Structural and mechanistic studies of VPS4 proteins

The ESCRT machinery: a cellular apparatus for sorting and scission

The ESCRT (endosomal sorting complex required for transport) machinery is a group of multisubunit protein complexes conserved across phyla that are involved in a range of diverse cellular processes. ESCRT proteins regulate the biogenesis of MVBs (multivesicular bodies) and the sorting of ubiquitinated cargos on to ILVs (intraluminal vesicles) within these MVBs. These proteins are also recruited to sites of retroviral particle assembly, where they provide an activity that allows release of these retroviruses. More recently, these proteins have been shown to be recruited to the intracellular bridge linking daughter cells at the end of mitosis, where they act to ensure the separation of these cells through the process of cytokinesis. Although these cellular processes are diverse, they share a requirement for a topologically unique membrane-fission step for their completion. Current models suggest that the ESCRT machinery catalyses this membrane fission.

Divergent pathways lead to ESCRT-III-catalyzed membrane fission

Endosomal sorting complexes required for transport (ESCRT) have been implicated in topologically similar but diverse cellular and pathological processes including multivesicular body (MVB) biogenesis, cytokinesis and enveloped virus budding. Although receptor sorting at the endosomal membrane producing MVBs employs the regulated assembly of ESCRT-0 followed by ESCRT-I, -II, -III and the vacuolar protein sorting (VPS)4 complex, other ESCRT-catalyzed processes require only a subset of complexes which commonly includes ESCRT-III and VPS4. Recent progress has shed light on the pathway of ESCRT assembly and highlights the separation of tasks of different ESCRT complexes and associated partners. The emerging picture suggests that among all ESCRT-catalyzed processes, divergent pathways lead to ESCRT-III assembly within the neck of a budding structure catalyzing membrane fission.

Autolytic activity of human calpain 7 is enhanced by ESCRT-III-related protein IST1 through MIT-MIM interaction

Calpain 7, a mammalian ortholog of yeast Cpl1/Rim13 and fungal PalB, is an atypical calpain that lacks a penta-EF-hand domain. Previously, we reported that a region containing a tandem repeat of microtubule-interacting and transport (MIT) domains in calpain 7 interacts with a subset of endosomal sorting complex required for transport (ESCRT)-III-related proteins, suggesting involvement of calpain 7 in the ESCRT system. Although yeast and fungal calpains are thought to be involved in alkaline adaptation via limited proteolysis of specific transcription factors, proteolytic activity of calpain 7 has not been demonstrated yet. In this study, we investigated the interaction between calpain 7 and a newly reported ESCRT-III family member, increased sodium tolerance-1 (IST1), which possesses two different types of MIT-interacting motifs (MIM1 and MIM2). We found that glutathione-S-transferase (GST)-fused tandem MIT domains of calpain 7 (calpain 7MIT) pulled down FLAG-tagged IST1 expressed in HEK293T cells. Coimmunoprecipitation assays with various deletion or point mutants of epitope-tagged calpain 7 and IST1 revealed that both repetitive MIT domains and MIMs are required for efficient interaction. Direct MIT-MIM binding was confirmed by a pulldown experiment with GST-fused IST1 MIM and purified recombinant calpain 7MIT. Furthermore, we found that the GST-MIM protein enhances the autolysis of purified Strep-tagged monomeric green fluorescent protein (mGFP)-fused calpain 7 (mGFP-calpain 7-Strep). The autolysis was almost completely abolished by 10 mmN-ethylmaleimide but only partially inhibited by 1 mm leupeptin or E-64. The putative catalytic Cys290-substituted mutant (mGFP-calpain 7(C290S)-Strep) showed no autolytic activity. These results demonstrate for the first time that human calpain 7 is proteolytically active, and imply that calpain 7 is activated in the ESCRT system.

Activation of human VPS4A by ESCRT-III proteins reveals ability of substrates to relieve enzyme autoinhibition

VPS4 proteins are AAA(+) ATPases required to form multivesicular bodies, release viral particles, and complete cytokinesis. They act by disassembling ESCRT-III heteropolymers during or after their proposed function in membrane scission. Here we show that purified human VPS4A is essentially inactive but can be stimulated to hydrolyze ATP by ESCRT-III proteins in a reaction that requires both their previously defined MIT interacting motifs and ∼50 amino acids of the adjacent sequence. Importantly, C-terminal fragments of all ESCRT-III proteins tested, including CHMP2A, CHMP1B, CHMP3, CHMP4A, CHMP6, and CHMP5, activated VPS4A suggesting that it disassembles ESCRT-III heteropolymers by affecting each component protein. VPS4A is thought to act as a ring-shaped cylindrical oligomer like other AAA(+) ATPases, but this has been difficult to directly demonstrate. We found that concentrating His(6)-VPS4A on liposomes containing Ni(2+)-nitrilotriacetic acid-tagged lipid increased ATP hydrolysis, confirming the importance of inter-subunit interactions for activity. We also found that mutating pore loops expected to line the center of a cylindrical oligomer changed the response of VPS4A to ESCRT-III proteins. Based on these data, we propose that ESCRT-III proteins facilitate assembly of functional but transient VPS4A oligomers and interact with sequences inside the pore of the assembled enzyme. Deleting the N-terminal MIT domain and adjacent linker from VPS4A increased both basal and liposome-enhanced ATPase activity, indicating that these elements play a role in autoinhibiting VPS4A until it encounters ESCRT-III proteins. These findings reveal new ways in which VPS4 activity is regulated and specifically directed to ESCRT-III polymers.

Structural role of the Vps4-Vta1 interface in ESCRT-III recycling

The ESCRT complexes are required for multivesicular body biogenesis, macroautophagy, cytokinesis, and the budding of HIV-1. The final step in the ESCRT cycle is the disassembly of the ESCRT-III lattice by the AAA+ ATPase Vps4. Vps4 assembles on its membrane-bound ESCRT-III substrate with its cofactor, Vta1. The crystal structure of the dimeric VSL domain of yeast Vta1 with the small ATPase and the betadomains of Vps4 was determined. Residues involved in structural interactions are conserved and are required for binding in vitro and for Cps1 sorting in vivo. Modeling of the Vta1 complex in complex with the lower hexameric ring of Vps4 indicates that the two-fold axis of the Vta1 VSL domain is parallel to within approximately 20 degrees of the six-fold axis of the hexamer. This suggests that Vta1 might not crosslink the two hexameric rings of Vps4, but rather stabilizes an array of Vps4-Vta1 complexes for ESCRT-III disassembly.

Coordination of substrate binding and ATP hydrolysis in Vps4-mediated ESCRT-III disassembly

Vps4 disassembly of ESCRT-III plays an important role in MVB sorting, viral budding, and cytokinesis. An in vitro system was developed to investigate this process. These studies revealed new insights into the mechanisms of Vps4 function.

