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

All roads lead to the vacuole-autophagic transport as part of the endomembrane trafficking network in plants

Plants regulate their development and response to the changing environment by sensing and interpreting environmental signals. Intracellular trafficking pathways including endocytic-, vacuolar-, and autophagic trafficking are important for the various aspects of responses in plants. Studies in the last decade have shown that the autophagic transport pathway uses common key components of endomembrane trafficking as well as specific regulators. A number of factors previously described for their function in endosomal trafficking have been discovered to be involved in the regulation of autophagy in plants. These include conserved endocytic machineries, such as the endosomal sorting complex required for transport (ESCRT), subunits of the HOPS and exocyst complexes, SNAREs, and RAB GTPases as well as plant-specific proteins. Defects in these factors have been shown to cause impairment of autophagosome formation, transport, fusion, and degradation, suggesting crosstalk between autophagy and other intracellular trafficking processes. In this review, we focus mainly on possible functions of endosomal trafficking components in autophagy.

Dividing the Archaeal Way: The Ancient Cdv Cell-Division Machinery

Cell division in most prokaryotes is mediated by the well-studied fts genes, with FtsZ as the principal player. In many archaeal species, however, division is orchestrated differently. The Crenarchaeota phylum of archaea features the action of the three proteins, CdvABC. This Cdv system is a unique and less-well-studied division mechanism that merits closer inspection. In vivo, the three Cdv proteins form a composite band that contracts concomitantly with the septum formation. Of the three Cdv proteins, CdvA is the first to be recruited to the division site, while CdvB and CdvC are thought to participate in the active part of the Cdv division machinery. Interestingly, CdvB shares homology with a family of proteins from the eukaryotic ESCRT-III complex, and CdvC is a homolog of the eukaryotic Vps4 complex. These two eukaryotic complexes are key factors in the endosomal sorting complex required for transport (ESCRT) pathway, which is responsible for various budding processes in eukaryotic cells and which participates in the final stages of division in Metazoa. There, ESCRT-III forms a contractile machinery that actively cuts the membrane, whereas Vps4, which is an ATPase, is necessary for the turnover of the ESCRT membrane-abscission polymers. In contrast to CdvB and CdvC, CdvA is unique to the archaeal Crenarchaeota and Thaumarchaeota phyla. The Crenarchaeota division mechanism has often been suggested to represent a simplified version of the ESCRT division machinery thus providing a model system to study the evolution and mechanism of cell division in higher organisms. However, there are still many open questions regarding this parallelism and the division mechanism of Crenarchaeota. Here, we review the existing data on the role of the Cdv proteins in the division process of Crenarchaeota as well as concisely review the ESCRT system in eukaryotes. We survey the similarities and differences between the division and abscission mechanisms in the two cases. We suggest that the Cdv system functions differently in archaea than ESCRT does in eukaryotes, and that, unlike the eukaryotic case, the Cdv system's main function may be related to surplus membrane invagination and cell-wall synthesis.

Image co-localization – co-occurrence versus correlation

Fluorescence image co-localization analysis is widely utilized to suggest biomolecular interaction. However, there exists some confusion as to its correct implementation and interpretation. In reality, co-localization analysis consists of at least two distinct sets of methods, termed co-occurrence and correlation. Each approach has inherent and often contrasting strengths and weaknesses. Yet, neither one can be considered to always be preferable for any given application. Rather, each method is most appropriate for answering different types of biological question. This Review discusses the main factors affecting multicolor image co-occurrence and correlation analysis, while giving insight into the types of biological behavior that are better suited to one approach or the other. Further, the limits of pixel-based co-localization analysis are discussed in the context of increasingly popular super-resolution imaging techniques.

Selection of housekeeping genes and demonstration of RNAi in cotton leafhopper, Amrasca biguttula biguttula (Ishida)

Amrasca biguttula biguttula (Ishida) commonly known as cotton leafhopper is a severe pest of cotton and okra. Not much is known on this insect at molecular level due to lack of genomic and transcriptomic data. To prepare for functional genomic studies in this insect, we evaluated 15 common housekeeping genes (Tub, B-Tub, EF alpha, GADPH, UbiCF, RP13, Ubiq, G3PD, VATPase, Actin, 18s, 28s, TATA, ETF, SOD and Cytolytic actin) during different developmental stages and under starvation stress. We selected early (1st and 2nd), late (3rd and 4th) stage nymphs and adults for identification of stable housekeeping genes using geNorm, NormFinder, BestKeeper and RefFinder software. Based on the different algorithms, RP13 and VATPase are identified as the most suitable reference genes for quantification of gene expression by reverse transcriptase quantitative PCR (RT-qPCR). Based on RefFinder which comprehended the results of three algorithms, RP13 in adults, Tubulin (Tub) in late nymphs, 28S in early nymph and UbiCF under starvation stress were identified as the most stable genes. We also developed methods for feeding double-stranded RNA (dsRNA) incorporated in the diet. Feeding dsRNA targeting Snf7, IAP, AQP1, and VATPase caused 56.17-77.12% knockdown of targeted genes compared to control and 16 to 48% mortality of treated insects when compared to control.

Fragile X Mental Retardation Protein Regulates Activity-Dependent Membrane Trafficking and Trans-Synaptic Signaling Mediating Synaptic Remodeling

Fragile X syndrome (FXS) is the leading monogenic cause of autism and intellectual disability. The disease arises through loss of fragile X mental retardation protein (FMRP), which normally exhibits peak expression levels in early-use critical periods, and is required for activity-dependent synaptic remodeling during this transient developmental window. FMRP canonically binds mRNA to repress protein translation, with targets that regulate cytoskeleton dynamics, membrane trafficking, and trans-synaptic signaling. We focus here on recent advances emerging in these three areas from the Drosophila disease model. In the well-characterized central brain mushroom body (MB) olfactory learning/memory circuit, FMRP is required for activity-dependent synaptic remodeling of projection neurons innervating the MB calyx, with function tightly restricted to an early-use critical period. FMRP loss is phenocopied by conditional removal of FMRP only during this critical period, and rescued by FMRP conditional expression only during this critical period. Consistent with FXS hyperexcitation, FMRP loss defects are phenocopied by heightened sensory experience and targeted optogenetic hyperexcitation during this critical period. FMRP binds mRNA encoding Drosophila ESCRTIII core component Shrub (human CHMP4 homolog) to restrict Shrub translation in an activity-dependent mechanism only during this same critical period. Shrub mediates endosomal membrane trafficking, and perturbing Shrub expression is known to interfere with neuronal process pruning. Consistently, FMRP loss and Shrub overexpression targeted to projection neurons similarly causes endosomal membrane trafficking defects within synaptic boutons, and genetic reduction of Shrub strikingly rescues Drosophila FXS model defects. In parallel work on the well-characterized giant fiber (GF) circuit, FMRP limits iontophoretic dye loading into central interneurons, demonstrating an FMRP role controlling core neuronal properties through the activity-dependent repression of translation. In the well-characterized Drosophila neuromuscular junction (NMJ) model, developmental synaptogenesis and activity-dependent synaptic remodeling both require extracellular matrix metalloproteinase (MMP) enzymes interacting with the heparan sulfate proteoglycan (HSPG) glypican dally-like protein (Dlp) to restrict trans-synaptic Wnt signaling, with FXS synaptogenic defects alleviated by both MMP and HSPG reduction. This new mechanistic axis spanning from activity to FMRP to HSPG-dependent MMP regulation modulates activity-dependent synaptogenesis. We discuss future directions for these mechanisms, and intersecting research priorities for FMRP in glial and signaling interactions.

