Differential irreversible insertion of protein kinase C into phospholipid vesicles by phorbol esters and related activators

Abstract

Incubation of protein kinase C (PKC) alpha with phorbol 12,13-dibutyrate and phospholipid vesicles promoted a time-dependent irreversible insertion of the enzyme into the vesicles and the generation of a calcium-independent kinase activity. Calcium neither caused insertion nor influenced the insertion induced by the phorbol ester. The effect was strongly dependent on the phosphatidylserine concentration in the vesicle and could also be supported by other anionic phospholipids. An analysis of the structure-activity relations of PKC activators for the calcium-independent kinase activity revealed marked relative differences in potencies for binding and for insertion. Compounds such as phorbol 13-myristate 12-acetate and mezerein were very efficient at inducing insertion. In contrast, 12-deoxyphorbol esters and diacylglycerol were relatively inefficient at inducing insertion, requiring higher concentrations than expected from their binding affinities. The insertion of PKC alpha depended substantially on the length of the aliphatic esters in the 12- and 13-positions of the phorbol derivatives, and once again, potencies for insertion and binding were not directly proportional. Our findings suggest two different sites for ligand interaction on the molecule of PKC alpha with different structure-activity requirements. We speculate that the differential ability of compounds to promote insertion could contribute to the documented marked differences in the biological behavior of PKC activators.