We have examined the phosphorylation of a 50,000-dalton cellular polypeptide associated with the Rous sarcoma virus (RSV) transforming protein pp60(src). It has been shown that pp60(src) forms a complex with two cellular polypeptides, an 89,000-dalton heat-shock protein (89K) and a 50,000-dalton phosphoprotein (50K). The pp60(src)-associated protein kinase activity phosphorylates at tyrosine residues, and the 50K polypeptide present in the complex contains phosphotyrosine and phosphoserine. These observations suggest that the 50K polypeptide may be a substrate for the protein kinase activity of pp60(src). To examine this possibility, we isolated the 50K polypeptide by two-dimensional polyacrylamide gel electrophoresis from lysates of uninfected or virally infected cells. Tryptic phosphopeptide analysis indicated that the 50K polypeptide isolated by this method was the same polypeptide as that complexed to pp60(src). In uninfected cells or cells infected by a transformation-defective mutant, the 50K polypeptide contained phosphoserine but little or no phosphotyrosine. In cells infected by Schmidt-Ruppin or Prague RSV, there was a 40- to 50-fold increase in the quantity of phosphotyrosine in the 50K protein. Thus, the phosphorylation of the 50K polypeptide at tyrosine is dependent on the presence of pp60(src). However, the 50K polypeptide isolated from cells infected by temperature-sensitive mutants of RSV was found to be phosphorylated at tyrosine at both permissive and nonpermissive temperatures; this behavior is different from that of other substrates or putative substrates of the pp60(src) kinase activity. It is possible that the 50K polypeptide is a high-affinity substrate of pp60(src).
|Original language||English (US)|
|Number of pages||8|
|Journal||Molecular and Cellular Biology|
|State||Published - 1982|
ASJC Scopus subject areas
- Cell Biology
- Molecular Biology