The KGD motif of Epstein-Barr virus gH/gL is bifunctional, orchestrating infection of B cells and epithelial cells

EBV-positive glycoproteins associated with nasopharyngeal carcinoma

Nasopharyngeal carcinoma (NPC) is closely related to Epstein-Barr virus (EBV) infection, and glycosylation of proteins is associated with precancerous lesions and carcinogenesis of NPC, and viral glycoproteins mediates the fusion of viruses with B cells or epithelial cells in the infection stage, promoting the conversion of normal epithelial cells into cancer cells. In the process of occurrence and development of NPC, various glycoproteins in the body promote or inhibit the proliferation, invasion, metastasis, and drug resistance of tumor cells, such as the tumor inhibitory effect of NGX6 and inhibin B (INHBB); the cancer-promoting effect of tenascin-C (TNC), fibronectin 1 (FN1), insulin-like growth factor binding protein-3 (IGFBP3), serglycin, and its core protein; and some effects of glycosylation of immune proteins on immunotherapy in NPC. This article provides an overview of the research progress on the interaction of glycoproteins associated with EBV infection with the occurrence and development of NPC.

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.

Molecular mimicry of host short linear motif-mediated interactions utilised by viruses for entry

Viruses are obligate intracellular parasites that depend on host cellular machinery for performing even basic biological functions. One of the many ways they achieve this is through molecular mimicry, wherein the virus mimics a host sequence or structure, thereby being able to hijack the host's physiological interactions for its pathogenesis. Such adaptations are specific recognitions that often confer tissue and species-specific tropisms to the virus, and enable the virus to utilise previously existing host signalling networks, which ultimately aid in further steps of viral infection, such as entry, immune evasion and spread. A common form of sequence mimicry utilises short linear motifs (SLiMs). SLiMs are short-peptide sequences that mediate transient interactions and are major elements in host protein interaction networks. This work is aimed at providing a comprehensive review of current literature of some well-characterised SLiMs that play a role in the attachment and entry of viruses into host cells, which mimic physiological receptor-ligand interactions already present in the host. Considering recent trends in emerging diseases, further research on such motifs involved in viral entry can help in the discovery of previously unknown cellular receptors utilised by viruses, as well as help in the designing of targeted therapeutics such as vaccines or inhibitors directed towards these interactions.

New insight into Epstein-Barr virus infection using models of stratified epithelium

Epstein-Barr virus (EBV) is a ubiquitous human pathogen that is transmitted in saliva. EBV transits through the oral epithelium to infect B cells, where it establishes a life-long latent infection. Reinfection of the epithelium is believed to be mediated by virus shed from B cells, but whether a latent reservoir can exist in the epithelia is unknown. We previously developed an in vitro organotypic model of stratified epithelium where EBV can readily replicate within the suprabasal layers of the epithelium following apical infection mediated by virus-producing B cells. Given that infected epithelial cells and cell-free virus are observed in saliva, we examined the ability of both of these to mediate infection in organotypic cultures. Epithelial-derived cell-free virus was able to infect organotypic cultures from the apical surface, but showed enhanced infection of B cells. Conversely, B cell-derived virus exhibited enhanced infection of epithelial cells. While EBV has been detected in basal cells in oral hairy leukoplakia, it is unknown whether EBV can be seen in undifferentiated primary keratinocytes in the basal layer. Undifferentiated epithelial cells expressed proposed EBV receptors in monolayer and were susceptible to viral binding and entry. Integrins, and occasionally ephrin A2, were expressed in the basal layer of gingiva and tonsil derived organotypic cultures, but the known B-cell receptors HLAII and CD21 were not detected. Following infection with cell-free virus or virus-producing B cells at either the apical or basolateral surface of preformed organotypic cultures, abundant infection was detected in differentiated suprabasal cells while more limited but readily detectable infection was observed in the undifferentiated basal cells. Together, our data has provided new insight into EBV infection in stratified epithelium.

Clinical and experimental bacteriophage studies: Recommendations for possible approaches for standing against SARS-CoV-2

In 2019, the world faced a serious health challenge, the rapid spreading of a life-threatening viral pneumonia, coronavirus disease 2019 (COVID-19) caused by a betacoronavirus severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). As of January 2022 WHO statistics shows more than 5.6 million death and about 350 million infection by SARS-CoV-2. One of the life threatening aspects of COVID-19 is secondary infections and reduced efficacy of antibiotics against them. Since the beginning of COVID-19 many researches have been done on identification, treatment, and vaccine development. Bacterial viruses (bacteriophages) could offer novel approaches to detect, treat and control COVID-19. Phage therapy and in particular using phage cocktails can be used to control or eliminate the bacterial pathogen as an alternative or complementary therapeutic agent. At the same time, phage interaction with the host immune system can regulate the inflammatory response. In addition, phage display and engineered synthetic phages can be utilized to develop new vaccines and antibodies, stimulate the immune system, and elicit a rapid and well-appropriate defense response. The emergence of SARS-CoV-2 new variants like delta and omicron has proved the urgent need for precise, efficient and novel approaches for vaccine development and virus detection techniques in which bacteriophages may be one of the plausible solutions. Therefore, phages with similar morphology and/or genetic content to that of coronaviruses can be used for ecological and epidemiological modeling of SARS-CoV-2 behavior and future generations of coronavirus, and in general new viral pathogens. This article is a comprehensive review/perspective of potential applications of bacteriophages in the fight against the present pandemic and the post-COVID era.

