Cell-surface expression of a mutated Epstein-Barr virus glycoprotein B allows fusion independent of other viral proteins

Epstein-Barr virus gp42 antibodies reveal sites of vulnerability for receptor binding and fusion to B cells

Epstein-Barr virus (EBV) causes infectious mononucleosis and is associated with B cell lymphomas. EBV glycoprotein 42 (gp42) binds HLA class II and activates membrane fusion with B cells. We isolated gp42-specific monoclonal antibodies (mAbs), A10 and 4C12, which use distinct mechanisms to neutralize virus infection. mAb A10 was more potent than the only known neutralizing gp42 mAb, F-2-1, in neutralizing EBV infection and blocking binding to HLA class II. mAb 4C12 was similar to mAb A10 in inhibiting glycoprotein-mediated B cell fusion but did not block receptor binding, and it was less effective in neutralizing infection. Crystallographic structures of gH/gL/gp42/A10 and gp42/4C12 complexes revealed two distinct sites of vulnerability on gp42 for receptor binding and B cell fusion. Passive transfer of mAb A10 into humanized mice conferred nearly 100% protection from viremia and EBV lymphomas after EBV challenge. These findings identify vulnerable sites on EBV that may facilitate therapeutics and vaccines.

Epstein-Barr Virus Envelope Glycoprotein gp110 Inhibits IKKi-Mediated Activation of NF-κB and Promotes the Degradation of β-Catenin

Epstein-Barr virus (EBV) infects host cells and establishes a latent infection that requires evasion of host innate immunity. A variety of EBV-encoded proteins that manipulate the innate immune system have been reported, but whether other EBV proteins participate in this process is unclear. EBV-encoded envelope glycoprotein gp110 is a late protein involved in virus entry into target cells and enhancement of infectivity. Here, we reported that gp110 inhibits RIG-I-like receptor pathway-mediated promoter activity of interferon-β (IFN-β) as well as the transcription of downstream antiviral genes to promote viral proliferation. Mechanistically, gp110 interacts with the inhibitor of NF-κB kinase (IKKi) and restrains its K63-linked polyubiquitination, leading to attenuation of IKKi-mediated activation of NF-κB and repression of the phosphorylation and nuclear translocation of p65. Additionally, gp110 interacts with an important regulator of the Wnt signaling pathway, β-catenin, and induces its K48-linked polyubiquitination degradation via the proteasome system, resulting in the suppression of β-catenin-mediated IFN-β production. Taken together, these results suggest that gp110 is a negative regulator of antiviral immunity, revealing a novel mechanism of EBV immune evasion during lytic infection. IMPORTANCE Epstein-Barr virus (EBV) is a ubiquitous pathogen that infects almost all human beings, and the persistence of EBV in the host is largely due to immune escape mediated by its encoded products. Thus, elucidation of EBV's immune escape mechanisms will provide a new direction for the design of novel antiviral strategies and vaccine development. Here, we report that EBV-encoded gp110 serves as a novel viral immune evasion factor, which inhibits RIG-I-like receptor pathway-mediated interferon-β (IFN-β) production. Furthermore, we found that gp110 targeted two key proteins, inhibitor of NF-κB kinase (IKKi) and β-catenin, which mediate antiviral activity and the production of IFN-β. gp110 inhibited K63-linked polyubiquitination of IKKi and induced β-catenin degradation via the proteasome, resulting in decreased IFN-β production. In summary, our data provide new insights into the EBV-mediated immune evasion surveillance strategy.

Epstein-Barr virus gH/gL has multiple sites of vulnerability for virus neutralization and fusion inhibition

Epstein-Barr virus (EBV) is nearly ubiquitous in adults. EBV causes infectious mononucleosis and is associated with B cell lymphomas, epithelial cell malignancies, and multiple sclerosis. The EBV gH/gL glycoprotein complex facilitates fusion of virus membrane with host cells and is a target of neutralizing antibodies. Here, we examined the sites of vulnerability for virus neutralization and fusion inhibition within EBV gH/gL. We developed a panel of human monoclonal antibodies (mAbs) that targeted five distinct antigenic sites on EBV gH/gL and prevented infection of epithelial and B cells. Structural analyses using X-ray crystallography and electron microscopy revealed multiple sites of vulnerability and defined the antigenic landscape of EBV gH/gL. One mAb provided near-complete protection against viremia and lymphoma in a humanized mouse EBV challenge model. Our findings provide structural and antigenic knowledge of the viral fusion machinery, yield a potential therapeutic antibody to prevent EBV disease, and emphasize gH/gL as a target for herpesvirus vaccines and therapeutics.

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.

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.

Epstein-Barr Virus gH/gL and Kaposi's Sarcoma-Associated Herpesvirus gH/gL Bind to Different Sites on EphA2 To Trigger Fusion