ESCRT-III undergoes dynamic assembly and disassembly to facilitate membrane exvagination processes including multivesicular body (MVB) formation, enveloped virus budding, and membrane abscission during cytokinesis. The AAA-ATPase Vps4 is required for ESCRT-III disassembly, however the coordination of Vps4 ATP hydrolysis with ESCRT-III binding and disassembly is not understood. Vps4 ATP hydrolysis has been proposed to execute ESCRT-III disassembly as either a stable oligomer or an unstable oligomer whose dissociation drives ESCRT-III disassembly. An in vitro ESCRT-III disassembly assay was developed to analyze Vps4 function during this process. The studies presented here support a model in which Vps4 acts as a stable oligomer during ATP hydrolysis and ESCRT-III disassembly. Moreover, Vps4 oligomer binding to ESCRT-III induces coordination of ATP hydrolysis at the level of individual Vps4 subunits. These results suggest that Vps4 functions as a stable oligomer that acts upon individual ESCRT-III subunits to facilitate ESCRT-III disassembly.

The ESCRT complexes

The ESCRT machinery consists of the peripheral membrane protein complexes ESCRT-0, -I, -II, -III, and Vps4-Vta1, and the ALIX homodimer. The ESCRT system is required for degradation of unneeded or dangerous plasma membrane proteins; biogenesis of the lysosome and the yeast vacuole; the budding of most membrane enveloped viruses; the membrane abscission step in cytokinesis; macroautophagy; and several other processes. From their initial discovery in 2001-2002, the literature on ESCRTs has grown exponentially. This review will describe the structure and function of the six complexes noted above and summarize current knowledge of their mechanistic roles in cellular pathways and in disease.

The M-domain controls Hsp104 protein remodeling activity in an Hsp70/Hsp40-dependent manner

Yeast Hsp104 is a ring-forming ATP-dependent protein disaggregase that, together with the cognate Hsp70 chaperone system, has the remarkable ability to rescue stress-damaged proteins from a previously aggregated state. Both upstream and downstream functions for the Hsp70 system have been reported, but it remains unclear how Hsp70/Hsp40 is coupled to Hsp104 protein remodeling activity. Hsp104 is a multidomain protein that possesses an N-terminal domain, an M-domain, and two tandem AAA(+) domains. The M-domain forms an 85-A long coiled coil and is a hallmark of the Hsp104 chaperone family. While the three-dimensional structure of Hsp104 has been determined, the function of the M-domain is unclear. Here, we demonstrate that the M-domain is essential for protein disaggregation, but dispensable for Hsp104 ATPase- and substrate-translocating activities. Remarkably, replacing the Hsp104 M-domain with that of bacterial ClpB, and vice versa, switches species specificity so that our chimeras now cooperate with the noncognate Hsp70/DnaK chaperone system. Our results demonstrate that the M-domain controls Hsp104 protein remodeling activities in an Hsp70/Hsp40-dependent manner, which is required to unleash Hsp104 protein disaggregating activity.

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

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.

ESCRT & Co

Components of the ESCRT (endosomal sorting complex required for transport) machinery mediate endosomal sorting of ubiquitinated membrane proteins. They are key regulators of biological processes important for cell growth and survival, such as growth-factor-mediated signalling and cytokinesis. In addition, enveloped viruses, such as HIV-1, hijack and utilize the ESCRTs for budding during virus release and infection. Obviously, the ESCRT-facilitated pathways require tight regulation, which is partly mediated by a group of interacting proteins, for which our knowledge is growing. In this review we discuss the different ESCRT-modulating proteins and how they influence ESCRT-dependent processes, for example, by acting as positive or negative regulators or by providing temporal and spatial control. A number of the interactors influence the classical ESCRT-mediated process of endosomal cargo sorting, for example, by modulating the interaction between ubiquitinated cargo and the ESCRTs. Certain accessory proteins have been implicated in regulating the activity or steady-state expression levels of the ESCRT components, whereas other interactors control the cellular localization of the ESCRTs, for example, by inducing shuttling between cytosol and nucleus or endosomes. In conclusion, the discovery of novel interactors has and will extend our knowledge of the biological roles of ESCRTs.

Assembly of the AAA ATPase Vps4 on ESCRT-III

A complex network of interactions mediates the recruitment of Vps4 to ESCRT-III and its subsequent assembly, two key steps in the ESCRT-dependent vesicle formation at the endosome. A model is presented depicting the order of events that lead to active, ESCRT-III–associated Vps4.

Vps4 is a key enzyme that functions in endosomal protein trafficking, cytokinesis, and retroviral budding. Vps4 activity is regulated by its recruitment from the cytoplasm to ESCRT-III, where the protein oligomerizes into an active ATPase. The recruitment and oligomerization steps are mediated by a complex network of at least 12 distinct interactions between Vps4, ESCRT-III, Ist1, Vta1, and Did2. The order of events leading to active, ESCRT-III–associated Vps4 is poorly understood. In this study we present a systematic in vivo analysis of the Vps4 interaction network. The data demonstrated a high degree of redundancy in the network. Although no single interaction was found to be essential for the localization or activity of Vps4, certain interactions proved more important than others. The most significant among these were the binding of Vps4 to Vta1 and to the ESCRT-III subunits Vps2 and Snf7. In our model we propose the formation of a recruitment complex in the cytoplasm that is composed of Did2-Ist1-Vps4, which upon binding to ESCRT-III recruits Vta1. Vta1 in turn is predicted to cause a rearrangement of the Vps4 interactions that initiates the assembly of the active Vps4 oligomer.

Microtubule-severing enzymes

In 1993, an enzyme with an ATP-dependent microtubule-severing activity was purified from sea urchin eggs and named katanin, after the Japanese word for sword. Now we know that katanin, spastin, and fidgetin form a family of closely related microtubule-severing enzymes that is widely distributed in eukaryotes ranging from Tetrahymena and Chlamydomonas to humans. Here we review the diverse in vivo functions of these proteins and the recent significant advances in deciphering the biophysical mechanism of microtubule severing.

A genome-wide deletion mutant screen identifies pathways affected by nickel sulfate in Saccharomyces cerevisiae

Background

The understanding of the biological function, regulation, and cellular interactions of the yeast genome and proteome, along with the high conservation in gene function found between yeast genes and their human homologues, has allowed for Saccharomyces cerevisiae to be used as a model organism to deduce biological processes in human cells. Here, we have completed a systematic screen of the entire set of 4,733 haploid S. cerevisiae gene deletion strains (the entire set of nonessential genes for this organism) to identify gene products that modulate cellular toxicity to nickel sulfate (NiSO₄).

