Membrane constriction and thinning by sequential ESCRT-III polymerization

Abstract

The Endosomal Sorting Complexes Required for Transport (ESCRTs) mediate diverse membrane remodeling events. These activities typically require ESCRT-III proteins to stabilize negatively-curved membranes, although recent work has indicated that certain ESCRT-IIIs also participate in positive-curvature membrane shaping reactions. ESCRT-IIIs polymerize into membrane-binding filaments, but the structural basis for negative versus positive membrane curvature shaping by these proteins remains poorly understood. To learn how ESCRT-IIIs shape membranes, we determined structures of human membrane-bound CHMP1B-only, membrane-bound CHMP1B+IST1, and IST1-only filaments by electron cryomicroscopy. Our structures show how CHMP1B first polymerizes into a single-stranded helical filament, shaping membranes into moderate-curvature tubules. Subsequently, IST1 assembles a second strand upon the CHMP1B filament, further constricting the membrane tube and reducing its diameter nearly to the fission point. Each step of constriction, moreover, thins the underlying bilayer and lowers the barrier to membrane fission. Together, our structures reveal how a two-component, sequential polymerization mechanism drives membrane tubulation, tube constriction, and bilayer thinning.