Principles of membrane remodeling by dynamic ESCRT-III polymers
2021; Elsevier BV; Volume: 31; Issue: 10 Linguagem: Inglês
10.1016/j.tcb.2021.04.005
ISSN1879-3088
AutoresAnna-Katharina Pfitzner, Joachim Moser von Filseck, Aurélien Roux,
Tópico(s)Extraction and Separation Processes
ResumoThe endosomal protein complex required for transport-III (ESCRT-III) proteins form membrane-remodeling (hetero)polymeric filaments, but how exactly they deform and mediate the fission of lipid membranes is unknown.ESCRT-III subunits adopt different conformations that polymerize into filaments with distinct polymer architectures.Depending on their subunit composition, ESCRT-III filaments display different shape parameters (curvature, torsion, rigidity) and different interfaces for protein–protein and protein–membrane interactions.Lateral interactions of two or more strands of different ESCRT-III homo- or heterofilaments affect overall filament shape parameters.Dynamic remodeling of ESCRT-III polymers by the AAA ATPase Vps4 can lead to changes in filament shapes and membrane interaction over time, converting the filament from one composition to another. Endosomal protein complex required for transport-III (ESCRT-III) polymers are involved in many crucial cellular functions, from cell division to endosome–lysosome dynamics. As a eukaryotic membrane remodeling machinery, ESCRT-III is unique in its ability to catalyze fission of membrane necks from their luminal side and to participate in membrane remodeling processes of essentially all cellular organelles. Found in Archaea, it is also the most evolutionary ancient membrane remodeling machinery. The simple protein structure shared by all of its subunits assembles into a large variety of filament shapes, limiting our understanding of how these filaments achieve membrane remodeling. Here, we review recent findings that discovered unpredicted properties of ESCRT-III polymers, which enable us to define general principles of the mechanism by which ESCRT-III filaments remodel membranes. Endosomal protein complex required for transport-III (ESCRT-III) polymers are involved in many crucial cellular functions, from cell division to endosome–lysosome dynamics. As a eukaryotic membrane remodeling machinery, ESCRT-III is unique in its ability to catalyze fission of membrane necks from their luminal side and to participate in membrane remodeling processes of essentially all cellular organelles. Found in Archaea, it is also the most evolutionary ancient membrane remodeling machinery. The simple protein structure shared by all of its subunits assembles into a large variety of filament shapes, limiting our understanding of how these filaments achieve membrane remodeling. Here, we review recent findings that discovered unpredicted properties of ESCRT-III polymers, which enable us to define general principles of the mechanism by which ESCRT-III filaments remodel membranes. Cellular and organellar membranes of eukaryotic cells undergo membrane remodeling events, such as membrane deformation, fission, and fusion, to maintain cell compartmentalization. In an increasing number of these membrane-remodeling events, ESCRT-III has been identified as a key player for deforming and breaking membranes. Importantly, ESCRT-III has the unique ability to catalyze membrane fission from within membrane necks, a process that is essential for many cellular functions; it allows the internalization of endosomal membranes for the formation of multivesicular endosomes, it is involved in cytokinetic abscission, and it releases budded viral particles from the plasma membrane. It is also required for resealing of the nuclear envelope after cell division, and it can repair punctured cellular membranes [1.Gatta A.T. Carlton J.G. The