Results

We have identified 149 genes whose gene deletion causes sensitivity to NiSO₄ and 119 genes whose gene deletion confers resistance. Pathways analysis with proteins whose absence renders cells sensitive and resistant to nickel identified a wide range of cellular processes engaged in the toxicity of S. cerevisiae to NiSO₄. Functional categories overrepresented with proteins whose absence renders cells sensitive to NiSO₄ include homeostasis of protons, cation transport, transport ATPases, endocytosis, siderophore-iron transport, homeostasis of metal ions, and the diphthamide biosynthesis pathway. Functional categories overrepresented with proteins whose absence renders cells resistant to nickel include functioning and transport of the vacuole and lysosome, protein targeting, sorting, and translocation, intra-Golgi transport, regulation of C-compound and carbohydrate metabolism, transcriptional repression, and chromosome segregation/division. Interactome analysis mapped seven nickel toxicity modulating and ten nickel-resistance networks. Additionally, we studied the degree of sensitivity or resistance of the 111 nickel-sensitive and 72 -resistant strains whose gene deletion product has a similar protein in human cells.

Conclusion

We have undertaken a whole genome approach in order to further understand the mechanism(s) regulating the cell's toxicity to nickel compounds. We have used computational methods to integrate the data and generate global models of the yeast's cellular response to NiSO₄. The results of our study shed light on molecular pathways associated with the cellular response of eukaryotic cells to nickel compounds and provide potential implications for further understanding the toxic effects of nickel compounds to human cells.

Dissecting the N-ethylmaleimide-sensitive factor: required elements of the N and D1 domains

N-Ethylmaleimide-sensitive factor (NSF) is a homo-hexameric member of the AAA(+) (ATPases associated with various cellular activities plus) family. It plays an essential role in most intracellular membrane trafficking through its binding to and disassembly of soluble NSF attachment protein (SNAP) receptor (SNARE) complexes. Each NSF protomer contains an N-terminal domain (NSF-N) and two AAA domains, a catalytic NSF-D1 and a structural NSF-D2. This study presents detailed mutagenesis analyses of NSF-N and NSF-D1, dissecting their roles in ATP hydrolysis, SNAP.SNARE binding, and complex disassembly. Our results show that a positively charged surface on NSF-N, bounded by Arg(67) and Lys(105), and the conserved residues in the central pore of NSF-D1 (Tyr(296) and Gly(298)) are involved in SNAP.SNARE binding but not basal ATP hydrolysis. Mutagenesis of Sensor 1 (Thr(373)-Arg(375)), Sensor 2 (Glu(440)-Glu(442)), and Arginine Fingers (Arg(385) and Arg(388)) in NSF-D1 shows that each region plays a discrete role. Sensor 1 is important for basal ATPase activity and nucleotide binding. Sensor 2 plays a role in ATP- and SNAP-dependent SNARE complex binding and disassembly but does so in cis and not through inter-protomer interactions. Arginine Fingers are important for SNAP.SNARE complex-stimulated ATPase activity and complex disassembly. Mutants at these residues have a dominant-negative phenotype in cells, suggesting that Arginine Fingers function in trans via inter-protomer interactions. Taken together, these data establish functional roles for many of the structural elements of the N domain and of the D1 ATP-binding site of NSF.

No strings attached: the ESCRT machinery in viral budding and cytokinesis

Since the initial discovery of the endosomal sorting complex required for transport (ESCRT) pathway, research in this field has exploded. ESCRT proteins are part of the endosomal trafficking system and play a crucial role in the biogenesis of multivesicular bodies by functioning in the formation of vesicles that bud away from the cytoplasm. Subsequently, a surprising role for ESCRT proteins was defined in the budding step of some enveloped retroviruses, including HIV-1. ESCRT proteins are also employed in this outward budding process, which results in the resolution of a membranous tether between the host cell and the budding virus particle. Remarkably, it has recently been described that ESCRT proteins also have a role in the topologically equivalent process of cell division. In the same way that viral particles recruit the ESCRT proteins to the site of viral budding, ESCRT proteins are also recruited to the midbody - the site of release of daughter cell from mother cell during cytokinesis. In this Commentary, we describe recent advances in the understanding of ESCRT proteins and how they act to mediate these diverse processes.

Ancient ESCRTs and the evolution of binary fission

Eukaryotic and prokaryotic orthologs of tubulin play key roles in DNA segregation and cell division processes. Remarkably, recent studies have revealed that cell division can occur in the absence of this highly conserved protein. Members of the hyperthermophilic crenarchaea, that lack tubulin-like proteins, undergo division by binary fission. Here we review how this process is dependent on archaeal homologs of the eukaryotic 'endosomal sorting complex required for transport' (ESCRT) system - an apparatus that plays a pivotal role in a wide range of membrane manipulation processes. Thus, two distinct machineries to drive binary fission have evolved in prokaryotes - one dependent on tubulin-like proteins and one dependent on the ESCRT system.

Cell biology of the ESCRT machinery

The ESCRT (endosomal sorting complex required for transport) machinery comprises a set of protein complexes that regulate sorting and trafficking into multivesicular bodies en route to the lysosome. The physical mechanism responsible for generating lumenal vesicles in this pathway is unknown. Here we review recent studies suggesting that components of the ESCRT-III complex drive lumenal vesicle formation and consider possible mechanisms for this reaction.

Three-dimensional structure of AAA ATPase Vps4: advancing structural insights into the mechanisms of endosomal sorting and enveloped virus budding

Vps4 is a AAA ATPase that mediates endosomal membrane protein sorting. It is also a host factor hijacked by a diverse set of clinically important viruses, including HIV and Ebola, to facilitate viral budding. Here we present the three-dimensional structure of the hydrolysis-defective Vps4p(E233Q) mutant. Single-particle analysis, multiangle laser light scattering, and the docking of independently determined atomic models of Vps4 monomers reveal a complex with C6 point symmetry, distinguishing between a range of previously suggested oligomeric states (8-14 subunits). The 3D reconstruction also reveals a tail-to-tail subunit organization between the two rings of the complex and identifies the location of domains critical to complex assembly and interaction with partner proteins. Our refined Vps4 structure is better supported by independent lines of evidence than those previously proposed, and provides insights into the mechanism of endosomal membrane protein sorting and viral envelope budding.

