The Mason-Pfizer monkey virus PPPY and PSAP motifs both contribute to virus release

CD81 fusion alters SARS-CoV-2 Spike trafficking

The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) pandemic caused the biggest public health crises in recent history. Many expect future coronavirus introductions into the human population. Hence, it is essential to understand the basic biology of these viruses. In natural infection, the SARS-CoV-2 Spike (S) glycoprotein is co-expressed with all other viral proteins, which modify cellular compartments to maximize virion assembly. By comparison, most of S is degraded when the protein is expressed in isolation, as in current molecular vaccines. To probe the maturation pathway of S, we redirected its maturation by fusing S to the tetraspanin protein CD81. CD81 is a defining constituent of extracellular vesicles (EVs) or exosomes. EVs are generated in large numbers by all cells, extruded into blood and lymph, and transfer cargo between cells and systemically (estimated 1012 EVs per mL plasma). EVs, like platelets, can be transfused between unrelated donors. When fusing the proline-stabilized form of strain Delta S into the flexible, large extracellular loop of CD81 rather than being degraded in the lysosome, S was extruded into EVs. CD81-S fusion containing EVs were produced in large numbers and could be isolated to high purity. Purified CD81::S EVs bound ACE2, and S displayed on individual EV was observed by cryogenic electron microscopy (EM). The CD81::S-fusion EVs were non-toxic and elicited an anti-S trimer and anti-RBD antibody response in mice. This report shows a design path to maximize viral glycoprotein assembly and release without relying on the co-expression of potentially pathogenic nonstructural viral proteins.

IMPORTANCE

The severe acute respiratory syndrome coronavirus 2 pandemic caused the biggest public health crises in recent history. To understand the maturation pathway of S, we fused S to the tetraspanin protein CD81. The resulting molecule is secreted in extracellular vesicles and induces antibodies in mice. This may be a general design path for viral glycoprotein vaccines.

Molecular architecture and conservation of an immature human endogenous retrovirus

The human endogenous retrovirus K (HERV-K) is the most recently acquired endogenous retrovirus in the human genome and is activated and expressed in many cancers and amyotrophic lateral sclerosis. We present the immature HERV-K capsid structure at 3.2 Å resolution determined from native virus-like particles using cryo-electron tomography and subtomogram averaging. The structure shows a hexamer unit oligomerized through a 6-helix bundle, which is stabilized by a small molecule analogous to IP6 in immature HIV-1 capsid. The HERV-K immature lattice is assembled via highly conserved dimer and trimer interfaces, as detailed through all-atom molecular dynamics simulations and supported by mutational studies. A large conformational change mediated by the linker between the N-terminal and the C-terminal domains of CA occurs during HERV-K maturation. Comparison between HERV-K and other retroviral immature capsid structures reveals a highly conserved mechanism for the assembly and maturation of retroviruses across genera and evolutionary time.

Molecular architecture and conservation of an immature human endogenous retrovirus

A significant part of the human genome consists of endogenous retroviruses sequences. Human endogenous retrovirus K (HERV-K) is the most recently acquired endogenous retrovirus, is activated and expressed in many cancers and amyotrophic lateral sclerosis and possibly contributes to the aging process. To understand the molecular architecture of endogenous retroviruses, we determined the structure of immature HERV-K from native virus-like particles (VLPs) using cryo-electron tomography and subtomogram averaging (cryoET STA). The HERV-K VLPs show a greater distance between the viral membrane and immature capsid lattice, correlating with the presence of additional peptides, SP1 and p15, between the capsid (CA) and matrix (MA) proteins compared to the other retroviruses. The resulting cryoET STA map of the immature HERV-K capsid at 3.2 Å resolution shows a hexamer unit oligomerized through a 6-helix bundle which is further stabilized by a small molecule in the same way as the IP6 in immature HIV-1 capsid. The HERV-K immature CA hexamer assembles into the immature lattice via highly conserved dimmer and trimer interfaces, whose interactions were further detailed through all-atom molecular dynamics simulations and supported by mutational studies. A large conformational change mediated by the flexible linker between the N-terminal and the C-terminal domains of CA occurs between the immature and the mature HERV-K capsid protein, analogous to HIV-1. Comparison between HERV-K and other retroviral immature capsid structures reveals a highly conserved mechanism for the assembly and maturation of retroviruses across genera and evolutionary time.

Converting Nanotoxicity Data to Information Using Artificial Intelligence and Simulation

Decades of nanotoxicology research have generated extensive and diverse data sets. However, data is not equal to information. The question is how to extract critical information buried in vast data streams. Here we show that artificial intelligence (AI) and molecular simulation play key roles in transforming nanotoxicity data into critical information, i.e., constructing the quantitative nanostructure (physicochemical properties)-toxicity relationships, and elucidating the toxicity-related molecular mechanisms. For AI and molecular simulation to realize their full impacts in this mission, several obstacles must be overcome. These include the paucity of high-quality nanomaterials (NMs) and standardized nanotoxicity data, the lack of model-friendly databases, the scarcity of specific and universal nanodescriptors, and the inability to simulate NMs at realistic spatial and temporal scales. This review provides a comprehensive and representative, but not exhaustive, summary of the current capability gaps and tools required to fill these formidable gaps. Specifically, we discuss the applications of AI and molecular simulation, which can address the large-scale data challenge for nanotoxicology research. The need for model-friendly nanotoxicity databases, powerful nanodescriptors, new modeling approaches, molecular mechanism analysis, and design of the next-generation NMs are also critically discussed. Finally, we provide a perspective on future trends and challenges.

Exosomal transmission of viruses, a two-edged biological sword

As a common belief, most viruses can egress from the host cells as single particles and transmit to uninfected cells. Emerging data have revealed en bloc viral transmission as lipid bilayer-cloaked particles via extracellular vesicles especially exosomes (Exo). The supporting membrane can be originated from multivesicular bodies during intra-luminal vesicle formation and autophagic response. Exo are nano-sized particles, ranging from 40-200 nm, with the ability to harbor several types of signaling molecules from donor to acceptor cells in a paracrine manner, resulting in the modulation of specific signaling reactions in target cells. The phenomenon of Exo biogenesis consists of multiple and complex biological steps with the participation of diverse constituents and molecular pathways. Due to similarities between Exo biogenesis and virus replication and the existence of shared pathways, it is thought that viruses can hijack the Exo biogenesis machinery to spread and evade immune cells. To this end, Exo can transmit complete virions (as single units or aggregates), separate viral components, and naked genetic materials. The current review article aims to scrutinize challenges and opportunities related to the exosomal delivery of viruses in terms of viral infections and public health. Video Abstract.

