SNARE complex

SNARE motifs spontaneously assemble into SNARE complexes. These consist of a bundle of four intertwined a-helices that are connected by a total of 16 layers of mostly hydrophobic amino acid side chains. In the middle of the bundle, there is a highly conserved and polar 0-layer consisting of three glutamine and one arginine residue. These residues are among the most conserved in the SNARE superfamily and led to a classification of SNAREs into Q- and R-SNAREs, respectively. Different fusion steps require different sets of SNAREs but some SNAREs can participate in different complexes, and some fusion steps involve several SNARE complexes that appear to operate in parallel and independently. 

In vitro, SNARE-complex formation is irreversible. Disassembly requires the concerted action of the chaperone-like ATPase NSF and SNAPs. 

Step 6 The general fusion machinery then assembles on the paired SNARE complex it includes an ATPase (NSF NEM-sensitive factor) and the SNAP (soluble NSF attachment factor) proteins. SNAPs bind to the SNARE (SNAP receptor) complex, enabling NSF to bind. 

Q Studies using v- and t-SNARE ptoteins reconsti-mted into separate lipid bilayer vesicles have indicated that they form SNAREpins, ie, SNARE complexes that hnk two membranes (vesicles). SNAPs and NSF are required for formation of SNAREpins, but once they have formed they can apparently lead to spontaneous fusion of membranes at physiologic temperamre, suggesting that they are the minimal machinery required for membrane fusion. 

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. 

Pabst S., Hazzard J. W., Antonin W., Sudhof T. C., Jahn R., Rizo J., and Fasshauer D. (2000). Selective interaction of complexin with the neuronal SNARE complex. Determination of the binding regions. J. Biol. Chem. 275 19808-19818. 

Each intracellular fusion reaction exhibits characteristic properties, and involves a different combination of SM and SNARE proteins. The specificity of fusion reactions appears to be independent of SNARE proteins because SNARE complex formation is nonspecific as long as the Q/R-rule is not violated (i.e., the fact that SNARE complexes need to be formed by SNARE proteins containing R-, Qa-, Qb-, and Qc-SNARE motifs), and of SM proteins because SM proteins often function in... 

Martin-Moutot N, Charvin N, Leveque C, Sato K, Nishiki T, Kozaki S, Takahashi M, Seagar M (1996) Interaction of SNARE complexes with P/Q-type calcium channels in rat cerebellar synaptosomes. J Biol Chem 271 6567-70... 

In the following sections, we limit our discussion to the neuronal SNARE complex that, however, is paradigmatic for most SNARE complexes studied so far. 

Qb-, Qc- and R-SNAREs (Bock and Scheller 2001). Following this classification, syntaxin 1A, SNAP-25, and synaptobrevin 2 represent the Qa-, Qb- and Qc-, and R-SNAREs, respectively (Fasshauer et al. 1998). It turned out later that actually all functional SNARE complexes assigned to trafficking steps in yeast and mammals have a QaQbQcR-composition (Hong 2005 Jahn and Scheller 2006). 

Acceptor complex Monomeric/dustered-SNAREs Cis-SNARE-complexes... 

Fig. 2 The conformational cycle of SNAREs. SNAREs cycle between two extreme conformations, the unstructured monomeric SNAREs and the fully assembled cis-SNARE complexes. Initially, SNAREs on the membranes destined to fuse establish trans-SNARE complexes between the opposed membranes. Proceeding SNARE complex assembly forces the membranes tightly together enforcing membrane fusion. The resulting cis-SNARE complexes are disassembled into free SNAREs by the ATPase NSF and its co-factor, a process that consumes ATP and fuels the SNAREs with energy for undergoing a new SNARE cycle (for details see text).

Fig. 3 muncl8-l SNARE-binding modes. The following muncl8-l interactions with monomeric/ assembled SNAREs have been proposed. From left, binding of muncl8-l to a closed conformation of syntaxin 1A (Misura et al. 2000), to a half-open conformation of syntaxin or to an acceptor complex formed by syntaxin and SNAP-25 (Zilly et al. 2006), and to an assembled SNARE complex (Dulubova et al. 2007). It is possible that each of the proposed complexes represents an intermediate on a munc 8-l controlled molecular pathway of specific SNARE complex assembly. 

Fig. 4 Stages in synaptic vesicle exocytosis. Putative intermediate steps on the molecular pathway to synaptic vesicle fusion. Vesicle delivery and tethering to the presynaptic membrane most likely involves Rab-proteins and their effectors. So far, the nature of a speculative docking complex (dc) is unclear, but docking appears to be independent from SNARE proteins. In the primed state, SNAREs have assembled into a complex probably stabilized by complexin (Cpx). The fusion reaction is arrested until the intracellular calcium concentration increases. The putative calcium sensor for fast neurotransmitter release, synaptotagmin 1 (Syt), binds to intracellular calcium and in turn triggers fusion by associating with the presynaptic membrane and interacting with the SNARE complex, thereby displacing complexin (Tang et al. 2006).

Tomosyn is a soluble protein of 130 kDa with a C-terminal R-SNARE motif that is capable of replacing synaptobrevin in the neuronal SNARE complex. Most available data indicate that tomosyn negatively regulates exocytosis by competing with synaptobrevin in the formation of SNARE complexes (Brunger 2005), thereby leading to the inhibition of synaptic vesicle priming (McEwen et al. 2006). 

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