Distinct Roles of Cellular ESCRT-I and ESCRT-III Proteins in Efficient Entry and Egress of Budded Virions of Autographa californica Multiple Nucleopolyhedrovirus

The endosomal sorting complex required for transport (ESCRT) machinery is necessary for budding of many enveloped viruses. Recently, it was demonstrated that Vps4, the key regulator for recycling of the ESCRT-III complex, is required for efficient infection by the baculovirus Autographa californica multiple nucleopolyhedrovirus (AcMNPV). However, ESCRT assembly, regulation, and function are complex, and little is known regarding the details of participation of specific ESCRT complexes in AcMNPV infection. In this study, the core components of ESCRT-I (Tsg101 and Vps28) and ESCRT-III (Vps2B, Vps20, Vps24, Snf7, Vps46, and Vps60) were cloned from Spodoptera frugiperda Using a viral complementation system and RNA interference (RNAi) assays, we found that ESCRT-I and ESCRT-III complexes are required for efficient entry of AcMNPV into insect cells. In cells knocking down or overexpressing dominant negative (DN) forms of the components of ESCRT-I and ESCRT-III complexes, entering virions were partially trapped within the cytosol. To examine only egress, cells were transfected with the double-stranded RNA (dsRNA) targeting an individual ESCRT-I or ESCRT-III gene and viral bacmid DNA or viral bacmid DNA that expressed DN forms of ESCRT-I and ESCRT-III components. We found that ESCRT-III components (but not ESCRT-I components) are required for efficient nuclear egress of progeny nucleocapsids. In addition, we found that several baculovirus core or conserved proteins (Ac11, Ac76, Ac78, GP41, Ac93, Ac103, Ac142, and Ac146) interact with Vps4 and components of ESCRT-III. We propose that these viral proteins may form an "egress complex" that is involved in recruiting ESCRT-III components to a virus egress domain on the nuclear membrane.IMPORTANCE The ESCRT system is hijacked by many enveloped viruses to mediate budding and release. Recently, it was found that Vps4, the key regulator of the cellular ESCRT machinery, is necessary for efficient entry and egress of Autographa californica multiple nucleopolyhedrovirus (AcMNPV). However, little is known about the roles of specific ESCRT complexes in AcMNPV infection. In this study, we demonstrated that ESCRT-I and ESCRT-III complexes are required for efficient entry of AcMNPV into insect cells. The components of ESCRT-III (but not ESCRT-I) are also necessary for efficient nuclear egress of progeny nucleocapsids. Several baculovirus core or conserved proteins were found to interact with Vps4 and components of ESCRT-III, and these interactions may suggest the formation of an "egress complex" involved in the nuclear release or transport of viral nucleocapsids.

Recruitment dynamics of ESCRT-III and Vps4 to endosomes and implications for reverse membrane budding

The ESCRT machinery mediates reverse membrane scission. By quantitative fluorescence lattice light-sheet microscopy, we have shown that ESCRT-III subunits polymerize rapidly on yeast endosomes, together with the recruitment of at least two Vps4 hexamers. During their 3–45 s lifetimes, the ESCRT-III assemblies accumulated 75–200 Snf7 and 15–50 Vps24 molecules. Productive budding events required at least two additional Vps4 hexamers. Membrane budding was associated with continuous, stochastic exchange of Vps4 and ESCRT-III components, rather than steady growth of fixed assemblies, and depended on Vps4 ATPase activity. An all-or-none step led to final release of ESCRT-III and Vps4. Tomographic electron microscopy demonstrated that acute disruption of Vps4 recruitment stalled membrane budding. We propose a model in which multiple Vps4 hexamers (four or more) draw together several ESCRT-III filaments. This process induces cargo crowding and inward membrane buckling, followed by constriction of the nascent bud neck and ultimately ILV generation by vesicle fission.

Cytokinesis in Metazoa and Fungi

SUMMARYCell division-cytokinesis-involves large-scale rearrangements of the entire cell. Primarily driven by cytoskeletal proteins, cytokinesis also depends on topological rearrangements of the plasma membrane, which are coordinated with nuclear division in both space and time. Despite the fundamental nature of the process, different types of eukaryotic cells show variations in both the structural mechanisms of cytokinesis and the regulatory controls. In animal cells and fungi, a contractile actomyosin-based structure plays a central, albeit flexible, role. Here, the underlying molecular mechanisms are summarized and integrated and common themes are highlighted.

ESCRT genes and regulation of developmental signaling

ESCRT (Endosomal Sorting Complex Required for Transport) proteins have been shown to control an increasing number of membrane-associated processes. Some of these, and prominently regulation of receptor trafficking, profoundly shape signal transduction. Evidence in fungi, plants and multiple animal models support the emerging concept that ESCRTs are main actors in coordination of signaling with the changes in cells and tissues occurring during development and homeostasis. Consistent with their pleiotropic function, ESCRTs are regulated in multiple ways to tailor signaling to developmental and homeostatic needs. ESCRT activity is crucial to correct execution of developmental programs, especially at key transitions, allowing eukaryotes to thrive and preventing appearance of congenital defects.

ESCRT-III Membrane Trafficking Misregulation Contributes To Fragile X Syndrome Synaptic Defects

The leading cause of heritable intellectual disability (ID) and autism spectrum disorders (ASD), Fragile X syndrome (FXS), is caused by loss of the mRNA-binding translational suppressor Fragile X Mental Retardation Protein (FMRP). In the Drosophila FXS disease model, we found FMRP binds shrub mRNA (human Chmp4) to repress Shrub expression, causing overexpression during the disease state early-use critical period. The FXS hallmark is synaptic overelaboration causing circuit hyperconnectivity. Testing innervation of a central brain learning/memory center, we found FMRP loss and Shrub overexpression similarly increase connectivity. The ESCRT-III core protein Shrub has a central role in endosome-to-multivesicular body membrane trafficking, with synaptic requirements resembling FMRP. Consistently, we found FMRP loss and Shrub overexpression similarly elevate endosomes and result in the arrested accumulation of enlarged intraluminal vesicles within synaptic boutons. Importantly, genetic correction of Shrub levels in the FXS model prevents synaptic membrane trafficking defects and strongly restores innervation. These results reveal a new molecular mechanism underpinning the FXS disease state.

The role of ESCRT during development and functioning of the nervous system

The endosomal sorting complex required for transport (ESCRT) is made of subcomplexes (ESCRT 0-III), crucial to membrane remodelling at endosomes, nuclear envelope and cell surface. ESCRT-III shapes membranes and in most cases cooperates with the ATPase VPS4 to mediate fission of membrane necks from the inside. The first ESCRT complexes mainly serve to catalyse the formation of ESCRT-III but can be bypassed by accessory proteins like the Alg-2 interacting protein-X (ALIX). In the nervous system, ALIX/ESCRT controls the survival of embryonic neural progenitors and later on the outgrowth and pruning of axons and dendrites, all necessary steps to establish a functional brain. In the adult brain, ESCRTs allow the endosomal turn over of synaptic vesicle proteins while stable ESCRT complexes might serve as scaffolds for the postsynaptic parts. The necessity of ESCRT for the harmonious function of the brain has its pathological counterpart, the mutations in CHMP2B of ESCRT-III giving rise to several neurodegenerative diseases.

