ANALYSIS OF THE ROLE OF THE SHC AND GRB2 PROTEINS IN SIGNAL-TRANSDUCTION BY THE V-ERBB PROTEIN

The epidermal growth factor receptor, EGFR, has been implicated in cel l transformation in both mammalian and avian species. The v-ErbB oncop rotein is an oncogenic form of the chicken EGFR. The tyrosine kinase a ctivity of this oncoprotein is required for transformation, but no tra nsformation-specific cellular substrates have been described to date. Recently activation of the ras signal transduction pathway by the EGFR has been shown to involve the Shc and Grb2 proteins. In this communic ation, we demonstrate that the She proteins are phosphorylated on tyro sine residues and are complexed with Grb2 and the chicken EGFR followi ng ligand activation of this receptor. In fibroblasts and erythroid ce lls transformed by the avian erythroblastosis virus (AEV) strains H an d ES4, the Shc proteins are found to be constitutively phosphorylated on tyrosine residues. The tyrosine-phosphorylated forms of the AEV str ain H v-ErbB protein are found in a complex with Shc and Grb2, but the Shc proteins do not bind to the AEV strain ES4 v-ErbB protein. Mutant forms of the v-ErbB protein (in which several of the tyrosines that b ecome autophosphorylated have been deleted by truncation) are unable t o transform erythroid cells but can still transform fibroblasts. Analy sis of cells transformed by one of these mutants revealed that the tru ncated v-ErbB protein could no longer bind to either Shc or Grb2, but this oncoprotein still gave rise to tyrosine-phosphorylated She protei ns that complexed with Grb2 and led to activation of mitogen-activated protein (MAP) kinase. The results suggest that stable binding of Grb2 and Shc to the v-ErbB protein is not necessary to activate this signa l transduction pathway and assuming that the mutants activate MAP kina se in erythroid cells in a manner similar to that of fibroblasts, that activation of this pathway is not sufficient to transform erythroid c ells.