Recent mechanistic and structural insights on class III viral fusion glycoproteins

Unveiling non-classical glycosylation patterns in Bombyx mori nucleopolyhedrovirus GP64: Insights into viral entry and fusion

The glycoprotein GP64 of alphabaculovirus is crucial for viral entry and fusion. Here, we investigated the N-glycosylation patterns of Bombyx mori nucleopolyhedrovirus (BmNPV) GP64 and its signal peptide (SP) cleaved form, SPΔnGP64, along with their impacts on viral infectivity and fusogenicity. Through deglycosylation assays, we confirmed N-glycosylation of BmNPV GP64 on multiple sites. Mutational analysis targeting predicted N-glycosylation sites revealed diverse effects on viral infectivity and cell fusion. Particularly noteworthy were mutations at sites 175, which resulted in complete loss of infectivity and fusion capacity. Furthermore, LC-MS/MS analysis uncovered unexpected non-classical N-glycosylation sites, including N252, N302, N367, and N471, with only N302 and N471 identified in SPΔnGP64. Subsequent investigation highlighted the critical roles of these residues in BmNPV amplification and fusion, underscoring the essentiality of N367 glycosylation for GP64 fusogenicity. Our findings provide valuable insights into the non-classical glycosylation landscape of BmNPV GP64 and its functional significance in viral biology.

Development of a highly effective combination monoclonal antibody therapy against Herpes simplex virus

BACKGROUND

Infections with Herpes simplex virus (HSV)-1 or -2 usually present as mild chronic recurrent disease, however in rare cases can result in life-threatening conditions with a large spectrum of pathology. Monoclonal antibody therapy has great potential especially to treat infections with virus resistant to standard therapies. HDIT101, a humanized IgG targeting HSV-1/2 gB was previously investigated in phase 2 clinical trials. The aim of this study was to develop a next-generation therapy by combining different antiviral monoclonal antibodies.

METHODS

A lymph-node derived phage display library (LYNDAL) was screened against recombinant gB from Herpes simplex virus (HSV) -1 and HDIT102 scFv was selected for its binding characteristics using bio-layer interferometry. HDIT102 was further developed as fully human IgG and tested alone or in combination with HDIT101, a clinically tested humanized anti-HSV IgG, in vitro and in vivo. T-cell stimulating activities by antigen-presenting cells treated with IgG-HSV immune complexes were analyzed using primary human cells. To determine the epitopes, the cryo-EM structures of HDIT101 or HDIT102 Fab bound to HSV-1F as well as HSV-2G gB protein were solved at resolutions < 3.5 Å.

RESULTS

HDIT102 Fab showed strong binding to HSV-1F gB with Kd of 8.95 × 10-11 M and to HSV-2G gB with Kd of 3.29 × 10-11 M. Neutralization of cell-free virus and inhibition of cell-to-cell spread were comparable between HDIT101 and HDIT102. Both antibodies induced internalization of gB from the cell surface into acidic endosomes by binding distinct epitopes in domain I of gB and compete for binding. CryoEM analyses revealed the ability to form heterogenic immune complexes consisting of two HDIT102 and one HDIT101 Fab bound to one gB trimeric molecule. Both antibodies mediated antibody-dependent phagocytosis by antigen presenting cells which stimulated autologous T-cell activation. In vivo, the combination of HDIT101 and HDIT102 demonstrated synergistic effects on survival and clinical outcome in immunocompetent BALB/cOlaHsd mice.

CONCLUSION

This biochemical and immunological study showcases the potential of an effective combination therapy with two monoclonal anti-gB IgGs for the treatment of HSV-1/2 induced disease conditions.

Neutralizing antibodies to block viral entry and for identification of entry inhibitors

Neutralizing antibodies (NAbs) are naturally produced by our immune system to combat viral infections. Clinically, neutralizing antibodies with potent efficacy and high specificity have been extensively used to prevent and treat a wide variety of viral infections, including Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2), Human Immunodeficiency Virus (HIV), Dengue Virus (DENV) and Hepatitis B Virus (HBV). An overwhelmingly large subset of clinically effective NAbs operates by targeting viral envelope proteins to inhibit viral entry into the host cell. Binding of viral envelope protein to the host receptor is a critical rate limiting step triggering a cascade of downstream events, including endocytosis, membrane fusion and pore formation to allow viral entry. In recent years, improved structural knowledge on these processes have allowed researchers to also leverage NAbs as an indispensable tool in guiding discovery of novel antiviral entry inhibitors, providing drug candidates with high efficacy and pan-genus specificity. This review will summarize the latest progresses on the applications of NAbs as effective entry inhibitors and as important tools to develop antiviral therapeutics by high-throughput drug screenings, rational design of peptidic entry inhibitor mimicking NAbs and in silico computational modeling approaches.

Virus-Induced Cell Fusion and Syncytia Formation

Most enveloped viruses encode viral fusion proteins to penetrate host cell by membrane fusion. Interestingly, many enveloped viruses can also use viral fusion proteins to induce cell-cell fusion, both in vitro and in vivo, leading to the formation of syncytia or multinucleated giant cells (MGCs). In addition, some non-enveloped viruses encode specialized viral proteins that induce cell-cell fusion to facilitate viral spread. Overall, viruses that can induce cell-cell fusion are nearly ubiquitous in mammals. Virus cell-to-cell spread by inducing cell-cell fusion may overcome entry and post-entry blocks in target cells and allow evasion of neutralizing antibodies. However, molecular mechanisms of virus-induced cell-cell fusion remain largely unknown. Here, I summarize the current understanding of virus-induced cell fusion and syncytia formation.

4'-(8-(4-Methylimidazole)-octyloxy)-arctigenin: The first inhibitor of fish rhabdovirus glycoprotein

Spring viraemia of carp virus (SVCV), a highly pathogenic rhabdovirus, could cause spring viraemia of carp (SVC) with up to 90% lethality. Like other rhabdoviruses, the entry of SVCV into susceptible cells was mediated by a single envelope glycoprotein G. Specific inhibitors targeting the glycoprotein were the most effective means to alleviate the epidemic. The programs including SWISS-MODEL, I-TASSER, Phyre2 and AlphaFold2 were used to build a three-dimensional structural model of glycoprotein. The structural comparison between SVCV-G and homology protein VSV-G revealed that the SVCV glycoprotein ectodomain (residues 19 to 466) folded into four distinct domains. Based on the potential small molecule binding sites on glycoprotein surfaces, virtual screening of the anti-SVCV drug libraries was performed using Autodock software and 4'-(8-(4-Methylimidazole)-octyloxy)-arctigenin (MOA) with a high binding affinity was identified. The solubility enhancer tags including trigger factor and maltose binding protein were fused with the ectodomain of glycoprotein, and the target protein with a purity of about 90% was successfully obtained. The interaction confirmation tests revealed that the fluorescence intensity of a characteristic peak induced by the endogenous chromophores in glycoprotein was decreased with the addition of MOA, indicating changes in the microenvironment of glycoprotein. Moreover, the interaction could cause a slight conformational change in glycoprotein, as shown by the content of β-turn, β-folding, and random coil of protein all increased with the decrease of α-helix content after the addition of MOA compound. These results demonstrated that MOA could act as a novel drug against fish rhabdovirus via direct targeting of glycoprotein.

