Plasma membrane Ca2+ pump isoform 3f is weakly stimulated by calmodulin

Isoform 3f of the plasma membrane Ca2+ pump is a major isoform of this pump in rat skeletal muscle. It has an unusual structure, with a short carboxyl -terminal regulatory region of only 33 residues when compared with the 77 t o 124 residues found in the other isoforms, Also, whereas the regulatory re gions of the other iso forms, downstream of the alternative splice, consist of two homologous groups, the sequence of 3f is not related to either grou p. A synthetic peptide representing the calmodulin binding domain of isofor m 3f had a much lower calmodulin affinity (with a K-d of 15 nM) than the co rresponding peptide of isoform 2b (K-d value was 0.2 nM). The characteristi cs of this domain were further studied by making chimeras of the 3f regulat ory region with the catalytic core of isoform 4 and by making the full-leng th isoform 3f, Both constructs bound to calmodulin-Sepharose. The chimera w as fully active without calmodulin, showing no stimulation of activity when calmodulin was added. The full-length isoform 3f was slightly activated by calmodulin, These data show that the regulatory region of isoform 3f is on ly a weak autoinhibitor of the enzyme, in contrast to the properties of all the other isoforms studied so far, Rather, this isoform is a special-purpo se, constitutively active form of the enzyme, expressed primarily in skelet al muscle and as a minor isoform in brain.