The linker region plays a regulatory role in assembly and activity of the Vps4 AAA ATPase

A Structural View on ESCRT-Mediated Abscission

The endosomal sorting complex required for transport (ESCRT) mediates cellular processes that are related to membrane remodeling, such as multivesicular body (MVB) formation, viral budding and cytokinesis. Abscission is the final stage of cytokinesis that results in the physical separation of the newly formed two daughter cells. Although abscission has been investigated for decades, there are still fundamental open questions related to the spatio-temporal organization of the molecular machinery involved in this process. Reviewing knowledge obtained from in vitro as well as in vivo experiments, we give a brief overview on the role of ESCRT components in abscission mainly focussing on mammalian cells.

Katanin Grips the β-Tubulin Tail through an Electropositive Double Spiral to Sever Microtubules

The AAA ATPase katanin severs microtubules. It is critical in cell division, centriole biogenesis, and neuronal morphogenesis. Its mutation causes microcephaly. The microtubule templates katanin hexamerization and activates its ATPase. The structural basis for these activities and how they lead to severing is unknown. Here, we show that β-tubulin tails are necessary and sufficient for severing. Cryoelectron microscopy (cryo-EM) structures reveal the essential tubulin tail glutamates gripped by a double spiral of electropositive loops lining the katanin central pore. Each spiral couples allosterically to the ATPase and binds alternating, successive substrate residues, with consecutive residues coordinated by adjacent protomers. This tightly couples tail binding, hexamerization, and ATPase activation. Hexamer structures in different states suggest an ATPase-driven, ratchet-like translocation of the tubulin tail through the pore. A disordered region outside the AAA core anchors katanin to the microtubule while the AAA motor exerts the forces that extract tubulin dimers and sever the microtubule.

The regulation of Endosomal Sorting Complex Required for Transport and accessory proteins in multivesicular body sorting and enveloped viral budding - An overview

ESCRT (Endosomal Sorting Complex Required for Transport) machinery drives different cellular processes such as endosomal sorting, organelle biogenesis, vesicular trafficking, maintenance of plasma membrane integrity, membrane fission during cytokinesis and enveloped virus budding. The normal cycle of assembly and disassembly of some ESCRT complexes at the membrane requires the AAA-ATPase vacuolar protein sorting 4 (Vps4p). A number of ESCRT proteins are hijacked by clinically significant enveloped viruses including Ebola, and Human Immunodeficiency Virus (HIV) to enable enveloped virus budding and Vps4p provides energy for the disassembly/recycling of these ESCRT proteins. Several years ago, the failure of the terminal budding process of HIV following Vps4 protein inhibition was published; although at that time a detailed understanding of the molecular players was missing. However, later it was acknowledged that the ESCRT machinery has a role in enveloped virus budding from cells due to its role in the multivesicular body (MVB) sorting pathway. The MVB sorting pathway facilitates several cellular activities in uninfected cells, such as the down-regulation of signaling through cell surface receptors as well as the process of viral budding from infected host cells. In this review, we focus on summarising the functional organisation of ESCRT proteins at the membrane and the role of ESCRT machinery and Vps4p during MVB sorting and enveloped viral budding.

The role of VPS4 in ESCRT-III polymer remodeling

The endosomal sorting complex required for transport-III (ESCRT-III) and VPS4 catalyze a variety of membrane-remodeling processes in eukaryotes and archaea. Common to these processes is the dynamic recruitment of ESCRT-III proteins from the cytosol to the inner face of a membrane neck structure, their activation and filament formation inside or at the membrane neck and the subsequent or concomitant recruitment of the AAA-type ATPase VPS4. The dynamic assembly of ESCRT-III filaments and VPS4 on cellular membranes induces constriction of membrane necks with large diameters such as the cytokinetic midbody and necks with small diameters such as those of intraluminal vesicles or enveloped viruses. The two processes seem to use different sets of ESCRT-III filaments. Constriction is then thought to set the stage for membrane fission. Here, we review recent progress in understanding the structural transitions of ESCRT-III proteins required for filament formation, the functional role of VPS4 in dynamic ESCRT-III assembly and its active role in filament constriction. The recent data will be discussed in the context of different mechanistic models for inside-out membrane fission.

