ArhGAP15, a Rac-specific GTPase-activating protein, plays a dual role in inhibiting small GTPase signaling.

Signaling from small GTPases is a tightly regulated process. In this work we used a protein microarray screen to identify the Rac-specific GAP, ArhGAP15, as a substrate of the Rac effectors Pak1 and Pak2. In addition to serving as a substrate of Pak1/2, we found that ArhGAP15, via its PH domain, bound to these kinases. The association of ArhGAP15 to Pak1/2 resulted in mutual inhibition of GAP and kinase catalytic activity, respectively. Knock-down of ArhGAP15 resulted in activation of Pak1/2, both indirectly, as a result of Rac activation, and directly, as a result of disruption of the ArhGAP15/Pak complex. Our data suggest that ArhGAP15 plays a dual negative role in regulating small GTPase signaling, by acting at the level of the GTPase itself, as well interacting with its effector, Pak kinase.