Characterization of a normal avian cell protein related to the avian sarcoma virus transforming gene product

Abstract

In this paper, we identify and characterize both structurally and functionally a protein from normal uninfected avian cells that is antigenically related to the pp60 src viral protein responsible for transformation by ASV. This protein was detected by immunoprecipitation of radiolabeled normal cell extracts with serum derived from marmosets bearing ASV-induced tumors. The normal avian cell protein, which has been detected in each of the four avian species tested (chicken, duck, quail and pheasant) is a phosphoprotein of 60,000 daltons. This protein is not related to any of the ASV structural proteins; however, its immunoprecipitation is prevented by preadsorption of the antiserum with cell extracts specifically containing pp60 src. Peptide analyses by partial proteolysis using chymotrypsin resulted in a map of the normal cell protein that was very similar to that of pp60 src. When Staphylococcus aureus V8 protease was used, however, one of the major cleavage products of the normal cell protein exhibited an altered migration with respect to the corresponding pp60 src product. Tryptic phosphopeptide analyses demonstrated that phosphorylation of the normal cell protein was also different from that seen in pp60 src. The expression of the normal cell protein did not seem to be affected by cellular growth conditions, maintaining a constant level which was approximately 30–50 fold lower than that of pp60 src in infected cells. The normal cell protein appeared to be functionally dissimilar to pp60 src lacking detectable protein kinase activity in the currently available assay system.