The ESCRT-related CHMP1A and B proteins mediate multivesicular body sorting of auxin carriers in Arabidopsis and are required for plant development

Plasma membrane proteins internalized by endocytosis and targeted for degradation are sorted into lumenal vesicles of multivesicular bodies (MVBs) by the endosomal sorting complexes required for transport (ESCRT) machinery. Here, we show that the Arabidopsis thaliana ESCRT-related CHARGED MULTIVESICULAR BODY PROTEIN/CHROMATIN MODIFYING PROTEIN1A (CHMP1A) and CHMP1B proteins are essential for embryo and seedling development. Double homozygous chmp1a chmp1b mutant embryos showed limited polar differentiation and failed to establish bilateral symmetry. Mutant seedlings show disorganized apical meristems and rudimentary true leaves with clustered stomata and abnormal vein patterns. Mutant embryos failed to establish normal auxin gradients. Three proteins involved in auxin transport, PINFORMED1 (PIN1), PIN2, and AUXIN-RESISTANT1 (AUX1) mislocalized to the vacuolar membrane of the mutant. PIN1 was detected in MVB lumenal vesicles of control cells but remained in the limiting membrane of chmp1a chmp1b MVBs. The chmp1a chmp1b mutant forms significantly fewer MVB lumenal vesicles than the wild type. Furthermore, CHMP1A interacts in vitro with the ESCRT-related proteins At SKD1 and At LIP5. Thus, Arabidopsis CHMP1A and B are ESCRT-related proteins with conserved endosomal functions, and the auxin carriers PIN1, PIN2, and AUX1 are ESCRT cargo proteins in the MVB sorting pathway.

Physiological involvement in pH signaling of Vps24-mediated recruitment of Aspergillus PalB cysteine protease to ESCRT-III

Activation of the Aspergillus nidulans transcription factor PacC, which mediates ambient pH regulation of gene expression and is recruited to ESCRT-III by the Vps32-interacting scaffold PalA, involves its ambient pH-dependent C-terminal proteolysis. This reaction is almost certainly catalyzed by the PalB calpain-like protease. Here we show that PalB associates with membranes and interacts specifically and directly with ESCRT-III Vps24. The PalB N-terminal MIT domain and the Vps24 C-terminal MIM motif are necessary and sufficient for this interaction. PalB(DeltaMIT), a mutant PalB lacking the MIT domain is inefficiently recruited to membranes and impaired in PacC proteolytic processing. Notably, membrane recruitment is promoted and PacC processing largely restored by covalent attachment of Vps24 to mutant PalB(DeltaMIT). This is the first reported evidence that calpain-like recruitment to ESCRT-III lattices plays a physiological role. It unambiguously positions the calpain-like protease PalB within the ESCRT-III-associated pH signaling complex, underlines the positive role of ESCRT-III in ambient pH signal transduction, and suggests a possible mechanism for PalB activation.

Structure and function of ESCRT-III

ESCRT-III (endosomal sorting complex required for transport III) is required for the formation and abscission of intraluminal endosomal vesicles, which gives rise to multivesicular bodies, budding of some enveloped viruses and cytokinesis. ESCRT-III is composed of 11 members in humans, which, except for one, correspond to the six ESCRT-III-like proteins in yeast. At least CHMP (charged multivesicular body protein) 2A and CHMP3 assemble into helical tubular structures that provide a platform for membrane interaction and VPS (vacuolar protein sorting) 4-catalysed effects leading to disassembly of ESCRT-III CHMP2A-CHMP3 polymers in vitro. Progress towards the understanding of the structures and function of ESCRT-III, its activation, its regulation by accessory factors and its role in abscission of membrane enveloped structures in concert with VPS4 are discussed.

Piecing together the ESCRTs

High-resolution structural analysis has characterized nearly all of the individual domains of ESCRT (endosomal sorting complex required for transport) subunits, all of the core structures of the soluble complexes and many of the interactions involving domains. Recent emphasis in structural studies has shifted towards efforts to integrate these structures into a larger-scale model. Molecular simulations, hydrodynamic analysis, small-angle X-ray scattering and cryo-EM (electron microscopy) techniques have all been brought to bear on the ESCRT system over the last year.

Disruption of Vps4 and JNK function in Drosophila causes tumour growth

Several regulators of endocytic trafficking have recently been identified as tumour suppressors in Drosophila. These include components of the endosomal sorting complex required for transport (ESCRT) machinery. Disruption of subunits of ESCRT-I and –II leads to cell-autonomous endosomal accumulation of ubiquitinated receptors, loss of apicobasal polarity and epithelial integrity, and increased cell death. Here we report that disruption of the ATPase dVps4, the most downstream component of the ESCRT machinery, causes the same array of cellular phenotypes. We find that loss of epithelial integrity and increased apoptosis, but not loss of cell polarity, require the activation of JNK signalling. Abrogation of JNK signalling prevents apoptosis in dVps4 deficient cells. Indeed double deficiency in dVps4 and JNK signalling leads to the formation of neoplastic tumours. We conclude that dvps4 is a tumour suppressor in Drosophila and that JNK is central to the cell-autonomous phenotypes of ESCRT-deficient cells.

Evolution and assembly of ESCRTs

The AAA (ATPase associated with various cellular activities) proteins participate in membrane trafficking, organelle biogenesis, DNA replication, intracellular locomotion, cytoskeletal remodelling, protein folding and proteolysis. The AAA Vps (vacuolar protein sorting) 4 is central to traffic to lysosomes, retroviral budding and mammalian cell division. It dissociates ESCRTs (endosomal sorting complexes required for transport) from endosomal membranes, enabling their recycling to the cytosol, and plays a role in fission of intraluminal vesicles within MVBs (multivesicular bodies). The mechanism of Vps4-catalysed disassembly of ESCRT networks is unknown; however, it requires interaction between Vps4 and ESCRT-III subunits. The 30 C-terminal residues of Vps2 and Vps46 (Did2) subunits are both necessary and sufficient for interaction with the Vps4 N-terminal MIT (microtubule-interacting and transport) domain, and the crystal structure of the Vps2 C-terminus in a complex with the Vps4 MIT domain shows that MIT helices α2 and α3 recognize a (D/E)XXLXXRLXXL(K/R) MIM (MIT-interacting motif). These Vps2–MIT interactions are essential for vacuolar sorting and for Vps4-catalysed disassembly of ESCRT-III networks in vitro. Electron microscopy of ESCRT-III filaments assembled in vitro has enabled us to identify surfaces of the Vps24 subunit that are critical for protein sorting in vivo. The ESCRT-III–Vps4 interaction predates the divergence of Archaea and Eukarya. The Crenarchaea have three classes of ESCRT-III-like subunits, and one of these subunits interacts with an archaeal Vps4-like protein in a manner closely related to the human Vps4–human ESCRT-III subunit Vps20 interaction. This archaeal Vps4–ESCRT-III interaction appears to have a fundamental role in cell division in the Crenarchaea.

