Binding of protein kinase C isoforms to actin in Aplysia

Recently, two of the 10 vertebrate protein kinase C (PKC) isoforms, PKC betaII and PKC epsilon, have been shown to bind specifically to actin filaments, suggesting that these kinases may regulate cytoskeletal dynamics. Here, we present evidence that two PKCs from the marine mollusk Aplysia californica, PKC Apl I, a Ca2+-activated PKC, and PKC Apl II, a Ca2+-independent PKC most similar to PKC epsilon and eta, also bind F-actin. First, they both cosedimented with purified actin filaments in a phorbol ester-dependent manner. Second, they both translocated to the Triton-insoluble fraction of the nervous system after phorbol ester treatment. PKC Apl II could also partially translocate to actin filaments and associate with the Triton-insoluble fraction in the absence of phorbol esters. Translocation to purified actin filaments was increased in the presence of a PKC inhibitor, suggesting that PKC phosphorylation reduces PKC bound to actin. Although both kinases bound F-actin, actin was not sufficient to activate the kinases. In support of a physiological role for actin-PKC interactions, immunochemical localization of PKC Apl II in neuronal growth cones revealed a striking colocalization with F-actin. Our results are consistent with a role for actin-PKC interactions in regulating cytoskeletal dynamics in Aplysia.