Ash/Grb-2, a SH2/SH3-containing protein, couples to signaling for mitogenesis and cytoskeletal reorganization by EGF and PDGF.

Abstract

The Src homology (SH) region 2 binds to phosphorylated tyrosine residues and SH3 domains may interact with cytoskeletal molecules and GTPase‐activating proteins for Rho/Rac proteins (the small GTP‐binding proteins related to Ras). The recently cloned Ash/Grb‐2 protein, a 25–28 kDa molecule composed entirely of SH2 and SH3 domains, is a mammalian homolog of the Caenorhabditis elegans Sem‐5 protein, which communicates between a receptor protein tyrosine kinase and a Ras protein. In the present study the function of Ash/Grb‐2 was investigated by microinjecting cells with an anti‐Ash antibody. The antibody abolished both S phase entry and the reorganization of actin assembly to ruffle formation upon stimulation with epidermal growth factor (EGF) and platelet‐derived growth factor (PDGF). On the other hand, anti‐Ash antibody had no effect on S phase entry or actin stress fiber formation induced by either serum or lysophosphatidic acid. Since the induction of DNA synthesis, ruffle induction and stress fiber formation involve a function of Ras, Rac activation and Rho activation respectively, the findings strongly suggest that Ash plays a critical role in the signaling of both pathways downstream from growth factor receptors to Ras and Rac. Consistent with this, Ash co‐precipitated with EGF receptor from EGF‐stimulated cells. Other proteins of approximately 21, 29, 135 and 160 kDa were also detected in the anti‐Ash antibody immunoprecipitates, suggesting a role of Ash as a linker molecule in signal transduction downstream of growth factor receptors.