The Fusion glycoprotein shell of Semliki Forest virus: an icosahedral assembly primed for fusogenic activation at endosomal pH

A molecular understanding of alphavirus entry

Alphaviruses cause severe human illnesses including persistent arthritis and fatal encephalitis. As alphavirus entry into target cells is the first step in infection, intensive research efforts have focused on elucidating aspects of this pathway, including attachment, internalization, and fusion. Herein, we review recent developments in the molecular understanding of alphavirus entry both in vitro and in vivo and how these advances might enable the design of therapeutics targeting this critical step in the alphavirus life cycle.

Mutation of N-glycosylation Sites in Salmonid Alphavirus (SAV) Envelope Proteins Attenuate the Virus in Cell Culture

Salmonid alphavirus (SAV) is the cause of pancreas disease and sleeping disease in farmed salmonid fish in Europe. The spread of these diseases has been difficult to control with biosecurity and current vaccination strategies, and increased understanding of the viral pathogenesis could be beneficial for the development of novel vaccine strategies. N-glycosylation of viral envelope proteins may be crucial for viral virulence and a possible target for its purposed attenuation. In this study, we mutated the N-glycosylation consensus motifs of the E1 and E2 glycoproteins of a SAV3 infectious clone using site-directed mutagenesis. Mutation of the glycosylation motif in E1 gave a complete inactivation of the virus as no viral replication could be detected in cell culture and infectious particles could not be rescued. In contrast, infectious virus particles could be recovered from the SAV3 E2 mutants (E2319Q, E2319A), but not if they were accompanied by lack of N-glycosylation in E1. Compared to the non-mutated infectious clone, the SAV3-E2319Q and SAV3-E2319A recombinant viruses produced less cytopathic effects in cell culture and lower amounts of infectious viral particles. In conclusion, the substitution in the N-linked glycosylation site in E2 attenuated SAV3 in cell culture. The findings could be useful for immunization strategies using live attenuated vaccines and testing in fish will be desirable to study the clone’s properties in vivo.

The Hantavirus Surface Glycoprotein Lattice and Its Fusion Control Mechanism

Hantaviruses are rodent-borne viruses causing serious zoonotic outbreaks worldwide for which no treatment is available. Hantavirus particles are pleomorphic and display a characteristic square surface lattice. The envelope glycoproteins Gn and Gc form heterodimers that further assemble into tetrameric spikes, the lattice building blocks. The glycoproteins, which are the sole targets of neutralizing antibodies, drive virus entry via receptor-mediated endocytosis and endosomal membrane fusion. Here we describe the high-resolution X-ray structures of the heterodimer of Gc and the Gn head and of the homotetrameric Gn base. Docking them into an 11.4-Å-resolution cryoelectron tomography map of the hantavirus surface accounted for the complete extramembrane portion of the viral glycoprotein shell and allowed a detailed description of the surface organization of these pleomorphic virions. Our results, which further revealed a built-in mechanism controlling Gc membrane insertion for fusion, pave the way for immunogen design to protect against pathogenic hantaviruses.

Expression of the Mxra8 Receptor Promotes Alphavirus Infection and Pathogenesis in Mice and Drosophila

Mxra8 is a recently described receptor for multiple alphaviruses, including Chikungunya (CHIKV), Mayaro (MAYV), Ross River (RRV), and O'nyong nyong (ONNV) viruses. To determine its role in pathogenesis, we generated mice with mutant Mxra8 alleles: an 8-nucleotide deletion that produces a truncated, soluble form (Mxra8^Δ8/Δ8) and a 97-nucleotide deletion that abolishes Mxra8 expression (Mxra8^Δ97/Δ97). Mxra8^Δ8/Δ8 and Mxra8^Δ97/Δ97 fibroblasts show reduced CHIKV infection in culture, and Mxra8^Δ8/Δ8 and Mxra8^Δ97/Δ97 mice have decreased infection of musculoskeletal tissues with CHIKV, MAYV, RRV, or ONNV. Less foot swelling is observed in CHIKV-infected Mxra8 mutant mice, which correlated with fewer infiltrating neutrophils and cytokines. A recombinant E2-D71A CHIKV with diminished binding to Mxra8 is attenuated in vivo in wild-type mice. Ectopic Mxra8 expression is sufficient to enhance CHIKV infection and lethality in transgenic flies. These studies establish a role for Mxra8 in the pathogenesis of multiple alphaviruses and suggest that targeting this protein may mitigate disease in humans.

Entry Inhibitors: Efficient Means to Block Viral Infection

The emerging and re-emerging viral infections are constant threats to human health and wellbeing. Several strategies have been explored to develop vaccines against these viral diseases. The main effort in the journey of development of vaccines is to neutralize the fusion protein using antibodies. However, significant efforts have been made in discovering peptides and small molecules that inhibit the fusion between virus and host cell, thereby inhibiting the entry of viruses. This class of inhibitors is called entry inhibitors, and they are extremely efficient in reducing viral infection as the entry of the virus is considered as the first step of infection. Nevertheless, these inhibitors are highly selective for a particular virus as antibody-based vaccines. The recent COVID-19 pandemic lets us ponder to shift our attention towards broad-spectrum antiviral agents from the so-called ‘one bug-one drug’ approach. This review discusses peptide and small molecule-based entry inhibitors against class I, II, and III viruses and sheds light on broad-spectrum antiviral agents.

Viral Membrane Fusion and the Transmembrane Domain

Initiation of host cell infection by an enveloped virus requires a viral-to-host cell membrane fusion event. This event is mediated by at least one viral transmembrane glycoprotein, termed the fusion protein, which is a key therapeutic target. Viral fusion proteins have been studied for decades, and numerous critical insights into their function have been elucidated. However, the transmembrane region remains one of the most poorly understood facets of these proteins. In the past ten years, the field has made significant advances in understanding the role of the membrane-spanning region of viral fusion proteins. We summarize developments made in the past decade that have contributed to the understanding of the transmembrane region of viral fusion proteins, highlighting not only their critical role in the membrane fusion process, but further demonstrating their involvement in several aspects of the viral lifecycle.

Nucleic acid nanotechnology for cancer treatment

Cancer is one of the most prevalent potentially lethal diseases. With the increase in the number of investigations into the uses of nanotechnology, many nucleic acid (NA)-based nanostructures such as small interfering RNA, microRNA, aptamers, and immune adjuvant NA have been applied to treat cancer. Here, we discuss studies on the applications of NA in cancer treatment, recent research trends, and the limitations and prospects of specific NA-mediated gene therapy and immunotherapy for cancer treatment. The NA structures used for cancer therapy consist only of NA or hybrids comprising organic or inorganic substances integrated with functional NA. We also discuss delivery vehicles for therapeutic NA and anti-cancer agents, and recent trends in NA-based gene therapy and immunotherapy against cancer.

