Calcium-dependent activation of protein kinase C. The role of the C2 domain in divalent cation selectivity.

Activation of certain isoforms of protein kinase C (cPKCs) requires Ca2+ and is associated with a conserved C2 domain that is not present in Ca(2+)-independent isoforms (nPKCs). The site(s) of Ca2+ binding and the role of the C2 domain have not been previously identified. We have analyzed phosphatidylserine-dependent Ca2+ binding to fusion proteins expressed in Escherichia coli that carry various modifications in the regulatory region of cPKC beta 1 or nPKC epsilon. Ca2+ is bound mainly to the C1 domain of PKC beta 1, but the C2 domain confers specificity for Ca2+ binding when compared with Mg2+ and Mn2+. We propose that in cPKCs there is selective binding of Ca2+ to a pocket formed by the C1 and C2 domains. This induces a change in conformation that activates the enzyme. In nPKCs, the cation binding pocket is less specific for Ca2+ because it lacks the C2 domain. Therefore, divalent cations like Mg2+ can bind to it, thereby abrogating the requirement of Ca2+ for enzyme activation.