Molecular basis of EphA2 recognition by gHgL from gammaherpesviruses

The human γ-herpesviruses Kaposi sarcoma associated herpesvirus (KSHV) and Epstein-Barr virus (EBV) are associated with many human malignancies. Viral glycoprotein H (gH) and glycoprotein L (gL) are crucial for the cell tropism by binding to specific receptors. Recently, EphA2 was identified as the specific entry receptor for both KSHV and EBV. Here, we characterized the crystal structures of KSHV gHgL or EBV gHgL in complex with the ligand binding domain (LBD) of EphA2. Both KSHV and EBV gHgL bind to the channel and peripheral regions of LBD primarily using gL. Extensive interactions with more contacts contribute to the higher affinity of KSHV gHgL to LBD than that of EBV gHgL. These binding characteristics were verified using cell-based fusion assays with mutations in key EphA2 residues. Our experiments suggest that multiple animal γ-herpesviruses could use EphA2 as an entry receptor, implying a potential threat to human health.

Epithelial cell infection by Epstein-Barr virus

Epstein-Barr Virus (EBV) is etiologically associated with multiple human malignancies including Burkitt lymphoma and Hodgkin disease as well as nasopharyngeal and gastric carcinoma. Entry of EBV into target cells is essential for virus to cause disease and is mediated by multiple viral envelope glycoproteins and cell surface associated receptors. The target cells of EBV include B cells and epithelial cells. The nature and mechanism of EBV entry into these cell types are different, requiring different glycoprotein complexes to bind to specific receptors on the target cells. Compared to the B cell entry mechanism, the overall mechanism of EBV entry into epithelial cells is less well known. Numerous receptors have been implicated in this process and may also be involved in additional processes of EBV entry, transport, and replication. This review summarizes EBV glycoproteins, host receptors, signal molecules and transport machinery that are being used in the epithelial cell entry process and also provides a broad view for related herpesvirus entry mechanisms.

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

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

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

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Cryptic envelope protein glycosylation interferes with immune detection

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

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

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.

Perspectives of Phage-Eukaryotic Cell Interactions to Control Epstein-Barr Virus Infections

Recently, leading medical journals emphasized the importance of further studies on the potential application of bacterial viruses (phages) for the treatment of antibiotics-resistant infections outlining the present status of the therapy and perspectives for the future. Furthermore, a leading scientific journal pointed to the recent progress in research on phage interactions with eukaryotic cells (especially cells of the immune system) and potential implications of their results for our broader understanding of the role of phages – not only as “bacteria eaters” – but also as an important part of our body defense protecting against external and internal pathogenic invaders (as suggested previously). This illustrates how our understanding of the actual role and potential of phages is expanding and how worldwide interest in their use in medicine is growing. In this article we envision how this advancement of our knowledge about phages could be translated into the progress in combating herpesvirus infections especially those caused by Epstein–Barr virus (EBV).

The Immunomodulatory Capacity of an Epstein-Barr Virus Abortive Lytic Cycle: Potential Contribution to Viral Tumorigenesis

Epstein-Barr virus (EBV) is characterized by a bipartite life cycle in which latent and lytic stages are alternated. Latency is compatible with long-lasting persistency within the infected host, while lytic expression, preferentially found in oropharyngeal epithelial tissue, is thought to favor host-to-host viral dissemination. The clinical importance of EBV relates to its association with cancer, which we think is mainly a consequence of the latency/persistency mechanisms. However, studies in murine models of tumorigenesis/lymphomagenesis indicate that the lytic cycle also contributes to cancer formation. Indeed, EBV lytic expression is often observed in established cell lines and tumor biopsies. Within the lytic cycle EBV expresses a handful of immunomodulatory (BCRF1, BARF1, BNLF2A, BGLF5 & BILF1) and anti-apoptotic (BHRF1 & BALF1) proteins. In this review, we discuss the evidence supporting an abortive lytic cycle in which these lytic genes are expressed, and how the immunomodulatory mechanisms of EBV and related herpesviruses Kaposi Sarcoma herpesvirus (KSHV) and human cytomegalovirus (HCMV) result in paracrine signals that feed tumor cells. An abortive lytic cycle would reconcile the need of lytic expression for viral tumorigenesis without relaying in a complete cycle that would induce cell lysis to release the newly formed infective viral particles.

Ephrin receptor A2 is a functional entry receptor for Epstein-Barr virus

Epstein-Barr virus (EBV) is an oncogenic virus that infects more than 90% of the world’s population¹. EBV predominantly infects human B cells and epithelial cells, which is initiated by fusion of the viral envelope with a host cellular membrane². The mechanism of EBV entry into B cells has been well characterized³. However, the mechanism for epithelial cell entry remains elusive. Here, we show that the integrins (αvβ5, αvβ6, and αvβ8) do not function as an entry and fusion receptor for epithelial cells whereas ephrin receptor tyrosine kinase A2 (EphA2) functions well for both. EphA2 overexpression significantly increased EBV infection of HEK 293 cells. Using a virus-free cell-cell fusion assay, we found that EphA2 dramatically promoted EBV but not HSV fusion with HEK293 cells. EphA2 silencing using shRNA or knockout by CRISPR/Cas9 blocked fusion with epithelial cells. This inhibitory effect was rescued by the expression of EphA2. Antibody against EphA2 blocked epithelial cell infection. Using label-free Surface Plasmon Resonance (SPR) binding studies, we confirmed that EphA2 but not EphA4 specifically bound to EBV gHgL and this interaction is through the EphA2 extracellular domain (EphA2-ECD). The discovery of EphA2 as an EBV epithelial cell receptor has important implications for EBV pathogenesis and may uncover new potential targets that can be used for the development of novel interventional strategies.