Galdieria sulphuraria: An Extremophilic Alga as a Source of Antiviral Bioactive Compounds

In the last decades, the interest in bioactive compounds derived from natural sources including bacteria, fungi, plants, and algae has significantly increased. It is well-known that aquatic or terrestrial organisms can produce, in special conditions, secondary metabolites with a wide range of biological properties, such as anticancer, antioxidant, anti-inflammatory, and antimicrobial activities. In this study, we focused on the extremophilic microalga Galdieria sulphuraria as a possible producer of bioactive compounds with antiviral activity. The algal culture was subjected to organic extraction with acetone. The cytotoxicity effect of the extract was evaluated by the 2,5-diphenyl-2H-tetrazolium bromide (MTT) assay. The antiviral activity was assessed through a plaque assay against herpesviruses and coronaviruses as enveloped viruses and poliovirus as a naked one. The monolayer was treated with different concentrations of extract, ranging from 1 µg/mL to 200 µg/mL, and infected with viruses. The algal extract displayed strong antiviral activity at non-toxic concentrations against all tested enveloped viruses, in particular in the virus pre-treatment against HSV-2 and HCoV-229E, with IC₅₀ values of 1.7 µg/mL and IC₉₀ of 1.8 µg/mL, respectively. However, no activity against the non-enveloped poliovirus has been detected. The inhibitory effect of the algal extract was confirmed by the quantitative RT-PCR of viral genes. Preliminary chemical profiling of the extract was performed using ultra-high-performance liquid chromatography coupled to high-resolution mass spectrometry (UHPLC-HRMS), revealing the enrichment in primary fatty acid amides (PFAA), such as oleamide, palmitamide, and pheophorbide A. These promising results pave the way for the further purification of the mixture to explore its potential role as an antiviral agent.

The cytomegalovirus gB/MF59 vaccine candidate induces antibodies against an antigenic domain controlling cell-to-cell spread

Vaccination against human cytomegalovirus (CMV) infection remains high priority. A recombinant form of a protein essential for CMV entry, glycoprotein B (gB), demonstrated partial protection in a clinical trial (NCT00299260) when delivered with the MF59 adjuvant. Although the antibody titre against gB correlated with protection poor neutralising responses against the 5 known antigenic domains (AD) of gB were evident. Here, we show that vaccination of CMV seronegative patients induces an antibody response against a region of gB we term AD-6. Responses to the polypeptide AD-6 are detected in >70% of vaccine recipients yet in <5% of naturally infected people. An AD-6 antibody binds to gB and to infected cells but not the virion directly. Consistent with this, the AD-6 antibody is non-neutralising but, instead, prevents cell-cell spread of CMV in vitro. The discovery of AD-6 responses has the potential to explain part of the protection mediated by gB vaccines against CMV following transplantation.

How EBV Infects: The Tropism and Underlying Molecular Mechanism for Viral Infection

The Epstein-Barr virus (EBV) is associated with a variety of human malignancies, including Burkitt's lymphoma, Hodgkin's disease, nasopharyngeal carcinoma and gastric cancers. EBV infection is crucial for the oncogenesis of its host cells. The prerequisite for the establishment of infection is the virus entry. Interactions of viral membrane glycoproteins and host membrane receptors play important roles in the process of virus entry into host cells. Current studies have shown that the main tropism for EBV are B cells and epithelial cells and that EBV is also found in the tumor cells derived from NK/T cells and leiomyosarcoma. However, the process of EBV infecting B cells and epithelial cells significantly differs, relying on heterogenous glycoprotein-receptor interactions. This review focuses on the tropism and molecular mechanism of EBV infection. We systematically summarize the key molecular events that mediate EBV cell tropism and its entry into target cells and provide a comprehensive overview.

The interaction of viral fusion peptides with lipid membranes

In this paper, we studied fusogenic peptides of class I-III fusion proteins, which are relevant to membrane fusion for certain enveloped viruses, in contact with model lipid membranes. We resolved the vertical structure and examined the adsorption or penetration behavior of the fusogenic peptides at phospholipid Langmuir monolayers with different initial surface pressures with x-ray reflectometry. We show that the fusion loops of tick-borne encephalitis virus (TBEV) glycoprotein E and vesicular stomatitis virus (VSV) G-protein are not able to insert deeply into model lipid membranes, as they adsorbed mainly underneath the headgroups with only limited penetration depths into the lipid films. In contrast, we observed that the hemagglutinin 2 fusion peptide (HA2-FP) and the VSV-transmembrane domain (VSV-TMD) can penetrate deeply into the membranes. However, in the case of VSV-TMD, the penetration was suppressed already at low surface pressures, whereas HA2-FP was able to insert even into highly compressed films. Membrane fusion is accompanied by drastic changes of the membrane curvature. To investigate how the peptides affect the curvature of model lipid membranes, we examined the effect of the fusogenic peptides on the equilibration of cubic monoolein structures after a phase transition from a lamellar state induced by an abrupt hydrostatic pressure reduction. We monitored this process in presence and absence of the peptides with small-angle x-ray scattering and found that HA2-FP and VSV-TMD drastically accelerate the equilibration, while the fusion loops of TBEV and VSV stabilize the swollen state of the lipid structures. In this work, we show that the class I fusion peptide of HA2 penetrates deeply into the hydrophobic region of membranes and is able to promote and accelerate the formation of negative curvature. In contrast, we found that the class II and III fusion loops of TBEV and VSV tend to counteract negative membrane curvature.

Ionization of D571 Is Coupled with SARS-CoV-2 Spike Up/Down Equilibrium Revealing the pH-Dependent Allosteric Mechanism of Receptor-Binding Domains

As a type I viral fusion protein, SARS-CoV-2 spike undergoes a pH-dependent switch to mediate the endosomal positioning of the receptor-binding domain to facilitate viral entry into cells and immune evasion. Gaps in our knowledge concerning the conformational transitions and key intramolecular motivations have hampered the development of effective therapeutics against the virus. To clarify the pH-sensitive elements on spike-gating the receptor-binding domain (RBD) opening and understand the details of the RBD opening transition, we performed microsecond-time scale constant pH molecular dynamics simulations in this study. We identified the deeply buried D571 with a clear pKa shift, suggesting a potential pH sensor, and showed the coupling of ionization of D571 with spike RBD-up/down equilibrium. We also computed the free-energy landscape for RBD opening and identified the crucial interactions that influence RBD dynamics. The atomic-level characterization of the pH-dependent spike activation mechanism provided herein offers new insights for a better understanding of the fundamental mechanisms of SARS-CoV-2 viral entry and infection and hence supports the discovery of novel therapeutics for COVID-19.