Role of Protein Glycosylation in Host-Pathogen Interaction

Host-pathogen interactions are fundamental to our understanding of infectious diseases. Protein glycosylation is one kind of common post-translational modification, forming glycoproteins and modulating numerous important biological processes. It also occurs in host-pathogen interaction, affecting host resistance or pathogen virulence often because glycans regulate protein conformation, activity, and stability, etc. This review summarizes various roles of different glycoproteins during the interaction, which include: host glycoproteins prevent pathogens as barriers; pathogen glycoproteins promote pathogens to attack host proteins as weapons; pathogens glycosylate proteins of the host to enhance virulence; and hosts sense pathogen glycoproteins to induce resistance. In addition, this review also intends to summarize the roles of lectin (a class of protein entangled with glycoprotein) in host-pathogen interactions, including bacterial adhesins, viral lectins or host lectins. Although these studies show the importance of protein glycosylation in host-pathogen interaction, much remains to be discovered about the interaction mechanism.

Optimal therapeutic activity of monoclonal antibodies against chikungunya virus requires Fc-FcγR interaction on monocytes

Chikungunya virus (CHIKV) is an emerging mosquito-borne virus that has caused explosive outbreaks worldwide. Although neutralizing monoclonal antibodies (mAbs) against CHIKV inhibit infection in animals, the contribution of Fc effector functions to protection remains unknown. Here, we evaluated the activity of therapeutic mAbs that had or lacked the ability to engage complement and Fcγ receptors (FcγR). When administered as post-exposure therapy in mice, the Fc effector functions of mAbs promoted virus clearance from infected cells and reduced joint swelling-results that were corroborated in antibody-treated transgenic animals lacking activating FcγR. The control of CHIKV infection by antibody-FcγR engagement was associated with an accelerated influx of monocytes. A series of immune cell depletions revealed that therapeutic mAbs required monocytes for efficient clearance of CHIKV infection. Overall, our study suggests that in mice, FcγR expression on monocytes is required for optimal therapeutic activity of antibodies against CHIKV and likely other related viruses.

Proteomics Computational Analyses Suggest That the Envelope Glycoproteins of Segmented Jingmen Flavi-Like Viruses are Class II Viral Fusion Proteins (b-Penetrenes) with Mucin-Like Domains

Jingmen viruses are newly described segmented flavi-like viruses that have a worldwide distribution in ticks and have been associated with febrile illnesses in humans. Computational analyses were used to predict that Jingmen flavi-like virus glycoproteins have structural features of class II viral fusion proteins, including an ectodomain consisting of beta-sheets and short alpha-helices, a fusion peptide with interfacial hydrophobicity and a three-domain architecture. Jingmen flavi-like virus glycoproteins have a sequence enriched in serine, threonine, and proline at the amino terminus, which is a feature of mucin-like domains. Several of the serines and threonines are predicted be modified by the addition of O-linked glycans. Some of the glycoproteins are predicted to have an additional mucin-like domain located prior to the transmembrane anchor, whereas others are predicted to have a stem consisting of two alpha-helices. The flavivirus envelope protein and Jingmen flavi-virus glycoproteins may have diverged from a common class II precursor glycoprotein with a mucin-like domain or domains acquired after divergence.

An Evolutionary Insertion in the Mxra8 Receptor-Binding Site Confers Resistance to Alphavirus Infection and Pathogenesis

Alphaviruses are emerging, mosquito-transmitted RNA viruses with poorly understood cellular tropism and species selectivity. Mxra8 is a receptor for multiple alphaviruses including chikungunya virus (CHIKV). We discovered that while expression of mouse, rat, chimpanzee, dog, horse, goat, sheep, and human Mxra8 enables alphavirus infection in cell culture, cattle Mxra8 does not. Cattle Mxra8 encodes a 15-amino acid insertion in its ectodomain that prevents Mxra8 binding to CHIKV. Identical insertions are present in zebu, yak, and the extinct auroch. As other Bovinae lineages contain related Mxra8 sequences, this insertion likely occurred at least 5 million years ago. Removing the Mxra8 insertion in Bovinae enhances alphavirus binding and infection, while introducing the insertion into mouse Mxra8 blocks CHIKV binding, prevents infection by multiple alphaviruses in cells, and mitigates CHIKV-induced pathogenesis in mice. Our studies on how this insertion provides resistance to CHIKV infection could facilitate countermeasures that disrupt Mxra8 interactions with alphaviruses.

The molecular foundations of zygosis

Zygosis is the generation of new biological individuals by the sexual fusion of gamete cells. Our current understanding of eukaryotic phylogeny indicates that sex is ancestral to all extant eukaryotes. Although sexual development is extremely diverse, common molecular elements have been retained. HAP2-GCS1, a protein that promotes the fusion of gamete cell membranes that is related in structure to certain viral fusogens, is conserved in many eukaryotic lineages, even though gametes vary considerably in form and behaviour between species. Similarly, although zygotes have dramatically different forms and fates in different organisms, diverse eukaryotes share a common developmental programme in which homeodomain-containing transcription factors play a central role. These common mechanistic elements suggest possible common evolutionary histories that, if correct, would have profound implications for our understanding of eukaryogenesis.

Cryo-EM Structures of Eastern Equine Encephalitis Virus Reveal Mechanisms of Virus Disassembly and Antibody Neutralization

Alphaviruses are enveloped pathogens that cause arthritis and encephalitis. Here, we report a 4.4-Å cryoelectron microscopy (cryo-EM) structure of eastern equine encephalitis virus (EEEV), an alphavirus that causes fatal encephalitis in humans. Our analysis provides insights into viral entry into host cells. The envelope protein E2 showed a binding site for the cellular attachment factor heparan sulfate. The presence of a cryptic E2 glycan suggests how EEEV escapes surveillance by lectin-expressing myeloid lineage cells, which are sentinels of the immune system. A mechanism for nucleocapsid core release and disassembly upon viral entry was inferred based on pH changes and capsid dissociation from envelope proteins. The EEEV capsid structure showed a viral RNA genome binding site adjacent to a ribosome binding site for viral genome translation following genome release. Using five Fab-EEEV complexes derived from neutralizing antibodies, our investigation provides insights into EEEV host cell interactions and protective epitopes relevant to vaccine design.

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EEEV cryo-EM structure shows the basis of receptor binding and pH-triggered disassembly

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Cryptic envelope protein glycosylation interferes with immune detection

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EEEV RNA genome binding site on capsid protein has an extended conformation

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Antibody inhibition of EEEV entry involves cross-linking of viral envelope proteins

Proteomics Computational Analyses Suggest that the Antennavirus Glycoprotein Complex Includes a Class I Viral Fusion Protein (α-Penetrene) with an Internal Zinc-Binding Domain and a Stable Signal Peptide

A metatranscriptomic study of RNA viruses in cold-blooded vertebrates identified two related viruses from frogfish (Antennarius striatus) that represent a new genus Antennavirus in the family Arenaviridae (Order: Bunyavirales). Computational analyses were used to identify features common to class I viral fusion proteins (VFPs) in antennavirus glycoproteins, including an N-terminal fusion peptide, two extended alpha-helices, an intrahelical loop, and a carboxyl terminal transmembrane domain. Like mammarenavirus and hartmanivirus glycoproteins, the antennavirus glycoproteins have an intracellular zinc-binding domain and a long virion-associated stable signal peptide (SSP). The glycoproteins of reptarenaviruses are also class I VFPs, but do not contain zinc-binding domains nor do they encode SSPs. Divergent evolution from a common progenitor potentially explains similarities of antennavirus, mammarenavirus, and hartmanivirus glycoproteins, with an ancient recombination event resulting in a divergent reptarenavirus glycoprotein.

