Avian pp60c-src is more active when expressed in yeast than in vertebrate fibroblasts

Avian pp60c-src, a proto-oncogene product, has been expressed in the yeast, Saccharomyces cerevisiae. pp60c-src from yeast is 10- to 15-fold more active than pp60c-src from mammalian fibroblasts. Extensive phosphorylation of pp60c-src occurs at tyrosine 527 in fibroblasts and inhibits pp60c-src kinase activity. In contrast, pp60c-src from yeast has low levels of phosphate at tyrosine 527. This phosphate may be added by an autophosphorylation reaction that is detectable when immunoprecipitates containing pp60c-src from yeast are incubated with ATP in vitro. This reaction is minor when compared with autophosphorylation at the principal site, tyrosine 416. These data suggest that pp60c-src in fibroblasts is normally inhibited by trans-acting factors, that either directly catalyze phosphate transfer to tyrosine 527 or augment autophosphorylation at this site. Such factors would have the potential to regulate pp60c-src kinase activity. The low but detectable ability of pp60c-src to autophosphorylate at an inhibitory site raises the possibility that pp60c-src may also be auto-regulated.