3BP-1, AN SH3 DOMAIN BINDING-PROTEIN, HAS GAP ACTIVITY FOR RAC AND INHIBITS GROWTH FACTOR-INDUCED MEMBRANE RUFFLING IN FIBROBLASTS

The SH3 binding protein, 3BP-1, was originally cloned as a partial cDN A from an expression library using the Abl SH3 domain as a probe. In a ddition to an SH3 binding domain, 3BP-1 displayed homology to a class of GTPase activating proteins (GAPs) active against Rac and Rho protei ns. We report here a full length cDNA of 3BP-1 which extends the homol ogy to GAP proteins previously noted. 3BP-1 functions in vitro as a GA P with a specificity for Rac-related G proteins. Microinjection of the 3BP-1 protein into serum-starved fibroblasts produces an inhibition o f platelet-derived growth factor (PDGF)-induced membrane ruffling medi ated by Rac. Co-injection of 3BP-1 with an activated Rac mutant that i s unresponsive to GAPs, counteracts this inhibition. 3BP-1 does not sh ow in vitro activity towards Rho and, in agreement with this finding, microinjection of 3BP-1 into fibroblasts has no effect on lysophosphat idic acid (LPA)-induced stress fiber assembly mediated by Rho. Thus 3B P-1 is a new and specific Rac GAP that can act in cells to counter Rac -mediated membrane ruffling. How its SH3 binding site interacts with i ts GAP activity remains to be understood.