Tyrosine phosphorylation of Grb2 by Bcr/Abl and epidermal growth factor receptor: a novel regulatory mechanism for tyrosine kinase signaling

Growth factor receptor-binding protein-2 (Grb2) plays a key role in signal transduction initiated by Bcr/Abl oncoproteins and growth factors, function ing as an adaptor protein through its Src homology 2 and 3 (SH2 and SH3) do mains. We found that Grb2 was tyrosine-phosphorylated in cells expressing B CR/ABL and in A431 cells stimulated with epidermal growth factor (EGF). Pho sphorylation of Grb2 by Bcr/Abl or EGF receptor reduced its SH3-dependent b inding to Sos in vivo, but not its SH2-dependent binding to Bcr/Abl. Tyr209 within the C-terminal SH3 domain of Grb2 was identified as one of the tyro sine phosphorylation sites, and phosphorylation of Tyr209 abolished the bin ding of the SH3 domain to a proline-rich Sos peptide in vitro. In vivo expr ession of a Grb2 mutant where Tyr209 was changed to phenylalanine enhanced BCR/ABL-induced ERK activation and fibroblast transformation, and potentiat ed and prolonged Grb2-mediated activation of Ras, mitogen-activated protein kinase and c-Jun N-terminal kinase in response to EGF stimulation. These r esults suggest that tyrosine phosphorylation of Grb2 is a novel mechanism o f down-regulation of tyrosine kinase signaling.