Syntaxin 17, an ancient SNARE paralog, plays different and conserved roles in different organisms

Abstract

Mammalian syntaxin 17 (Stx17) has several functions, other than membrane fusion, including mitochondrial division, autophagosome formation and lipid droplet expansion. Different from conventional syntaxins, Stx17 has a long C-terminal hydrophobic region with a hairpin-like structure flanked by a basic amino acid-enriched C-terminal tail. Although Stx17 is one of the six ancient SNAREs and present in diverse eukaryotic organisms, it has been lost in multiple lineages during evolution. In the present study, we compared the localization and function of fly and nematode Stx17s expressed in HeLa cells with those of human Stx17. We found that fly Stx17 predominantly localizes to the cytosol and mediates autophagy, but not mitochondrial division. Nematode Stx17, on the other hand, is predominantly present in mitochondria and facilitates mitochondrial division, but is irrelevant to autophagy. These differences are likely due to different structures in the C-terminal tail. Non-participation of fly Stx17 and nematode Stx17 in mitochondrial division and autophagy, respectively, was demonstrated in individual organisms. Our results provide an insight into the evolution of Stx17 in metazoa.

MitoSNARE Assembly and Disassembly Factors Regulate Basal Autophagy and Aging in C. elegans

“…Expression of SYX-17::mCherry driven by syx-17 endogenous promoter is evident in various somatic tissues, including muscles, intestine and neuronal cells ( Figure S2A,B ). In agreement with previous studies in flies and humans, we find that the C. elegans SYX-17 colocalizes with mitochondria both in muscle and neuronal cells ( Figure 1 C and Figure S2A , respectively) [ 31 , 39 ]. Next, we generated transgenic animals expressing VAMP-7 tagged with GFP in muscle cells and found that a significant portion of VAMP-7 is found in proximity to mitochondria while it co-localizes with small TOMM-20-positive vesicles, reminiscent of mitochondrial-derived vesicles (MDVs)(TOMM-20) ( Figure 1 D) [ 31 , 40 ].…”

“…Although the presence of SYX-17 on mitochondria facilitating mitochondrial fission was shown to be conserved in nematodes, flies and mammals, its contribution to autophagy has been disputed [ 39 ]. Therefore, we sought to determine whether endogenous SYX-17 regulates basal autophagy in C. elegans.…”

“…Interestingly, we experimentally verified the mitochondrial presence of SYX-17, VAMP-7, SNB-6 and USO-1 in C. elegans ( Figure 1 C and Figure S2C ). Paradoxically, while the nematode SYX-17 heterologously expressed in HeLa cells, was present in mitochondria, it was neither colocalized with LC3 dots nor could it compensate for the absence of human STX17 regarding autophasome formation upon starvation [ 39 ]. This discrepancy could be due to the different physiology of this specific cancer cell line or to the lack of its cognate assembly and disassembly factors and needs to be further explained.…”