The purified product of the transforming gene of avian sarcoma virus phosphorylates tyrosine.

The product of the avian sarcoma virus transforming gene (src) is a phosphoprotein of 60,000 daltons (pp60src) which is responsible for the oncogenic potential of the virus. Recent findings indicate that this protein possesses an affiliated protein kinase activity. We have determined by hydrodynamic measurements and gel filtration that this kinase activity tracks with a highly asymmetric molecule of 60,000 daltons, strengthening the idea that pp60src alone (as opposed to a complex) possesses the enzymatic activity. To more fully characterize the properties of this kinase activity, we undertook its purification by two independent methods. In each case, a protein related to pp60src was extensively purified from contaminating cellular proteins. The yields from one of the procedures were sufficient to induce high titer monospecific antibodies against pp60src in mice. We have shown that purified pp60src is able to phosphorylate several protein substrates other than IgG. The conclusion that pp60src possesses the responsible enzymatic activity was strengthened by demonstrating that a temperature-sensitive conditional mutation in src affected the thermal stability of the purified protein. It has recently been shown that the protein kinase activity affiliated with pp60src phosphorylates tyrosine residues on IgG. We have examined the target specificity of the purified protein on several substrates other than IgG, and show that in every case, the phosphorylation occurs exclusively at a tyrosine residue; it therefore appears that tyrosine phosphorylatin is not an artifact of phosphorylation in th immunoprecipitate, but instead represents the general substrate specificity of pp60src.