The amino-terminal region of pp60c-src has a modulatory role and contains multiple sites of tyrosine phosphorylation.

The c-src gene product from either platelet-derived growth factor treated cells or from polyomavirus-infected cells migrates anomalously on gels, displays enhanced kinase activity, and contains additional sites of tyrosine phosphorylation within its amino-terminus. To probe the importance of these post-translational modifications, each of the five amino-terminal tyrosine residues (residues 90, 92, 131, 136, and 149) was altered to phenylalanine using site-directed mutagenesis. Except for the 136F variant, the mutants displayed enhanced kinase activity, albeit at a low level shown insufficient to induce focus formation in NIH3T3 cells. Mutagenesis of residues adjacent to tyrosines 90 and 92 also resulted in variants with enhanced kinase activity indicating that the region and not the tyrosines per se, may be involved in regulating this activity. Fingerprinting analysis demonstrated that the enhanced kinase activity brought about by these mutations occurred through a mechanism which appears to be independent of the phosphorylation state of Tyr 416 and possibly Tyr 527. Upon treatment of cells with vanadate both amino-terminal variants and transforming mutants of pp60c-src displayed a slower migrating form, designated p60+. Hence, the appearance of these p60+ proteins may be elicited either by mutations that enhance the transforming activity of pp60c-src or by perturbations within the amino-terminal region of the enzyme. The retarded mobility of p60+ was shown to be due in part to additional tyrosine phosphorylations residing at its amino-terminus. The demonstration that p60+ could be resolved into multiple bands and that amino-terminal fragments containing phosphorylated tyrosine residues were obtained regardless of which of the five tyrosines in the region was altered to phenylalanine indicates that there are multiple sites of tyrosine phosphorylation within the amino-terminal region of pp60c-src.