Phosphorylation and associated translocation of the 87-kDa protein, a major protein kinase C substrate, in isolated nerve terminals

Abstract

A protein of 87 kilodaltons (87 kDa) was previously identified as a major specific substrate for protein kinase C in neuronal and other tissues. We have now studied the effect of protein kinase C-catalyzed phosphorylation of this protein on its association with membranes in isolated nerve terminals (synaptosomes) from rat cerebral cortex. Incubation of synaptosomal membranes under conditions associated with activation of protein kinase C led to the release of the phosphorylated 87-kDa protein into the incubation medium. In intact synaptosomes, activation of protein kinase C by phorbol esters or by depolarization-induced Ca2+ influx caused an increased phosphorylation of the 87-kDa protein and its translocation from membrane to cytosol. This translocation showed time courses, calcium dependency, and reversibility similar to those observed for the protein kinase C-induced phosphorylation of the protein. These results suggest that protein kinase C-catalyzed phosphorylation of the 87-kDa protein is responsible for its subcellular translocation into the cytosol of nerve terminals.