Identification and isolation of the phosphorylated intermediate of the calcium pump in rat intestinal basolateral membranes

Primary structure of rat plasma membrane Ca(2+)-ATPase isoform 4 and analysis of alternative splicing patterns at splice site A

We have determined the primary structure of the rat plasma membrane Ca(2+)-ATPase isoform 4 (PMCA4), and have analysed its mRNA tissue distribution and alternative splicing patterns at splice site A. Rat PMCA4 (rPMCA4) genomic clones were isolated and used to determine the coding sequences and intron/exon organization of the 5'-end of the gene, and the remaining coding sequence was determined from PCR-amplified cDNA fragments. Pairwise comparisons reveal that the amino acid sequence of rPMCA4 has diverged substantially from those of rPMCA isoforms 1, 2 and 3 (73-76% identity) and from that of human PMCA4 (87%). Despite the high degree of sequence divergence between the two species, comparisons of intron and untranslated mRNA sequences with the corresponding human sequences confirm the identity of this rat isoform as PMCA4. Northern blot studies demonstrate that the PMCA4 mRNA is expressed in all rat tissues examined except liver, with the highest levels in uterus and stomach. A combination of PCR analysis of alternative splicing patterns and sequence analysis of the gene demonstrate that a 36 nt exon at site A is included in PMCA4 mRNAs of most tissues but is largely excluded in heart and testis. Alternative splicing of both the 36 nt exon and a previously characterized 175 nt exon at splice site C, each of which can be either included or excluded in a highly tissue-specific manner, leads to the production of four different PMCA4 variants ranging in size from 1157 to 1203 amino acids.

Investigation of the Calcium-Transporting ATPases at the Endoplasmic Reticulum and Plasma Membrane of Red Beet (Beta vulgaris)

Calcium-transporting ATPases were compared in endoplasmic reticulum (ER)- and plasma membrane-enriched fractions of red beet (Beta vulgaris L.) storage tissue by measuring 45Ca uptake and calcium-dependent phosphoenzyme formation. The plasma membrane fraction was prepared by aqueous two-phase partitioning of a microsomal fraction and collecting the upper phase. The ER-enriched fraction was obtained by submitting a sucrose-gradient ER-enriched fraction to aqueous two-phase partitioning and collecting the lower phase; this reduced contaminating plasma membrane, which partitioned into the upper phase. The ATP-dependent calcium uptake observed in both fractions was released by the calcium ionophore A23187. Calcium uptake showed saturation kinetics for calcium with Km values of 0.92 mmol m-3 for the ER fraction and 1.24 mmol m-3 for the plasma membrane fraction. Uptake into both fractions was inhibited by vanadate and erythrosin B, although the plasma membrane system was slightly more sensitive to both inhibitors. Cyclopiazonic acid and thapsigargin, at low concentrations, had no marked effect on uptake. The plasma membrane system was less substrate-specific for ATP than the ER system, since it was able to use GTP and ITP to drive calcium transport at up to 50% of the level obtained with ATP. Following phosphorylation with [[gamma]-32P]ATP, two high molecular mass, calcium-dependent phosphoproteins (119 and 124 kD) and a low molecular mass, calcium-independent phosphoprotein (17 kD) were observed in the plasma membrane fraction. The ER fraction showed one high molecular mass phosphoprotein (119 kD) in the presence of calcium and two low molecular mass phosphoproteins (17 and 20 kD) that showed no calcium dependence. The low molecular mass phosphoproteins were insensitive to hydroxyl-amine, but they did show turnover. The identity of these proteins is unknown, but they do not have the properties of phosphorylated intermediates of calcium-ATPases. In contrast, the high molecular mass phosphoproteins displayed properties consistent with their representing phosphorylated intermediates of E1E2-type ATPases; they were hydroxylamine-sensitive, showed rapid turnover, and were inhibited by vanadate. Because they showed calcium-dependent phosphorylation and were sensitive to erythrosin B, the 119- and 124-kD phosphoproteins may be phosphorylated intermediates of the ER and plasma membrane calcium ATPases. These phosphoproteins were characterized further with respect to inhibitor sensitivity, responses to ions, and substrate specificity.