Two unique membrane-bound AAA proteins from Sulfolobus solfataricus

Thermoacidophilic crenarchaea of the genus Sulfolobus contain six AAA (ATPase associated with various cellular activities) proteins, including a proteasome-associated ATPase, a Vps4 (vacuolar protein sorting 4) homologue, and two Cdc48 (cell-division cycle 48)-like proteins. The last two AAA proteins are deeply branching divergent members of this family without close relatives outside the Sulfolobales. Both proteins have two nucleotide-binding domains and, unlike other members of the family, they seem to lack folded N-terminal domains. Instead, they contain N-terminal extensions of approx. 50 residues, which are predicted to be unstructured, except for a single transmembrane helix. We have analysed the two proteins, MBA (membrane-bound AAA) 1 and MBA2, by computational and experimental means. They appear to be monophyletic and to share a common ancestor with the Cdc48 clade. Both are membrane-bound and active as nucleotidases upon heterologous expression in Escherichia coli. They form ring complexes, which are stable after solubilization in a mild detergent and whose formation is dependent on the presence of the N-terminal extensions.

Biochemical analyses of human IST1 and its function in cytokinesis

The newly described yeast endosomal sorting complexes required for transport (ESCRT) protein increased sodium tolerance-1 (Ist1p) binds the late-acting ESCRT proteins Did2p/charged MVB protein (CHMP) 1 and Vps4p and exhibits synthetic vacuolar protein sorting defects when combined with mutations in the Vta1p/LIP5-Vps60p/CHMP5 complex. Here, we report that human IST1 also functions in the ESCRT pathway and is required for efficient abscission during HeLa cell cytokinesis. IST1 binding interactions with VPS4, CHMP1, LIP5, and ESCRT-I were characterized, and the IST1-VPS4 interaction was investigated in detail. Mutational and NMR spectroscopic studies revealed that the IST1 terminus contains two distinct MIT interacting motifs (MIM1 and MIM2) that wrap around and bind in different groves of the MIT helical bundle. IST1, CHMP1, and VPS4 were recruited to the midbodies of dividing cells, and depleting either IST1 or CHMP1 proteins blocked VPS4 recruitment and abscission. In contrast, IST1 depletion did not inhibit human immunodeficiency virus-1 budding. Thus, IST1 and CHMP1 act together to recruit and modulate specific VPS4 activities required during the final stages of cell division.

Essential role of hIST1 in cytokinesis

The last steps of multivesicular body (MVB) formation, human immunodeficiency virus (HIV)-1 budding and cytokinesis require a functional endosomal sorting complex required for transport (ESCRT) machinery to facilitate topologically equivalent membrane fission events. Increased sodium tolerance (IST) 1, a new positive modulator of the ESCRT pathway, has been described recently, but an essential function of this highly conserved protein has not been identified. Here, we describe the previously uncharacterized KIAA0174 as the human homologue of IST1 (hIST1), and we report its conserved interaction with VPS4, CHMP1A/B, and LIP5. We also identify a microtubule interacting and transport (MIT) domain interacting motif (MIM) in hIST1 that is necessary for its interaction with VPS4, LIP5 and other MIT domain-containing proteins, namely, MITD1, AMSH, UBPY, and Spastin. Importantly, hIST1 is essential for cytokinesis in mammalian cells but not for HIV-1 budding, thus providing a novel mechanism of functional diversification of the ESCRT machinery. Last, we show that the hIST1 MIM activity is essential for cytokinesis, suggesting possible mechanisms to explain the role of hIST1 in the last step of mammalian cell division.

Structure and disassembly of filaments formed by the ESCRT-III subunit Vps24

The ESCRT machinery mediates sorting of ubiquitinated transmembrane proteins to lysosomes via multivesicular bodies (MVBs) and also has roles in cytokinesis and viral budding. The ESCRT-III subunits are metastable monomers that transiently assemble on membranes. However, the nature of these assemblies is unknown. Among the core yeast ESCRT-III subunits, Snf7 and Vps24 spontaneously form ordered polymers in vitro. Single-particle EM reconstruction of helical Vps24 filaments shows both parallel and head-to-head subunit arrangements. Mutations of regions involved in intermolecular assembly in vitro result in cargo-sorting defects in vivo, suggesting that these homopolymers mimic interactions formed by ESCRT-III heteropolymers during MVB biogenesis. The C terminus of Vps24 is at the surface of the filaments and is not required for filament assembly. When this region is replaced by the MIT-interacting motif from the Vps2 subunit of ESCRT-III, the AAA-ATPase Vps4 can both bundle and disassemble the chimeric filaments in a nucleotide-dependent fashion.

Ordered assembly of the ESCRT-III complex on endosomes is required to sequester cargo during MVB formation

The sequential action of the Vps27/HRS complex, ESCRT-I, -II, and -III is required to sort ubiquitinated transmembrane proteins to the lumen of lysosomes via the multivesicular body (MVB) pathway. While Vps27/HRS, ESCRT-I, and -II are recruited to endosomes as preformed complexes, the ESCRT-III subunits Vps20, Snf7, Vps24, and Vps2 only assemble into a complex on endosomes. We have addressed the pathway and the regulation for ESCRT-III assembly. Our findings indicate the ordered assembly of a transient 450 kDa ESCRT-III complex on endosomes. Despite biochemical and structural similarity, each subunit contributes a specific function. Vps20 nucleates transient oligomerization of Snf7, which appears to sequester MVB cargo. Vps24 terminates Snf7 oligomerization by recruiting Vps2, which subsequently engages the AAA-ATPase Vps4 to dissociate ESCRT-III. We propose that the ordered assembly and disassembly of ESCRT-III delineates an MVB sorting domain to sequester cargo and complete the last steps of MVB sorting.

Nucleotide-dependent conformational changes and assembly of the AAA ATPase SKD1/VPS4B

SKD1/VPS4B belongs to the adenosine triphosphatases associated with diverse cellular activities (AAA) family and regulates multivesicular body (MVB) biogenesis. SKD1 changes its oligomeric state during the ATPase cycle and subsequently releases endosomal sorting complex required for transport (ESCRT) complexes from endosomes during the formation of MVBs. In this study, we describe domain motions in monomeric SKD1 on ATP and ADP binding. Nucleotides bind between the alpha/beta and the alpha-helical domains of SKD1, inducing a approximately 20 degrees domain rotation and closure of the binding site, which are similar to the changes observed in the AAA+ ATPase, HslU. Gel filtration and small-angle X-ray scattering experiments showed that the ATP-bound form of SKD1 oligomerizes in solution, whereas ADP-bound and apo forms of SKD1 exist as monomers, even though the conformations of the ADP- and ATP-bound forms are nearly identical. Nucleotide-bound SKD1 structures are compatible with a hexameric ring arrangement reminiscent of the AAA ATPase p97 D1 ring. In the hexameric ring model of SKD1, Arg290 from a neighboring molecule binds to the gamma-phosphate of ATP, which promotes oligomerization of the ATP-bound form. ATP hydrolysis would eliminate this interaction and subsequent nucleotide release causes the domains to rotate, which together lead to the disassembly of the SKD1 oligomer.

