Fig. 4. (A) `Control by curvature' model for Arf function and vesicle formation. (1) Arf is activated by GTP exchange for GDP at the membrane. (2a) Coat is recruited to the membrane by Arf1-GTP. (2b) GAP binds to the membrane by guanine nucleotide exchange factor (GEF). (3) The GAP is incorporated into the Arf1-GTP-coat complex. (4) The coat polymerizes, driving budding from the membrane. (5) The GAP is activated on the curved surface with consequent inactivation and dissociation of Arf1. The coat then becomes metastable. The coat proteins on the convex surface remain bound because they are part of a polymer that is anchored to the membranes at the base of the bud where Arf GAP1 is not active. The GAP is active on the entire surface once the bud is released as a vesicle and the entire surface is convex. Cargo is incorporated through low-affinity interactions with the coat and GAP. (B) `Proofreading' model for Arf function. (1) Arf is activated through GTP exchange for GDP by GEF. (2) Coat proteins and Arf GAPs are recruited to the site of vesicle formation. In the schematic, the complex of coat protein and Arf GAP is shown to be recruited en bloc to the membrane. However, this has not been explicitly tested and it is possible that the Arf GAP and coat proteins are independently recruited to the membrane after which they associate. (3a) Coat proteins bind to cargo on the membrane, displacing Arf-GTP from coat proteins. (3b) Arf-GTP binds to Arf GAP, GTP on Arf is hydrolyzed by Arf GAP and Arf-GDP dissociates from the membrane. (4) The coat polymerizes, leading to membrane budding and fission of coated vesicles.