Structural basis for endosomal targeting by the Bro1 domain

SRP RNA remodeling by SRP68 explains its role in protein translocation

The signal recognition particle (SRP) is central to membrane protein targeting; SRP RNA is essential for SRP assembly, elongation arrest, and activation of SRP guanosine triphosphatases. In eukaryotes, SRP function relies on the SRP68-SRP72 heterodimer. We present the crystal structures of the RNA-binding domain of SRP68 (SRP68-RBD) alone and in complex with SRP RNA and SRP19. SRP68-RBD is a tetratricopeptide-like module that binds to a RNA three-way junction, bends the RNA, and inserts an α-helical arginine-rich motif (ARM) into the major groove. The ARM opens the conserved 5f RNA loop, which in ribosome-bound SRP establishes a contact to ribosomal RNA. Our data provide the structural basis for eukaryote-specific, SRP68-driven RNA remodeling required for protein translocation.

Virus budding and the ESCRT pathway

Enveloped viruses escape infected cells by budding through limiting membranes. In the decade since the discovery that HIV recruits cellular ESCRT (endosomal sorting complexes required for transport) machinery to facilitate viral budding, this pathway has emerged as the major escape route for enveloped viruses. In cells, the ESCRT pathway catalyzes analogous membrane fission events required for the abscission stage of cytokinesis and for a series of "reverse topology" vesiculation events. Studies of enveloped virus budding are therefore providing insights into the complex cellular mechanisms of cell division and membrane protein trafficking (and vice versa). Here, we review how viruses mimic cellular recruiting signals to usurp the ESCRT pathway, discuss mechanistic models for ESCRT pathway functions, and highlight important research frontiers.

The yeast Alix homolog Bro1 functions as a ubiquitin receptor for protein sorting into multivesicular endosomes

Sorting of ubiquitinated membrane proteins into lumenal vesicles of multivesicular bodies is mediated by the Endosomal Sorting Complex Required for Transport (ESCRT) apparatus and accessory proteins such as Bro1, which recruits the deubiquitinating enzyme Doa4 to remove ubiquitin from cargo. Here we propose that Bro1 works as a receptor for the selective sorting of ubiquitinated cargoes. We found synthetic genetic interactions between BRO1 and ESCRT-0, suggesting that Bro1 functions similarly to ESCRT-0. Multiple structural approaches demonstrated that Bro1 binds ubiquitin via the N-terminal trihelical arm of its middle V domain. Mutants of Bro1 that lack the ability to bind Ub were dramatically impaired in their ability to sort Ub-cargo membrane proteins, but only when combined with hypomorphic alleles of ESCRT-0. These data suggest that Bro1 and other Bro1 family members function in parallel with ESCRT-0 to recognize and sort Ub-cargoes.

Identification and characterization of the nano-sized vesicles released by muscle cells

Several cell types secrete small membranous vesicles that contain cell-specific collections of proteins, lipids, and genetic material. The function of these vesicles is to allow cell-to-cell signaling and the horizontal transfer of their cargo molecules. Here, we demonstrate that muscle cells secrete nano-sized vesicles and that their release increases during muscle differentiation. Analysis of these nanovesicles allowed us to characterize them as exosome-like particles and to define the potential role of the multifunctional protein Alix in their biogenesis.

Alix serves as an adaptor that allows human parainfluenza virus type 1 to interact with the host cell ESCRT system

The cellular ESCRT (endosomal sorting complex required for transport) system functions in cargo-sorting, in the formation of intraluminal vesicles that comprise multivesicular bodies (MVB), and in cytokinesis, and this system can be hijacked by a number of enveloped viruses to promote budding. The respiratory pathogen human parainfluenza virus type I (HPIV1) encodes a nested set of accessory C proteins that play important roles in down-regulating viral transcription and replication, in suppressing the type I interferon (IFN) response, and in suppressing apoptosis. Deletion or mutation of the C proteins attenuates HPIV1 in vivo, and such mutants are being evaluated preclinically and clinically as vaccines. We show here that the C proteins interact and co-localize with the cellular protein Alix, which is a member of the class E vacuolar protein sorting (Vps) proteins that assemble at endosomal membranes into ESCRT complexes. The HPIV1 C proteins interact with the Bro1 domain of Alix at a site that is also required for the interaction between Alix and Chmp4b, a subunit of ESCRT-III. The C proteins are ubiquitinated and subjected to proteasome-mediated degradation, but the interaction with Alix_Bro1 protects the C proteins from degradation. Neither over-expression nor knock-down of Alix expression had an effect on HPIV1 replication, although this might be due to the large redundancy of Alix-like proteins. In contrast, knocking down the expression of Chmp4 led to an approximately 100-fold reduction in viral titer during infection with wild-type (WT) HPIV1. This level of reduction was similar to that observed for the viral mutant, P(C-) HPIV1, in which expression of the C proteins were knocked out. Chmp4 is capable of out-competing the HPIV1 C proteins for binding Alix. Together, this suggests a possible model in which Chmp4, through Alix, recruits the C proteins to a common site on intracellular membranes and facilitates budding.

Membrane fission reactions of the mammalian ESCRT pathway

The endosomal sorting complexes required for transport (ESCRT) pathway was initially defined in yeast genetic screens that identified the factors necessary to sort membrane proteins into intraluminal endosomal vesicles. Subsequent studies have revealed that the mammalian ESCRT pathway also functions in a series of other key cellular processes, including formation of extracellular microvesicles, enveloped virus budding, and the abscission stage of cytokinesis. The core ESCRT machinery comprises Bro1 family proteins and ESCRT-I, ESCRT-II, ESCRT-III, and VPS4 complexes. Site-specific adaptors recruit these soluble factors to assemble on different cellular membranes, where they carry out membrane fission reactions. ESCRT-III proteins form filaments that draw membranes together from the cytoplasmic face, and mechanistic models have been advanced to explain how ESCRT-III filaments and the VPS4 ATPase can work together to catalyze membrane fission.

