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First published online March 22, 2006
doi: 10.1242/10.1242/jcs.02924

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Arf GAPs and membrane traffic

Laboratory of Cellular Oncology, Center for Cancer Research, National Cancer Institute, National Institutes of Health, Department of Health and Human Services, Bethesda, MD 20892, USA

View larger version (21K): [in a new window]   Fig. 1. Model for the role of Arf1-GTP in the generation of transport vesicles. The productive pathway has the essential elements of the classical model for Arf1-dependent membrane trafficking. GAP recruitment was introduced when the model was modified to account for GTP-hydrolysis-dependent sorting. (1) Activation of Arf1 through GTP exchange for GDP by guanine nucleotide exchange factors at the membrane. (2) Arf1-GTP recruits coat proteins to membrane sites of vesicle formation. (3) Coat proteins bind to and concentrate cargo. (4) Arf GAPs are recruited to membrane sites of vesicle formation through binding to coat proteins or cargo proteins. (5) If the right cargos are present, GAP activity is inhibited and coat complexes polymerize to deform membrane. (6) Coated vesicle detaches from the membrane. (7) GTP on Arf is hydrolyzed by Arf GAP. Arf-GDP and Arf GAP dissociate. (8) Coat proteins dissociate to leave vesicles competent for docking and fusion with acceptor membranes. The discard pathway represents a modification of the classical model that explains a requirement for GTP hydrolysis in cargo sorting. In the text, we also refer to this modification as the `sorting by GAP inhibition' model. In the absence of correct cargo, Arf-GTP and coat proteins are recruited as in steps 1 and 2 of the productive pathway. (3) Arf GAP is recruited. (4) Since there is no cargo to inhibit GAP activity, GTP on Arf is hydrolyzed and Arf-GDP and Arf GAP dissociate from membrane. (5) Coat proteins dissociate from membrane.

View larger version (23K): [in a new window]   Fig. 2. Classification and domain structures of Arf GAPs. Three major groups of Arf GAPs are indicated. Alternate names for the different Arf GAPs are included in parentheses. Accession numbers for the Arf GAPs are listed along with the species of origin indicated; h, human; r, rat; and m, mouse. A, ankyrin repeat; BAR, Bin, amphiphysin and Rvs 167 and Rvs 161; GLD, GTP-binding protein-like domain; PBS, paxillin-binding sequence; PH, pleckstrin homology; SAM, sterile motif; SH3, src homology 3; SHD, spa-homology domain.

View larger version (43K): [in a new window]   Fig. 3. Mechanisms of Arf GAP1 activation. (A) Effect of diacylglycerol on lipid packing. Diacylglycerol has a small head group compared with other lipids. When present in the membrane, lipid head groups pack less tightly, allowing peripheral membrane proteins, such as Arf GAP1, access to the central, hydrophobic portion of the bilayer. Arf GAP1 is more active in this environment. (B) Effect of membrane curvature on lipid packing and Arf GAP1 binding. On a tightly packed flat surface, Arf GAP1 cannot penetrate the bilayer and, therefore, is inactive. On the convex part of the bud, the packing of lipids is loosened allowing Arf GAP1 to penetrate the bilayer and consequently become active. GTP on Arf is therefore hydrolyzed and Arf-GDP dissociates from membrane.

View larger version (23K): [in a new window]   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.