Conformational changes in Chikungunya virus E2 protein upon heparan sulfate receptor binding explain mechanism of E2-E1 dissociation during viral entry

Receptor binding is the first step in viral cell entry. In enveloped virus cell entry, viral and host membrane fusion follows receptor binding. Viral surface receptor-binding protein associates with membrane fusion protein and masks its structure, to prevent pre-mature fusion activity. Dissociation of receptor-binding protein from fusion protein is an essential step before membrane fusion. Mechanism of receptor binding leading to dissociation of receptor binding and fusion protein is poorly understood in alphaviruses. Chikungunya virus (CHIKV), an alphavirus, re-emerged as a global pathogen in recent past. CHIKV surface envelope proteins, E2 and E1, function as receptor binding and fusion protein, respectively. Site of heparan sulfate (HS) receptor binding on E2–E1 heterodimer and its effect on E2–E1 heterodimer conformation is not known. Using molecular docking, we mapped HS binding to a positively charged pocket on E2 that is structurally conserved in alphaviruses. Based on our results from docking and sequence analysis, we identified a novel HS-binding sequence motif in E2. Purified E2 binds to heparin and HS specifically through charge interactions. Binding affinity of E2 to HS is comparable with other known HS–protein interactions (K_d ∼ 1.8 μM). Mutation of charged residues in the predicted HS-binding motif of E2 to alanine resulted in reduction of HS binding. Molecular dynamics (MD) simulations on E2, after docking HS, predicted allosteric domain movements. Fluorescence spectroscopy, far-UV circular dichroism spectroscopy, fluorescence resonance energy transfer experiments on HS-bound E2 corroborate our findings from MD simulations. We propose a mechanism where receptor-binding results in allosteric domain movements in E2, explaining E2–E1 dissociation.

Vaccinia viral A26 protein is a fusion suppressor of mature virus and triggers membrane fusion through conformational change at low pH

Vaccinia mature virus requires A26 envelope protein to mediate acid-dependent endocytosis into HeLa cells in which we hypothesized that A26 protein functions as an acid-sensitive membrane fusion suppressor. Here, we provide evidence showing that N-terminal domain (aa1-75) of A26 protein is an acid-sensitive region that regulates membrane fusion. Crystal structure of A26 protein revealed that His48 and His53 are in close contact with Lys47, Arg57, His314 and Arg312, suggesting that at low pH these His-cation pairs could initiate conformational changes through protonation of His48 and His53 and subsequent electrostatic repulsion. All the A26 mutant mature viruses that interrupted His-cation pair interactions of His48 and His 53 indeed have lost virion infectivity. Isolation of revertant viruses revealed that second site mutations caused frame shifts and premature termination of A26 protein such that reverent viruses regained cell entry through plasma membrane fusion. Together, we conclude that viral A26 protein functions as an acid-sensitive fusion suppressor during vaccinia mature virus endocytosis.

Vaccinia virus is a complex large DNA virus with a large number of viral membrane proteins to facilitate cell entry. Although it is well established that vaccinia mature virus uses endocytosis to enter cells, it remains unclear how it triggers membrane fusion in the acidic environment of endosomes. Recently, we hypothesized that A26 protein in vaccinia mature virus functions as an acid-sensitive membrane fusion suppressor, which suggests a novel viral regulation not present in other enveloped viruses. We postulated that conformational changes of A26 protein at low pH result in de-repression of viral fusion complex activity to trigger viral and endosomal membrane fusion. Here, we provide structural, biochemical and biological evidence demonstrating that vaccinia A26 protein does indeed function as an acid-sensitive fusion suppressor protein to regulate vaccinia mature virus membrane fusion during endocytosis. Our data reveal an important and unique “checkpoint” for vaccinia mature virus endocytosis that has not been described for other viruses. Furthermore, by isolating adaptive vaccinia mutants that escaped endocytic blockage, we discovered that mutations within the A26L gene serve as an effective strategy for switching the viral infection route from endocytosis to plasma membrane fusion, expanding viral host range.

Molecular organization and dynamics of the fusion protein Gc at the hantavirus surface

The hantavirus envelope glycoproteins Gn and Gc mediate virion assembly and cell entry, with Gc driving fusion of viral and endosomal membranes. Although the X-ray structures and overall arrangement of Gn and Gc on the hantavirus spikes are known, their detailed interactions are not. Here we show that the lateral contacts between spikes are mediated by the same 2-fold contacts observed in Gc crystals at neutral pH, allowing the engineering of disulfide bonds to cross-link spikes. Disrupting the observed dimer interface affects particle assembly and overall spike stability. We further show that the spikes display a temperature-dependent dynamic behavior at neutral pH, alternating between ‘open’ and ‘closed’ forms. We show that the open form exposes the Gc fusion loops but is off-pathway for productive Gc-induced membrane fusion and cell entry. These data also provide crucial new insights for the design of optimized Gn/Gc immunogens to elicit protective immune responses.

Hantaviruses infect rodents and other small mammals, but do not harm them. When transmitted to humans, often through rodent urine, feces or saliva, they can cause serious and even fatal diseases. Currently, there are no known methods that effectively prevent hantavirus infections or treat the diseases that they cause.

During an infection, viruses invade the cells of their host. A hantavirus interacts with target cells through proteins on its surface called Gn and Gc glycoproteins. Previous work has shown that these glycoproteins are organized in bundles of four Gn and four Gc proteins, termed spikes, which project from the membrane that surrounds the virus. The Gc protein changes shape when it is activated and exposes a hidden region that can insert into the membrane of the target cell. The Gc proteins then change shape again to force the cell to fuse with the viral membrane. This process allows the virus to be taken up into the cell, where it can replicate.

While the structures of each viral glycoprotein have been determined in isolation, it was not known how they interact within the Gn/Gc spike. Such information is crucial to understand how the viruses infect cells and which areas are exposed to the immune system of the host – and so could be targeted by antiviral treatments.

Bignon et al. have now identified the molecular contacts that occur between spikes and interconnect them into a grid-like lattice on the surface of the virus. Genetically altering specific sections of the Gc glycoprotein strengthened or weakened these contacts, which correspondingly increased or decreased how stable the spike was. Preventing the contacts from forming resulted in cells releasing fewer virus-like particles.

Bignon et al. also show that at the body temperature of mammals, the shape of the spike fluctuates between an ‘open’ form that exposes the region of Gc that inserts into the cell membrane, and a ‘closed’ form that hides this region. However, when Gc is activated, the open form becomes unable to cause the viral and cell membranes to fuse together.

Together, the results presented by Bignon et al. help us to understand how changes to the hantavirus surface enable the virus to infect cells. This knowledge will help researchers to design vaccines that protect against hantavirus infections.

Molecular Basis of Arthritogenic Alphavirus Receptor MXRA8 Binding to Chikungunya Virus Envelope Protein

Arthritogenic alphaviruses, such as Chikungunya virus (CHIKV), cause severe and debilitating rheumatic diseases worldwide, resulting in severe morbidity and economic costs. Recently, MXRA8 was reported as an entry receptor. Here, we present the crystal structures of the mouse MXRA8, human MXRA8 in complex with the CHIKV E protein, and the cryo-electron microscopy structure of human MXRA8 and CHIKV virus-like particle. MXRA8 has two Ig-like domains with unique structural topologies. This receptor binds in the "canyon" between two protomers of the E spike on the surface of the virion. The atomic details at the interface between the two binding entities reveal that both the two domains and the hinge region of MXRA8 are involved in interaction with CHIKV E1-E2 residues from two protomers. Notably, the stalk region of MXRA8 is critical for CHIKV virus entry. This finding provides important information regarding the development of therapeutic countermeasures against those arthritogenic alphaviruses.