Importance of Viral Late Domains in Budding and Release of Enveloped RNA Viruses

Late assembly (L) domains are conserved sequences that are necessary for the late steps of viral replication, acting like cellular adaptors to engage the ESCRT membrane fission machinery that promote virion release. These short sequences, whose mutation or deletion produce the accumulation of immature virions at the plasma membrane, were firstly identified within retroviral Gag precursors, and in a further step, also in structural proteins of many other enveloped RNA viruses including arenaviruses, filoviruses, rhabdoviruses, reoviruses, and paramyxoviruses. Three classes of L domains have been identified thus far (PT/SAP, YPXnL/LXXLF, and PPxY), even if it has recently been suggested that other motifs could act as L domains. Here, we summarize the current state of knowledge of the different types of L domains and their cellular partners in the budding events of RNA viruses, with a particular focus on retroviruses.

When in Need of an ESCRT: The Nature of Virus Assembly Sites Suggests Mechanistic Parallels between Nuclear Virus Egress and Retroviral Budding

The proper assembly and dissemination of progeny virions is a fundamental step in virus replication. As a whole, viruses have evolved a myriad of strategies to exploit cellular compartments and mechanisms to ensure a successful round of infection. For enveloped viruses such as retroviruses and herpesviruses, acquisition and incorporation of cellular membrane is an essential process during the formation of infectious viral particles. To do this, these viruses have evolved to hijack the host Endosomal Sorting Complexes Required for Transport (ESCRT-I, -II, and -III) to coordinate the sculpting of cellular membrane at virus assembly and dissemination sites, in seemingly different, yet fundamentally similar ways. For instance, at the plasma membrane, ESCRT-I recruitment is essential for HIV-1 assembly and budding, while it is dispensable for the release of HSV-1. Further, HSV-1 was shown to recruit ESCRT-III for nuclear particle assembly and egress, a process not used by retroviruses during replication. Although the cooption of ESCRTs occurs in two separate subcellular compartments and at two distinct steps for these viral lifecycles, the role fulfilled by ESCRTs at these sites appears to be conserved. This review discusses recent findings that shed some light on the potential parallels between retroviral budding and nuclear egress and proposes a model where HSV-1 nuclear egress may occur through an ESCRT-dependent mechanism.

The Interplay between ESCRT and Viral Factors in the Enveloped Virus Life Cycle

Viruses are obligate parasites that rely on host cellular factors to replicate and spread. The endosomal sorting complexes required for transport (ESCRT) system, which is classically associated with sorting and downgrading surface proteins, is one of the host machineries hijacked by viruses across diverse families. Knowledge gained from research into ESCRT and viruses has, in turn, greatly advanced our understanding of many other cellular functions in which the ESCRT pathway is involved, e.g., cytokinesis. This review highlights the interplay between the ESCRT pathway and the viral factors of enveloped viruses with a special emphasis on retroviruses.

Analysis of the Cell Type-Dependence on the Arenavirus Z-Mediated Virus-Like Particle Production

Several arenaviruses are highly pathogenic to humans, causing hemorrhagic fever. Discovery of anti-arenavirus drug candidates is urgently needed, although the molecular basis of the host- and organ-specific pathogenicity remains to be fully elucidated. The arenavirus Z protein facilitates production of virus-like particles (VLPs), providing an established method to assess virus budding. In this study, we examined the efficiency of VLP production by solely expressing Z protein of several different arenaviruses. In addition, we analyzed the role of the late (L)-domain of the arenavirus Z protein, which is essential for the interaction with ESCRT proteins, in VLP production among different cell lines. VLP assay was performed using Z proteins of Junín virus (JUNV), Machupo virus (MACV), Tacaribe virus (TCRV), Latino virus (LATV), Pichinde virus (PICV), and Lassa virus (LASV) in six different cell lines: HEK293T, Huh-7, A549, Vero76, BHK-21, and NIH3T3 cells. JUNV, MACV, and LASV Z proteins efficiently produced VLPs in all tested cell lines, while the efficiencies of VLP production by the other arenavirus Z proteins were cell type-dependent. The contribution of the L-domain(s) within Z protein to VLP production also highly depended on the cell type. These results suggested that each arenavirus has its own particle-production mechanism, which is different among the cell types.

The Integrity of the YxxL Motif of Ebola Virus VP24 Is Important for the Transport of Nucleocapsid-Like Structures and for the Regulation of Viral RNA Synthesis

While it is well appreciated that late domains in the viral matrix proteins are crucial to mediate efficient virus budding, little is known about roles of late domains in the viral nucleocapsid proteins. Here, we characterized the functional relevance of a YxxL motif with potential late-domain function in the Ebola virus nucleocapsid protein VP24. Mutations in the YxxL motif had two opposing effects on the functions of VP24. On the one hand, the mutation affected the regulatory function of VP24 in viral RNA transcription and replication, which correlated with an increased incorporation of minigenomes into released transcription- and replication-competent virus-like particles (trVLPs). Consequently, cells infected with those trVLPs showed higher levels of viral transcription. On the other hand, mutations of the YxxL motif greatly impaired the intracellular transport of nucleocapsid-like structures (NCLSs) composed of the viral proteins NP, VP35, and VP24 and the length of released trVLPs. Attempts to rescue recombinant Ebola virus expressing YxxL-deficient VP24 failed, underlining the importance of this motif for the viral life cycle.IMPORTANCE Ebola virus (EBOV) causes a severe fever with high case fatality rates and, so far, no available specific therapy. Understanding the interplay between viral and host proteins is important to identify new therapeutic approaches. VP24 is one of the essential nucleocapsid components and is necessary to regulate viral RNA synthesis and condense viral nucleocapsids before their transport to the plasma membrane. Our functional analyses of the YxxL motif in VP24 suggested that it serves as an interface between nucleocapsid-like structures (NCLSs) and cellular proteins, promoting intracellular transport of NCLSs in an Alix-independent manner. Moreover, the YxxL motif is necessary for the inhibitory function of VP24 in viral RNA synthesis. A failure to rescue EBOV encoding VP24 with a mutated YxxL motif indicated that the integrity of the YxxL motif is essential for EBOV growth. Thus, this motif might represent a potential target for antiviral interference.

A PLPPV sequence in the p8 region of Gag provides late domain function for mouse mammary tumor virus

The late (L) domain sequence used by mouse mammary tumor virus (MMTV) remains undefined. Similar to other L domain-containing proteins, MMTV p8 and p14NC proteins are monoubiquitinated, suggesting L domain function. Site-directed mutagenesis of p8, PLPPV, and p14NC, PLPPL, sequences in MMTV Gag revealed a requirement only for the PLPPV sequence in virion release in a position-dependent manner. Electron microscopy of a defective Gag mutant confirmed an L domain budding defect morphology. The equine infectious anemia virus (EIAV) YPDL core L domain sequence and PLPPV provided L domain function in reciprocal MMTV and EIAV Gag exchange mutants, respectively. Alanine scanning of the PLPPV sequence revealed a strict requirement for the valine residue but only minor requirements for any one of the other residues. Thus, PLPPV provides MMTV L domain function, representing a fourth type of retroviral L domain that enables MMTV Gag proteins to co-opt cellular budding pathways for release.

