Regulated exocytosis of neurotransmitters and hormones, as well as intracellular traffic, requires fusion of two lipid bilayers. SNARE proteins are thought to form a protein bridge, the SNARE complex, between an incoming vesicle and the acceptor compartment. SNARE proteins contribute to the specificity of membrane fusion, implying that the mechanisms by which SNAREs are targeted to subcellular compartments are important for specific docking and fusion of vesicles. This mechanism involves a family of conserved proteins, members of which appear to function at all sites of constitutive and regulated secretion in eukaryotes []. Among them are 2 types of cytosolic protein, NSF (N-ethyl-maleimide-sensitive protein) and the SNAPs (alpha-, beta- and gamma-soluble NSF attachment proteins). The yeast vesicular fusion protein, sec17, a cytoplasmic peripheral membrane protein involved in vesicular transport between the endoplasmic reticulum and the golgi apparatus, shows a high degree of sequence similarity to the alpha-SNAP family.Alpha-SNAP is universally present in eukaryotes and acts as an adaptor protein between SNARE (integral membrane SNAP receptor) and NSF for recruitment to the 20S complex. Beta-SNAP is brain-specific and shares high sequence identity (about 85%) with alpha-SNAP. Gamma-SNAP is weakly related (about 20-25% identity) to the two other isoforms, and is ubiquitous. It may help regulate the activity of the 20S complex. The X-ray structures of vertebrate gamma-SNAP and Sec17 show similar all-helical structures consisting of an N-terminal extended twisted sheet of four tetratricopeptide repeat (TPR)-like helical hairpins and a C-terminal helical bundle [, , , , , , , ].SNAP-25 and its non-neuronal homologue Syndet/SNAP-23 are synthesized as soluble proteins in the cytosol. Both SNAP-25 and Syndet/SNAP-23 are palmitoylated at cysteine residues clustered in a loop between two N- and C-terminal coils and palmitoylation is essential for membrane binding and plasma membrane targeting. The C-terminal and the N-terminal helices of SNAP-25, are each targeted to the plasma membrane by two distinct cysteine-rich domains and appear to regulate the availability of SNAP to form complexes with SNARE [].
Vesicle-associated membrane protein 5 (Vamp5) is part of the Vamp family of SNAREs (soluble NSF attachment protein receptor), whose members are proteins responsible for the last stage of docking and subsequent fusion in diverse intracellular membrane transport events [].
This entry represents a family of proteins, approximately 300 residues in length, involved in vesicle transport. They have a single C-terminal transmembrane domain and a SNARE [soluble NSF (N-ethylmaleimide-sensitive fusion protein) attachment protein receptor]domain of approximately 60 residues. The SNARE domains are essential for membrane fusion and are conserved from yeasts to humans. Use1 is one of the three protein subunits that make up the SNARE complex and it is specifically required for Golgi-endoplasmic reticulum retrograde transport [].
This family of vesicle-fusing ATPases includes NSF, an essential protein in membrane fusion events. NSF playsthe role of a chaperone by activating SNAP receptor proteins (SNAREs) so that they can participate in membrane fusion. For subsequent rounds of fusion, the SNARE complex must be disassembled, and NSF, in conjunction with SNAPs, utilises its intrinsic ATPase activity to provide a driving force that disassembles SNARE complexes [, , ].
Syntaxin-16 (STX16) belongs to the syntaxin family, which is a group of the membrane integrated proteins participating in exocytosis. Syntaxins are associated with various intracellular membrane compartments and have been implicated in various physiological processes such as axonal growth and cell division []. This entry includes STX16 from mammals and it's homologue, Tlg2, from yeasts. Human STX16 is a component of a soluble NSF attachment protein receptor (SNARE) complex (consisting of STX10, STX16, Vti1a, and VAMP3) that is required for the mannose 6-phosphate receptor transport from early endosomes to the Golgi []. Budding yeast Tlg2 is a Syntaxin-like t-SNARE that regulates membrane traffic through the endocytic system [, ].
This entry includes Sec5 from yeasts and EXOC2 from animals. In S. cerevisiae, Sec5 is an essential component of the exocyst complex, which is composed of Sec3, Sec5, Sec6, Sec8, Sec10, Sec15, Exo70 and Exo84 []. The exocyst complex tethers post-Golgi secretory vesicles to the plasma membrane before soluble NSF attachment protein receptor (SNARE)-mediated membrane fusion [].In mammals, active form (GTP-bound) of Ral GTPase interacts with EXOC2 and Exo84 and regulates the assembly interface of a full octameric exocyst complex []. Similar to yeast exocyst, the mammalian exocyst has been directly linked to post-Golgi targeting of secretory vesicles to discreet membrane sites [].
This entry represent a domain found at the N terminus of Sec5 from budding yeasts and EXOC2/VPS51 from animals.In S. cerevisiae, Sec5 is an essential component of the exocyst complex, which is composed of Sec3, Sec5, Sec6, Sec8, Sec10, Sec15, Exo70 and Exo84 []. The exocyst complex tethers post-Golgi secretory vesicles to the plasma membrane before soluble NSF attachment protein receptor (SNARE)-mediated membrane fusion [].In mammals, active form (GTP-bound) of Ral GTPase interacts with EXOC2 and Exo84 and regulates the assembly interface of a full octameric exocyst complex []. Similar to yeast exocyst, the mammalian exocyst has been directly linked to post-Golgi targeting of secretory vesicles to discreet membrane sites [].
The process of vesicular membrane fusion in eukaryotic cells depends on a conserved fusion machinery called SNARE (soluble N-ethylmaleimide-sensitive factor (NSF) attachment protein (SNAP) receptors). In the process of vesicle docking, proteins present on the vesicle (v-SNARE) have to bind to their counterpart on the target membrane (t-SNARE) to form a core complex that can then recruit the soluble proteins NSF and SNAP. This so called fusion complex can then disassemble after ATP hydrolysis mediated by the ATPase NSF in a process that leads to membrane fusion and the release of the vesicle contents. v-SNAREs include proteins homologous to synaptobrevin [, , ].Structurally the SNARE complex is generally a four-helix bundle comprised of three coiled-coil-forming domains from t-SNAREs and one fromv-SNARE. Although sequence similarity in the t- and v-SNARE coiled-coil homology domains are low there is a striking conservation of theso-called heptad repeat that is of central importance in forming a coiled-coil structure. In a coiled-coil motif, seven residues constitute a canonicalheptad and are designated 'a' through 'g', with 'a' and 'd' being occupied by hydrophobic residues. The association of the four α-helices in the SNARE fusion complex structure produces highly conserved layers of interacting amino acid side chains in the centre of the four-helix bundle. The centre of the bundle is made up of 15 hydrophobic layers from the 'a' and 'd' positions of the heptad repeats of the coiled-coil-forming domains, whereas the central 'ionic' layer is highly conserved and polar in nature, containing a glutamine residue in the three t-SNAREs and an arginine in the v-SNARE, hence the classification of v- and t-SNAREs as R- and Q-SNAREs, respectively. The v-SNARE coiled-coil homology domain is around 60 amino acids in length [, , ].The entry represents the entire v-SNARE coiled-coil homology domain.
