Host Cell Plasma Membrane Phosphatidylserine Regulates the Assembly and Budding of Ebola Virus

A cationic, C-terminal patch and structural rearrangements in Ebola virus matrix VP40 protein control its interactions with phosphatidylserine

Ebola virus (EBOV) is a filamentous lipid-enveloped virus that causes hemorrhagic fever with a high fatality rate. Viral protein 40 (VP40) is the major EBOV matrix protein and regulates viral budding from the plasma membrane. VP40 is a transformer/morpheein that can structurally rearrange its native homodimer into either a hexameric filament that facilitates viral budding or an RNA-binding octameric ring that regulates viral transcription. VP40 associates with plasma-membrane lipids such as phosphatidylserine (PS), and this association is critical to budding from the host cell. However, it is poorly understood how different VP40 structures interact with PS, what essential residues are involved in this association, and whether VP40 has true selectivity for PS among different glycerophospholipid headgroups. In this study, we used lipid-binding assays, MD simulations, and cellular imaging to investigate the molecular basis of VP40-PS interactions and to determine whether different VP40 structures (i.e. monomer, dimer, and octamer) can interact with PS-containing membranes. Results from quantitative analysis indicated that VP40 associates with PS vesicles via a cationic patch in the C-terminal domain (Lys^{224, 225} and Lys^{274, 275}). Substitutions of these residues with alanine reduced PS-vesicle binding by >40-fold and abrogated VP40 localization to the plasma membrane. Dimeric VP40 had 2-fold greater affinity for PS-containing membranes than the monomer, whereas binding of the VP40 octameric ring was reduced by nearly 10-fold. Taken together, these results suggest the different VP40 structures known to form in the viral life cycle harbor different affinities for PS-containing membranes.

Ebola virus requires a host scramblase for externalization of phosphatidylserine on the surface of viral particles

Cell surface receptors for phosphatidylserine contribute to the entry of Ebola virus (EBOV) particles, indicating that the presence of phosphatidylserine in the envelope of EBOV is important for the internalization of EBOV particles. Phosphatidylserine is typically distributed in the inner layer of the plasma membrane in normal cells. Progeny virions bud from the plasma membrane of infected cells, suggesting that phosphatidylserine is likely flipped to the outer leaflet of the plasma membrane in infected cells for EBOV virions to acquire it. Currently, the intracellular dynamics of phosphatidylserine during EBOV infection are poorly understood. Here, we explored the role of XK-related protein (Xkr) 8, which is a scramblase responsible for exposure of phosphatidylserine in the plasma membrane of apoptotic cells, to understand its significance in phosphatidylserine-dependent entry of EBOV. We found that Xkr8 and transiently expressed EBOV glycoprotein GP often co-localized in intracellular vesicles and the plasma membrane. We also found that co-expression of GP and viral major matrix protein VP40 promoted incorporation of Xkr8 into ebolavirus-like particles (VLPs) and exposure of phosphatidylserine on their surface, although only a limited amount of phosphatidylserine was exposed on the surface of the cells expressing GP and/or VP40. Downregulating Xkr8 or blocking caspase-mediated Xkr8 activation did not affect VLP production, but they reduced the amount of phosphatidylserine on the VLPs and their uptake in recipient cells. Taken together, our findings indicate that Xkr8 is trafficked to budding sites via GP-containing vesicles, is incorporated into VLPs, and then promote the entry of the released EBOV to cells in a phosphatidylserine-dependent manner.

Although Ebola virus causes severe hemorrhagic fever with a high mortality rate, there are no approved therapeutics. The viral entry process is one of the targets for antiviral development. Previous studies suggest that binding of phosphatidylserine, a component of the viral envelop, to the receptors promotes the entry of Ebola virus. Ebola virus is released from the surface membrane of infected cells. However, phosphatidylserine normally distributes in the inner layer of the cell surface membrane, suggesting that phosphatidylserine is likely flipped to the outer leaflet of the membrane in infected cells for Ebola virus to acquire it. Because the mechanism by which phosphatidylserine changes its orientation in Ebola virus-infected cells is poorly understood, we studied and identified a cellular enzyme, XK-related protein 8 (Xkr8), as a responsible factor involved in this process. We demonstrated that the Ebola virus glycoprotein promoted the incorporation of Xkr8 in viral particles, which flips phosphatidylserine on their surface, enhancing their entry to cells. Our findings provide new insights into the mechanism of Ebola virus infection, which may be exploited for the development of therapeutics against Ebola virus infection.

