A RAS-GTPASE-ACTIVATING PROTEIN SH3-DOMAIN-BINDING PROTEIN

We report the purification of a Ras-GTPase-activating protein (GAP)-bi nding protein, G3BP, a ubiquitously expressed cytosolic 68-kDa protein that coimmunoprecipitates with GAP. G3BP physically associates with t he SH3 domain of GAP, which previously had been shown to be essential for Ras signaling. The G3BP cDNA revealed that G3BP is a novel 466-ami no-acid protein that shares several features with heterogeneous nuclea r RNA-binding proteins, including ribonucleoprotein (RNP) motifs RNP1 and RNP2, an RG-rich domain, and acidic sequences. Recombinant G3BP bi nds effectively to the GAP SH3 domain G3BP coimmunoprecipitates with G AP only when cells are in a proliferating state, suggesting a recruitm ent of a GAP-G3BP complex when Ras is in its activated conformation.