The unique mechanism of SNX9 BAR domain for inducing membrane tubulation.

Sorting nexin 9 (SNX9) is a member of the sorting nexin family of proteins and plays a critical role in clathrin-mediated endocytosis. It has a Bin-Amphiphysin-Rvs (BAR) domain which can form a crescent-shaped homodimer structure that induces deformation of the plasma membrane. While other BAR-domain containing proteins such as amphiphysin and endophilin have an amphiphatic helix in front of the BAR domain which plays a critical role in membrane penetration, SNX9 does not. Thus, whether and how SNX9 BAR domain could induce the deformation of the plasma membrane is not clear. The present study identified the internal putative amphiphatic stretch in the 1(st) α-helix of the SNX9 BAR domain and proved that together with the N-terminal helix (H0) region, this internal putative amphiphatic stretch is critical for inducing membrane tubulation. Therefore, our study shows that SNX9 uses a unique mechanism to induce the tubulation of the plasma membrane which mediates proper membrane deformation during clathrin-mediated endocytosis.