Herpes simplex viruses activate phospholipid scramblase to redistribute phosphatidylserines and Akt to the outer leaflet of the plasma membrane and promote viral entry

Herpes simplex virus (HSV) entry is associated with Akt translocation to the outer leaflet of the plasma membrane to promote a complex signaling cascade. We hypothesized that phospholipid scramblase-1 (PLSCR1), a calcium responsive enzyme that flips phosphatidylserines between membrane leaflets, might redistribute Akt to the outside during entry. Confocal imaging, biotinylation of membrane proteins and flow cytometric analysis demonstrated that HSV activates PLSCR1 and flips phosphatidylserines and Akt to the outside shortly following HSV-1 or HSV-2 exposure. Translocation was blocked by addition of a cell permeable calcium chelator, pharmacological scramblase antagonist, or transfection with small interfering RNA targeting PLSCR1. Co-immunoprecipitation and proximity ligation studies demonstrated that PLSCR1 associated with glycoprotein L at the outer leaflet and studies with gL deletion viruses indicate that this interaction facilitates subsequent restoration of the plasma membrane architecture. Ionomycin, a calcium ionophore, also induced PLSCR1 activation resulting in Akt externalization, suggesting a previously unrecognized biological phenomenon.

Defining the mechanisms by which herpes simplex viruses (HSV) enter cells will facilitate the development of new strategies to prevent or treat infections and provide insights into cell biology. We report the novel observation that HSV activates the enzyme, scramblase, which redistributes phosphatidylserines, the major component of the inner leaflet of the plasma membrane, and the associated protein, Akt, between the inner and outer leaflet of the plasma membrane, to promote viral entry.

Lipids at membrane contact sites: cell signaling and ion transport

Communication between organelles is essential to coordinate cellular functions and the cell's response to physiological and pathological stimuli. Organellar communication occurs at membrane contact sites (MCSs), where the endoplasmic reticulum (ER) membrane is tethered to cellular organelle membranes by specific tether proteins and where lipid transfer proteins and cell signaling proteins are located. MCSs have many cellular functions and are the sites of lipid and ion transfer between organelles and generation of second messengers. This review discusses several aspects of MCSs in the context of lipid transfer, formation of lipid domains, generation of Ca²⁺ and cAMP second messengers, and regulation of ion transporters by lipids.

Membrane association and localization dynamics of the Ebola virus matrix protein VP40

The Ebola virus matrix protein VP40 is a major structural protein that provides the scaffolding for new Ebola virus particles. For this, VP40 is first trafficked to the lower leaflet of the plasma membrane (PM) in its dimeric form. Once associated with the PM, the VP40 dimers undergo structural rearrangements and oligomerize into hexamers and filaments that make up the virus matrix. Therefore, association of the VP40 dimers and their stabilization at the PM is a crucial step in the Ebola life-cycle. To understand the molecular details of the VP40 dimer-PM interactions, we investigated the dimer association with the inner leaflet of the PM using detailed all-atom molecular dynamics (MD) simulations. The formation of the dimer-PM complex is facilitated by the interactions of the VP40 lysine residues and the anionic lipids POPS, POPI, and PIP₂ in the PM. In contrast, the dimer fails to associate with a membrane without POPS, POPI, or PIP₂ lipids. We explored the mechanisms of the association and identified important residues and lipids involved in localization and stabilization of VP40 dimers at the PM. MD simulations elucidate the role of a C-terminal α-helix alignment parallel to the lipid bilayer surface as well as the creation of membrane defects that allow partial insertion of the hydrophobic residue V276 into the membrane to further stabilize the VP40 dimer-PM complex. Understanding the mechanisms of the VP40 dimer-PM association that facilitate oligomerization can be important for potentially targeting the VP40 for small molecules that can interfere with the virus life-cycle.

The lifecycle of the Ebola virus in host cells

Ebola haemorrhagic fever causes deadly disease in humans and non-human primates resulting from infection with the Ebola virus (EBOV) genus of the family Filoviridae. However, the mechanisms of EBOV lifecycle in host cells, including viral entry, membrane fusion, RNP formation, GP-tetherin interaction, and VP40-inner leaflet association remain poorly understood. This review describes the biological functions of EBOV proteins and their roles in the lifecycle, summarizes the factors related to EBOV proteins or RNA expression throughout the different phases, and reviews advances with regards to the molecular events and mechanisms of the EBOV lifecycle. Furthermore, the review outlines the aspects remain unclear that urgently need to be solved in future research.

