THE SH2 AND SH3 DOMAINS OF MAMMALIAN GRB2 COUPLE THE EGF RECEPTOR TO THE RAS ACTIVATOR MSOS1

MANY tyrosine kinases, including the receptors for hormones such as ep idermal growth factor (EGF), nerve growth factor and insulin, transmit intracellular signals through Ras proteins1-4. Ligand binding to such receptors stimulates Ras guanine-nucleotide-exchange activity5-9 and increases the level of GTP-bound Ras10-12, suggesting that these tyros ine kinases may activate a guanine-nucleotide releasing protein (GNRP) . In Caenorhabditis elegans and Drosophila, genetic studies have shown that Ras activation by tyrosine kinases requires the protein Sem-5/dr k, which contains a single Src-homology (SH) 2 domain and two flanking SH3 domains13-15. Sem-5 is homologous to the mammallian protein Grb2, which binds the autophosphorylated EGF receptor and other phosphotyro sine-containing proteins such as Shc through its SH2 domain16-17. Here we show that in rodent fibroblasts, the SH3 domains of Grb2 are bound to the proline-rich carboxy-terminal tail of mSos1, a protein homolog ous to Drosophila Sos. Sos is required for Ras signalling18-20 and con tains a central domain related to known Ras-GNRPs21-23. EGF stimulatio n induces binding of the Grb2-mSos1 complex to the autophosphorylated EGF receptor, and mSos1 phosphorylation. Grb2 therefore appears to lin k tyrosine kinases to a Ras-GNRP in mammalian cells.