Glycoprotein B Antibodies Completely Neutralize EBV Infection of B Cells

The Epstein–Barr virus (EBV) is the first reported oncogenic herpesvirus that establishes persistent infection in B lymphocytes in 95% of adults worldwide. Glycoprotein B (gB) plays a predominant role in the fusion of the viral envelope with the host cell membrane. Hence, it is of great significance to isolate gB-specific fusion-inhibiting neutralizing antibodies (NAbs). AMMO5 is the only gB NAb but fails to antagonize B-cell infection. It is essential to isolate potent NAbs that can completely block EBV infection of B cells. Using hybridoma technology and neutralization assay, we isolate two gB NAbs 8A9 and 8C12 that are capable of completely neutralizing B-cell infection in vitro. In addition, 8A9 shows cross-reactivity with rhesus lymphocryptovirus (rhLCV) gB. Competitive binding experiments demonstrate that 8A9 and 8C12 recognize novel epitopes that are different from the AMMO5 epitope. The epitopes of 8A9 and 8C12 are mapped to gB D-II, and the AMMO5 epitope is located precisely at gB aa 410–419. We find that 8A9 and 8C12 significantly inhibit gB-derived membrane fusion using a virus-free fusion assay. In summary, this study identifies two gB-specific NAbs that potently block EBV infection of B cells. Our work highlights the importance of gB D-II as a predominant neutralizing epitope, and aids in the rational design of therapeutics or vaccines based on gB.

A surface pocket in the cytoplasmic domain of the herpes simplex virus fusogen gB controls membrane fusion

Membrane fusion during the entry of herpesviruses is carried out by the viral fusogen gB that is activated by its partner protein gH in some manner. The fusogenic activity of gB is controlled by its cytoplasmic (or intraviral) domain (gB_CTD) and, according to the current model, the gB_CTD is a trimeric, inhibitory clamp that restrains gB in the prefusion conformation. But how the gB_CTD clamp is released by gH is unclear. Here, we identified two new regulatory elements within gB and gH from the prototypical herpes simplex virus 1: a surface pocket within the gB_CTD and residue V831 within the gH cytoplasmic tail. Mutagenesis and structural modeling suggest that gH V831 interacts with the gB pocket. The gB pocket is located above the interface between adjacent protomers, and we hypothesize that insertion of the gH V831 wedge into the pocket serves to push the protomers apart, which releases the inhibitory clamp. In this manner, gH activates the fusogenic activity of gB. Both gB and gH are conserved across all herpesviruses, and this activation mechanism could be used by other gB homologs. Our proposed mechanism emphasizes a central role for the cytoplasmic regions in regulating the activity of a viral fusogen.

Herpes simplex virus 1 (HSV-1) establishes lifelong infections in over a half of people and causes diseases ranging from oral or genital sores to blindness and brain inflammation. No vaccines or curative treatments are currently available. To infect cells, HSV-1 must first penetrate them by merging its lipid envelope with the membrane of the target cell. This process requires the collective actions of several viral and cellular proteins, notably, viral glycoproteins B and H (gB and gH). gH is thought to activate the fusogenic function of gB, but how the two proteins interact is unclear. Here, using mutational analysis, we have identified two new functional elements within the cytoplasmic regions of gB and gH: a surface pocket in gB and a single residue in gH, both of which are important for membrane fusion. Based on structural modeling, we propose that the gB pocket is the binding site for the gH residue, and that their interaction activates gB to cause membrane fusion. These findings extend our knowledge of the HSV-1 membrane fusion mechanism. Mechanistic understanding of HSV-1 entry is essential for understanding its pathogenesis and developing new strategies to prevent infections.

Inhibition of Viral Membrane Fusion by Peptides and Approaches to Peptide Design

Fusion of lipid-enveloped viruses with the cellular plasma membrane or the endosome membrane is mediated by viral envelope proteins that undergo large conformational changes following binding to receptors. The HIV-1 fusion protein gp41 undergoes a transition into a "six-helix bundle" after binding of the surface protein gp120 to the CD4 receptor and a co-receptor. Synthetic peptides that mimic part of this structure interfere with the formation of the helix structure and inhibit membrane fusion. This approach also works with the S spike protein of SARS-CoV-2. Here we review the peptide inhibitors of membrane fusion involved in infection by influenza virus, HIV-1, MERS and SARS coronaviruses, hepatitis viruses, paramyxoviruses, flaviviruses, herpesviruses and filoviruses. We also describe recent computational methods used for the identification of peptide sequences that can interact strongly with protein interfaces, with special emphasis on SARS-CoV-2, using the PePI-Covid19 database.

Innate Immune Signaling and Role of Glial Cells in Herpes Simplex Virus- and Rabies Virus-Induced Encephalitis

The environment of the central nervous system (CNS) represents a double-edged sword in the context of viral infections. On the one hand, the infectious route for viral pathogens is restricted via neuroprotective barriers; on the other hand, viruses benefit from the immunologically quiescent neural environment after CNS entry. Both the herpes simplex virus (HSV) and the rabies virus (RABV) bypass the neuroprotective blood-brain barrier (BBB) and successfully enter the CNS parenchyma via nerve endings. Despite the differences in the molecular nature of both viruses, each virus uses retrograde transport along peripheral nerves to reach the human CNS. Once inside the CNS parenchyma, HSV infection results in severe acute inflammation, necrosis, and hemorrhaging, while RABV preserves the intact neuronal network by inhibiting apoptosis and limiting inflammation. During RABV neuroinvasion, surveilling glial cells fail to generate a sufficient type I interferon (IFN) response, enabling RABV to replicate undetected, ultimately leading to its fatal outcome. To date, we do not fully understand the molecular mechanisms underlying the activation or suppression of the host inflammatory responses of surveilling glial cells, which present important pathways shaping viral pathogenesis and clinical outcome in viral encephalitis. Here, we compare the innate immune responses of glial cells in RABV- and HSV-infected CNS, highlighting different viral strategies of neuroprotection or Neuroinflamm. in the context of viral encephalitis.

Identification of a pH-Sensitive Switch in VSV-G and a Crystal Structure of the G Pre-fusion State Highlight the VSV-G Structural Transition Pathway

VSV fusion machinery, like that of many other enveloped viruses, is triggered at low pH in endosomes after virion endocytosis. It was suggested that some histidines could play the role of pH-sensitive switches. By mutating histidine residues H22, H60, H132, H162, H389, H397, H407, and H409, we demonstrate that residues H389 and D280, facing each other in the six-helix bundle of the post-fusion state, and more prominently H407, located at the interface between the C-terminal part of the ectodomain and the fusion domain, are crucial for fusion. Passages of recombinant viruses bearing mutant G resulted in the selection of compensatory mutations. Thus, the H407A mutation in G resulted in two independent compensatory mutants, L396I and S422I. Together with a crystal structure of G, presented here, which extends our knowledge of G pre-fusion structure, this indicates that the conformational transition is initiated by refolding of the C-terminal part of the G ectodomain.