The AAA protein spastin possesses two levels of basal ATPase activity

The AAA ATPase spastin is a microtubule-severing enzyme that plays important roles in various cellular events including axon regeneration. Herein, we found that the basal ATPase activity of spastin is negatively regulated by spastin concentration. By determining a spastin crystal structure, we demonstrate the necessity of intersubunit interactions between spastin AAA domains. Neutralization of the positive charges in the microtubule-binding domain (MTBD) of spastin dramatically decreases the ATPase activity at low concentration, although the ATP-hydrolyzing potential is not affected. These results demonstrate that, in addition to the AAA domain, the MTBD region of spastin is also involved in regulating ATPase activity, making interactions between spastin protomers more complicated than expected.

NMR studies on the interactions between yeast Vta1 and Did2 during the multivesicular bodies sorting pathway

As an AAA-ATPase, Vps4 is important for function of multivesicular bodies (MVB) sorting pathway, which involves in cellular phenomena ranging from receptor down-regulation to viral budding to cytokinesis. The activity of Vps4 is stimulated by the interactions between Vta1 N-terminus (named as Vta1NTD) and Did2 fragment (176–204 aa) (termed as Did2_176–204) or Vps60 (128–186 aa) (termed as Vps60_128–186). The structural basis of how Vta1NTD binds to Did2_176–204 is still unclear. To address this, in this report, the structure of Did2_176–204 in complex with Vta1NTD was determined by NMR techniques, demonstrating that Did2_176–204 interacts with Vta1NTD through its helix α6′ extending over the 2^nd and the 3^rd α-helices of Vta1NTD microtubule interacting and transport 1 (MIT1) domain. The residues within Did2_176–204 helix α6′ in the interface make up of an amino acid sequence as E₁₉₂′xxL₁₉₅′xxR₁₉₈′L₁₉₉′xxL₂₀₂′R₂₀₃′, identical to type 1 MIT-interacting motif (MIM1) (D/E)xxLxxRLxxL(K/R) of CHMP1A_180–196 observed in Vps4-CHMP1A complex structure, indicating that Did2 binds to Vta1NTD through canonical MIM1 interactions. Moreover, the Did2 binding does not result in Vta1NTD significant conformational changes, revealing that Did2, similar to Vps60, enhances Vta1 stimulation of Vps4 ATPase activity in an indirect manner.

Asymmetric ring structure of Vps4 required for ESCRT-III disassembly

The vacuolar protein sorting 4 AAA-ATPase (Vps4) recycles endosomal sorting complexes required for transport (ESCRT-III) polymers from cellular membranes. Here we present a 3.6-Å X-ray structure of ring-shaped Vps4 from Metallosphera sedula (MsVps4), seen as an asymmetric pseudohexamer. Conserved key interface residues are shown to be important for MsVps4 assembly, ATPase activity in vitro, ESCRT-III disassembly in vitro and HIV-1 budding. ADP binding leads to conformational changes within the protomer, which might propagate within the ring structure. All ATP-binding sites are accessible and the pseudohexamer binds six ATP with micromolar affinity in vitro. In contrast, ADP occupies one high-affinity and five low-affinity binding sites in vitro, consistent with conformational asymmetry induced on ATP hydrolysis. The structure represents a snapshot of an assembled Vps4 conformation and provides insight into the molecular motions the ring structure undergoes in a concerted action to couple ATP hydrolysis to ESCRT-III substrate disassembly.

Conformational Changes in the Endosomal Sorting Complex Required for the Transport III Subunit Ist1 Lead to Distinct Modes of ATPase Vps4 Regulation

Intralumenal vesicle formation of the multivesicular body is a critical step in the delivery of endocytic cargoes to the lysosome for degradation. Endosomal sorting complex required for transport III (ESCRT-III) subunits polymerize on endosomal membranes to facilitate membrane budding away from the cytoplasm to generate these intralumenal vesicles. The ATPase Vps4 remodels and disassembles ESCRT-III, but the manner in which Vps4 activity is coordinated with ESCRT-III function remains unclear. Ist1 is structurally homologous to ESCRT-III subunits and has been reported to inhibit Vps4 function despite the presence of a microtubule-interacting and trafficking domain-interacting motif (MIM) capable of stimulating Vps4 in the context of other ESCRT-III subunits. Here we report that Ist1 inhibition of Vps4 ATPase activity involves two elements in Ist1: the MIM itself and a surface containing a conserved ELYC sequence. In contrast, the MIM interaction, in concert with a more open conformation of the Ist1 core, resulted in stimulation of Vps4. Addition of the ESCRT-III subunit binding partner of Ist1, Did2, also converted Ist1 from an inhibitor to a stimulator of Vps4 ATPase activity. Finally, distinct regulation of Vps4 by Ist1 corresponded with altered ESCRT-III disassembly in vitro. Together, these data support a model in which Ist1-Did2 interactions during ESCRT-III polymerization coordinate Vps4 activity with the timing of ESCRT-III disassembly.

