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

Abstract

Growth factor receptor‐binding protein‐2 (Grb2) plays a key role in signal transduction initiated by Bcr/Abl oncoproteins and growth factors, functioning as an adaptor protein through its Src homology 2 and 3 (SH2 and SH3) domains. We found that Grb2 was tyrosine‐phosphorylated in cells expressing BCR/ABL and in A431 cells stimulated with epidermal growth factor (EGF). Phosphorylation of Grb2 by Bcr/Abl or EGF receptor reduced its SH3‐dependent binding 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 tyrosine phosphorylation sites, and phosphorylation of Tyr209 abolished the binding of the SH3 domain to a proline‐rich Sos peptide in vitro. In vivo expression of a Grb2 mutant where Tyr209 was changed to phenylalanine enhanced BCR/ABL‐induced ERK activation and fibroblast transformation, and potentiated and prolonged Grb2‐mediated activation of Ras, mitogen‐activated protein kinase and c‐Jun N‐terminal kinase in response to EGF stimulation. These results suggest that tyrosine phosphorylation of Grb2 is a novel mechanism of down‐regulation of tyrosine kinase signaling.