Detection of lipid-induced structural changes of the Marburg virus matrix protein VP40 using hydrogen/deuterium exchange-mass spectrometry

Marburg virus (MARV) is a lipid-enveloped virus from the Filoviridae family containing a negative sense RNA genome. One of the seven MARV genes encodes the matrix protein VP40, which forms a matrix layer beneath the plasma membrane inner leaflet to facilitate budding from the host cell. MARV VP40 (mVP40) has been shown to be a dimeric peripheral protein with a broad and flat basic surface that can associate with anionic phospholipids such as phosphatidylserine. Although a number of mVP40 cationic residues have been shown to facilitate binding to membranes containing anionic lipids, much less is known on how mVP40 assembles to form the matrix layer following membrane binding. Here we have used hydrogen/deuterium exchange (HDX) mass spectrometry to determine the solvent accessibility of mVP40 residues in the absence and presence of phosphatidylserine and phosphatidylinositol 4,5-bisphosphate. HDX analysis demonstrates that two basic loops in the mVP40 C-terminal domain make important contributions to anionic membrane binding and also reveals a potential oligomerization interface in the C-terminal domain as well as a conserved oligomerization interface in the mVP40 N-terminal domain. Lipid binding assays confirm the role of the two basic patches elucidated with HD/X measurements, whereas molecular dynamics simulations and membrane insertion measurements complement these studies to demonstrate that mVP40 does not appreciably insert into the hydrocarbon region of anionic membranes in contrast to the matrix protein from Ebola virus. Taken together, we propose a model by which association of the mVP40 dimer with the anionic plasma membrane facilitates assembly of mVP40 oligomers.

MPLEx: a method for simultaneous pathogen inactivation and extraction of samples for multi-omics profiling

The continued emergence and spread of infectious agents is of great concern, and systems biology approaches to infectious disease research can advance our understanding of host-pathogen relationships and facilitate the development of new therapies and vaccines. Molecular characterization of infectious samples outside of appropriate biosafety containment can take place only subsequent to pathogen inactivation. Herein, we describe a modified Folch extraction using chloroform/methanol that facilitates the molecular characterization of infectious samples by enabling simultaneous pathogen inactivation and extraction of proteins, metabolites, and lipids for subsequent mass spectrometry-based multi-omics measurements. This single-sample metabolite, protein and lipid extraction (MPLEx) method resulted in complete inactivation of clinically important bacterial and viral pathogens with exposed lipid membranes, including Yersinia pestis, Salmonella Typhimurium, and Campylobacter jejuni in pure culture, and Yersinia pestis, Campylobacter jejuni, and West Nile, MERS-CoV, Ebola, and influenza H7N9 viruses in infection studies. In addition, >99% inactivation, which increased with solvent exposure time, was also observed for pathogens without exposed lipid membranes including community-associated methicillin-resistant Staphylococcus aureus, Clostridium difficile spores and vegetative cells, and adenovirus type 5. The overall pipeline of inactivation and subsequent proteomic, metabolomic, and lipidomic analyses was evaluated using a human epithelial lung cell line infected with wild-type and mutant influenza H7N9 viruses, thereby demonstrating that MPLEx yields biomaterial of sufficient quality for subsequent multi-omics analyses. Based on these experimental results, we believe that MPLEx will facilitate systems biology studies of infectious samples by enabling simultaneous pathogen inactivation and multi-omics measurements from a single specimen with high success for pathogens with exposed lipid membranes.

Plasma membrane association facilitates conformational changes in the Marburg virus protein VP40 dimer

The membrane binding interface of the Marburg virus protein mVP40 dimer differs from that of the Ebola virus eVP40 dimer but membrane binding allows conformational changes in mVP40 that makes it structurally similar to the eVP40 dimer.