Biochemical and structural studies of yeast Vps4 oligomerization

The ESCRT (endosomal sorting complexes required for transport) pathway functions in vesicle formation at the multivesicular body, the budding of enveloped RNA viruses such as HIV-1, and the final abscission stage of cytokinesis. As the only known enzyme in the ESCRT pathway, the AAA ATPase (ATPase associated with diverse cellular activities) Vps4 provides the energy required for multiple rounds of vesicle formation. Like other Vps4 proteins, yeast Vps4 cycles through two states: a catalytically inactive disassembled state that we show here is a dimer and a catalytically active higher-order assembly that we have modeled as a dodecamer composed of two stacked hexameric rings. We also report crystal structures of yeast Vps4 proteins in the apo- and ATPgammaS [adenosine 5'-O-(3-thiotriphosphate)]-bound states. In both cases, Vps4 subunits assembled into continuous helices with 6-fold screw axes that are analogous to helices seen previously in other Vps4 crystal forms. The helices are stabilized by extensive interactions between the large and small AAA ATPase domains of adjacent Vps4 subunits, suggesting that these contact surfaces may be used to build both the catalytically active dodecamer and catalytically inactive dimer. Consistent with this model, we have identified interface mutants that specifically inhibit Vps4 dimerization, dodecamerization, or both. Thus, the Vps4 dimer and dodecamer likely form distinct but overlapping interfaces. Finally, our structural studies have allowed us to model the conformation of a conserved loop (pore loop 2) that is predicted to form an arginine-rich pore at the center of one of the Vps4 hexameric rings. Our mutational analyses demonstrate that pore loop 2 residues Arg241 and Arg251 are required for efficient HIV-1 budding, thereby supporting a role for this "arginine collar" in Vps4 function.

Helical structures of ESCRT-III are disassembled by VPS4

During intracellular membrane trafficking and remodeling, protein complexes known as the ESCRTs (endosomal sorting complexes required for transport) interact with membranes and are required for budding processes directed away from the cytosol, including the budding of intralumenal vesicles to form multivesicular bodies; for the budding of some enveloped viruses; and for daughter cell scission in cytokinesis. We found that the ESCRT-III proteins CHMP2A and CHMP3 (charged multivesicular body proteins 2A and 3) could assemble in vitro into helical tubular structures that expose their membrane interaction sites on the outside of the tubule, whereas the AAA-type adenosine triphosphatase VPS4 could bind on the inside of the tubule and disassemble the tubes upon adenosine triphosphate hydrolysis. CHMP2A and CHMP3 copolymerized in solution, and their membrane targeting was cooperatively enhanced on planar lipid bilayers. Such helical CHMP structures could thus assemble within the neck of an inwardly budding vesicle, catalyzing late steps in budding under the control of VPS4.

Two distinct modes of ESCRT-III recognition are required for VPS4 functions in lysosomal protein targeting and HIV-1 budding

The ESCRT pathway mediates membrane remodeling during enveloped virus budding, cytokinesis, and intralumenal endosomal vesicle formation. Late in the pathway, a subset of membrane-associated ESCRT-III proteins display terminal amphipathic "MIM1" helices that bind and recruit VPS4 ATPases via their MIT domains. We now report that VPS4 MIT domains also bind a second, "MIM2" motif found in a different subset of ESCRT-III subunits. The solution structure of the VPS4 MIT-CHMP6 MIM2 complex revealed that MIM2 elements bind in extended conformations along the groove between the first and third helices of the MIT domain. Mutations that block VPS4 MIT-MIM2 interactions inhibit VPS4 recruitment, lysosomal protein targeting, and HIV-1 budding. MIT-MIM2 interactions appear to be common throughout the ESCRT pathway and possibly elsewhere, and we suggest how these interactions could contribute to a mechanism in which VPS4 and ESCRT-III proteins function together to constrict the necks of budding vesicles.

Endosomal sorting complex required for transport proteins in cancer pathogenesis, vesicular transport, and non-endosomal functions

Endosomal sorting complex required for transport (ESCRT) proteins form a multicomplex sorting machinery that controls multivesicular body (MVB) formation and the sorting of ubiquitinated membrane proteins to the endosomes. Being sorted to the MVB generally results in the lysosome-dependent degradation of cell-surface receptors, and defects in this machinery induce dysregulated receptor traffic and turnover. Recent lessons from gene targeting and silencing methodologies have implicated the ESCRT in normal development, cell differentiation, and growth, as well as in the budding of certain enveloped viruses. Furthermore, it is becoming apparent that the dysregulation of ESCRT proteins is involved in the development of various human diseases, including many types of cancers and neurodegenerative disorders. Here, we summarize the roles of ESCRT proteins in MVB sorting processes and the regulation of tumor cells, and we discuss some of their other functions that are unrelated to vesicular transport.

Mechanism of homotropic control to coordinate hydrolysis in a hexameric AAA+ ring ATPase

AAA(+) proteins are ubiquitous mechanochemical ATPases that use energy from ATP hydrolysis to remodel their versatile substrates. The AAA(+) characteristic hexameric ring assemblies raise important questions about if and how six often identical subunits coordinate hydrolysis and associated motions. The PspF AAA(+) domain, PspF(1-275), remodels the bacterial sigma(54)-RNA polymerase to activate transcription. Analysis of ATP substrate inhibition kinetics on ATP hydrolysis in hexameric PspF(1-275) indicates negative homotropic effects between subunits. Functional determinants required for allosteric control identify: (i) an important link between the ATP bound ribose moiety and the SensorII motif that would allow nucleotide-dependent *-helical */beta subdomain dynamics; and (ii) establishes a novel regulatory role for the SensorII helix in PspF, which may apply to other AAA(+) proteins. Consistent with functional data, homotropic control appears to depend on nucleotide state-dependent subdomain angles imposing dynamic symmetry constraints in the AAA(+) ring. Homotropic coordination is functionally important to remodel the sigma(54) promoter. We propose a structural symmetry-based model for homotropic control in the AAA(+) characteristic ring architecture.