ESCRT-machinery: closing holes and expanding roles.Curr. Opin. Cell Biol. 2019; 59: 121-132Crossref PubMed Scopus (51) Google Scholar, 2.Schöneberg J. et al.Reverse-topology membrane scission by the ESCRT proteins.Nat. Rev. Mol. Cell Biol. 2017; 18: 5-17Crossref PubMed Scopus (211) Google Scholar, 3.Vietri M. et al.The many functions of ESCRTs.Nat. Rev. Mol. Cell Biol. 2020; 21: 25-42Crossref PubMed Scopus (233) Google Scholar]. Unlike other membrane-remodeling machineries, ESCRT-III can also function with an inverse geometry, as it was reported to catalyze membrane fission from the outside of membrane necks during the release of peroxisomes, recycling endosomes, and possibly lipid droplets [4.Allison R. et al.An ESCRT-spastin interaction promotes fission of recycling tubules from the endosome.J. Cell Biol. 2013; 202: 527-543Crossref PubMed Scopus (91) Google Scholar, 5.Chang C.-L. et al.Spastin tethers lipid droplets to peroxisomes and directs fatty acid trafficking through ESCRT-III.J. Cell Biol. 2019; 218: 2583-2599Crossref PubMed Scopus (68) Google Scholar, 6.Mast F.D. et al.ESCRT-III is required for scissioning new peroxisomes from the endoplasmic reticulum.J. Cell Biol. 2018; 217: 2087-2102Crossref PubMed Scopus (30) Google Scholar]. A minimal functional unit composed of different ESCRT-III subunits and the AAA ATPase Vps4 (VPS4A/B in mammals) is involved in all ESCRT-III-mediated membrane-remodeling events. Here, we review recent findings on the structural features of ESCRT-III subunits and filaments as well as their dynamics. We will then draft the general principles of membrane remodeling by ESCRT-III. Humans have eight ESCRT-III proteins with a total of 12 isoforms named charged multivesicular body proteins (CHMPs) (names of the eight Saccharomyces cerevisiae protein homologs are noted in parentheses): CHMP1A/B (Did2), CHMP2A/B (Vps2), CHMP3 (Vps24), CHMP4A/B/C (Snf7/Vps32), CHMP5 (Vps60), CHMP6 (Vps20), CHMP7 (Chm7), and CHMP8/IST1 (Ist1). These proteins all share a core structure that is proposed to adopt different conformations. In their closed state, they form a four-helix bundle; the longer helices, α1 and α2, form a hairpin, with the shorter helices, α3 and α4, packed against the hairpin, as shown for CHMP3 [7.Bajorek M. et al.Structural basis for ESCRT-III protein autoinhibition.Nat. Struct. Mol. Biol. 2009; 16: 754-762Crossref PubMed Scopus (155) Google Scholar,8.Muzioł T. et al.Structural basis for budding by the ESCRT-III factor CHMP3.Dev. Cell. 2006; 10: 821-830Abstract Full Text Full Text PDF PubMed Scopus (178) Google Scholar] and IST1 [9.McCullough J. et al.Structure and membrane remodeling activity of ESCRT-III helical polymers.Science. 2015; 350: 1548-1551Crossref PubMed Scopus (138) Google Scholar,10.Nguyen H.C. et al.Membrane constriction and thinning by sequential ESCRT-III polymerization.Nat. Struct. Mol. Biol. 2020; 27: 392-399Crossref PubMed Scopus (19) Google Scholar] (Figure 1A ). In their open state, helices α2 and α3 merge into a single helix α2–3 while maintaining the hairpin of the closed state, hence disrupting the interaction between helix α4 and the hairpin and leading to a more elongated form, as shown for full-length human CHMP1B [9.McCullough J. et al.Structure and membrane remodeling activity of ESCRT-III helical polymers.Science. 2015; 350: 1548-1551Crossref PubMed Scopus (138) Google Scholar,10.Nguyen H.C. et al.Membrane constriction and thinning by sequential ESCRT-III polymerization.Nat. Struct. Mol. Biol. 2020; 27: 392-399Crossref PubMed Scopus (19) Google Scholar] and truncated forms of CHMP4/Shrub/Snf7 from Drosophila melanogaster [11.McMillan B.J. et al.Electrostatic interactions between elongated monomers drive filamentation of Drosophila Shrub, a metazoan ESCRT-III protein.Cell Rep. 2016; 16: 1211-1217Abstract Full Text Full Text PDF PubMed Scopus (19) Google Scholar] and S. cerevisiae [12.Tang S. et al.Structural basis for activation, assembly and membrane binding of ESCRT-III Snf7 filaments.eLife. 