N-terminal SNARE motifs of SNAP25 and SNAP23, members of the Qb/Qc subfamily of SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) proteins. SNAP23 interacts with STX4 (Qa) and the lysosomal R-SNARE VAMP8. The complex plays a role in transport of secretory granule from trans-Golgi network to the plasma membrane. SNAP25 interacts with Syntaxin-1 (Qa) and the R-SNARE VAMP2 (also called synaptobrevin-2). The complex plays a role in transport of secretory granule from trans-Golgi network to the plasma membrane. Qb/Qc SNAREs consist of 2 coiled-coil helices (called SNARE motifs, one belonging to the Qb subgroup and one belonging to the Qc subgroup), which mediate the interactions with other SNARE proteins, and a transmembrane domain. In general, the SNARE complex mediates membrane fusion, important for trafficking of newly synthesized proteins, recycling of pre-existing proteins and organelle formation. SNARE proteins are classified into four groups, Qa-, Qb-, Qc- and R-SNAREs, depending on whether the residue in the hydrophilic center layer of the four-helical bundle is a glutamine (Q) or arginine (R). Qa-, as well as Qb- and Qc-SNAREs, are localized to target organelle membranes, while R-SNARE is localized to vesicle membranes. They form unique complexes consisting of one member of each subgroup, that mediate fusion between a specific type of vesicles and their target organelle. Their SNARE motifs form twisted and parallel heterotetrameric helix bundles. Other members of the Qb/Qc SNAREs are SNAP29, SNAP47 and SEC9.
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