Structural Basis for ESCRT-III Protein Autoinhibition

ESCRT-III (endosomal sorting complexes required for transport-III) subunits cycle between two states: soluble monomers and higher-order assemblies that bind and remodel membranes during endosomal vesicle formation, midbody abscission and enveloped virus budding. Here, we show that the N-terminal core domains of IST1 (increased sodium tolerance-1) and CHMP3 (charged multivesicularbody protein-3) form equivalent four-helix bundles, revealing that IST1 is a previously unrecognized ESCRT-III family member. IST1 and its ESCRT-III binding partner, CHMP1B, both form higher-order helical structures in vitro, and IST1-CHMP1 interactions are required for abscission. The IST1 and CHMP3 structures also reveal that equivalent downstream α5 helices can fold back against the core domains. Mutations within the CHMP3 core-α5 interface stimulate the protein’s in vitro assembly and HIV inhibition activities, indicating that dissociation of the autoinhibitory α5 helix from the core activates ESCRT-III proteins for assembly at membranes.