The v-Src and c-Src tyrosine kinases immunoprecipitated from Rous sarcoma virus-transformed cells display different peptide substrate specificities

In the cells transformed by Rous sarcoma virus (RSV), two Src proteins are expressed: the ubiquitous tyrosine kinase c-Src and the v-Src, the product of the transforming gene of the virus. Using three synthetic peptide substrates widely used for testing Src kinase activity, we show that they are phosphorylated with different efficiencies by the v-Src and c-Src tyrosine kinases immunoprecipitated from the tumor cell line H19. The v-Src displays higher efficiency (Vmax/Km ratio) toward all three peptides used, but the Vmax of v-Src is much lower than Vmax of c-Src with two peptides out of three. This difference in substrate specificity, if ignored, may cause misestimation of the amounts of active c-Src and v-Src in RSV-transformed cells. On the other hand, the different peptide substrate specificities may also reflect different protein substrate specificities of the v-Src and c-Src kinases in vivo.