Membrane constriction and thinning by sequential ESCRT-III polymerization

Abstract

The Endosomal Sorting Complexes Required for Transport (ESCRTs) mediate diverse membrane remodeling events. These typically require ESCRT-III proteins to stabilize negatively-curved membranes, although recent work has indicated that certain ESCRT-IIIs also participate in positive-curvature membrane shaping reactions. ESCRT-IIIs polymerize into membrane-binding filaments, but the structural basis for negative versus positive membrane remodeling by these proteins remains poorly understood. To learn how certain ESCRT-IIIs shape positively-curved membranes, we determined structures of human membrane-bound CHMP1B-only, membrane-bound CHMP1B+IST1, and IST1-only filaments by electron cryomicroscopy. Our structures show how CHMP1B first polymerizes into a single-stranded helical filament, shaping membranes into moderate-curvature tubules. Subsequently, IST1 assembles a second strand upon CHMP1B, further constricting the membrane tube and reducing its diameter nearly to the fission point. Each step of constriction, thins the underlying bilayer, lowering the barrier to membrane fission. Our structures reveal how a two-component, sequential polymerization mechanism drives membrane tubulation, constriction, and bilayer thinning.

Design principles of the ESCRT-III Vps24-Vps2 module

“…Among the different subcomplexes of ESCRTs, ESCRT-III is required in all processes, while the requirement of the other upstream complexes 0, I, and II is variable ( Henne et al, 2013 ). Our understanding of the mechanisms of ESCRT-III assembly has increased substantially in the last decade ( Henne et al, 2012 ; Shen et al, 2014 ; Cashikar et al, 2014 ; Chiaruttini et al, 2015 ; McCullough et al, 2015 ; Tang et al, 2015 ; Adell et al, 2017 ; Mierzwa et al, 2017 ; Schöneberg et al, 2018 ; Goliand et al, 2018 ; Maity et al, 2019 ; Bertin et al, 2020 ; Moser von Filseck et al, 2020 ; Nguyen et al, 2020 ; Pfitzner et al, 2020 ). However, important questions remain.…”

A Structural View on ESCRT-Mediated Abscission

“…A number of in vitro ultrastructural studies of ESCRT-III components are based on single-particle analyses by cryo-EM (Su et al, 2017;Maity et al, 2019;Bertin et al, 2020;Huber et al, 2020;Moser von Filseck et al, 2020;Nguyen et al, 2020;Pfitzner et al, 2020). While in vitro experiments provide useful structural information about the building blocks of the abscission machinery under artificial conditions, it is difficult to correlate this information with the dynamic attributes of the cytokinetic apparatus under physiological conditions.…”

“…In a study of ESCRT-III-driven shaping of positively curved membranes, the membrane-bound complex CHMP1B-IST1 was shown to constrict the underlying membrane bilayer nearly to the fission point by a two-component sequential polymerization mechanism (Nguyen et al, 2020). Spontaneous co-assembly between CHMP1 and truncated IST1 frequently resulted in conical helix shapes with decreasing diameter under low ionic strength, whereas an increasing ionic strength favored the monomeric form of the subunits (McCullough et al, 2015).…”

The ESCRT-III isoforms CHMP2A and CHMP2B display different effects on membranes upon polymerization

“…SLBs were also produced in the presence of PIP lipids (PI(4,5)P2 or PI(3-5)P3), with PIP2-SUV or PIP3-SUV lipid compositions, respectively. SLB formation was achieved in a buffer containing 150 mM KCl, 20 mM citrate pH = 4.8 [ 42 ]. Following SLB formation, CHMP proteins were injected at a concentration of 200 nM in BP buffer.…”

“…ESCRT-III proteins cycle between an inactive cytosolic state [ 18 – 20 ] and an activated state [ 21 – 23 ] leading to filamentous polymers forming spirals or helical tubular structures in vitro and in vivo [ 19 , 24 – 42 ].…”