Epstein-Barr Virus Fusion with Epithelial Cells Triggered by gB Is Restricted by a gL Glycosylation Site

Epstein-Barr virus (EBV) entry into epithelial cells is mediated by the conserved core fusion machinery, composed of the fusogen gB and the receptor-binding complex gH/gL. The heterodimeric gH/gL complex binds to the EBV epithelial cell receptor or gp42, which binds to the B-cell receptor, triggering gB-mediated fusion of the virion envelope with cellular membranes. Our previous study found that the gL glycosylation mutant N69L/S71V had an epithelial cell-specific hyperfusogenic phenotype. To study the influence of this gL mutant on the initiation and kinetics of gB-driven epithelial cell fusion, we established a virus-free split-green fluorescent protein cell-cell fusion assay that enables real-time measurements of membrane fusion using live cells. The gL_N69L/S71V mutant had a large increase in epithelial cell fusion activity of up to 300% greater than that of wild-type gL starting at early time points. The hyperfusogenicity of the gL mutant was not a result of alterations in complex formation with gH or alterations in cellular localization. Moreover, the hyperfusogenic phenotype of the gL mutant correlated with the formation of enlarged syncytia. In summary, our present findings highlight an important role of gL in the kinetics of gB-mediated epithelial cell fusion, adding to previous findings indicating a direct interaction between gL and gB in EBV membrane fusion.IMPORTANCE EBV predominantly infects epithelial cells and B lymphocytes, which are the cells of origin for the EBV-associated malignancies Hodgkin and Burkitt lymphoma as well as nasopharyngeal carcinoma. Contrary to the other key players of the core fusion machinery, gL has the most elusive role during EBV-induced membrane fusion. We found that the glycosylation site N69/S71 of gL is involved in restricting epithelial cell fusion activity, strongly correlating with syncytium size. Interestingly, our data showed that the gL glycosylation mutant increases the fusion activity of the hyperfusogenic gB mutants, indicating that this gL mutant and the gB mutants target different steps during fusion. Our studies on how gL and gB work together to modulate epithelial cell fusion kinetics are essential to understand the highly tuned tropism of EBV for epithelial cells and B lymphocytes and may result in novel strategies for therapies preventing viral entry into target host cells. Finally, making our results of particular interest is the absence of gL syncytial mutants in other herpesviruses.

A comparative review of viral entry and attachment during large and giant dsDNA virus infections

Viruses enter host cells via several mechanisms, including endocytosis, macropinocytosis, and phagocytosis. They can also fuse at the plasma membrane and can spread within the host via cell-to-cell fusion or syncytia. The mechanism used by a given viral strain depends on its external topology and proteome and the type of cell being entered. This comparative review discusses the cellular attachment receptors and entry pathways of dsDNA viruses belonging to the families Adenoviridae, Baculoviridae, Herpesviridae and nucleocytoplasmic large DNA viruses (NCLDVs) belonging to the families Ascoviridae, Asfarviridae, Iridoviridae, Phycodnaviridae, and Poxviridae, and giant viruses belonging to the families Mimiviridae and Marseilleviridae as well as the proposed families Pandoraviridae and Pithoviridae. Although these viruses have several common features (e.g., topology, replication and protein sequence similarities) they utilize different entry pathways to infect wide-range of hosts, including humans, other mammals, invertebrates, fish, protozoa and algae. Similarities and differences between the entry methods used by these virus families are highlighted, with particular emphasis on viral topology and proteins that mediate viral attachment and entry. Cell types that are frequently used to study viral entry are also reviewed, along with other factors that affect virus-host cell interactions.

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.

Pathogenesis of Gastric Cancer: Genetics and Molecular Classification

Gastric cancer is the fifth most incident and the third most common cause of cancer-related death in the world. Infection with Helicobacter pylori is the major risk factor for this disease. Gastric cancer is the final outcome of a cascade of events that takes decades to occur and results from the accumulation of multiple genetic and epigenetic alterations. These changes are crucial for tumor cells to expedite and sustain the array of pathways involved in the cancer development, such as cell cycle, DNA repair, metabolism, cell-to-cell and cell-to-matrix interactions, apoptosis, angiogenesis, and immune surveillance. Comprehensive molecular analyses of gastric cancer have disclosed the complex heterogeneity of this disease. In particular, these analyses have confirmed that Epstein-Barr virus (EBV)-positive gastric cancer is a distinct entity. The identification of gastric cancer subtypes characterized by recognizable molecular profiles may pave the way for a more personalized clinical management and to the identification of novel therapeutic targets and biomarkers for screening, prognosis, prediction of response to treatment, and monitoring of gastric cancer progression.

