Protein kinase C βII specifically binds to and is activated by F-actin

The two most closely related isoenzymes of protein kinase C (PKC), PKC βI and βII, are distinct but highly homologous isoenzymes derived via alternative splicing of the same gene product. In this study, PKC βII, but not PKC βI, translocated to the actin cytoskeleton upon stimulation of cells with phorbol esters. In cells, antibodies to PKC βII, but not to PKC βI, co-immunoprecipitated actin. Using an actin-binding co-sedimentation assay, we show in vitro that PKC βII, but not PKC βI, binds to actin specifically. This binding was inhibited by peptides based on sequences unique to PKC βII; thus defining an actin-binding site in PKC βII that is not present in PKC βI. The binding of PKC βII to actin was not inhibited by kinase inhibitors of PKC (sphingosine and staurosporine), suggesting that prior activation and/or substrate phosphorylation are not required for the interaction of PKC βII with actin. On the other hand, the interaction of PKC βII with actin resulted in marked enhancement of autophosphorylation of PKC βII and in an alteration in substrate specificity. These studies serve to define a novel functional domain in the carboxyl-terminal region of PKC β, which is involved in directing isoenzyme-specific protein-protein interactions, and consequently, isoenzyme-specific functions in vivo.