Structure of HIV-1 gp41 with its membrane anchors targeted by neutralizing antibodies

The HIV-1 gp120/gp41 trimer undergoes a series of conformational changes in order to catalyze gp41-induced fusion of viral and cellular membranes. Here, we present the crystal structure of gp41 locked in a fusion intermediate state by an MPER-specific neutralizing antibody. The structure illustrates the conformational plasticity of the six membrane anchors arranged asymmetrically with the fusion peptides and the transmembrane regions pointing into different directions. Hinge regions located adjacent to the fusion peptide and the transmembrane region facilitate the conformational flexibility that allows high-affinity binding of broadly neutralizing anti-MPER antibodies. Molecular dynamics simulation of the MPER Ab-stabilized gp41 conformation reveals a possible transition pathway into the final post-fusion conformation with the central fusion peptides forming a hydrophobic core with flanking transmembrane regions. This suggests that MPER-specific broadly neutralizing antibodies can block final steps of refolding of the fusion peptide and the transmembrane region, which is required for completing membrane fusion.

Development of monoclonal antibody against glycoprotein of hirame novirhabdovirus (HIRRV) with virus neutralizing activity

Hirame rhabdovirus (HIRRV) is one of the most important viruses of fish, posing a great threat to the fish industry in Asia and Europe. The glycoprotein (G) of HIRRV is known to play important roles in virus attachment and entry, making it an ideal target for both diagnosis and therapy. In this study, a truncated G of HIRRV was expressed as a fusion protein in Escherichia coli. Using the recombinant G protein (rG), monoclonal antibodies (mAbs) were prepared by the hybridoma technology. Subsequently, positive clones were screened by indirect enzyme-linked immunosorbent assay (ELISA) and further characterized by Western blot and immunofluorescence assay (IFA). ELISA results showed that two mAbs (3E5 and 4D10) could react with the rG, as well as the purified HIRRV. Western blot analysis showed that the mAbs belong to the IgG isotype and could recognize a 60 kDa viral protein, which is consistent with the molecular weight of G protein and determined to be the G protein of HIRRV by mass spectrometry. The virions in HIRRV-infected EPC could also be recognized by two mAbs in IFA. Moreover, neutralization assay showed that mAb 4D10 could significantly inhibit the proliferation of HIRRV and delay the development of cytopathic effect in viral-infected EPC cells, and in vivo neutralization assay also showed that mAb 4D10 could significantly reduce the mortality of HIRRV-infected flounder, indicating that mAb 4D10 can partially neutralize the HIRRV infection. Western blot analysis showed that mAb 4D10 could specifically bind the C-terminal domain of HIRRV-G protein. These results demonstrated that the produced mAbs could specifically recognize the G protein of HIRRV and displayed virus-neutralizing activity in vitro and in vivo, which could serve as effective detection probes and potential neutralizing antibodies for HIRRV.

Light-induced lipid mixing implies a causal role of lipid splay in membrane fusion

The fusion of lipid membranes is central to many biological processes and requires substantial structural reorganization of lipids brought about by the action of fusogenic proteins. Previous molecular dynamics simulations have suggested that splayed lipids, whose tails transiently contact the headgroup region of the bilayer, initiate lipid mixing. Here, we explore the lipid splay hypothesis experimentally. We show that the light-induced trans/cis conversion of the azobenzene-based tail of a model lipid molecule enhances the probability by which its own acyl chains, or the acyl chains of the host lipid, transiently contact the lipid headgroup in a liposomal bilayer. At the same time, the trans/cis conversion triggers lipid mixing of sonicated or extruded liposomes, without requiring fusogenic proteins. This establishes a causal relationship between lipid splay and membrane fusion.

Critical Residues and Contacts within Domain IV of Autographa californica Multiple Nucleopolyhedrovirus GP64 Contribute to Its Refolding during Membrane Fusion

Autographa californica multiple nucleopolyhedrovirus (AcMNPV) GP64 is a class III viral fusion protein that mediates low-pH-triggered membrane fusion during virus entry. Although the structure of GP64 in a postfusion conformation has been solved, its prefusion structure and the mechanism of how the protein refolds to execute fusion are unknown. In its postfusion structure, GP64 is composed of five domains (domains I to V). Domain IV (amino acids [aa] 374 to 407) contains two loops (loop 1 and loop 2) that form a hydrophobic pocket at the membrane-distal end of the molecule. To determine the roles of domain IV, we used alanine-scanning mutagenesis to replace each of the individual residues and the contact-forming residues within domain IV and evaluate their contributions to GP64-mediated membrane fusion and virus infection. In many cases, replacement of a single amino acid had no significant impact on GP64. However, replacement of R392 or disruption of the N381-N385, N384-Y388, N385-W393, or K389-W393 contact resulted in poor cell surface expression and fusion loss of the modified GP64, whereas replacement of E390 or G391 or disruption of the N381-K389, N381-Q401, or N381-I403 contact reduced the cell surface expression level of the constructs and the ability of GP64 to mediate fusion pore expansion. In contrast, replacement of N407 or disruption of contact D404-S406 appeared to restrict fusion pore expansion without affecting expression. Combined with the finding that these constructs remain in the prefusion conformation or have a dramatically less efficient transition from the prefusion to the postfusion state under acidic conditions, we proposed that domain IV is necessary for refolding of GP64 during membrane fusion.IMPORTANCE Baculovirus GP64 is grouped with rhabdovirus G, herpesvirus gB, and thogotovirus glycoproteins as a class III viral fusion protein. In their postfusion structures, these proteins contain five domains (domains I to V). Distinct from domain IV of rhabdovirus G and herpesvirus gB proteins, which is composed of β-sheets, domain IV of GP64 is a loop region; the same domain in thogotovirus glycoproteins has not been solved. In addition, domain IV is proximal to domain I (fusion domain) in prefusion structures of vesicular stomatitis virus (VSV) G and human cytomegalovirus (HCMV) gB but resides at the domain I-distal end of the molecule in a postfusion conformation. In this study, we identified that highly conserved residues and contacts within domain IV of AcMNPV GP64 are necessary for low-pH-triggered conformational change and fusion pore expansion. Our results highlight the roles of domain IV of class III viral fusion proteins in refolding during membrane fusion.

The prefusion structure of herpes simplex virus glycoprotein B

Cell entry of enveloped viruses requires specialized viral proteins that mediate fusion with the host membrane by substantial structural rearrangements from a metastable pre- to a stable postfusion conformation. This metastability renders the herpes simplex virus 1 (HSV-1) fusion glycoprotein B (gB) highly unstable such that it readily converts into the postfusion form, thereby precluding structural elucidation of the pharmacologically relevant prefusion conformation. By identification of conserved sequence signatures and molecular dynamics simulations, we devised a mutation that stabilized this form. Functionally locking gB allowed the structural determination of its membrane-embedded prefusion conformation at sub-nanometer resolution and enabled the unambiguous fit of all ectodomains. The resulting pseudo-atomic model reveals a notable conservation of conformational domain rearrangements during fusion between HSV-1 gB and the vesicular stomatitis virus glycoprotein G, despite their very distant phylogeny. In combination with our comparative sequence-structure analysis, these findings suggest common fusogenic domain rearrangements in all class III viral fusion proteins.