Pathogenesis of Gastric Cancer: Genetics and Molecular Classification

Gastric cancer is the fifth most incident and the third most common cause of cancer-related death in the world. Infection with Helicobacter pylori is the major risk factor for this disease. Gastric cancer is the final outcome of a cascade of events that takes decades to occur and results from the accumulation of multiple genetic and epigenetic alterations. These changes are crucial for tumor cells to expedite and sustain the array of pathways involved in the cancer development, such as cell cycle, DNA repair, metabolism, cell-to-cell and cell-to-matrix interactions, apoptosis, angiogenesis, and immune surveillance. Comprehensive molecular analyses of gastric cancer have disclosed the complex heterogeneity of this disease. In particular, these analyses have confirmed that Epstein-Barr virus (EBV)-positive gastric cancer is a distinct entity. The identification of gastric cancer subtypes characterized by recognizable molecular profiles may pave the way for a more personalized clinical management and to the identification of novel therapeutic targets and biomarkers for screening, prognosis, prediction of response to treatment, and monitoring of gastric cancer progression.

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 Cytoplasmic Tail Domain of Epstein-Barr Virus gH Regulates Membrane Fusion Activity through Altering gH Binding to gp42 and Epithelial Cell Attachment

Epstein-Barr virus (EBV) is associated with infectious mononucleosis and a variety of cancers as well as lymphoproliferative disorders in immunocompromised patients. EBV mediates viral entry into epithelial and B cells using fusion machinery composed of four glycoproteins: gB, the gH/gL complex, and gp42. gB and gH/gL are required for both epithelial and B cell fusion. The specific role of gH/gL in fusion has been the most elusive among the required herpesvirus entry glycoproteins. Previous mutational studies have focused on the ectodomain of EBV gH and not on the gH cytoplasmic tail domain (CTD). In this study, we chose to examine the function of the gH CTD by making serial gH truncation mutants as well as amino acid substitution mutants to determine the importance of the gH CTD in epithelial and B cell fusion. Truncation of 8 amino acids (aa 698 to 706) of the gH CTD resulted in diminished fusion activity using a virus-free syncytium formation assay and fusion assay. The importance of the amino acid composition of the gH CTD was also investigated by amino acid substitutions that altered the hydrophobicity or hydrophilicity of the CTD. These mutations also resulted in diminished fusion activity. Interestingly, some of the gH CTD truncation mutants and hydrophilic tail substitution mutants lost the ability to bind to gp42 and epithelial cells. In summary, our studies indicate that the gH CTD is an important functional domain.

Infection with Epstein-Barr virus (EBV) causes diseases ranging from the fairly benign infectious mononucleosis to life-threatening cancer. Entry into target cells is the first step for viral infection and is important for EBV to cause disease. Understanding the EBV entry mechanism is useful for the development of infection inhibitors and developing EBV vaccine approaches. Epithelial and B cells are the main target cells for EBV infection. The essential glycoproteins for EBV entry include gB, gH/gL, and gp42. We characterized the function of the EBV gH C-terminal cytoplasmic tail domain (CTD) in fusion using a panel of gH CTD truncation or substitution mutants. We found that the gH CTD regulates fusion by altering gp42 and epithelial cell attachment. Our studies may lead to a better understanding of EBV fusion and entry, which may result in novel therapies that target the EBV entry step.

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.

The BDLF3 gene product of Epstein-Barr virus, gp150, mediates non-productive binding to heparan sulfate on epithelial cells and only the binding domain of CD21 is required for infection

The cell surface molecules used by Epstein-Barr virus (EBV) to attach to epithelial cells are not well-defined, although when CD21, the B cell receptor for EBV is expressed epithelial cell infection increases disproportionately to the increase in virus bound. Many herpesviruses use low affinity charge interactions with molecules such as heparan sulfate to attach to cells. We report here that the EBV glycoprotein gp150 binds to heparan sulfate proteoglycans, but that attachment via this glycoprotein is not productive of infection. We also report that only the aminoterminal two short consensus repeats of CD21 are required for efficient infection, This supports the hypothesis that, when expressed on an epithelial cell CD21 serves primarily to cluster the major attachment protein gp350 in the virus membrane and enhance access of other important glycoproteins to the epithelial cell surface.

Scanning Mutagenesis of Human Cytomegalovirus Glycoprotein gH/gL

The core, conserved function of the herpesvirus gH/gL is to promote gB-mediated membrane fusion during entry, although the mechanism is poorly understood. The human cytomegalovirus (HCMV) gH/gL can exist as either the gH/gL/gO trimer or the gH/gL/UL128/UL130/UL131 (gH/gL/UL128-131) pentamer. One model suggests that gH/gL/gO provides the core fusion role during entry into all cells within the broad tropism of HCMV, whereas gH/gL/UL128-131 acts at an earlier stage, by a distinct receptor-binding mechanism to enhance infection of select cell types, such as epithelial cells, endothelial cells, and monocytes/macrophages. To further study the distinct functions of these complexes, mutants with individual charged cluster-to-alanine (CCTA) mutations of gH and gL were combined to generate a library of 80 mutant gH/gL heterodimers. The majority of the mutant gH/gL complexes were unable to facilitate gB-mediated membrane fusion in transient-expression cell-cell fusion experiments. In contrast, these mutants supported the formation of gH/gL/UL128-131 complexes that could block HCMV infection in receptor interference experiments. These results suggest that receptor interactions with gH/gL/UL128-131 involve surfaces contained on the UL128-131 proteins but not on gH/gL. gH/gL/UL128-131 receptor interference could be blocked with anti-gH antibodies, suggesting that interference is a cell surface phenomenon and that anti-gH antibodies can block gH/gL/UL128-131 in a manner that is distinct from that for gH/gL/gO.

