Syntaxin binding protein 5 (STXBP5, also called Tomosyn), as well as its relative Syntaxin binding protein 6 (STXBP6, also called Amisyn) contains a C-terminal R-SNARE-like domain, which allows it to assemble into SNARE complexes, which in turn makes the complexes inactive and inhibits exocytosis. Tomosyn contains an N-terminal WD40 repeat region and has been shown to form complexes with SNAP-25 and syntaxin 1a, as well as SNAP-23 and syntaxin 4. In general, SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) 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.
Jiyao W et al.(2023). "The conserved domain database in 2023.",