Filovirus infections cause hemorrhagic fever in humans and non-human primates that often results in high fatality rates. The Marburg virus is a lipid-enveloped virus from the Filoviridae family and is closely related to the Ebola virus. The viral matrix layer underneath the lipid envelope is formed by the matrix protein VP40 (VP40), which is also involved in other functions during the viral life-cycle. As in the Ebola virus VP40 (eVP40), the recently determined X-ray crystal structure of the Marburg virus VP40 (mVP40) features loops containing cationic residues that form a lipid binding basic patch. However, the mVP40 basic patch is significantly flatter with a more extended surface than in eVP40, suggesting the possibility of differences in the plasma membrane interactions and phospholipid specificity between the VP40 dimers. In this paper, we report on molecular dynamics simulations that investigate the roles of various residues and lipid types in PM association as well as the conformational changes of the mVP40 dimer facilitated by membrane association. We compared the structural changes of the mVP40 dimer with the mVP40 dimer in both lipid free and membrane associated conditions. Despite the significant structural differences in the crystal structure, the Marburg VP40 dimer is found to adopt a configuration very similar to the Ebola VP40 dimer after associating with the membrane. This conformational rearrangement upon lipid binding allows Marburg VP40 to localize and stabilize at the membrane surface in a manner similar to the Ebola VP40 dimer. Consideration of the structural information in its lipid-interacting condition may be important in targeting mVP40 for novel drugs to inhibit viral budding from the plasma membrane.

Filovirus Structural Biology: The Molecules in the Machine

In this chapter, we describe what is known thus far about the structures and functions of the handful of proteins encoded by filovirus genomes. Amongst the fascinating findings of the last decade is the plurality of functions and structures that these polypeptides can adopt. Many of the encoded proteins can play multiple, distinct roles in the virus life cycle, although the mechanisms by which these functions are determined and controlled remain mostly veiled. Further, some filovirus proteins are multistructural: adopting different oligomeric assemblies and sometimes, different tertiary structures to achieve their separate, and equally essential functions. Structures, and the functions they dictate, are described for components of the nucleocapsid, the matrix, and the surface and secreted glycoproteins.

The Ebola virus protein VP40 hexamer enhances the clustering of PI(4,5)P2 lipids in the plasma membrane

The Ebola virus is a lipid-enveloped virus that obtains its lipid coat from the plasma membrane of the host cell it infects during the budding process. The Ebola virus protein VP40 localizes to the inner leaflet of the plasma membrane and forms the viral matrix, which provides the major structure for the Ebola virus particles. VP40 is initially a dimer that rearranges to a hexameric structure that mediates budding. VP40 hexamers and larger filaments have been shown to be stabilized by PI(4,5)P2 in the plasma membrane inner leaflet. Reduction in the plasma membrane levels of PI(4,5)P2 significantly reduce formation of VP40 oligomers and virus-like particles. We investigated the lipid-protein interactions in VP40 hexamers at the plasma membrane. We quantified lipid-lipid self-clustering by calculating the fractional interaction matrix and found that the VP40 hexamer significantly enhances the PI(4,5)P2 clustering. The radial pair distribution functions suggest a strong interaction between PI(4,5)P2 and the VP40 hexamer. The cationic Lys side chains are found to mediate the PIP2 clustering around the protein, with cholesterol filling the space between the interacting PIP2 molecules. These computational studies support recent experimental data and provide new insights into the mechanisms by which VP40 assembles at the plasma membrane inner leaflet, alters membrane curvature, and forms new virus-like particles.

Could Bat Cell Temperature and Filovirus Filament Length Explain the Emergence of Ebola Virus in Mammals? Predictions of a Thermodynamic Model

The host reservoir of Zaire ebolavirus (EBOV) remains elusive. One suggestion is that EBOV emerges in mammals when the precursor virus jumps from mayflies (or other riverine insects) to insectivorous bats. However, this does not fit with the current view that filoviruses cannot infect arthropods. Here, it is first argued that the evidence that arthropods are refractory is not definitive. Second, it is proposed that a combination of filovirus filament length and the high temperature (~42°C) experienced by an insect virus ingested by a flying bat, together with the large number of insects eaten by bats (e.g. during an ephemeral mass emergence of mayflies), facilitate jumping the species barrier. The length of a filovirus filament is related to the number of genome copies (GC). Predictions from a preliminary thermodynamic model developed here suggest that filament length could greatly affect EBOV infectivity to mammalian cells with infectivity peaking for filaments of a certain length. Importantly, the infectivity to mammals of even short filaments may be more than one million-fold higher than that for the single GC virion. Third, it is proposed that at the high temperature within the bat, the phospholipid phosphatidylserine in the virus envelope promotes filament formation through fusion of single GC particles within the ingested insect, thus hugely increasing their infectivity to bats. Forth, according to the thermodynamic model, increasing the temperature from 27°C (insect cell temperature at average air temperature in Guinea, West Africa) to 42°C (bat) could increase the affinity of the filaments for bat cells by 1-2 orders of magnitude, while having no effect on the binding affinity of the single GC virions. The thermodynamic model developed here is supported by the counterintuitive observation that high glycoprotein densities on the EBOV surface reduce its infectivity in contrast to other viruses such as HIV.

