pp60c-src is a substrate for phosphorylation when cells are stimulated to enter cycle.

The endogenous cellular oncogene products, pp60c-src, exhibits a protein kinase activity, but is itself a phosphoprotein. Based on the assumption that pp60c-src might play a role in the control of cell proliferation, we have studied its behaviour as a substrate for phosphorylation known to occur when quiescent, serum-deprived cells are stimulated to enter cell cycle following addition of either serum, platelet-derived growth factor or the phorbol ester derivative, 12-O-tetradecanoyl-phorbol-13-acetate. For this purpose a partial purification of pp60c-src on DEAE ion-exchange chromatography was combined with immune precipitation. A 2-4-fold increase in serine phosphorylation of pp60c-src was consistently observed after stimulation of quiescent cells to growth.