Growing functions of the ESCRT machinery in cell biology and viral replication

The vast expansion in recent years of the cellular processes promoted by the endosomal sorting complex required for transport (ESCRT) machinery has reinforced its identity as a modular system that uses multiple adaptors to recruit the core membrane remodelling activity at different intracellular sites and facilitate membrane scission. Functional connections to processes such as the aurora B-dependent abscission checkpoint also highlight the importance of the spatiotemporal regulation of the ESCRT machinery. Here, we summarise the role of ESCRTs in viral budding, and what we have learned about the ESCRT pathway from studying this process. These advances are discussed in the context of areas of cell biology that have been transformed by research in the ESCRT field, including cytokinetic abscission, nuclear envelope resealing and plasma membrane repair.

Dynamic and elastic shape transitions in curved ESCRT-III filaments

The ESCRT-III complex is an evolutionary ancient and conserved complex that catalyzes fission of lipid membranes from the lumen of the neck in many, if not all processes requiring this specific fission reaction. The ESCRT-III membrane remodeling complex is unique as its molecular and polymeric structures do not intuitively suggests how it could deform and break lipid membranes. Here we review the common structural features of the ESCRT-III subunits, and the shape diversity of the various filamentous forms. We propose a simple geometry and elasticity framework that could help to isolate which features of the ESCRT-III filaments are common to all filamentous forms as well as to explain their diversity. We speculate on how these features could provide mechanistic insights into the many functions of the ESCRT-III complex.

Dynamic subunit turnover in ESCRT-III assemblies is regulated by Vps4 to mediate membrane remodelling during cytokinesis

The Endosomal Sorting Complex Required for Transport (ESCRT)-III mediates membrane fission in fundamental cellular processes, including cytokinesis. ESCRT-III is thought to form persistent filaments that over time increase their curvature to constrict membranes. Unexpectedly, we found that ESCRT-III at the midbody of human cells rapidly turns over subunits with cytoplasmic pools while gradually forming larger assemblies. ESCRT-III turnover depended on the ATPase VPS4, which accumulated at the midbody simultaneously with ESCRT-III subunits, and was required for assembly of functional ESCRT-III structures. In vitro, the Vps2/Vps24 subunits of ESCRT-III formed side-by-side filaments with Snf7 and inhibited further polymerization, but the growth inhibition was alleviated by the addition of Vps4 and ATP. High-speed atomic force microscopy further revealed highly dynamic arrays of growing and shrinking ESCRT-III spirals in presence of Vps4. Continuous ESCRT-III remodeling by subunit turnover might facilitate shape adaptions to variable membrane geometries, with broad implications for diverse cellular processes.

ESCRT-mediated vesicle concatenation in plant endosomes

ESCRT proteins play essential functions by remodeling cellular membranes. Buono et al. report on a novel ESCRT-dependent mechanism in plant endosomes that leads to sequential concatenation of vesicle buds by temporally uncoupling membrane constriction from membrane fission. During this process, ESCRT-III proteins remain inside endosomes after intralumenal vesicle release.

Ubiquitinated plasma membrane proteins (cargo) are delivered to endosomes and sorted by endosomal sorting complex required for transport (ESCRT) machinery into endosome intralumenal vesicles (ILVs) for degradation. In contrast to the current model that postulates that ILVs form individually from inward budding of the endosomal limiting membrane, plant ILVs form as networks of concatenated vesicle buds by a novel vesiculation mechanism. We ran computational simulations based on experimentally derived diffusion coefficients of an ESCRT cargo protein and electron tomograms of Arabidopsis thaliana endosomes to measure cargo escape from budding ILVs. We found that 50% of the ESCRT cargo would escape from a single budding profile in 5–20 ms and from three concatenated ILVs in 80–200 ms. These short cargo escape times predict the need for strong diffusion barriers in ILVs. Consistent with a potential role as a diffusion barrier, we find that the ESCRT-III protein SNF7 remains associated with ILVs and is delivered to the vacuole for degradation.

Regulation of yeast ESCRT-III membrane scission activity by the Doa4 ubiquitin hydrolase

Doa4 is the ubiquitin hydrolase in yeast that deubiquitinates transmembrane proteins sorted by ESCRTs. Results support a model for bidirectional regulation between Doa4 and the ESCRT-III complex.

ESCRT-III executes membrane scission during the budding of intralumenal vesicles (ILVs) at endosomes. The scission mechanism is unknown but appears to be linked to the cycle of assembly and disassembly of ESCRT-III complexes at membranes. Regulating this cycle is therefore expected to be important for determining the timing of ESCRT-III–mediated membrane scission. We show that in Saccharomyces cerevisiae, ESCRT-III complexes are stabilized and ILV membrane scission is delayed by Doa4, which is the ubiquitin hydrolase that deubiquitinates transmembrane proteins sorted as cargoes into ILVs. These results suggest a mechanism to delay ILV budding while cargoes undergo deubiquitination. We further show that deubiquitination of ILV cargoes is inhibited via Doa4 binding to Vps20, which is the subunit of ESCRT-III that initiates assembly of the complex. Current models suggest that ESCRT-III complexes surround ubiquitinated cargoes to trap them at the site of ILV budding while the cargoes undergo deubiquitination. Thus our results also propose a mechanism to prevent the onset of ILV cargo deubiquitination at the initiation of ESCRT-III complex assembly.

Cell surface recycling in yeast: mechanisms and machineries

Sorting internalized proteins and lipids back to the cell surface controls the supply of molecules throughout the cell and regulates integral membrane protein activity at the surface. One central process in mammalian cells is the transit of cargo from endosomes back to the plasma membrane (PM) directly, along a route that bypasses retrograde movement to the Golgi. Despite recognition of this pathway for decades we are only beginning to understand the machinery controlling this overall process. The budding yeastSaccharomyces cerevisiae, a stalwart genetic system, has been routinely used to identify fundamental proteins and their modes of action in conserved trafficking pathways. However, the study of cell surface recycling from endosomes in yeast is hampered by difficulties that obscure visualization of the pathway. Here we briefly discuss how recycling is likely a more prevalent process in yeast than is widely appreciated and how tools might be built to better study the pathway.

Reciprocal Regulation of Target of Rapamycin Complex 1 and Potassium Accumulation

The proper maintenance of potassium homeostasis is crucial for cell viability. Among the major determinants of potassium uptake in the model organism Saccharomyces cerevisiae are the Trk1 high affinity potassium transporter and the functionally redundant Hal4 (Sat4) and Hal5 protein kinases. These kinases are required for the plasma membrane accumulation of not only Trk1 but also several nutrient permeases. Here, we show that overexpression of the target of rapamycin complex 1 (TORC1) effector NPR1 improves hal4 hal5 growth defects by stabilizing nutrient permeases at the plasma membrane. We subsequently found that internal potassium levels and TORC1 activity are linked. Specifically, growth under limiting potassium alters the activities of Npr1 and another TORC1 effector kinase, Sch9; hal4 hal5 and trk1 trk2 mutants display hypersensitivity to rapamycin, and reciprocally, TORC1 inhibition reduces potassium accumulation. Our results demonstrate that in addition to carbon and nitrogen, TORC1 also responds to and regulates potassium fluxes.

