Isolation and characterization of two distinct forms of protein kinase C

Protein kinase C (Ca2+- and phospholipid-dependent protein kinase) has been purified from rat brain by a three-step, 18-h procedure resulting in the isolation of milligram quantities of enzyme. Unlike previous preparations from published protocols, which yield a single polypeptide, this procedure yields a protein which consists of a 78/80-kDa doublet upon sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The two polypeptides have been characterized with respect to structure and function and are very similar in both regards. However, the two forms can be distinguished immunologically by polyclonal antisera generated against purified protein kinase C. The 78- and 80-kDa proteins do not appear to be related to one another by proteolytic cleavage or by differential phosphorylation, although the two purified proteins do contain stoichiometric amounts of phosphate. The 78- and 80-kDa polypeptides therefore appear to represent two distinct forms of protein kinase C, thus providing evidence for the existence of multiple isozymes of this key regulatory protein.