Doa4 function in ILV budding is restricted through its interaction with the Vps20 subunit of ESCRT-III

Assembly of the endosomal sorting complex required for transport (ESCRT)-III executes the formation of intralumenal vesicles (ILVs) at endosomes. Repeated cycles of ESCRT-III function requires disassembly of the complex by Vps4, an ATPase with a microtubule interaction and trafficking (MIT) domain that binds MIT-interacting motifs (MIM1 or MIM2) in ESCRT-III subunits. We identified a putative MIT domain at the N-terminus of Doa4, which is the ubiquitin (Ub) hydrolase in Saccharomyces cerevisiae that deubiquitinates ILV cargo proteins. The Doa4 N-terminus is predicted to have the α-helical structure common to MIT domains, and it binds directly to a MIM1-like sequence in the Vps20 subunit of ESCRT-III. Disrupting this interaction does not prevent endosomal localization of Doa4 but enhances the defect in ILV cargo protein deubiquitination observed in cells lacking Bro1, which is an ESCRT-III effector protein that stimulates Doa4 catalytic activity. Deletion of the BRO1 gene (bro1Δ) blocks ILV budding, but ILV budding was rescued upon disrupting the interaction between Vps20 and Doa4. This rescue in ILV biogenesis requires Doa4 expression but is independent of its Ub hydrolase activity. Thus, binding of Vps20 to the Doa4 N-terminus inhibits a non-catalytic function of Doa4 that promotes ILV formation.

Protein tyrosine phosphatases--from housekeeping enzymes to master regulators of signal transduction

There are many misconceptions surrounding the roles of protein phosphatases in the regulation of signal transduction, perhaps the most damaging of which is the erroneous view that these enzymes exert their effects merely as constitutively active housekeeping enzymes. On the contrary, the phosphatases are critical, specific regulators of signalling in their own right and serve an essential function, in a coordinated manner with the kinases, to determine the response to a physiological stimulus. This review is a personal perspective on the development of our understanding of the protein tyrosine phosphatase family of enzymes. I have discussed various aspects of the structure, regulation and function of the protein tyrosine phosphatase family, which I hope will illustrate the fundamental importance of these enzymes in the control of signal transduction.

Identification and characterization of PlAlix, the Alix homologue from the Mediterranean sea urchin Paracentrotus lividus

The sea urchin provides a relatively simple and tractable system for analyzing the early stages of embryo development. Here, we use the sea urchin species, Paracentrotus lividus, to investigate the role of Alix in key stages of embryogenesis, namely the egg fertilization and the first cleavage division. Alix is a multifunctional protein involved in different cellular processes including endocytic membrane trafficking, filamentous (F)-actin remodeling, and cytokinesis. Alix homologues have been identified in different metazoans; in these organisms, Alix is involved in oogenesis and in determination/differentiation events during embryo development. Herein, we describe the identification of the sea urchin homologue of Alix, PlAlix. The deduced amino acid sequence shows that Alix is highly conserved in sea urchins. Accordingly, we detect the PlAlix protein cross-reacting with monoclonal Alix antibodies in extracts from P. lividus, at different developmental stages. Focusing on the role of PlAlix during early embryogenesis we found that PlAlix is a maternal protein that is expressed at increasingly higher levels from fertilization to the 2-cell stage embryo. In sea urchin eggs, PlAlix localizes throughout the cytoplasm with a punctuated pattern and, soon after fertilization, accumulates in larger puncta in the cytosol, and in microvilli-like protrusions. Together our data show that PlAlix is structurally conserved from sea urchin to mammals and may open new lines of inquiry into the role of Alix during the early stages of embryo development.

Wrapping up the bad news: HIV assembly and release

The late Nobel Laureate Sir Peter Medawar once memorably described viruses as ‘bad news wrapped in protein’. Virus assembly in HIV is a remarkably well coordinated process in which the virus achieves extracellular budding using primarily intracellular budding machinery and also the unusual phenomenon of export from the cell of an RNA. Recruitment of the ESCRT system by HIV is one of the best documented examples of the comprehensive way in which a virus hijacks a normal cellular process. This review is a summary of our current understanding of the budding process of HIV, from genomic RNA capture through budding and on to viral maturation, but centering on the proteins of the ESCRT pathway and highlighting some recent advances in our understanding of the cellular components involved and the complex interplay between the Gag protein and the genomic RNA.

Dynamics of ESCRT proteins

Proteins of the ESCRT (endosomal sorting complex required for transport) complex function in membrane fission processes, such as multivesicular body (MVBs) formation, the terminal stages of cytokinesis, and separation of enveloped viruses from the plasma membrane. In mammalian cells, the machinery consists of a network of more than 20 proteins, organized into three complexes (ESCRT-I, -II, and -III), and other associated proteins such as the ATPase vacuolar protein sorting 4 (Vps4). Early biochemical studies of MVBs biogenesis in yeast support a model of sequential recruitment of ESCRT complexes on membranes. Live-cell imaging of ESCRT protein dynamics during viral budding and cytokinesis now reveal that this long-standing model of sequential assembly and disassembly holds true in mammalian cells.

Identification of the HIV-1 NC binding interface in Alix Bro1 reveals a role for RNA

HIV-1 recruits members of ESCRT, the cell membrane fission machinery that promotes virus exit. HIV-1 Gag protein gains access to ESCRT directly by binding Alix, an ESCRT-associated protein that promotes budding. The Alix Bro1 and V domains bind Gag NC and p6 regions, respectively. Whereas V-p6 binding and function are well characterized, residues in Bro1 that interact with NC and their functional contribution to Alix-mediated HIV-1 budding are unknown. We mapped Bro1 residues that constitute the NC binding interface and found that they are critical for function. Intriguingly, residues involved in interactions on both sides of the Bro1-NC interface are positively charged, suggesting the involvement of a negatively charged cellular factor serving as a bridge. Nuclease treatment eliminated Bro1-NC interactions, revealing the involvement of RNA. These findings establish a direct role for NC in mediating interactions with ESCRT necessary for virus release and report the first evidence of RNA involvement in such recruitments.

Multivesicular body morphogenesis

Multivesicular bodies (MVBs) are unique organelles in the endocytic pathway that contain vesicles in their lumen. Sorting and incorporation of material into such vesicles is a critical cellular process that has been intensely studied following discovery of the ESCRT (endosomal sorting complex required for transport) machinery just more than a decade ago. In this review, we summarize current understanding of the cellular functions of MVBs and how the ESCRT machinery contributes to MVB morphogenesis. We also highlight the importance of MVBs and ESCRTs in human health. We identify critical areas in which further mechanistic and spatiotemporal studies in living cells will advance this exciting area of research.

