Soluble N-ethylmaleimide attachment protein receptor (SNARE) proteins are a family of membrane-associated proteins characterised by an α-helical coiled-coil domain called the SNARE motif []. These proteins are classified as v-SNAREs and t-SNAREs based on their localisation on vesicle or target membrane; another classification scheme defines R-SNAREs and Q-SNAREs, as based on the conserved arginine or glutamine residue in the centre of the SNARE motif []. SNAREs are localised to distinct membrane compartments of the secretory and endocytic trafficking pathways, and contribute to the specificity of intracellular membrane fusion processes.The t-SNARE domain consists of a 4-helical bundle with a coiled-coil twist. The SNARE motif contributes to the fusion of two membranes. SNARE motifs fall into four classes: homologues of syntaxin 1a (t-SNARE), VAMP-2 (v-SNARE), and the N- and C-terminal SNARE motifs of SNAP-25.This entry represents the N-terminal SNARE motif found in plant SNAP25 homologues, SNAP29, SNAP30 and SNAP33 [, , ], and in the novel plant SNAREs (NPSNs) 11, 12 and 13 from Arabidopsis thaliana [, ]. SNAP33 and NPS11 play a key role in cytokinesis [, ]. SNAP33, the most studied SNAP25 homologue in Arabidopsis, is also involved in triggering innate immune responses. SNAP29 and SNAP30 have not been well-characterised yet [].
Snapin is a component of the biogenesis of lysosomal organelles complex-1 (BLOC-1), a complex required for the normal biogenesis of lysosome-related organelles, such as melanosomes and platelet dense granules [], and endosomal cargo sorting [].Snapin binds to SNAP-25, part of the SNARE complex that mediates the synaptic vesicle docking and fusion []. Snapin may modulate a step between vesicle priming, fusion and calcium-dependent neurotransmitter release through its ability to potentiate the interaction of synaptotagmin with the SNAREs and the plasma-membrane-associated protein SNAP25 []. Its phosphorylation state influences exocytotic protein interactions and may regulate synaptic vesicle exocytosis [, ]. It may also have a role in the mechanisms of SNARE-mediated membrane fusion in non-neuronal cells [].
This entry represents a domain found in the SNAP-25 family members. SNAP-25 (synaptosome-associated protein 25kDa) proteins are components of SNARE complexes, which are proposed to account for the specificity of membrane fusion and todirectly execute fusion by forming a tight complex (the SNARE or corecomplex) that brings the synaptic vesicle and plasma membranestogether. The SNAREs constitute a large family of proteins thatare characterised by 60-residue sequences known as SNARE motifs (),which have a high propensity to form coiled coils and often precedecarboxy-terminal transmembrane regions. The synaptic core complex is formed by four SNARE motifs (two fromSNAP25 and one each from synaptobrevin and syntaxin 1) that areunstructured in isolation but form a parallel four-helix bundle onassembly. The crystal structure of the core complex revealedthat the helix bundle is highly twisted and contains several salt bridges onthe surface, as well as layers of interior hydrophobic residues.However, a polar layer in the centre of the complex is formed by threeglutamines (two from SNAP25 and one from syntaxin 1) and one arginine(from synaptobrevin) [].Members of the SNAP-25 family contain a cluster of cysteine residues that can be palmitoylated for membrane attachment [].