Chm7 and Heh1 collaborate to link nuclear pore complex quality control with nuclear envelope sealing

The integrity of the nuclear envelope barrier relies on membrane remodeling by the ESCRTs, which seal nuclear envelope holes and contribute to the quality control of nuclear pore complexes (NPCs); whether these processes are mechanistically related remains poorly defined. Here, we show that the ESCRT-II/III chimera, Chm7, is recruited to a nuclear envelope subdomain that expands upon inhibition of NPC assembly and is required for the formation of the storage of improperly assembled NPCs (SINC) compartment. Recruitment to sites of NPC assembly is mediated by its ESCRT-II domain and the LAP2-emerin-MAN1 (LEM) family of integral inner nuclear membrane proteins, Heh1 and Heh2. We establish direct binding between Heh2 and the "open" forms of both Chm7 and the ESCRT-III, Snf7, and between Chm7 and Snf7. Interestingly, Chm7 is required for the viability of yeast strains where double membrane seals have been observed over defective NPCs; deletion of CHM7 in these strains leads to a loss of nuclear compartmentalization suggesting that the sealing of defective NPCs and nuclear envelope ruptures could proceed through similar mechanisms.

ESCRTs and associated proteins in lysosomal fusion with endosomes and autophagosomes

Endolysosomal and autophagosomal degradation pathways are highly connected at various levels, sharing multiple molecular effectors that modulate them individually or simultaneously. These two lysosomal degradative pathways are primarily involved in the disposal of cargo internalized from the cell surface or long-lived proteins or aggregates and aged organelles present in the cytosol. Both of these pathways involve a number of carefully regulated vesicular fusion events that are dependent on ESCRT proteins. The ESCRT proteins especially ESCRT-I and III participate in the regulation of fusion events between autophagosome/amphisome and lysosome. Along with these, a number of functionally diverse ESCRT associated and regulatory proteins such as, endosomal PtdIns (3) P 5-kinase Fab1, ALIX, mahogunin ring finger 1, atrogin 1, syntaxin 17, ATG12-ATG3 complex, and protein kinase CK2α are involved in fusion events in either or both the lysosomal degradative pathways.

Reverse-topology membrane scission by the ESCRT proteins

The narrow membrane necks formed during viral, exosomal and intra-endosomal budding from membranes, as well as during cytokinesis and related processes, have interiors that are contiguous with the cytosol. Severing these necks involves action from the opposite face of the membrane as occurs during the well-characterized formation of coated vesicles. This 'reverse' (or 'inverse')-topology membrane scission is carried out by the endosomal sorting complex required for transport (ESCRT) proteins, which form filaments, flat spirals, tubes and conical funnels that are thought to direct membrane remodelling and scission. Their assembly, and their disassembly by the ATPase vacuolar protein sorting-associated 4 (VPS4) have been intensively studied, but the mechanism of scission has been elusive. New insights from cryo-electron microscopy and various types of spectroscopy may finally be close to rectifying this situation.

Overexpression of CHMP7 from rapeseed and Arabidopsis causes dwarfism and premature senescence in Arabidopsis

Endosomal sorting complexes required for transport (ESCRT) are well known in mammalians and yeast and plays an essential role in the formation of multi-vesicular bodies. Accumulating evidence has shown that ESCRT proteins contribute to proper plant development. CHMP7 (charged multi-vesicular body protein 7) is an ESCRT-III-related protein and functions in the endosomal sorting pathway in humans. However, its function in plants has not been explored in detail. In this study, we isolate the putative homolog of CHMP7 from rapeseed, BnCHMP7, which contains eight exons and encodes a protein consisting of 423 amino acid residues. Compared with the wild-type, overexpression of BnCHMP7 in Arabidopsis disturbs plant growth and decreases seed yield. Moreover, the transgenic plants also display early leaf senescence and hypersensitivity to dark treatment due to defects in autophagic degradation. Further study showed that BnCHMP7 is highly expressed in leaves and that YFP-BnCHMP7 is predominantly localized in endosome. Compared with human CHMP7, we found that BnCHMP7 not only interacts with ESCRT-III subunits SNF7.2 (CHMP4B), but also with VPS2.2 and CHMP1B. As expected, microarray analysis revealed that the expression of ESCRT transport genes is significantly affected. Additionally, the expression of some genes that are involved in senescence, protein synthesis and protein degradation is also altered in BnCHMP7-overexpressing plants. Taken together, BnCHMP7 encodes an endosome-localized protein, which causes dwarfism and leaf senescence as an ESCRT-III-related component.

Phosphorylation-Dependent Activation of the ESCRT Function of ALIX in Cytokinetic Abscission and Retroviral Budding

The modular adaptor protein ALIX is a key player in multiple ESCRT-III-mediated membrane remodeling processes. ALIX is normally present in a closed conformation due to an intramolecular interaction that renders ALIX unable to perform its ESCRT functions. Here we demonstrate that M phase-specific phosphorylation of the intramolecular interaction site within the proline-rich domain (PRD) of ALIX transforms cytosolic ALIX from closed to open conformation. Defining the role of this mechanism of ALIX regulation in three classical ESCRT-mediated processes revealed that phosphorylation of the intramolecular interaction site in the PRD is required for ALIX to function in cytokinetic abscission and retroviral budding, but not in multivesicular body sorting of activated epidermal growth factor receptor. Thus, phosphorylation of the intramolecular interaction site in the PRD is one of the major mechanisms that activates the ESCRT function of ALIX.

Electrostatic Interactions between Elongated Monomers Drive Filamentation of Drosophila Shrub, a Metazoan ESCRT-III Protein

The endosomal sorting complex required for transport (ESCRT) is a conserved protein complex that facilitates budding and fission of membranes. It executes a key step in many cellular events, including cytokinesis and multi-vesicular body formation. The ESCRT-III protein Shrub in flies, or its homologs in yeast (Snf7) or humans (CHMP4B), is a critical polymerizing component of ESCRT-III needed to effect membrane fission. We report the structural basis for polymerization of Shrub and define a minimal region required for filament formation. The X-ray structure of the Shrub core shows that individual monomers in the lattice interact in a staggered arrangement using complementary electrostatic surfaces. Mutations that disrupt interface salt bridges interfere with Shrub polymerization and function. Despite substantial sequence divergence and differences in packing interactions, the arrangement of Shrub subunits in the polymer resembles that of Snf7 and other family homologs, suggesting that this intermolecular packing mechanism is shared among ESCRT-III proteins.

ALIX and ESCRT-I/II function as parallel ESCRT-III recruiters in cytokinetic abscission

Cytokinetic abscission, the final stage of cell division, is mediated by the ESCRT machinery. Here, Christ et al. dissect the regulation of ESCRT-III recruitment and abscission timing and identify an intersection with abscission checkpoint signaling in cells with chromatin bridges.

