Syntaxin 17 cycles between the ER and ERGIC and is required to maintain the architecture of ERGIC and Golgi

Syntaxin 17 is a SNARE (soluble N-ethylmaleimide-sensitive-factor-attachment protein receptor) protein that predominantly localizes to the ER (endoplasmic reticulum) and to some extent in the ERGIC (ER-Golgi intermediate compartment). Syntaxin 17 has been suggested to function as a receptor at the ER membrane that mediates trafficking between the ER and post-ER compartments. It has a unique 33 amino acid luminal tail whose function is not known. Here we have investigated the structural requirements for localization of syntaxin 17 to the ERGIC and its role in trafficking.

Deletion analysis showed that syntaxin 17 required its cytoplasmic domain to exit the ER and localize to the ERGIC. Mutation of a conserved tyrosine residue in the cytoplasmic domain resulted in reduced localization of syntaxin 17 in the ERGIC and ER-exit sites, suggesting the presence of a tyrosine-based ER export motif. Syntaxin 17 also required its C-terminal tail to localize to the ERES (ER exit sites) and ERGIC. Knockdown of syntaxin 17 destabilized the ERGIC organization and also caused fragmentation of the Golgi complex. Syntaxin 17 showed direct interaction with transmembrane proteins p23 and p25 (cargo receptors that cycle between the ER and Golgi) with the help of its C-terminal tail. Overexpression of syntaxin 17 redistributed β-COP (β-coatomer protein) which required its C-terminal tail. Overexpression of syntaxin 17 also blocked the anterograde transport of VSVG (vesicular stomatitis virus G-protein) in the ERGIC.

We show that syntaxin 17 has a tyrosine-based motif which is required for its incorporation into COPII (coatomer protein II) vesicles, exit from the ER and localization to the ERGIC. Our results suggest that syntaxin 17 cycles between the ER and ERGIC through classical trafficking pathways involving COPII and COPI (coatomer protein I) vesicles, which requires its unique C-terminal tail. We also show that syntaxin 17 is essential for maintaining the architecture of ERGIC and Golgi.