Structural basis for ESCRT-III protein autoinhibition

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Abstract
ESCRT-III proteins play important roles in multivesicular body (MVB) formation, cytokinesis, and enveloped virus budding. The structure of Ist1, which also functions in cytokinesis and MVB sorting, reveals that it, too, is an ESCRT-III family member and suggests that this protein family uses a common mode of autoinhibition. Endosomal sorting complexes required for transport-III (ESCRT-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 increased sodium tolerance-1 (IST1) and charged multivesicular body protein-3 (CHMP3) 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.