ALIX binds a YPX(3)L motif of the GPCR PAR1 and mediates ubiquitin-independent ESCRT-III/MVB sorting

The sorting of signaling receptors to lysosomes is an essential regulatory process in mammalian cells. During degradation, receptors are modified with ubiquitin and sorted by endosomal sorting complex required for transport (ESCRT)-0, -I, -II, and -III complexes into intraluminal vesicles (ILVs) of multivesicular bodies (MVBs). However, it remains unclear whether a single universal mechanism mediates MVB sorting of all receptors. We previously showed that protease-activated receptor 1 (PAR1), a G protein-coupled receptor (GPCR) for thrombin, is internalized after activation and sorted to lysosomes independent of ubiquitination and the ubiquitin-binding ESCRT components hepatocyte growth factor-regulated tyrosine kinase substrate and Tsg101. In this paper, we report that PAR1 sorted to ILVs of MVBs through an ESCRT-III-dependent pathway independent of ubiquitination. We further demonstrate that ALIX, a charged MVB protein 4-ESCRT-III interacting protein, bound to a YPX(3)L motif of PAR1 via its central V domain to mediate lysosomal degradation. This study reveals a novel MVB/lysosomal sorting pathway for signaling receptors that bypasses the requirement for ubiquitination and ubiquitin-binding ESCRTs and may be applicable to a subset of GPCRs containing YPX(n)L motifs.

Two distinct binding modes define the interaction of Brox with the C-terminal tails of CHMP5 and CHMP4B

Interactions of the CHMP protein carboxyl terminal tails with effector proteins play important roles in retroviral budding, cytokinesis, and multivesicular body biogenesis. Here we demonstrate that hydrophobic residues at the CHMP4B C-terminal amphipathic α helix bind a concave surface of Brox, a mammalian paralog of Alix. Unexpectedly, CHMP5 was also found to bind Brox and specifically recruit endogenous Brox to detergent-resistant membrane fractions through its C-terminal 20 residues. Instead of an α helix, the CHMP5 C-terminal tail adopts a tandem β-hairpin structure that binds Brox at the same site as CHMP4B. Additional Brox:CHMP5 interface is furnished by a unique CHMP5 hydrophobic pocket engaging the Brox residue Y348 that is not conserved among the Bro1 domains. Our studies thus unveil a β-hairpin conformation of the CHMP5 protein C-terminal tail, and provide insights into the overlapping but distinct binding profiles of ESCRT-III and the Bro1 domain proteins.

The signaling mechanism of ambient pH sensing and adaptation in yeast and fungi

The four protein complexes termed endosomal sorting complexes required for transport (ESCRT) are key mediators of multivesicular body sorting/formation, retroviral budding and cell abscission, which share a membrane deformation process with the same topological change: vesicles budding away from the cytoplasm. Independent studies of the signal transduction pathways that mediate ambient pH sensing and adaptation in yeast and fungi revealed that these pathways share a conserved signaling mechanism that utilizes ESCRT complexes for its activation. This pathway in Saccharomyces cerevisiae, termed the Rim101 pathway, consists of both a sensing complex, which senses ambient alkaline pH, and a proteolytic complex, which proteolyzes and thereby activates the key transcription factor Rim101. Since the proteolytic complex is thought to be formed and activated on a platform of a multimerized ESCRT-III component Snf7, the organization, regulation and function of this pathway are dependent on the function of ESCRT complexes.

EhADH112 is a Bro1 domain-containing protein involved in the Entamoeba histolytica multivesicular bodies pathway

EhADH112 is an Entamoeba histolytica Bro1 domain-containing protein, structurally related to mammalian ALIX and yeast BRO1, both involved in the Endosomal Sorting Complexes Required for Transport (ESCRT)-mediated multivesicular bodies (MVB) biogenesis. Here, we investigated an alternative role for EhADH112 in the MVB protein trafficking pathway by overexpressing 166 amino acids of its N-terminal Bro1 domain in trophozoites. Trophozoites displayed diminished phagocytosis rates and accumulated exogenous Bro1 at cytoplasmic vesicles which aggregated into aberrant complexes at late stages of phagocytosis, probably preventing EhADH112 function. Additionally, the existence of a putative E. histolytica ESCRT-III subunit (EhVps32) presumably interacting with EhADH112, led us to perform pull-down experiments with GST-EhVps32 and [³⁵S]-labeled EhADH112 or EhADH112 derivatives, confirming EhVps32 binding to EhADH112 through its Bro1 domain. Our overall results define EhADH112 as a novel member of ESCRT-accessory proteins transiently present at cellular surface and endosomal compartments, probably contributing to MVB formation during phagocytosis.

An ordered pathway for the assembly of fungal ESCRT-containing ambient pH signalling complexes at the plasma membrane

The fungal pal/RIM signalling pathway, which regulates gene expression in response to environmental pH involves, in addition to dedicated proteins, several components of ESCRT complexes, which suggested that pH signalling proteins assemble on endosomal platforms. In Aspergillus nidulans, dedicated Pal proteins include the plasma membrane receptor PalH and its coupled arrestin, PalF, which becomes ubiquitylated in alkaline pH conditions, and three potentially endosomal ESCRT-III associates, including Vps32 interactors PalA and PalC and Vps24 interactor calpain-like PalB. We studied the subcellular locations at which signalling takes place after activating the pathway by shifting ambient pH to alkalinity. Rather than localising to endosomes, Vps32 interactors PalA and PalC transiently colocalise at alkaline-pH-induced cortical structures in a PalH-, Vps23- and Vps32-dependent but Vps27-independent manner. These cortical structures are much more stable when Vps4 is deficient, indicating that their half-life depends on ESCRT-III disassembly. Pull-down studies revealed that Vps23 interacts strongly with PalF, but co-immunoprecipitates exclusively with ubiquitylated PalF forms from extracts. We demonstrate that Vps23-GFP, expressed at physiological levels, is also recruited to cortical structures, very conspicuous in vps27Δ cells in which the prominent signal of Vps23-GFP on endosomes is eliminated, in a PalF- and alkaline pH-dependent manner. Dual-channel epifluorescence microscopy showed that PalC arrives at cortical complexes before PalA. As PalC recruitment is PalA independent and PalA recruitment is PalC dependent but PalB independent, these data complete the participation order of Pal proteins in the pathway and strongly support a model in which pH signalling takes place in ESCRT-containing, plasma-membrane-associated, rather than endosome-associated, complexes.