Cryo-EM Structure of Chikungunya Virus in Complex with the Mxra8 Receptor

Mxra8 is a receptor for multiple arthritogenic alphaviruses that cause debilitating acute and chronic musculoskeletal disease in humans. Herein, we present a 2.2 Å resolution X-ray crystal structure of Mxra8 and 4 to 5 Å resolution cryo-electron microscopy reconstructions of Mxra8 bound to chikungunya (CHIKV) virus-like particles and infectious virus. The Mxra8 ectodomain contains two strand-swapped Ig-like domains oriented in a unique disulfide-linked head-to-head arrangement. Mxra8 binds by wedging into a cleft created by two adjacent CHIKV E2-E1 heterodimers in one trimeric spike and engaging a neighboring spike. Two binding modes are observed with the fully mature VLP, with one Mxra8 binding with unique contacts. Only the high-affinity binding mode was observed in the complex with infectious CHIKV, as viral maturation and E3 occupancy appear to influence receptor binding-site usage. Our studies provide insight into how Mxra8 binds CHIKV and creates a path for developing alphavirus entry inhibitors.

Assessment of Immunogenicity and Neutralisation Efficacy of Viral-Vectored Vaccines Against Chikungunya Virus

Chikungunya virus (CHIKV) has caused extensive outbreaks in several countries within the Americas, Asia, Oceanic/Pacific Islands, and Europe. In humans, CHIKV infections cause a debilitating disease with acute febrile illness and long-term polyarthralgia. Acute and chronic symptoms impose a major economic burden to health systems and contribute to poverty in affected countries. An efficacious vaccine would be an important step towards decreasing the disease burden caused by CHIKV infection. Despite no licensed vaccine is yet available for CHIKV, there is strong evidence of effective asymptomatic viral clearance due to neutralising antibodies against the viral structural proteins. We have designed viral-vectored vaccines to express the structural proteins of CHIKV, using the replication-deficient chimpanzee adenoviral platform, ChAdOx1. Expression of the CHIKV antigens results in the formation of chikungunya virus-like particles. Our vaccines induce high frequencies of anti-chikungunya specific T-cell responses as well as high titres of anti-CHIKV E2 antibodies with high capacity for in vitro neutralisation. Our results indicate the potential for further clinical development of the ChAdOx1 vaccine platform in CHIKV vaccinology.

Antiviral Functions of Monoclonal Antibodies against Chikungunya Virus

Chikungunya virus (CHIKV) is the most common alphavirus infecting humans worldwide. Antibodies play pivotal roles in the immune response to infection. Increasingly, therapeutic antibodies are becoming important for protection from pathogen infection for which neither vaccine nor treatment is available, such as CHIKV infection. The new generation of ultra-potent and/or broadly cross-reactive monoclonal antibodies (mAbs) provides new opportunities for intervention. In the past decade, several potent human and mouse anti-CHIKV mAbs were isolated and demonstrated to be protective in vivo. Mechanistic studies of these mAbs suggest that mAbs exert multiple modes of action cooperatively. Better understanding of these antiviral mechanisms for mAbs will help to optimize mAb therapies.

Function of the HCV E1 envelope glycoprotein in viral entry and assembly

HCV envelope glycoproteins, E1 and E2, are multifunctional proteins. Until recently, E2 glycoprotein was thought to be the fusion protein and was the focus of investigations. However, the recently obtained partial structures of E2 and E1 rather support a role for E1 alone or in association with E2 in HCV fusion. Moreover, they suggest that HCV harbors a new fusion mechanism, distinct from that of other members of the Flaviviridae family. In this context, E1 aroused a renewed interest. Recent functional characterizations of E1 revealed a more important role than previously thought in entry and assembly. Thus, E1 is involved in the viral genome encapsidation step and influences the association of the virus with lipoprotein components. Moreover, E1 modulates HCV–receptor interaction and participates in a late entry step potentially fusion. In this review, we outline our current knowledge on E1 functions in HCV assembly and entry.

Third Tofo Advanced Study Week on Emerging and Re-emerging Viruses, 2018

The Third Tofo Advanced Study Week on Emerging and Re-Emerging Viruses (3rd TASW) was held in Praia do Tofo, Mozambique, from September 02 to 06, 2018. It brought together 55 participants from 10 African countries as well as from Belgium, China, Germany, Singapore, and the USA. Meeting sessions covered aspects of the epidemiology, diagnosis, molecular and structural biology, vaccine development, and antiviral drug discovery for emerging RNA viruses that are current threats in Africa and included flaviviruses (dengue and Zika), alphaviruses (chikungunya), coronaviruses, filoviruses (Ebola), influenza viruses, Crimean Congo hemorrhagic fever virus, Rift Valley fever Virus, Lassa virus, and others. Data were presented on recent flavivirus and/or chikungunyavirus outbreaks in Angola, Burkina Faso, and Mozambique. In addition, these viruses are endemic in many sub-Saharan countries. The TASW series on emerging viruses is unique in Africa and successful in promoting collaborations between researchers in Africa and other parts of the world, as well as among African scientists. This report summarizes the lectures held at the meeting and highlights advances in the field.

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The 3rd Tofo Advanced Study Week on Emerging and Re-emerging Viruses took place from September 2–6, 2018.

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African attendees came from Angola, Botswana, Burkina Faso, the CAR, Mozambique, Nigeria, S Africa, Tanzania and Zimbabwe.

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Other participants were from Europe, China, Singapore, and the USA.

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This unique meeting enabled scientists from Africa and elsewhere to discuss problems and initiate new collaborations.

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Presentations covered dengue virus, Zika, chikungunya, coronaviruses, Ebola, influenza, Rift Valley fever, CCHF, and RSV.

Implication for alphavirus host-cell entry and assembly indicated by a 3.5Å resolution cryo-EM structure

Alphaviruses are enveloped RNA viruses that contain several human pathogens. Due to intrinsic heterogeneity of alphavirus particles, a high resolution structure of the virion is currently lacking. Here we provide a 3.5 Å cryo-EM structure of Sindbis virus, using block based reconstruction method that overcomes the heterogeneity problem. Our structural analysis identifies a number of conserved residues that play pivotal roles in the virus life cycle. We identify a hydrophobic pocket in the subdomain D of E2 protein that is stabilized by an unknown pocket factor near the viral membrane. Residues in the pocket are conserved in different alphaviruses. The pocket strengthens the interactions of the E1/E2 heterodimer and may facilitate virus assembly. Our study provides structural insights into alphaviruses that may inform the design of drugs and vaccines.