2015; 4e12548Crossref PubMed Scopus (74) Google Scholar] (Figure 1A). Yeast Vps24 displays an intermediate, semi-open conformation without the packing of α3 and α4 to the hairpin of the closed conformation, but also without the α2–3 merger and full elongation of the open conformation [13.Huber S.T. et al.Structure and assembly of ESCRT-III helical Vps24 filaments.Sci. Adv. 2020; 6eaba4897Crossref PubMed Scopus (10) Google Scholar] (Figure 1A). So far, Vps24 and CHMP3 are the only homologs whose structures were solved in two different conformations. Importantly, ESCRT-III proteins in all three conformations were observed to assemble into filaments; in CHMP1B/IST1 double-layered tubes, CHMP1B polymerizes through a domain swap in the open conformation, while IST1, in the closed conformation, assembles through head-to-tail interactions [9.McCullough J. et al.Structure and membrane remodeling activity of ESCRT-III helical polymers.Science. 2015; 350: 1548-1551Crossref PubMed Scopus (138) Google Scholar,10.Nguyen H.C. et al.Membrane constriction and thinning by sequential ESCRT-III polymerization.Nat. Struct. Mol. Biol. 2020; 27: 392-399Crossref PubMed Scopus (19) Google Scholar] (Figure 1B). In the semi-open conformation, Vps24 dimers are formed by a variation of the domain swap mechanism, which then assemble laterally into double-stranded helical filaments [13.Huber S.T. et al.Structure and assembly of ESCRT-III helical Vps24 filaments.Sci. Adv. 2020; 6eaba4897Crossref PubMed Scopus (10) Google Scholar] (Figure 1B). The dynamic transition between the opened and the closed conformation is required for Snf7 functionality [14.Buchkovich N.J. et al.Essential N-terminal insertion motif anchors the ESCRT-III filament during MVB vesicle formation.Dev. Cell. 2013; 27: 201-214Abstract Full Text Full Text PDF PubMed Scopus (63) Google Scholar,15.Henne W.M. et al.The endosomal sorting complex ESCRT-II mediates the assembly and architecture of ESCRT-III helices.Cell. 2012; 151: 356-371Abstract Full Text Full Text PDF PubMed Scopus (143) Google Scholar], presumably because it is in their open state that these subunits most efficiently self-assemble into flexible, highly curved filaments [12.Tang S. et al.Structural basis for activation, assembly and membrane binding of ESCRT-III Snf7 filaments.eLife. 2015; 4e12548Crossref PubMed Scopus (74) Google Scholar,16.Banjade S. et al.Electrostatic lateral interactions drive ESCRT-III heteropolymer assembly.eLife. 2019; 8e46207Crossref PubMed Google Scholar]. In the open conformation, intersubunit contacts spanning up to eight subunits are established that likely allow for a high flexibility of the formed filaments, as subunits can slide along each other and even be removed while maintaining the overall filament integrity (Figure 1B). Indeed, such polymer flexibility would open the possibility of adapting to a wide range of diverse curvatures, including highly curved filaments, by the same polymer as well as accumulating elastic energy within the filaments (Figure 1C). In support of this, the recent structures of the plastid and bacterial homologs of ESCRT-III called Vipp1 (vesicle inducing protein in plastids 1, also named IM30) and PspA, respectively, show that many intermediates from semi-open to fully open conformations can polymerize in rings of different curvatures [17.Liu J. et al.Bacterial Vipp1 and PspA are members of the ancient ESCRT-III membrane-remodelling superfamily.bioRxiv. 