Modifications in the phosphorylation of cell proteins related to the expression of src gene in chicken embryo fibroblasts.

The possible physiological targets of pp60src in chicken embryo fibroblasts transformed by Rous sarcoma virus were looked for by a general screening of the modifications of phosphorylation of the major cell phosphoproteins. These modifications were analyzed quantitatively by SDS-PAGE of total cell proteins on gradient gels, combined with a computerized densitometric evaluation of gel autoradiographies, using cells labeled with either [14C]-amino acids or with [32P]-phosphate. A large numbers of proteins, 37 out of the 68 studied, were found to be more phosphorylated in virally transformed cells. The determination of the phosphoamino acid content of proteins which were more phosphorylated in transformed cells and the study of the kinetics of protein phosphorylation in cells infected by a ts mutant, after a shift from the nonpermissive to the permissive temperature, showed that, among these proteins, five displayed a large increase in phosphotyrosine content and an early increase in phosphorylation after the temperature shift. These proteins of 36 K, 41 K, 46 K, 65 K and 280-300 K doublet are therefore good candidates for being physiological targets of pp60src in the cell.