Budding of retroviruses utilizing divergent L domains requires nucleocapsid

We recently reported that human immunodeficiency virus type 1 (HIV-1) carrying PTAP and LYPX(n)L L domains ceased budding when the nucleocapsid (NC) domain was mutated, suggesting a role for NC in HIV-1 release. Here we investigated whether NC involvement in virus release is a property specific to HIV-1 or a general requirement of retroviruses. Specifically, we examined a possible role for NC in the budding of retroviruses relying on divergent L domains and structurally homologous NC domains that harbor diverse protein sequences. We found that NC is critical for the release of viruses utilizing the PTAP motif whether it functions within its native Gag in simian immunodeficiency virus cpzGAB2 (SIVcpzGAB2) or SIVsmmE543 or when it is transplanted into the heterologous Gag protein of equine infectious anemia virus (EIAV). In both cases, virus release was severely diminished even though NC mutant Gag proteins retained the ability to assemble spherical particles. Moreover, budding-defective NC mutants, which displayed particles tethered to the plasma membrane, were triggered to release virus when access to the cell endocytic sorting complex required for transport pathway was restored (i.e., in trans expression of Nedd4.2s). We also examined the role of NC in the budding of EIAV, a retrovirus relying exclusively on the (L)YPX(n)L-type L domain. We found that EIAV late budding defects were rescued by overexpression of the isolated Alix Bro1 domain (Bro1). Bro1-mediated rescue of EIAV release required the wild-type NC. EIAV NC mutants lost interactions with Bro1 and failed to produce viruses despite retaining the ability to self-assemble. Together, our studies establish a role for NC in the budding of retroviruses harboring divergent L domains and evolutionarily diverse NC sequences, suggesting the utilization of a common conserved mechanism and/or cellular factor rather than a specific motif.

Alix protein is substrate of Ozz-E3 ligase and modulates actin remodeling in skeletal muscle

Alix/AIP1 is a multifunctional adaptor protein that participates in basic cellular processes, including membrane trafficking and actin cytoskeleton assembly, by binding selectively to a variety of partner proteins. However, the mechanisms regulating Alix turnover, subcellular distribution, and function in muscle cells are unknown. We now report that Alix is expressed in skeletal muscle throughout myogenic differentiation. In myotubes, a specific pool of Alix colocalizes with Ozz, the substrate-binding component of the muscle-specific ubiquitin ligase complex Ozz-E3. We found that interaction of the two endogenous proteins in the differentiated muscle fibers changes Alix conformation and promotes its ubiquitination. This in turn regulates the levels of the protein in specific subcompartments, in particular the one containing the actin polymerization factor cortactin. In Ozz(-/-) myotubes, the levels of filamentous (F)-actin is perturbed, and Alix accumulates in large puncta positive for cortactin. In line with this observation, we show that the knockdown of Alix expression in C2C12 muscle cells affects the amount and distribution of F-actin, which consequently leads to changes in cell morphology, impaired formation of sarcolemmal protrusions, and defective cell motility. These findings suggest that the Ozz-E3 ligase regulates Alix at sites where the actin cytoskeleton undergoes remodeling.

A combination of exosomes carrying TSA derived from HLA-A2-positive human white buffy coat and polyI:C for use as a subcellular antitumor vaccination

To improve its antitumor effect, we used human leukocyte antigen -A2 (HLA-A2)-positive human dendritic cell (DC)-derived DEXs (DC-derived exosomes) to support NY-ESO-1 antigen and polyI:C, with the aim of increasing the proliferation of specific cytotoxic T lymphocytes (CTL) in transgenic mice. Mature dendritic cells derived from peripheral blood mononuclear cells (PBMC) were isolated from the blood of healthy adults with positive HLA-2A. Using centrifuge and membrane ultrafiltration, EXO (exosomes) were extracted from the supernatant of DCs secretions. Transgenic C57 mice were immunized with human-derived tumor testis antigen NY-ESO-1/EXO, with or without polyI:C. Mice were sacrificed four weeks after immunization, and spleen cells were isolated and tested for function. The experiments included antigen-specific CTL proliferation, as tested by dimerization and antitumor effects for K562 cells as well as melanoma, tested at different ratios of effected cells:target cells (0:1, 10:1, 50:1, and 100:1). Dimerization experiments indicated that the effect of DEX/TSA (tumor specific antigens) + PolyI:C was 2.36 ± 1.10% and the control was 0.38 ± 0.31%, while the effect of DEX/TSA was 1.97 ± 0.63% and the control was 0.36 ± 0.07%. Antitumor effects by DEX/TSA: PolyI:C for the cell ratios of 0:1, 10:1, 50:1, and 100:1 were 11.14 ± 1.36%, 14.17 ± 0.62%, 15.71 ± 2.48%, and 24.31 ± 2.91%, respectively, for K562 cells. The antitumor effects for DEX/TSA for the cell ratios of 0:1, 10:1, 50:1, and 100:1 were 12.23 ± 2.25%, 13.10 ± 1.57%, 15.27 ± 2.93%, and 19.87 ± 2.72%, respectively, for K562 cells. With ratios of 10:1 and 100:1, the antitumor effects of DEX/TSA + PolyI:C were better than for the DEX/TSA group (P < 0.05). However, higher ratios of effecter cells to target cells increased, and there were no significant improvements in antitumor effect for control cells. Combining PolyI:C with DEX/TSA derived from healthy human blood positive for HLA-A2 is a promising strategy for developing new subcellular antitumor vaccination.

Genetic interactions with mutations affecting septin assembly reveal ESCRT functions in budding yeast cytokinesis

Membrane trafficking via targeted exocytosis to the Saccharomyces cerevisiae bud neck provides new membrane and membrane-associated factors that are critical for cytokinesis. It remains unknown whether yeast plasma membrane abscission, the final step of cytokinesis, occurs spontaneously following extensive vesicle fusion, as in plant cells, or requires dedicated membrane fission machinery, as in cultured mammalian cells. Components of the endosomal sorting complexes required for transport (ESCRT) pathway, or close relatives thereof, appear to participate in cytokinetic abscission in various cell types, but roles in cell division had not been documented in budding yeast, where ESCRTs were first characterized. By contrast, the septin family of filament-forming cytoskeletal proteins were first identified by their requirement for yeast cell division. We show here that mutations in ESCRT-encoding genes exacerbate the cytokinesis defects of cla4Δ or elm1Δ mutants, in which septin assembly is perturbed at an early stage in cell division, and alleviate phenotypes of cells carrying temperature-sensitive alleles of a septin-encoding gene, CDC10. Elevated chitin synthase II (Chs2) levels coupled with aberrant morphogenesis and chitin deposition in elm1Δ cells carrying ESCRT mutations suggest that ESCRTs normally enhance the efficiency of cell division by promoting timely endocytic turnover of key cytokinetic enzymes.

