Syntaxin 5 (Syn5) regulates the transport from the ER to the Golgi, as well as the early/recycling endosomes to the trans-Golgi network and participates in the assembly of transitional ER and the Golgi, lipid droplet fusion, and cytokinesis. Syn5 exists in 2 isoforms, long (42 kDa) and short (35 kDa). The short form is localized in the Golgi complex, whereas the long form is additionally found in the endoplasmic reticulum (ER). The syntaxin-5 SNARE complexes, which also contain Bet1 (Qc) and either GS27 (Qb) and Sec22B (R-SNARE) or GS28 (Qb) and Ykt6 (R-SNARE), regulate the early secretory pathway of eukaryotic cells at the level of endoplasmic reticulum (ER) to Golgi transport. The syntaxin-5 SNARE complex, which also contains GS15 (Qc), GS28 (Qb) and Ykt6 (R-SNAREs) is involved in the transport from the trans-Golgi network to the cis-Golgi. Syn5 is member of the Qa subgroup of SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) proteins, which consist of coiled-coil helices (called SNARE motifs) that 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.
Jiyao W et al.(2023). "The conserved domain database in 2023.",