In vivo phosphorylation states and kinase activities of transforming p60c-src mutants

To further investigate regulation of p60src by tyrosine phosphorylation, the in vivo phosphorylation states and kinase activities of transforming mutants derived from p60c-src were examined. One set of six in vitro recombinants encoded chimeric proteins containing the carboxyl terminus of chicken p60c-src (including Tyr 527) in combination with various isolated amino acid substitutions present in p60v-src. Another set of seven viral isolates were randomly selected from soft-agar colonies infected with replicating retroviruses that originally encoded normal p60c-src. Results show that mutations at multiple sites throughout amino- and carboxy-terminal domains are sufficient to alter the in vivo phosphorylation state and kinase activity of p60c-src. Moreover, all of the transforming p60c-src mutants exhibited elevated in vivo kinase activities, as assayed by immune blot analysis of cellular proteins using antibodies to phosphotyrosine, which correlated well with enhanced auto-phosphorylation at Tyr 416. By contrast, Tyr 527 phosphorylation in the transforming mutants varied from low to high levels that were correlated to a limited extent with levels of in vivo kinase activities. Taken together, results suggest that, in addition to complete dephosphorylation of Tyr 527, other mechanisms contribute to p60c-src activation.