Evolutionary diversification of the HAP2 membrane insertion motifs to drive gamete fusion across eukaryotes

HAPLESS2 (HAP2) is a broadly conserved, gamete-expressed transmembrane protein that was shown recently to be structurally homologous to viral class II fusion proteins, which initiate fusion with host cells via insertion of fusion loops into the host membrane. However, the functional conformation of the HAP2 fusion loops has remained unknown, as the reported X-ray structure of Chlamydomonas reinhardtii HAP2 lacked this critical region. Here, we report a structure-guided alignment that reveals diversification of the proposed HAP2 fusion loops. Representative crystal structures show that in flowering plants, HAP2 has a single prominent fusion loop projecting an amphipathic helix at its apex, while in trypanosomes, three small nonpolar loops of HAP2 are poised to interact with the target membrane. A detailed structure-function analysis of the Arabidopsis HAP2 amphipathic fusion helix defines key residues that are essential for membrane insertion and for gamete fusion. Our study suggests that HAP2 may have evolved multiple modes of membrane insertion to accommodate the diversity of membrane environments it has encountered during eukaryotic evolution.

The fusion of gamete plasma membranes is the fundamental cellular event that brings two parental cells together to form a new individual, yet we know surprisingly little about this process at the molecular level. HAPLESS 2 (HAP2) is a conserved sperm plasma membrane protein that is essential for gamete fusion in a diverse array of eukaryotes. It was recently shown to share a common ancestor with viral proteins that drive fusion of the viral envelope with host membranes, but its mechanism of action remained elusive, since the reported structure did not resolve the proposed membrane interaction surface. Here, we report two new HAP2 structures revealing that HAP2 has evolved diverse membrane interaction surfaces. In the flowering plants, HAP2 uses an amphipathic helix that presents nonpolar residues to the target membrane; in trypanosomes, the membrane interaction surface comprises three shallow nonpolar loops.

Role of the vacuolar ATPase in the Alphavirus replication cycle

We have shown that Alphaviruses can enter cells by direct penetration at the plasma membrane (R. Vancini, G. Wang, D. Ferreira, R. Hernandez, and D. Brown, J Virol, 87:4352–4359, 2013). Direct penetration removes the requirement for receptor-mediated endocytosis exposure to low pH and membrane fusion in the process of RNA entry. Endosomal pH as well as the pH of the cell cytoplasm is maintained by the activity of the vacuolar ATPase (V-ATPase). Bafilomycin is a specific inhibitor of V-ATPase. To characterize the roll of the V-ATPase in viral replication we generated a Bafilomycin A1(BAF) resistant mutant of Sindbis virus (BRSV). BRSV produced mature virus and virus RNA in greater amounts than parent virus in BAF-treated cells. Sequence analysis revealed mutations in the E2 glycoprotein, T15I/Y18H, were responsible for the phenotype. These results show that a functional V-ATPase is required for efficient virus RNA synthesis and virus maturation in Alphavirus infection.

An attenuated replication-competent chikungunya virus with a fluorescently tagged envelope

BACKGROUND

Chikungunya virus (CHIKV) is the most common alphavirus infecting humans worldwide, causing acute and chronically debilitating arthralgia at a great economic expense.

METHODOLOGY/PRINCIPAL FINDINGS

To facilitate our study of CHIKV, we generated a mCherry tagged replication-competent chimeric virus, CHIKV 37997-mCherry. Single particle cryoEM demonstrated icosahedral organization of the chimeric virus and the display of mCherry proteins on virus surface. CHIKV 37997-mCherry is attenuated in both IFNαR knockout and wild-type mice. Strong anti-CHIKV and anti-mCherry antibody responses were induced in CHIKV 37997-mCherry infected mice.

CONCLUSIONS/SIGNIFICANCE

Our work suggests that chimeric alphaviruses displaying foreign antigen can serve as vaccines against both aphaviruses and other pathogens and diseases.

A compendium of small molecule direct-acting and host-targeting inhibitors as therapies against alphaviruses

Alphaviruses were amongst the first arboviruses to be isolated, characterized and assigned a taxonomic status. They are globally widespread, infecting a large variety of terrestrial animals, birds, insects and even fish. Moreover, they are capable of surviving and circulating in both sylvatic and urban environments, causing considerable human morbidity and mortality. The re-emergence of Chikungunya virus (CHIKV) in almost every part of the world has caused alarm to many health agencies throughout the world. The mosquito vector for this virus, Aedes, is globally distributed in tropical and temperate regions and capable of thriving in both rural and urban landscapes, giving the opportunity for CHIKV to continue expanding into new geographical regions. Despite the importance of alphaviruses as human pathogens, there is currently no targeted antiviral treatment available for alphavirus infection. This mini-review discusses some of the major features in the replication cycle of alphaviruses, highlighting the key viral targets and host components that participate in alphavirus replication and the molecular functions that were used in drug design. Together with describing the importance of these targets, we review the various direct-acting and host-targeting inhibitors, specifically small molecules that have been discovered and developed as potential therapeutics as well as their reported in vitro and in vivo efficacies.

Monomeric Intermediates Formed by Vesiculovirus Glycoprotein during Its Low-pH-induced Structural Transition

•Vesiculovirus G is the prototype of class III viral fusion glycoproteins. •The structures of both G pre- and post-fusion conformation have been determined. •The structure of monomeric intermediates reveals the pathway of the transition. •A fusion-loop-exposing antiparallel dimer may initiate the fusion process. •Those data challenge the current model proposed for viral membrane fusion.

Potassium is a trigger for conformational change in the fusion spike of an enveloped RNA virus

Many enveloped viruses enter cells through the endocytic network, from which they must subsequently escape through fusion of viral and endosomal membranes. This membrane fusion is mediated by virus-encoded spikes that respond to the dynamic endosomal environment, which triggers conformational changes in the spikes that initiate the fusion process. Several fusion triggers have been identified and include pH, membrane composition, and endosome-resident proteins, and these cues dictate when and where viral fusion occurs. We recently reported that infection with an enveloped bunyavirus requires elevated potassium ion concentrations [K⁺], controlled by cellular K⁺ channels, that are encountered during viral transit through maturing endosomes. Here we reveal the molecular basis for the K⁺ requirement of bunyaviruses through the first direct visualization of a member of the Nairoviridae family, namely Hazara virus (HAZV), using cryo-EM. Using cryo-electron tomography, we observed HAZV spike glycoproteins within infectious HAZV particles exposed to both high and low [K⁺], which showed that exposure to K⁺ alone results in dramatic changes to the ultrastructural architecture of the virion surface. In low [K⁺], the spikes adopted a compact conformation arranged in locally ordered arrays, whereas, following exposure to high [K⁺], the spikes became extended, and spike–membrane interactions were observed. Viruses exposed to high [K⁺] also displayed enhanced infectivity, thus identifying K⁺ as a newly defined trigger that helps promote viral infection. Finally, we confirmed that K⁺ channel blockers are inhibitory to HAZV infection, highlighting the potential of K⁺ channels as anti-bunyavirus targets.

A glycerophospholipid-specific pocket in the RVFV class II fusion protein drives target membrane insertion

The Rift Valley fever virus (RVFV) is transmitted by infected mosquitoes, causing severe disease in humans and livestock across Africa. We determined the x-ray structure of the RVFV class II fusion protein Gc in its postfusion form and in complex with a glycerophospholipid (GPL) bound in a conserved cavity next to the fusion loop. Site-directed mutagenesis and molecular dynamics simulations further revealed a built-in motif allowing en bloc insertion of the fusion loop into membranes, making few nonpolar side-chain interactions with the aliphatic moiety and multiple polar interactions with lipid head groups upon membrane restructuring. The GPL head-group recognition pocket is conserved in the fusion proteins of other arthropod-borne viruses, such as Zika and chikungunya viruses, which have recently caused major epidemics worldwide.

