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3.A.31.  The Endosomal Sorting Complexes Required for Transport III (ESCRT-III) Family 

Four distinct ESCRT protein complexes have been identified, each orchestrating a discrete step in multivesicular body (MVB) vesicle formation (Babst 2011). ESCRT-0, together with flat clathrin coats, forms a protein network on endosomal membranes, capturing ubiquitinated cargo proteins and initiating their sorting into the MVB pathway. ESCRT-I binds to both ESCRT-0 and to ubiqutinated cargo proteins, suggesting that it functions as an additional cargo sorting system. ESCRT-I also interacts with ESCRT-II, which is important for downstream ESCRT-II functions, specifically in initiating ESCRT-III recruitment and assembly. ESCRT-III is comprised of several subunits, a subset of which forms linear polymers that have been implicated in cargo trapping, membrane deformation and vesicle abscission. The final step of MVB vesicle formation requires the Vps4 complex. This ATPase dissociates ESCRT-III, which is essential for the recycling of the ESCRT machinery for subsequent rounds of sorting and may also be involved in the scission of the forming vesicle (Babst 2011).

The endosomal sorting complexes required for transport (ESCRTs) catalyze reverse-topological scission from the inner face of membrane necks in HIV budding, multivesicular endosome biogenesis, cytokinesis, and other pathways. Schöneberg et al. 2018 encapsulated ESCRT-III subunits, Snf7, Vps24, and Vps2 and the AAA+ ATPase, Vps4, in giant vesicles from which membrane nanotubes reflecting the correct topology of scission could be pulled. Upon ATP release, this system generates forces within the nanotubes that leads to membrane scission in a manner dependent upon Vps4 catalytic activity and Vps4 coupling to the ESCRT-III proteins. Imaging of scission revealed Snf7 and Vps4 puncta within nanotubes whose presence followed ATP release, correlated with force generation and nanotube constriction, and preceded scission. These observations verified long-standing predictions that ATP-hydrolyzing assemblies of ESCRT-III and Vps4 sever membranes (Schöneberg et al. 2018). The reformation of sealed nuclei after cell division requires ESCRTs and LEM2, a transmembrane ESCRT adaptor (von Appen et al. 2020). The ability of LEM2 to condense on microtubules governs the activation of ESCRTs and coordinates spindle disassembly (von Appen et al. 2020).

The ESCRT III complex consists of at least 18 proteins and is required for the sorting and concentration of proteins resulting in the entry of these proteins into the invaginating vesicles of the multivesicular body (Babst et al. 2002). The sequential action of ESCRT-0, -I, and -II together with the ordered assembly of ESCRT-III links membrane invagination to cargo sorting. Membrane scission in the neck of the growing vesicle releases mature, cargo-laden vesicles into the lumen (Buchkovich et al. 2013, Adell et al. 2014). ESCRT-III is critical for late steps in MVB sorting, such as membrane invagination and final cargo sorting and recruitment of late-acting components of the sorting machinery (Adell et al. 2014). SNF7 is the most abundant ESCRT-III subunit which forms membrane-sculpting filaments with 30 Å periodicity and an exposed cationic membrane-binding surface (Tang et al. 2015). Its activation requires a prominent conformational rearrangement to expose protein-membrane and protein-protein interfaces. SNF7 filaments then form spirals that may function as spiral springs (Chiaruttini et al. 2015). The elastic expansion of compressed SNF7 spirals generates an area difference between the two sides of the membrane, and thus, curvature, which could be the origin of membrane deformation leading eventually to fission. SNF7 recruits BRO1, which in turn recruits DOA4, which deubiquitinates cargos before their enclosure within MVB vesicles (Amerik et al. 2000, Kim et al. 2005). ESCRT-III is also recruited to the nuclear envelope (NE) by integral INM proteins to surveil and clear defective nuclear pore complex (NPC) assembly intermediates to ensure the fidelity of NPC assembly (Webster et al. 2014).Vsp4 is an ATPase that provides the force generation and membrane scission by ESCRT-III (Schöneberg et al. 2018).

As noted above, the sorting of transmembrane proteins (e.g., cell surface receptors) into the multivesicular body (MVB) pathway to the lysosomal/vacuolar lumen requires the function of the ESCRT protein complexes. The soluble coiled-coil-containing proteins Vps2, Vps20, Vps24, and Snf7 are recruited from the cytoplasm to endosomal membranes where they oligomerize into a protein complex, ESCRT-III. ESCRT-III contains two functionally distinct subcomplexes. The Vps20-Snf7 subcomplex binds to the endosomal membrane, in part via the myristoyl group of Vps20. The Vps2-Vps24 subcomplex binds to the Vps20-Snf7 complex and thereby serves to recruit additional cofactors to this site of protein sorting. Evidence for a role for ESCRT-III in sorting and/or concentration of MVB cargoe has been forthcoming (Babst et al. 2002).