ITCH E3 Ubiquitin Ligase Interacts with Ebola Virus VP40 To Regulate Budding

Ebola virus (EBOV) and Marburg virus (MARV) belong to the Filoviridae family and can cause outbreaks of severe hemorrhagic fever, with high mortality rates in humans. The EBOV VP40 (eVP40) and MARV VP40 (mVP40) matrix proteins play a central role in virion assembly and egress, such that independent expression of VP40 leads to the production and egress of virus-like particles (VLPs) that accurately mimic the budding of infectious virus. Late (L) budding domains of eVP40 recruit host proteins (e.g., Tsg101, Nedd4, and Alix) that are important for efficient virus egress and spread. For example, the PPxY-type L domain of eVP40 and mVP40 recruits the host Nedd4 E3 ubiquitin ligase via its WW domains to facilitate budding. Here we sought to identify additional WW domain host interactors and demonstrate that the PPxY L domain motif of eVP40 interacts specifically with the WW domain of the host E3 ubiquitin ligase ITCH. ITCH, like Nedd4, is a member of the HECT class of E3 ubiquitin ligases, and the resultant physical and functional interaction with eVP40 facilitates VLP and virus budding. Identification of this novel eVP40 interactor highlights the functional interplay between cellular E3 ligases, ubiquitination, and regulation of VP40-mediated egress.

IMPORTANCE

The unprecedented magnitude and scope of the recent 2014-2015 EBOV outbreak in West Africa and its emergence here in the United States and other countries underscore the critical need for a better understanding of the biology and pathogenesis of this emerging pathogen. We have identified a novel and functional EBOV VP40 interactor, ITCH, that regulates VP40-mediated egress. This virus-host interaction may represent a new target for our previously identified small-molecule inhibitors of virus egress.

Roles of the three L-domains in β-retrovirus budding

Retroviral Gag protein plays a critical role during the late stage of virus budding and possesses a so-called L-domain containing PT/SAP, PPxY, YxxL or FPIV motifs that are critical for efficient budding. Mason-Pfizer monkey virus (M-PMV) contains PSAP, PPPY, and YADL sequences in Gag. This study was performed to investigate the roles of these three L-domain-like sequences in virus replication in three different cell lines, 293T, COS-7 and HeLa cells. It was found that the PPxY motif plays an essential role in progeny virus production as a major L-domain in all three cell lines. The PSAP sequence was shown to function as an additional L-domain in HeLa cells and to promote efficient release of M-PMV; however, this sequence was dispensable for M-PMV production in 293T and COS-7 cells, suggesting that the role of the PSAP motif as an L-domain in M-PMV budding is cell type-dependent. Viruses possessing multiple L-domains appear to change the L-domain usage to replicate in various cells. On the other hand, the YADL motif was required for M-PMV production as a transport signal of Gag to the plasma membrane, but not as an L-domain.

Tracking the Fate of Genetically Distinct Vesicular Stomatitis Virus Matrix Proteins Highlights the Role for Late Domains in Assembly

Vesicular stomatitis virus (VSV) assembly requires condensation of the viral ribonucleoprotein (RNP) core with the matrix protein (M) during budding from the plasma membrane. The RNP core comprises the negative-sense genomic RNA completely coated by the nucleocapsid protein (N) and associated by a phosphoprotein (P) with the large polymerase protein (L). To study the assembly of single viral particles, we tagged M and P with fluorescent proteins. We selected from a library of viruses with insertions in the M gene a replication-competent virus containing a fluorescent M and combined that with our previously described virus containing fluorescent P. Virus particles containing those fusions maintained the same bullet shape appearance as wild-type VSV but had a modest increase in particle length, reflecting the increased genome size. Imaging of the released particles revealed a variation in the amount of M and P assembled into the virions, consistent with a flexible packaging mechanism. We used the recombinants to further study the importance of the late domains in M, which serve to recruit the endosomal sorting complex required for transport (ESCRT) machinery during budding. Mutations in late domains resulted in the accumulation of virions that failed to pinch off from the plasma membrane. Imaging of single virions released from cells that were coinfected with M tagged with enhanced green fluorescent protein and M tagged with mCherry variants in which the late domains of one virus were inactivated by mutation showed a strong bias against the incorporation of the late-domain mutant into the released virions. In contrast, the intracellular expression and membrane association of the two variants were unaltered. These studies provide new tools for imaging particle assembly and enhance our resolution of existing models for assembly of VSV.

IMPORTANCE

Assembly of vesicular stomatitis virus (VSV) particles requires the separate trafficking of the viral replication machinery, a matrix protein (M) and a glycoprotein, to the plasma membrane. The matrix protein contains a motif termed a "late domain" that engages the host endosomal sorting complex required for transport (ESCRT) machinery to facilitate the release of viral particles. Inactivation of the late domains through mutation results in the accumulation of virions arrested at the point of release. In the study described here, we developed new tools to study VSV assembly by fusing fluorescent proteins to M and to a constituent of the replication machinery, the phosphoprotein (P). We used those tools to show that the late domains of M are required for efficient incorporation into viral particles and that the particles contain a variable quantity of M and P.

Biomembranes in atomistic and coarse-grained simulations

The architecture of biological membranes is tightly coupled to the localization, organization, and function of membrane proteins. The organelle-specific distribution of lipids allows for the formation of functional microdomains (also called rafts) that facilitate the segregation and aggregation of membrane proteins and thus shape their function. Molecular dynamics simulations enable to directly access the formation, structure, and dynamics of membrane microdomains at the molecular scale and the specific interactions among lipids and proteins on timescales from picoseconds to microseconds. This review focuses on the latest developments of biomembrane force fields for both atomistic and coarse-grained molecular dynamics (MD) simulations, and the different levels of coarsening of biomolecular structures. It also briefly introduces scale-bridging methods applicable to biomembrane studies, and highlights selected recent applications.

Angiomotin functions in HIV-1 assembly and budding

Many retroviral Gag proteins contain PPXY late assembly domain motifs that recruit proteins of the NEDD4 E3 ubiquitin ligase family to facilitate virus release. Overexpression of NEDD4L can also stimulate HIV-1 release but in this case the Gag protein lacks a PPXY motif, suggesting that NEDD4L may function through an adaptor protein. Here, we demonstrate that the cellular protein Angiomotin (AMOT) can bind both NEDD4L and HIV-1 Gag. HIV-1 release and infectivity are stimulated by AMOT overexpression and inhibited by AMOT depletion, whereas AMOT mutants that cannot bind NEDD4L cannot function in virus release. Electron microscopic analyses revealed that in the absence of AMOT assembling Gag molecules fail to form a fully spherical enveloped particle. Our experiments indicate that AMOT and other motin family members function together with NEDD4L to help complete immature virion assembly prior to ESCRT-mediated virus budding.

DOI:http://dx.doi.org/10.7554/eLife.03778.001

To multiply and spread infections, viruses must enter and exit cells. Once inside a cell, many viruses conscript the cell's machinery to produce new viral particles and release them into the surroundings. Some viruses—like HIV-1—exit the cell in a way that leads to them being wrapped (or ‘enveloped’) in membrane from the host cell.

