THE SIGNALING PATHWAY COUPLING EPIDERMAL GROWTH-FACTOR RECEPTORS TO ACTIVATION OF P21(RAS)

Epidermal growth factor (EGF) treatment causes autophosphorylation of the epidermal growth factor receptor (EGFR) leading to increased guani ne nucleotide exchange factor (GEF; Sos) activity and enhanced formati on of p21(ras)-GTP. The connection of the EGFR to p21(ras) activation can occur through binding of Grb2 Sos complexes to the EGFR or through the adaptor protein Shc via EGFR.Shc.Grb2.Sos multimeric complexes. T herefore, we investigated the importance of She in coupling the EGFR t o activation of ras GEF (Sos). EGF treatment led to rapid tyrosine pho sphorylation of She. Although phosphorylated EGFR can bind to both She and Grb2, the predominant linkage was observed between EGFR and She. Similarly, more Grb2 was associated with She than with EGFR after EGF stimulation. Immunoprecipitation of She from EGF-stimulated cells remo ved almost all EGFR-associated Grb2. Furthermore, immunodepletion of S he proteins from membrane fractions of EGF-stimulated cells removed 93 % of the ras GEF activity, whereas, precipitation of EGFR had only a s mall effect on ras GEF activity. These data indicate that coupling to She provides the major pathway linking activated EGFRs to Grb2.Sos and stimulation of the p21(ras) pathway,