Stuck in the middle: structural insights into the role of the gH/gL heterodimer in herpesvirus entry

An In Silico Approach to Discover Efficient Natural Inhibitors to Tie Up Epstein-Barr Virus Infection

Epstein-Barr virus (EBV), also known as human herpesvirus 4, is a member of the herpes virus family. EBV is a widespread virus and causes infectious mononucleosis, which manifests with symptoms such as fever, fatigue, lymphadenopathy, splenomegaly, and hepatomegaly. Additionally, EBV is associated with different lymphocyte-associated non-malignant, premalignant, and malignant diseases. So far, no effective treatment or therapeutic drug is known for EBV-induced infections and diseases. This study investigated natural compounds that inhibit EBV glycoprotein L (gL) and block EBV fusion in host cells. We utilised computational approaches, including molecular docking, in silico ADMET analysis, and molecular dynamics simulation. We docked 628 natural compounds against gL and identified the four best compounds based on binding scores and pharmacokinetic properties. These four compounds, with PubChem CIDs 4835509 (CHx-HHPD-Ac), 2870247 (Cyh-GlcNAc), 21206004 (Hep-HHPD-Ac), and 51066638 (Und-GlcNAc), showed several interactions with EBV gL. However, molecular dynamics simulations indicated that the protein-ligand complexes of CID: 4835509 (CHx-HHPD-Ac) and CID: 2870247 (Cyh-GlcNAc) are more stable than those of the other two compounds. Therefore, CIDs 4835509 and 2870247 (Cyh-GlcNAc) may be potent natural inhibitors of EBV infection. These findings can open a new way for effective drug design against EBV and its associated infections and diseases.

GB and gH/gL fusion machinery: a promising target for vaccines to prevent Epstein-Barr virus infection

Epstein‒Barr virus (EBV) is a double-stranded DNA virus belonging to the family Orthoherpesviridae that is associated with the development of various tumors, such as lymphoma, nasopharyngeal carcinoma, and gastric cancer. There are no uniformly effective treatments for human EBV infection, and vaccines and immunotherapies are currently the main research directions. The glycoproteins gB and gH/gL are surface glycoproteins that are common to all herpesviruses, with subtle differences in structure and function between different viruses. The core membrane fusion machinery constituted by EBV gB and gH/gL is an important target of neutralizing antibodies in epithelial EBV infection due to its essential role in the fusion of viral and target cell membranes. In this article, we review the main modes of EBV infection, the structure and function of the core fusion machinery gB and gH/gL, and the development of neutralizing antibodies and prophylactic vaccines based on this target.

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.

Targeted mutagenesis of the herpesvirus fusogen central helix captures transition states

Herpesviruses remain a burden for animal and human health, including the medically important varicella-zoster virus (VZV). Membrane fusion mediated by conserved core glycoproteins, the fusogen gB and the heterodimer gH-gL, enables herpesvirus cell entry. The ectodomain of gB orthologs has five domains and is proposed to transition from a prefusion to postfusion conformation but the functional relevance of the domains for this transition remains poorly defined. Here we describe structure-function studies of the VZV gB DIII central helix targeting residues 526EHV528. Critically, a H527P mutation captures gB in a prefusion conformation as determined by cryo-EM, a loss of membrane fusion in a virus free assay, and failure of recombinant VZV to spread in cell monolayers. Importantly, two predominant cryo-EM structures of gB[H527P] are identified by 3D classification and focused refinement, suggesting they represented gB conformations in transition. These studies reveal gB DIII as a critical element for herpesvirus gB fusion function.

Whole genome sequence analysis of equid gammaherpesvirus -2 field isolates reveals high levels of genomic diversity and recombination

Background

Equid gammaherpesvirus 2 (EHV2) is a gammaherpesvirus with a widespread distribution in horse populations globally. Although its pathogenic significance can be unclear in most cases of infection, EHV2 infection can cause upper respiratory tract disease in foals. Co-infection of different strains of EHV2 in an individual horse is common. Small regions of the EHV2 genome have shown considerable genetic heterogeneity. This could suggest genomic recombination between different strains of EHV2, similar to the extensive recombination networks that have been demonstrated for some alphaherpesviruses. This study examined natural recombination and genome diversity of EHV2 field isolates.

Results

Whole genome sequencing analysis of 18 EHV2 isolates, along with analysis of two publicly available EHV2 genomes, revealed variation in genomes sizes (from 173.7 to 184.8 kbp), guanine plus cytosine content (from 56.7 to 57.8%) and the size of the terminal repeat regions (from 17,196 to 17,551 bp). The nucleotide sequence identity between the genomes ranged from 86.2 to 99.7%. The estimated average inter-strain nucleotide diversity between the 20 EHV2 genomes was 2.9%. Individual gene sequences showed varying levels of nucleotide diversity and ranged between 0 and 38.1%. The ratio of nonsynonymous substitutions, Ka, to synonymous substitutions, Ks, (Ka/Ks) suggests that over 50% of EHV2 genes are undergoing diversifying selection. Recombination analyses of the 20 EHV2 genome sequences using the recombination detection program (RDP4) and SplitsTree revealed evidence of viral recombination.

Conclusions

Analysis of the 18 new EHV2 genomes alongside the 2 previously sequenced genomes revealed a high degree of genetic diversity and extensive recombination networks. Herpesvirus genome diversification and virus evolution can be driven by recombination, and our findings are consistent with recombination being a key mechanism by which EHV2 genomes may vary and evolve.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-022-08789-x.

Herpes Simplex Virus 1 Entry Glycoproteins Form Complexes before and during Membrane Fusion

Herpesviruses—ubiquitous pathogens that cause persistent infections—have some of the most complex cell entry mechanisms. Entry of the prototypical herpes simplex virus 1 (HSV-1) requires coordinated efforts of 4 glycoproteins, gB, gD, gH, and gL. The current model posits that the glycoproteins do not interact before receptor engagement and that binding of gD to its receptor causes a “cascade” of sequential pairwise interactions, first activating the gH/gL complex and subsequently activating gB, the viral fusogen. But how these glycoproteins interact remains unresolved. Here, using a quantitative split-luciferase approach, we show that pairwise HSV-1 glycoprotein complexes form before fusion, interact at a steady level throughout fusion, and do not depend on the presence of the cellular receptor. Based on our findings, we propose a revised “conformational cascade” model of HSV-1 entry. We hypothesize that all 4 glycoproteins assemble into a complex before fusion, with gH/gL positioned between gD and gB. Once gD binds to a cognate receptor, the proximity of the glycoproteins within this complex allows for efficient transmission of the activating signal from the receptor-activated gD to gH/gL to gB through sequential conformational changes, ultimately triggering the fusogenic refolding of gB. Our results also highlight previously unappreciated contributions of the transmembrane and cytoplasmic domains to glycoprotein interactions and fusion. Similar principles could be at play in other multicomponent viral entry systems, and the split-luciferase approach used here is a powerful tool for investigating protein-protein interactions in these and a variety of other systems.

