Identification of Phosphotyrosine-Containing Proteins in Untransformed and Rous Sarcoma Virus-Transformed Chicken Embryo Fibroblasts

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Abstract
Phosphorylation on tyrosine residues mediated by pp60src appears to be a primary biochemical event leading to the establishment of the transformed phenotype in Rous sarcoma virus (RSV)-infected cells. To identify the cellular proteins that undergo tyrosine phosphorylation during transformation, a 32P-labeled RSV-transformed chicken embryo cell extract was analyzed by electrophoresis on a polyacrylamide gel. After slicing the gel into approximately 60 slices, phosphoamino acid analyses were carried out on the protein recovered from each gel slice. Phosphotyrosine was found in every gel slice, with two major peaks of this phosphoamino acid around Mr's of 59 and 36 kilodaltons. When the same analysis was performed with cells infected with a transformation-defective src deletion mutant of RSV (tdNY101), significant and reproducible peaks of phosphotyrosine were found in only 2 of 60 gel slices. These gel slices corresponded to Mr's of 42 and 40 kilodaltons. Identical results were obtained with normal uninfected chicken embryo fibroblasts. We conclude from these observations that pp60src or the combined action of pp60src and pp60src -activated cellular protein kinases cause the tyrosine-specific phosphorylation of a very large number of cellular polypeptides in RSV-transformed cells. In addition, untransformed cells appear to possess one or more active tyrosine-specific protein kinases which are responsible for the phosphorylation of a limited number of proteins. These proteins are different from the major phosphotyrosine-containing proteins of the transformed cells.