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.