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

Plasma membrane Ca2+ ATPase isoform 4b binds to membrane-associated guanylate kinase (MAGUK) proteins via their PDZ (PSD-95/Dlg/ZO-1) domains

Plasma membrane Ca2+ ATPases are P-type pumps important for intracellular Ca2+ homeostasis. The extreme C termini of alternatively spliced "b"-type Ca2+ pump isoforms resemble those of K+ channels and N-methyl-D-aspartate receptor subunits that interact with channel-clustering proteins of the membrane-associated guanylate kinase (MAGUK) family via PDZ domains. Yeast two-hybrid assays demonstrated strong interaction of Ca2+ pump 4b with the PDZ1 + 2 domains of several mammalian MAGUKs. Pump 4b and PSD-95 could be co-immunoprecipitated from COS-7 cells overexpressing these proteins. Surface plasmon resonance revealed that a C-terminal pump 4b peptide interacted with the PDZ1 + 2 domains of hDlg with nanomolar affinity (KD = 1.6 nM), whereas binding to PDZ3 was in the micromolar range (KD = 1.2 microM). In contrast, the corresponding C-terminal peptide of Ca2+ pump 2b interacted weakly with PDZ1 + 2 and not at all with PDZ3 of hDlg. Ca2+ pump 4b bound strongly to PDZ1 + 2 + 3 of hDlg on filter assays, whereas isoform 2b bound weakly, and the splice variants 2a and 4a failed to bind. Together, these data demonstrate a direct physical binding of Ca2+ pump isoform 4b to MAGUKs via their PDZ domains and reveal a novel role of alternative splicing within the family of plasma membrane Ca2+ pumps. Alternative splicing may dictate their specific interaction with PDZ domain-containing proteins, potentially influencing their localization and incorporation into functional multiprotein complexes at the plasma membrane.

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.

Characterization of the Golgi complex cleared of proteins in transit and examination of calcium uptake activities

To characterize endogenous molecules and activities of the Golgi complex, proteins in transit were > 99% cleared from rat hepatocytes by using cycloheximide (CHX) treatment. The loss of proteins in transit resulted in condensation of the Golgi cisternae and stacks. Isolation of a stacked Golgi fraction is equally efficient with or without proteins in transit [control (CTL SGF1) and cycloheximide (CHX SGF1)]. Electron microscopy and morphometric analysis showed that > 90% of the elements could be positively identified as Golgi stacks or cisternae. Biochemical analysis showed that the cis-, medial-, trans-, and TGN Golgi markers were enriched over the postnuclear supernatant 200- to 400-fold with and 400- to 700-fold without proteins in transit. To provide information on a mechanism for import of calcium required at the later stages of the secretory pathway, calcium uptake into CTL SGF1 and CHX SGF1 was examined. All calcium uptake into CTL SGF1 was dependent on a thapsigargin-resistant pump not resident to the Golgi complex and a thapsigargin-sensitive pump resident to the Golgi. Experiments using CHX SGF1 showed that the thapsigargin-resistant activity was a plasma membrane calcium ATPase isoform in transit to the plasma membrane and the thapsigargin-sensitive pump was a sarcoplasmic/endoplasmic reticulum calcium ATPase isoform. In vivo both of these calcium ATPases function to maintain millimolar levels of calcium within the Golgi lumen.

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.