Immunization with a self-assembling nanoparticle vaccine displaying EBV gH/gL protects humanized mice against lethal viral challenge

Epstein-Barr virus (EBV) is a cancer-associated pathogen responsible for 165,000 deaths annually. EBV is also the etiological agent of infectious mononucleosis and is linked to multiple sclerosis and rheumatoid arthritis. Thus, an EBV vaccine would have a significant global health impact. EBV is orally transmitted and has tropism for epithelial and B cells. Therefore, a vaccine would need to prevent infection of both in the oral cavity. Passive transfer of monoclonal antibodies against the gH/gL glycoprotein complex prevent experimental EBV infection in humanized mice and rhesus macaques, suggesting that gH/gL is an attractive vaccine candidate. Here, we evaluate the immunogenicity of several gH/gL nanoparticle vaccines. All display superior immunogenicity relative to monomeric gH/gL. A nanoparticle displaying 60 copies of gH/gL elicits antibodies that protect against lethal EBV challenge in humanized mice, whereas antibodies elicited by monomeric gH/gL do not. These data motivate further development of gH/gL nanoparticle vaccines for EBV.

A Neutralizing Antibody Targeting gH Provides Potent Protection against EBV Challenge In Vivo

Epstein-Barr virus (EBV) is an oncogenic herpesvirus that is associated with 200,000 new cases of cancer and 140,000 deaths annually. To date, there are no available vaccines or therapeutics for clinical usage. Recently, the viral heterodimer glycoprotein gH/gL has become a promising target for the development of prophylactic vaccines against EBV. Here, we developed the anti-gH antibody 6H2 and its chimeric version C6H2, which had full neutralizing activity in epithelial cells and partial neutralizing activity in B cells. C6H2 exhibited potent protection against lethal EBV challenge in a humanized mouse model. The cryo-electron microscopy (cryo-EM) structure further revealed that 6H2 recognized a previously unidentified epitope on gH/gL D-IV that is critical for viral attachment and subsequent membrane fusion with epithelial cells. Our results suggest that C6H2 is a promising candidate in the prevention of EBV-induced lymphoproliferative diseases (LPDs) and may inform the design of an EBV vaccine. IMPORTANCE Epstein-Barr virus (EBV) is a ubiquitous gammaherpesvirus that establishes lifelong persistence and is related to multiple diseases, including cancers. Neutralizing antibodies (NAbs) have proven to be highly effective in preventing EBV infection and subsequent diseases. Here, we developed an anti-EBV-gH NAb, 6H2, which blocked EBV infection in vitro and in vivo. This 6H2 neutralizing epitope should be helpful to understand EBV infection mechanisms and guide the development of vaccines and therapeutics against EBV infection.

Well Put Together-A Guide to Accessorizing with the Herpesvirus gH/gL Complexes

Herpesviruses are enveloped, double-stranded DNA viruses that infect a variety of hosts across the animal kingdom. Nine of these establish lifelong infections in humans, for which there are no cures and few vaccine or treatment options. Like all enveloped viruses, herpesviruses enter cells by fusing their lipid envelopes with a host cell membrane. Uniquely, herpesviruses distribute the functions of receptor engagement and membrane fusion across a diverse cast of glycoproteins. Two glycoprotein complexes are conserved throughout the three herpesvirus subfamilies: the trimeric gB that functions as a membrane fusogen and the heterodimeric gH/gL, the role of which is less clearly defined. Here, we highlight the conserved and divergent functions of gH/gL across the three subfamilies of human herpesviruses by comparing its interactions with a broad range of accessory viral proteins, host cell receptors, and neutralizing or inhibitory antibodies. We propose that the intrinsic structural plasticity of gH/gL enables it to function as a signal integration machine that can accept diverse regulatory inputs and convert them into a "trigger" signal that activates the fusogenic ability of gB.

Two Sides to Every Story: Herpes Simplex Type-1 Viral Glycoproteins gB, gD, gH/gL, gK, and Cellular Receptors Function as Key Players in Membrane Fusion

Herpes simplex virus type-1 (HSV-1) and type-2 (HSV-2) are prototypical alphaherpesviruses that are characterized by their unique properties to infect trigeminal and dorsal root ganglionic neurons, respectively, and establish life-long latent infections. These viruses initially infect mucosal epithelial tissues and subsequently spread to neurons. They are associated with a significant disease spectrum, including orofacial and ocular infections for HSV-1 and genital and neonatal infections for HSV-2. Viral glycoproteins within the virion envelope bind to specific cellular receptors to mediate virus entry into cells. This is achieved by the fusion of the viral envelope with the plasma membrane. Similarly, viral glycoproteins expressed on cell surfaces mediate cell-to-cell fusion and facilitate virus spread. An interactive complex of viral glycoproteins gB, gD/gH/gL, and gK and other proteins mediate these membrane fusion phenomena with glycoprotein B (gB), the principal membrane fusogen. The requirement for the virion to enter neuronal axons suggests that the heterodimeric protein complex of gK and membrane protein UL20, found only in alphaherpesviruses, constitute a critical determinant for neuronal entry. This hypothesis was substantiated by the observation that a small deletion in the amino terminus of gK prevents entry into neuronal axons while allowing entry into other cells via endocytosis. Cellular receptors and receptor-mediated signaling synergize with the viral membrane fusion machinery to facilitate virus entry and intercellular spread. Unraveling the underlying interactions among viral glycoproteins, envelope proteins, and cellular receptors will provide new innovative approaches for antiviral therapy against herpesviruses and other neurotropic viruses.

A Novel Strain-Specific Neutralizing Epitope on Glycoprotein H of Human Cytomegalovirus

Human cytomegalovirus (HCMV) is a ubiquitous pathogen that causes severe clinical disease in immunosuppressed patients and congenitally infected newborn infants. Viral envelope glycoproteins represent attractive targets for vaccination or passive immunotherapy. To extend the knowledge of mechanisms of virus neutralization, monoclonal antibodies (MAbs) were generated following immunization of mice with HCMV virions. Hybridoma supernatants were screened for in vitro neutralization activity, yielding three potent MAbs, 6E3, 3C11, and 2B10. MAbs 6E3 and 3C11 blocked infection of all viral strains that were tested, while MAb 2B10 neutralized only 50% of the HCMV strains analyzed. Characterization of the MAbs using indirect immunofluorescence analyses demonstrated their reactivity with recombinantly derived gH. While MAbs 6E3 and 3C11 reacted with gH when expressed alone, 2B10 detected gH only when it was coexpressed with gB and gL. Recognition of gH by 3C11 was dependent on the expression of the entire ectodomain of gH, whereas 6E3 required residues 1 to 629 of gH. The strain-specific determinant for neutralization by Mab 2B10 was identified as a single Met→Ile amino acid polymorphism within gH, located within the central part of the protein. The polymorphism is evenly distributed among described HCMV strains. The 2B10 epitope thus represents a novel strain-specific antibody target site on gH of HCMV. The dependence of the reactivity of 2B10 on the simultaneous presence of gB/gH/gL will be of value in the structural definition of this tripartite complex. The 2B10 epitope may also represent a valuable tool for diagnostics to monitor infections/reinfections with different HCMV strains during pregnancy or after transplantation. IMPORTANCE HCMV infections are life threatening to people with compromised or immature immune systems. Understanding the antiviral antibody repertoire induced during HCMV infection is a necessary prerequisite to define protective antibody responses. Here, we report three novel anti-gH MAbs that potently neutralized HCMV infectivity. One of these MAbs (2B10) targets a novel strain-specific conformational epitope on gH that only becomes accessible upon coexpression of the minimal fusion machinery gB/gH/gL. Strain specificity is dependent on a single amino acid polymorphism within gH. Our data highlight the importance of strain-specific neutralizing antibody responses against HCMV. The 2B10 epitope may also represent a valuable tool for diagnostics to monitor infections/reinfections with different HCMV strains during pregnancy or after transplantation. In addition, the dependence of the reactivity of 2B10 on the simultaneous presence of gB/gH/gL will be of value in the structural definition of this tripartite complex.

