To ensure an orderly flow of vesicle traffic, transport vesicles must be highly accurate in recognizing the correct target membrane with which to fuse. Because of the diversity and crowding of membrane systems in the cytoplasm, a vesicle is likely to encounter many potential target membranes before it finds the correct one. Specificity in targeting is ensured because all transport vesicles display surface markers that identify them according to their origin and type of cargo, and target membranes display complementary receptors that recognize the appropriate markers. This crucial process occurs in two steps. First, Rab proteins and Rab effectors direct the vesicle to specific spots on the correct target membrane. Second, SNARE proteins and SNARE regulators mediate the fusion of the lipid bilayers.
Rab proteins play a central part in the specificity of vesicle transport. Like the coat-recruitment GTPases discussed earlier (see Figure 13–14), Rab proteins are also monomeric GTPases. With over 60 known members, the Rab subfamily is the largest of the monomeric GTPase subfamilies. Each Rab protein is associated with one or more membrane-enclosed organelles of the secretory or endocytic pathways, and each of these organelles has at least one Rab protein on its cytosolic surface (Table 13–1). Their highly selective distribution on these membrane systems makes Rab proteins ideal molecular markers for identifying each membrane type and guiding vesicle traffic between them. Rab proteins can function on transport vesicles, on target membranes, or both.
Like the coat-recruitment GTPases, Rab proteins cycle between a membrane and the cytosol and regulate the reversible assembly of protein complexes on the membrane. In their GDP-bound state, they are inactive and bound to another protein (Rab-GDP dissociation inhibitor, or GDI) that keeps them soluble in the
Figure 13–15 Packaging of procollagen into large tubular COPII-coated vesicles.
The cartoons show models for two copII coat assembly modes. The models are based on cryoelectron tomography images of reconstituted copII vesicles. on a spherical membrane (left), the sec23/24 inner coat proteins assemble in patches that anchor the sec13/31 outer coat protein cage. The sec13/31 rods assemble a cage of triangles, squares, and pentagons. when procollagen needs to be packaged (right), special packaging proteins sense the cargo and modify the coat assembly process. This interaction recruits the copII inner coat protein sec24 and locally enhances the rate with which sar1 cycles on and off the membrane (not shown). In addition, a monoubiquitin is added to the sec31 protein, changing the assembly properties of the outer cage. sec23/24 proteins arrange in larger arrays and sec13/31 arrange in a regular lattice of diamond shapes. as the result, a large tubular vesicle is formed that can accommodate the large cargo molecules. The packaging proteins are not part of the budding vesicle but remain in the er. (Modified from g. Zanetti et al., eLife 2:e00951, 2013.) cytosol; in their GTP-bound state, they are active and tightly associated with the membrane of an organelle or transport vesicle. Membrane-bound Rab-GEFs activate Rab proteins on both transport vesicle and target membranes; for some membrane fusion events, activated Rab molecules are required on both sides of the reaction. Once in the GTP-bound state and membrane-bound through a now-exposed lipid anchor, Rab proteins bind to other proteins, called Rab effectors, which are the downstream mediators of vesicle transport, membrane tethering, and membrane fusion (Figure 13–16). The rate of GTP hydrolysis sets the concentration of active Rab and, consequently, the concentration of its effectors on the membrane.