The calcium pump of the plasma membrane: membrane targeting, calcium binding sites, tissue-specific isoform expression.

The two Ca2+ pumps of higher eucaryotes are strictly targeted to different membrane systems: the plasma membrane (PMCA) and the sarco(endo)plasmic reticulum (SERCA). Chimeric constructs of the two pumps expressed in COS-7 cells have revealed a strong signal for endoplasmic reticulum retention in the N-terminal cytosolic portion of the SERCA pump: the signal is contained in a stretch of 28 amino acids that follows the N-terminus. A second, but masked, endoplasmic reticulum retention signal is contained in a cytosolic C-terminal sequence immediately preceding the calmodulin-binding domain of the Ca2+ pump. Selective mutations on the SERCA pump have led to the conclusion that 5 conserved residue membrane domains (TM)4, 5, and 6 form the Ca2+ channel through the pump protein. A comparative sequence inspection has failed to reveal any of these residues in TM5 of the PMCA pump. Mutation of the conserved residue in TM4 and of two in TM6 abolished the ability of the pump to form the Ca(2+)-dependent phosphoenzyme. However, one of the mutations (N979, TM6) also caused retention of the PMCA pump in the reticulum, suggesting structural alterations. Of the four basic isoforms of the pump, two (1, 4) are ubiquitously expressed, two (2, 3) are essentially brain specific. Isoform 2 has the highest calmodulin affinity. Primary cultures of cerebellar granule cells from newborn rats did not express isoforms 2 and 3 at plating time. Incubation of the cells in depolarizing concentrations of KCl, which promote Ca2+ influx, promoted the expression of isoforms 2 and 3, and of a brain specific spliced variant of isoform 1. Incubation of the cells in L-type Ca2+ channel blockers abolished the upregulation of the pump genes.