class="head no_bottom_margin" id="sec1title">IntroductionESCRT-III (endosomal sorting complex required for transport) has been implicated in the formation of intralumenal vesicles (ILVs) during biogenesis of multi-vesicular bodies (MVBs) by genetic (, ) and biochemical assays (, , , , ). ESCRT-III budding occurs in an opposite direction than in endocytosis: the limiting membrane is pushed outward from the cytoplasm instead of curving inward. ESCRT-III has been proposed to play a role in membrane deformation () and fission of ILVs (). Consistent with this, ESCRT-III is also required for geometrically similar fission reactions such as viral budding () and abscission during cytokinesis (, , ). ESCRT-III nucleation is promoted by ESCRT-II and its disassembly by the ATPase Vps4 ().It is unclear how ESCRT-III deforms lipid membranes. Because of their polymerization abilities, ESCRT-III proteins (Vps20, Snf7, Vps2, Vps24) have been proposed to generate membrane curvature by scaffolding (, href="#bib13" rid="bib13" class=" bibr popnode">Fabrikant et al., 2009, href="#bib15" rid="bib15" class=" bibr popnode">Hanson et al., 2008, href="#bib20" rid="bib20" class=" bibr popnode">Lata et al., 2008). In this mode, polymers coating the membrane usually adopt a single specific shape, or, at least, a set of geometrically similar shapes. ESCRT-III filaments adopt instead a wide variety of shapes in vivo and in vitro: concentric circles, rings, spirals, helices, or linear filaments have been observed (href="#bib15" rid="bib15" class=" bibr popnode">Hanson et al., 2008, href="#bib16" rid="bib16" class=" bibr popnode">Henne et al., 2012, href="#bib26" rid="bib26" class=" bibr popnode">Pires et al., 2009). Furthermore, no unique shape for the assembly of ESCRT-III proteins arises from the molecular structure of ESCRT-III proteins (href="#bib24" rid="bib24" class=" bibr popnode">McCullough et al., 2013). Instead, curvature could be generated by other mechanisms: for example, it has been proposed that the amphipathic insertion of the N-terminal part of Snf7 could participate in the generation of membrane curvature (href="#bib4" rid="bib4" class=" bibr popnode">Buchkovich et al., 2013). We were thus interested in studying how ESCRT-III polymerization could drive membrane curvature.
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