The Arf-GAP domain (IPR001164) is an ARF1-directed GTPase-activating protein, the cycle-regulating GTPase-activating protein (GAP) GCS1. It is essential for the regulation of the ADP ribosylation factor ARF, a Ras superfamily GTP-binding protein member . GAP is a protein that activates GTP hydrolysis. Arf-GAP participates in the recruitment of coat proteins necessary for membrane budding and fission, requiring the hydrolysis of ARF-associated GTPs. There is no apparent similarity between the Arf GAP domain and other GAP proteins. Unlike Arf, the GAP domain does not donate arginine to the active site, indicating that it plays a more indirect role in GTPase hydrolysis .