Syntaxin 18, a SNAP receptor that functions in the endoplasmic reticulum, intermediate compartment, and cis-Golgi vesicle trafficking

Members of the syntaxin family are target-soluble N-ethylmaleimide-sensitiv e factor-attachment protein receptors involved in vesicle docking and/or fu sion within the exocytic and endocytotic pathways. By using the yeast two-h ybrid system, we have identified a novel member of the syntaxin family, syn taxin 18, that binds to alpha-soluble N-ethylmaleimide-sensitive factor-att achment protein. Subcellular fractionation and immunocytochemical analysis revealed that syntaxin 18 is principally located in the endoplasmic reticul um, We examined the effect of overexpression of FLAG-tagged syntaxin 18 and a mutant lacking the N-terminal 81 amino acid residues on protein transpor t and organelles in the early secretory pathway. Both expressed proteins lo calized to the endoplasmic reticulum, and the expressed FLAG-syntaxin 18 ca used remarkable aggregation of endoplasmic reticulum membranes. Although ex pression of the FLAG-syntaxin 18 lacking the N-terminal region produced les s effect on the morphology of the endoplasmic reticulum, dispersion of the endoplasmic reticulum-Golgi intermediate compartment and cis-Golgi was elic ited. Moreover, overexpression of the FLAG-syntaxin 18 mutant inhibited pro tein export from the endoplasmic reticulum, These results taken together su ggest that syntaxin 18 functions in transport between the endoplasmic retic ulum and Golgi.