The ESCRT machinery drives membrane scission for diverse cellular functions that require budding away from the cytosol, including cell division and transmembrane receptor trafficking and degradation. The ESCRT machinery is also hijacked by retroviruses, such as HIV-1, to release virions from infected cells. The ESCRT machinery initiates virus abscission by scaffolding early-acting ESCRT-I within the head of the budding virus (Hoffman et al. 2019). In certaion archaea such as Sulfolobus acidocaldarius,ss the proteasome controls ESCRT-III-mediated cell division (Schöneberg et al. 2018). The reformation of sealed nuclei after cell division requires ESCRTs and LEM2, a transmembrane ESCRT adaptor (von Appen et al. 2020). The ability of LEM2 to condense on microtubules governs the activation of ESCRTs and coordinates spindle disassembly (von Appen et al. 2020).

The ESCRT III complex consists of at least 18 proteins and is required for the sorting and concentration of proteins resulting in the entry of these proteins into the invaginating vesicles of the multivesicular body (Babst et al. 2002). The sequential action of ESCRT-0, -I, and -II together with the ordered assembly of ESCRT-III links membrane invagination to cargo sorting. Membrane scission in the neck of the growing vesicle releases mature, cargo-laden vesicles into the lumen (Buchkovich et al. 2013, Adell et al. 2014). ESCRT-III is critical for late steps in MVB sorting, such as membrane invagination and final cargo sorting and recruitment of late-acting components of the sorting machinery (Adell et al. 2014). SNF7 is the most abundant ESCRT-III subunit which forms membrane-sculpting filaments with 30 Å periodicity and an exposed cationic membrane-binding surface (Tang et al. 2015). Its activation requires a prominent conformational rearrangement to expose protein-membrane and protein-protein interfaces. SNF7 filaments then form spirals that may function as spiral springs (Chiaruttini et al. 2015). The elastic expansion of compressed SNF7 spirals generates an area difference between the two sides of the membrane, and thus, curvature, which could be the origin of membrane deformation leading eventually to fission. SNF7 recruits BRO1, which in turn recruits DOA4, which deubiquitinates cargos before their enclosure within MVB vesicles (Amerik et al. 2000, Kim et al. 2005). ESCRT-III is also recruited to the nuclear envelope (NE) by integral INM proteins to surveil and clear defective nuclear pore complex (NPC) assembly intermediates to ensure the fidelity of NPC assembly (Webster et al. 2014).Vsp4 is an ATPase that provides the force generation and membrane scission by ESCRT-III (Schöneberg et al. 2018).

As noted above, the sorting of transmembrane proteins (e.g., cell surface receptors) into the multivesicular body (MVB) pathway to the lysosomal/vacuolar lumen requires the function of the ESCRT protein complexes. The soluble coiled-coil-containing proteins Vps2, Vps20, Vps24, and Snf7 are recruited from the cytoplasm to endosomal membranes where they oligomerize into a protein complex, ESCRT-III. ESCRT-III contains two functionally distinct subcomplexes. The Vps20-Snf7 subcomplex binds to the endosomal membrane, in part via the myristoyl group of Vps20. The Vps2-Vps24 subcomplex binds to the Vps20-Snf7 complex and thereby serves to recruit additional cofactors to this site of protein sorting. Evidence for a role for ESCRT-III in sorting and/or concentration of MVB cargoe has been forthcoming (Babst et al. 2002).

The ESCRT machinery drives membrane scission for diverse cellular functions that require budding away from the cytosol, including cell division and transmembrane receptor trafficking and degradation. The ESCRT machinery is also hijacked by retroviruses, such as HIV-1, to release virions from infected cells. The ESCRT machinery initiates virus abscission by scaffolding early-acting ESCRT-I within the head of the budding virus (Hoffman et al. 2019). In certaion archaea such as Sulfolobus acidocaldarius,ss the proteasome controls ESCRT-III-mediated cell division ( Adell, M.A., G.F. Vogel, M. Pakdel, M. Müller, H. Lindner, M.W. Hess, and D. Teis. (2014). Coordinated binding of Vps4 to ESCRT-III drives membrane neck constriction during MVB vesicle formation. J. Cell Biol. 205: 33-49. 24711499

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