IMPORTANCE

Interest in the gH/gL complexes of HCMV (especially gH/gL/UL128-131) as vaccine targets has far outpaced our understanding of the mechanism by which they facilitate entry and contribute to broad cellular tropism. For Epstein-Barr virus (EBV), gH/gL and gH/gL/gp42 are both capable of promoting gB fusion for entry into epithelial cells and B cells, respectively. In contrast, HCMV gH/gL/gO appears to be the sole fusion cofactor that promotes gB fusion activity, whereas gH/gL/UL128-131 expands cell tropism through a distinct yet unknown mechanism. This study suggests that the surfaces of HCMV gH/gL are critical for promoting gB fusion but are dispensable for gH/gL/UL128-131 receptor interaction. This underscores the importance of gH/gL/gO in HCMV entry into all cell types and reaffirms the complex as a candidate target for vaccine development. The two functionally distinct forms of gH/gL present in HCMV make for a useful model with which to study the fundamental mechanisms by which herpesvirus gH/gL regulates gB fusion.

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.

Nonmuscle myosin heavy chain IIA mediates Epstein-Barr virus infection of nasopharyngeal epithelial cells

EBV causes B lymphomas and undifferentiated nasopharyngeal carcinoma (NPC). Although the mechanisms by which EBV infects B lymphocytes have been extensively studied, investigation of the mechanisms by which EBV infects nasopharyngeal epithelial cells (NPECs) has only recently been enabled by the successful growth of B lymphoma Mo-MLV insertion region 1 homolog (BMI1)-immortalized NPECs in vitro and the discovery that neuropilin 1 expression positively affects EBV glycoprotein B (gB)-mediated infection and tyrosine kinase activations in enhancing EBV infection of BMI1-immortalized NPECs. We have now found that even though EBV infected NPECs grown as a monolayer at extremely low efficiency (<3%), close to 30% of NPECs grown as sphere-like cells (SLCs) were infected by EBV. We also identified nonmuscle myosin heavy chain IIA (NMHC-IIA) as another NPEC protein important for efficient EBV infection. EBV gH/gL specifically interacted with NMHC-IIA both in vitro and in vivo. NMHC-IIA densely aggregated on the surface of NPEC SLCs and colocalized with EBV. EBV infection of NPEC SLCs was significantly reduced by NMHC-IIA siRNA knock-down. NMHC-IIA antisera also efficiently blocked EBV infection. These data indicate that NMHC-IIA is an important factor for EBV NPEC infection.

Membrane anchoring of Epstein-Barr virus gp42 inhibits fusion with B cells even with increased flexibility allowed by engineered spacers

We recently described the architecture of the Epstein-Barr virus (EBV) fusion-triggering complex consisting of the EBV B cell receptor human leukocyte antigen (HLA) class II and the EBV-encoded proteins gp42 and gH/gL. The architecture of this structure positioned the main body of gp42, comprising the C-type lectin domain (CTLD), away from the membrane and distant from where the membrane-bound form of gp42 might be tethered. gp42 is a type II membrane glycoprotein, with functional gp42 formed by cleavage near the gp42 amino-terminal transmembrane domain. This cleavage results in an approximately 50-amino-acid unstructured region that is responsible for binding gH/gL with nanomolar affinity. Our previous studies had shown that membrane-bound gp42 is not functional in B cell fusion. To investigate whether we could restore gp42 function by extending it from the membrane, we introduced one, two, and four structured immunoglobulin-like domains from muscle protein titin into a membrane-bound form of gp42 and tested function in binding to gHgL and HLA class II and function in fusion. We hypothesized that cleavage of gp42 generates a soluble functional form that relieves steric hindrance imposed on gHgL by membrane-bound gp42. All of the linker mutants had a dominant-negative effect on gp42 function, indicating that gp42 fusion function could not be restored simply by the addition of one to four titin domains.

IMPORTANCE

Epstein-Barr virus (EBV) is associated with numerous diseases from benign mononucleosis to Burkitt's and Hodgkin's lymphoma, nasopharyngeal and gastric carcinoma, and lymphoproliferative disorders in patients with immune dysfunction resulting from immune suppression. Among the glycoproteins important for fusion, gp42, along with gH/gL, determines EBV tropism between epithelial and B cells. The function of gp42 is dependent on N-terminal cleavage, since membrane-bound gp42 cannot mediate fusion. We further investigated whether insertion of a linker into membrane-bound gp42 would relieve steric hindrance imposed on membrane-bound gp42 and restore fusion function. However, adding one, two, or four structured immunoglobulin-like domains to membrane gp42 did not restore fusion activity, indicating that the architecture and membrane orientation of the B cell fusion-triggering complex of EBV may be easily perturbed and that gp42 cleavage is essential for B cell fusion.