The Alphavirus Exit Pathway: What We Know and What We Wish We Knew

Alphaviruses are enveloped positive sense RNA viruses and include serious human pathogens, such as the encephalitic alphaviruses and Chikungunya virus. Alphaviruses are transmitted to humans primarily by mosquito vectors and include species that are classified as emerging pathogens. Alphaviruses assemble highly organized, spherical particles that bud from the plasma membrane. In this review, we discuss what is known about the alphavirus exit pathway during a cellular infection. We describe the viral protein interactions that are critical for virus assembly/budding and the host factors that are involved, and we highlight the recent discovery of cell-to-cell transmission of alphavirus particles via intercellular extensions. Lastly, we discuss outstanding questions in the alphavirus exit pathway that may provide important avenues for future research.

Nonstructural Proteins of Alphavirus-Potential Targets for Drug Development

Alphaviruses are enveloped, positive single-stranded RNA viruses, typically transmitted by arthropods. They often cause arthralgia or encephalitic diseases in infected humans and there is currently no targeted antiviral treatment available. The re-emergence of alphaviruses in Asia, Europe, and the Americas over the last decade, including chikungunya and o'nyong'nyong viruses, have intensified the search for selective inhibitors. In this review, we highlight key molecular determinants within the alphavirus replication complex that have been identified as viral targets, focusing on their structure and functionality in viral dissemination. We also summarize recent structural data of these viral targets and discuss how these could serve as templates to facilitate structure-based drug design and development of small molecule inhibitors.

Shielding and activation of a viral membrane fusion protein

Entry of enveloped viruses relies on insertion of hydrophobic residues of the viral fusion protein into the host cell membrane. However, the intermediate conformations during fusion remain unknown. Here, we address the fusion mechanism of Rift Valley fever virus. We determine the crystal structure of the Gn glycoprotein and fit it with the Gc fusion protein into cryo-electron microscopy reconstructions of the virion. Our analysis reveals how the Gn shields the hydrophobic fusion loops of the Gc, preventing premature fusion. Electron cryotomography of virions interacting with membranes under acidic conditions reveals how the fusogenic Gc is activated upon removal of the Gn shield. Repositioning of the Gn allows extension of Gc and insertion of fusion loops in the outer leaflet of the target membrane. These data show early structural transitions that enveloped viruses undergo during host cell entry and indicate that analogous shielding mechanisms are utilized across diverse virus families.

Viral fusion proteins undergo extensive conformational changes during entry but intermediate conformations often remain unknown. Here, the authors show how Gn of Rift Valley fever virus fusion protein shields hydrophobic fusion loops of Gc and how these loops embed in the target membrane at acidic conditions.

Structural biology of Zika virus and other flaviviruses

Zika virus (ZIKV) is an enveloped, icosahedral flavivirus that has structural and functional similarities to other human flavivirus pathogens such as dengue (DENV), West Nile (WNV) and Japanese encephalitis (JEV) viruses. ZIKV infections have been linked to fetal microcephaly and the paralytic Guillain-Barré syndrome. This review provides a comparative structural analysis of the assembly, maturation and host-cell entry of ZIKV with other flaviviruses, especially DENV. We also discuss the mechanisms of neutralization by antibodies.

Detection, isolation, and characterization of chikungunya viruses associated with the Pakistan outbreak of 2016-2017

The chikungunya virus (CHIKV) is a mosquito-transmitted alphavirus, which has infected millions of people in Africa, Asia, Americas, and Europe since it reemerged in India and Indian Ocean regions in 2005-2006. Starting in the middle of November 2016, CHIKV has been widely spread, and more than 4,000 cases of infections in humans were confirmed in Pakistan. Here, we report the first isolation and characterization of CHIKV from the Pakistan outbreak. Eight CHIKV strains were newly isolated from human serum samples using a cell culture procedure. A full-length genome sequence and eight complete envelope (E1) sequences of CHIKV from Pakistan were obtained in this study. Alignment of the CHIKV E1 sequences revealed that the eight new CHIKV isolates were highly homogeneous, with only two nonsynonymous substitutions found at generally conserved sites (E99 and Q235). Based on the comparison of 342 E1 sequences, the two nonsynonymous mutations were located in well-recognized domains associated with viral functions such as the cell fusion and vector specificity, suggesting their potential functional importance. Phylogenetic analysis indicated that the CHIKV strains from Pakistan originated from CHIKV circulating in the Indian region. This study helps elucidate the epidemics of CHIKV in Pakistan and also provides a foundation for studies of evolution and expansion of CHIKV in South Asia.

Both Sphingomyelin and Cholesterol in the Host Cell Membrane Are Essential for Rubella Virus Entry

Rubella virus (RuV) causes a systemic infection, and transplacental fetal infection causes congenital rubella syndrome. In this study, we showed that treatment of cells with sphingomyelinase inhibited RuV infection. Assays using inhibitors of serine palmitoyl transferase and ceramide transport protein demonstrated the contribution of sphingomyelin (SM) to RuV infection. Compelling evidence for direct binding of RuV to lipid membranes at neutral pH was obtained using liposome coflotation assays. The absence of either SM or cholesterol (Chol) abrogated the RuV-liposome interaction. SM and Chol (SM/Chol) were also critical for RuV binding to erythrocytes and lymphoid cells. Removal of Ca²⁺ from the assay buffer or mutation of RuV envelope E1 protein Ca²⁺-binding sites abrogated RuV binding to liposomes, erythrocytes, and lymphoid cells. However, RuV bound to various nonlymphoid adherent cell lines independently of extracellular Ca²⁺ or SM/Chol. Even in these adherent cell lines, both the E1 protein Ca²⁺-binding sites and cellular SM/Chol were essential for the early stage of RuV infection, possibly affecting envelope-membrane fusion in acidic compartments. Myelin oligodendrocyte glycoprotein (MOG) has recently been identified as a cellular receptor for RuV. However, RuV bound to MOG-negative cells in a Ca²⁺-independent manner. Collectively, our data demonstrate that RuV has two distinct binding mechanisms: one is Ca²⁺ dependent and the other is Ca²⁺ independent. Ca²⁺-dependent binding observed in lymphoid cells occurs by the direct interaction between E1 protein fusion loops and SM/Chol-enriched membranes. Clarification of the mechanism of Ca²⁺-independent RuV binding is an important next step in understanding the pathology of RuV infection.IMPORTANCE Rubella has a significant impact on public health as infection during early pregnancy can result in babies being born with congenital rubella syndrome. Even though effective rubella vaccines are available, rubella outbreaks still occur in many countries. We studied the entry mechanism of rubella virus (RuV) and found that RuV binds directly to the host plasma membrane in the presence of Ca²⁺ at neutral pH. This Ca²⁺-dependent binding is specifically directed to membranes enriched in sphingomyelin and cholesterol and is critical for RuV infection. Importantly, RuV also binds to many cell lines in a Ca²⁺-independent manner. An unidentified RuV receptor(s) is involved in this Ca²⁺-independent binding. We believe that the data presented here may aid the development of the first anti-RuV drug.

