Stimulation of ras GTPase activity by an anti-ras monoclonal antibody

Abstract

Wild-type ras has GTPase activity, and this activity is accelerated substantially by GTPase-activating proteins (GAPs). Oncogenic ras species have an abnormally low intrinsic GTPase activity, and this activity is insensitive to GAPs. We confirmed that the anti-ras monoclonal antibody Y13-238 inhibited GAP activity in vitro, but we also noted that this antibody had GAP activity of its own. We studied the GAP activity of Y13-238 in circumstances in which ras GTPase activity was influenced by the GTPase-inhibitory antibody Y13-259 or by substitutions in ras. The GTPase-inhibitory antibody Y13-259 blocked the GAP associated with Y13-238. A ras species with a substitution in the effector loop that blocked conventional GAP activity was sensitive to stimulation by Y13-238. Both Y13-238 and Y13-259 stimulated the autophosphorylation of Ala59Thr ras. We interpreted these data in terms of a model in which the extrinsic factors influence the ras GTPase reaction by affecting the balance between "committed" and "uncommitted" states. We suggest that there is a mechanism distinct from that exploited by conventional GAPs for stimulating ras GTPase activity.