DIFFERENTIAL SELECTIVITY OF LIGANDS FOR THE C1A AND C1B PHORBOL ESTERBINDING DOMAINS OF PROTEIN-KINASE C-DELTA - POSSIBLE CORRELATION WITHTUMOR-PROMOTING ACTIVITY

Protein kinase C (PKC) represents the major, high-affinity receptor fo r the phorbol esters as well as for a series of structurally diverse n atural products, The phorbol esters function by binding to the tandem C1a and C1b domains in PKC, leading to enzyme activation. Although the typical phorbol esters represent the paradigm for tumor promoters in mouse skin, it is now clear that different high affinity ligands for P KC have distinct biological effects. Thus, the daphnane analogue mezer ein is a second-stage promoter, the macrolide bryostatin 1 is a partia l antagonist, and certain 12-deoxyphorbol 13-monoesters also function as partial antagonists but with a different pattern of activity, The b iochemical basis for these differences is an area of active investigat ion, In this report, we have examined the relative interaction of liga nds differing in structure and pattern of biological response with the Cia and C1b domains of PKC delta. We mutated either or both of the C1 domains of PKC delta, expressed the constructs in NM 3T3 cells, and m onitored the interaction of the ligands by their ability to induce tra nslocation of the mutated PKC delta from the cytosol to the particulat e fraction, We found that different ligands showed different dependenc e on the C1a and C1b domains for translocation, Whereas phorbol 12-myr istate 13-acetate and the indole alkaloids indolactam and octylindolac tam were selectively dependent on the C1b domain, selectivity was not observed for mezerein, for the 12-deoxyphorbol 13-monoesters prostrati n or 12-deoxyphorbol 13-phenylacetate, or for the macrocyclic lactone bryostatin 1, Provocatively, the pattern of response corresponds with the activity of the compounds as complete tumor promoters.