Membrane budding and scission by the ESCRT machinery: it's all in the neck

Abstract

The endosomal sorting complexes required for transport (ESCRTs) catalyse one of the most unusual membrane remodelling events in cell biology. ESCRT-I and ESCRT-II direct membrane budding away from the cytosol by stabilizing bud necks without coating the buds and without being consumed in the buds. ESCRT-III cleaves the bud necks from their cytosolic faces. ESCRT-III-mediated membrane neck cleavage is crucial for many processes, including the biogenesis of multivesicular bodies, viral budding, cytokinesis and, probably, autophagy. Recent studies of ultrastructures induced by ESCRT-III overexpression in cells and the in vitro reconstitution of the budding and scission reactions have led to breakthroughs in understanding these remarkable membrane reactions.

Figure 1

Biological roles of the ESCRTs. a. Plasma membrane proteins such as EGFR are ubiquitylated and endocytosed following their stimulation. The initial endocytosis of ubiquitylated receptors does not require the ESCRTs. b. The early endosome is a branch point from which housekeeping receptors and other cargoes not bound for the lysosome are recycled, again not requiring the ESCRTs. c. MVB biogenesis involves the formation of membrane buds, the sorting of ubiquitylated cargo into the buds and cleavage of the buds to form ILVs. MVB biogenesis requires all of the ESCRT complexes. d. HIV-1 budding needs ESCRT-I, ESCRT-III, VPS4 and, to a lesser extent, ALIX. Other ESCRT-dependent viruses have varying requirements for specific components of the ESCRT machinery, but all require ESCRT-III and VPS4. e. Cytokinesis in animal cells requires ESCRT-I, ESCRT-III, VPS4 and ALIX, which have been shown to localize to the midbody during cytokinesis in the appropriate time and place to carry out the final cleavage of the membrane neck, separating the two daughter cells. Cytokinesis in the Crenarchaea requires homologues of ESCRT-III and Vps4. f. Autophagy requires all of the ESCRT complexes, similar to MVB biogenesis, although it is not clear whether their roles in autophagy are direct or indirect. There is an intriguing analogy between the closure of the phagophore neck and the formation of ILVs, making it tempting to speculate that the ESCRTs are involved in closing this neck. However the presence of ESCRTs at the phagophore neck has never been visualized.

Figure 2

ESCRT-I and ESCRT-II. a. Composite structures of ESCRT-I and II modelled on the basis of crystal structures of the core complexes and flexibly attached domains. Regions with known functions that bring them close to the membrane are highlighted to show that these regions are concentrated at either end of the ESCRT-I stalk and at the three tips of the Y-shaped ESCRT-II scaffold. Above the scale bar at left, Ub and ESCRT-0 bind to the UEV domain of ESCRT-I, and the N-terminal basic helix of ESCRT-I contributes to membrane binding. Above the scale bar at right, the C-terminal domain of Vps28 binds to ESCRT-II, at least in yeast. Below the scale bar at left and right, the WH2 domains of both molecules of Vps25 bind to the ESCRT-III subunit Vps20. At bottom center, the basic helix 0 of Vps22 and the GLUE domain of Vps36 contribute to membrane binding, and in yeast, the NZF inserts into the GLUE domain bind to ubiquitin and ESCRT-I. b. Schematic version of the structures.

Figure 3

ESCRT-III. a. Assembled ESCRT-III polymer is visualized in this electron micrograph showing the filaments underlying buds and tubules emerging from the top of a cell overexpressing human SNF7 and an inactive mutant of VPS4B. Image is reproduced, with permission, from. The SNF7-containing filaments are seen because of post-fixation extraction with detergent. b. Helical organization of individual ESCRT-III proteins based on the observed structure of human VPS24,. Helices α1– α4 form the core of the protein responsible for membrane binding and polymer assembly and are regulated by autoinhibitory sequences in α5 and the C-terminal MIM1 (not resolved in the CHMP3 structure). All ESCRT-III proteins contain the α1– α4 core and the α5 autoinhibitory sequence. Vps2, Did2, and Vps24 contain a C-terminal MIM1 motif, whereas Vps20 and Snf7 contain a C-terminal MIM2 motif. Ist1 has both MIM1 and MIM2 motifs at its C-terminus. Both types of MIM bind to the MIT domain of Vps4, but they do so at separate sites and do not compete with one another. c. Cycle of ESCRT-III assembly and VPS4-mediated disassembly. ESCRT-III proteins are in a `closed' and monomeric conformation in the cytosol, with intramolecular autoinhibitory interactions preventing membrane binding and polymer assembly. In their `open' conformation, these autoinhibitory interactions are released, and the subunits bind to each other and to the membrane as they assemble into functional ESCRT-III polymers. -This could be redrawn with colors that fit better w/ rest of figs

Figure 3

ESCRT-III. a. Assembled ESCRT-III polymer is visualized in this electron micrograph showing the filaments underlying buds and tubules emerging from the top of a cell overexpressing human SNF7 and an inactive mutant of VPS4B. Image is reproduced, with permission, from. The SNF7-containing filaments are seen because of post-fixation extraction with detergent. b. Helical organization of individual ESCRT-III proteins based on the observed structure of human VPS24,. Helices α1– α4 form the core of the protein responsible for membrane binding and polymer assembly and are regulated by autoinhibitory sequences in α5 and the C-terminal MIM1 (not resolved in the CHMP3 structure). All ESCRT-III proteins contain the α1– α4 core and the α5 autoinhibitory sequence. Vps2, Did2, and Vps24 contain a C-terminal MIM1 motif, whereas Vps20 and Snf7 contain a C-terminal MIM2 motif. Ist1 has both MIM1 and MIM2 motifs at its C-terminus. Both types of MIM bind to the MIT domain of Vps4, but they do so at separate sites and do not compete with one another. c. Cycle of ESCRT-III assembly and VPS4-mediated disassembly. ESCRT-III proteins are in a `closed' and monomeric conformation in the cytosol, with intramolecular autoinhibitory interactions preventing membrane binding and polymer assembly. In their `open' conformation, these autoinhibitory interactions are released, and the subunits bind to each other and to the membrane as they assemble into functional ESCRT-III polymers. -This could be redrawn with colors that fit better w/ rest of figs