Vesicle formation within endosomes: An ESCRT marks the spot

Vesicle-mediated cargo transport within the endomembrane system requires precise coordination between adaptor molecules, which recognize sorting signals on substrates, and factors that promote changes in membrane architecture. At endosomal compartments, a set of protein complexes collectively known as the ESCRT machinery sequesters transmembrane cargoes that harbor a ubiquitin modification and packages them into vesicles that bud into the endosome lumen. Several models have been postulated to describe this process. However, consensus in the field remains elusive. Here, we discuss recent findings regarding the structure and function of the ESCRT machinery, highlighting specific roles for ESCRT-0 and ESCRT-III in regulating cargo selection and vesicle formation.

The Bro1 domain-containing Myopic/HDPTP coordinates with Rab4 to regulate cell adhesion and migration

Protein tyrosine phosphatases (PTPs) are a group of tightly regulated enzymes that coordinate with protein tyrosine kinases to control protein phosphorylation during various cellular processes. Using genetic analysis in Drosophila non-transmembrane PTPs, we identified one role that Myopic (Mop), the Drosophila homolog of the human His domain phosphotyrosine phosphatase (HDPTP), plays in cell adhesion. Depletion of Mop results in aberrant integrin distribution and border cell dissociation during Drosophila oogenesis. Interestingly, Mop phosphatase activity is not required for its role in maintaining border cell cluster integrity. We further identified Rab4 GTPase as a Mop interactor in a yeast two-hybrid screen. Expression of the Rab4 dominant negative mutant leads to border cell dissociation and suppresses Mop-induced wing-blade adhesion defects, suggesting a critical role of Rab4 in Mop-mediated signaling. In mammals, it has been shown that Rab4-dependent recycling of integrins is necessary for cell adhesion and migration. We found that human HDPTP regulates the spatial distribution of Rab4 and integrin trafficking. Depletion of HDPTP resulted in actin reorganization and increased cell motility. Together, our findings suggest an evolutionarily conserved function of HDPTP-Rab4 in the regulation of endocytic trafficking, cell adhesion and migration.

Structure of the Bro1 domain protein BROX and functional analyses of the ALIX Bro1 domain in HIV-1 budding

Background

Bro1 domains are elongated, banana-shaped domains that were first identified in the yeast ESCRT pathway protein, Bro1p. Humans express three Bro1 domain-containing proteins: ALIX, BROX, and HD-PTP, which function in association with the ESCRT pathway to help mediate intraluminal vesicle formation at multivesicular bodies, the abscission stage of cytokinesis, and/or enveloped virus budding. Human Bro1 domains share the ability to bind the CHMP4 subset of ESCRT-III proteins, associate with the HIV-1 NC^Gag protein, and stimulate the budding of viral Gag proteins. The curved Bro1 domain structure has also been proposed to mediate membrane bending. To date, crystal structures have only been available for the related Bro1 domains from the Bro1p and ALIX proteins, and structures of additional family members should therefore aid in the identification of key structural and functional elements.

Methodology/Principal Findings

We report the crystal structure of the human BROX protein, which comprises a single Bro1 domain. The Bro1 domains from BROX, Bro1p and ALIX adopt similar overall structures and share two common exposed hydrophobic surfaces. Surface 1 is located on the concave face and forms the CHMP4 binding site, whereas Surface 2 is located at the narrow end of the domain. The structures differ in that only ALIX has an extended loop that projects away from the convex face to expose the hydrophobic Phe105 side chain at its tip. Functional studies demonstrated that mutations in Surface 1, Surface 2, or Phe105 all impair the ability of ALIX to stimulate HIV-1 budding.

Conclusions/Significance

Our studies reveal similarities in the overall folds and hydrophobic protein interaction sites of different Bro1 domains, and show that a unique extended loop contributes to the ability of ALIX to function in HIV-1 budding.

The Phe105 loop of Alix Bro1 domain plays a key role in HIV-1 release

Alix and cellular paralogs HD-PTP and Brox contain N-terminal Bro1 domains that bind ESCRT-III CHMP4. In contrast to HD-PTP and Brox, expression of the Bro1 domain of Alix alleviates HIV-1 release defects that result from interrupted access to ESCRT. In an attempt to elucidate this functional discrepancy, we solved the crystal structures of the Bro1 domains of HD-PTP and Brox. They revealed typical "boomerang" folds they share with the Bro1 Alix domain. However, they each contain unique structural features that may be relevant to their specific function(s). In particular, phenylalanine residue in position 105 (Phe105) of Alix belongs to a long loop that is unique to its Bro1 domain. Concurrently, mutation of Phe105 and surrounding residues at the tip of the loop compromise the function of Alix in HIV-1 budding without affecting its interactions with Gag or CHMP4. These studies identify a new functional determinant in the Bro1 domain of Alix.

Proline-rich regions and motifs in trafficking: from ESCRT interaction to viral exploitation

Most membrane-enveloped viruses bud from infected cells by hijacking the host ESCRT machinery. The ESCRTs are recruited to the budding sites by viral proteins that contain short proline (Pro)-rich motifs (PRMs) known as late domains. The late domains probably evolved by co-opting host PRMs involved in the normal functions of ESCRTs in endosomal sorting and cytokinesis. The solution and crystal structures of PRMs bound to their interaction partners explain the conserved roles of Pro and other residues that predominate in these sequences. PRMs are often grouped together in much larger Pro-rich regions (PRRs) of as many as 150 residues. The PRR of the ESCRT-associated protein, ALIX, autoregulates its conformation and activity. The robustness of different viral budding and host pathways to impairments in Pro-based interactions varies considerably. The known biology of PRM recognition in the ESCRT pathway seems, in principle, compatible with antiviral development, given our increasingly nuanced understanding of the relative weakness and robustness of the host and viral processes.

A screen for conditional growth suppressor genes identifies the Drosophila homolog of HD-PTP as a regulator of the oncoprotein Yorkie

Mammalian cancers depend on "multiple hits," some of which promote growth and some of which block apoptosis. We screened for mutations that require a synergistic block in apoptosis to promote tissue overgrowth and identified myopic (mop), the Drosophila homolog of the candidate tumor-suppressor and endosomal regulator His-domain protein tyrosine phosphatase (HD-PTP). We find that Myopic regulates the Salvador/Warts/Hippo (SWH) tumor suppressor pathway: Myopic PPxY motifs bind conserved residues in the WW domains of the transcriptional coactivator Yorkie, and Myopic colocalizes with Yorkie at endosomes. Myopic controls Yorkie endosomal association and protein levels, ultimately influencing expression of some Yorkie target genes. However, the antiapoptotic gene diap1 is not affected, which may explain the conditional nature of the myopic growth phenotype. These data establish Myopic as a Yorkie regulator and implicate Myopic-dependent association of Yorkie with endosomal compartments as a regulatory step in nuclear outputs of the SWH pathway.