A virus protein called Gag is required for the release of HIV-1 and other enveloped viruses. In some cases, Gag proteins bind directly to members of the NEDD4 protein family to enable the viruses to be released. However, the Gag protein from HIV-1 does not appear to be able to interact directly with NEDD4 proteins, so it was not clear how Gag works in this case.

Mercenne et al. studied how HIV-1 is released from human cells grown in the laboratory. The experiments show that members of a human protein family called the Angiomotins bind to both Gag and NEDD4L (a member of the NEDD4 family) and are required for the efficient release of viruses. Using a technique called electron microscopy, Mercenne et al. observed that when Angiomotins are present, Gag proteins assemble in spheres at the cell membrane and viruses are able to exit the cell. However, when Angiomotins are depleted or absent, incomplete spheres of Gag proteins accumulate on the inner membrane surface and viruses are not released.

These findings show that Angiomotins act as a link between Gag and NEDD4L to promote the release of HIV-1 from human cells. The next step will be to learn precisely how this works. There are indications that the Angiomotins may also be involved in the release of other enveloped viruses, so the findings may be useful for the development of treatments for a variety of viral infections.

DOI:http://dx.doi.org/10.7554/eLife.03778.002

NEDD4: The founding member of a family of ubiquitin-protein ligases

Ubiquitination plays a crucial role in regulating proteins post-translationally. The focus of this review is on NEDD4, the founding member of the NEDD4 family of ubiquitin ligases that is evolutionarily conserved in eukaryotes. Many potential substrates of NEDD4 have been identified and NEDD4 has been shown to play a critical role in the regulation of a number of membrane receptors, endocytic machinery components and the tumour suppressor PTEN. In this review we will discuss the diverse pathways in which NEDD4 is involved, and the patho-physiological significance of this important ubiquitin ligase.

"Ménage à Trois": the evolutionary interplay between JSRV, enJSRVs and domestic sheep

Sheep betaretroviruses represent a fascinating model to study the complex evolutionary interplay between host and pathogen in natural settings. In infected sheep, the exogenous and pathogenic Jaagsiekte sheep retrovirus (JSRV) coexists with a variety of highly related endogenous JSRVs, referred to as enJSRVs. During evolution, some of them were co-opted by the host as they fulfilled important biological functions, including placental development and protection against related exogenous retroviruses. In particular, two enJSRV loci, enJS56A1 and enJSRV-20, were positively selected during sheep domestication due to their ability to interfere with the replication of related competent retroviruses. Interestingly, viruses escaping these transdominant enJSRVs have recently emerged, probably less than 200 years ago. Overall, these findings suggest that in sheep the process of endogenization is still ongoing and, therefore, the evolutionary interplay between endogenous and exogenous sheep betaretroviruses and their host has not yet reached an equilibrium.

Suppression of production of baboon endogenous virus by dominant negative mutants of cellular factors involved in multivesicular body sorting pathway

Baboon endogenous virus (BaEV) is an infectious endogenous gammaretrovirus isolated from a baboon placenta. BaEV-related sequences have been identified in both Old World monkeys and African apes, but not in humans or Asian apes. Recently, it was reported that BaEV-like particles were produced from Vero cells derived from African green monkeys by chemical induction, and thus BaEV-like particles may contaminate biological products manufactured using Vero cells. In this study, we constructed an infectious molecular clone of BaEV strain M7. We found two putative L-domain motifs, PPPY and PSAP, in the pp15 region of Gag. To examine the function of the L-domain motifs, we conducted virus budding assay using L-domain motif mutants. We revealed that the PPPY motif, but not the PSAP motif, plays a major role as the L-domain in BaEV budding. We also demonstrated that Vps4A/B are involved in BaEV budding. These data suggest that BaEV Gag recruits the cellular endosomal sorting complex required for transport (ESCRT) machinery through the interaction of the PPPY L-domain with cellular factors. These data will be useful for controlling contamination of BaEV-like particles in biological products in the future.

Small-molecule probes targeting the viral PPxY-host Nedd4 interface block egress of a broad range of RNA viruses

Budding of filoviruses, arenaviruses, and rhabdoviruses is facilitated by subversion of host proteins, such as Nedd4 E3 ubiquitin ligase, by viral PPxY late (L) budding domains expressed within the matrix proteins of these RNA viruses. As L domains are important for budding and are highly conserved in a wide array of RNA viruses, they represent potential broad-spectrum targets for the development of antiviral drugs. To identify potential competitive blockers, we used the known Nedd4 WW domain-PPxY interaction interface as the basis of an in silico screen. Using PPxY-dependent budding of Marburg (MARV) VP40 virus-like particles (VLPs) as our model system, we identified small-molecule hit 1 that inhibited Nedd4-PPxY interaction and PPxY-dependent budding. This lead candidate was subsequently improved with additional structure-activity relationship (SAR) analog testing which enhanced antibudding activity into the nanomolar range. Current lead compounds 4 and 5 exhibit on-target effects by specifically blocking the MARV VP40 PPxY-host Nedd4 interaction and subsequent PPxY-dependent egress of MARV VP40 VLPs. In addition, lead compounds 4 and 5 exhibited antibudding activity against Ebola and Lassa fever VLPs, as well as vesicular stomatitis and rabies viruses (VSV and RABV, respectively). These data provide target validation and suggest that inhibition of the PPxY-Nedd4 interaction can serve as the basis for the development of a novel class of broad-spectrum, host-oriented antivirals targeting viruses that depend on a functional PPxY L domain for efficient egress.

IMPORTANCE

There is an urgent and unmet need for the development of safe and effective therapeutics against biodefense and high-priority pathogens, including filoviruses (Ebola and Marburg) and arenaviruses (e.g., Lassa and Junin) which cause severe hemorrhagic fever syndromes with high mortality rates. We along with others have established that efficient budding of filoviruses, arenaviruses, and other viruses is critically dependent on the subversion of host proteins. As disruption of virus budding would prevent virus dissemination, identification of small-molecule compounds that block these critical viral-host interactions should effectively block disease progression and transmission. Our findings provide validation for targeting these virus-host interactions as we have identified lead inhibitors with broad-spectrum antiviral activity. In addition, such inhibitors might prove useful for newly emerging RNA viruses for which no therapeutics would be available.

