The functional importance of the extreme C-terminal tail in the gene 2 organellar Ca(2+)-transport ATPase (SERCA2a/b).

Ca(2+)-uptake experiments in microsomal fractions from transfected COS-1 cells have revealed a functional difference between the non-muscle SERCA2b Ca2+ pump and its muscle-specific SERCA2a splice variant. Structurally, the two pumps differ only in their C-terminal tail. The last four amino acids of SERCA2a are replaced in SERCA2b by a 49-residue-long peptide chain containing a very hydrophobic stretch which could be an additional transmembrane segment. The functionally important subdomains in the SERCA2b tail were analysed by constructing three SERCA2b deletion mutants lacking 12, 31 or 49 amino acids. The mutants and the parental SERCA2 pumps were expressed in COS-1 cells and analysed for functional difference. SERCA2b had a twofold higher Ca2+ affinity, a twofold lower turnover rate and a 10-fold lower vanadate-sensitivity than SERCA2a and the mutants. Since each of the three truncated versions of SERCA2b acquire the characteristic properties of SERCA2a, it is concluded that the stretch of the last 12 residues of SERCA2b is of critical importance.