Characterization of EGTA-washed synaptosomal membrane with emphasis on its calmodulin-binding proteins. Demonstration of possible reconstitution with added calcium/calmodulin

Endogenous calmodulin (CaM) in the EGTA-washed cerebral-cortical synaptosomal membrane (SM) preparation was estimated below 3 micrograms/ml protein by the semiquantitative immunoblot analysis (Natsukari, N., Ohta, H. and Fujita, M. (1989) J. Immunol. Methods 125, 159-166). Membrane-bound CaM was immunoelectron-microscopically demonstrated in EGTA-washed, non-treated (control), and Ca(2+)-treated cerebral-cortical synaptosomal membranes (SM) as well as for the SM enriched with added CaM. The density of CaM increased in the above order. CaM-dependent adenylate cyclase and CaM-dependent protein kinase II (CaM-kinase II) activities were restored, whereas the phosphodiesterase (PDE) activity was not affected by exogenous CaM over all the Ca2+ concentrations tested. Adenylate cyclase at pCa 6.2 was synergistically activated either by GTP and CaM or by CaM and beta-adrenergic agonist, (+/-)-isoproterenol, reflecting the intactness of signal transduction pathway in the SM. Also demonstrated were the presence of protein kinase A, CaM-kinase II, and their endogenous substrates in the SM. Based on 32P-autoradiography and 125I-CaM overlay data certain CaM-binding proteins such as CaM-kinase II and synapsin I were identified on SDS-PAGE. Ca(2+)-dependent and -independent CaMBPs were distinguished by 125I-CaM gel overlay with and without Ca2+. The former had bigger molecular size (greater than or equal to 49 kDa) than the latter (less than or equal to 34 kDa). Yield of Ca(2+)-dependent CaMBPs was not affected by Ca2+ concentration during preparation of the SM while that of Ca(2+)-independent CaMBPs was reduced by exposure to 100 microM Ca2+. In contrast with the CaMBPs of brain SM, those of enterocyte and eyrthrocyte plasma membranes especially, microvillous membrane of the enterocyte, showed quite distinct CaMBP profiles. The present findings suggested that the EGTA-washed SM preparation made a useful system for studying the role of CaM in the brain SM.

The effect of vitamin D on rat intestinal plasma membrane CA-pump mRNA

The effects of vitamin D on steady-state levels of rat intestinal Ca-pump mRNA were examined in RNA extracted from isolated cell fractions of the crypt-to-villus gradient of differentiation. Northern blots revealed three different size mRNAs. Vitamin D deficient animals showed a decrease in these Ca-pump mRNAs, which increased markedly after 1,25-(OH)2D3 repletion, particularly for the villus cell. The data suggest that one of the effects of 1,25-(OH)2D3 may be to modulate enterocyte Ca-pump mRNA and that this effect is partly dependent on the stage of cell differentiation.

Cellular and segmental distribution of Ca2(+)-pump epitopes in rat intestine

We used a monoclonal antibody (5F10) specific for the human erythrocyte plasma membrane Ca(++)-pump to demonstrate the presence and distribution of Ca(++)-pump epitopes in rat intestine. In paraffin embedded tissue sections, antibody 5F10 binds to epitopes in the basolateral membranes of absorptive cells in rat duodenum and portions of jejunum but not ileum. Western blot analysis of intestinal mucosal proteins with antibody 5F10 shows binding of antibody to major bands of Mr approximately 135,000 and Mr approximately 72,000, and to lesser bands of Mr approximately 125,000 and Mr approximately 27,000. This pattern was seen in mucosal homogenates of rat duodenal and jejunal cells and to a lesser extent in ileal cells. The Mr approximately 135,000 band corresponds to the molecular weight of Ca(++)-pumps in other tissues. The other bands correspond in size to known proteolytic fragments of the Ca(++)-pump. Slot-blot analysis of nitrocellulose immobilized mucosal homogenates shows binding of 5F10 to be greatest in duodenum and least in ileum. Ca(++)-transport studies by the everted gut sac technique show a correlation between vitamin D induction of active Ca(++)-transport and the segmental distribution of Ca(++)-pump epitopes.