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 sc reened a mouse T-cell lymphoma cDNA library using the yeast two-hybrid system. As a result, we obtained several positive clones including th e Son of Sevenless gene which encodes a mammalian homolog of Drosophil a 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) (P505) but not with wild type p56(lck) (E'505), indi cating the requirement for an active conformation of p56(lck) for bind ing to Sos. Subsequently, we have demonstrated in vitro that the SH3 d omain of p56(lck) as well as the proline-rich sequences of Sos are res ponsible 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-gamma, 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.