In Vitro Viral Evolution Identifies a Critical Residue in the Alphaherpesvirus Fusion Glycoprotein B Ectodomain That Controls gH/gL-Independent Entry

Herpesvirus entry and spread requires fusion of viral and host cell membranes, which is mediated by the conserved surface glycoprotein B (gB). Upon activation, gB undergoes a major conformational change and transits from a metastable prefusion to a stable postfusion conformation. Although gB is a structural homolog of low-pH-triggered class III fusogens, its fusion activity depends strictly on the presence of the conserved regulatory gH/gL complex and nonconserved receptor binding proteins, which ensure that fusion occurs at the right time and space. How gB maintains its prefusion conformation and how gB fusogenicity is controlled remain poorly understood. Here, we report the isolation and characterization of a naturally selected pseudorabies virus (PrV) gB able to mediate efficient gH/gL-independent virus-cell and cell-cell fusion. We found that the control exerted on gB by the accompanying viral proteins is mediated via its cytosolic domain (CTD). Whereas gB variants lacking the CTD are inactive, a single mutation of a conserved asparagine residue in an alpha-helical motif of the ectodomain recently shown to be at the core of the gB prefusion trimer compensated for CTD absence and uncoupled gB from regulatory viral proteins, resulting in a hyperfusion phenotype. This phenotype was transferred to gB homologs from different alphaherpesvirus genera. Overall, our data propose a model in which the central helix acts as a molecular switch for the gB pre-to-postfusion transition by conveying the structural status of the endo- to the ectodomain, thereby governing their cross talk for fusion activation, providing a new paradigm for herpesvirus fusion regulation.

In silico analysis of antiviral phytochemicals efficacy against Epstein-Barr virus glycoprotein H

Epstein-Barr virus is a tumor-associated, enveloped virus with glycoprotein receptor gHgL on its surface. gH attaches to epithelial or B cells and mediates internalization. Till date, no specific anti-EBV FDA approved drug is available. Targeting gH may aid in designing virus-specific therapeutics and reducing the drug induced complications in host. We investigated the influence of antiviral phytochemicals on gH using computational approaches. Through molecular docking, we performed binding energy analysis of cellocidin, bruceantin, EGCG, formononetin and sesquiterpene lactones with gH DII/DIII interface, crucial for gH functions. Further, to cause any perturbations in the protein function, the molecules must bind stably to gH. Bruceantin and EGCG interacted with high affinities to gH. Simulation of these two molecules revealed stable binding with gH throughout 100 ns moreover, van der Waal interactions stabilized overall binding. Mutation of amino acids like V265, L269, L315, I423, I459, L474 and F475 involved in stable binding to gH was predicted deleterious to protein function. We obtained no difference in RMSD between these two ligands and minor deviations in the RMSF were noticed compared to gH. Conclusively, our study provided insights into the potential of bruceantin and EGCG to target gH. Different amino acids are involved in binding of each ligand to gH, engagement of certain amino acids may affect the virus binding with epithelial or B cells. The interaction of the ligand with gH may trap it in its native conformation or induce structural flexibility thereby inhibiting the interaction with host receptors or other glycoproteins.Communicated by Ramaswamy H. Sarma.

Influence of N-glycosylation on Expression and Function of Pseudorabies Virus Glycoprotein gB

Envelope glycoprotein (g)B is conserved throughout the Herpesviridae and mediates fusion of the viral envelope with cellular membranes for infectious entry and spread. Like all viral envelope fusion proteins, gB is modified by asparagine (N)-linked glycosylation. Glycans can contribute to protein function, intracellular transport, trafficking, structure and immune evasion. gB of the alphaherpesvirus pseudorabies virus (PrV) contains six consensus sites for N-linked glycosylation, but their functional relevance is unknown. Here, we investigated the occupancy and functional relevance of N-glycosylation sites in PrV gB. To this end, all predicted N-glycosylation sites were inactivated either singly or in combination by the introduction of conservative mutations (N➔Q). The resulting proteins were tested for expression, fusion activity in cell-cell fusion assays and complementation of a gB-deficient PrV mutant. Our results indicate that all six sites are indeed modified. However, while glycosylation at most sites was dispensable for gB expression and fusogenicity, inactivation of N154 and N700 affected gB processing by furin cleavage and surface localization. Although all single mutants were functional in cell-cell fusion and viral entry, simultaneous inactivation of all six N-glycosylation sites severely impaired fusion activity and viral entry, suggesting a critical role of N-glycans for maintaining gB structure and function.

Virus-Mediated Cell-Cell Fusion

Cell-cell fusion between eukaryotic cells is a general process involved in many physiological and pathological conditions, including infections by bacteria, parasites, and viruses. As obligate intracellular pathogens, viruses use intracellular machineries and pathways for efficient replication in their host target cells. Interestingly, certain viruses, and, more especially, enveloped viruses belonging to different viral families and including human pathogens, can mediate cell-cell fusion between infected cells and neighboring non-infected cells. Depending of the cellular environment and tissue organization, this virus-mediated cell-cell fusion leads to the merge of membrane and cytoplasm contents and formation of multinucleated cells, also called syncytia, that can express high amount of viral antigens in tissues and organs of infected hosts. This ability of some viruses to trigger cell-cell fusion between infected cells as virus-donor cells and surrounding non-infected target cells is mainly related to virus-encoded fusion proteins, known as viral fusogens displaying high fusogenic properties, and expressed at the cell surface of the virus-donor cells. Virus-induced cell-cell fusion is then mediated by interactions of these viral fusion proteins with surface molecules or receptors involved in virus entry and expressed on neighboring non-infected cells. Thus, the goal of this review is to give an overview of the different animal virus families, with a more special focus on human pathogens, that can trigger cell-cell fusion.