Binding of Substrates to the Central Pore of the Vps4 ATPase Is Autoinhibited by the Microtubule Interacting and Trafficking (MIT) Domain and Activated by MIT Interacting Motifs (MIMs)

The endosomal sorting complexes required for transport (ESCRT) pathway drives reverse topology membrane fission events within multiple cellular pathways, including cytokinesis, multivesicular body biogenesis, repair of the plasma membrane, nuclear membrane vesicle formation, and HIV budding. The AAA ATPase Vps4 is recruited to membrane necks shortly before fission, where it catalyzes disassembly of the ESCRT-III lattice. The N-terminal Vps4 microtubule-interacting and trafficking (MIT) domains initially bind the C-terminal MIT-interacting motifs (MIMs) of ESCRT-III subunits, but it is unclear how the enzyme then remodels these substrates in response to ATP hydrolysis. Here, we report quantitative binding studies that demonstrate that residues from helix 5 of the Vps2p subunit of ESCRT-III bind to the central pore of an asymmetric Vps4p hexamer in a manner that is dependent upon the presence of flexible nucleotide analogs that can mimic multiple states in the ATP hydrolysis cycle. We also find that substrate engagement is autoinhibited by the Vps4p MIT domain and that this inhibition is relieved by binding of either Type 1 or Type 2 MIM elements, which bind the Vps4p MIT domain through different interfaces. These observations support the model that Vps4 substrates are initially recruited by an MIM-MIT interaction that activates the Vps4 central pore to engage substrates and generate force, thereby triggering ESCRT-III disassembly.

Vps4 stimulatory element of the cofactor Vta1 contacts the ATPase Vps4 α7 and α9 to stimulate ATP hydrolysis

The endosomal sorting complexes required for transport (ESCRTs) function in a variety of membrane remodeling processes including multivesicular body sorting, abscission during cytokinesis, budding of enveloped viruses, and repair of the plasma membrane. Vps4 ATPase activity modulates ESCRT function and is itself modulated by its cofactor Vta1 and its substrate ESCRT-III. The carboxyl-terminal Vta1/SBP-1/Lip5 (VSL) domain of Vta1 binds to the Vps4 β-domain to promote Vps4 oligomerization-dependent ATP hydrolysis. Additionally, the Vps4 stimulatory element (VSE) of Vta1 contributes to enhancing Vps4 oligomer ATP hydrolysis. The VSE is also required for Vta1-dependent stimulation of Vps4 by ESCRT-III subunits. However, the manner by which the Vta1 VSE contributes to Vps4 activation is unknown. Existing structural data were used to generate a model of the Vta1 VSE in complex with Vps4. This model implicated residues within the small ATPase associated with various activities (AAA) domain, specifically α-helices 7 and 9, as relevant contact sites. Rational generation of Vps4 mutants defective for VSE-mediated stimulation, as well as intergenic compensatory mutations, support the validity of this model. These findings have uncovered the Vps4 surface responsible for coordinating ESCRT-III-stimulated Vta1 input during ESCRT function and identified a novel mechanism of Vps4 stimulation.

Coordinated binding of Vps4 to ESCRT-III drives membrane neck constriction during MVB vesicle formation

Five endosomal sorting complexes required for transport (ESCRTs) mediate the degradation of ubiquitinated membrane proteins via multivesicular bodies (MVBs) in lysosomes. ESCRT-0, -I, and -II interact with cargo on endosomes. ESCRT-II also initiates the assembly of a ringlike ESCRT-III filament consisting of Vps20, Snf7, Vps24, and Vps2. The AAA-adenosine triphosphatase Vps4 disassembles and recycles the ESCRT-III complex, thereby terminating the ESCRT pathway. A mechanistic role for Vps4 in intraluminal vesicle (ILV) formation has been unclear. By combining yeast genetics, biochemistry, and electron tomography, we find that ESCRT-III assembly on endosomes is required to induce or stabilize the necks of growing MVB ILVs. Yet, ESCRT-III alone is not sufficient to complete ILV biogenesis. Rather, binding of Vps4 to ESCRT-III, coordinated by interactions with Vps2 and Snf7, is coupled to membrane neck constriction during ILV formation. Thus, Vps4 not only recycles ESCRT-III subunits but also cooperates with ESCRT-III to drive distinct membrane-remodeling steps, which lead to efficient membrane scission at the end of ILV biogenesis in vivo.