Structural insights into the cooperative remodeling of membranes by amphiphysin/BIN1

Amphiphysin2/BIN1 is a crescent-shaped N-BAR protein playing a key role in forming deeply invaginated tubes in muscle T-tubules. Amphiphysin2/BIN1 structurally stabilizes tubular formations in contrast to other N-BAR proteins involved in dynamic membrane scission processes; however, the molecular mechanism of the stabilizing effect is poorly understood. Using cryo-EM, we investigated the assembly of the amphiphysin/BIN1 on a membrane tube. We found that the N-BAR domains self-assemble on the membrane surface in a highly cooperative manner. Our biochemical assays and 3D reconstructions indicate that the N-terminal amphipathic helix Ho plays an important role in the initiation of the tube assembly and further in organizing BAR-mediated polymerization by locking adjacent N-BAR domains. Mutants that lack Ho or the tip portion, which is also involved in interactions of the neighboring BAR unit, lead to a disruption of the polymer organization, even though tubulation can still be observed. The regulatory region of amphiphysin/BIN1 including an SH3 domain does not have any apparent involvement in the polymer lattice. Our study indicates that the Ho helix and the BAR tip are necessary for efficient and organized self-assembly of amphiphysin/NBAR.