of PMCA isoforms in the kidney can be selectively modulated in response to pathophysiologic stimuli. The association of a decrease in mRNA encoding PMCA2b and PMCA3(a + c) with hypercalciuria suggests that the two PMCA isoforms may be operant in tubular reabsorption of Ca2+ and its regulation.

An autoinhibitory peptide from the erythrocyte Ca-ATPase aggregates and inhibits both muscle Ca-ATPase isoforms

We have studied the effects of C28R2, a basic peptide derived from the autoinhibitory domain of the plasma membrane Ca-ATPase, on enzyme activity, oligomeric state, and E1-E2 conformational equilibrium of the Ca-ATPase from skeletal and cardiac sarcoplasmic reticulum (SR). Time-resolved phosphorescence anisotropy (TPA) was used to determine changes in the distribution of Ca-ATPase among its different oligomeric species in SR. C28R2, at a concentration of 1-10 microM, inhibits the Ca-ATPase activity of both skeletal and cardiac SR (CSR). In skeletal SR, this inhibition by C28R2 is much greater at low (0.15 microM) than at high (10 microM) Ca2+, whereas in CSR the inhibition is the same at low and high Ca2+. The effects of the peptide on the rotational mobility of the Ca-ATPase correlated well with function, indicating that C28R2-induced protein aggregation and Ca-ATPase inhibition are much more Ca-dependent in skeletal than in CSR. In CSR at low Ca2+, phospholamban (PLB) antibody (functionally equivalent to PLB phosphorylation) increased the inhibitory effect of C28R2 slightly. Fluorescence of fluorescein 5-isothiocyanate-labeled SR suggests that C28R2 stabilizes the E1 conformation of the Ca-ATPase in skeletal SR, whereas in CSR it stabilizes E2. After the addition of PLB antibody, C28R2 still stabilizes the E2 conformational state of CSR. Therefore, we conclude that C28R2 affects Ca-ATPase activity, conformation, and self-association differently in cardiac and skeletal SR and that PLB is probably not responsible for the differences.

Protein kinase C phosphorylates plasma membrane Ca2+ pump isoform 4a at its calmodulin binding domain

Phosphorylation by protein kinase C of isoform 4a of the human plasma membrane Ca2+ pump (hPMCA4a) was studied using the COS cell expression system. Phosphorylation of several truncated mutants of hPMCA4a indicated that a single phosphorylation site lies in a region between residues 1113 and 1125. This region is within the calmodulin binding domain and contains a single phosphorylatable residue, serine 1115. Converting this serine to an alanine diminished phosphorylation greatly. Phosphorylation, done in the absence of calmodulin, did not affect subsequent calmodulin binding, but previous binding of calmodulin did inhibit phosphorylation. Moreover, no significant shift in the calmodulin response curve of hPMCA4a was observed when phosphorylation was mimicked by converting serine 1115 to an acidic residue. The calmodulin binding domain of hPMCA4a is much longer than other calmodulin binding domains and has been suggested to consist of two binding lobes interrupted by a short nonbinding region. The findings of this study indicate that serine 1115 is the residue phosphorylated by protein kinase C, and that it lies within the nonbinding region of the calmodulin binding domain.

Modulation of the plasma membrane Ca2+ pump

The plasma membrane calcium pump, which ejects Ca2+ from the cell, is regulated by calmodulin. In the absence of calmodulin, the pump is relatively inactive; binding of calmodulin to a specific domain stimulates its activity. Phosphorylation of the pump with protein kinase C or A may modify this regulation. Most of the regulatory functions of the enzyme are concentrated in a region at the carboxyl terminus. This region varies substantially between different isoforms of the pump, causing substantial differences in regulatory properties. The pump shares some motifs of the carboxyl terminus with otherwise unrelated proteins: The calmodulin-binding domain is a modified IQ motif (a motif which is present in myosins) and the last 3 residues of isoform 4b are a PDZ target domain. The pump is ubiquitous, with isoforms 1 and 4 of the pump being more widely distributed than 2 and 3. In some kinds of cells isoform 1 or 4 is missing, and is replaced by another isoform.

