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Introduction
Important biological processes, such as synaptic transmission and cellular trafficking in eukaryotes require SNARE proteins that are thought to play a crucial role in membrane fusion. To connect membranes and allow their fusion, SNARE (soluble N-ethylmaleimide-sensitive-factor attachment protein receptor) proteins assemble into a core-complex of four helices. The SNARE complex assembly is mediated by a conserved SNARE motif consisting of 60-70 amino acids. The best-studied SNAREs are the neuronal and the endosomal SNARE complexes. The Neuronal SNARE complex mediates exocytosis of synaptic vesicles in the neurons and includes the vesicle protein synaptobrevin (also called VAMP), the membrane proteins SNAP-25 and syntaxin 1. The Endosomal SNARE complex includes syntaxin7, syntaxin8, vti1b and andobrevin (also referred as VAMP-8) and is responsible for the fusion of late endosomes/lysosomes. Moreover, SNAREs can be devided into two categories: the v-(vesicle) SNAREs, which are found in the vesicle membrane and the t-(target) SNAREs, which are anchored in the target membrane.
Membrane fusion mechanism
Membrane Fusion requires the assembly of the core complex. Free t-SNAREs that are organized in clusters first assemble into acceptor complexes thanks to SM (Sec1/Munc18-related) proteins. Acceptor complexes can then interact with the v-SNAREs through the N-terminal domain of the SNARE motif. This enables the formation of four-helical trans-complexes, in which only the N-terminal portions of the SNARE motifs are bound. This binding evolves from a loose to a tight state, thus leading to the formation of a fusion pore. During the fusion, the conformation relaxes to a cis-configuration. Cis-complexes dissociate thanks to proteins and cofactors. T- and v-SNAREs can be separated and recycled.
Structure
SNARE domain
The SNARE domain is approximately 60-70 residues long and it is located immediately adjacent to a C-terminal transmembrane anchor. It contains a repeating heptad pattern of hydrophobic residues. Their orientation place them in an alpha-helical structure so that all the hydrophobic side chains are located on the same face of the helix. SNARE domains allow the four SNAREs protein to assemble into parallel four-helix bundles. This parallel arrangement brings the transmembrane anchors and the membranes closer.
Structure of individual layers
The centre of the four-helix bundle is constituted of 16 layers. These layers are composed of hydrophobic side chains, which are perpendicular to the axis of the four-helix bundle, except for the central “0”-layer. This last one consists of three glutamine (Q) and one arginine (R) highly conserved residues. Glutamine residues are found in syntaxin and both SNAP-25 and arginine residues in synaptobrevin. Those highly conserved residues have led to a new classification of SNAREs into Q- and R-SNAREs.
Surface interactions
Polar and ionic surface interactions help stabilisation of the helix bundle. These include six pairs of opposed charges that are conserved between SNAREs families. For instance, Glu 179 in syntaxin 7 forms a salt bridge with Arg 148 in vti1b. Another example of a conserved interaction is a salt bridge between Arg 32 of endobrevin and Asp 177 of syntaxin 8. In the neuronal complex, the corresponding residues are Lys 52 in synaptobrevin 2 and Asp 172 in SNAP-25. A positively charged amino acid in this position (mostly Lys) is found in all synaptobrevins, except for members of the VAMP7-subfamily, but is missing from the ykt6- (Ser/Asn/Ala) and sec22-subfamilies (Asp/Glu). Most d-helices contain a negatively charged residue at this position.A third example is Glu 207 of syntaxin 7, which forms a salt bridge with Arg 176 in vti1b. Again, this interaction is conserved: most syntaxins contain an Asp or Glu at this position, which is matched by an Arg or Lys (occasionally also Asn) in the c-helix SNARE motifs (except for members of the membrin and gs28/Gos1-subfamilies).
Regulation
2NPS in cancers/diseases
External ressources
References
Proteopedia page contributors and editors
Florence HERMAL and Camille ROESCH