Virus budding and the ESCRT pathway

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

A host-oriented inhibitor of Junin Argentine hemorrhagic fever virus egress

There are currently no U.S. Food and Drug Administration (FDA)-approved vaccines or therapeutics to prevent or treat Argentine hemorrhagic fever (AHF). The causative agent of AHF is Junin virus (JUNV); a New World arenavirus classified as a National Institute of Allergy and Infectious Disease/Centers for Disease Control and Prevention category A priority pathogen. The PTAP late (L) domain motif within JUNV Z protein facilitates virion egress and transmission by recruiting host Tsg101 and other ESCRT complex proteins to promote scission of the virus particle from the plasma membrane. Here, we describe a novel compound (compound 0013) that blocks the JUNV Z-Tsg101 interaction and inhibits budding of virus-like particles (VLPs) driven by ectopic expression of the Z protein and live-attenuated JUNV Candid-1 strain in cell culture. Since inhibition of the PTAP-Tsg101 interaction inhibits JUNV egress, compound 0013 serves as a prototype therapeutic that could reduce virus dissemination and disease progression in infected individuals. Moreover, since PTAP l-domain-mediated Tsg101 recruitment is utilized by other RNA virus pathogens (e.g., Ebola virus and HIV-1), PTAP inhibitors such as compound 0013 have the potential to function as potent broad-spectrum, host-oriented antiviral drugs.

IMPORTANCE

There are currently no FDA-approved vaccines or therapeutics to prevent or treat Argentine hemorrhagic fever (AHF). The causative agent of AHF is Junin virus (JUNV); a New World arenavirus classified as an NIAID/CDC category A priority pathogen. Here, we describe a prototype therapeutic that blocks budding of JUNV and has the potential to function as a broad-spectrum antiviral drug.

The S40 residue in HIV-1 Gag p6 impacts local and distal budding determinants, revealing additional late domain activities

Background

HIV-1 budding is directed primarily by two motifs in Gag p6 designated as late domain-1 and −2 that recruit ESCRT machinery by binding Tsg101 and Alix, respectively, and by poorly characterized determinants in the capsid (CA) domain. Here, we report that a conserved Gag p6 residue, S40, impacts budding mediated by all of these determinants.

Results

Whereas budding normally results in formation of single spherical particles ~100 nm in diameter and containing a characteristic electron-dense conical core, the substitution of Phe for S40, a change that does not alter the amino acids encoded in the overlapping pol reading frame, resulted in defective CA-SP1 cleavage, formation of strings of tethered particles or filopodia-like membrane protrusions containing Gag, and diminished infectious particle formation. The S40F-mediated release defects were exacerbated when the viral-encoded protease (PR) was inactivated or when L domain-1 function was disrupted or when budding was almost completely obliterated by the disruption of both L domain-1 and −2. S40F mutation also resulted in stronger Gag-Alix interaction, as detected by yeast 2-hybrid assay. Reducing Alix binding by mutational disruption of contact residues restored single particle release, implicating the perturbed Gag-Alix interaction in the aberrant budding events. Interestingly, introduction of S40F partially rescued the negative effects on budding of CA NTD mutations EE75,76AA and P99A, which both prevent membrane curvature and therefore block budding at an early stage.

Conclusions

The results indicate that the S40 residue is a novel determinant of HIV-1 egress that is most likely involved in regulation of a critical assembly event required for budding in the Tsg101-, Alix-, Nedd4- and CA N-terminal domain affected pathways.

Identification of late assembly domains of the human endogenous retrovirus-K(HML-2)

Background

Late assembly (L)-domains are protein interaction motifs, whose dysfunction causes characteristic budding defects in enveloped viruses. Three different amino acid motifs, namely PT/SAP, PPXY and YPX_nL have been shown to play a major role in the release of exogenous retroviruses. Although the L-domains of exogenous retroviruses have been studied comprehensively, little is known about these motifs in endogenous human retroviruses.

Results

Using a molecular clone of the human endogenous retrovirus K113 that had been engineered to reverse the presumed non-synonymous postinsertional mutations in the major genes, we identified three functional L-domains of the virus, all located in the Gag p15 protein. A consensus PTAP tetrapeptide serves as the core of a main L-domain for the virus and its inactivation reduces virus release in HEK 293T cells by over 80%. Electron microscopy of cells expressing the PTAP mutant revealed predominantly late budding structures and budding chains at the plasma membrane. The fact that this motif determines subcellular colocalization with Tsg101, an ESCRT-I complex protein known to bind to the core tetrapeptide, supports its role as an L-domain. Moreover, two YPX_nL motifs providing additional L-domain function were identified in the p15 protein. One is adjacent to the PTAP sequence and the other is in the p15 N-terminus. Mutations in either motif diminishes virus release and induces an L-domain phenotype while inactivation of all three L-domains results in a complete loss of particle release in HEK 293T cells. The flexibility of the virus in the use of L-domains for gaining access to the ESCRT machinery is demonstrated by overexpression of Tsg101 which rescues the release of the YPX_nL mutants. Similarly, overexpression of Alix not only enhances release of the PTAP mutant by a factor of four but also the release of a triple mutant, indicating that additional cryptic YPX_nL domains with a low affinity for Alix may be present. No L-domain activity is provided by the proline-rich peptides at the Gag C-terminus.

Conclusions

Our data demonstrate that HERV-K(HML-2) release is predominantly mediated through a consensus PTAP motif and two auxiliary YPX_nL motifs in the p15 protein of the Gag precursor.

Wrapping up the bad news: HIV assembly and release

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

ESCRT-independent budding of HIV-1 gag virus-like particles from Saccharomyces cerevisiae spheroplasts

Heterologous expression of HIV-1 Gag in a variety of host cells results in its packaging into virus-like particles (VLPs) that are subsequently released into the extracellular milieu. This phenomenon represents a useful tool for probing cellular factors required for viral budding and has contributed to the discovery of roles for ubiquitin ligases and the endosomal sorting complexes required for transport (ESCRTs) in viral budding. These factors are highly conserved throughout eukaryotes and have been studied extensively in the yeast Saccharomyces cerevisiae, a model eukaryote previously utilized as a host for the production of VLPs. We used heterologous expression of HIV Gag in yeast spheroplasts to examine the role of ESCRTs and associated factors (Rsp5, a HECT ubiquitin ligase of the Nedd4 family; Bro1, a homolog of Alix; and Vps4, the AAA-ATPase required for ESCRT function in all contexts/organisms investigated) in the generation of VLPs. Our data reveal: 1) characterized Gag-ESCRT interaction motifs (late domains) are not required for VLP budding, 2) loss of function alleles of the essential HECT ubiquitin ligase Rsp5 do not display defects in VLP formation, and 3) ESCRT function is not required for VLP formation from spheroplasts. These results suggest that the egress of HIV Gag from yeast cells is distinct from the most commonly described mode of exit from mammalian cells, instead mimicking ESCRT-independent VLP formation observed in a subset of mammalian cells. As such, budding of Gag from yeast cells appears to represent ESCRT-independent budding relevant to viral replication in at least some situations. Thus the myriad of genetic and biochemical tools available in the yeast system may be of utility in the study of this aspect of viral budding.

Budding of Enveloped Viruses: Interferon-Induced ISG15-Antivirus Mechanisms Targeting the Release Process

Pathogenic strains of viruses that infect humans are encapsulated in membranes derived from the host cell in which they infect. After replication, these viruses are released by a budding process that requires cell/viral membrane scission. As such, this represents a natural target for innate immunity mechanisms to interdict enveloped virus spread and recent advances in this field will be the subject of this paper.

