Avian sarcoma virus-transforming protein, pp60^src shows protein kinase activity specific for tyrosine

Abstract

The protein responsible for malignant transformation by avian sarcoma viruses (ASVs) has been identified as a phos-phoprotein of molecular weight 60,000 designated pp60^src (refs 1–4). It has been suggested that this protein has a functional role in cellular transformation involving the phosphorylation of cellular proteins, for it was discovered that specific immunoprecipitates from ASV-transformed cells that contain pp60src catalysed the transfer of phosphate from [γ-³²P]ATP to the heavy chain of rabbit immunoglobulin^5,6. Additional studies involving the cell-free synthesis of the ASV src protein further demonstrated that the presence of the src polypeptide correlated with that presence of a phosphotransf erase activity⁷. Our studies, involving the biochemical purification of this protein, have demonstrated that the ASV-transforming gene product, pp60^src, is itself a protein kinase. We have purified the pp60^src protein approximately 5,000-fold using either conventional ion-exchange chromatography or immunoaffinity chromatography^8,9. The resultant partially purified preparations contain a cyclic AMP-independent protein kinase activity⁸. We report here that the soluble phosphotransferase activity of partially purified pp60^src results in the phosphorylation of exclusively tyrosine residues in a variety of proteins that serve as substrates.