STRUCTURALLY AND FUNCTIONALLY MODIFIED FORMS OF PP60V-SRC IN ROUS-SARCOMA VIRUS-TRANSFORMED CELL LYSATES

When analyzed from transformed cell lysates, pp60v-src, the product of the Rous sarcoma virus src gene, typically appears as a single polypeptide of 60,000 MW, phosphorylated at 2 major sites, an amino-terminal region serine residue and carboxy-terminal region tyrosine residue. The identification of variant forms of pp60v-src present in [Rous sarcoma virus] transformed vole cell lysates that exhibited an altered electrophoretic mobility in sodium dodecyl sulfate-polyacrylamide gels is described. This change in migration appeared to be the result of some alteration in the amino-terminal portion of the molecule and paralleled the appearance of extensive amino-terminal region tyrosine phosphorylation on the pp60v-src molecule. These structural modifications were further correlated with a dramatic increase in the protein kinase-specific activity of pp60v-src. The detection of these variant forms of pp60v-src depended on the prior treatment of the transformed cell cultures with V2+ ions or the inclusion in the cell disruption buffer of Mg2+ or ATP-Mg2+. The implication is that modified, highly active forms of the pp60v-src protein exist in transformed cells, but are transient and rapidly converted to stable forms, possibly by specific dephosphorylation. Evidently, amino-terminal region tyrosine phosphorylation of pp60v-src, presumably the result of autophosphorylation, serves to greatly enhance src protein enzymatic activity, but much of the regulation of this transforming protein''s function may involve a phosphotyrosyl protein phosphatase.