Localization of the Interaction Site of Herpes Simplex Virus Glycoprotein D (gD) on the Membrane Fusion Regulator, gH/gL

A cascade of protein-protein interactions between four herpes simplex virus (HSV) glycoproteins (gD, gH/gL, and gB) drive fusion between the HSV envelope and host membrane, thereby allowing for virus entry and infection. Specifically, binding of gD to one of its receptors induces a conformational change that allows gD to bind to the regulatory complex gH/gL, which then activates the fusogen gB, resulting in membrane fusion. Using surface plasmon resonance and a panel of anti-gD monoclonal antibodies (MAbs) that sterically blocked the interaction, we previously showed that gH/gL binds directly to gD at sites distinct from the gD receptor binding site. Here, using an analogous strategy, we first evaluated the ability of a panel of uncharacterized anti-gH/gL MAbs to block binding to gD and/or inhibit fusion. We found that the epitopes of four gD-gH/gL-blocking MAbs were located within flexible regions of the gH N terminus and the gL C terminus, while the fifth was placed around gL residue 77. Taken together, our data localized the gD binding region on gH/gL to a group of gH and gL residues at the membrane distal region of the heterodimer. Surprisingly, a second set of MAbs did not block gD-gH/gL binding but instead stabilized the complex by altering the kinetic binding. However, despite this prolonged gD-gH/gL interaction, "stabilizing" MAbs also inhibited cell-cell fusion, suggesting a unique mechanism by which the fusion process is halted. Our findings support targeting the gD-gH/gL interaction to prevent fusion in both therapeutic and vaccine strategies against HSV.IMPORTANCE Key to developing a human HSV vaccine is an understanding of the virion glycoproteins involved in entry. HSV employs multiple glycoproteins for attachment, receptor interaction, and membrane fusion. Determining how these proteins function was resolved, in part, by structural biology coupled with immunological and biologic evidence. After binding, virion gD interacts with a receptor to activate the regulator gH/gL complex, triggering gB to drive fusion. Multiple questions remain, one being the physical location of each glycoprotein interaction site. Using protective antibodies with known epitopes, we documented the long-sought interaction between gD and gH/gL, detailing the region on gD important to create the gD-gH/gL triplex. Now, we have identified the corresponding gD contact sites on gH/gL. Concurrently we discovered a novel mechanism whereby gH/gL antibodies stabilize the complex and inhibit fusion progression. Our model for the gD-gH/gL triplex provides a new framework for studying fusion, which identifies targets for vaccine development.

Tetrameric glycoprotein complex gH/gL/gQ1/gQ2 is a promising vaccine candidate for human herpesvirus 6B

Primary infection of human herpesvirus 6B (HHV-6B) occurs in infants after the decline of maternal immunity and causes exanthema subitum accompanied by a high fever, and it occasionally develops into encephalitis resulting in neurological sequelae. There is no effective prophylaxis for HHV-6B, and its development is urgently needed. The glycoprotein complex gH/gL/gQ1/gQ2 (called 'tetramer of HHV-6B') on the virion surface is a viral ligand for its cellular receptor human CD134, and their interaction is thus essential for virus entry into the cells. Herein we examined the potency of the tetramer as a vaccine candidate against HHV-6B. We designed a soluble form of the tetramer by replacing the transmembrane domain of gH with a cleavable tag, and the tetramer was expressed by a mammalian cell expression system. The expressed recombinant tetramer is capable of binding to hCD134. The tetramer was purified to homogeneity and then administered to mice with aluminum hydrogel adjuvant and/or CpG oligodeoxynucleotide adjuvant. After several immunizations, humoral and cellular immunity for HHV-6B was induced in the mice. These results suggest that the tetramer together with an adjuvant could be a promising candidate HHV-6B vaccine.

Human herpesvirus 6B (HHV-6B) is known as the cause of the common childhood febrile illness exanthem subitum in its primary infection, and it develops into a lifelong latent infection in almost all individuals. Severe complications such as meningitis and encephalitis can occur in both the primary infection and reactivation. There is no established treatment or vaccine. The tetrameric glycoprotein complex gH/gL/gQ1/gQ2 (tetramer) on the viral envelope is the ligand for the entry of HHV-6B, which is the critical part for its infection. Here, we established a soluble form of the tetramer and purified it to homogeneity. After several immunizations of tetramer along with different combinations of adjuvants in mice, we observed that it greatly induced defensive immunity against HHV-6B, indicating that the tetramer has the potential to become a vaccine candidate. Moreover, our results also revealed that combinations of distinct adjuvants with the tetramer would be useful as an HHV-6B vaccine strategy for different purposes.

Expression, Purification, and Crystallization of HSV-1 Glycoproteins for Structure Determination

Herpes simplex viruses utilize glycoproteins displayed on the viral envelope to perform a variety of functions in the viral infectious cycle. Structural and functional studies of these viral glycoproteins can benefit from biochemical, biophysical, and structural analysis of purified proteins. Here, we describe a general protocol for expression and purification of viral glycoproteins from insect cells based on those developed for the HSV-1 gB and HSV-2 gH/gL ectodomains as well as the protocol for crystallization of these glycoproteins. This protocol can be used for generating milligram amounts of wild-type (WT) or mutant gB and gH/gL ectodomains or can be adapted to produce purified ectodomains of glycoproteins from HSV or other herpesviruses for biochemical and structural studies.

Expression, Purification, and Crystallization of Full-Length HSV-1 gB for Structure Determination

HSV glycoproteins play important roles in the viral life cycle, particularly viral cell entry. Here we describe the protocol for expression, purification, and crystallization of full-length HSV-1 glycoprotein B. The protocol provides a framework for incorporating transmembrane domain-stabilizing amphipols into the crystallization setup and can be adapted to isolate other complete HSV glycoproteins.

Distribution of the CMV glycoprotein gH/gL/gO and gH/gL/pUL128/pUL130/pUL131A complex variants and associated clinical manifestations in infants infected congenitally or postnatally

Human cytomegalovirus (CMV) is a major cause of morbidity in fetuses following intrauterine infection. The glycoprotein (g) envelope trimeric gH/gL/gO and pentameric gH/gL/pUL128/pUL130/pUL131A complexes are required for CMV entry into fibroblasts and endothelial/epithelial cells, respectively, and both are targets for neutralizing antibodies. The role of sequence variability among viral strains in the outcome of congenital CMV infection is controversial. Variation in the CMV UL75 gene encoding glycoprotein H (gH), the UL115 (gL), the UL74 (gO), and the UL128 locus (UL128L) encoding three structural proteins (pUL128, pUL130, and pUL131A) was determined in 82 newborns with congenital CMV infection and 113 infants with postnatal or unproven congenital CMV infection. Genotyping was performed by sequencing analysis of PCR-amplified fragments and the PCR-restriction fragment length polymorphism (RFLP) method, and the viral load was measured by quantitative real-time PCR. The obtained results demonstrated that (1) different CMV variants and mixed CMV infections can be detected in newborns infected congenitally; (2) the gH1 genotype, UL130 variant 6, and UL131A variant 1 were associated with some signs/symptoms within cohort of pediatric patients, mainly consisting of infants with symptomatic CMV infection. The results revealed that pUL130, pUL131A, and gH polymorphisms seemed to be associated with the outcome of CMV infection in infants.

Gammaherpesvirus entry and fusion: A tale how two human pathogenic viruses enter their host cells

The prototypical human γ-herpesviruses Epstein-Barr virus (EBV) and Kaposi Sarcoma-associated herpesvirus (KSHV) are involved in the development of malignancies. Like all herpesviruses, they share the establishment of latency, the typical architecture, and the conserved fusion machinery to initiate infection. The fusion machinery reflects virus-specific adaptations due to the requirements of the respective herpesvirus. For example, EBV evolved a tropism switch involving either the B- or epithelial cell-tropism complexes to activate fusion driven by gB. Most of the EBV entry proteins and their cellular receptors have been crystallized providing molecular details of the initial steps of infection. For KSHV, a variety of entry and binding receptors has also been reported but the mechanism how receptor binding activates gB-driven fusion is not as well understood as that for EBV. However, the downstream signaling pathways that promote the early steps of KSHV entry are well described. This review summarizes the current knowledge of the key players involved in EBV and KSHV entry and the cell-type specific mechanisms that allow infection of a wide variety of cell types.

