Translocation of protein kinase Cs in Aplysia neurons: evidence for complex regulation.

We examined the activation of protein kinase C (PKC) produced by phorbol esters in Aplysia nervous tissue. Translocation of PKC in intact ganglia requires higher concentrations of phorbol esters than would be expected from: (1) their affinity for Aplysia PKCs measured in vitro; (2) their physiological effects on cultured Aplysia neurons; and (3) their actions on PKC in synaptosomes. Although phorbol esters enter intact ganglia slowly, delayed access to neurons is insufficient to account for the high concentrations needed for translocation. Increasing accessibility to the neural components of ganglia increases the rate at which translocation occurs, but does not affect the concentration of phorbol ester required. We suggest that this might best be explained by the presence of a competitive inhibitor at the binding site for phorbol esters in PKC. An indication for an inhibitor is that the concentration of phorbol esters needed for translocation in homogenates of nervous tissue is markedly decreased by diluting the extract. Preliminary characterization shows that the inhibitory activity is unusual: in addition to being competitive with lipid activators, it is soluble and tissue-specific. This type of inhibitor may be an important regulator of protein phosphorylation by PKC in neurons.