Structural basis for Vipp1 membrane binding: From loose coats and carpets to ring and rod assemblies

Vipp1 (also known as IM30) is essential in most oxygenic photoautotrophic organisms. It is involved in membrane remodeling and fusion and is critical for thylakoid membrane biogenesis and maintenance. Vipp1 has recently been identified as a member of the ESCRT-III superfamily of membrane remodeling proteins, albeit it still is elusive how Vipp1 interacts with and finally remodels membranes. Here we present a series of cryo-EM structures of cyanobacterial Vipp1 interacting with bacterial membranes: first, we solved seven structures between 5 and 7 Å resolution of three unique helical and four types of stacked-ring assemblies engulfing membranes, and, second, using sub-tomogram averaging, we determined three ∼20 Å resolution structures compatible with previously observed carpet structures at three different membrane curvatures. By analyzing ten additional unique structures of N-terminally truncated Vipp1, we could show that helix α0 is essential for membrane tubulation and forms the membrane anchoring domain of Vipp1. Using a conformation-restrained Vipp1 mutant, we were able to reduce the structural plasticity of Vipp1 assemblies in the presence of lipids and determined two structures of Vipp1 at 3.0 Å resolution, resolving the molecular details of membrane anchoring and intersubunit contacts of helix α0. Our data reveal the molecular details of how Vipp1 interacts with membranes, showing membrane curvature-dependent structural transitions from carpets to rings and rods, some of which are capable of inducing and/or stabilizing high local membrane curvature triggering membrane fusion.