The phagophore (also called isolation membrane) elongates and encloses a portion of cytoplasm, resulting in formation of the autophagosome. After completion of autophagosome formation, the outer autophagosomal membrane becomes ready to fuse with the lysosome for degradation of enclosed cytoplasmic materials. However, the molecular mechanism for how the fusion of completed autophagosomes with the lysosome is regulated has not been fully understood. We discovered syntaxin 17 (STX17) as an autophagosomal soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE). STX17 has a hairpin-type structure mediated by two transmembrane domains, each containing glycine zipper motifs. This unique transmembrane structure contributes to its specific localization to completed autophagosomes but not to phagophores. STX17 interacts with SNAP29 and the lysosomal SNARE VAMP8, and all of these proteins are required for autophagosome-lysosome fusion. The late recruitment of STX17 to completed autophagosomes could prevent premature fusion of the lysosome with unclosed phagophores.
Keywords: SNARE; autophagosome; glycine zipper motif; hairpin-type structure; syntaxin 17.