RHO FAMILY GTPASE-ACTIVATING PROTEINS P190, BCR AND RHOGAP SHOW DISTINCT SPECIFICITIES IN-VITRO AND IN-VIVO

rho family GTPases link extracellular signals to changes in the organi zation of cytoskeletal actin. Serum stimulation of quiescent Swiss 3T3 fibroblasts leads to rho-dependent actin stress fibre formation and f ocal adhesions, whilst several growth factors initiate signalling path ways leading to rac-dependent actin polymerization at the plasma membr ane, and membrane ruffling. The product of the breakpoint cluster regi on gene bcr, rho GTPase accelerating protein (rhoGAP) and rasGAP-assoc iated p190 share structurally related rho GAP domains, and possess GAP activity for rho family members in vitro. We have directly compared t he activities of the isolated GAP domains of these three proteins in r egulating different rho family GTPases, both by in vitro assays and by microinjection, to address their possible physiologic functions. We s how that bcr accelerates the GTPase activity of rac, but not rho in vi tro, and inhibits rac-mediated membrane ruffling, but not rho-mediated stress fibre formation, after microinjection into Swiss 3T3 fibroblas ts. In vitro, rhoGAP has a striking preference for G25K as a substrate , whilst p190GAP has marked preferential activity for rho. Furthermore , p190 preferentially inhibits rho-mediated stress fibre formation in vivo. Our data suggest that p190, rhoGAP and bcr play distinct roles i n signalling pathways mediated through different rho family GTPases.