2020; (Published online August 14, 2020. https://doi.org/10.1101/2020.08.13.249979)Google Scholar, 18.Gupta T.K. et al.Structural basis for VIPP1 oligomerization and maintenance of thylakoid membrane integrity.bioRxiv. 2020; (Published online August 11, 2020. https://doi.org/10.1101/2020.08.11.243204)Google Scholar, 19.Junglas B. et al.PspA maintains membrane integrity by an ESCRT-like membrane remodeling activity.bioRxiv. 2020; (Published online September 23, 2020. https://doi.org/10.1101/2020.09.23.309765)Google Scholar]. By contrast, the head-to-tail arrangement of the closed conformation displays much less intersubunit contacts and can be assumed to result in a more rigid filament, which is less resistant to torsional stress (Figure 1B). Importantly, as well as their differences in flexibility, these filaments might differentiate in their ability to bind membranes, as open-conformation polymers display extended membrane-binding interfaces whereas no membrane-binding interface has yet been observed for semi-open and closed-conformation polymers (Figure 1D). In general, ESCRT-III polymers are curved and flexible, and most possess a membrane-binding interface. They can take a variety of shapes on membranes in vitro and in vivo, including rings, flat spirals, open and closed helices, as well as cones. Most intriguingly, different ESCRT-III subunits can copolymerize, and the shapes of single-subunit polymers can vary significantly from those of multisubunit polymers, even if they contain the same subunits [9.McCullough J. et al.Structure and membrane remodeling activity of ESCRT-III helical polymers.Science. 2015; 350: 1548-1551Crossref PubMed Scopus (138) Google Scholar,10.Nguyen H.C. et al.Membrane constriction and thinning by sequential ESCRT-III polymerization.Nat. Struct. Mol. Biol. 2020; 27: 392-399Crossref PubMed Scopus (19) Google Scholar,15.Henne W.M. et al.The endosomal sorting complex ESCRT-II mediates the assembly and architecture of ESCRT-III helices.Cell. 2012; 151: 356-371Abstract Full Text Full Text PDF PubMed Scopus (143) Google Scholar,20.Bertin A. et al.Human ESCRT-III polymers assemble on positively curved membranes and induce helical membrane tube formation.Nat. Commun. 2020; 11: 2663Crossref PubMed Scopus (30) Google Scholar, 21.Chiaruttini N. et al.Relaxation of loaded ESCRT-III spiral springs drives membrane deformation.Cell. 2015; 163: 866-879Abstract Full Text Full Text PDF PubMed Scopus (186) Google Scholar, 22.Effantin G. et al.ESCRT-III CHMP2A and CHMP3 form variable helical polymers in vitro and act synergistically during HIV-1 budding.Cell. Microbiol. 2013; 15: 213-226Crossref PubMed Scopus (45) Google Scholar, 23.Lata S. et al.Helical structures of ESCRT-III are disassembled by VPS4.Science. 2008; 321: 1354-1357Crossref PubMed Scopus (241) Google Scholar, 24.Moser von Filseck J. et al.Anisotropic ESCRT-III architecture governs helical membrane tube formation.Nat. Commun. 2020; 11: 1516Crossref PubMed Scopus (21) Google Scholar, 25.Shen Q.-T. et al.Structural analysis and modeling reveals new mechanisms governing ESCRT-III spiral filament assembly.J. Cell Biol. 2014; 206: 763-777Crossref PubMed Scopus (78) Google Scholar] (Figure 1C). Of note, the ESCRT-III cones are often observed in vivo in various functions [26.Guizetti J. et al.Cortical constriction during abscission involves helices of ESCRT-III-dependent filaments.Science. 2011; 331: 1616-1620Crossref PubMed Scopus (333) Google Scholar, 27.Goliand I. et al.Resolving ESCRT-III spirals at the intercellular bridge of dividing cells using 3D STORM.Cell Rep. 2018; 24: 1756-1764Abstract Full Text Full Text PDF PubMed Scopus (33) Google Scholar, 28.Cashikar A.G. et al.Structure of cellular ESCRT-III spirals and their relationship to HIV budding.eLife. 