Pathogen at the Gates: Human Cytomegalovirus Entry and Cell Tropism

The past few years have brought substantial progress toward understanding how human cytomegalovirus (HCMV) enters the remarkably wide spectrum of cell types and tissues that it infects. Neuropilin-2 and platelet-derived growth factor receptor alpha (PDGFRα) were identified as receptors, respectively, for the trimeric and pentameric glycoprotein H/glycoprotein L (gH/gL) complexes that in large part govern HCMV cell tropism, while CD90 and CD147 were also found to play roles during entry. X-ray crystal structures for the proximal viral fusogen, glycoprotein B (gB), and for the pentameric gH/gL complex (pentamer) have been solved. A novel virion gH complex consisting of gH bound to UL116 instead of gL was described, and findings supporting the existence of a stable complex between gH/gL and gB were reported. Additional work indicates that the pentamer promotes a mode of cell-associated spread that resists antibody neutralization, as opposed to the trimeric gH/gL complex (trimer), which appears to be broadly required for the infectivity of cell-free virions. Finally, viral factors such as UL148 and US16 were identified that can influence the incorporation of the alternative gH/gL complexes into virions. We will review these advances and their implications for understanding HCMV entry and cell tropism.

Natural Selection of Glycoprotein B Mutations That Rescue the Small-Plaque Phenotype of a Fusion-Impaired Herpes Simplex Virus Mutant

Glycoprotein B (gB) is a conserved viral fusion protein that is required for herpesvirus entry. To mediate fusion with the cellular membrane, gB refolds from a prefusion to a postfusion conformation. We hypothesize that an interaction between the C-terminal arm and the central coiled coil of the herpes simplex virus 1 (HSV-1) gB ectodomain is critical for fusion. We previously reported that three mutations in the C-terminal arm (I671A/H681A/F683A, called gB3A) greatly reduced cell-cell fusion and that virus carrying these mutations had a small-plaque phenotype and delayed entry into cells. By serially passaging gB3A virus, we selected three revertant viruses with larger plaques. These revertant viruses acquired mutations in gB that restore the fusion function of gB3A, including gB-A683V, gB-S383F/G645R/V705I/A855V, and gB-T509M/N709H. V705I and N709H are novel mutations that map to the portion of domain V that enters domain I in the postfusion structure. S383F, G645R, and T509M are novel mutations that map to an intersection of three domains in a prefusion model of gB. We introduced these second-site mutations individually and in combination into wild-type gB and gB3A to examine the impact of the mutations on fusion and expression. V705I and A855V (a known hyperfusogenic mutation) restored the fusion function of gB3A, whereas S383F and G645R dampened fusion and T509M and N709H worked in concert to restore gB3A fusion. The results identify two regions in the gB ectodomain that modulate the fusion activity of gB, potentially by impacting intramolecular interactions and stability of the prefusion and/or postfusion gB trimer.IMPORTANCE Glycoprotein B (gB) is an essential viral protein that is conserved in all herpesviruses and is required for virus entry. gB is thought to undergo a conformational change that provides the energy to fuse the viral and cellular membranes; however, the details of this conformational change and the structure of the prefusion and intermediate conformations of gB are not known. Previously, we demonstrated that mutations in the gB "arm" region inhibit fusion and impart a small-plaque phenotype. Using serial passage of a virus carrying these mutations, we identified revertants with restored plaque size. The revertant viruses acquired novel mutations in gB that restored fusion function and mapped to two sites in the gB ectodomain. This work supports our hypothesis that an interaction between the gB arm and the core of gB is critical for gB refolding and provides details about the function of gB in herpesvirus-mediated fusion and subsequent virus entry.

From recognition to execution-the HCMV Pentamer from receptor binding to fusion triggering

The β-herpesvirus human cytomegalovirus (HCMV) is the leading viral cause of neonatal developmental disabilities. In HCMV, the conserved herpesvirus glycoprotein B (gB) mediates membrane fusion between the viral and host cell membranes, whereas the trimeric gH/gL/gO or the pentameric gH/gL/UL128/UL130/UL31A complexes (Pentamer) bind to cell-specific receptors and provide the triggering signal to gB. Recent structural and functional studies have provided new insights into Pentamer structure, conformational flexibility, location of epitopes for neutralizing antibodies and potential binding sites for cell surface receptors. Together, these data suggest a model where receptor binding triggers a conformational change in Pentamer, allowing it to interact with gB and initiate the membrane fusion process.

An Antibody Targeting the Fusion Machinery Neutralizes Dual-Tropic Infection and Defines a Site of Vulnerability on Epstein-Barr Virus

Epstein-Barr virus (EBV) is a causative agent of infectious mononucleosis and is associated with 200,000 new cases of cancer and 140,000 deaths annually. Subunit vaccines against this pathogen have focused on the gp350 glycoprotein and remain unsuccessful. We isolated human antibodies recognizing the EBV fusion machinery (gH/gL and gB) from rare memory B cells. One anti-gH/gL antibody, AMMO1, potently neutralized infection of B cells and epithelial cells, the two major cell types targeted by EBV. We determined a cryo-electron microscopy reconstruction of the gH/gL-gp42-AMMO1 complex and demonstrated that AMMO1 bound to a discontinuous epitope formed by both gH and gL at the Domain-I/Domain-II interface. Integrating structural, biochemical, and infectivity data, we propose that AMMO1 inhibits fusion of the viral and cellular membranes. This work identifies a crucial epitope that may aid in the design of next-generation subunit vaccines against this major public health burden.

Global sensing of the antigenic structure of herpes simplex virus gD using high-throughput array-based SPR imaging

While HSV-2 typically causes genital lesions, HSV-1 is increasingly the cause of genital herpes. In addition, neonatal HSV infections are associated with a high rate of mortality and HSV-2 may increase the risk for HIV or Zika infections, reinforcing the need to develop an effective vaccine. In the GSK Herpevac trial, doubly sero-negative women were vaccinated with a truncated form of gD2 [gD2(284t)], then examined for anti-gD serum titers and clinical manifestations of disease. Surprisingly, few vaccinees were protected against genital HSV-2 but 86% were protected from genital HSV-1. These observations suggest that subtle differences in gD structure might influence a protective response. To better understand the antigenic structure of gD and how it impacts a protective response, we previously utilized several key anti-gD monoclonal antibodies (mAbs) to dissect epitopes in vaccinee sera. Several correlations were observed but the methodology limited the number of sera and mAbs that could be tested. Here, we used array-based surface plasmon imaging (SPRi) to simultaneously measure a larger number of protein-protein interactions. We carried out cross-competition or "epitope binning" studies with 39 anti-gD mAbs and four soluble forms of gD, including a form [gD2(285t)] that resembles the Herpevac antigen. The results from these experiments allowed us to organize the mAbs into four epitope communities. Notably, relationships within and between communities differed depending on the form of gD, and off-rate analysis suggested differences in mAb-gD avidity depending on the gD serotype and length. Together, these results show that gD1 and gD2 differ in their structural topography. Consistent with the Herpevac results, several mAbs that bind both gD1 and gD2 neutralize only HSV-1. Thus, this technology provides new insights into the antigenic structure of gD and provides a rationale as to how vaccination with a gD2 subunit may lead to protection from HSV-1 infection.

