A model illustrating ESCRT-III spiral assembly and constriction during membrane scission. Based on our findings, ESCRT-III subunits adopt a head-to-tail interacting mode to assemble into single-stranded filaments. Flexibility of the hinge region within each monomer facilitates the formation of curved spiral polymers. (top left) The carboxyl-terminal helices function as spacers between neighboring filaments. We propose that displacement of the carboxyl-terminal helices in ESCRT-III subunits away from the interface between individual filaments within spirals results in their compaction and a concomitant release of free energy that may contribute to membrane scission.