Viral Entry

Epstein-Barr virus primarily, though not exclusively, infects B cells and epithelial cells. Many of the virus and cell proteins that are involved in entry into these two cell types in vitro have been identified, and their roles in attachment and fusion are being explored. This chapter discusses what is known about entry at the cellular level in vitro and describes what little is known about the process in vivo. It highlights some of the questions that still need to be addressed and considers some models that need further testing.

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.

The conserved disulfide bond within domain II of Epstein-Barr virus gH has divergent roles in membrane fusion with epithelial cells and B cells

Epstein-Barr virus (EBV) infects target cells via fusion with cellular membranes. For entry into epithelial cells, EBV requires the herpesvirus conserved core fusion machinery, composed of glycoprotein B (gB) and gH/gL. In contrast, for B cell fusion it requires gB and gH/gL with gp42 serving as a cell tropism switch. The available crystal structures for gH/gL allow the targeted analysis of structural determinants of gH to identify functional regions critical for membrane fusion. Domain II of EBV gH contains two disulfide bonds (DBs). The first is unique for EBV and closely related gammaherpesviruses. The second is conserved across the beta- and gammaherpesviruses and is positioned to stabilize a putative syntaxin-like bundle motif. To analyze the role of these DBs in membrane fusion, gH was mutated by amino acid substitution of the DB cysteines. Mutation of the EBV-specific DB resulted in diminished gH/gL cell surface expression that correlated with diminished B cell and epithelial cell fusion. In contrast, mutation of the conserved DB resulted in wild-type-like B cell fusion, whereas epithelial cell fusion was greatly reduced. The gH mutants bound well to gp42 but had diminished binding to epithelial cells. Tyrosine 336, located adjacent to cysteine 335 of the conserved DB, also was found to be important for DB stabilization and gH/gL function. We conclude that the conserved DB has a cell type-specific function, since it is important for the binding of gH to epithelial cells initiating epithelial cell fusion but not for fusion with B cells and gp42 binding.

IMPORTANCE

EBV predominantly infects epithelial and B cells in humans, which can result in EBV-associated cancers, such as Burkitt and Hodgkin lymphoma, as well as nasopharyngeal carcinoma. EBV is also associated with a variety of lymphoproliferative disorders, typically of B cell origin, observed in immunosuppressed individuals, such as posttransplant or HIV/AIDS patients. The gH/gL complex plays an essential but still poorly characterized role as an important determinant for EBV cell tropism. In the current studies, we found that mutants in the DB C278/C335 and the neighboring tyrosine 336 have cell type-specific functional deficits with selective decreases in epithelial cell, but not B cell, binding and fusion. The present study brings new insights into the gH function as a determinant for epithelial cell tropism during herpesvirus-induced membrane fusion and highlights a specific gH motif required for epithelial cell fusion.

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.

Epstein-Barr virus glycoprotein gB and gHgL can mediate fusion and entry in trans, and heat can act as a partial surrogate for gHgL and trigger a conformational change in gB

Epstein-Barr virus (EBV) fusion with an epithelial cell requires virus glycoproteins gHgL and gB and is triggered by an interaction between gHgL and integrin αvβ5, αvβ6, or αvβ8. Fusion with a B cell requires gHgL, gp42, and gB and is triggered by an interaction between gp42 and human leukocyte antigen class II. We report here that, like alpha- and betaherpesviruses, EBV, a gammaherpesvirus, can mediate cell fusion if gB and gHgL are expressed in trans. Entry of a gH-null virus into an epithelial cell is possible if the epithelial cell expresses gHgL, and entry of the same virus, which phenotypically lacks gHgL and gp42, into a B cell expressing gHgL is possible in the presence of a soluble integrin. Heat is capable of inducing the fusion of cells expressing only gB, and the proteolytic digestion pattern of gB in virions changes in the same way following the exposure of virus to heat or to soluble integrins. It is suggested that the Gibbs free energy released as a result of the high-affinity interaction of gHgL with an integrin contributes to the activation energy required to cause the refolding of gB from a prefusion to a postfusion conformation.

IMPORTANCE

The core fusion machinery of herpesviruses consists of glycoproteins gB and gHgL. We demonstrate that as in alpha- and betaherpesvirus, gB and gHgL of the gammaherpesvirus EBV can mediate fusion and entry when expressed in trans in opposing membranes, implicating interactions between the ectodomains of the proteins in the activation of fusion. We further show that heat and exposure to a soluble integrin, both of which activate fusion, result in the same changes in the proteolytic digestion pattern of gB, possibly representing the refolding of gB from its prefusion to its postfusion conformation.

