Friction-driven membrane scission by the human ESCRT-III proteins CHMP1B and IST1
The endosomal sorting complexes required for transport (ESCRT) system is an ancient and ubiquitous membrane scission machinery that catalyzes the budding and scission of membranes. ESCRT-mediated scission events, exemplified by those involved in the budding of HIV-1, are usually directed away from the cytosol ( reverse-topology), but they can also be directed towards the cytosol ( normal-topology). Of the ESCRT complexes 0-III, ESCRT-III is most directly implicated in membrane severing. Various subunits of ESCRT-III recruit the AAA+ ATPase VPS4, which is essential for ESCRT disassembly and reverse topology membrane scission. The ESCRT-III subunits CHMP1B and IST1 can coat and constrict positively curved membrane tubes, suggesting that these subunits could catalyze normal topology membrane severing, perhaps in conjunction with a AAA+ ATPase. CHMP1B and IST1 bind and recruit the microtubule-severing AAA+ ATPase spastin, a close relative of VPS4, suggesting that spastin could have a VPS4-like role in normal topology membrane scission. In order to determine whether CHMP1B and IST1 are capable of membrane severing on their own or in concert with VPS4 or spastin, we sought to reconstitute the process in vitro using membrane nanotubes pulled from giant unilamellar vesicles (GUVs) using an optical trap. CHMP1B and IST1 copolymerize on membrane nanotubes, forming stable scaffolds that constrict the tubes, but do not, on their own, lead to scission. However, CHMP1B-IST1-scaffolded tubes were severed when an additional extensional force was applied, consistent with a friction-driven scission mechanism. Spastin colocalized with CHMP1B enriched sites but did not disassemble the CHMP1B-IST1 coat from the membrane. VPS4 resolubilized CHMP1B and IST1 but did not lead to scission. These data show that the CHMP1B and IST1 tubular coat contributes to membrane scission. Constriction alone is insufficient for scission. However, the dynamical extension of the coated tube does lead to scission. Finally, we find that in the normal topology setting analyzed here, scission is independent of VPS4 and spastin. These observations show that the CHMP1B-IST1 ESCRT-III combination is capable of severing membranes by a friction-driven mechanism.