Ca(2+)-modulated phosphorylation of a low-molecular-mass polypeptide in rat liver mitochondria: evidence that it is identical with subunit c of F(0)F(1)-ATPase.

A 3.5-kDa polypeptide associated with the inner membrane of rat liver was found to be phosphorylated by [gamma-(32)P]ATP, presumably via a cAMP-dependent kinase. The phosphorylation was modulated by [Ca(2+)] in the physiological range, with a minimum at 1 microM and rising fourfold toward lower (10 nM) and higher (10 microM) concentrations. Further characterization of the 3.5-kDa component showed that the polypeptide has the same electrophoretic mobility as subunit c of F(0)F(1)-ATPase and that it selectively binds to antibodies against subunit c.