A unique combination of plasma membrane Ca2+-ATPase isoforms is expressed in islets of Langerhans and pancreatic β-cell lines

摘要

pChanges in free intracellular Casup2+/sup concentration regulate insulin secretion from pancreatic β-cells. The existence of steep Casup2+/sup gradients within the β-cell requires the presence of specialized Casup2+/sup exclusion systems. In this study we have characterized the plasma membrane Casup2+/sup-ATPases (PMCAs) which extrude Casup2+/sup from the cytoplasm. PMCA isoform- and subtype-specific mRNA expression was investigated in rodent pancreatic α- and β-cell lines, and in human and rat islets of Langerhans using reverse-transcription PCR with primers flanking the calmodulin-binding region of rat PMCA. The expression pattern of PMCA 1 and 2 was conserved in different species and islet-cell types since both rat and human islets of Langerhans and all cell lines tested contained the 1b and 2b forms. PMCA 4 isoform subtypes, however, were expressed in a cell-type-specific manner since β-cells expressed PMCA 4b only, whereas in islets of Langerhans, which contain α, β, δ and polypeptide-secreting cells, PMCA 4a and 4b were simultaneously present. No evidence was obtained for the expression of PMCA 3. Characterization of the β-cell Casup2+/sup-pump protein showed that it shared several similarities with the erythrocyte PMCA. It is a P-type ATPase; its phosphorylated intermediate was stabilized by Lasup3+/sup; it reacted with a PMCA-specific antibody; and it was not N-glycosylated. However, the β-cell PMCA had a higher molecular mass than that of the erythrocyte; this difference could be explained by either predominant translation of the PMCA 2 form, which has a molecular mass 3–8 kDa higher than the erythrocyte PMCA 1 and 4 proteins, or by a possible sequence insertion. Thus a unique combination of functionally distinct PMCA isoforms (1b, 2b, 4b) participates in Casup2+/sup homoeostasis in the β-cell./p