Structural studies of Chikungunya virus maturation

Cleavage of the alphavirus precursor glycoprotein p62 into the E2 and E3 glycoproteins before assembly with the nucleocapsid is the key to producing fusion-competent mature spikes on alphaviruses. Here we present a cryo-EM, 6.8-Å resolution structure of an "immature" Chikungunya virus in which the cleavage site has been mutated to inhibit proteolysis. The spikes in the immature virus have a larger radius and are less compact than in the mature virus. Furthermore, domains B on the E2 glycoproteins have less freedom of movement in the immature virus, keeping the fusion loops protected under domain B. In addition, the nucleocapsid of the immature virus is more compact than in the mature virus, protecting a conserved ribosome-binding site in the capsid protein from exposure. These differences suggest that the posttranslational processing of the spikes and nucleocapsid is necessary to produce infectious virus.

Current Progress of Virus-mimicking Nanocarriers for Drug Delivery

Nanomedicines often involve the use of nanocarriers as a delivery system for drugs or genes for maximizing the therapeutic effect and/or minimizing the adverse effect. From drug administration to therapeutic activity, nanocarriers must evade the host's immune system, specifically and efficiently target and enter the cell, and release their payload into the cell cytoplasm by endosomal escape. These processes constitute the early infection stage of viruses. Viruses are a powerful natural nanomaterial for the efficient delivery of genetic information by sophisticated mechanisms. Over the past two decades, many virus-inspired nanocarriers have been generated to permit successful drug and gene delivery. In this review, we summarize the early infection machineries of viruses, of which the part has so far been utilized for delivery systems. Furthermore, we describe basics and applications of the bio-nanocapsule, which is a hepatitis B virus-mimicking nanoparticle harboring nearly all activities involved in the early infection machineries (i.e., stealth activity, targeting activity, cell entry activity, endosomal escaping activity).

The Bridges and Blockades to Evolutionary Convergence on the Road to Predicting Chikungunya Virus Evolution

Chikungunya virus, first isolated in the 1950s, has since reemerged to cause several epidemics and millions of infections throughout the world. What was once blurred and confused with dengue virus in both diagnosis and name has since become one of the best-characterized arboviral diseases. In this review, we cover the history of this virus, its evolution into distinct genotypes and lineages, and, most notably, the convergent evolution observed in recent years. We highlight research that reveals to what extent convergent evolution, and its inherent predictability, may occur and what genetic or environmental factors may hinder it.

Synthesis of p-methoxyphenyl sulfated β-GalNAc derivatives with inhibitory activity against Japanese encephalitis virus

The N-acetylgalactosamine (GalNAc) residue is one of the units of chondroitin sulfate E (CS-E) which has been reported to have inhibitory activity against Japanese encephalitis virus (JEV). Herein, we describe the synthesis of a series of p-methoxyphenyl β-GalNAc derivatives with a sulfate group at 3-, 4-, and/or 6-positions using an efficient route through a common synthetic intermediate. By measuring the inhibition activity of these compounds that bear different numbers and positions of sulfate groups, the effect of position specificity for interaction with the virus was determined. From these results, GalNAc6S and GalNAc4S6S derivatives inhibited JEV infections well; we suggest the 6-O-sulfate group is necessary for selective recognition by the virus.

Characterization and Role of Lentivirus-Associated Host Proteins

Enveloped viruses obtain their envelopes during the process of budding from infected cells. During this process, however, these viruses acquire parts of the host cell membranes and host cell-derived proteins as integral parts of their mature envelopes. These host-derived components of viral envelopes may subsequently exhibit various effects on the life cycle of the virus; virus cell interactions, especially host response to virus-incorporated self-proteins; and the pathogenesis of the disease induced by these viruses. Although it was known for some time that various viruses incorporate host cell-derived proteins, the issue of the role of these proteins has received increased attention, specifically in connection with human immunodeficiency virus (HIV) infection and development of acquired immunodeficiency syndrome (AIDS) in humans. The aim of this review is to summarize our current knowledge of the analysis and role of host-derived proteins associated with enveloped viruses, with emphasis on the potential role of these proteins in the pathogenesis of AIDS. Clearly, differences in the clinical outcome of those nonhuman primates infected with simian immunodeficiency virus (SIV) that are disease resistant compared with SIV-infected species that are disease susceptible provide a unique opportunity to determine whether differences in the incorporation of distinct sets of host proteins play a role with distinct clinical outcomes.

Alteration of a Second Putative Fusion Peptide of Structural Glycoprotein E2 of Classical Swine Fever Virus Alters Virus Replication and Virulence in Swine

E2, the major envelope glycoprotein of classical swine fever virus (CSFV), is involved in several critical virus functions, including cell attachment, host range susceptibility, and virulence in natural hosts. Functional structural analysis of E2 based on a Wimley-White interfacial hydrophobicity distribution predicted the involvement of a loop (residues 864 to 881) stabilized by a disulfide bond (⁸⁶⁹CKWGGNWTCV⁸⁷⁸, named FPII) in establishing interactions with the host cell membrane. This loop further contains an ⁸⁷²GG⁸⁷³ dipeptide, as well as two aromatic residues (⁸⁷¹W and ⁸⁷⁵W) accessible to solvent. Reverse genetics utilizing a full-length infectious clone of the highly virulent CSFV strain Brescia (BICv) was used to evaluate how amino acid substitutions within FPII may affect replication of BICv in vitro and virus virulence in swine. Recombinant CSFVs containing mutations in different residues of FPII were constructed. A particular construct, harboring amino acid substitutions W871T, W875D, and V878T (FPII.2), demonstrated a significantly decreased ability to replicate in a swine cell line (SK6) and swine macrophage primary cell cultures. Interestingly, mutated virus FPII.2 was completely attenuated in pigs. Also, animals infected with FPII.2 virus were protected against virulent challenge with Brescia virus at 21 days postvaccination. Supporting a role for the E2 the loop from residues 864 to 881 in membrane fusion, only synthetic peptides that were based on the native E2 functional sequence were competent for insertion into model membranes and perturbation of their integrity, and this functionality was lost in synthetic peptides harboring amino acid substitutions W871T, W875D, and V878T in FPII.2.

IMPORTANCE

This report describes the identification and characterization of a putative fusion peptide (FP) in the major structural protein E2 of classical swine fever virus (CSFV). The FP identification was performed by functional structural analysis of E2. We characterized the functional significance of this FP by using artificial membranes. Replacement of critical amino acid residues within the FP radically alters how it interacts with the artificial membranes. When we introduced the same mutations into the viral sequence, there was a reduction in replication in cell cultures, and when we infected domestic swine, the natural host of CSFV host, we observed that the virus was now completely attenuated in swine. In addition, the virus mutant that was attenuated in vivo efficiently protected pigs against wild-type virus. These results provide the proof of principle to support as a strategy for vaccine development the discovery and manipulation of FPs.