Cytokinetic abscission, the final stage of cell division where the two daughter cells are separated, is mediated by the endosomal sorting complex required for transport (ESCRT) machinery. The ESCRT-III subunit CHMP4B is a key effector in abscission, whereas its paralogue, CHMP4C, is a component in the abscission checkpoint that delays abscission until chromatin is cleared from the intercellular bridge. How recruitment of these components is mediated during cytokinesis remains poorly understood, although the ESCRT-binding protein ALIX has been implicated. Here, we show that ESCRT-II and the ESCRT-II–binding ESCRT-III subunit CHMP6 cooperate with ESCRT-I to recruit CHMP4B, with ALIX providing a parallel recruitment arm. In contrast to CHMP4B, we find that recruitment of CHMP4C relies predominantly on ALIX. Accordingly, ALIX depletion leads to furrow regression in cells with chromosome bridges, a phenotype associated with abscission checkpoint signaling failure. Collectively, our work reveals a two-pronged recruitment of ESCRT-III to the cytokinetic bridge and implicates ALIX in abscission checkpoint signaling.

Biogenesis of Plant Prevacuolar Multivesicular Bodies

Plant prevacuolar compartments (PVCs), or multivesicular bodies (MVBs), are single membrane-bound organelles that play important roles in mediating protein trafficking to vacuoles in the secretory pathway. PVC/MVB also serves as a late endosome in the endocytic pathway in plants. Since the plant PVC was identified as an MVB more than 10 years ago, great progress has been made toward the understanding of PVC/MVB function and biogenesis in plants. In this review, we first summarize previous research into the identification and characterization of plant PVCs/MVBs, and then highlight recent advances on the mechanisms underlying intraluminal vesicle formation and maturation of plant PVCs/MVBs. In addition, we discuss the possible crosstalk that appears to occur between PVCs/MVBs and autophagosomes during autophagy in plants. Finally, we list some open questions and present future perspectives in this field.

ESCRT-III drives the final stages of CUPS maturation for unconventional protein secretion

The unconventional secretory pathway exports proteins that bypass the endoplasmic reticulum. In Saccharomyces cerevisiae, conditions that trigger Acb1 secretion via this pathway generate a Grh1 containing compartment composed of vesicles and tubules surrounded by a cup-shaped membrane and collectively called CUPS. Here we report a quantitative assay for Acb1 secretion that reveals requirements for ESCRT-I, -II, and -III but, surprisingly, without the involvement of the Vps4 AAA-ATPase. The major ESCRT-III subunit Snf7 localizes transiently to CUPS and this was accelerated in vps4Δ cells, correlating with increased Acb1 secretion. Microscopic analysis suggests that, instead of forming intraluminal vesicles with the help of Vps4, ESCRT-III/Snf7 promotes direct engulfment of preexisting Grh1 containing vesicles and tubules into a saccule to generate a mature Acb1 containing compartment. This novel multivesicular / multilamellar compartment, we suggest represents the stable secretory form of CUPS that is competent for the release of Acb1 to cells exterior.

DOI:http://dx.doi.org/10.7554/eLife.16299.001

Cells produce thousands of different proteins with a variety of different roles in the body. Some proteins, for example the hormone insulin, perform roles outside of the cell and are released from cells in a process that has several stages. In the first step, newly-made insulin and many other “secretory” proteins enter a compartment called the endoplasmic reticulum. Once inside, these proteins can then be loaded into other compartments and transported to the edge of the cell.

There is another class of secretory proteins that are released from the cell without first entering the endoplasmic reticulum, in a process termed “unconventional protein secretion”. A protein called Acb1 is released from yeast cells in this manner. Previous research identified a compartment that might be involved in this process. However, it is not clear how this compartment (named CUPS) forms, and what role it plays in unconventional protein secretion.

Curwin et al. investigated how CUPS form in yeast cells, and whether the compartment contains Acb1 proteins. The experiments reveal that after CUPS form they need to mature into a form that is involved in the release of Acb1 proteins from the cell. This maturation process involves some, but not all, of the same genes as those involved in producing another type of compartment in cells called a multivesicular body. Acb1 is only found in the mature CUPS and multivesicular bodies are not involved in the release of this protein from the cell.

Curwin et al.’s findings shed some light on how Acb1 and other secretory proteins can be released from cells without involving the endoplasmic reticulum. Future challenges are to reveal how CUPS capture cargo and find out how Acb1 leaves the CUPS to exit the cell.

DOI:http://dx.doi.org/10.7554/eLife.16299.002

ESCRT-III activation by parallel action of ESCRT-I/II and ESCRT-0/Bro1 during MVB biogenesis

The endosomal sorting complexes required for transport (ESCRT) pathway facilitates multiple fundamental membrane remodeling events. Previously, we determined X-ray crystal structures of ESCRT-III subunit Snf7, the yeast CHMP4 ortholog, in its active and polymeric state (Tang et al., 2015). However, how ESCRT-III activation is coordinated by the upstream ESCRT components at endosomes remains unclear. Here, we provide a molecular explanation for the functional divergence of structurally similar ESCRT-III subunits. We characterize novel mutations in ESCRT-III Snf7 that trigger activation, and identify a novel role of Bro1, the yeast ALIX ortholog, in Snf7 assembly. We show that upstream ESCRTs regulate Snf7 activation at both its N-terminal core domain and the C-terminus α6 helix through two parallel ubiquitin-dependent pathways: the ESCRT-I-ESCRT-II-Vps20 pathway and the ESCRT-0-Bro1 pathway. We therefore provide an enhanced understanding for the activation of the spatially unique ESCRT-III-mediated membrane remodeling.

DOI:http://dx.doi.org/10.7554/eLife.15507.001

ESCRT-III and Vps4: a dynamic multipurpose tool for membrane budding and scission

Complex molecular machineries bud, scission and repair cellular membranes. Components of the multi-subunit endosomal sorting complex required for transport (ESCRT) machinery are enlisted when multivesicular bodies are generated, extracellular vesicles are formed, the plasma membrane needs to be repaired, enveloped viruses bud out of host cells, defective nuclear pores have to be cleared, the nuclear envelope must be resealed after mitosis and for final midbody abscission during cytokinesis. While some ESCRT components are only required for specific processes, the assembly of ESCRT-III polymers on target membranes and the action of the AAA-ATPase Vps4 are mandatory for every process. In this review, we summarize the current knowledge of structural and functional features of ESCRT-III/Vps4 assemblies in the growing pantheon of ESCRT-dependent pathways. We describe specific recruitment processes for ESCRT-III to different membranes, which could be useful to selectively inhibit ESCRT function during specific processes, while not affecting other ESCRT-dependent processes. Finally, we speculate how ESCRT-III and Vps4 might function together and highlight how the characterization of their precise spatiotemporal organization will improve our understanding of ESCRT-mediated membrane budding and scission in vivo.

ESCRT Filaments as Spiral Springs

In a recent issue of Cell, Chiaruttini et al. (2015) reveal the mechanical properties of the mysterious spiral filaments formed by the yeast ESCRT-III protein Snf7. The spirals are shown to be springs whose bending drives membrane deformation and perhaps membrane scission.