The Epstein-Barr virus (EBV) glycoprotein B cytoplasmic C-terminal tail domain regulates the energy requirement for EBV-induced membrane fusion

The entry of enveloped viruses into host cells is preceded by membrane fusion, which in Epstein-Barr virus (EBV) is thought to be mediated by the refolding of glycoprotein B (gB) from a prefusion to a postfusion state. In our current studies, we characterized a gB C-terminal tail domain (CTD) mutant truncated at amino acid 843 (gB843). This truncation mutant is hyperfusogenic as monitored by syncytium formation and in a quantitative fusion assay and is dependent on gH/gL for fusion activity. gB843 can rescue the fusion function of other glycoprotein mutants that have null or decreased fusion activity in epithelial and B cells. In addition, gB843 requires less gp42 and gH/gL for fusion, and can function in fusion at a lower temperature than wild-type gB, indicating a lower energy requirement for fusion activation. Since a key step in fusion is the conversion of gB from a prefusion to an active postfusion state by gH/gL, gB843 may access this activated gB state more readily. Our studies indicate that the gB CTD may participate in the fusion function by maintaining gB in an inactive prefusion form prior to activation by receptor binding. Importance: Diseases resulting from Epstein-Barr virus (EBV) infection in humans range from the fairly benign disease infectious mononucleosis to life-threatening cancer. As an enveloped virus, EBV must fuse with a host cell membrane for entry and infection by using glycoproteins gH/gL, gB, and gp42. Among these glycoproteins, gB is thought to be the protein that executes fusion. To further characterize the function of the EBV gB cytoplasmic C-terminal tail domain (CTD) in fusion, we used a previously constructed CTD truncation mutant and studied its fusion activity in the context of other EBV glycoprotein mutants. From these studies, we find that the gB CTD regulates fusion by altering the energy requirements for the triggering of fusion mediated by gH/gL or gp42. Overall, our studies may lead to a better understanding of EBV fusion and entry, which may result in novel therapies that target the EBV entry step.

Chlamydia trachomatis polymorphic membrane protein D is a virulence factor involved in early host-cell interactions

Chlamydia trachomatis is an obligate intracellular mucosotropic pathogen of significant medical importance. It is the etiological agent of blinding trachoma and bacterial sexually transmitted diseases, infections that afflict hundreds of millions of people globally. The C. trachomatis polymorphic membrane protein D (PmpD) is a highly conserved autotransporter and the target of broadly cross-reactive neutralizing antibodies; however, its role in host-pathogen interactions is unknown. Here we employed a targeted reverse genetics approach to generate a pmpD null mutant that was used to define the role of PmpD in the pathogenesis of chlamydial infection. We show that pmpD is not an essential chlamydial gene and the pmpD null mutant has no detectable deficiency in cultured murine cells or in a murine mucosal infection model. Notably, however, the pmpD null mutant was significantly attenuated for macaque eyes and cultured human cells. A reduction in pmpD null infection of human endocervical cells was associated with a deficiency in chlamydial attachment to cells. Collectively, our results show that PmpD is a chlamydial virulence factor that functions in early host-cell interactions. This study is the first of its kind using reverse genetics to evaluate the contribution of a C. trachomatis gene to disease pathogenesis.

Epstein Barr virus entry; kissing and conjugation

Epstein Barr virus (EBV) is a highly prevalent human gamma 1 lymphocryptovirus which infects both B lymphocytes and epithelial cells. In the healthy host, infection of these different cell lineages broadly reflects the different phases of the virus lifecycle. Memory B cells are the reservoir for latent EBV, in which viral gene expression is highly restricted to maintain an asymptomatic lifelong infection. In contrast, epithelial cells may be a major site of the virus lytic cycle, where infectious virus is propagated and transmitted via saliva to uninfected hosts. To achieve this dual tropism, EBV has evolved a unique set of glycoproteins in addition to a highly conserved set, which interact with cell lineage-specific receptors and switch cellular tropism during infection.

Implication of human herpesviruses in oncogenesis through immune evasion and supression

All human herpesviruses (HHVs) have been implicated in immune system evasion and suppression. Moreover, two HHV family members, i.e. EBV and KSHV, are recognised as oncogenic viruses. Our literature review summarises additional examples of possible oncogenic mechanisms that have been attributed to other HHVs. In general, HHVs affect almost every cancer-implicated branch of the immune system, namely tumour-promoting inflammation, immune evasion, and immunosuppression. Some HHVs accomplish these effects by inhibiting apoptotic pathways and by promoting proliferation. Mechanisms related to immunosupression and low grade chronic inflammation could eventually result in the initiation and progression of cancer. In this article we open a discussion on the members of Herpesviridae, their immune evasion and suppression mechanisms, and their possible role in cancer development. We conclude that discerning the mechanisms of interplay between HHV, immune system, and cancer is essential for the development of novel preventative and therapeutic approaches for cancer treatment and prophylaxis.

Identification of the human herpesvirus 6A gQ1 domain essential for its functional conformation

