P190 RHOGAP, THE MAJOR RASGAP-ASSOCIATED PROTEIN, BINDS GTP DIRECTLY

In mitogenically stimulated cells, a specific complex forms between th e pas GTPase-activating protein (RasGAP) and the cellular protein p190 . We have previously reported that p190 contains a carboxy-terminal do main that functions as a GAP for the Rho family GTPases. Thus, the Ras GAP-p190 complex may serve to couple Ras- and Rho-mediated signalling pathways. In addition to its RhoGAP domain, p190 contains an amino-ter minal domain that contains sequence motifs found in all known GTPases. Here, we report that p190 binds GTP and GDP through this conserved do main and that the structural requirements for binding are similar to t hose seen with other GTPases. While the purified protein is unable to hydrolyze GTP, we detect an activity in cell lysates that can promote GTP hydrolysis by p190. A mutated form of p190 that fails to bind nucl eotide retains its RasGAP binding and RhoGAP activities, indicating th at GTP binding by p190 is not required for these functions. The sequen ce of p190 in the GTP-binding domain, which shares structural features with both the Ras-like small GTPases and the larger G proteins, sugge sts that this protein defines a novel class of guanine nucleotide-bind ing proteins.