Intra-host dynamics of Ebola virus during 2014

Since 2013, West Africa has encountered the largest Ebola virus (EBOV) disease outbreak on record, and Sierra Leone is the worst-affected country, with nearly half of the infections. By means of next-generation sequencing and phylogeographic analysis, the epidemiology and transmission of EBOV have been well elucidated. However, the intra-host dynamics that mainly reflect viral-host interactions still need to be studied. Here, we show a total of 710 intra-host single nucleotide variations (iSNVs) from deep-sequenced samples from EBOV-infected patients, through a well-tailored bioinformatics pipeline. We present a comprehensive distribution of iSNVs during this outbreak and along the EBOV genome. Analyses of iSNV and its allele frequency reveal that VP40 is the most conserved gene during this outbreak, and thus it would be an ideal therapeutic target. In the co-occurring iSNV network, varied iSNV sites present different selection features. Intriguingly, the T-to-C substitutions at the 3'-UTR of the nucleoprotein (NP; positions 3008 and 3011), observed in many patients, result in the upregulation of the transcription of NP through an Ebola mini-genome reporting system. Additionally, no iSNV enrichment within B-cell epitopes of GP has been observed.

Interdomain salt-bridges in the Ebola virus protein VP40 and their role in domain association and plasma membrane localization

The Ebola virus protein VP40 is a transformer protein that possesses an extraordinary ability to accomplish multiple functions by transforming into various oligomeric conformations. The disengagement of the C-terminal domain (CTD) from the N-terminal domain (NTD) is a crucial step in the conformational transformations of VP40 from the dimeric form to the hexameric form or octameric ring structure. Here, we use various molecular dynamics (MD) simulations to investigate the dynamics of the VP40 protein and the roles of interdomain interactions that are important for the domain-domain association and dissociation, and report on experimental results of the behavior of mutant variants of VP40. The MD studies find that various salt-bridge interactions modulate the VP40 domain dynamics by providing conformational specificity through interdomain interactions. The MD simulations reveal a novel salt-bridge between D45-K326 when the CTD participates in a latch-like interaction with the NTD. The D45-K326 salt-bridge interaction is proposed to help domain-domain association, whereas the E76-K291 interaction is important for stabilizing the closed-form structure. The effects of the removal of important VP40 salt-bridges on plasma membrane (PM) localization, VP40 oligomerization, and virus like particle (VLP) budding assays were investigated experimentally by live cell imaging using an EGFP-tagged VP40 system. It is found that the mutations K291E and D45K show enhanced PM localization but D45K significantly reduced VLP formation.

Characterization of Human and Murine T-Cell Immunoglobulin Mucin Domain 4 (TIM-4) IgV Domain Residues Critical for Ebola Virus Entry

Phosphatidylserine (PtdSer) receptors that are responsible for the clearance of dying cells have recently been found to mediate enveloped virus entry. Ebola virus (EBOV), a member of the Filoviridae family of viruses, utilizes PtdSer receptors for entry into target cells. The PtdSer receptors human and murine T-cell immunoglobulin mucin (TIM) domain proteins TIM-1 and TIM-4 mediate filovirus entry by binding to PtdSer on the virion surface via a conserved PtdSer binding pocket within the amino-terminal IgV domain. While the residues within the TIM-1 IgV domain that are important for EBOV entry are characterized, the molecular details of virion-TIM-4 interactions have yet to be investigated. As sequences and structural alignments of the TIM proteins suggest distinct differences in the TIM-1 and TIM-4 IgV domain structures, we sought to characterize TIM-4 IgV domain residues required for EBOV entry. Using vesicular stomatitis virus pseudovirions bearing EBOV glycoprotein (EBOV GP/VSVΔG), we evaluated virus binding and entry into cells expressing TIM-4 molecules mutated within the IgV domain, allowing us to identify residues important for entry. Similar to TIM-1, residues in the PtdSer binding pocket of murine and human TIM-4 (mTIM-4 and hTIM-4) were found to be important for EBOV entry. However, additional TIM-4-specific residues were also found to impact EBOV entry, with a total of 8 mTIM-4 and 14 hTIM-4 IgV domain residues being critical for virion binding and internalization. Together, these findings provide a greater understanding of the interaction of TIM-4 with EBOV virions.

