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SNARE motif of Vti1b-like Vti1b (vesicle transport through interaction with t-SNAREs homolog 1B) belongs to the Qb subgroup of SNAREs (soluble N-ethylmaleimide-sensitive factor attachment protein receptor). Vti1b interacts with syntaxin 7 (Qa), syntaxin 8 (Qc), and the lysosomal R-SNARE VAMP8 or VAMP7 to form the endosomal SNARE core complexes that mediate transport from the early endosomes and the MVBs (multivesicular bodies), and from the MVBs to the lysosomes, respectively. 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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