The membrane topology of the amino-terminal domain of the red cell calcium pump

A systematic study of the membrane-associated regions in the plasma membrane Ca2+ pump of erythrocytes has been performed by hydrophobic photolabeling. Purified Ca2+ pump was labeled with 3-(trifluoromethyl)-3-(m-[125I]iodophenyl)-diazirine ([125I]TID), a generic photoactivatable hydrophobic probe. These results were compared with the enzyme labeled with a strictly membrane-bound probe, [3H]bis-phosphatidylethanolamine (trifluoromethyl) phenyldiazirine. A significant light-dependent labeling of an M(r) 135,000-140,000 peptide, corresponding to the full Ca2+ pump, was observed with both probes. After proteolysis of the pump labeled with each probe and isolation of fragments by SDS-PAGE, a common pattern of labeled peptides was observed. Similarly, labeling of the Ca2+ pump with [125I]TID, either in isolated red blood cell membranes or after the enzyme was purified, yields a similar pattern of labeled peptides. Taken together, these results validate the use of either probe to study the lipid interface of the membrane-embedded region of this protein, and sustain the notion that the conformation of the pump is maintained throughout the procedures of solubilization, affinity purification, and reconstitution into proteoliposomes. In this work, we put special emphasis on a detailed analysis of the N-terminal domain of the Ca2+ pump. A labeled peptide of M(r) 40,000 belonging to this region was purified and further digested with V8 protease. The specific incorporation of [125I]TID to proteolytic fragments pertaining to the amino-terminal region indicates the existence of two transmembrane stretches in this domain. A theoretical analysis based on the amino acid sequence 1-322 predicts two segments with high probability of membrane insertion, in agreement with the experimental data. Each segment shows a periodicity pattern of hydrophobicity and variability compatible with alpha-helical structure. These results strongly suggest the existence of a transmembrane helical hairpin motif near the N-terminus of the Ca2+ pump.

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.

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.

Change in plasma membrane Ca2(+)-ATPase splice-variant expression in response to a rise in intracellular Ca2+

BACKGROUND

Most eukaryotic genes are divided into introns and exons. Upon transcription, the intronic segments are eliminated and the exonic sequences spliced together through a series of complex processing events. Alternative splicing refers to the optional inclusion or exclusion of specific exons in transcripts derived from a single gene, which leads to structural and functional changes in the encoded proteins. Although many components of the machinery directing the physical excision of introns and joining of exons have been elucidated in recent years, the signaling pathways regulating the activity of the machinery remain largely unexplored.

RESULTS

A calcium-mediated signaling pathway regulates alternative splicing at a specific site of human plasma membrane calcium pump-2 transcripts. This site consists of three exons, which are differentially used in a tissue-specific manner. In IMR32 neuroblastoma cells, a transient elevation of intracellular calcium changed the predominant pattern from one in which all three exons are included to the coexpression of a variant including only the third exon. Western-blot analysis demonstrated that the newly expressed mRNAs are faithfully translated. Once induced, the new splicing pattern was maintained over multiple cell divisions. Protein synthesis was not required to induce the alternative splice change, indicating that all components necessary for a rapid cellular response are present in the cells.

CONCLUSIONS

Calcium signaling exerts a direct influence on the regulation of alternative splicing. Notably, a calcium-mediated change in the expression of alternatively spliced variants of a calcium regulatory protein was discovered. The change in splicing occurs quickly, is persistent but reversible and leads to a corresponding change in protein expression. The specific nature in which differently spliced protein variants are expressed, and now the fact that their expression can be regulated by distinct intracellular signaling pathways, suggests that the regulation of alternative splicing by physiological stimuli is a widespread regulatory mechanism by which a cell may coordinate its responses to environmental cues.

Halothane alters electrical activity and calcium dynamics in cultured mouse cortical, spinal cord, and dorsal root ganglion neurons

Halothane inhibits neural plasma membrane Ca(2+)-ATPase, a pump that ejects Ca2+ from the cell after influx through voltage- or ligand-activated channels. Intracellular microelectrode recordings in mouse embryonic cortical and spinal cord neurons showed that halothane and eosin, a pump inhibitor, prolonged repolarization associated with spontaneous bursts of depolarization. These agents also prolonged the repolarization phases of electrically induced action potentials and of capsaicin-mediated Ca(2+)-dependent depolarization in mouse adult dorsal root ganglion neurons. In keeping with these findings, confocal microfluorimetry showed that halothane delayed clearance of intracellular Ca2+ accumulated by N-methyl-D-aspartate stimulation of single neurons.