Deletions and mutations in the acidic lipid-binding region of the plasma membrane Ca2+ pump: a study on different splicing variants of isoform 2.

Acidic phospholipids increase the affinity of the plasma membrane Ca(2+)-ATPase pump for Ca(2+). They interact with the C-terminal region of the pump and with a domain in the loop connecting transmembrane domains 2 and 3 (A(L) region) next to site A of alternative splicing. The contribution of the two phospholipid-binding sites and the possible interference of splicing inserts at site A with the regulation of the ATPase activity of isoform 2 of the pump by phospholipids have been analyzed. The activity of the full-length z/b variant (no insert at site A), the w/b (with insert at site A), and the w/a variant, containing both the 45-amino acid A-site insert and a C-site insert that truncates the pump in the calmodulin binding domain, has been analyzed in microsomal membranes of overexpressing CHO cells. The A-site insertion did not modify the phospholipid sensitivity of the pump, but the doubly inserted w/a variant became insensitive to acidic phospholipids, even if containing the intact A(L) phospholipid binding domain. Pump mutants in which 12 amino acids had been deleted, or single lysine mutations introduced, in the A(L) region were studied by monitoring agonist-induced Ca(2+) transients in overexpressing CHO cells. The 12-residue deletion completely abolished the ATPase activity of the w/a variant but only reduced that of the z/b variant, which was also affected by the single lysine substitutions in the same domain. A structural interpretation of the interplay of the pump with phospholipids, and of the mechanism of their activation, is proposed on the basis of molecular modeling studies.