The Hairpin-type Tail-Anchored SNARE Syntaxin 17 Targets to Autophagosomes for Fusion with Endosomes/Lysosomes

The lysosome is a degradative organelle, and its fusion with other organelles is strictly regulated. In contrast to fusion with the late endosome, the mechanisms underlying autophagosome-lysosome fusion remain unknown. Here, we identify syntaxin 17 (Stx17) as the autophagosomal SNARE required for fusion with the endosome/lysosome. Stx17 localizes to the outer membrane of completed autophagosomes but not to the isolation membrane (unclosed intermediate structures); for this reason, the lysosome does not fuse with the isolation membrane. Stx17 interacts with SNAP-29 and the endosomal/lysosomal SNARE VAMP8. Depletion of Stx17 causes accumulation of autophagosomes without degradation. Stx17 has a unique C-terminal hairpin structure mediated by two tandem transmembrane domains containing glycine zipper-like motifs, which is essential for its association with the autophagosomal membrane. These findings reveal a mechanism by which the SNARE protein is available to the completed autophagosome. [Display omitted] ► Syntaxin 17 is present on completed autophagosomes but not isolation membranes ► Stx17 is required for autophagosomal fusion with the endosome/lysosome ► Stx17 has two tandem transmembrane domains containing glycine zipper-like motifs ► The hairpin-like transmembrane structure is required for autophagosomal targeting Syntaxin 17 localizes to the outer membrane of completed autophagosomes via its C-terminal hairpin-like structure, which consists of two tandem transmembrane domains containing glycine zipper-like motifs. Syntaxin 17, together with SNAP-29 and VAMP8, is required for fusion between the autophagosome and endosome/lysosome.