Abstract
We provide direct evidence that serine 17 is the major site of serine phosphorylation in p60v-src, the transforming protein of Rous sarcoma virus, and in its cellular homolog, p60c-src. The amino acid composition of the tryptic peptide containing the major site of serine phosphorylation in p60v-src was deduced by peptide map analysis of the protein labeled biosynthetically with a variety of radioactive amino acids. Manual Edman degradation revealed that the phosphorylated serine in this peptide was the amino terminal residue. These data are consistent only with the phosphorylation of serine 17. The major site of serine phosphorylation in chicken p60c-src, the cellular homolog of p60v-src, is contained in a tryptic peptide identical to that containing serine 17 in p60v-src of Schmidt Ruppin Rous sarcoma virus of subgroup A. Serine 17 is therefore also phosphorylated in p60c-src. The p60v-src protein encoded by Prague Rous sarcoma virus was found to contain two sites of tyrosine phosphorylation. The previously unrecognized site of tyrosine phosphorylation may be tyrosine 205 or possibly tyrosine 208. Treatment of Prague Rous sarcoma virus-infected cells with vanadyl ions stimulated the protein kinase activity of p60v-src and increased the phosphorylation of tyrosine 416 but not the phosphorylation of the additional site of tyrosine phosphorylation.