Cell Fusion Induced by a Fusion-Active Form of Human Cytomegalovirus Glycoprotein B (gB) Is Inhibited by Antibodies Directed at Antigenic Domain 5 in the Ectodomain of gB

Human cytomegalovirus (HCMV) is a ubiquitous pathogen that can cause severe clinical disease in allograft recipients and infants infected in utero Virus-neutralizing antibodies defined in vitro have been proposed to confer protection against HCMV infection, and the virion envelope glycoprotein B (gB) serves as a major target of neutralizing antibodies. The viral fusion protein gB is nonfusogenic on its own and requires glycoproteins H (gH) and L (gL) for membrane fusion, which is in contrast to requirements of related class III fusion proteins, including vesicular stomatitis virus glycoprotein G (VSV-G) or baculovirus gp64. To explore requirements for gB's fusion activity, we generated a set of chimeras composed of gB and VSV-G or gp64, respectively. These gB chimeras were intrinsically fusion active and led to the formation of multinucleated cell syncytia when expressed in the absence of other viral proteins. Utilizing a panel of virus-neutralizing gB-specific monoclonal antibodies (MAbs), we could demonstrate that syncytium formation of the fusogenic gB/VSV-G chimera can be significantly inhibited by only a subset of neutralizing MAbs which target antigenic domain 5 (AD-5) of gB. This observation argues for differential modes of action of neutralizing anti-gB MAbs and suggests that blocking the membrane fusion function of gB could be one mechanism of antibody-mediated virus neutralization. In addition, our data have important implications for the further understanding of the conformation of gB that promotes membrane fusion as well as the identification of structures in AD-5 that could be targeted by antibodies to block this early step in HCMV infection.IMPORTANCE HCMV is a major global health concern, and antiviral chemotherapy remains problematic due to toxicity of available compounds and the emergence of drug-resistant viruses. Thus, an HCMV vaccine represents a priority for both governmental and pharmaceutical research programs. A major obstacle for the development of a vaccine is a lack of knowledge of the nature and specificities of protective immune responses that should be induced by such a vaccine. Glycoprotein B of HCMV is an important target for neutralizing antibodies and, hence, is often included as a component of intervention strategies. By generation of fusion-active gB chimeras, we were able to identify target structures of neutralizing antibodies that potently block gB-induced membrane fusion. This experimental system provides an approach to screen for antibodies that interfere with gB's fusogenic activity. In summary, our data will likely contribute to both rational vaccine design and the development of antibody-based therapies against HCMV.

Components and Architecture of the Rhabdovirus Ribonucleoprotein Complex

Rhabdoviruses, as single-stranded, negative-sense RNA viruses within the order Mononegavirales, are characterised by bullet-shaped or bacteroid particles that contain a helical ribonucleoprotein complex (RNP). Here, we review the components of the RNP and its higher-order structural assembly.

A tool with many applications: vesicular stomatitis virus in research and medicine

INTRODUCTION

Vesicular stomatitis virus (VSV) has long been a useful research tool in virology and recently become an essential part of medicinal products. Vesiculovirus research is growing quickly following its adaptation to clinical gene and cell therapy and oncolytic virotherapy.

AREAS COVERED

This article reviews the versatility of VSV as a research tool and biological reagent, its use as a viral and vaccine vector delivering therapeutic and immunogenic transgenes and an oncolytic virus aiding cancer treatment. Challenges such as the immune response against such advanced therapeutic medicinal products and manufacturing constraints are also discussed.

EXPERT OPINION

The field of in vivo gene and cell therapy is advancing rapidly with VSV used in many ways. Comparison of VSV's use as a versatile therapeutic reagent unveils further prospects and problems for each application. Overcoming immunological challenges to aid repeated administration of viral vectors and minimizing harmful host-vector interactions remains one of the major challenges. In the future, exploitation of reverse genetic tools may assist the creation of recombinant viral variants that have improved onco-selectivity and more efficient vaccine vector activity. This will add to the preferential features of VSV as an excellent advanced therapy medicinal product (ATMP) platform.

Anti-HSV-1 activity of Aspergillipeptide D, a cyclic pentapepetide isolated from fungus Aspergillus sp. SCSIO 41501

Background

Herpes simplex virus 1, an enveloped DNA virus belonging to the Herpesviridae family, spreads to neurons and causes pathological changes in the central nervous system. The purpose of this study was to investigate the potency and mechanism of antiviral activity of Aspergillipeptide D, a cyclic pentapeptide isolated from a culture broth of marine gorgonian-derived fungus Aspergillus sp. SCSIO 41501, At present, there are many studies on the anti-tumor, anti-clotting, anti-oxidant and immunoinflammatory effects of Aspergillipeptide D, but little research has been done on the anti-HSV-1 activity of Aspergillipeptide D.

Methods

The anti-HSV-1 activity of Aspergillipeptide D was evaluated by plaque reduction assay. The mechanism of action against HSV-1 was determined from the effective stage. Then we assayed the viral DNA replication, viral RNA synthesis and protein expression, respectively. We also identified the proteins that interact with gB by mass spectrometry, and assayed the effect of Aspergillipeptide D on the interaction between the virus gB protein and cell proteins.

Results

Plaque reduction experiments showed that Aspergillipeptide D did not affect HSV-1 early infection events, including viral inactivation, attachment and penetration. Interestingly, Aspergillipeptide D dramatically reduced both the gene and protein levels of viral late protein gB, and suppressed its location in the endoplasmic reticulum and Golgi apparatus. In contrast, overexpression of gB restored viral production. Finally, proteomic analysis revealed that the numbers of cellular proteins that interacted with gB protein was largely decreased by Aspergillipeptide D. These results suggested that Aspergillipeptide D inhibited gB function to affect HSV-1 intercellular spread.

Conclusions

Our results indicated that Aspergillipeptide D might be a potential candidate for HSV-1 therapy, especially for ACV-resistant strains.

Herpes Simplex Virus Glycoprotein C Regulates Low-pH Entry

Herpes simplex viruses (HSVs) cause significant morbidity and mortality in humans worldwide. Herpesviruses mediate entry by a multicomponent virus-encoded machinery. Herpesviruses enter cells by endosomal low-pH and pH-neutral mechanisms in a cell-specific manner. HSV mediates cell entry via the envelope glycoproteins gB and gD and the heterodimer gH/gL regardless of pH or endocytosis requirements. Specifics concerning HSV envelope proteins that function selectively in a given entry pathway have been elusive. Here, we demonstrate that gC regulates cell entry and infection by a low-pH pathway. Conformational changes in the core herpesviral fusogen gB are critical for membrane fusion. The presence of gC conferred a higher pH threshold for acid-induced antigenic changes in gB. Thus, gC may selectively facilitate low-pH entry by regulating conformational changes in the fusion protein gB. We propose that gC modulates the HSV fusion machinery during entry into pathophysiologically relevant cells, such as human epidermal keratinocytes.IMPORTANCE Herpesviruses are ubiquitous pathogens that cause lifelong latent infections and that are characterized by multiple entry pathways. We propose that herpes simplex virus (HSV) gC plays a selective role in modulating HSV entry, such as entry into epithelial cells, by a low-pH pathway. gC facilitates a conformational change of the main fusogen gB, a class III fusion protein. We propose a model whereby gC functions with gB, gD, and gH/gL to allow low-pH entry. In the absence of gC, HSV entry occurs at a lower pH, coincident with trafficking to a lower pH compartment where gB changes occur at more acidic pHs. This report identifies a new function for gC and provides novel insight into the complex mechanism of HSV entry and fusion.

