Several lines of evidence suggest that phosphorylation of tyrosine residue 416 (Tyr416) has a positive regulatory influence on the functional activity of the protein tyrosine kinase pp60c-src. To further examine the functional importance of phosphorylation at Tyr416, we have eliminated this phosphoacceptor site in four functionally unique mutant forms of the c-src gene product that are phosphorylated on Tyr416 in vivo. Substitution of phenylalanine for Tyr416 suppressed the biological activity of all of the mutant proteins as assayed by colony formation in soft agar and the induction of morphological alterations. However, the extent of this effect, and the degree to which the substitution affected the phosphorylation of substrates in vivo and in vitro, varied considerably in each of the mutants. These results support the notion that phosphorylation of Tyr416 has a positive regulatory effect on the biological activity of c-src; however, this effect does not directly correlate with a general effect on the total level of tyrosine kinase activity in vitro or the level of tyrosine phosphorylation of cellular proteins in vivo.
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