Expression of hPMCA4b, the major form of the plasma membrane calcium pump in megakaryoblastoid cells is greatly reduced in mature human platelets

Antibodies 5F10 and JA3 (raised against the erythrocyte Ca2+ pump) were used to identify hPMCA4b as the major form of the plasma membrane Ca2+ pump in human platelets and in three human megakaryoblastoid cell lines, MEG 01, DAMI and CHRF 288-11. 5F10 was used because it has been shown to recognize all known isoforms of the hPMCA and JA3 because it reacts exclusively with hPMCA4b [Caride A.J., Filoteo A.G., Enyedi A., Verma A.K., Penniston J.T. Detection of isoform 4 of the plasma membrane calcium pump in human tissues by using isoform-specific monoclonal antibodies. Biochem J 1996; 316: 353-359]. In addition to hPMCA4b, hPMCA1b was also detected in the megakaryoblastoid cells by using isoform-specific polyclonal antibodies. The apparent size of this isoform, however, was smaller than that seen in HeLa and COS-7 cell membranes indicating the presence of a modified form of hPMCA1b. In platelets, no evidence of the expression of hPMCA1b could be found. The amount of PMCA in these cells was compared with that of the constitutive form of the sarco/endoplasmic reticulum Ca2+ pump in non-muscle cells (SERCA2b) and also with the amount of PMCA in human erythrocytes. A very low level of the plasma membrane Ca2+ pump was found in platelets while in their precursor cells the expression of this Ca2+ pump was much more abundant. Whereas the expression level of PMCA decreased dramatically in mature human platelets, the expression of SERCA2b did not change substantially upon megakaryocytic differentiation.

mRNA encoding four isoforms of the plasma membrane calcium pump and their variants in rat kidney and nephron segments

To survey the presence of the four different isoforms of the plasma membrane calcium pump (PMCA) and their alternative splicing variants in the rat kidney, three major zones (cortex, outer medulla, and inner medulla) were macrodissected and probed for the presence of mRNA encoding these isoforms and their variants at the splicing site C by using reverse transcription-polymerase chain reaction (RT-PCR). Both the cortex and the outer medulla showed PMCA 1b, 2b, 3(a and c), and 4b. Semiquantitative comparisons indicated that isoform 2b is more abundant in the cortex than in the outer medulla and conversely, that isoform 3 (a and c) is more abundant in the outer medulla than in the cortex. The inner medulla showed only mRNA for isoforms 1b and 4b. The nephron segments present in the cortex and outer medulla were microdissected and analyzed by RT-PCR. Isoforms 1b, 2b, and 4b were found in all nephron segments but were found more frequently in tubular segments with high rates of Ca2+ reabsorption, suggesting that these isoforms may be involved in transepithelial transport. On the other hand, mRNA encoding isoform 3 (a and c) was most abundant in descending thin limb of Henle but was detected also in glomeruli and cortical thin ascending limb. Its distinct localization suggests that this isoform might have another function, such as in intracellular signalling.

Protein kinase C phosphorylates the "a" forms of plasma membrane Ca2+ pump isoforms 2 and 3 and prevents binding of calmodulin

Phosphorylation by protein kinase C of the "a" and "b" variants of plasma membrane Ca2+ pump isoforms 2 and 3 was studied. Full-length versions of these isoforms were assembled and expressed in COS cells. Whereas the "a" forms were phosphorylated easily with PKC, isoform 2b was phosphorylated only a little, and isoform 3b was not phosphorylated at all. Phosphorylation of isoforms 2a and 3a did not affect their basal activity, but prevented the stimulation of their activity by calmodulin and their binding to calmodulin-Sepharose. This indicated that phosphorylation prevented activation of these isoforms by preventing calmodulin binding. Based on these results, phosphorylation of the pump with PKC would be expected to increase free intracellular Ca2+ levels in those cells where isoforms 2a and 3a are expressed.

Plasma membrane Ca2+ pump in rat brain. Patterns of alternative splices seen by isoform-specific antibodies

The expression at the protein level of plasma membrane calcium pump (PMCA) isoforms in rat brain was detected by new antibodies that distinguished the four gene products and their alternatively spliced variants. All four gene products were distributed throughout hippocampus, cortex, and cerebellum, but the alternate splices showed more distinct distribution patterns. The b splice of isoform 1 was not detectable in any of the brain regions, which makes it unlikely that this isoform performs an essential housekeeping role as is frequently proposed. The b splices of isoforms 3 and 4, although expressed in all three regions, showed evidence of proteolysis, which removed a portion of the carboxyl terminus. In contrast, isoform 2b retained its full length, indicating that PMCA2b is more resistant to proteolysis than the other b forms. Whereas substantial amounts of isoforms 1a, 2a, and 3a were expressed in all regions, 4a was found only in frontal cortex. The distinct patterns of expression of the PMCA isoforms in brain suggest that some of them play a special role in intracellular Ca regulation.

