The specific interaction of the Rous sarcoma virus transforming protein, pp60src, with two cellular proteins

Sera from rabbits bearing tumors induced by Rous sarcoma virus (RSV) were previously found to contain antibody to the RSV transforming protein, pp60^src. Two additional transformation-specific phosphoproteins from RSV-transformed avian cells are immunoprecipitated with these sera. These proteins, having molecular weights of 90,000 (pp90) and 50,000 (pp50), are not precipitated from uninfected or transformation-defective virus-infected cells and are not related to any RSV structural proteins. Neither pp50 nor pp90 shares any partial or complete proteolytic cleavage peptides with pp60^src, suggesting that pp90 and pp50 do not represent either a precursor or a cleavage product of pp60^src. Sedimentation analysis of RSV-transformed cell lysates on glycerol gradients revealed that the RSV pp60^src protein is present as two forms, one of which represents the majority (95%) of pp60^src and sediments as a monomer, 60,000 molecular weight protein and the other of which sediments with pp90 and pp50 as an apparent 200,000 molecular weight complex. Lysates from cells transformed by viruses containing a temperature-sensitive defect in the src gene contain a greater percentage of pp60^src associated with pp90 and pp50 under both permissive (35°C) and nonpermissive (41°C) conditions compared to wild-type virus-infected cell lysates. Phosphoserine and phosphotyrosine were found associated with pp60^src molecules that sedimented as a monomer, whereas pp60^src molecules that are complexed with pp90 and pp50 contain phosphoserine and greatly reduced amounts of phosphotyrosine. Only the monomer form of pp60^src is capable of phosphorylating IgG in the immune complex phosphotransferase reaction. Normal uninfected chicken cells contain a protein that shares identical partial proteolytic cleavage peptides with the pp90 protein immunoprecipitated from RSV-transformed cells. This pp90 protein is one of the major cytoplasmic proteins in uninfected cells. Antibody directed against pp90 also immunoprecipitates pp60^src and pp50 from lysates of RSV-transformed chicken cells.