Homologous tyrosine phosphorylation sites in transformation-specific gene products of distinct avian sarcoma viruses

Three classes of avian sarcoma viruses have been defined recently (Table 1)1–5. Transformation-specific proteins of all three viral classes have associated tyrosine-specific protein kinase activity4–12. The protein kinase activity of Rous sarcoma viruses (RSV), representing class I, seems to be a property of the transformation-specific protein, pp60src, itself13 and seems to be essential for cellular transformation, as the activity is labile or absent in transformation mutants6,7,14,15. This connection between transformation-specific protein, enzyme activity and oncogenic cellular changes has yet to be established for the viruses of classes II and III, which code for polyproteins in which gag-derived and transformation-specific sequences are fused. We report here an analysis of the phosphorylation sites on the transformation-specific proteins encoded by RSV and the class III viruses which shows that, although there is no appreciable sequence relationship between large portions of these molecules, a tryptic peptide containing the phosphotyrosine seems to be identical in classes I and III