Activation of the retroviral budding factor ALIX

The cellular ALIX protein functions within the ESCRT pathway to facilitate intralumenal endosomal vesicle formation, the abscission stage of cytokinesis, and enveloped virus budding. Here, we report that the C-terminal proline-rich region (PRR) of ALIX folds back against the upstream domains and auto-inhibits V domain binding to viral late domains. Mutations designed to destabilize the closed conformation of the V domain opened the V domain, increased ALIX membrane association, and enhanced virus budding. These observations support a model in which ALIX activation requires dissociation of the autoinhibitory PRR and opening of the V domain arms.

Bro1 binding to Snf7 regulates ESCRT-III membrane scission activity in yeast

The ubiquitin hydrolase activating factor Bro1 enhances ESCRT-III stability by inhibiting Vps4-mediated disassembly.

Endosomal sorting complexes required for transport (ESCRTs) promote the invagination of vesicles into the lumen of endosomes, the budding of enveloped viruses, and the separation of cells during cytokinesis. These processes share a topologically similar membrane scission event facilitated by ESCRT-III assembly at the cytosolic surface of the membrane. The Snf7 subunit of ESCRT-III in yeast binds directly to an auxiliary protein, Bro1. Like ESCRT-III, Bro1 is required for the formation of intralumenal vesicles at endosomes, but its role in membrane scission is unknown. We show that overexpression of Bro1 or its N-terminal Bro1 domain that binds Snf7 enhances the stability of ESCRT-III by inhibiting Vps4-mediated disassembly in vivo and in vitro. This stabilization effect correlates with a reduced frequency in the detachment of intralumenal vesicles as observed by electron tomography, implicating Bro1 as a regulator of ESCRT-III disassembly and membrane scission activity.

Decoding the intrinsic mechanism that prohibits ALIX interaction with ESCRT and viral proteins

The adaptor protein ALIX [ALG-2 (apoptosis-linked-gene-2 product)-interacting protein X] links retroviruses to ESCRT (endosomal sorting complex required for transport) machinery during retroviral budding. This function of ALIX requires its interaction with the ESCRT-III component CHMP4 (charged multivesicular body protein 4) at the N-terminal Bro1 domain and retroviral Gag proteins at the middle V domain. Since cytoplasmic or recombinant ALIX is unable to interact with CHMP4 or retroviral Gag proteins under non-denaturing conditions, we constructed ALIX truncations and mutations to define the intrinsic mechanism through which ALIX interactions with these partner proteins are prohibited. Our results demonstrate that an intramolecular interaction between Patch 2 in the Bro1 domain and the TSG101 (tumour susceptibility gene 101 protein)-docking site in the proline-rich domain locks ALIX into a closed conformation that renders ALIX unable to interact with CHMP4 and retroviral Gag proteins. Relieving the intramolecular interaction of ALIX, by ectopically expressing a binding partner for one of the intramolecular interaction sites or by deleting one of these sites, promotes ALIX interaction with these partner proteins and facilitates ALIX association with the membrane. Ectopic expression of a GFP (green fluorescent protein)-ALIX mutant with a constitutively open conformation, but not the wild-type protein, increases EIAV (equine infectious anaemia virus) budding from HEK (human embryonic kidney)-293 cells. These findings predict that relieving the autoinhibitory intramolecular interaction of ALIX is a critical step for ALIX to participate in retroviral budding.

Alix regulates egress of hepatitis B virus naked capsid particles in an ESCRT-independent manner

Hepatitis B virus (HBV) is an enveloped DNA virus that exploits the endosomal sorting complexes required for transport (ESCRT) pathway for budding. In addition to infectious particles, HBV‐replicating cells release non‐enveloped (nucleo)capsids, but their functional implication and pathways of release are unclear. Here, we focused on the molecular mechanisms and found that the sole expression of the HBV core protein is sufficient for capsid release. Unexpectedly, released capsids are devoid of a detectable membrane bilayer, implicating a non‐vesicular exocytosis process. Unlike virions, naked capsid budding does not require the ESCRT machinery. Rather, we identified Alix, a multifunctional protein with key roles in membrane biology, as a regulator of capsid budding. Ectopic overexpression of Alix enhanced capsid egress, while its depletion inhibited capsid release. Notably, the loss of Alix did not impair HBV production, furthermore indicating that virions and capsids use diverse export routes. By mapping of Alix domains responsible for its capsid release‐mediating activity, its Bro1 domain was found to be required and sufficient. Alix binds to core via its Bro1 domain and retained its activity even if its ESCRT‐III binding site is disrupted. Together, the boomerang‐shaped Bro1 domain of Alix appears to escort capsids without ESCRT.

Basic residues in the nucleocapsid domain of Gag are critical for late events of HIV-1 budding

The p6 region of HIV-1 Gag contains two late (L) domains, PTAP and LYPXnL, that bind the cellular proteins Tsg101 and Alix, respectively. These interactions are thought to recruit members of the host fission machinery (ESCRT) to facilitate HIV-1 release. Here we report a new role for the p6-adjacent nucleocapsid (NC) domain in HIV-1 release. The mutation of basic residues in NC caused a pronounced decrease in virus release from 293T cells, although NC mutant Gag proteins retained the ability to interact with cellular membranes and RNAs. Remarkably, electron microscopy analyses of these mutants revealed arrested budding particles at the plasma membrane, analogous to those seen following the disruption of the PTAP motif. This result indicated that the basic residues in NC are important for virus budding. When analyzed in physiologically more relevant T-cell lines (Jurkat and CEM), NC mutant viruses remained tethered to the plasma membrane or to each other by a membranous stalk, suggesting membrane fission impairment. Remarkably, NC mutant release defects were alleviated by the coexpression of a Gag protein carrying a wild-type (WT) NC domain but devoid of all L domain motifs and by providing alternative access to the ESCRT pathway, through the in trans expression of the ubiquitin ligase Nedd4.2s. Since NC mutant Gag proteins retained the interaction with Tsg101, we concluded that NC mutant budding arrests might have resulted from the inability of Gag to recruit or utilize members of the host ESCRT machinery that act downstream of Tsg101. Together, these data support a model in which NC plays a critical role in HIV-1 budding.

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.

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.

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.