Structure of the prefusion-locking broadly neutralizing antibody RVC20 bound to the rabies virus glycoprotein

Rabies virus (RABV) causes fatal encephalitis in more than 59,000 people yearly. Upon the bite of an infected animal, the development of clinical disease can be prevented with post-exposure prophylaxis (PEP), which includes the administration of Rabies immunoglobulin (RIG). However, the high cost and limited availability of serum-derived RIG severely hamper its wide use in resource-limited countries. A safe low-cost alternative is provided by using broadly neutralizing monoclonal antibodies (bnAbs). Here we report the X-ray structure of one of the most potent and most broadly reactive human bnAbs, RVC20, in complex with its target domain III of the RABV glycoprotein (G). The structure reveals that the RVC20 binding determinants reside in a highly conserved surface of G, rationalizing its broad reactivity. We further show that RVC20 blocks the acid-induced conformational change required for membrane fusion. Our results may guide the future development of direct antiviral small molecules for Rabies treatment.

Common characteristics and unique features: A comparison of the fusion machinery of the alphaherpesviruses Pseudorabies virus and Herpes simplex virus

Membrane fusion is a fundamental biological process that allows different cellular compartments delimited by a lipid membrane to release or exchange their respective contents. Similarly, enveloped viruses such as alphaherpesviruses exploit membrane fusion to enter and infect their host cells. For infectious entry the prototypic human Herpes simplex viruses 1 and 2 (HSV-1 and -2, collectively termed HSVs) and the porcine Pseudorabies virus (PrV) utilize four different essential envelope glycoproteins (g): the bona fide fusion protein gB and the regulatory heterodimeric gH/gL complex that constitute the "core fusion machinery" conserved in all members of the Herpesviridae; and the subfamily specific receptor binding protein gD. These four components mediate attachment and fusion of the virion envelope with the host cell plasma membrane through a tightly regulated sequential activation process. Although PrV and the HSVs are closely related and employ the same set of glycoproteins for entry, they show remarkable differences in the requirements for fusion. Whereas the HSVs strictly require all four components for membrane fusion, PrV can mediate cell-cell fusion without gD. Moreover, in contrast to the HSVs, PrV provides a unique opportunity for reversion analyses of gL-negative mutants by serial cell culture passaging, due to a limited cell-cell spread capacity of gL-negative PrV not observed in the HSVs. This allows a more direct analysis of the function of gH/gL during membrane fusion. Unraveling the molecular mechanism of herpesvirus fusion has been a goal of fundamental research for years, and yet important mechanistic details remain to be uncovered. Nevertheless, the elucidation of the crystal structures of all key players involved in PrV and HSV membrane fusion, coupled with a wealth of functional data, has shed some light on this complex puzzle. In this review, we summarize and discuss the contemporary knowledge on the molecular mechanism of entry and membrane fusion utilized by the alphaherpesvirus PrV, and highlight similarities but also remarkable differences in the requirements for fusion between PrV and the HSVs.

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.

Common Features of Enveloped Viruses and Implications for Immunogen Design for Next-Generation Vaccines

Enveloped viruses enter cells by inducing fusion of viral and cellular membranes, a process catalyzed by a specialized membrane-fusion protein expressed on their surface. This review focuses on recent structural studies of viral fusion proteins with an emphasis on their metastable prefusion form and on interactions with neutralizing antibodies. The fusion glycoproteins have been difficult to study because they are present in a labile, metastable form at the surface of infectious virions. Such metastability is a functional requirement, allowing these proteins to refold into a lower energy conformation while transferring the difference in energy to catalyze the membrane fusion reaction. Structural studies have shown that stable immunogens presenting the same antigenic sites as the labile wild-type proteins efficiently elicit potently neutralizing antibodies, providing a framework with which to engineer the antigens for stability, as well as identifying key vulnerability sites that can be used in next-generation subunit vaccine design.

Structural and cellular biology of rhabdovirus entry

Rhabdoviruses are enveloped viruses with a negative-sense single strand RNA genome and are widespread among a great variety of organisms. In their membrane, they have a single glycoprotein (G) that mediates both virus attachment to cellular receptors and fusion between viral and endosomal membranes allowing viral genome release in the cytoplasm. We present structural and cellular aspects of Rhabdovirus entry into their host cell with a focus on vesicular stomatitis virus (VSV) and rabies virus (RABV) for which the early events of the viral cycle have been extensively studied. Recent data have shown that the only VSV receptors are the members of the LDL-R family. This is in contrast with RABV for which multiple receptors belonging to unrelated families have been identified. Despite having different receptors, after attachment, rhabdovirus internalization occurs through clathrin-mediated endocytosis (CME) in an actin-dependent manner. There are still debates about the exact endocytic pathway of VSV in the cell and on RABV transport in the neuronal axon. In any case, fusion is triggered in the endosomal vesicle via a low-pH induced structural rearrangement of G from its pre- to its postfusion conformation. Vesiculovirus G is one of the best characterized fusion glycoproteins as the previously reported crystal structures of the pre- and postfusion states have been recently completed by those of intermediates during the structural transition. Understanding the entry pathway of rhabdoviruses may have strong impact in biotechnologies as, for example, VSV G is used for pseudotyping lentiviruses to promote efficient transduction, and VSV is a promising oncolytic virus.

Characterization of Antibody Interactions with the G Protein of Vesicular Stomatitis Virus Indiana Strain and Other Vesiculovirus G Proteins

VSVind.G is currently regarded as the gold-standard envelope glycoprotein to pseudotype lentiviral vectors. However, recently other G proteins derived from vesiculoviruses have been proposed as alternative envelopes. Here, we investigated two commercially available anti-VSVind.G monoclonal antibodies for their ability to cross-react with other vesiculovirus G proteins, identified the epitopes they recognize, and explored their neutralization activity. We have identified 8G5F11, for the first time, as a cross-neutralizing antibody against several vesiculovirus G proteins. Furthermore, we elucidated the two different neutralization mechanisms employed by these two monoclonal antibodies. Understanding how cross-neutralizing antibodies interact with other G proteins may be of interest in the context of host-pathogen interaction and coevolution, as well as providing the opportunity to modify the G proteins and improve G protein-containing medicinal products and vaccine vectors.