2014; 3e02184Crossref Scopus (80) Google Scholar], while in vitro, helices, rings, and spirals are more common. Reviewing those shapes raises the question of whether ESCRT-III filaments possess one or several membrane-binding interfaces [29.Chiaruttini N. Roux A. Dynamic and elastic shape transitions in curved ESCRT-III filaments.Curr. Opin. Cell Biol. 2017; 47: 126-135Crossref PubMed Scopus (26) Google Scholar]. If one considers a single filament forming a conical shape, either its membrane-binding interface changes from the bottom to the tip of the structure, or the filament acquires a torsion along its axis to keep the orientation of its membrane interface parallel to the membrane plane (Figures 1D and 2A ). Indeed, diverse filament-to-membrane orientations have been reported for different ESCRT-III filaments. These diverse orientations can explain the capacity of different ESCRT-III assemblies to bind inside or outside membrane tubes. Whereas CHMP2A and CHMP2B alone, as well as CHMP2/CHMP3 together, form helical filaments that bind membranes on their outside [23.Lata S. et al.Helical structures of ESCRT-III are disassembled by VPS4.Science. 2008; 321: 1354-1357Crossref PubMed Scopus (241) Google Scholar,30.Bodon G. et al.Charged multivesicular body protein 2B (CHMP2B) of the endosomal sorting complex required for transport-III (ESCRT-III) polymerizes into helical structures deforming the plasma membrane.J. Biol. Chem. 2011; 286: 40276-40286Abstract Full Text Full Text PDF PubMed Scopus (76) Google Scholar], the polymer–membrane interface is located on the inside of CHMP1B and CHMP1B/IST1 helices [9.McCullough J. et al.Structure and membrane remodeling activity of ESCRT-III helical polymers.Science. 2015; 350: 1548-1551Crossref PubMed Scopus (138) Google Scholar,10.Nguyen H.C. et al.Membrane constriction and thinning by sequential ESCRT-III polymerization.Nat. Struct. Mol. Biol. 2020; 27: 392-399Crossref PubMed Scopus (19) Google Scholar]. De facto, different filament-to-membrane orientations can be due to distinct membrane-binding interfaces along the polymerized subunits. Alternatively, changes of subunit organization within the polymer might establish different filament-to-membrane orientations while using the same membrane-binding interface at the level of a single subunit (e.g., τx; Figure 2A). Intriguingly, in Snf7/Vps24/Vps2 filaments, two distinct membrane-binding interfaces were reported, suggesting the possibility of multiple membrane-binding conformations within the same polymer [24.Moser von Filseck J. et al.Anisotropic ESCRT-III architecture governs helical membrane tube formation.Nat. Commun. 2020; 11: 1516Crossref PubMed Scopus (21) Google Scholar,31.Mierzwa B.E. et al.Dynamic subunit turnover in ESCRT-III assemblies is regulated by Vps4 to mediate membrane remodelling during cytokinesis.Nat. Cell Biol. 2017; 19: 787-798Crossref PubMed Scopus (119) Google Scholar]. It is thus tempting to speculate about a possible transfer of membrane-binding interfaces and/or curvature along helical ESCRT-III polymers during membrane remodeling. Such helical polymers have been observed in several sites of ESCRT-III function in vivo [26.Guizetti J. et al.Cortical constriction during abscission involves helices of ESCRT-III-dependent filaments.Science. 2011; 331: 1616-1620Crossref PubMed Scopus (333) Google Scholar, 27.Goliand I. et al.Resolving ESCRT-III spirals at the intercellular bridge of dividing cells using 3D STORM.Cell Rep. 2018; 24: 1756-1764Abstract Full Text Full Text PDF PubMed Scopus (33) Google Scholar, 28.Cashikar A.G. et al.Structure of cellular ESCRT-III spirals and their relationship to HIV budding.eLife. 2014; 3e02184Crossref Scopus (80) Google Scholar,32.Hanson P.I. et al.Plasma membrane deformation by circular arrays of ESCRT-III protein filaments.J. Cell Biol. 2008; 180: 389-402Crossref PubMed Scopus (327) Google Scholar,33.Ladinsky M.S. et al.Electron tomography of HIV-1 infection in gut-associated lymphoid tissue.PLoS Pathog. 