Deletions in the AL region of the h4xb plasma membrane Ca2+ pump: High apparent affinity for Ca2+ of a deletion mutant resembling the alternative spliced form h4zb

Mutants of the plasma membrane Ca2+ pump (human isoform 4xb) with deletions in the linker between domain A and transmembrane segment M3 (AL region) were constructed and expressed in Chinese hamster ovary cells. The total or partial removal of the amino acid segment 300–349 did not change the maximal Ca2+ transport activity, but mutants with deletions involving residues 300–338 exhibited a higher apparent affinity for Ca2+ than the wild type h4xb enzyme. Deletion of the putative acidic lipid interacting sequence (residues 339–349) had no observable functional consequences. The removal of either residues 300–314 or 313–338 resulted in a similar increase in the apparent Ca2+ affinity of the pump although the increase was somewhat lower than that obtained by the deletion 300–349 suggesting that both deletions affected the same structural determinant. The results show that alterations in the region of the alternative splicing site A change the sensitivity to Ca2+ of the human isoform 4 of the PMCA.