Detection of the Endosomal Sorting Complex Required for Transport in Entamoeba histolytica and Characterization of the EhVps4 Protein

Eukaryotic endocytosis involves multivesicular bodies formation, which is driven by endosomal sorting complexes required for transport (ESCRT). Here, we showed the presence and expression of homologous ESCRT genes in Entamoeba histolytica. We cloned and expressed the Ehvps4 gene, an ESCRT member, to obtain the recombinant EhVps4 and generate specific antibodies, which immunodetected EhVps4 in cytoplasm of trophozoites. Bioinformatics and biochemical studies evidenced that rEhVps4 is an ATPase, whose activity depends on the conserved E211 residue. Next, we generated trophozoites overexpressing EhVps4 and mutant EhVps4-E211Q FLAG-tagged proteins. The EhVps4-FLAG was located in cytosol and at plasma membrane, whereas the EhVps4-E211Q-FLAG was detected as abundant cytoplasmic dots in trophozoites. Erythrophagocytosis, cytopathic activity, and hepatic damage in hamsters were not improved in trophozoites overexpressing EhVps4-FLAG. In contrast, EhVps4-E211Q-FLAG protein overexpression impaired these properties. The localization of EhVps4-FLAG around ingested erythrocytes, together with our previous results, strengthens the role for EhVps4 in E. histolytica phagocytosis and virulence.

Endocytic pathway is required for Drosophila Toll innate immune signaling

The Toll signaling pathway is required for the innate immune response against fungi and Gram-positive bacteria in Drosophila. Here we show that the endosomal proteins Myopic (Mop) and Hepatocyte growth factor-regulated tyrosine kinase substrate (Hrs) are required for the activation of the Toll signaling pathway. This requirement is observed in cultured cells and in flies, and epistasis experiments show that the Mop protein functions upstream of the MyD88 adaptor and the Pelle kinase. Mop and Hrs, which are critical components of the ESCRT-0 endocytosis complex, colocalize with the Toll receptor in endosomes. We conclude that endocytosis is required for the activation of the Toll signaling pathway.

An LYPSL late domain in the gag protein contributes to the efficient release and replication of Rous sarcoma virus

The efficient release of newly assembled retrovirus particles from the plasma membrane requires the recruitment of a network of cellular proteins (ESCRT machinery) normally involved in the biogenesis of multivesicular bodies and in cytokinesis. Retroviruses and other enveloped viruses recruit the ESCRT machinery through three classes of short amino acid consensus sequences termed late domains: PT/SAP, PPXY, and LYPX(n)L. The major late domain of Rous sarcoma virus (RSV) has been mapped to a PPPY motif in Gag that binds members of the Nedd4 family of ubiquitin ligases. RSV Gag also contains a second putative late domain motif, LYPSL, positioned 5 amino acids downstream of PPPY. LYPX(n)L motifs have been shown to support budding in other retroviruses by binding the ESCRT adaptor protein Alix. To investigate a possible role of the LYPSL motif in RSV budding, we constructed PPPY and LYPSL mutants in the context of an infectious virus and then analyzed the budding rates, spreading profiles, and budding morphology. The data imply that the LYPSL motif acts as a secondary late domain and that its role in budding is amplified in the absence of a fully functional PPPY motif. The LYPXL motif proved to be a stronger late domain when an aspartic acid was substituted for the native serine, recapitulating the properties of the LYPDL late domain of equine infectious anemia virus. The overexpression of human Alix in the absence of a fully functional PPPY late domain partially rescued both the viral budding rate and viral replication, supporting a model in which the RSV LYPSL motif mediates budding through an interaction with the ESCRT adaptor protein Alix.

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.

Intervention of Bro1 in pH-responsive Rim20 localization in Saccharomyces cerevisiae

Yeast cells contain two Bro1 domain proteins: Bro1, which is required for endosomal trafficking, and Rim20, which is required for the response to the external pH via the Rim101 pathway. Rim20 associates with endosomal structures under alkaline growth conditions, when it promotes activation of Rim101 through proteolytic cleavage. We report here that the pH-dependent localization of Rim20 is contingent on the amount of Bro1 in the cell. Cells that lack Bro1 have increased endosomal Rim20-green fluorescent protein (GFP) under acidic conditions; cells that overexpress Bro1 have reduced endosomal Rim20-GFP under acidic or alkaline conditions. The novel endosomal association of Rim20-GFP in the absence of Bro1 requires ESCRT components including Vps27 but not specific Rim101 pathway components such as Dfg16. Vps27 influences the localization of Bro1 but is not required for RIM101 pathway activation in wild-type cells, thus suggesting that Rim20 enters the Bro1 localization pathway when a vacancy exists. Despite altered localization of Rim20, the lack of Bro1 does not bypass the need for signaling protein Dfg16 to activate Rim101, as evidenced by the expression levels of the Rim101 target genes RIM8 and SMP1. Therefore, endosomal association of Rim20 is not sufficient to promote Rim101 activation.

Mutational analysis of Candida albicans SNF7 reveals genetically separable Rim101 and ESCRT functions and demonstrates divergence in bro1-domain protein interactions

The opportunistic pathogen Candida albicans can grow over a wide pH range, which is associated with its ability to colonize and infect distinct host niches. C. albicans growth in neutral-alkaline environments requires proteolytic activation of the transcription factor Rim101. Rim101 activation requires Snf7, a member of the endosomal sorting complex required for transport (ESCRT) pathway. We hypothesized that Snf7 has distinct functions in the Rim101 and ESCRT pathways, which we tested by alanine-scanning mutagenesis. While some snf7 alleles conferred no defects, we identified alleles with solely ESCRT-dependent, solely Rim101-dependent, or both Rim101- and ESCRT-dependent defects. Thus, Snf7 function in these two pathways is at least partially separable. Both Rim101- and ESCRT-dependent functions require Snf7 recruitment to the endosomal membrane and alleles that disrupted both pathways were found to localize normally, suggesting a downstream defect. Most alleles that conferred solely Rim101-dependent defects were still able to process Rim101 normally under steady-state conditions. However, these same strains did display a kinetic defect in Rim101 processing. Several alleles with solely Rim101-dependent defects mapped to the C-terminal end of Snf7. Further analyses suggested that these mutations disrupted interactions with bro-domain proteins, Rim20 and Bro1, in overlapping but slightly divergent Snf7 domains.

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.

