Syntaxin18 (also known as Ufe1p) is involved in retrograde transport of CopI coatomer coated vesicles from the Golgi to the ER. It forms a complex with USE1 (SLT1, Qc), Bnip1 (Sec20p, Qb) and Sec22b (R-SNARE). Syntaxin18 is a member of the Qa subfamily of SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) protein family. 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). Qc-, as well as Qa- and Qb-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.
Jiyao W et al.(2023). "The conserved domain database in 2023.",