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.

Structure-Based Mutations in the Herpes Simplex Virus 1 Glycoprotein B Ectodomain Arm Impart a Slow-Entry Phenotype

Glycoprotein B (gB) is the conserved herpesvirus fusion protein, and it is required for the entry of herpesviruses. The structure of the postfusion conformation of gB has been solved for several herpesviruses; however, the gB prefusion crystal structure and the details of how the protein refolds from a prefusion to a postfusion form to mediate fusion have not been determined. Using structure-based mutagenesis, we previously reported that three mutations (I671A, H681A, and F683A) in the C-terminal arm of the gB ectodomain greatly reduced cell-cell fusion. This fusion deficit could be rescued by the addition of a hyperfusogenic mutation, suggesting that the gB triple mutant was not misfolded. Using a bacterial artificial chromosome (BAC), we constructed two independent herpes simplex virus 1 mutant strains (gB 3A) carrying the three arm mutations. The gB 3A viruses have 200-fold smaller plaques than the wild-type virus and demonstrate remarkably delayed entry into cells. Single-step and multistep growth curves show that gB 3A viruses have delayed replication kinetics. Interestingly, incubation at 40°C promoted the entry of the gB 3A viruses. We propose that the gB 3A viruses' entry deficit is due to a loss of interactions between residues in the gB C-terminal arm and the coiled-coil core of gB. The results suggest that the triple alanine mutation may destabilize the postfusion gB conformation and/or stabilize the prefusion gB conformation and that exposure to elevated temperatures can overcome the defect in gB 3A viruses.IMPORTANCE Because of its complexity, the mechanism of herpesvirus entry into cells is not well understood. Our study investigated one of the most important unanswered questions about herpesvirus entry; i.e., how does the herpesvirus fusion protein gB mediate membrane fusion? gB is an essential protein that is conserved in all herpesviruses and is thought to undergo a conformational change to provide the energy to fuse the viral and cellular membranes. Using our understanding of the structure of gB, we designed mutations in the gB "arm" region that we predicted would impede gB function. We introduced these mutations into herpes simplex virus 1 by using a bacterial artificial chromosome, and the mutant virus exhibited a drastically delayed rate of entry. This entry defect was rescued by incubation at elevated temperatures, supporting a hypothesis that the engineered mutations altered the energetics of gB refolding. This study supports our hypothesis that an interaction between the gB arm and the core of gB is critical for gB refolding and the execution of membrane fusion, thus providing key details about the function of gB in herpesvirus-mediated fusion and subsequent virus entry.

Infectivity inhibition by overlapping synthetic peptides derived from the gH/gL heterodimer of herpes simplex virus type 1

Herpes simplex virus (HSV) is a human pathogen that infects epithelial cells. The cutaneous lesions, caused by the virus, spread to the nervous system creating several complications. Fusion of host membranes with the viral envelope is mandatory and mediated by a group of glycoproteins conserved in all Herpesviridae subfamilies, such as the glycoproteins B (gB), H (gH), L (gL) and D (gD). We investigated the inhibitory activity mediated by synthetic overlapping peptides spanning the entire ectodomains of gH and gL glycoproteins. We have performed a brute analysis of the complete gH/gL heterodimer in order to explore the inhibitory activity of peptides modelled on these glycoproteins against HSV‐1 infection. Twenty‐four of the gH peptides at a concentration of 150 μM reached the 50% of inhibition cut‐off. Interestingly, they are mainly located in the gH carboxy‐terminal domain. None of the gL peptides had a clear inhibiting effect. No peptide toxicity was observed by lactate dehydrogenase assay at the concentrations used in our experimental conditions. HSV‐1 therapy is based on acyclovir treatment, but some resistant strains are emerging. In this scenario, innovative approaches for HSV‐1 treatment are necessary. Our data support the direct involvement of the described domains in the process of virus penetration; therefore, these results are of relevance to the potential development of novel therapeutic compounds to prevent HSV‐1 infections. Copyright © 2017 European Peptide Society and John Wiley & Sons, Ltd.

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.

Regulation of Herpes Simplex Virus Glycoprotein-Induced Cascade of Events Governing Cell-Cell Fusion

Receptor-dependent herpes simplex virus (HSV)-induced cell-cell fusion requires glycoproteins gD, gH/gL, and gB. Our current model posits that during fusion, receptor-activated conformational changes in gD activate gH/gL, which subsequently triggers the transformation of the prefusion form of gB into a fusogenic state. To examine the role of each glycoprotein in receptor-dependent cell-cell fusion, we took advantage of our discovery that fusion by wild-type herpes simplex virus 2 (HSV-2) glycoproteins occurs twice as fast as that achieved by HSV-1 glycoproteins. By sequentially swapping each glycoprotein between the two serotypes, we established that fusion speed was governed by gH/gL, with gH being the main contributor. While the mutant forms of gB fuse at distinct rates that are dictated by their molecular structure, these restrictions can be overcome by gH/gL of HSV-2 (gH2/gL2), thereby enhancing their activity. We also found that deregulated forms of gD of HSV-1 (gD1) and gH2/gL2 can alter the fusogenic potential of gB, promoting cell fusion in the absence of a cellular receptor, and that deregulated forms of gB can drive the fusion machinery to even higher levels. Low pH enhanced fusion by affecting the structure of both gB and gH/gL mutants. Together, our data highlight the complexity of the fusion machinery, the impact of the activation state of each glycoprotein on the fusion process, and the critical role of gH/gL in regulating HSV-induced fusion. IMPORTANCE Cell-cell fusion mediated by HSV glycoproteins requires gD, gH/gL, gB, and a gD receptor. Here, we show that fusion by wild-type HSV-2 glycoproteins occurs twice as fast as that achieved by HSV-1 glycoproteins. By sequentially swapping each glycoprotein between the two serotypes, we found that the fusion process was controlled by gH/gL. Restrictions imposed on the gB structure by mutations could be overcome by gH2/gL2, enhancing the activity of the mutants. Under low-pH conditions or when using deregulated forms of gD1 and gH2/gL2, the fusogenic potential of gB could only be increased in the absence of receptor, underlining the exquisite regulation that occurs in the presence of receptor. Our data highlight the complexity of the fusion machinery, the impact of the activation state of each glycoprotein on the fusion process, and the critical role of gH/gL in regulating HSV-induced fusion.

Integrins as Herpesvirus Receptors and Mediators of the Host Signalosome

The repertoire of herpesvirus receptors consists of nonintegrin and integrin molecules. Integrins interact with the conserved glycoproteins gH/gL or gB. This interaction is a conserved biology across the Herpesviridae family, likely directed to promote virus entry and endocytosis. Herpesviruses exploit this interaction to execute a range of critical functions that include (a) relocation of nonintegrin receptors (e.g., herpes simplex virus nectin1 and Kaposi's sarcoma-associated herpesvirus EphA2), or association with nonintegrin receptors (i.e., human cytomegalovirus EGFR), to dictate species-specific entry pathways; (b) activation of multiple signaling pathways (e.g., Ca²⁺ release, c-Src, FAK, MAPK, and PI3K); and (c) association with Rho GTPases, tyrosine kinase receptors, Toll-like receptors, which result in cytoskeletal remodeling, differential cell type targeting, and innate responses. In turn, integrins can be modulated by viral proteins (e.g., Epstein-Barr virus LMPs) to favor spread of transformed cells. We propose that herpesviruses evolved a multipartite entry system to allow interaction with multiple receptors, including integrins, required for their sophisticated life cycle.