Plasmalemmal ATPase calcium pump localizes to inner and outer hair bundles

Recent studies demonstrate calcium ion influx at the tips of hair cell stereocilia during mechano-transduction. These ions must be either pumped from the cytosol into the extracellular space or endoplasmic envelope, or else sequestered by binding to specific proteins. A plasma membrane calcium pump (ATPase-type) was analysed in whole-mounts of rat organ of Corti using a monoclonal antibody to a large cytoplasmic loop of this protein. The reactivity was particularly high on the tips of longer stereocilia and was found along the shafts. Inner hair cell stereocilia had much less reactivity than outer hair cells. The reactivity lined the plasma membrane of inner hair cell bodies while a higher reactivity appeared in the cytoplasm of outer hair cells. Supporting cells were unreactive. Ultrastructural examination confirmed the plasma membrane calcium pump location on stereocilia and along the endolymph surface of receptor cells. Reaction product lined the plasma membrane of stereocilia as intense puncta. More reactive puncta occurred near the distal ends of stereocilia and the number decreased toward the ciliary base. The endolymph plasma membrane over the cuticular notch was especially reactive. The finding of more intense pump reactivity at the tips of stereocilia than the base is consistent with the hypothesis that during transduction, calcium ions enter stereocilia, distally, and the ATPase plasma membrane calcium pump rapidly extrudes these ions to the extracellular space.

Plasma membrane Ca2+ pump isoforms 2a and 2b are unusually responsive to calmodulin and Ca2+

The full-length a and b variants of the rat plasma membrane calcium pump, isoform 2 (rPMCA2a and rPMCA2b), were constructed and expressed in COS-7 cells. To characterize these isoforms, calcium transport was determined in a microsomal fraction. Both rPMCA2a and rPMCA2b had a much higher affinity for calmodulin than the corresponding forms of hPMCA4, and rPMCA2b had the highest affinity among the isoforms that have been tested so far. When analyzed at a relatively high calmodulin concentration, rPMCA2b and, to a lesser extent, rPMCA2a showed higher apparent calcium affinity; i.e. they were more active at lower Ca2+ concentrations than hPMCA4b. This indicates that these two variants of rat isoform 2 will tend to maintain a lower free cytosolic Ca2+ level in cells where they are expressed. Both variants also showed a higher level of basal activity (in the complete absence of calmodulin) than hPMCA4b, a property which would reinforce their ability to maintain a low free cytosolic Ca2+ concentration. Experiments designed to determine the source of the higher apparent Ca2+ affinity of rPMCA2b showed that it came from the properties of the carboxyl terminus, rather than from any difference in the catalytic core.

Replacement of Val674 by Pro increases the sensitivity of the plasma membrane Ca2+ pump to inhibition by Mg2+

A cDNA encoding a plasma membrane Ca2+ pump mutant V674P(ct120) was constructed and expressed in COS-1 cells. Immunoblots of transfected COS-1 membranes showed that the V674P(ct120) and the wild-type hPMCA4b(ct120) proteins were expressed at similar levels. The change of Val674 to Pro reduced the activity of the hPMCA4b(ct120) to an extent similar to that observed previously in the full-length Ca2+ pump (Adamo et al. (1995) J. Biol. Chem. 270, 30111-30114). Despite its lower activity, the apparent affinity for Ca2+ of the V674P(ct120) enzyme was at least as high as that of hPMCA4b(ct120), indicating that substitution of Val674 by Pro did not impair the interaction of the enzyme with Ca2+. The sensitivity of the V674P(ct120) enzyme to inhibition by vanadate was not significantly different from that of the hPMCA4b(ct120), supporting the idea that the mutation did not alter the equilibrium between E2-E1. The study of the Mg2+ dependency of the Ca2+ transport showed that the V674P(ct120) mutant reached maximum activation at 100 microM Mg2+ in contrast with 500 microM in the hPMCA4b(ct120). Furthermore, while at 2 mM Mg2+ the hPMCA4b(ct120) showed no sign of inhibition, the activity of the mutant decreased to less than 50% of the maximum activity observed at 100 microM Mg2+. These results indicate that the decrease in the activity observed upon substitution of Val674 by Pro was due to a higher sensitivity to Mg2+ as inhibitor.

Protein kinase C activates the plasma membrane Ca2+ pump isoform 4b by phosphorylation of an inhibitory region downstream of the calmodulin-binding domain

The carboxyl-terminal region of the plasma membrane Ca2+ pump isoform 4b contains two autoinhibitory regions which keep the pump inactive in the absence of activators such as calmodulin. One of these regions is approximately coterminous with the calmodulin-binding domain, while the second region is downstream (Verma, A. K., Enyedi, A., Filoteo, A. G., and Penniston, J. T. (1994) J. Biol. Chem. 269, 1687-1691). The carboxyl-terminal region has also been identified as the site for phosphorylation of this isoform by protein kinase C (Wang, K. K. W., Wright, L. C., Machan, C. L., Allen, B. G., Conigrave, A. D., and Roufogalis, B. D. (1991) J. Biol. Chem. 266, 9078-9085). Using constructs lacking various numbers of residues at the carboxyl terminus, we studied the degree of phosphorylation by protein kinase C and the resultant activation of Ca2+ transport. The results showed that the most specific and easy phosphorylation occurred in a region of about 20 residues which is downstream of the calmodulin-binding domain, and that the downstream inhibitory domain had also about the same size and location. Phosphorylation partially activated the pump by removing only the inhibition due to this region. Binding of calmodulin to the calmodulin-binding domain activated the pump more fully by removing the inhibition due to both regions, regardless of the state of phosphorylation at the downstream inhibitory region.