Structural basis of Vta1 function in the multivesicular body sorting pathway

The MVB pathway plays essential roles in several eukaryotic cellular processes. Proper function of the MVB pathway requires reversible membrane association of the ESCRTs, a process catalyzed by Vps4 ATPase. Vta1 regulates the Vps4 activity, but its mechanism of action was poorly understood. We report the high-resolution crystal structures of the Did2- and Vps60-binding N-terminal domain and the Vps4-binding C-terminal domain of S. cerevisiae Vta1. The C-terminal domain also mediates Vta1 dimerization and both subunits are required for its function as a Vps4 regulator. Emerging from our analysis is a mechanism of regulation by Vta1 in which the C-terminal domain stabilizes the ATP-dependent double ring assembly of Vps4. In addition, the MIT motif-containing N-terminal domain, projected by a long disordered linker, allows contact between the Vps4 disassembly machinery and the accessory ESCRT-III proteins. This provides an additional level of regulation and coordination for ESCRT-III assembly and disassembly.

ESCRT-III family members stimulate Vps4 ATPase activity directly or via Vta1

The AAA-ATPase Vps4 is critical for function of the MVB sorting pathway, which in turn impacts cellular phenomena ranging from receptor downregulation to viral budding to cytokinesis. Vps4 dissociates ESCRTs from endosomal membranes during MVB sorting, but it is unclear how Vps4 ATPase activity is synchronized with ESCRT release. Vta1 potentiates Vps4 activity and interacts with ESCRT-III family members. We have investigated the impact of Vta1 and ESCRT-III family members on Vps4 ATPase activity. Two distinct mechanisms of Vps4 stimulation are described: Vps2 can directly stimulate Vps4 via its MIT domain, whereas Vps60 stimulates via Vta1. Moreover, Did2 can stimulate Vps4 by both mechanisms in distinct contexts. Recent structural determination of the ESCRT-III-binding region of Vta1 unexpectedly revealed a MIT-like region. These data support a model wherein a network of MIT and MIT-like domain interactions with ESCRT-III subunits contributes to the regulation of Vps4 activity during MVB sorting.

Novel interactions of ESCRT-III with LIP5 and VPS4 and their implications for ESCRT-III disassembly

The AAA+ ATPase VPS4 plays an essential role in multivesicular body biogenesis and is thought to act by disassembling ESCRT-III complexes. VPS4 oligomerization and ATPase activity are promoted by binding to LIP5. LIP5 also binds to the ESCRT-III like protein CHMP5/hVps60, but how this affects its function remains unclear. Here we confirm that LIP5 binds tightly to CHMP5, but also find that it binds well to additional ESCRT-III proteins including CHMP1B, CHMP2A/hVps2-1, and CHMP3/hVps24 but not CHMP4A/hSnf7-1 or CHMP6/hVps20. LIP5 binds to a different region within CHMP5 than within the other ESCRT-III proteins. In CHMP1B and CHMP2A, its binding site encompasses sequences at the proteins' extreme C-termini that overlap with "MIT interacting motifs" (MIMs) known to bind to VPS4. We find unexpected evidence of a second conserved binding site for VPS4 in CHMP2A and CHMP1B, suggesting that LIP5 and VPS4 may bind simultaneously to these proteins despite the overlap in their primary binding sites. Finally, LIP5 binds preferentially to soluble CHMP5 but instead to polymerized CHMP2A, suggesting that the newly defined interactions between LIP5 and ESCRT-III proteins may be regulated by ESCRT-III conformation. These studies point to a role for direct binding between LIP5 and ESCRT-III proteins that is likely to complement LIP5's previously described ability to regulate VPS4 activity.

Human calpain 7/PalBH associates with a subset of ESCRT-III-related proteins in its N-terminal region and partly localizes to endocytic membrane compartments

Calpain 7 (also known as PalBH) is a mammalian homologue of the Aspergillus, atypical calpain PalB. Knowledge of the biochemical properties of calpain 7 is limited and its function is not yet known. In this study, we investigated the interactions of calpain 7 with all 11 ESCRT-III-related proteins, named charged multivesicular body proteins (CHMPs), and the subcellular localization of calpain 7. Pulldown assays using stable HEK293T transfectants of Strep-tagged calpain 7 revealed interactions of calpain 7 with a subset of FLAG-tagged CHMPs, among which CHMP1B was selected for further analyses. The N-terminal region containing a tandem repeat of MIT domains of calpain 7 was found to be necessary and sufficient for interaction with CHMP1B. Direct interaction was confirmed by a pulldown assay using recombinant proteins. Fluorescence microscopic analysis using HeLa cells revealed that overexpression of GFP-fused CHMPs or a dominant-negative construct of SKD1/Vps4B caused accumulation of epitope-tagged calpain 7 in a punctate pattern in the perinuclear area. Subcellular fractionation revealed that the most of endogenous calpain 7 is present in the cytosol but a small portion is present in particulate fractions. Punctate fluorescence signals of monomeric GFP-fused calpain 7 partly merged with those of endocytosed tetramethylrhodamine-labelled EGF. These results suggest that calpain 7 plays roles in the endosomal pathway by interacting with a subset of ESCRT-III-related proteins.

The Sulfolobus solfataricus AAA protein Sso0909, a homologue of the eukaryotic ESCRT Vps4 ATPase

Sso0909 is a protein of the thermo-acidophilic crenarchaeon Sulfolobus solfataricus, annotated as a p60 katanin-like ATPase. We present here results supporting the hypothesis that Sso0909 is an orthologue of the eukaryotic ESCRT (endosomal sorting complex required for transport) ATPase Vps4 (vacular protein sorting 4). The spectrum of Sso0909 homologues is limited to several orders of Crenarchaea and to three euryarchaeal Thermoplasmata species, where they were presumably acquired by lateral gene transfer. Almost invariably, Sso0909 homologues occur in the genomic vicinity of homologues of eukaryotic ESCRT-III components, which are the targets of disassembly by Vps4, as well as with a creanarchaeal-specific coiled-coil protein. S. solfataricus sso0909 is constitutively expressed under normal growth conditions and appears to be essential, as judged by the failure to obtain stable deletion mutants. We expressed Sso0909 in Escherichia coli and S. solfataricus, but have not obtained preparations with ATPase activity so far.