Structure and function of the ESCRT-II-III interface in multivesicular body biogenesis

The ESCRT-II-ESCRT-III interaction coordinates the sorting of ubiquitinated cargo with the budding and scission of intralumenal vesicles into multivesicular bodies. The interacting regions of these complexes were mapped to the second winged helix domain of human ESCRT-II subunit VPS25 and the first helix of ESCRT-III subunit VPS20. The crystal structure of this complex was determined at 2.0 A resolution. Residues involved in structural interactions explain the specificity of ESCRT-II for Vps20, and are critical for cargo sorting in vivo. ESCRT-II directly activates ESCRT-III-driven vesicle budding and scission in vitro via these structural interactions. VPS20 and ESCRT-II bind membranes with nanomolar affinity, explaining why binding to ESCRT-II is dispensable for the recruitment of Vps20 to membranes. Docking of the ESCRT-II-VPS20(2) supercomplex reveals a convex membrane-binding surface, suggesting a hypothesis for negative membrane curvature induction in the nascent intralumenal vesicle.

A crescent-shaped ALIX dimer targets ESCRT-III CHMP4 filaments

ALIX recruits ESCRT-III CHMP4 and is involved in membrane remodeling during endosomal receptor sorting, budding of some enveloped viruses, and cytokinesis. We show that ALIX dimerizes via the middle domain (ALIX(-V)) in solution. Structural modeling based on small angle X-ray scattering (SAXS) data reveals an elongated crescent-shaped conformation for dimeric ALIX lacking the proline-rich domain (ALIX(BRO1-V)). Mutations at the dimerization interface prevent dimerization and induce an open elongated monomeric conformation of ALIX(-V) as determined by SAXS modeling. ALIX dimerizes in vivo and dimeric ALIX colocalizes with CHMP4B upon coexpression. We show further that ALIX dimerization affects HIV-1 budding. C-terminally truncated activated CHMP4B retaining the ALIX binding site forms linear, circular, and helical filaments in vitro, which can be bridged by ALIX. Our data suggest that dimeric ALIX represents the active form that interacts with ESCRT-III CHMP4 polymers and functions as a scaffolding protein during membrane remodeling processes.

Scratching the niche that controls Caenorhabditis elegans germline stem cells

The Caenorhabditis elegans gonad provides a well-defined model for a stem cell niche and its control of self-renewal and differentiation. The distal tip cell (DTC) forms a mesenchymal niche that controls germline stem cells (GSCs), both to generate the germline tissue during development and to maintain it during adulthood. The DTC uses GLP-1/Notch signaling to regulate GSCs; germ cells respond to Notch signaling with a network of RNA regulators to control the decision between self-renewal and entry into the meiotic cell cycle.

Divergent Bro1 domains share the capacity to bind human immunodeficiency virus type 1 nucleocapsid and to enhance virus-like particle production

To promote the release of infectious virions, human immunodeficiency virus type 1 (HIV-1) exploits the endosomal sorting complex required for transport (ESCRT) pathway by engaging Tsg101 and ALIX through late assembly (L) domains in p6 Gag. An LYPx(n)L motif in p6 serves as docking site for the central V domain of ALIX and is required for its ability to stimulate HIV-1 budding. Additionally, the nucleocapsid (NC) domain of Gag binds to the N-terminal Bro1 domain of ALIX, which connects ALIX to the membrane-deforming ESCRT-III complex via its CHMP4 subunits. Since the isolated Bro1 domain of ALIX is sufficient to markedly stimulate virus-like particle (VLP) production in a minimal Gag rescue assay, we examined whether the Bro1 domains of other human proteins possess a similar activity. We now show that the Bro1 domain-only protein Brox and the isolated Bro1 domains of HD-PTP and rhophilin all bind to HIV-1 NC. Furthermore, all shared the capacity to stimulate VLP production by a minimal HIV-1 Gag molecule, and Brox in particular was as potent as the Bro1 domain of ALIX in this assay. Unexpectedly, Brox retained significant activity even if its CHMP4 binding site was disrupted. Thus, the ability to assist in VLP production may be an intrinsic property of the boomerang-shaped Bro1 domain.

The nucleocapsid region of HIV-1 Gag cooperates with the PTAP and LYPXnL late domains to recruit the cellular machinery necessary for viral budding

HIV-1 release is mediated through two motifs in the p6 region of Gag, PTAP and LYPX(n)L, which recruit cellular proteins Tsg101 and Alix, respectively. The Nucleocapsid region of Gag (NC), which binds the Bro1 domain of Alix, also plays an important role in HIV-1 release, but the underlying mechanism remains unclear. Here we show that the first 202 residues of the Bro1 domain (Bro(i)) are sufficient to bind Gag. Bro(i) interferes with HIV-1 release in an NC-dependent manner and arrests viral budding at the plasma membrane. Similar interrupted budding structures are seen following over-expression of a fragment containing Bro1 with the adjacent V domain (Bro1-V). Although only Bro1-V contains binding determinants for CHMP4, both Bro(i) and Bro1-V inhibited release via both the PTAP/Tsg101 and the LYPX(n)L/Alix pathways, suggesting that they interfere with a key step in HIV-1 release. Remarkably, we found that over-expression of Bro1 rescued the release of HIV-1 lacking both L domains. This rescue required the N-terminal region of the NC domain in Gag and the CHMP4 binding site in Bro1. Interestingly, release defects due to mutations in NC that prevented Bro1 mediated rescue of virus egress were rescued by providing a link to the ESCRT machinery via Nedd4.2s over-expression. Our data support a model in which NC cooperates with PTAP in the recruitment of cellular proteins necessary for its L domain activity and binds the Bro1-CHMP4 complex required for LYPX(n)L-mediated budding.

The CHMP4b- and Src-docking sites in the Bro1 domain are autoinhibited in the native state of Alix

The Bro1 domain of Alix [ALG-2 (apoptosis-linked gene 2)-interacting protein X], which plays important roles in endosomal sorting and multiple ESCRT (endosomal sorting complex required for transport)-linked processes, contains the docking sites for the ESCRT-III component CHMP4b (charged multivesicular body protein 4b) and the regulatory tyrosine kinase, Src. Although the structural bases for these docking sites have been defined by crystallography studies, it has not been determined whether these sites are available in the native state of Alix. In the present study, we demonstrate that these two docking sites are unavailable in recombinant Alix under native conditions and that their availabilities can be induced by detergents. In HEK (human embryonic kidney)-293 cell lysates, these two docking sites are not available in cytosolic Alix, but are available in membrane-bound Alix. These findings show that the native state of Alix does not have a functional Bro1 domain and predict that Alix's involvement in endosomal sorting and other ESCRT-linked processes requires an activation step that relieves the autoinhibition of the Bro1 domain.

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.