Tetraspanin CD151 Promotes Initial Events in Human Cytomegalovirus Infection

Human cytomegalovirus (HCMV), a betaherpesvirus, can cause life-threatening disease in immunocompromised individuals. Viral envelope glycoproteins that mediate binding to and penetration into target cells have been identified previously. In contrast, cellular proteins supporting HCMV during entry are largely unknown. In order to systematically identify host genes affecting initial steps of HCMV infection, a targeted RNA interference screen of 96 cellular genes was performed in endothelial cells by use of a virus strain expressing the full set of known glycoprotein H and L (gH/gL) complexes. The approach yielded five proviral host factors from different protein families and eight antiviral host factors, mostly growth factor receptors. The tetraspanin CD151 was uncovered as a novel proviral host factor and was analyzed further. Like endothelial cells, fibroblasts were also less susceptible to HCMV infection after CD151 depletion. Virus strains with different sets of gH/gL complexes conferring either broad or narrow cell tropism were equally impaired. Infection of CD151-depleted cells by a fluorescent virus with differentially labeled capsid and envelope proteins revealed a role of CD151 in viral penetration but not in adsorption to the cell. In conclusion, the tetraspanin CD151 has emerged as a novel host factor in HCMV entry and as a putative antiviral target.

IMPORTANCE

At present, the events at the virus-cell interface and the cellular proteins involved during the HCMV entry steps are scarcely understood. In this study, several host factors with putative roles in this process were identified. The tetraspanin CD151 was discovered as a previously unrecognized proviral host factor for HCMV and was found to support viral penetration into the target cells. The findings of this study shed light on the cellular contribution during the initial steps of HCMV infection and open a new direction in HCMV research.

HVint: A Strategy for Identifying Novel Protein-Protein Interactions in Herpes Simplex Virus Type 1

Human herpesviruses are widespread human pathogens with a remarkable impact on worldwide public health. Despite intense decades of research, the molecular details in many aspects of their function remain to be fully characterized. To unravel the details of how these viruses operate, a thorough understanding of the relationships between the involved components is key. Here, we present HVint, a novel protein-protein intraviral interaction resource for herpes simplex virus type 1 (HSV-1) integrating data from five external sources. To assess each interaction, we used a scoring scheme that takes into consideration aspects such as the type of detection method and the number of lines of evidence. The coverage of the initial interactome was further increased using evolutionary information, by importing interactions reported for other human herpesviruses. These latter interactions constitute, therefore, computational predictions for potential novel interactions in HSV-1. An independent experimental analysis was performed to confirm a subset of our predicted interactions. This subset covers proteins that contribute to nuclear egress and primary envelopment events, including VP26, pUL31, pUL40, and the recently characterized pUL32 and pUL21. Our findings support a coordinated crosstalk between VP26 and proteins such as pUL31, pUS9, and the CSVC complex, contributing to the development of a model describing the nuclear egress and primary envelopment pathways of newly synthesized HSV-1 capsids. The results are also consistent with recent findings on the involvement of pUL32 in capsid maturation and early tegumentation events. Further, they open the door to new hypotheses on virus-specific regulators of pUS9-dependent transport. To make this repository of interactions readily accessible for the scientific community, we also developed a user-friendly and interactive web interface. Our approach demonstrates the power of computational predictions to assist in the design of targeted experiments for the discovery of novel protein-protein interactions.

Structural and Mechanistic Insights into the Tropism of Epstein-Barr Virus

Epstein-Barr virus (EBV) is the prototypical γ-herpesvirus and an obligate human pathogen that infects mainly epithelial cells and B cells, which can result in malignancies. EBV infects these target cells by fusing with the viral and cellular lipid bilayer membranes using multiple viral factors and host receptor(s) thus exhibiting a unique complexity in its entry machinery. To enter epithelial cells, EBV requires minimally the conserved core fusion machinery comprised of the glycoproteins gH/gL acting as the receptor-binding complex and gB as the fusogen. EBV can enter B cells using gp42, which binds tightly to gH/gL and interacts with host HLA class II, activating fusion. Previously, we published the individual crystal structures of EBV entry factors, such as gH/gL and gp42, the EBV/host receptor complex, gp42/HLA-DR1, and the fusion protein EBV gB in a postfusion conformation, which allowed us to identify structural determinants and regions critical for receptor-binding and membrane fusion. Recently, we reported different low resolution models of the EBV B cell entry triggering complex (gHgL/gp42/HLA class II) in "open" and "closed" states based on negative-stain single particle electron microscopy, which provide further mechanistic insights. This review summarizes the current knowledge of these key players in EBV entry and how their structures impact receptor-binding and the triggering of gB-mediated 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.

Comparative Mutagenesis of Pseudorabies Virus and Epstein-Barr Virus gH Identifies a Structural Determinant within Domain III of gH Required for Surface Expression and Entry Function

Herpesviruses infect cells using the conserved core fusion machinery composed of glycoprotein B (gB) and gH/gL. The gH/gL complex plays an essential but still poorly characterized role in membrane fusion and cell tropism. Our previous studies demonstrated that the conserved disulfide bond (DB) C278/C335 in domain II (D-II) of Epstein-Barr virus (EBV) gH has an epithelial cell-specific function, whereas the interface of D-II/D-III is involved in formation of the B cell entry complex by binding to gp42. To extend these studies, we compared gH of the alphaherpesvirus pseudorabies virus (PrV) with gH of the gammaherpesvirus EBV to identify functionally equivalent regions critical for gH function during entry. We identified several conserved amino acids surrounding the conserved DB that connects three central helices of D-III of PrV and EBV gH. The present study verified that the conserved DB and several contacting amino acids in D-III modulate cell surface expression and thereby contribute to gH function. In line with this finding, we found that DB C404/C439 and T401 are important for cell-to-cell spread and efficient entry of PrV. This parallel comparison between PrV and EBV gH function brings new insights into how gH structure impacts fusion function during herpesvirus entry.

IMPORTANCE

The alphaherpesvirus PrV is known for its neuroinvasion, whereas the gammaherpesvirus EBV is associated with cancer of epithelial and B cell origin. Despite low amino acid conservation, PrV gH and EBV gH show strikingly similar structures. Interestingly, both PrV gH and EBV gH contain a structural motif composed of a DB and supporting amino acids which is highly conserved within the Herpesviridae. Our study verified that PrV gH uses a minimal motif with the DB as the core, whereas the DB of EBV gH forms extensive connections through hydrogen bonds to surrounding amino acids, ensuring the cell surface expression of gH/gL. Our study verifies that the comparative analysis of distantly related herpesviruses, such as PrV and EBV, allows the identification of common gH functions. In addition, we provide an understanding of how functional domains can evolve over time, resulting in subtle differences in domain structure and function.

EBV glycoproteins: where are we now?