IMPORTANCE

With more than 28,000 cases and over 11,000 deaths during the largest and most recent Ebola virus (EBOV) outbreak, there has been increased emphasis on the development of therapeutics against filoviruses. Many therapies under investigation target EBOV cell entry. T-cell immunoglobulin mucin (TIM) domain proteins are cell surface factors important for the entry of many enveloped viruses, including EBOV. TIM family member TIM-4 is expressed on macrophages and dendritic cells, which are early cellular targets during EBOV infection. Here, we performed a mutagenesis screening of the IgV domain of murine and human TIM-4 to identify residues that are critical for EBOV entry. Surprisingly, we identified more human than murine TIM-4 IgV domain residues that are required for EBOV entry. Defining the TIM IgV residues needed for EBOV entry clarifies the virus-receptor interactions and paves the way for the development of novel therapeutics targeting virus binding to this cell surface receptor.

The Ebola Virus matrix protein, VP40, requires phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2) for extensive oligomerization at the plasma membrane and viral egress

VP40 is one of eight proteins encoded by the Ebola Virus (EBOV) and serves as the primary matrix protein, forming virus like particles (VLPs) from mammalian cells without the need for other EBOV proteins. While VP40 is required for viral assembly and budding from host cells during infection, the mechanisms that target VP40 to the plasma membrane are not well understood. Phosphatidylserine is required for VP40 plasma membrane binding, VP40 hexamer formation, and VLP egress, However, PS also becomes exposed on the outer membrane leaflet at sites of VP40 budding, raising the question of how VP40 maintains an interaction with the plasma membrane inner leaflet when PS is flipped to the opposite side. To address this question, cellular and in vitro assays were employed to determine if phosphoinositides are important for efficient VP40 localization to the plasma membrane. Cellular studies demonstrated that PI(4,5)P₂ was an important component of VP40 assembly at the plasma membrane and subsequent virus like particle formation. Additionally, PI(4,5)P₂ was required for formation of extensive oligomers of VP40, suggesting PS and PI(4,5)P₂ have different roles in VP40 assembly where PS regulates formation of hexamers from VP40 dimers and PI(4,5)P₂ stabilizes and/or induces extensive VP40 oligomerization at the plasma membrane.

Investigation of the Lipid Binding Properties of the Marburg Virus Matrix Protein VP40

Marburg virus (MARV), which belongs to the virus family Filoviridae, causes hemorrhagic fever in humans and nonhuman primates that is often fatal. MARV is a lipid-enveloped virus that during the replication process extracts its lipid coat from the plasma membrane of the host cell it infects. MARV carries seven genes, one of which encodes its matrix protein VP40 (mVP40), which regulates the assembly and budding of the virions. Currently, little information is available on mVP40 lipid binding properties. Here, we have investigated the in vitro and cellular mechanisms by which mVP40 associates with lipid membranes. mVP40 associates with anionic membranes in a nonspecific manner that is dependent upon the anionic charge density of the membrane. These results are consistent with recent structural determination of mVP40, which elucidated an mVP40 dimer with a flat and extensive cationic lipid binding interface.

IMPORTANCE

Marburg virus (MARV) is a lipid-enveloped filamentous virus from the family Filoviridae. MARV was discovered in 1967, and yet little is known about how its seven genes are used to assemble and form a new viral particle in the host cell it infects. The MARV matrix protein VP40 (mVP40) underlies the inner leaflet of the virus and regulates budding from the host cell plasma membrane. In vitro and cellular assays in this study investigated the mechanism by which mVP40 associates with lipids. The results demonstrate that mVP40 interactions with lipid vesicles or the inner leaflet of the plasma membrane are electrostatic but nonspecific in nature and are dependent on the anionic charge density of the membrane surface. Small molecules that can disrupt lipid trafficking or reduce the anionic charge of the plasma membrane interface may be useful in inhibiting assembly and budding of MARV.