Tyrosine phosphorylation regulates the biochemical and biological properties of pp60^c-src

To investigate the importance of tyrosine phosphorylation in the regulation of pp60^c-src, we have substituted phenylalanine for tyrosine at positions 416, 519, and 527. Cells expressing the 527 or the 519 527 mutant but not the 416 or the 519 mutant were morphologically transformed, grew in soft agar, and formed foci. In addition, the 527 and 519 527 mutants had elevated kinase activities in vitro. Modifying Tyr 416 to phenylalanine in the 527 or the 519 527 mutants only partially inhibited their kinase activities yet abolished their ability to induce focus formation and promote growth in soft agar. These results suggest that two events must occur to activate the full transforming potential of pp60^c-src: hypophosphorylation at Tyr 527 and hyperphosphorylation at Tyr 416.