Crystal Structure of Glycoprotein C from a Hantavirus in the Post-fusion Conformation

Hantaviruses are important emerging human pathogens and are the causative agents of serious diseases in humans with high mortality rates. Like other members in the Bunyaviridae family their M segment encodes two glycoproteins, G_N and G_C, which are responsible for the early events of infection. Hantaviruses deliver their tripartite genome into the cytoplasm by fusion of the viral and endosomal membranes in response to the reduced pH of the endosome. Unlike phleboviruses (e.g. Rift valley fever virus), that have an icosahedral glycoprotein envelope, hantaviruses display a pleomorphic virion morphology as G_N and G_C assemble into spikes with apparent four-fold symmetry organized in a grid-like pattern on the viral membrane. Here we present the crystal structure of glycoprotein C (G_C) from Puumala virus (PUUV), a representative member of the Hantavirus genus. The crystal structure shows G_C as the membrane fusion effector of PUUV and it presents a class II membrane fusion protein fold. Furthermore, G_C was crystallized in its post-fusion trimeric conformation that until now had been observed only in Flavi- and Togaviridae family members. The PUUV G_C structure together with our functional data provides intriguing evolutionary and mechanistic insights into class II membrane fusion proteins and reveals new targets for membrane fusion inhibitors against these important pathogens.

Hantaviruses (family: Bunyaviridae) encompass pathogens responsible to serious human diseases and economic burden worldwide. Following endocytosis, these enveloped RNA viruses are directed to an endosomal compartment where a sequence of pH-dependent conformational changes of the viral envelope glycoproteins mediates the fusion between the viral and endosomal membranes. The lack of high-resolution structural information for the entry of hantaviruses impair our ability to rationalize new treatments and prevention strategies. We determined the three-dimensional structure of a glycoprotein C from Puumala virus (PUUV) using X-ray crystallography. The two structures (at pH 6.0 and 8.0) were determined to 1.8 Å and 2.3 Å resolutions, respectively. Both structures reveal a class II membrane fusion protein in its post-fusion trimeric conformation with novel structural features in the trimer assembly and stabilization. Our structures suggest that neutralizing antibodies against G_C target its conformational changes as inhibition mechanism and highlight new molecular targets for hantavirus-specific membrane fusion inhibitors. Furthermore, combined with the available structures of other class II proteins, we remodeled the evolutionary relationships between virus families encompassing these proteins.

Mechanistic Insight into Bunyavirus-Induced Membrane Fusion from Structure-Function Analyses of the Hantavirus Envelope Glycoprotein Gc

Hantaviruses are zoonotic viruses transmitted to humans by persistently infected rodents, giving rise to serious outbreaks of hemorrhagic fever with renal syndrome (HFRS) or of hantavirus pulmonary syndrome (HPS), depending on the virus, which are associated with high case fatality rates. There is only limited knowledge about the organization of the viral particles and in particular, about the hantavirus membrane fusion glycoprotein Gc, the function of which is essential for virus entry. We describe here the X-ray structures of Gc from Hantaan virus, the type species hantavirus and responsible for HFRS, both in its neutral pH, monomeric pre-fusion conformation, and in its acidic pH, trimeric post-fusion form. The structures confirm the prediction that Gc is a class II fusion protein, containing the characteristic β-sheet rich domains termed I, II and III as initially identified in the fusion proteins of arboviruses such as alpha- and flaviviruses. The structures also show a number of features of Gc that are distinct from arbovirus class II proteins. In particular, hantavirus Gc inserts residues from three different loops into the target membrane to drive fusion, as confirmed functionally by structure-guided mutagenesis on the HPS-inducing Andes virus, instead of having a single "fusion loop". We further show that the membrane interacting region of Gc becomes structured only at acidic pH via a set of polar and electrostatic interactions. Furthermore, the structure reveals that hantavirus Gc has an additional N-terminal "tail" that is crucial in stabilizing the post-fusion trimer, accompanying the swapping of domain III in the quaternary arrangement of the trimer as compared to the standard class II fusion proteins. The mechanistic understandings derived from these data are likely to provide a unique handle for devising treatments against these human pathogens.

Capsid-deficient alphaviruses generate propagative infectious microvesicles at the plasma membrane

Alphavirus budding is driven by interactions between nucleocapsids assembled in the cytoplasm and envelope proteins present at the plasma membrane. So far, the expression of capsid and envelope proteins in infected cells has been considered an absolute requirement for alphavirus budding and propagation. In the present study, we show that Semliki Forest virus and Sindbis virus lacking the capsid gene can propagate in mammalian and insect cells. This propagation is mediated by the release of infectious microvesicles (iMVs), which are pleomorphic and have a larger size and density than wild-type virus. iMVs, which contain viral RNA inside and viral envelope proteins on their surface, are released at the plasma membrane and infect cells using the endocytic pathway in a similar way to wild-type virus. iMVs are not pathogenic in immunocompetent mice when injected intravenously, but can infect different organs like lungs and heart. Finally, we also show that alphavirus genomes without capsid can mediate the propagation of heterologous genes, making these vectors potentially interesting for gene therapy or vaccination studies. The minimalist infectious system described in this study shows that a self-replicating RNA able to express membrane proteins with binding and fusion properties is able to propagate, providing some insights into virus evolution.

The Interferon-Stimulated Gene IFITM3 Restricts Infection and Pathogenesis of Arthritogenic and Encephalitic Alphaviruses

Host cells respond to viral infections by producing type I interferon (IFN), which induces the expression of hundreds of interferon-stimulated genes (ISGs). Although ISGs mediate a protective state against many pathogens, the antiviral functions of the majority of these genes have not been identified. IFITM3 is a small transmembrane ISG that restricts a broad range of viruses, including orthomyxoviruses, flaviviruses, filoviruses, and coronaviruses. Here, we show that alphavirus infection is increased in Ifitm3(-/-) and Ifitm locus deletion (Ifitm-del) fibroblasts and, reciprocally, reduced in fibroblasts transcomplemented with Ifitm3. Mechanistic studies showed that Ifitm3 did not affect viral binding or entry but inhibited pH-dependent fusion. In a murine model of chikungunya virus arthritis, Ifitm3(-/-) mice sustained greater joint swelling in the ipsilateral ankle at days 3 and 7 postinfection, and this correlated with higher levels of proinflammatory cytokines and viral burden. Flow cytometric analysis suggested that Ifitm3(-/-) macrophages from the spleen were infected at greater levels than observed in wild-type (WT) mice, results that were supported by experiments with Ifitm3(-/-) bone marrow-derived macrophages. Ifitm3(-/-) mice also were more susceptible than WT mice to lethal alphavirus infection with Venezuelan equine encephalitis virus, and this was associated with greater viral burden in multiple organs. Collectively, our data define an antiviral role for Ifitm3 in restricting infection of multiple alphaviruses.

IMPORTANCE

The interferon-induced transmembrane protein 3 (IFITM3) inhibits infection of multiple families of viruses in cell culture. Compared to other viruses, much less is known about the antiviral effect of IFITM3 on alphaviruses. In this study, we characterized the antiviral activity of mouse Ifitm3 against arthritogenic and encephalitic alphaviruses using cells and animals with a targeted gene deletion of Ifitm3 as well as deficient cells transcomplemented with Ifitm3. Based on extensive virological analysis, we demonstrate greater levels of alphavirus infection and disease pathogenesis when Ifitm3 expression is absent. Our data establish an inhibitory role for Ifitm3 in controlling infection of alphaviruses.