2014; 10e1003899Crossref PubMed Scopus (34) Google Scholar]. Heteropolymeric ESCRT-III filaments are also curved and flexible with at least one membrane-binding interface (Figure 2A). Depending on the subunit composition, however, filament curvature and rigidity, as well as orientation of their membrane-binding interface, can differ between polymers. In a previous review [29.Chiaruttini N. Roux A. Dynamic and elastic shape transitions in curved ESCRT-III filaments.Curr. Opin. Cell Biol. 2017; 47: 126-135Crossref PubMed Scopus (26) Google Scholar], we explored the geometrical possibilities that arise from single-stranded polymers with these filament properties. In the following, we extend this geometrical description to explore effects and opportunities of filament bundling and lateral hetero-copolymerization on ESCRT-III filaments. Several recent studies reported the formation of double-stranded and three-stranded ESCRT-III filaments [9.McCullough J. et al.Structure and membrane remodeling activity of ESCRT-III helical polymers.Science. 2015; 350: 1548-1551Crossref PubMed Scopus (138) Google Scholar,10.Nguyen H.C. et al.Membrane constriction and thinning by sequential ESCRT-III polymerization.Nat. Struct. Mol. Biol. 2020; 27: 392-399Crossref PubMed Scopus (19) Google Scholar,16.Banjade S. et al.Electrostatic lateral interactions drive ESCRT-III heteropolymer assembly.eLife. 2019; 8e46207Crossref PubMed Google Scholar,20.Bertin A. et al.Human ESCRT-III polymers assemble on positively curved membranes and induce helical membrane tube formation.Nat. Commun. 2020; 11: 2663Crossref PubMed Scopus (30) Google Scholar,24.Moser von Filseck J. et al.Anisotropic ESCRT-III architecture governs helical membrane tube formation.Nat. Commun. 2020; 11: 1516Crossref PubMed Scopus (21) Google Scholar,31.Mierzwa B.E. et al.Dynamic subunit turnover in ESCRT-III assemblies is regulated by Vps4 to mediate membrane remodelling during cytokinesis.Nat. Cell Biol. 2017; 19: 787-798Crossref PubMed Scopus (119) Google Scholar,34.Pfitzner A.-K. et al.An ESCRT-III polymerization sequence drives membrane deformation and fission.Cell. 2020; 182: 1140-1155Abstract Full Text Full Text PDF PubMed Scopus (30) Google Scholar]. The most intuitive consequence following the transition from a single-stranded to a multistranded filament is an increase of the overall polymer rigidity. Since Snf7 spirals were shown to store elastic energy [21.Chiaruttini N. et al.Relaxation of loaded ESCRT-III spiral springs drives membrane deformation.Cell. 2015; 163: 866-879Abstract Full Text Full Text PDF PubMed Scopus (186) Google Scholar,35.Lenz M. et al.Membrane buckling induced by curved filaments.Phys. Rev. Lett. 2009; 103038101Crossref PubMed Scopus (38) Google Scholar], the formation of flat membrane-bound spirals can be best explained by a highly flexible helical polymer, which undergoes an elastic stretch when bound to membrane (see [29.Chiaruttini N. Roux A. Dynamic and elastic shape transitions in curved ESCRT-III filaments.Curr. Opin. Cell Biol. 2017; 47: 126-135Crossref PubMed Scopus (26) Google Scholar] for details; spiral under tension Figure 2B). When the rigidity of such a spiral filament is increased through the addition of another laterally bound filament, buckling into a helix with its membrane-binding interface (depicted as colored area in Figure 2B) perpendicular to the helix axis is expected. Such double-stranded helical polymers [15.Henne W.M. et al.The endosomal sorting complex ESCRT-II mediates the assembly and architecture of ESCRT-III helices.Cell. 2012; 151: 356-371Abstract Full Text Full Text PDF PubMed Scopus (143) Google Scholar,16.Banjade S. et al.Electrostatic lateral interactions drive ESCRT-III heteropolymer assembly.eLife. 