STIMULATION OF RAS GTPASE ACTIVITY BY AN ANTI-RAS MONOCLONAL-ANTIBODY

Wild-type ras has GTPase activity, and this activity is accelerated su bstantially by GTPase-activating proteins (GAPs). Oncogenic ras specie s have an abnormally low intrinsic GTPase activity, and this activity is insensitive to GAPs. We confirmed that the anti-ras monoclonal anti body Y13-238 inhibited GAP activity in vitro, but we also noted that t his antibody had GAP activity of its own. We studied the GAP activity of Y13-238 in circumstances in which ras GTPase activity was influence d by the GTPase-inhibitory antibody Y13-259 or by substitutions in ras . The GTPase-inhibitory antibody Y13-259 blocked the GAP associated wi th Y13-238. A ras species with a substitution in the effector loop tha t blocked conventional GAP activity was sensitive to stimulation by Y1 3-238. Both Y13-238 and Y13-259 stimulated the autophosphorylation of Ala59Thr ras. We interpreted these data in terms of a model in which t he extrinsic factors influence the ras GTPase reaction by affecting th e balance between ''committed'' and ''uncommitted'' states. We suggest that there is a mechanism distinct from that exploited by conventiona l GAPs for stimulating ras GTPase activity. (C) 1996 Wiley-Liss, Inc.