SNX9: An Unconventional Bar Domain Protein?

Abstract

BAR domain protein sorting nexin 9 (SNX9) has been reported to play an important role in both clathrin-mediated and clathrin-independent endocytosis. Recent cell biology studies have shown that SNX9 self-assembles at the endocytic (tubular) membrane necks and it was suggested that this assembly of SNX9 can constrict the necks. However, the mechanism underlying the recruitment of SNX9 to the endocytic neck remains unclear. Moreover, a direct observation of SNX9 constricting tubular membranes is still missing. Here, to study the recruitment of SNX9 and its mechanical effect on tubular membranes, we used an in vitro approach where highly curved membrane tubes with controlled radii are pulled from giant unilamellar vesicles in the presence of purified full-length SNX9 injected adjacent to the tubes. We demonstrated that SNX9 can sense positive membrane curvature, thus accounting for the recruitment of SNX9 to endocytic membrane necks. However, we observed that SNX9 does not constrict membrane tubes. This observation is surprising given that in vitro the BAR domain of endophilin and amphiphysin has been shown to sense membrane curvature and constrict membrane tubes, and structurally the BAR domain of SNX9 is similar to that of endophilin and amphiphysin but with a shallower curvature. Our results thus further indicate that SNX9 is not a conventional BAR domain protein.