Identification of a Fish Protein Associated with a Kinase Activity and Related to the Rous Sarcoma Virus Transforming Protein1

Cells of all vertebrates thus far investigated contain a cellular src gene, which appears to be a highly conserved homolog of the Rous sarcoma virus oncogene src. Similar to the Rous sarcoma virus oncogene src, this endogenous cellular src gene codes for a Mr 60,000 phosphoprotein (pp60c~src) which is associated with a kinase activity. This kinase specifically phosphorylates tyrosine residues of proteins. We have systemati cally looked for pp60c"src in specific strains of certain teleostean fish, including the genera Xiphophorus and Girardinus. These strains are known to contain chromosomal oncogenes causing neoplastic transformation in certain mutants and hybrids. Since the pp60c"src found in other species has been shown to be antigenically related to the viral src gene product, a phosphoprotein with a molecular weight of 60,000 (pp60v"sro), we have used tumor-bearing rabbit serum which recognizes Rous sarcoma virus pp60src to detect such a related protein in fish. Tumor-bearing rabbit serum precipitated two proteins with molecular weights of 60,000 and 52,000. The M, 60,000 phosphoprotein was associated with a protein kinase activity that phosphorylates tyrosine residues of the immunoprecipitating immunoglobulin G heavy chain (M, 53,000). The antigenic relationship of this fish protein to viral pp60src was further documented by an immune competition assay. We conclude, therefore, that the fish M, 60,000 phosphoprotein is a pp60c"src encoded for by a fish cellular src gene that corresponds to the viral src oncogene. The M, 52,000 phosphoprotein is probably a proteolytic degradation product of pp60c"src. All animals of the various strains of the fish investigated in this study showed kinase activity that was reactive upon ¡mmunoprecipitation with tumor-bearing rabbit serum, indicating that the pp60c"src protein is a normal constituent of the fish. Measurable quantitative differences were revealed between the various strains as well as in different tissues. Whereas brain and melanoma tissue showed a high kinase activity when compared to skin, liver, spleen, and testes, muscle tissue displayed no significant activity.