α-Synuclein interferes with the ESCRT-III complex contributing to the pathogenesis of Lewy body disease

α-Synuclein (α-syn) has been implicated in neurological disorders with parkinsonism, including Parkinson's disease and Dementia with Lewy body. Recent studies have shown α-syn oligomers released from neurons can propagate from cell-to-cell in a prion-like fashion exacerbating neurodegeneration. In this study, we examined the role of the endosomal sorting complex required for transport (ESCRT) pathway on the propagation of α-syn. α-syn, which is transported via the ESCRT pathway through multivesicular bodies for degradation, can also target the degradation of the ESCRT protein-charged multivesicular body protein (CHMP2B), thus generating a roadblock of endocytosed α-syn. Disruption of the ESCRT transport system also resulted in increased exocytosis of α-syn thus potentially increasing cell-to-cell propagation of synuclein. Conversely, delivery of a lentiviral vector overexpressing CHMP2B rescued the neurodegeneration in α-syn transgenic mice. Better understanding of the mechanisms of intracellular trafficking of α-syn might be important for understanding the pathogenesis and developing new treatments for synucleinopathies.

Cell layer-specific distribution of transiently expressed barley ESCRT-III component HvVPS60 in developing barley endosperm

The significance of the endosomal sorting complexes required for transport (ESCRT)-III in cereal endosperm has been shown by the identification of the recessive mutant supernumerary aleurone layer1 (SAL1) in maize. ESCRT-III is indispensable in the final membrane fission step during biogenesis of multivesicular bodies (MVBs), responsible for protein sorting to vacuoles and to the cell surface. Here, we annotated barley ESCRT-III members in the (model) crop Hordeum vulgare and show that all identified members are expressed in developing barley endosperm. We used fluorescently tagged core ESCRT-III members HvSNF7a/CHMP4 and HvVPS24/CHMP3 and the associated ESCRT-III component HvVPS60a/CHMP5 for transient localization studies in barley endosperm. In vivo confocal microscopic analyses show that the localization of recombinantly expressed HvSNF7a, HvVPS24 and HvVPS60a differs within barley endosperm. Whereas HvSNF7a induces large agglomerations, HvVPS24 shows mainly cytosolic localization in aleurone and subaleurone. In contrast, HvVPS60a localizes strongly at the plasma membrane in aleurone. In subaleurone, HvVPS60a was found to a lesser extent at the plasma membrane and at vacuolar membranes. These results indicate that the steady-state association of ESCRT-III may be influenced by cell layer-specific protein deposition or trafficking and remodelling of the endomembrane system in endosperm. We show that sorting of an artificially mono-ubiquitinated Arabidopsis plasma membrane protein is inhibited by HvVPS60a in aleurone. The involvement of HvVPS60a in different cell layer-specific trafficking pathways, reflected by localization of HvVPS60a at the plasma membrane in aleurone and at the PSV membrane in subaleurone, is discussed.

Structural basis for activation, assembly and membrane binding of ESCRT-III Snf7 filaments

The endosomal sorting complexes required for transport (ESCRTs) constitute hetero-oligomeric machines that catalyze multiple topologically similar membrane-remodeling processes. Although ESCRT-III subunits polymerize into spirals, how individual ESCRT-III subunits are activated and assembled together into a membrane-deforming filament remains unknown. Here, we determine X-ray crystal structures of the most abundant ESCRT-III subunit Snf7 in its active conformation. Using pulsed dipolar electron spin resonance spectroscopy (PDS), we show that Snf7 activation requires a prominent conformational rearrangement to expose protein-membrane and protein-protein interfaces. This promotes the assembly of Snf7 arrays with ~30 Å periodicity into a membrane-sculpting filament. Using a combination of biochemical and genetic approaches, both in vitro and in vivo, we demonstrate that mutations on these protein interfaces halt Snf7 assembly and block ESCRT function. The architecture of the activated and membrane-bound Snf7 polymer provides crucial insights into the spatially unique ESCRT-III-mediated membrane remodeling.

LUCID: A Quantitative Assay of ESCRT-Mediated Cargo Sorting into Multivesicular Bodies

Endosomes are transportation nodes, mediating selective transport of soluble and transmembrane cargos to and from the Golgi apparatus, plasma membrane and lysosomes. As endosomes mature to become multivesicular bodies (MVBs), Endosomal Sorting Complexes Required for Transport (ESCRTs) selectively incorporate transmembrane cargos into vesicles that bud into the endosome lumen. Luminal vesicles and their cargoes are targeted for destruction when MVBs fuse with lysosomes. Common assays of endosomal luminal targeting, including fluorescence microscopy and monitoring of proteolytic cargo maturation, possess significant limitations. We present a quantitative assay system called LUCID (LUCiferase reporter of Intraluminal Deposition) that monitors exposure of chimeric luciferase-cargo reporters to cytosol. Luciferase-chimera signal increases when sorting to the endosome lumen is disrupted, and silencing of signal from the chimera depends upon luminal delivery of the reporter rather than proteolytic degradation. The system presents several advantages, including rapidity, microscale operation and a high degree of reproducibility that enables detection of subtle phenotypic differences. Luciferase reporters provide linear signal over an extremely broad dynamic range, allowing analysis of reporter traffic even at anemic levels of expression. Furthermore, LUCID reports transport kinetics when applied to inducible trafficking reporters.

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.

Evidence for a Nonendosomal Function of the Saccharomyces cerevisiae ESCRT-III-Like Protein Chm7

Endosomal sorting complex required for transport (ESCRT) proteins are involved in a number of cellular processes, such as endosomal protein sorting, HIV budding, cytokinesis, plasma membrane repair, and resealing of the nuclear envelope during mitosis. Here we explored the function of a noncanonical member of the ESCRT-III protein family, the Saccharomyces cerevisiae ortholog of human CHMP7. Very little is known about this protein. In silico analysis predicted that Chm7 (yeast ORF YJL049w) is a fusion of an ESCRT-II and ESCRT-III-like domain, which would suggest a role in endosomal protein sorting. However, our data argue against a role of Chm7 in endosomal protein sorting. The turnover of the endocytic cargo protein Ste6 and the vacuolar protein sorting of carboxypeptidase S (CPS) were not affected by CHM7 deletion, and Chm7 also responded very differently to a loss in Vps4 function compared to a canonical ESCRT-III protein. Our data indicate that the Chm7 function could be connected to the endoplasmic reticulum (ER). In line with a function at the ER, we observed a strong negative genetic interaction between the deletion of a gene function (APQ12) implicated in nuclear pore complex assembly and messenger RNA (mRNA) export and the CHM7 deletion. The patterns of genetic interactions between the APQ12 deletion and deletions of ESCRT-III genes, two-hybrid interactions, and the specific localization of mCherry fusion proteins are consistent with the notion that Chm7 performs a novel function at the ER as part of an alternative ESCRT-III complex.