Figure 3

ESCRT-III. a. Assembled ESCRT-III polymer is visualized in this electron micrograph showing the filaments underlying buds and tubules emerging from the top of a cell overexpressing human SNF7 and an inactive mutant of VPS4B. Image is reproduced, with permission, from. The SNF7-containing filaments are seen because of post-fixation extraction with detergent. b. Helical organization of individual ESCRT-III proteins based on the observed structure of human VPS24,. Helices α1– α4 form the core of the protein responsible for membrane binding and polymer assembly and are regulated by autoinhibitory sequences in α5 and the C-terminal MIM1 (not resolved in the CHMP3 structure). All ESCRT-III proteins contain the α1– α4 core and the α5 autoinhibitory sequence. Vps2, Did2, and Vps24 contain a C-terminal MIM1 motif, whereas Vps20 and Snf7 contain a C-terminal MIM2 motif. Ist1 has both MIM1 and MIM2 motifs at its C-terminus. Both types of MIM bind to the MIT domain of Vps4, but they do so at separate sites and do not compete with one another. c. Cycle of ESCRT-III assembly and VPS4-mediated disassembly. ESCRT-III proteins are in a `closed' and monomeric conformation in the cytosol, with intramolecular autoinhibitory interactions preventing membrane binding and polymer assembly. In their `open' conformation, these autoinhibitory interactions are released, and the subunits bind to each other and to the membrane as they assemble into functional ESCRT-III polymers. -This could be redrawn with colors that fit better w/ rest of figs

Figure 4

Membrane budding by ESCRTs a. Buds induced by ESCRT-I and II. Image reproduced, with permission, from. The first panel at left shows the membrane labelled with rhodamine-PE. The second and fourth panels show that ESCRT-II (Alexa-488) is localized exclusively to the neck of the bud. The third panel shows that ubiquitin (CFP) is localized throughout the bud, but with some additional concentration at the neck coinciding with ESCRT-II. The same has been found for ESCRT-I, Vps20 and Snf7 (not shown). The scale bar is 2 microns. b. Model for bud stabilization by ESCRT-I and II. In this model, the various membrane-proximal domains of ESCRT-I and –II as shown in Fig. 2 bind to one side of the rim of the bud neck. The rigid stalk of ESCRT-I and the rigid Y-shaped core of ESCRT-II stabilize the opening of the bud neck. c. Putative pathway of bud formation in vivo, derived from a series of images of fixed, negatively stained cells, and arranged into a putative time series, reproduced by permission from.

Figure 5

Membrane scission by ESCRT-III. a. Imaging of Snf7 in the act of membrane scission at a bud neck in vitro, reproduced by permission from. The merged image at top combines red (membrane, rhodamine), blue (ubiquitylated cargo, CFP) and green (Snf7, Alexa 488) channels. The lower panel shows a close-up of the membrane bud, with the arrow highlighting the localization of Snf7 to the bud neck. ESCRT-I, -II, and Vps20 are present but not labeled in this experiment. b. Cartoon for scission, inspired by spiral SNF7 structures. At top, a molecule of Vps20 initiates ESCRT-III assembly. In yeast cells, at least two copies of Vps20 are required to initiate the scission-competent assembly of ESCRT-III,. At 3 o'clock, multiple Snf7 molecules polymerize downstream of Vps20. At bottom, one or a few molecules of Vps24 and Vps2 close the spiral, with only Vps24 minimally required for scission. At 9 o'clock, Vps4 breaks up the ESCRT-III polymer at the expense of ATP hydrolysis. c. Integration of budding and scission. After assembling in the bud neck as depicted in Fig. 4, the working model postulates that the ESCRT-III spiral assembles on the bud-proximal side of ESCRT-II to carry out scission.

Figure 6

VPS4 structure and function. a. Bottom and side views of a proposed VPS4 hexamer placed into the cryo-EM reconstruction of full-length Vps4p, indicating assembly of Vps4p into a dodecamer. Reproduced with permission from. b. Working model of VPS4 function, showing delivery of ESCRT-III substrate into the enzyme's central pore following interaction between the enzyme's MIT domains and MIM motifs present at the C-terminus of an ESCRT-III protein. Cycles of ATP hydrolysis lead to movements in loops lining the pore and are proposed to physically induce conformational changes in ESCRT-III proteins that release the proteins from the assembled polymer. -Please redraw with color of cylinder matching color on Fig. 6a (replace red with purple outline and blue fill)

Figure 6

VPS4 structure and function. a. Bottom and side views of a proposed VPS4 hexamer placed into the cryo-EM reconstruction of full-length Vps4p, indicating assembly of Vps4p into a dodecamer. Reproduced with permission from. b. Working model of VPS4 function, showing delivery of ESCRT-III substrate into the enzyme's central pore following interaction between the enzyme's MIT domains and MIM motifs present at the C-terminus of an ESCRT-III protein. Cycles of ATP hydrolysis lead to movements in loops lining the pore and are proposed to physically induce conformational changes in ESCRT-III proteins that release the proteins from the assembled polymer. -Please redraw with color of cylinder matching color on Fig. 6a (replace red with purple outline and blue fill)