Phosphotyrosine-containing 120,000-dalton protein coimmunoprecipitated with pp60v-src from Rous sarcoma...

Serum from rabbits bearing tumors (TBR serum) induced by the Rous sarcoma virus (RSV) was originally developed to identify the RSV src gene protein-pp60v-src. It is also capable of directly or indirectly immunoprecipitating a number of proteins besides pp60v-src from lysates of RSV-transformed cells. This report describes a highly phosphorylated protein of approximately 120,000 Da (pp120) which is specifically immunoprecipitated from RSV-transformed mammalian cells by TBR sera and monospecific antibodies against pp60v-src. However, it was not immunoprecipitated by TBR serum from RSV-transformed chicken embryo fibroblasts. Phosphoamino acid analysis revealed that pp120 contains phosphoserine and phosphotyrosine in relative amounts similar to that found in pp60v-src. Various experimental results indicate that pp120 is not structurally related to RSV virion proteins or to the putative pp60v-src substrate-vinculin. Furthermore, proteolytic peptide mapping and immunoblotting experiments indicate that although pp120 is immunoprecipitated by various anti-pp60v-src antibodies, it does not appear to be related to the RSV transforming protein. These results suggest that pp120 is distinct from previously reported substrates of pp60v-src and may exist in association with the transforming protein in mammalian cell lysates.