Structural basis of microtubule severing by the hereditary spastic paraplegia protein spastin

Spastin, the most common locus for mutations in hereditary spastic paraplegias, and katanin are related microtubule-severing AAA ATPases involved in constructing neuronal and non-centrosomal microtubule arrays and in segregating chromosomes. The mechanism by which spastin and katanin break and destabilize microtubules is unknown, in part owing to the lack of structural information on these enzymes. Here we report the X-ray crystal structure of the Drosophila spastin AAA domain and provide a model for the active spastin hexamer generated using small-angle X-ray scattering combined with atomic docking. The spastin hexamer forms a ring with a prominent central pore and six radiating arms that may dock onto the microtubule. Helices unique to the microtubule-severing AAA ATPases surround the entrances to the pore on either side of the ring, and three highly conserved loops line the pore lumen. Mutagenesis reveals essential roles for these structural elements in the severing reaction. Peptide and antibody inhibition experiments further show that spastin may dismantle microtubules by recognizing specific features in the carboxy-terminal tail of tubulin. Collectively, our data support a model in which spastin pulls the C terminus of tubulin through its central pore, generating a mechanical force that destabilizes tubulin-tubulin interactions within the microtubule lattice. Our work also provides insights into the structural defects in spastin that arise from mutations identified in hereditary spastic paraplegia patients.

The Vps4 C-terminal helix is a critical determinant for assembly and ATPase activity and has elements conserved in other members of the meiotic clade of AAA ATPases

Sorting of membrane proteins into intralumenal endosomal vesicles, multivesicular body (MVB) sorting, is critical for receptor down regulation, antigen presentation and enveloped virus budding. Vps4 is an AAA ATPase that functions in MVB sorting. Although AAA ATPases are oligomeric, mechanisms that govern Vps4 oligomerization and activity remain elusive. Vps4 has an N-terminal microtubule interacting and trafficking domain required for endosome recruitment, an AAA domain containing the ATPase catalytic site and a beta domain, and a C-terminal alpha helix positioned close to the catalytic site in the 3D structure. Previous attempts to identify the role of the C-terminal helix have been unsuccessful. Here, we show that the C-terminal helix is important for Vps4 assembly and ATPase activity in vitro and function in vivo, but not endosome recruitment or interactions with Vta1 or ESCRT-III. Unlike the beta domain, which is also important for Vps4 assembly, the C-terminal helix is not required in vivo for Vps4 homotypic interaction or dominant-negative effects of Vps4-E233Q, carrying a mutation in the ATP hydrolysis site. Vta1 promotes assembly of hybrid complexes comprising Vps4-E233Q and Vps4 lacking an intact C-terminal helix in vitro. Formation of catalytically active hybrid complexes demonstrates an intersubunit catalytic mechanism for Vps4. One end of the C-terminal helix lies in close proximity to the second region of homology (SRH), which is important for assembly and intersubunit catalysis in AAA ATPases. We propose that Vps4 SRH function requires an intact C-terminal helix. Co-evolution of a distinct Vps4 SRH and C-terminal helix in meiotic clade AAA ATPases supports this possibility.

ESCRT complexes and the biogenesis of multivesicular bodies

Multivesicular bodies (MVBs) are crucial intermediates in the trafficking of ubiquitinated receptors and other cargo destined for lysosomes. The formation of MVBs by invagination of the endosomal limiting membrane is catalyzed by the endosomal sorting complex required for transport (ESCRT) complexes, a process that has recently been visualized in three-dimensional detail by electron tomography. Structural and biochemical analysis of the upstream components, Vps27-Hse1, ESCRT-I, and ESCRT-II, shows how these complexes assemble and cluster cargo. Rapid progress has been made in understanding the assembly and disassembly of the ESCRT-III complex and the interactions of its subunits with MIT domain and other proteins. A key role for deubiquitination in the regulation of the system has been demonstrated. One central question remains largely unanswered, which is how the ESCRTs actually promote the invagination of the endosomal membrane.

Plasma membrane deformation by circular arrays of ESCRT-III protein filaments

Endosomal sorting complex required for transport III (ESCRT-III) proteins function in multivesicular body biogenesis and viral budding. They are recruited from the cytoplasm to the membrane, where they assemble into large complexes. We used “deep-etch” electron microscopy to examine polymers formed by the ESCRT-III proteins hSnf7-1 (CHMP4A) and hSnf7-2 (CHMP4B). When overexpressed, these proteins target to endosomes and the plasma membrane. Both hSnf7 proteins assemble into regular approximately 5-nm filaments that curve and self-associate to create circular arrays. Binding to a coexpressed adenosine triphosphate hydrolysis–deficient mutant of VPS4B draws these filaments together into tight circular scaffolds that bend the membrane away from the cytoplasm to form buds and tubules protruding from the cell surface. Similar buds develop in the absence of mutant VPS4B when hSnf7-1 is expressed without its regulatory C-terminal domain. We demonstrate that hSnf7 proteins form novel membrane-attached filaments that can promote or stabilize negative curvature and outward budding. We suggest that ESCRT-III polymers delineate and help generate the luminal vesicles of multivesicular bodies.

Vacuolar protein sorting: two different functional states of the AAA-ATPase Vps4p

The vacuolar protein sorting (Vps) pathway, in which Vps4 class I AAA-ATPases play a central role, regulates growth factor receptors, immune response, and developmental signaling, and participates in tumor suppression, apoptosis, and retrovirus budding. We present the first atomic structure of the nucleotide-free yeast His(6)DeltaNVps4p dimer and its AMPPNP (5'-adenylyl-beta,gamma-imidodiphosphate)-bound tetradecamer, derived from a cryo electron microscopy map. Vps4p dimers form two distinct heptameric rings and accommodate AAA cassettes in a head-to-head--not in a head-to-tail-fashion as in class II AAA-ATPases. Our model suggests a mechanism for disassembling ESCRT (endosomal sorting complex required for transport) complexes by movements of substrate-binding domains located at the periphery of the tetradecamer during ATP hydrolysis in one ring, followed by translocation through the central pore and ATP hydrolysis in the second ring.

Cryo-EM structure of dodecameric Vps4p and its 2:1 complex with Vta1p

The type I AAA (ATPase associated with a variety of cellular activities) ATPase Vps4 and its co-factor Vta1p/LIP5 function in membrane remodeling events that accompany cytokinesis, multivesicular body biogenesis, and retrovirus budding, apparently by driving disassembly and recycling of membrane-associated ESCRT (endosomal sorting complex required for transport)-III complexes. Here, we present electron cryomicroscopy reconstructions of dodecameric yeast Vps4p complexes with and without their microtubule interacting and transport (MIT) N-terminal domains and Vta1p co-factors. The ATPase domains of Vps4p form a bowl-like structure composed of stacked hexameric rings. The two rings adopt dramatically different conformations, with the "upper" ring forming an open assembly that defines the sides of the bowl and the lower ring forming a closed assembly that forms the bottom of the bowl. The N-terminal MIT domains of the upper ring localize on the symmetry axis above the cavity of the bowl, and the binding of six extended Vta1p monomers causes additional density to appear both above and below the bowl. The structures suggest models in which Vps4p MIT and Vta1p domains engage ESCRT-III substrates above the bowl and help transfer them into the bowl to be pumped through the center of the dodecameric assembly.