The product of the protooncogene c-src is modified during the cellular response to platelet-derived growth factor

We have observed a modification of the cellular protein kinase pp60c-src, elicited in murine 3T3 fibroblasts by platelet-derived growth factor (PDGF). The modification occurred rapidly after addition of PDGF to the culture medium and was first detected as a reduction in the electrophoretic mobility of a portion of the pp60c-src molecules. A similarly modified form of the viral homologue pp60v-src occurs in vivo in the absence of stimulation by PDGF. The occurrence of modified forms of both pp60c-src and pp60v-src was associated with a novel phosphorylation at tyrosine in the amino-terminal domains of the proteins. The time-course and dose-response for this modification of pp60c-src paralleled PDGF-induced increases in phosphorylation of pp36, a major cellular substrate for several tyrosine-specific protein kinases. In parallel experiments, treatment of cells with PDGF increased the kinase activity of pp60c-src in an immunocomplex assay. These results suggest pp60c-src may play a role in the mitogenic response to PDGF.