Son of Sevenless Binds to the SH3 Domain of Src-Type Tyrosine Kinase

To identify molecules which bind to the SH3 domains of p56^lck, we screened a mouse T-cell lymphoma cDNA library using the yeast two-hybrid system. As a result, we obtained several positive clones including the Son of Sevenless gene which encodes a mammalian homolog of Drosophila Ras GDP/GTP exchange factor. In a subsequent analysis with the yeast two-hybrid system, Sos associated only with the constitutively active form of p56^lck (F505) but not with wild type P56^lck (Y505), indicating the requirement for an active conformation of p56^lck for binding to Sos. Subsequently, we have demonstrated in vitro that the SH3 domain of p56^lck as well as the proline-rich sequences of Sos are responsible for this association. In addition, the proline-rich domain of Sos also bound to the SH3 domains of other src-type tyrosine kinases, src and fyn, but not to that of PLC-γ. More importantly, the p56^lck SH3-Sos interaction was enhanced by serum stimulation, suggesting the possibility that the direct interaction between p56^lck SH3 and Sos may contribute to the regulation of the Ras pathway.