ArfGAPs: Not Only for the Termination

While Arf-family small GTPases (Arf-GTPases) consist of 5 members in humans, 31 human genes have been identified that encode proteins bearing the GTPase-activating protein (GAP) domain for Arf-GTPases. Interestingly, Arf1, the first identified Arf, was shown to substantially lack intrinsic GTPase activity, which other Ras-superfamily members of small GTPases generally bear. Likewise, ArfGAP domains primarily consist of zinc-finger structures, and do not resemble GAP domains for other small GTPases. Arfs primarily function in intracellular vesicle/membrane trafficking. A general model shows that Arfs play roles in membrane budding, in which GTP-Arfs recruit coatomer proteins to generate and maintain membrane curvature to initiate the budding. Coatomers are thought to be separated from Arf-mediated vesicles before they reach the target membrane, while this separation may or may not be coupled with the GTP hydrolysis activity. We have shown that several ArfGAPs, such as AMAP1 and AMAP2, have the ability to bind stably to GTP-Arf6, without immediate GTP hydrolysis. They each contain a BAR domain and hence may act as coatomers for Arf-mediated vesicles. These ArfGAPs moreover act to recruit their binding proteins to sites of Arf6 activation, which are not coatomer components. These findings have amended the classical, general model of the functions of ArfGAPs, as well as Arf-GTPases. In this review, we will describe the recent information revealed about ArfGAPs, with the aim to decipher and discuss their fundamental roles.