Vesicular stomatitis virus Indiana strain G protein (VSVind.G) is the most commonly used envelope glycoprotein to pseudotype lentiviral vectors (LV) for experimental and clinical applications. Recently, G proteins derived from other vesiculoviruses (VesG), for example, Cocal virus, have been proposed as alternative LV envelopes with possible advantages over VSVind.G. Well-characterized antibodies that recognize VesG will be useful for vesiculovirus research, development of G protein-containing advanced therapy medicinal products (ATMPs), and deployment of VSVind-based vaccine vectors. Here, we show that one commercially available monoclonal antibody, 8G5F11, binds to and neutralizes G proteins from three strains of VSV, as well as Cocal and Maraba viruses, whereas the other commercially available monoclonal anti-VSVind.G antibody, IE9F9, binds to and neutralizes only VSVind.G. Using a combination of G protein chimeras and site-directed mutations, we mapped the binding epitopes of IE9F9 and 8G5F11 on VSVind.G. IE9F9 binds close to the receptor binding site and competes with soluble low-density lipoprotein receptor (LDLR) for binding to VSVind.G, explaining its mechanism of neutralization. In contrast, 8G5F11 binds close to a region known to undergo conformational changes when the G protein moves to its postfusion structure, and we propose that 8G5F11 cross-neutralizes VesGs by inhibiting this.

IMPORTANCE VSVind.G is currently regarded as the gold-standard envelope glycoprotein to pseudotype lentiviral vectors. However, recently other G proteins derived from vesiculoviruses have been proposed as alternative envelopes. Here, we investigated two commercially available anti-VSVind.G monoclonal antibodies for their ability to cross-react with other vesiculovirus G proteins, identified the epitopes they recognize, and explored their neutralization activity. We have identified 8G5F11, for the first time, as a cross-neutralizing antibody against several vesiculovirus G proteins. Furthermore, we elucidated the two different neutralization mechanisms employed by these two monoclonal antibodies. Understanding how cross-neutralizing antibodies interact with other G proteins may be of interest in the context of host-pathogen interaction and coevolution, as well as providing the opportunity to modify the G proteins and improve G protein-containing medicinal products and vaccine vectors.

Transmembrane Helix Induces Membrane Fusion through Lipid Binding and Splay

The fusion of biological membranes may require splayed lipids whose tails transiently visit the headgroup region of the bilayer, a scenario suggested by molecular dynamics simulations. Here, we examined the lipid splay hypothesis experimentally by relating liposome fusion and lipid splay induced by model transmembrane domains (TMDs). Our results reveal that a conformationally flexible transmembrane helix promotes outer leaflet mixing and lipid splay more strongly than a conformationally rigid one. The lipid dependence of basal as well as of TMD-driven lipid mixing and splay suggests that the cone-shaped phosphatidylethanolamine stimulates basal fusion via enhancing lipid splay and that the negatively charged phosphatidylserine inhibits fusion via electrostatic repulsion. Phosphatidylserine also strongly differentiates basal and helix-driven fusion, which is related to its preferred interaction with the conformationally more flexible transmembrane helix. Thus, the contribution of a transmembrane helix to membrane fusion appears to depend on lipid binding, which results in lipid splay.

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.

Structure-Function Dissection of Pseudorabies Virus Glycoprotein B Fusion Loops

Conserved across the family Herpesviridae, glycoprotein B (gB) is responsible for driving fusion of the viral envelope with the host cell membrane for entry upon receptor binding and activation by the viral gH/gL complex. Although crystal structures of the gB ectodomains of several herpesviruses have been reported, the membrane fusion mechanism has remained elusive. Here, we report the X-ray structure of the pseudorabies virus (PrV) gB ectodomain, revealing a typical class III postfusion trimer that binds membranes via its fusion loops (FLs) in a cholesterol-dependent manner. Mutagenesis of FL residues allowed us to dissect those interacting with distinct subregions of the lipid bilayer and their roles in membrane interactions. We tested 15 gB variants for the ability to bind to liposomes and further investigated a subset of them in functional assays. We found that PrV gB FL residues Trp187, Tyr192, Phe275, and Tyr276, which were essential for liposome binding and for fusion in cellular and viral contexts, form a continuous hydrophobic patch at the gB trimer surface. Together with results reported for other alphaherpesvirus gBs, our data suggest a model in which Phe275 from the tip of FL2 protrudes deeper into the hydrocarbon core of the lipid bilayer, while the side chains of Trp187, Tyr192, and Tyr276 form a rim that inserts into the more superficial interfacial region of the membrane to catalyze the fusion process. Comparative analysis with gBs from beta- and gamma-herpesviruses suggests that this membrane interaction model is valid for gBs from all herpesviruses.IMPORTANCE Herpesviruses are common human and animal pathogens that infect cells by entering via fusion of viral and cellular membranes. Central to the membrane fusion event is glycoprotein B (gB), which is the most conserved envelope protein across the herpesvirus family. Like other viral fusion proteins, gB anchors itself in the target membrane via two polypeptide segments called fusion loops (FLs). The molecular details of how gB FLs insert into the lipid bilayer have not been described. Here, we provide structural and functional data regarding key FL residues of gB from pseudorabies virus, a porcine herpesvirus of veterinary concern, which allows us to propose, for the first time, a molecular model to understand how the initial interactions by gBs from all herpesviruses with target membranes are established.

Sequence analysis of malacoherpesvirus proteins: Pan-herpesvirus capsid module and replication enzymes with an ancient connection to "Megavirales"

The order Herpesvirales includes animal viruses with large double-strand DNA genomes replicating in the nucleus. The main capsid protein in the best-studied family Herpesviridae contains a domain with HK97-like fold related to bacteriophage head proteins, and several virion maturation factors are also homologous between phages and herpesviruses. The origin of herpesvirus DNA replication proteins is less well understood. While analyzing the genomes of herpesviruses in the family Malacohepresviridae, we identified nearly 30 families of proteins conserved in other herpesviruses, including several phage-related domains in morphogenetic proteins. Herpesvirus DNA replication factors have complex evolutionary history: some are related to cellular proteins, but others are closer to homologs from large nucleocytoplasmic DNA viruses. Phylogenetic analyses suggest that the core replication machinery of herpesviruses may have been recruited from the same pool as in the case of other large DNA viruses of eukaryotes.

Inhibition of EBV-mediated membrane fusion by anti-gHgL antibodies

Herpesvirus entry into cells requires the coordinated action of multiple virus envelope glycoproteins, including gH, gL, and gB. For EBV, the gp42 protein assembles into complexes with gHgL heterodimers and binds HLA class II to activate gB-mediated membrane fusion with B cells. EBV tropism is dictated by gp42 levels in the virion, as it inhibits entry into epithelial cells while promoting entry into B cells. The gHgL and gB proteins are targets of neutralizing antibodies and potential candidates for subunit vaccine development, but our understanding of their neutralizing epitopes and the mechanisms of inhibition remain relatively unexplored. Here we studied the structures and mechanisms of two anti-gHgL antibodies, CL40 and CL59, that block membrane fusion with both B cells and epithelial cells. We determined the structures of the CL40 and CL59 complexes with gHgL using X-ray crystallography and EM to identify their epitope locations. CL59 binds to the C-terminal domain IV of gH, while CL40 binds to a site occupied by the gp42 receptor binding domain. CL40 binding to gHgL/gp42 complexes is not blocked by gp42 and does not interfere with gp42 binding to HLA class II, indicating that its ability to block membrane fusion with B cells represents a defect in gB activation. These data indicate that anti-gHgL neutralizing antibodies can block gHgL-mediated activation of gB through different surface epitopes and mechanisms.