Distal leucines are key functional determinants of Alix-binding simian immunodeficiency virus SIV(smE543) and SIV(mac239) type 3 L domains

In addition to PTAP L domains, primate lentiviruses carry Alix-binding motifs that include the recently described type 3 SREKPYKEVTEDLLHLNSLF sequence. We examined the requirements for the type 3 sequence motif in simian immunodeficiency virus SIV(smE543) and identified the (499)LNSLF(503) sequence as a key functional determinant. Mutation of distal leucines (499)L and (502)L (LL mutant) caused an inhibitory effect on Alix-dependent SIV(smE543) release that was quantitatively similar to that observed following disruption of the type 3 L domain or RNA interference (RNAi) depletion of Alix. Similar results were obtained with the SIV(mac239) LL mutant. Thus, distal leucines are key determinants of SIV(smE543) and SIV(mac239) type 3 L domains.

The role of ITCH protein in human T-cell leukemia virus type 1 release

Human T-cell leukemia virus type 1 (HTLV-1) has two late domain (LD) motifs, PPPY and PTAP, which are important for viral budding. Mutations in the PPPY motif are more deleterious for viral release than changes in the PTAP motif. Several reports have shown that the interaction of PPPY with the WW domains of a Nedd4 (neuronal precursor cell-expressed developmentally down-regulated-4) family ubiquitin ligase (UL) is a critical event in virus release. We tested nine members of the Nedd4 family ULs and found that ITCH is the main contributor to HTLV-1 budding. ITCH overexpression strongly inhibited release and infectivity of wild-type (wt) HTLV-1, but rescued the release of infectious virions with certain mutations in the PPPY motif. Electron microscopy showed either fewer or misshapen virus particles when wt HTLV-1 was produced in the presence of overexpressed ITCH, whereas mutants with changes in the PPPY motif yielded normal looking particles at wt level. The other ULs had significantly weaker or no effects on HTLV-1 release and infectivity except for SMURF-1, which caused enhanced release of wt and all PPPY(-) mutant particles. These particles were poorly infectious and showed abnormal morphology by electron microscopy. Budding and infectivity defects due to overexpression of ITCH and SMURF-1 were correlated with higher than normal ubiquitination of Gag. Only silencing of ITCH, but not of WWP1, WWP2, and Nedd4, resulted in a reduction of HTLV-1 budding from 293T cells. The binding efficiencies between the HTLV-1 LD and WW domains of different ULs as measured by mammalian two-hybrid interaction did not correlate with the strength of their effect on HTLV-1 budding.

Mechanism of inhibition of retrovirus release from cells by interferon-induced gene ISG15

Budding of retroviruses from cell membranes requires ubiquitination of Gag and recruitment of cellular proteins involved in endosome sorting, including endosome sorting complex required for transport III (ESCRT-III) protein complex and vacuolar protein sorting 4 (VPS4) and its ATPase. In response to infection, a cellular mechanism has evolved that blocks virus replication early and late in the budding process through expression of interferon-stimulated gene 15 (ISG15), a dimer homologue of ubiquitin. Interferon treatment of DF-1 cells blocks avian sarcoma/leukosis virus release, demonstrating that this mechanism is functional under physiological conditions. The late block to release is caused in part by a loss in interaction between VPS4 and its coactivator protein LIP5, which is required to promote the formation of the ESCRT III-VPS4 double-hexamer complex to activate its ATPase. ISG15 is conjugated to two different LIP5-ESCRT-III-binding charged multivesicular body proteins, CHMP2A and CHMP5. Upon ISGylation of each, interaction with LIP5 is no longer detected. Two other ESCRT-III proteins, CHMP4B and CHMP6, are also conjugated to ISG15. ISGylation of CHMP2A, CHMP4B, and CHMP6 weakens their binding directly to VPS4, thereby facilitating the release of this protein from the membrane into the cytosol. The remaining budding complex fails to release particles from the cell membrane. Introducing a mutant of ISG15 into cells that cannot be conjugated to proteins prevents the ISG15-dependent mechanism from blocking virus release. CHMP5 is the primary switch to initiate the antiviral mechanism, because removal of CHMP5 from cells prevents ISGylation of CHMP2A and CHMP6.

TANK-binding kinase 1 attenuates PTAP-dependent retroviral budding through targeting endosomal sorting complex required for transport-I

Retroviruses need to bud from producer cells to spread infection. To facilitate its budding, some virus hijacks the multivesicular body (MVB) pathway that is normally used to cargo and degrade ubiquitylated cellular proteins, through interaction between the late domain of Gag polyproteins and the components of MVB machinery. In this study, we demonstrated that TANK-binding kinase 1 (TBK1) directly interacted with VPS37C, a subunit of endosomal sorting complex required for transport-I (ESCRT-I) in the MVB pathway, without affecting the ultrastructure or general function of MVB. Interestingly, overexpression of TBK1 attenuated, whereas short hairpin RNA interference of TBK1 enhanced HIV-1 pseudovirus release from Vero cells in type I IFN (IFN-I)-independent manner. Down-regulation of TBK1 by short hairpin RNA in TZM-bl cells also enhanced live HIV-1 NL4-3 or JR-CSF virus budding without involvement of IFN-I induction. Furthermore, infection of TBK1-deficient mouse embryonic fibroblast cells with a chimeric murine leukemia virus/p6, whose PPPY motif was replaced by PTAP motif of HIV-1, showed that lack of TBK1 significantly enhanced PTAP-dependent, but not PPPY-dependent retrovirus budding. Finally, phosphorylation of VPS37C by TBK1 might regulate the viral budding efficiency, because overexpression of the kinase-inactive mutant of TBK1 (TBK1-K38A) in Vero cells accelerated HIV-1 pseudovirus budding. Therefore, through tethering to VPS37C of the ESCRT-I complex, TBK1 controlled the speed of PTAP-dependent retroviral budding through phosphorylation of VPS37C, which would serve as a novel mechanism of host cell defense independent of IFN-I signaling.

Budding capability of the influenza virus neuraminidase can be modulated by tetherin

We have determined that, in addition to its receptor-destroying activity, the influenza virus neuraminidase is capable of efficiently forming virus-like particles (VLPs) when expressed individually from plasmid DNA. This observation applies to both human subtypes of neuraminidase, N1 and N2. However, it is not found with every strain of influenza virus. Through gain-of-function and loss-of-function analyses, a critical determinant within the neuraminidase ectodomain was identified that contributes to VLP formation but is not sufficient to accomplish release of plasmid-derived VLPs. This sequence lies on the plasma membrane-proximal side of the neuraminidase globular head. Most importantly, we demonstrate that the antiviral restriction factor tetherin plays a role in determining the strain-specific limitations of release competency. If tetherin is counteracted by small interfering RNA knockdown or expression of the HIV anti-tetherin factor vpu, budding and release capability is bestowed upon an otherwise budding-deficient neuraminidase. These data suggest that budding-competent neuraminidase proteins possess an as-yet-unidentified means of counteracting the antiviral restriction factor tetherin and identify a novel way in which the influenza virus neuraminidase can contribute to virus release.

