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The endosomal sorting complexes required for transport (ESCRTs) constitute hetero-oligomeric machines that mediate topologically similar membrane-sculpting processes, including cytokinesis, retroviral egress, and multivesicular body (MVB) biogenesis. Although ESCRT-III drives membrane remodeling that creates MVBs, its structure and the mechanism of vesicle formation are unclear. Using electron microscopy, we visualize an ESCRT-II:ESCRT-III supercomplex and propose how it mediates vesicle formation. We define conformational changes that activate ESCRT-III subunit Snf7 and show that it assembles into spiraling ~9 nm protofilaments on lipid monolayers. A high-content flow cytometry assay further demonstrates that mutations halting ESCRT-III assembly block ESCRT function. Strikingly, the addition of Vps24 and Vps2 transforms flat Snf7 spirals into membrane-sculpting helices. Finally, we show that ESCRT-II and ESCRT-III coassemble into ~65 nm diameter rings indicative of a cargo-sequestering supercomplex. We propose that ESCRT-III has distinct architectural stages that are modulated by ESCRT-II to mediate cargo capture and vesicle formation by ordered assembly., (Copyright © 2012 Elsevier Inc. All rights reserved.)