The COMPLEXity in herpesvirus entry

Enveloped viruses have evolved diverse transmembrane proteins and protein complexes to enable host cell entry by regulating and activating membrane fusion in a target cell-specific manner. In general terms, the entry process requires a receptor binding step, an activation step and a membrane fusion step, which can be encoded within a single viral protein or distributed among multiple viral proteins. HIV and influenza virus, for example, encode all of these functions in a single trimeric glycoprotein, HIV env or influenza virus hemagglutinin (HA). In contrast, herpesviruses have the host receptor binding, activation and fusogenic roles distributed among multiple envelope glycoproteins (ranging from three to six), which must coordinate their functions at the site of fusion. Despite the apparent complexity in the number of viral entry proteins, herpesvirus entry is fundamentally built around two core glycoprotein entities: the gHgL complex, which appears to act as an 'activator' of entry, and the gB protein, which is thought to act as the membrane 'fusogen'. Both are required for all herpesvirus fusion and entry. In many herpesviruses, gHgL either binds host receptors directly or assembles into larger complexes with additional viral proteins that bind host receptors, conferring specificity to the cells that are targeted for infection. These gHgL entry complexes (ECs) are centrally important to activating gB-mediated membrane fusion and establishing viral tropism, forming membrane bridging intermediates before gB triggering. Here we review recent structural and functional studies of Epstein-Barr virus (EBV) and Cytomegalovirus (CMV) gHgL complexes that provide a framework for understanding the role of gHgL in herpesvirus entry. Furthermore, a recently determined EM model of Herpes Simplex virus (HSV) gB embedded in exosomes highlights how gB conformational changes may promote viral and cellular membrane fusion.

Structural intermediates in the fusion-associated transition of vesiculovirus glycoprotein

Vesiculoviruses enter cells by membrane fusion, driven by a large, low-pH-induced, conformational change in the fusion glycoprotein G that involves transition from a trimeric pre-fusion toward a trimeric post-fusion state via monomeric intermediates. Here, we present the structure of the G fusion protein at intermediate pH for two vesiculoviruses, vesicular stomatitis virus (VSV) and Chandipura virus (CHAV), which is responsible for deadly encephalopathies. First, a CHAV G crystal structure shows two intermediate conformations forming a flat dimer of heterodimers. On virions, electron microscopy (EM) and tomography reveal monomeric spikes similar to one of the crystal conformations. In solution, mass spectrometry shows dimers of G. Finally, mutations at a dimer interface, involving fusion domains associated in an antiparallel manner to form an intermolecular β-sheet, affect G fusion properties. The location of the compensatory mutations restoring fusion activity strongly suggests that this interface is functionally relevant. This work reveals the range of G structural changes and suggests that G monomers can re-associate, through antiparallel interactions between fusion domains, into dimers that play a role at some early stage of the fusion process.

Structural basis for Epstein-Barr virus host cell tropism mediated by gp42 and gHgL entry glycoproteins

Herpesvirus entry into host cells is mediated by multiple virally encoded receptor binding and membrane fusion glycoproteins. Despite their importance in host cell tropism and associated disease pathology, the underlying and essential interactions between these viral glycoproteins remain poorly understood. For Epstein–Barr virus (EBV), gHgL/gp42 complexes bind HLA class II to activate membrane fusion with B cells, but gp42 inhibits fusion and entry into epithelial cells. To clarify the mechanism by which gp42 controls the cell specificity of EBV infection, here we determined the structure of gHgL/gp42 complex bound to an anti-gHgL antibody (E1D1). The critical regulator of EBV tropism is the gp42 N-terminal domain, which tethers the HLA-binding domain to gHgL by wrapping around the exterior of three gH domains. Both the gp42 N-terminal domain and E1D1 selectively inhibit epithelial-cell fusion; however, they engage distinct surfaces of gHgL. These observations clarify key determinants of EBV host cell tropism.

The entry of herpesviruses (such as Epstein-Barr virus) into host cells is mediated by a multitude of glycoproteins. Here, the authors show the structure of a viral glycoprotein complex, gHgL/gp42, bound to an anti-gHgL antibody, clarifying determinants of EBV host cell tropism.

The Microtubule Inhibitor Podofilox Inhibits an Early Entry Step of Human Cytomegalovirus

Human cytomegalovirus is a ubiquitous β-herpesvirus that infects many different cell types through an initial binding to cell surface receptors followed by a fusion event at the cell membrane or endocytic vesicle. A recent high-throughput screen to identify compounds that block a step prior to viral gene expression identified podofilox as a potent and nontoxic inhibitor. Time-of-addition studies in combination with quantitative-PCR analysis demonstrated that podofilox limits an early step of virus entry at the cell surface. Podofilox was also able to drastically reduce infection by herpes simplex 1, an α-herpesvirus with a very similar entry process to CMV. Podofilox caused a reduced maximal plateau inhibition of infection by viruses with single step binding processes prior to fusion-like Newcastle disease virus, Sendai virus, and influenza A virus or viruses that enter via endocytosis like vesicular stomatitis virus and a clinical-like strain of CMV. These results indicate that microtubules appear to be participating in the post-binding step of virus entry including the pre- and post-penetration events. Modulation of the plasma membrane is required to promote virus entry for herpesviruses, and that podofilox, unlike colchicine or nocodazole, is able to preferentially target microtubule networks at the plasma membrane.

Fusion of Enveloped Viruses in Endosomes

Ari Helenius launched the field of enveloped virus fusion in endosomes with a seminal paper in the Journal of Cell Biology in 1980. In the intervening years, a great deal has been learned about the structures and mechanisms of viral membrane fusion proteins as well as about the endosomes in which different enveloped viruses fuse and the endosomal cues that trigger fusion. We now recognize three classes of viral membrane fusion proteins based on structural criteria and four mechanisms of fusion triggering. After reviewing general features of viral membrane fusion proteins and viral fusion in endosomes, we delve into three characterized mechanisms for viral fusion triggering in endosomes: by low pH, by receptor binding plus low pH and by receptor binding plus the action of a protease. We end with a discussion of viruses that may employ novel endosomal fusion‐triggering mechanisms. A key take‐home message is that enveloped viruses that enter cells by fusing in endosomes traverse the endocytic pathway until they reach an endosome that has all of the environmental conditions (pH, proteases, ions, intracellular receptors and lipid composition) to (if needed) prime and (in all cases) trigger the fusion protein and to support membrane fusion.

gH/gL supercomplexes at early stages of herpesvirus entry

Membrane fusion during herpesvirus entry into host cells is a complex process where multiple glycoproteins interact to relay the triggering signal from a receptor-binding protein to the conserved fusogen gB through the conserved heterodimer gH/gL. Crystal structures of individual glycoproteins are available, yet high-order 'supercomplexes' have been elusive. Recent structures of complexes between gH/gL from human cytomegalovirus or Epstein-Barr virus and the receptor-binding proteins that form at early stages of herpesviral entry highlighted mechanisms that control tropism and revealed dynamic intermediate complexes containing gH/gL that may directly participate in membrane deformation and juxtaposition. Determining how the triggering signal reaches the fusogen gB represents the next frontier in structural biology of herpesvirus entry.