Regulation of Membrane Trafficking Structural Insights from a Rab/Effector Complex

membrane compartments (reviewed by Novick and ZeIntroduction rial, 1997). Eukaryotic cells contain a highly dynamic set of memConcomitant with the SNARE cycle, Rab proteins unbrane compartments that are responsible for packaging, dergo a intricate cycle of membrane and protein interacsorting, secreting, and recycling proteins and other moltions. Rabs are posttranslationally modified at C-termiecules. Trafficking between organelles within the secrenal cysteines by the addition of two geranylgeranyl tory pathway occurs as vesicles derived from a donor groups, which mediate membrane association when the compartment fuse with specific acceptor membranes, Rab is in the GTP-bound state (Figure 1). After guanine resulting in the directional transfer of cargo molecules. nucleotide hydrolysis occurs, the Rab is extracted from This process is tightly controlled by the Rab/Ypt family the membrane upon forming a complex with a cytosolic of proteins (reviewed by Novick and Zerial, 1997), a GDP-dissociation inhibitor (GDI). This cytosolic intermebranch of the superfamily of small GTPases. Rab prodiate is then recycled onto a newly forming vesicle, most teins regulate a variety of functions, including vesicle likely through a secondary factor termed a GDI dissociatranslocation and docking at specific fusion sites. Rabs tion factor (GDF), which displaces GDI. After the Rab may also play critical roles in higher order processes becomes membrane bound, a guanidine nucleotide exsuch as modulating the levels of neurotransmitter rechange factor (GEF) promotes release of GDP and the lease in neurons, a likely mechanism in synaptic plasticsubsequent loading of GTP. In its GTP-bound conformaity that underlies learning and memory (Geppert and tion, the Rab is then free to associate with its specific Südhof, 1998). set of effectors, which can in turn trigger events leading Small GTPases share a common three-dimensional to the eventual fusion of the vesicle with a target memfold that, in the GTP bound state, can bind a variety of brane. To complete the cycle, perhaps after or concurdownstream effector proteins. GTP hydrolysis leads to rent with membrane fusion, a GTPase activating protein a conformational change in the “switch” regions that (GAP) accelerates nucleotide hydrolysis, switching off renders the GTPase unrecognizable to its effectors. In the GTPase. The remaining GDP-bound Rab can then this way, by localizing and activating a select set of participate in a new round of fusion. effectors, a common structural motif is used to control Rab interactions with effectors are likely to regulate a wide array of distinct cellular processes. Recently, the vesicle targeting and membrane fusion in three ways first structure of a Rab/effector complex was reported (Figure 1). First, a Rab may specifically facilitate vectorial for the Rab-binding domain (RBD) of the Rab effector vesicle transport. Vesicles are transported from their protein Rabphilin-3A bound to Rab3A (Ostermeier and site of origin to acceptor compartments likely through Brünger, 1999). This structure highlights a novel RBD associations with cytoskeletal elements and transport and sheds light on the specificity of effector interactions motors. A protein has been identified with a domain within the Rab family, interactions that may be critical structure that suggests a connection between the cyfor understanding the specificity and regulation of memtoskeleton and the Rabs. This protein, called Rabbrane trafficking in cells. kinesin-6, contains a kinesin-like ATPase motor domain The Role of Rabs in Regulating followed by a coiled-coil stalk region and a RBD that Membrane Trafficking specifically binds Rab6 (Echard et al., 1998). An addiThe final steps in membrane fusion are likely to be driven tional link with the cytoskeleton is provided by the Rab by a set of proteins known as SNAREs (Figure 1). After effector, Rabphilin-3A. Rabphilin-3A has been shown in a vesicle becomes docked, the cytoplasmic domains vitro to interact with a-actinin, an actin-bundling protein, of VAMP (also termed synaptobrevin) and syntaxin on but only when not bound to Rab3A (Kato et al., 1996).