Human herpesvirus 6 is a T lymphotropic herpesvirus, long classified into variants A and B (HHV-6A and HHV-6B) based on differences in sequence and pathogenicity. Recently, however, HHV-6A and HHV-6B were reclassified as different species. Here, we isolated a neutralizing monoclonal antibody (Mab) named AgQ 1-1 that was specific for HHV-6A glycoprotein Q1 (AgQ1), and we showed that amino acid residues 494 to 497 of AgQ1 were critical for its recognition by this Mab. This region was also essential for AgQ1's complex formation with gH, gL, and gQ2, which might be important for viral binding to the cellular receptor, CD46. In addition, amino acid residues 494 to 497 are essential for viral replication. Interestingly, this sequence corresponds to the domain on HHV-6B gQ1 that is critical for recognition by an HHV-6B-specific neutralizing Mab. Within this domain, only Q at position 496 of HHV-6A is distinct from the HHV-6B sequence; however, the mutant AgQ1(Q496E) was still clearly recognized by the Mab AgQ 1-1. Surprisingly, replacement of the adjacent amino acid, in mutant AgQ1(C495A), resulted in poor recognition by Mab AgQ 1-1, and AgQ1(C495A) could not form the gH/gL/gQ1/gQ2 complex. Furthermore, the binding ability of mutant AgQ1(L494A) with CD46 decreased, although it could form the gH/gL/gQ1/gQ2 complex and it showed clear reactivity to Mab AgQ 1-1. These data indicated that amino acid residues 494 to 497 of AgQ1 were critical for the recognition by Mab AgQ 1-1 and essential for AgQ1's functional conformation.

The large groove found in the gH/gL structure is an important functional domain for Epstein-Barr virus fusion

Epstein-Barr virus (EBV) mediates viral entry into cells using four glycoproteins-gB, the gH/gL complex, and gp42-and fusion is cell type specific. gB and gH/gL are required for epithelial cell fusion; B cell fusion also requires gp42. To investigate functional domains within the gH/gL structure, we constructed site-directed EBV gH/gL mutants with alterations of residues located in a large groove that separates domain I (D-I) from domain II (D-II) within the gH/gL structure. We found that substitution of alanine for leucine 207 reduces both epithelial and B cell fusion and is accompanied by reduced gp42 binding. We also observed that substitution of alanine for arginine 152, histidine 154, or threonine 174 reduces fusion with epithelial cells but not with B cells. To test whether flexibility of the region between D-I and D-II of gH/gL could be important for membrane fusion activity and to allow potential interactions across the D-I/D-II groove, we mutated D-I amino acids V47, P48, and G49 to cysteine, allowing novel intersubunit disulfide bonds to form with the free C153 located in D-II. We found that the G49C mutant, predicted to bridge D-I and D-II with C153 of gH/gL, had normal B cell fusion activity but reduced epithelial cell fusion activity, which was partially restored by treatment with dithiothreitol. We conclude that structural rearrangements and/or interactions across the D-I/D-II groove of gH/gL are required for fusion with epithelial cells but not for fusion with B cells.

A soluble form of Epstein-Barr virus gH/gL inhibits EBV-induced membrane fusion and does not function in fusion

We investigated whether soluble EBV gH/gL (sgH/gL) functions in fusion and made a series of truncations of gH/gL domains based on the gH/gL crystal structure. We found sgH/gL failed to mediate cell-cell fusion both when co-expressed with the other entry glycoproteins and when added exogenously to fusion assays. Interestingly, sgH/gL inhibited cell-cell fusion in a dose dependent manner when co-expressed. sgH/gL from HSV was unable to inhibit EBV fusion, suggesting the inhibition was specific to EBV gH/gL. sgH/gL stably binds gp42, but not gB nor gH/gL. The domain mutants, DI/gL, DI-II/gL and DI-II-III/gL were unable to bind gp42. Instead, DI-II/gL, DI-II-III/gL and sgH/gL but not DI/gL decreased the expression of gp42, resulting in decreased overall fusion. Overall, our results suggest that domain IV may be required for proper folding and the transmembrane domain and cytoplasmic tail of EBV gH/gL are required for the most efficient fusion.

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

Enveloped viruses enter cells by fusing the viral and cellular membranes, and most use a single viral envelope protein that combines receptor-binding and fusogenic functions. In herpesviruses, these functions are distributed among multiple proteins: the conserved fusion protein gB, various non-conserved receptor-binding proteins, and the conserved gH/gL heterodimer that curiously lacks an apparent counterpart in other enveloped viruses. Recent structural studies of gH/gL from HSV-2 and EBV revealed a unique complex with no structural or functional similarity to other viral proteins. Here we analyzed gH/gL structures and highlighted important functional regions. We propose that gH/gL functions as an adaptor that transmits the triggering signals from various non-conserved inputs to the highly conserved fusion protein gB.

Herpes virus fusion and entry: a story with many characters

Herpesviridae comprise a large family of enveloped DNA viruses all of whom employ orthologs of the same three glycoproteins, gB, gH and gL. Additionally, herpesviruses often employ accessory proteins to bind receptors and/or bind the heterodimer gH/gL or even to determine cell tropism. Sorting out how these proteins function has been resolved to a large extent by structural biology coupled with supporting biochemical and biologic evidence. Together with the G protein of vesicular stomatitis virus, gB is a charter member of the Class III fusion proteins. Unlike VSV G, gB only functions when partnered with gH/gL. However, gH/gL does not resemble any known viral fusion protein and there is evidence that its function is to upregulate the fusogenic activity of gB. In the case of herpes simplex virus, gH/gL itself is upregulated into an active state by the conformational change that occurs when gD, the receptor binding protein, binds one of its receptors. In this review we focus primarily on prototypes of the three subfamilies of herpesviruses. We will present our model for how herpes simplex virus (HSV) regulates fusion in series of highly regulated steps. Our model highlights what is known and also provides a framework to address mechanistic questions about fusion by HSV and herpesviruses in general.