Deletion of amino acid residues 18-75 inactivates the plasma membrane Ca2+ pump

A mutant of the plasma membrane Ca2+ pump hPMCA4b(d18-75)(ct120) containing a deletion of the N-terminal amino acid residues 18-75 and lacking the C-terminal 120 amino acid residues was expressed in COS-1 cells. The deletion in the N-terminal region did not significantly affect the level of expression of the Ca2+ pump. Tryptic digestion of the hPMCA4b(d18-75)(ct120) mutant resulted in the appearance of the same fragments obtained by proteolysis of the hPMCA4b(ct120) enzyme, suggesting that deletion of residues 18-75 neither impeded the insertion in the membrane nor extensively affected the folding of the mutant protein. The functional competence of the hPMCA4b(d18-75)(ct120) enzyme was examined by measuring the Ca2+ transport and the Ca2+ ATPase activity of COS-1 cell microsomes expressing the mutant protein. Both tests proved the mutant to be inactive. Under conditions in which hPMCA4b(ct120) becomes phosphorylated, hPMCA4b(d18-75)(ct120) was incapable of reacting with ATP and Ca2+ to form the phosphoenzyme. Taken together these results suggest that the segment of amino acids 18-75 is essential for the activity of the plasma membrane Ca2+ pump.

The plasma membrane Ca2+ pump mutant lysine591 --> arginine retains some activity, but is still inactivated by fluorescein isothiocyanate

Inactivation of the wild-type human plasma membrane Ca2+ pump (isoform 4b) by fluorescein isothiocyanate is accompanied by covalent modification of Lys591. The mutation of Lys591 to arginine reduced the Ca2+ transport activity to 35% of the wild-type, and diminished the amount of acylphosphate formed from ATP by a corresponding amount. When this mutant was treated with fluorescein isothiocyanate; the enzyme was still irreversibly inactivated, even though no reactive residue was available at position 591. The results show that, although Ca2+ pump function is sensitive to the residue at position 591, Lys591 is not essential for enzyme activity. They also demonstrate that irreversible inhibition of the plasma membrane Ca2+ pump by fluorescein isothiocyanate does not require the covalent modification of Lys591. This indicates that fluorescein isothiocyanate reacts with lysine residues at other positions in addition to Lys591.

Detection of isoform 4 of the plasma membrane calcium pump in human tissues by using isoform-specific monoclonal antibodies

The epitope location and specificity of monoclonal antibodies JA9, 5F10 and JA3, raised against the human plasma membrane Ca2+ pump (hPMCA), were analysed by using synthetic peptides of the corresponding epitopes as well as the complete isoforms, hPMCA4b, hPMCA4a and hPMCA1b, expressed in COS-1 cells. The experiments with the peptides showed that JA9 reacted specifically with a region containing residues 51-75 of hPMCA4 (a or b), but not with the same region of isoforms 1, 2 or 3. JA3 reacted with residues 1156-1180, a region unique to hPMCA4b. 5F10 reacted in the region of residues 719-738, which is highly conserved in all PMCA isoforms. Indeed, 5F10 recognized all three isoforms expressed in COS-1 cells. JA9, in contrast, reacted with both variants a and b of hPMCA4 but not with hPMCA1, and JA3 recognized exclusively hPMCA4b. We used these antibodies to discern the distribution of hPMCA4a and hPMCA4b in human brain, heart, kidney and lung. In Western blots of human brain samples, we could identify both hPMCA4a and hPMCA4b. Heart tissue also showed isoform 4b, and probably 4a. In contrast, kidney and lung showed primarily hPMCA4b. In brain, overlapping bands that did not correspond to either variant of hPMCA4 were detected, and in kidney a band migrating in the same position as hPMCA1b was observed. The distribution of the a and b forms of hPMCA4 at the protein level, as analysed by these antibodies, is consistent with the available data about the abundance of mRNAs for the hPMCA isoforms. The presence of hPMCA4b in all the samples supports the proposed role of this isoenzyme as a constitutive form of the pump.