Glycoproteins are critical to virus entry, to spread within and between hosts and can modify the behavior of cells. Many viruses carry only a few, most found in the virion envelope. EBV makes more than 12, providing flexibility in how it colonizes its human host. Some are dedicated to getting the virus through the cell membrane and on toward the nucleus of the cell, some help guide the virus back out and on to the next cell in the same or a new host. Yet others undermine host defenses helping the virus persist for a lifetime, maintaining a presence that is mostly tolerated and serves to perpetuate EBV as one of the most common infections of man.

Soluble Human Cytomegalovirus gH/gL/pUL128-131 Pentameric Complex, but Not gH/gL, Inhibits Viral Entry to Epithelial Cells and Presents Dominant Native Neutralizing Epitopes

Congenital infection of human cytomegalovirus (HCMV) is one of the leading causes of nongenetic birth defects, and development of a prophylactic vaccine against HCMV is of high priority for public health. The gH/gL/pUL128-131 pentameric complex mediates HCMV entry into endothelial and epithelial cells, and it is a major target for neutralizing antibody responses. To better understand the mechanism by which antibodies interact with the epitopes of the gH/gL/pUL128-131 pentameric complex resulting in viral neutralization, we expressed and purified soluble gH/gL/pUL128-131 pentameric complex and gH/gL from Chinese hamster ovary cells to >95% purity. The soluble gH/gL, which exists predominantly as (gH/gL)2 homodimer with a molecular mass of 220 kDa in solution, has a stoichiometry of 1:1 and a pI of 6.0-6.5. The pentameric complex has a molecular mass of 160 kDa, a stoichiometry of 1:1:1:1:1, and a pI of 7.4-8.1. The soluble pentameric complex, but not gH/gL, adsorbs 76% of neutralizing activities in HCMV human hyperimmune globulin, consistent with earlier reports that the most potent neutralizing epitopes for blocking epithelial infection are unique to the pentameric complex. Functionally, the soluble pentameric complex, but not gH/gL, blocks viral entry to epithelial cells in culture. Our results highlight the importance of the gH/gL/pUL128-131 pentameric complex in HCMV vaccine design and emphasize the necessity to monitor the integrity of the pentameric complex during the vaccine manufacturing process.

Epstein-Barr virus infection mechanisms

Epstein-Barr virus (EBV) infection occurs by distinct mechanisms across different cell types. EBV infection of B cells in vitro minimally requires 5 viral glycoproteins and 2 cellular proteins. By contrast, infection of epithelial cells requires a minimum of 3 viral glycoproteins, which are capable of interacting with one or more of 3 different cellular proteins. The full complement of proteins involved in entry into all cell types capable of being infected in vivo is unknown. This review discusses the events that occur when the virus is delivered into the cytoplasm of a cell, the players known to be involved in these events, and the ways in which these players are thought to function.

Assembly and architecture of the EBV B cell entry triggering complex

Epstein-Barr Virus (EBV) is an enveloped double-stranded DNA virus of the gammaherpesvirinae sub-family that predominantly infects humans through epithelial cells and B cells. Three EBV glycoproteins, gH, gL and gp42, form a complex that targets EBV infection of B cells. Human leukocyte antigen (HLA) class II molecules expressed on B cells serve as the receptor for gp42, triggering membrane fusion and virus entry. The mechanistic role of gHgL in herpesvirus entry has been largely unresolved, but it is thought to regulate the activation of the virally-encoded gB protein, which acts as the primary fusogen. Here we study the assembly and function of the reconstituted B cell entry complex comprised of gHgL, gp42 and HLA class II. The structure from negative-stain electron microscopy provides a detailed snapshot of an intermediate state in EBV entry and highlights the potential for the triggering complex to bring the two membrane bilayers into proximity. Furthermore, gHgL interacts with a previously identified, functionally important hydrophobic pocket on gp42, defining the overall architecture of the complex and playing a critical role in membrane fusion activation. We propose a macroscopic model of the initiating events in EBV B cell fusion centered on the formation of the triggering complex in the context of both viral and host membranes. This model suggests how the triggering complex may bridge the two membrane bilayers, orienting critical regions of the N- and C- terminal ends of gHgL to promote the activation of gB and efficient membrane fusion.

Mechanism for neutralizing activity by the anti-CMV gH/gL monoclonal antibody MSL-109

Cytomegalovirus (CMV) is a widespread opportunistic pathogen that causes birth defects when transmitted transplacentally and severe systemic illness in immunocompromised individuals. MSL-109, a human monoclonal IgG isolated from a CMV seropositive individual, binds to the essential CMV entry glycoprotein H (gH) and prevents infection of cells. Here, we suggest a mechanism for neutralization activity by MSL-109. We define a genetic basis for resistance to MSL-109 and have generated a structural model of gH that reveals the epitope of this neutralizing antibody. Using surface-based, time-resolved FRET, we demonstrate that gH/gL interacts with glycoprotein B (gB). Additionally, we detect homodimers of soluble gH/gL heterodimers and confirm this novel oligomeric assembly on full-length gH/gL expressed on the cell surface. We show that MSL-109 perturbs the dimerization of gH/gL:gH/gL, suggesting that dimerization of gH/gL may be required for infectivity. gH/gL homodimerization may be conserved between alpha- and betaherpesviruses, because both CMV and HSV gH/gL demonstrate self-association in the FRET system. This study provides evidence for a novel mechanism of action for MSL-109 and reveals a previously undescribed aspect of viral entry that may be susceptible to therapeutic intervention.

Expression, purification, and crystallization of HSV-1 glycoproteins for structure determination

HSV glycoproteins play important roles in the viral infectious cycle, particularly viral entry into the cell. Here we describe the protocol for expression, purification, and crystallization of viral glycoproteins based on those developed for the HSV-1 gB and HSV-2 gH/gL ectodomains. These protocols can be used for generating milligram amounts of wild-type (WT) or mutant gB and gH/gL ectodomains or can be adapted to produce purified ectodomains of other HSV glycoproteins for biochemical and structural studies.

Extensive mutagenesis of the HSV-1 gB ectodomain reveals remarkable stability of its postfusion form

Viral fusogens mediate the merger of the viral envelope and cellular membrane during viral entry. These proteins share little sequence similarity but all are thought to act by refolding through a series of conformational intermediates from the metastable prefusion form to the stable postfusion form. Crystal structures of both prefusion and postfusion forms have illuminated the conformational pathways of several viral fusogens. By contrast, only the structure of the postfusion form is available for glycoprotein B (gB), the conserved fusogen of herpesviruses. To gain insight into the nature of the fusogenic conformational changes in gB, we used several approaches aimed at engineering the prefusion form of the herpes simplex virus type 1 gB ectodomain, including modifications intended to stabilize the prefusion form and novel mutations aimed at destabilizing the postfusion form. We found that the postfusion conformation of gB is remarkably stable and resistant to perturbations. Several mutations successfully destabilized the gB trimer, identifying regions that are critical for the stability of the postfusion form. Yet, none of the constructs adopted the prefusion conformation. We propose that the soluble ectodomain of gB folds into the postfusion form without first adopting the prefusion intermediate. These results suggest that other regions of gB, including the transmembrane region and the cytoplasmic domain, may be necessary to establish and maintain the metastable prefusion conformation.