STX17: an ancient SNARE protein whose roles have not been conserved

Mammalian STX17 (syntaxin 17) plays different cellular roles, including in mitochondrial fission, lipid droplet expansion and macroautophagy/autophagy, depending on the nutritional status. STX17 has a long C-terminal hydrophobic domain (CHD) with a hairpin-like structure, flanked by a basic amino acid-enriched C-terminal tail (C-tail). STX17 is present in a wide variety of eukaryotes, but has been lost in several lineages during evolution. Moreover, the structure of its C-tail remarkably varies, although the CHD is highly conserved. Recently, we compared the localization and function of fly and nematode Syx17/SYX-17 proteins expressed in mammalian cells with that of human STX17. Fly Syx17 expressed in mammalian cells localizes almost exclusively to the cytosol and translocates to autophagosomes upon starvation, whereas nematode SYX-17 is mainly distributed to mitochondria and promotes mitochondrial fission, but does not participate in autophagy. In vivo experiments showed that fly and nematode STX17 homologs are not involved in mitochondrial fission and autophagy, respectively. These results provide important insights into the localization and function of STX17, which acts as a molecular sensor for different nutritional conditions.