2019; 8e46207Crossref PubMed Google Scholar,24.Moser von Filseck J. et al.Anisotropic ESCRT-III architecture governs helical membrane tube formation.Nat. Commun. 2020; 11: 1516Crossref PubMed Scopus (21) Google Scholar] and corresponding protein-covered helical membrane tubes have been described recently [20.Bertin A. et al.Human ESCRT-III polymers assemble on positively curved membranes and induce helical membrane tube formation.Nat. Commun. 2020; 11: 2663Crossref PubMed Scopus (30) Google Scholar,24.Moser von Filseck J. et al.Anisotropic ESCRT-III architecture governs helical membrane tube formation.Nat. Commun. 2020; 11: 1516Crossref PubMed Scopus (21) Google Scholar]. Another consequence of adding a second filament is the emergence of a ribbon-like structure, particularly in cases where lateral clustering of double-stranded filaments into groups of four, six, and higher numbers of filaments occurs [20.Bertin A. et al.Human ESCRT-III polymers assemble on positively curved membranes and induce helical membrane tube formation.Nat. Commun. 2020; 11: 2663Crossref PubMed Scopus (30) Google Scholar,24.Moser von Filseck J. et al.Anisotropic ESCRT-III architecture governs helical membrane tube formation.Nat. Commun. 2020; 11: 1516Crossref PubMed Scopus (21) Google Scholar,31.Mierzwa B.E. et al.Dynamic subunit turnover in ESCRT-III assemblies is regulated by Vps4 to mediate membrane remodelling during cytokinesis.Nat. Cell Biol. 2017; 19: 787-798Crossref PubMed Scopus (119) Google Scholar,34.Pfitzner A.-K. et al.An ESCRT-III polymerization sequence drives membrane deformation and fission.Cell. 2020; 182: 1140-1155Abstract Full Text Full Text PDF PubMed Scopus (30) Google Scholar]. A ribbon will probably bend more easily along its face and less easily along its edges. Comparing the dimensions of nonconstrained helices of Snf7/Vps24/Vps2 with their dimensions when bound to helical membrane tubes, we found that these ribbon-like filaments have different rigidities along their edge and face [24.Moser von Filseck J. et al.Anisotropic ESCRT-III architecture governs helical membrane tube formation.Nat. Commun. 2020; 11: 1516Crossref PubMed Scopus (21) Google Scholar]. This implies that the heterogeneous rigidities of the entire ESCRT-III assembly contribute to the generation of unique membrane shapes. ESCRT-III filaments, although sharing the same general architecture, were demonstrated to vary in their curvatures [9.McCullough J. et al.Structure and membrane remodeling activity of ESCRT-III helical polymers.Science. 2015; 350: 1548-1551Crossref PubMed Scopus (138) Google Scholar,10.Nguyen H.C. et al.Membrane constriction and thinning by sequential ESCRT-III polymerization.Nat. Struct. Mol. Biol. 2020; 27: 392-399Crossref PubMed Scopus (19) Google Scholar,15.Henne W.M. et al.The endosomal sorting complex ESCRT-II mediates the assembly and architecture of ESCRT-III helices.Cell. 2012; 151: 356-371Abstract Full Text Full Text PDF PubMed Scopus (143) Google Scholar,20.Bertin A. et al.Human ESCRT-III polymers assemble on positively curved membranes and induce helical membrane tube formation.Nat. Commun. 2020; 11: 2663Crossref PubMed Scopus (30) Google Scholar, 21.Chiaruttini N. et al.Relaxation of loaded ESCRT-III spiral springs drives membrane deformation.Cell. 2015; 163: 866-879Abstract Full Text Full Text PDF PubMed Scopus (186) Google Scholar, 22.Effantin G. et al.ESCRT-III CHMP2A and CHMP3 form variable helical polymers in vitro and act synergistically during HIV-1 budding.Cell. Microbiol. 2013; 15: 213-226Crossref PubMed Scopus (45) Google Scholar, 23.Lata S. et al.Helical structures of ESCRT-III are disassembled by VPS4.Science. 