Vti1a (vesicle transport through interaction with t-SNAREs homolog 1A) belongs to the Qb subgroup of SNAREs (soluble N-ethylmaleimide-sensitive factor attachment protein receptor). Vti1a interacts with syntaxin 16 (Qa), syntaxin 6 (Qc), and the lysosomal R-SNARE VAMP4 to form an endosomal SNARE core complex that mediates transport from the early endosomes to the TGN (trans-Golgi network). SNARE proteins consist of coiled-coil helices (called SNARE motifs) which mediate the interactions between SNARE proteins, and a transmembrane domain. 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. Examples for members of the Qb SNAREs are N-terminal domains of SNAP23 and SNAP25, Vti1, Sec20 and GS27.
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