Diverse mitogenic agents induce rapid phosphorylation of a common set of cellular proteins at tyrosine in quiescent mammalian cells.

Protein phosphorylation of quiescent human skin fibroblasts was analyzed following stimulation by epidermal growth factor, fibroblast growth factor, platelet-derived growth factor, serum, or 12-O-tetradecanoyl-phorbol-13-acetate. In mitogen-treated cells, a markedly increased phosphorylation of two Mr = 43,000 proteins and two Mr = 41,000 proteins was always detected by two-dimensional gel electrophoresis. More acidic forms were the dominant species and contained phosphotyrosine, phosphoserine, and phosphothreonine, while the basic forms contained phosphotyrosine and phosphoserine. The two Mr = 41,000 proteins were structurally related to each other. All mitogens seemed to stimulate the phosphorylation of each protein with the same site specificity. Induction of the same set of phosphoproteins was observed in mitogen-stimulated rat and mouse fibroblasts as well. These stimulated phosphorylations occurred rapidly, were maximal 5 min after exposure of cells to mitogens, and diminished gradually after 30 min. Mitogen-induced phosphorylation of these proteins was correlated to the extent of mitogen-stimulated DNA synthesis. In addition, such increased protein phosphorylation was not observed in exponentially growing cells, nor in Rous sarcoma virus-transformed rat cells. Thus, phosphorylation of the Mr = 43,000 and 41,000 proteins, which represents a common and specific response of cells to mitogens, could constitute an early event involved in the control of cellular G0----G1 transition.