2008; 321: 1354-1357Crossref PubMed Scopus (241) Google Scholar, 24.Moser von Filseck J. et al.Anisotropic ESCRT-III architecture governs helical membrane tube formation.Nat. Commun. 2020; 11: 1516Crossref PubMed Scopus (21) Google Scholar, 25.Shen Q.-T. et al.Structural analysis and modeling reveals new mechanisms governing ESCRT-III spiral filament assembly.J. Cell Biol. 2014; 206: 763-777Crossref PubMed Scopus (78) Google Scholar,32.Hanson P.I. et al.Plasma membrane deformation by circular arrays of ESCRT-III protein filaments.J. Cell Biol. 2008; 180: 389-402Crossref PubMed Scopus (327) Google Scholar,36.Ghazi-Tabatabai S. et al.Structure and disassembly of filaments formed by the ESCRT-III subunit Vps24.Structure. 2008; 16: 1345-1356Abstract Full Text Full Text PDF PubMed Scopus (107) Google Scholar]. Thus, it seems conceivable that changes in curvature occur as a result of copolymerization of two filament strands (Figure 2C). Prominently, CHMP1B filaments were found to be constricted when copolymerized with IST1 [9.McCullough J. et al.Structure and membrane remodeling activity of ESCRT-III helical polymers.Science. 2015; 350: 1548-1551Crossref PubMed Scopus (138) Google Scholar,10.Nguyen H.C. et al.Membrane constriction and thinning by sequential ESCRT-III polymerization.Nat. Struct. Mol. Biol. 2020; 27: 392-399Crossref PubMed Scopus (19) Google Scholar]. Apart from increasing the curvature radial to the helix axis, copolymerization could alter the twist of the filament along the helix axis and thereby modify the helical pitch (Figure 2D,E). One example is that of flat Snf7 spirals that, following recruitment of Vps24/Vps2, transform into open helices [15.Henne W.M. et al.The endosomal sorting complex ESCRT-II mediates the assembly and architecture of ESCRT-III helices.Cell. 2012; 151: 356-371Abstract Full Text Full Text PDF PubMed Scopus (143) Google Scholar,16.Banjade S. et al.Electrostatic lateral interactions drive ESCRT-III heteropolymer assembly.eLife. 2019; 8e46207Crossref PubMed Google Scholar,24.Moser von Filseck J. et al.Anisotropic ESCRT-III architecture governs helical membrane tube formation.Nat. Commun. 2020; 11: 1516Crossref PubMed Scopus (21) Google Scholar]. Similarly, human proteins show a transition from flat CHMP4B spirals to helices following the addition of CHMP2/CHMP3 [20.Bertin A. et al.Human ESCRT-III polymers assemble on positively curved membranes and induce helical membrane tube formation.Nat. Commun. 2020; 11: 2663Crossref PubMed Scopus (30) Google Scholar,32.Hanson P.I. et al.Plasma membrane deformation by circular arrays of ESCRT-III protein filaments.J. Cell Biol. 2008; 180: 389-402Crossref PubMed Scopus (327) Google Scholar]. Likewise, recruitment of Did2/Vps2 to flat Snf7 spirals results in the formation of open helices [34.Pfitzner A.-K. et al.An ESCRT-III polymerization sequence drives membrane deformation and fission.Cell. 2020; 182: 1140-1155Abstract Full Text Full Text PDF PubMed Scopus (30) Google Scholar]. Such shape changes in response to binding of a secondary filament may occur via conformational distortion or rearrangement of subunits in the initial filament to increase its curvature or twist. In case of a change in helical pitch of a membrane-bound spiral, an increase of the filament rigidity, as discussed earlier, might simply be the underlying cause of the shape transition, since the twist of initial Snf7 filaments could be zero in the spirals due to its flexible nature which allows for formation of a spiral spring upon membrane binding of the filament (Figure 2B, spiral under tension). Addition of Vsp24/Vps2 or Vps2/Did2 would not change the twist but rather rigidify the filament such that
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