Marburg virus budding: ESCRT of progeny virion to the outside of the cell

The major virion matrix protein of the Marburg virus (MARV), VP40, plays a key role in MARV assembly and budding, and its sole expression can produce enveloped virus-like particles. VP40 possesses only the PPXY motif as an L-domain critical for efficient virus budding, and interacts with the cellular ubiquitin ligase Nedd4. Functional abrogation of the cellular components of the endosomal sorting complexes required for transport complexes that participate in budding of multivesicular bodies into late endosomes by dominant-negative mutants or siRNA inhibited virus-like particle release, suggest that MARV budding utilizes the multivesicular bodies sorting pathway. In addition, tetherin/BST-2 was recently identified as an antiviral cellular factor that reduces MARV virus-like particle production. These findings may contribute to development of novel anti-MARV therapeutic strategies.

The noncanonical Gag domains p8 and n are critical for assembly and release of mouse mammary tumor virus

The mouse mammary tumor virus (MMTV) Gag contains the unique domains pp21, p3, p8, and n. We investigated the contribution of these domains to particle assembly and found that the region spanning the p8 and n domains is critical for shape determination and assembly. Deletion of pp21 and p3 reduced the number of released particles, but deletion of the n domain resulted in frequent formation of aberrant particles, while deletion of p8 severely impaired assembly. Further investigation of p8 revealed that both the basic and the proline-rich motifs within p8 contribute to MMTV assembly.

Antiviral activity of innate immune protein ISG15

The host innate immune response, including the production of type-I IFN, represents the primary line of defense against invading viral pathogens. Of the hundreds of IFN-stimulated genes (ISGs) discovered to date, ISG15 was one of the first identified and shown to encode a ubiquitin-like protein that functions, in part, as a modifier of protein function. Evidence implicating ISG15 as an innate immune protein with broad-spectrum antiviral activity continues to accumulate rapidly. This review will summarize recent findings on the innate antiviral activity of ISG15, with a focus on the interplay between ubiquitination and ISGylation pathways resulting in modulation of RNA virus assembly/budding. Indeed, ubiquitination is known to be proviral for some RNA viruses, whereas the parallel ISGylation pathway is known to be antiviral. A better understanding of the antiviral activities of ISG15 will enhance our fundamental knowledge of host innate responses to viral pathogens and may provide insight useful for the development of novel therapeutic approaches designed to enhance the immune response against such pathogens.

The ESCRT complexes

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

The ESCRT-associated protein Alix recruits the ubiquitin ligase Nedd4-1 to facilitate HIV-1 release through the LYPXnL L domain motif

The p6 region of HIV-1 Gag contains two late (L) domains, PTAP and LYPX(n)L, that bind Tsg101 and Alix, respectively. Interactions with these two cellular proteins recruit members of the host's fission machinery (ESCRT) to facilitate HIV-1 release. Other retroviruses gain access to the host ESCRT components by utilizing a PPXY-type L domain that interacts with cellular Nedd4-like ubiquitin ligases. Despite the absence of a PPXY motif in HIV-1 Gag, interaction with the ubiquitin ligase Nedd4-2 was recently shown to stimulate HIV-1 release. We show here that another Nedd4-like ubiquitin ligase, Nedd4-1, corrected release defects resulting from the disruption of PTAP (PTAP(-)), suggesting that HIV-1 Gag also recruits Nedd4-1 to facilitate virus release. Notably, Nedd4-1 remediation of HIV-1 PTAP(-) budding defects is independent of cellular Tsg101, implying that Nedd4-1's function in HIV-1 release does not involve ESCRT-I components and is therefore distinct from that of Nedd4-2. Consistent with this finding, deletion of the p6 region decreased Nedd4-1-Gag interaction, and disruption of the LYPX(n)L motif eliminated Nedd4-1-mediated restoration of HIV-1 PTAP(-). This result indicated that both Nedd4-1 interaction with Gag and function in virus release occur through the Alix-binding LYPX(n)L motif. Mutations of basic residues located in the NC domain of Gag that are critical for Alix's facilitation of HIV-1 release, also disrupted release mediated by Nedd4-1, further confirming a Nedd4-1-Alix functional interdependence. In fact we found that Nedd4-1 binds Alix in both immunoprecipitation and yeast-two-hybrid assays. In addition, Nedd4-1 requires its catalytic activity to promote virus release. Remarkably, RNAi knockdown of cellular Nedd4-1 eliminated Alix ubiquitination in the cell and impeded its ability to function in HIV-1 release. Together our data support a model in which Alix recruits Nedd4-1 to facilitate HIV-1 release mediated through the LYPX(n)L/Alix budding pathway via a mechanism that involves Alix ubiquitination.

Fluctuations of the Casimir-like force between two membrane inclusions

Although Casimir forces are inseparable from their fluctuations, little is known about these fluctuations in soft matter systems. We use the membrane stress tensor to study the fluctuations of the membrane-mediated Casimir-like force. This method enables us to recover the Casimir force between two inclusions and to calculate its variance. We show that the Casimir force is dominated by its fluctuations. Furthermore, when the distance d between the inclusions is decreased from infinity, the variance of the Casimir force decreases as -1/d2. This distance dependence shares a common physical origin with the Casimir force itself.

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

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

A capsid-encoded PPxY-motif facilitates adenovirus entry

Viruses use cellular machinery to enter and infect cells. In this study we address the cell entry mechanisms of nonenveloped adenoviruses (Ads). We show that protein VI, an internal capsid protein, is rapidly exposed after cell surface attachment and internalization and remains partially associated with the capsid during intracellular transport. We found that a PPxY motif within protein VI recruits Nedd4 E3 ubiquitin ligases to bind and ubiquitylate protein VI. We further show that this PPxY motif is involved in rapid, microtubule-dependent intracellular movement of protein VI. Ads with a mutated PPxY motif can efficiently escape endosomes but are defective in microtubule-dependent trafficking toward the nucleus. Likewise, depletion of Nedd4 ligases attenuates nuclear accumulation of incoming Ad particles and infection. Our data provide the first evidence that virus-encoded PPxY motifs are required during virus entry, which may be of significance for several other pathogens.

