Speaker
Description
Biological membranes undergo dynamic remodeling that is essential for cellular homeostasis. BAR protein SNX9 is known to play a key role in actin-driven membrane remodeling, for example at saddle-shaped or tubular membrane necks connecting vesicular buds to the plasma membrane during endocytosis, and in the more complex membrane remodeling of membrane ruffling during macropinocytosis. SNX9 has a BAR domain that is thought to sense membrane curvature, a PX domain that is known to bind to various phosphoinositide (PI) lipids, and a SH3 domain that can trigger actin assembly by interacting with actin nucleation promoting factors (NPFs) such as N-WASP. However, it remains unclear whether and how SNX9 can sense membrane curvature and modulate actin assembly at curved membranes. In this study, we developed in vitro reconstitution assays to quantitatively characterize the dual functions of SNX9 in curvature sensing and actin assembly. We found that SNX9 can sense both tubular and saddle-shaped membranes. On tubular membranes, SNX9 has a comparable curvature sensing ability on PI(3,4)P2 and PI(4,5)P2 containing membranes. On saddle-shaped membrane necks, SNX9 has a higher enrichment at the necks with a radius of ~200 nm than at necks with ~150 nm and ~300 nm. Furthermore, we showed that SNX9 at membrane necks promotes the assembly of branched actin networks by enriching pVCA, a truncated NPF. Taken together, our results provide a comprehensive framework for SNX9-mediated actin assembly at curved membranes.
