The potential for isoenzyme-selective modulation of protein kinase C.

Protein kinase C (PKC) is a phospholipid-dependent serine/threonine kinase family consisting of at least 11 closely related isoenzymes. The different PKC isoenzymes play important roles in signal transduction pathways. The exact significance of each isoenzyme is not known at present; therefore, the elucidation of the roles of the various PKC isoenzymes is important. To explain the function of distinct PKC isoenzymes, the availability of isoenzyme-specific inhibibitors or activators would be an advantage. PKC inhibitors have been known for some time, but these compounds are not isoenzyme-specific and also inhibit other kinases. Recently, an inhibitor selective for PKC alpha and another one selective for PKCbetaI and betaII were described. Both compounds compete with the ATP binding sites that exhibit high homologies among the different PKC isoenzymes. Among others, the phosporyl transfer region, the pseudosubstrate domain, the phorbolester binding sequences, and the phosphorylation sites may also be targets for modulation of isoenzyme-specific PKC activity. The question is whether the differences in these domains and the substrate specificity of the PKC isoenzymes will allow isoenzyme-specific inhibition. In this review the human sequences of these sites, isoenzyme-specific substrates, inhibitory compounds, and inhibitory peptides are summarized.