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Fig. 3. (A) Vesicle budding and fusion. Activation of a small G protein (such as Arf1 or Sar1) (red) by the exchange of GDP for GTP results in the recruitment of a coat complex (blue) to the membrane by the GTP-bound form of the G protein. Membrane curvature and sorting of cargo (yellow) into the forming bud ensue, followed by fission of the coated bud to form a vesicle. Hydrolysis of GTP eventually leads to release of the coat from the vesicle. The vesicle is targeted to the acceptor-compartment membrane by tethering complexes (long coiled-coil, green; multi-subunit, purple). For simplicity, the coat is not shown at this stage, but note that the coat may in some cases remain on the vesicle during the tethering process, with uncoating occurring after tethering. The vesicle SNARE (v-SNARE; dark blue bars) on the vesicle engages the tripartite target-localized SNARE (t-SNARE; maroon bars), which leads to fusion of the vesicle and acceptor-compartment membranes, and to the release of cargo into the acceptor compartment. (B) Tethering of highly curved and flat membranes by GMAP-210. The C-terminus of GMAP-210 contains a GRAB domain (yellow) that binds to membranes containing Arf1-GTP (red circles). In the absence of highly curved membranes, the N-terminal ALPS motif (orange) of GMAP-210 is unstructured. When the ALPS motif comes into contact with a highly curved membrane such as a vesicle (green), it folds into an amphipathic helix that binds tightly to the vesicle membrane. Note that the C-terminal GRAB domain of GMAP-210 binds only to flat membranes, owing to curvature-stimulated ArfGAP1 activity on Arf1-GTP. (C) Model for the role of GMAP-210 in cells. A donor compartment (green; top) produces coated vesicles that are directed towards an acceptor compartment (dark blue; bottom). GMAP-210, via its GRAB domain (yellow), binds to Arf1-GTP (red circles) on the flat membrane of the acceptor compartment. The ALPS motif (orange) of GMAP-210 binds to the incoming vesicle (shown here after uncoating) and directs it to the fusion site of the acceptor-compartment membrane. After fusion, the loss of curvature in the vesicle membrane results in release from the GMAP-210 ALPS motif. Arf1-GTP is involved in the sorting of components that are to be recycled back to the donor compartment through formation of retrograde transport vesicles. When these vesicles begin to form on the acceptor-compartment membrane, hydrolysis of GTP on Arf1 by ArfGAP1 (pink) at regions of positive curvature in the membrane of the budding vesicle results in release of the GMAP-210 GRAB domain from the membrane.
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