Liaison alcaline: Pals entice non-endosomal ESCRTs to the plasma membrane for pH signaling

The alkaline pH-responsive Pal/Rim signal transduction pathway mediating regulation of gene expression by ambient pH has been extensively studied in Aspergillus nidulans and Saccharomyces cerevisiae. In A. nidulans, PalH, PalI, PalF, PalC, PalA and PalB are required for the proteolytic activation of the executing transcription factor PacC. Although necessary, Pal proteins are insufficient to transmit the signal, which additionally requires ESCRT-I, II and Vps20 with Snf7 in ESCRT-III. Although this initially suggested cooperation between a plasma membrane sensor and an ESCRT-containing Pal complex on endosomes, recent evidence convincingly indicates that pH signaling actually takes place in plasma membrane-associated foci in which Pal proteins and an ESCRT-III polymer scaffold cooperate for pH signaling purposes, representing another non-endosomal role of ESCRT components.

A family of tetraspans organizes cargo for sorting into multivesicular bodies

The abundance of cell-surface membrane proteins is regulated by internalization and delivery into intralumenal vesicles (ILVs) of multivesicular bodies (MVBs). Many cargoes are ubiquitinated, allowing access to an ESCRT-dependent pathway into MVBs. Yet how nonubiquitinated proteins, such as glycosylphosphatidylinositol-anchored proteins, enter MVBs is unclear, supporting the possibility of mechanistically distinct ILV biogenesis pathways. Here we show that a family of highly ubiquitinated tetraspan Cos proteins provides a Ub signal in trans, allowing sorting of nonubiquitinated MVB cargo into the canonical ESCRT- and Ub-dependent pathway. Cos proteins create discrete endosomal subdomains that concentrate Ub cargo prior to their envelopment into ILVs, and the activity of Cos proteins is required not only for efficient sorting of canonical Ub cargo but also for sorting nonubiquitinated cargo into MVBs. Expression of these proteins increases during nutrient stress through an NAD(+)/Sir2-dependent mechanism that in turn accelerates the downregulation of a broad range of cell-surface proteins.

Membrane manipulations by the ESCRT machinery

The endosomal sorting complexes required for transport (ESCRTs) collectively comprise a machinery that was first known for its function in the degradation of transmembrane proteins in the endocytic pathway of eukaryotic cells. Since their discovery, however, ESCRTs have been recognized as playing important roles at the plasma membrane, which appears to be the original site of function for the ESCRT machinery. This article reviews some of the major research findings that have shaped our current understanding of how the ESCRT machinery controls membrane dynamics and considers new roles for the ESCRT machinery that might be driven by these mechanisms.

EhVps32 Is a Vacuole-Associated Protein Involved in Pinocytosis and Phagocytosis of Entamoeaba histolytica

Here, we investigated the role of EhVps32 protein (a member of the endosomal-sorting complex required for transport) in endocytosis of Entamoeba histolytica, a professional phagocyte. Confocal microscopy, TEM and cell fractionation revealed EhVps32 in cytoplasmic vesicles and also located adjacent to the plasma membrane. Between 5 to 30 min of phagocytosis, EhVps32 was detected on some erythrocytes-containing phagosomes of acidic nature, and at 60 min it returned to cytoplasmic vesicles and also appeared adjacent to the plasma membrane. TEM images revealed it in membranous structures in the vicinity of ingested erythrocytes. EhVps32, EhADH (an ALIX family member), Gal/GalNac lectin and actin co-localized in the phagocytic cup and in some erythrocytes-containing phagosomes, but EhVps32 was scarcely detected in late phagosomes. During dextran uptake, EhVps32, EhADH and Gal/GalNac lectin, but not actin, co-localized in pinosomes. EhVps32 recombinant protein formed oligomers composed by rings and filaments. Antibodies against EhVps32 monomers stained cytoplasmic vesicles but not erythrocytes-containing phagosomes, suggesting that in vivo oligomers are formed on phagosome membranes. The involvement of EhVps32 in phagocytosis was further study in pNeoEhvps32-HA-transfected trophozoites, which augmented almost twice their rate of erythrophagocytosis as well as the membranous concentric arrays built by filaments, spirals and tunnel-like structures. Some of these structures apparently connected phagosomes with the phagocytic cup. In concordance, the EhVps32-silenced G3 trophozoites ingested 80% less erythrocytes than the G3 strain. Our results suggest that EhVps32 participates in E. histolytica phagocytosis and pinocytosis. It forms oligomers on erythrocytes-containing phagosomes, probably as a part of the scission machinery involved in membrane invagination and intraluminal vesicles formation.

Trophozoites of E. histolytica represent an excellent model to study endosomal-sorting complex required for transport components due to their high endocytic activity and vesicle trafficking. The key role of EhVps32 on phagocytosis is supported by: i) its presence on phagosomes, ii) its interaction with EhADH (an erythrocytes receptor), Gal/GalNac lectin and actin, iii) the higher rate of erythrophagocytosis showed by EhVps32 overexpressing trophozoites, iv) the diminish rate of phagocytosis in EhVps32-silenced G3 trophozoites, and v) its location in erythrocytes-containing acidic phagosomes. Here, we discovered the presence of membranous concentric helicoidally and tunnel-like structures constituted by EhVps32 and EhADH that may have a dynamic role in membrane remodeling and in the generation of intraluminal vesicles in the phagosomes. Elucidating molecular mechanisms of endocytosis-exocytosis pathways will help us to better understand the pathogenic process of E. histolytica and develop new drugs for diagnosis and vaccine methods.

Solution structure of the Atg1 complex: implications for the architecture of the phagophore assembly site

The biogenesis of autophagosomes commences at the phagophore assembly site (PAS), a protein-vesicle ultrastructure that is organized by the Atg1 complex. The Atg1 complex consists of the Atg1 protein kinase, the intrinsically disordered region-rich Atg13, and the dimeric double crescent-shaped Atg17-Atg31-Atg29 subcomplex. We show that the PAS contains a relatively uniform ∼28 copies of Atg17, and upon autophagy induction, similar numbers of Atg1 and Atg13 molecules. We then apply ensemble refinement of small-angle X-ray scattering to determine the solution structures of the Atg1-Atg13 and Atg17-Atg31-Atg29 subcomplexes and the Atg1 complex, using a trimmed minipentamer tractable to biophysical studies. We observe tetramers of Atg1 pentamers that assemble via Atg17-Atg31-Atg29. This leads to a model for the higher organization of the Atg1 complex in PAS scaffolding.

The VPS-20 subunit of the endosomal sorting complex ESCRT-III exhibits an open conformation in the absence of upstream activation

Members of the endosomal sorting complex required for transport (ESCRT) machinery function in membrane remodelling processes during multivesicular endosome (MVE) biogenesis, cytokinesis, retroviral budding and plasma membrane repair. During luminal vesicle formation at endosomes, the ESCRT-II complex and the ESCRT-III subunit vacuolar protein sorting (VPS)-20 play a specific role in regulating assembly of ESCRT-III filaments, which promote vesicle scission. Previous work suggests that Vps20 isoforms, like other ESCRT-III subunits, exhibits an auto-inhibited closed conformation in solution and its activation depends on an association with ESCRT-II specifically at membranes [1]. However, we show in the present study that Caenorhabditis elegans ESCRT-II and VPS-20 interact directly in solution, both in cytosolic cell extracts and in using recombinant proteins in vitro. Moreover, we demonstrate that purified VPS-20 exhibits an open extended conformation, irrespective of ESCRT-II binding, in contrast with the closed auto-inhibited architecture of another ESCRT-III subunit, VPS-24. Our data argue that individual ESCRT-III subunits adopt distinct conformations, which are tailored for their specific functions during ESCRT-mediated membrane reorganization events.

