Peptide analysis of the transformation-specific antigen from avian sarcoma virus-transformed cells

Sera from rabbits bearing tumors induced by avian sarcoma virus (ASV) were ussed to immunopecipitate virus-specific proteins from extracts of chicken, hamster, and field vole cells transformed by ASV. Two virus-specific proteins having molecular weights of 76,000 and 60,000 were found in all cell lines examined. The 76,000-molecular-weight protein, Pr76, is the precursor to the internal core proteins of ASV. The 60,000-molecular-weight (60K) transformation-specific antigen from each cell line was subjected to peptide analysis, using chymotrypsin and Staphylococcus aureus V8 protease. The resulting peptide maps of the 60K protein from the different ASV-infected cell types were similar for each enzyme, strongly suggesting that the 60K protein is virus coded. Two-dimensional analysis of chymotryptic peptides from Pr76 and 60K reveals that 60K is not related to the gs antigen precursor. Radiolabeling of ASV-transformed cells with inorganic phosphate revealed that 60K is phosphorylated in vivo. The 60K proteins isolated from both ASV-transformed chicken and field vole cells were found to contain one tryptic phosphopeptide. The tryptic phosphopeptides of 60K from both cell lines migrated identically upon two-dimensional peptide analyses, and their migration differed from that of the principal phosphopeptide of Pr76.