V-SNARE proteins

Based largely on a proposal by Rothman and colleagues, anterograde vesicular transport can be considered to occur in eight steps (Figure 46-7). The basic concept is that each transport vesicle bears a unique address marker consisting of one or more v-SNARE proteins, while each target membrane bears one or more complementary t-SNARE proteins with which the former interact specifically. 

Each of the 21 v-SNARE proteins could interact with each of 7 t-SNARE partners. Multiplication gives the total number of different interacting pairs 7 X 21 = 147 different v-SNARE—t-SNARE pairs. 

The Integral membrane proteins In a budding vesicle Include v-SNAREs, which are crucial to eventual fusion of the vesicle with the correct target membrane. Shortly after formation of a vesicle Is completed, the coat Is shed exposing Its v-SNARE proteins. The specific joining of v-SNAREs In... 

FIGURE 5.19 Role of cholesterol in the exocytosis of synaptic vesicles. Cholesterol constrains the conformation of v-SNARE proteins in such a way that their transmembrane domains adopt a parallel ("closed scissors") orientation. This particular conformation is required for the fusion process because it induces a fusion-compatible curvatiue of the s)fnaptic vesicle. In this case, the interaction between v-SNARE and t-SNARE proteins triggers the fusion mechanism (lower inset, 2). Without cholesterol, v-SNARE remains in an "open scissors" conformation (upper inset, 1), and the curvature of the vesicle is not appropriate for fusion. ... 

Seel A member of a family of proteins that attach to t-SNAREs and are displaced from them by Rab proteins, thereby allowing v-SNARE-t-SNARE interactions to occur. 

Step 5 Vesicle targeting is achieved via members of a family of integral proteins, termed v-SNAREs, that tag the vesicle during its budding. v-SNAREs pair with cognate t-SNAREs in the target membrane to dock the vesicle. 

Step 8 Retrograde transport occurs to restart the cycle. This last step may retrieve certain proteins or recycle v-SNAREs. Nocodazole, a microtubule-disrupting agent, inhibits this step. 

The decisive element in exocytosis is the interaction between proteins known as SNAREs that are located on the vesicular membrane (v-SNAREs) and on the plasma membrane (t-SNAREs). In the resting state (1), the v-SNARE synaptobrevin is blocked by the vesicular protein synaptotagmin. When an action potential reaches the presynaptic membrane, voltage-gated Ca "" channels open (see p. 348). Ca "" flows in and triggers the machinery by conformational changes in proteins. Contact takes place between synaptobrevin and the t-SNARE synaptotaxin (2). Additional proteins known as SNAPs bind to the SNARE complex and allow fusion between the vesicle and the plasma membrane (3). The process is supported by the hydrolysis of GTP by the auxiliary protein Rab. 

In eukaryotes, soluble N-ethylmaleimide-sensitive factor (NSF) adaptor proteins (SNAPs) receptors (SNAREs) are known to be required for docking and fusion of intracellular transport vesicles with acceptor/target membranes. The fusion of vesicles in the secretory pathway involves target-SNAREs (t-SNAREs) on the target membrane and vesicle-SNAREs (v-SNAREs) on vesicle membranes that recognize each other and assemble into trans-SNARE complexes (Sollner et al., 1993). 

Collins, N.C., Thordal-Christensen, H., Lipka, V., Bau, S., Kombrink, E., Qiu, J.L., Hiickelhoven, R., Stein, M., Freialdenhoven, A., Somerville, S.C., Schulze-Lefert, P. SNARE-protein-mediated disease resistance at the plant cell wall. Nature 425 (2003) 973-977. 

Protein transport between intracellular compartments is mediated by a mechanism that is well-conserved among all eukaryotes, from yeast to man. The transport mechanism involves carrier vesicles that bud from one organelle and fuse selectively to another. Specialized proteins are required for vesicle transport, docking, and fusion, and they have been generically named SNAREs (an acronym for soluble N-ethylma-leimide-sensitive fusion attachment protein receptor). SNAREs have been divided into those associated with the vesicle (termed v-SNAREs), and those associated with the target (termed t-SNAREs). The key protein, which led to the discovery of SNAREs was NSF, an ATPase found ubiquitously in all cells, and involved in numerous intracellular transport events. The subsequent identification of soluble proteins stably bound to NSF, the so-called SNARE complex, led to the formulation of the SNARE hypothesis, which posits that all intracellular fusion events are mediated by SNAREs (Rothman, 2002). 

In neurons, the SNARE complex consists of three main proteins the v-SNARE synaptobrevin or VAMP (vesicle-associated membrane protein), and two t-SNAREs, syntaxin and SNAP-25 (synaptosomal associated protein of 25 kD). Synaptobrevins traverse the synaptic vesicle membrane in an asymmetric manner a few amino acids are found inside the vesicle, but most of the molecule lies outside the vesicle, within the cytoplasm. Synaptobrevin makes contact with another protein anchored to the plasma membrane of the presynaptic neuron, syntaxin, which is associated with SNAP-25. Via these interactions, the SNARE proteins play a role in the docking and fusion of synaptic vesicles to the active zone. 

Kiessling V, Tamm LK. Measuring distances in supported bilayers 51. by fluorescence interference-contrast microscopy polymer supports and SNARE proteins. Biophys. J. 2003 84 408-418. 

As will be appreciated, in all of these cases neurotransmitters are ultimately packaged into synaptic vesicles. These synaptic vesicles are released fr om the terminal in response to a rise in intracellular Ca +. In most instances the source of this Ca is the extracellular medium, and Ca moves into the nerve terminal through voltage sensitive Ca channels. A complex of proteins holds the vesicle in a primed state atpresynaptic release sites, the active zones discussed above. Some of the proteins involved in this complex are provided by the vesicle (v-SNAREs) and... 

---