Regulation of postsynaptic function by the dementia-related ESCRT-III subunit CHMP2B

The charged multivesicular body proteins (Chmp1-7) are an evolutionarily conserved family of cytosolic proteins that transiently assembles into helical polymers that change the curvature of cellular membrane domains. Mutations in human CHMP2B cause frontotemporal dementia, suggesting that this protein may normally control some neuron-specific process. Here, we examined the function, localization, and interactions of neuronal Chmp2b. The protein was highly expressed in mouse brain and could be readily detected in neuronal dendrites and spines. Depletion of endogenous Chmp2b reduced dendritic branching of cultured hippocampal neurons, decreased excitatory synapse density in vitro and in vivo, and abolished activity-induced spine enlargement and synaptic potentiation. To understand the synaptic effects of Chmp2b, we determined its ultrastructural distribution by quantitative immuno-electron microscopy and its biochemical interactions by coimmunoprecipitation and mass spectrometry. In the hippocampus in situ, a subset of neuronal Chmp2b was shown to concentrate beneath the perisynaptic membrane of dendritic spines. In synaptoneurosome lysates, Chmp2b was stably bound to a large complex containing other members of the Chmp family, as well as postsynaptic scaffolds. The supramolecular Chmp assembly detected here corresponds to a stable form of the endosomal sorting complex required for transport-III (ESCRT-III), a ubiquitous cytoplasmic protein complex known to play a central role in remodeling of lipid membranes. We conclude that Chmp2b-containing ESCRT-III complexes are also present at dendritic spines, where they regulate synaptic plasticity. We propose that synaptic ESCRT-III filaments may function as a novel element of the submembrane cytoskeleton of spines.

Distinct mechanisms of recognizing endosomal sorting complex required for transport III (ESCRT-III) protein IST1 by different microtubule interacting and trafficking (MIT) domains

The endosomal sorting complex required for transport (ESCRT) machinery is responsible for membrane remodeling in a number of biological processes including multivesicular body biogenesis, cytokinesis, and enveloped virus budding. In mammalian cells, efficient abscission during cytokinesis requires proper function of the ESCRT-III protein IST1, which binds to the microtubule interacting and trafficking (MIT) domains of VPS4, LIP5, and Spartin via its C-terminal MIT-interacting motif (MIM). Here, we studied the molecular interactions between IST1 and the three MIT domain-containing proteins to understand the structural basis that governs pairwise MIT-MIM interaction. Crystal structures of the three molecular complexes revealed that IST1 binds to the MIT domains of VPS4, LIP5, and Spartin using two different mechanisms (MIM1 mode versus MIM3 mode). Structural comparison revealed that structural features in both MIT and MIM contribute to determine the specific binding mechanism. Within the IST1 MIM sequence, two phenylalanine residues were shown to be important in discriminating MIM1 versus MIM3 binding. These observations enabled us to deduce a preliminary binding code, which we applied to provide CHMP2A, a protein that normally only binds the MIT domain in the MIM1 mode, the additional ability to bind the MIT domain of Spartin in the MIM3 mode.

Constitutively active ESCRT-II suppresses the MVB-sorting phenotype of ESCRT-0 and ESCRT-I mutants

This study is a step toward understanding the minimal requirements for MVB biogenesis and for ESCRT machinery in this process. Early ESCRT complexes (ESCRT-0 and -I) seem to have redundant functions in cargo sorting. Furthermore, vesicle formation seems to require cargo sorting in addition to late ESCRT machinery.

The endosomal sorting complex required for transport (ESCRT) protein complexes function at the endosome in the formation of intraluminal vesicles (ILVs) containing cargo proteins destined for the vacuolar/lysosomal lumen. The early ESCRTs (ESCRT-0 and -I) are likely involved in cargo sorting, whereas ESCRT-III and Vps4 function to sever the neck of the forming ILVs. ESCRT-II links these functions by initiating ESCRT-III formation in an ESCRT-I–regulated manner. We identify a constitutively active mutant of ESCRT-II that partially suppresses the phenotype of an ESCRT-I or ESCRT-0 deletion strain, suggesting that these early ESCRTs are not essential and have redundant functions. However, the ESCRT-III/Vps4 system alone is not sufficient for ILV formation but requires cargo sorting mediated by one of the early ESCRTs.

ESCRT-II/Vps25 constrains digit number by endosome-mediated selective modulation of FGF-SHH signaling

Sorting and degradation of receptors and associated signaling molecules maintain homeostasis of conserved signaling pathways during cell specification and tissue development. Yet, whether machineries that sort signaling proteins act preferentially on different receptors and ligands in different contexts remains mysterious. Here, we show that Vacuolar protein sorting 25, Vps25, a component of ESCRT-II (Endosomal Sorting Complex Required for Transport II), directs preferential endosome-mediated modulation of FGF signaling in limbs. By ENU-induced mutagenesis, we isolated a polydactylous mouse line carrying a hypomorphic mutation of Vps25 (Vps25(ENU)). Unlike Vps25-null embryos we generated, Vps25(ENU/ENU) mutants survive until late gestation. Their limbs display FGF signaling enhancement and consequent hyperactivation of the FGF-SHH feedback loop causing polydactyly, whereas WNT and BMP signaling remain unperturbed. Notably, Vps25(ENU/ENU) Mouse Embryonic Fibroblasts exhibit aberrant FGFR trafficking and degradation; however, SHH signaling is unperturbed. These studies establish that the ESCRT-II machinery selectively limits FGF signaling in vertebrate skeletal patterning.

Structural analysis and modeling reveals new mechanisms governing ESCRT-III spiral filament assembly

Cryo-EM and molecular dynamics simulations reveal unexpected flexibility in individual monomers and a stable interface between monomers in the spiral filaments formed by the ESCRT-III subunit Vps32/CHMP4B.

The scission of biological membranes is facilitated by a variety of protein complexes that bind and manipulate lipid bilayers. ESCRT-III (endosomal sorting complex required for transport III) filaments mediate membrane scission during the ostensibly disparate processes of multivesicular endosome biogenesis, cytokinesis, and retroviral budding. However, mechanisms by which ESCRT-III subunits assemble into a polymer remain unknown. Using cryogenic electron microscopy (cryo-EM), we found that the full-length ESCRT-III subunit Vps32/CHMP4B spontaneously forms single-stranded spiral filaments. The resolution afforded by two-dimensional cryo-EM combined with molecular dynamics simulations revealed that individual Vps32/CHMP4B monomers within a filament are flexible and able to accommodate a range of bending angles. In contrast, the interface between monomers is stable and refractory to changes in conformation. We additionally found that the carboxyl terminus of Vps32/CHMP4B plays a key role in restricting the lateral association of filaments. Our findings highlight new mechanisms by which ESCRT-III filaments assemble to generate a unique polymer capable of membrane remodeling in multiple cellular contexts.