Late domain-independent rescue of a release-deficient Moloney murine leukemia virus by the ubiquitin ligase itch

Moloney murine leukemia virus (MoMLV) Gag utilizes its late (L) domain motif PPPY to bind members of the Nedd4-like ubiquitin ligase family. These interactions recruit components of the cell's budding machinery that are critical for virus release. MoMLV Gag contains two additional L domains, PSAP and LYPAL, that are believed to drive residual MoMLV release via interactions with cellular proteins Tsg101 and Alix, respectively. We found that overexpression of Tsg101 or Alix failed to rescue the release of PPPY-deficient MoMLV via these other L domains. However, low-level expression of the ubiquitin ligase Itch potently rescued the release and infectivity of MoMLV lacking PPPY function. In contrast, other ubiquitin ligases such as WWP1, Nedd4.1, Nedd4.2, and Nedd4.2s did not rescue this release-deficient virus. Efficient rescue required the ubiquitin ligase activity of Itch and an intact C2 domain but not presence of the endophilin-binding site. Additionally, we found Itch to immunoprecipitate with MoMLV Gag lacking the PPPY motif and to be incorporated into rescued MoMLV particles. The PSAP and LYPAL motifs were dispensable for Itch-mediated virus rescue, and their absence did not affect the incorporation of Itch into the rescued particles. Itch-mediated rescue of release-defective MoMLV was sensitive to inhibition by dominant-negative versions of ESCRT-III components and the VPS4 AAA ATPase, indicating that Itch-mediated correction of MoMLV release defects requires the integrity of the host vacuolar sorting protein pathway. RNA interference knockdown of Itch suppressed the residual release of the MoMLV lacking the PPPY motif. Interestingly, Itch stimulation of the PPPY-deficient MoMLV release was accompanied by the enhancement of Gag ubiquitination and the appearance of new ubiquitinated Gag proteins in virions. Together, these results suggest that Itch can facilitate MoMLV release in an L domain-independent manner via a mechanism that requires the host budding machinery and involves Gag ubiquitination.

Regulation of Marburg virus (MARV) budding by Nedd4.1: a different WW domain of Nedd4.1 is critical for binding to MARV and Ebola virus VP40

The VP40 matrix protein of Marburg virus (MARV) has been shown to be the driving force behind MARV budding, a process in which the PPPY L-domain motif of VP40 plays a critical role. Here, we report that Vps4B and Nedd4.1 play critical roles in MARV VP40-mediated budding. We showed that unidentified activities of the Nedd4.1 HECT domain, along with its E3 ubiquitin ligase activity, may be required for MARV budding. Moreover, we showed that the first WW domain of Nedd4.1, WW1, is critical for binding to MARV VP40, indicating that MARV VP40 and Ebola virus VP40 are recognized by a different WW domain of Nedd4.1. This is the first report showing that the viral L-domains containing PPxY have specificities for binding to WW domains. Our findings provide new insights into MARV budding, which may contribute to the development of novel anti-MARV therapeutic strategies.

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

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

Human cytomegalovirus exploits ESCRT machinery in the process of virion maturation

The endosomal sorting complex required for transport (ESCRT) machinery controls the incorporation of cargo into intraluminal vesicles of multivesicular bodies. This machinery is used during envelopment of many RNA viruses and some DNA viruses, including herpes simplex virus type 1. Other viruses mature independent of ESCRT components, instead relying on the intrinsic behavior of viral matrix and envelope proteins to drive envelopment. Human cytomegalovirus (HCMV) maturation has been reported to proceed independent of ESCRT components (A. Fraile-Ramos et al. Cell. Microbiol. 9:2955-2967, 2007). A virus complementation assay was used to evaluate the role of dominant-negative (DN) form of a key ESCRT ATPase, vacuolar protein sorting-4 (Vps4DN) in HCMV replication. Vps4DN specifically inhibited viral replication, whereas wild-type-Vps4 had no effect. In addition, a DN form of charged multivesicular body protein 1 (CHMP1DN) was found to inhibit HCMV. In contrast, DN tumor susceptibility gene-101 (Tsg101DN) did not impact viral replication despite the presence of a PTAP motif within pp150/ppUL32, an essential tegument protein involved in the last steps of viral maturation and release. Either Vps4DN or CHMP1DN blocked viral replication at a step after the accumulation of late viral proteins, suggesting that both are involved in maturation. Both Vps4A and CHMP1A localized in the vicinity of viral cytoplasmic assembly compartments, sites of viral maturation that develop in CMV-infected cells. Thus, ESCRT machinery is involved in the final steps of HCMV replication.

Nonmyristoylated matrix protein from the Mason-Pfizer monkey virus forms oligomers

We studied the oligomeric properties of betaretroviral nonmyristoylated matrix protein (MA) and its R55F mutant from the Mason-Pfizer monkey virus in solution by means of chemical crosslinking and NMR spectroscopy. By analyzing crosslinked products and using concentration-dependent NMR chemical shift mapping, we have proven that the wild-type (WT) MA forms oligomers in solution. Conversely, no oligomerization was observed for the R55F mutant. Structural comparison of MAs explained their different behaviors in solution, concluding that the key residues involved in intermonomeric interaction are exposed in the WT MA but buried in the mutant, preventing the oligomerization of R55F. The final model of oligomerization of the WT MA was derived by concerted use of chemical shift mapping and diffusion-ordered spectroscopy measured on a set of protein samples with varying concentrations. We found that the Mason-Pfizer monkey virus WT MA exists in a monomer-dimer-trimer equilibrium in solution, with the corresponding dissociation constants of 2.3 and 0.24 mM, respectively. Structures of the oligomers calculated with HADDOCK software are closely related to the structures of other retroviral MA trimers.

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

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

Role of multivesicular bodies and their components in the egress of enveloped RNA viruses

As an enveloped virus buds, the nascent viral capsid becomes wrapped in a plasma membrane-derived lipid envelope, and a membrane fission event is thus necessary to separate the virion from the host cell. This membrane fission event is well characterised in the case of enveloped RNA viruses, where it is promoted by late assembly domains (L-domains) present at the level of specific viral structural proteins. Research conducted over the past 10 years has demonstrated that L-domains represent docking sites for cellular proteins essential for the biogenesis of a cellular organelle, the multivesicular body (MVB). In this way, enveloped RNA viruses hijack the MVB components to the cellular site where the budding is executed. This review will focus on the cellular machinery exploited by enveloped RNA viruses in order to be released from infected cells. The role of ubiquitin and lipids in viral budding will also be discussed.

Three-dimensional analysis of budding sites and released virus suggests a revised model for HIV-1 morphogenesis

Current models of HIV-1 morphogenesis hold that newly synthesized viral Gag polyproteins traffic to and assemble at the cell membrane into spherical protein shells. The resulting late-budding structure is thought to be released by the cellular ESCRT machinery severing the membrane tether connecting it to the producer cell. Using electron tomography and scanning transmission electron microscopy, we find that virions have a morphology and composition distinct from late-budding sites. Gag is arranged as a continuous but incomplete sphere in the released virion. In contrast, late-budding sites lacking functional ESCRT exhibited a nearly closed Gag sphere. The results lead us to propose that budding is initiated by Gag assembly, but is completed in an ESCRT-dependent manner before the Gag sphere is complete. This suggests that ESCRT functions early in HIV-1 release--akin to its role in vesicle formation--and is not restricted to severing the thin membrane tether.