Synapsin I in PC12 cells. II. Evidence for regulation by NGF of phosphorylation at a novel site.

NGF treatment of PC12 cells caused a rapid increase in the state of phosphorylation of synapsin I. This phosphorylation of synapsin I is accompanied by a decrease in its electrophoretic mobility on SDS-PAGE. Phosphopeptide fingerprint analysis of the synapsin I revealed that this phosphorylation occurred on a particular phosphopeptide, designated peptide N. Phosphoserine was the only phosphoamino acid detected in peptide N. Partially purified PC12 synapsin I was a substrate for several protein kinases known to be capable of phosphorylating brain synapsin I, but none of these kinases phosphorylated synapsin I on peptide N. The results suggest that the NGF-stimulated phosphorylation of synapsin I may be mediated by a novel protein kinase.