Both Epstein-Barr virus (EBV) and Kaposi's sarcoma-associated herpesvirus (KSHV) are human gammaherpesviruses and are important in a variety of malignancies. Eph family receptor tyrosine kinase A2 (EphA2) is a cellular receptor for KSHV and EBV. Previous studies identified five conserved residues (ELEFN^50-54) in the N-terminal domain of KSHV gH that are critical for Eph binding and KSHV infection. However, the specific domains of EBV gH/gL important for EphA2 binding are not well described. We found that the KSHV gH (ELEFN^50-54) motif is important for higher KSHV fusion and that EBV gH/gL does not utilize a similar motif for fusion activity. We previously identified that an EBV gL N-glycosylation mutant (gL-N⁶⁹L/S⁷¹V) was hyperfusogenic in epithelial cells but not in B cells. To determine whether this glycosylation site may be the binding region for EphA2, we compared the EphA2 binding activity of EBV gH/gL and the EBV gH/gL-N⁶⁹L/S⁷¹V mutant. We found that EBV gH/gL-N⁶⁹L/S⁷¹V had higher binding affinity for EphA2, indicating that the EBV gL N-glycosylation site might be responsible for inhibiting the binding of gH/gL to EphA2. Loss of N-glycosylation at this site may remove steric hindrance that reduces EBV gH/gL binding to EphA2. In addition, the mutations located in the large groove of EBV gH/gL (R¹⁵²A and G⁴⁹C) also have decreased binding with EphA2. Taken together, our data indicate that the binding site of EphA2 on EBV gH/gL is at least in part proximal to the EBV gL glycosylation site, which in part accounts for differences in EphA2 binding affinity by KSHV.IMPORTANCE Virus entry into target cells is the first step for virus infection. Understanding the overall entry mechanism, including the binding mechanism of specific virus glycoproteins with cellular receptors, can be useful for the design of small molecule inhibitors and vaccine development. Recently, EphA2 was identified as an important entry receptor for both KSHV and EBV. In the present study, we investigated the required binding sites within EphA2 and EBV gH/gL that mediate the interaction of these two proteins allowing entry into epithelial cells and found that it differed in compared to the interaction of KSHV gH/gL with EphA2. Our discoveries may uncover new potential interventional strategies that block EBV and KSHV infection of target epithelial cells.

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.

Immunization with Components of the Viral Fusion Apparatus Elicits Antibodies That Neutralize Epstein-Barr Virus in B Cells and Epithelial Cells

Epstein-Barr virus (EBV) causes infectious mononucleosis and is associated with epithelial-cell cancers and B cell lymphomas. An effective EBV vaccine is not available. We found that antibodies to the EBV glycoprotein gH/gL complex were the principal components in human plasma that neutralized infection of epithelial cells and that antibodies to gH/gL and gp42 contributed to B cell neutralization. Immunization of mice and nonhuman primates with nanoparticle vaccines that displayed components of the viral-fusion machinery EBV gH/gL or gH/gL/gp42 elicited antibodies that potently neutralized both epithelial-cell and B cell infection. Immune serum from nonhuman primates inhibited EBV-glycoprotein-mediated fusion of epithelial cells and B cells and targeted an epitope critical for virus-cell fusion. Therefore, unlike the leading EBV gp350 vaccine candidate, which only protects B cells from infection, these EBV nanoparticle vaccines elicit antibodies that inhibit the virus-fusion apparatus and provide cell-type-independent protection from virus infection.

Ephrin Receptor A4 is a New Kaposi's Sarcoma-Associated Herpesvirus Virus Entry Receptor

Kaposi's sarcoma-associated herpesvirus (KSHV) is a human gammaherpesvirus associated with the development of Kaposi's sarcoma (KS). KSHV target cells include endothelial cells, B cells, monocytes, epithelial cells, dendritic cells, macrophages, and fibroblasts. KSHV entry into target cells is a complex multistep process and is initiated by the binding and interaction of viral envelope glycoproteins with the cellular receptors. In the current studies, we have found that EphA4 promotes KSHV glycoprotein H/glycoprotein L (gH/gL)-mediated fusion and infection better than does ephrin A2 (EphA2) in HEK293T cells, indicating that EphA4 is a new KSHV entry receptor. To confirm that epithelial cells express EphA2 and EphA4, we analyzed the expression of EphA2 and EphA4 in epithelial cells, endothelial cells, B cells, monocytes, fibroblasts using RNA sequencing (RNA-seq) data analysis of existing data sets. We found that these cell types broadly express both EphA2 and EphA4, with the exception of monocytes and B cells. To confirm EphA4 is important for KSHV fusion and infection, we generated EphA2 and EphA4 single- and double-knockout cells. We found that both EphA2 and EphA4 play a role in KSHV fusion and infection, since EphA2-EphA4 double-knockout cells had the greatest decrease in fusion activity and infection compared to single-knockout cells. Fusion and infection of KSHV were rescued in the EphA2-EphA4 double-knockout cells upon overexpression of EphA2 and/or EphA4. EphA2 binds to both Epstein-Barr virus (EBV) and KSHV gH/gL; however, EphA4 binds only to KSHV gH/gL. Taken together, our results identify EphA4 as a new entry receptor for KSHV.IMPORTANCE The overall entry mechanism for herpesviruses is not completely known, including those for the human gammaherpesviruses Kaposi's sarcoma-associated herpesvirus (KSHV) and Epstein-Barr virus (EBV). To fully understand the herpesvirus entry process, functional receptors need to be identified. In the current study, we found that EphA4 can also function for a KSHV entry receptor along with EphA2. Interestingly, we found that EphA4 does not function as an entry receptor for EBV, whereas EphA2 does. The discovery of EphA4 as a KSHV entry receptor has important implications for KSHV pathogenesis in humans, may prove useful in understanding the unique pathogenesis of KSHV infection in humans, and may uncover new potential targets that can be used for the development of novel interventional strategies.

Ovine Herpesvirus 2 Glycoproteins B, H, and L Are Sufficient for, and Viral Glycoprotein Ov8 Can Enhance, Cell-Cell Membrane Fusion

Ovine herpesvirus 2 (OvHV-2) is a gammaherpesvirus in the genus Macavirus that is carried asymptomatically by sheep. Infection of poorly adapted animals with OvHV-2 results in sheep-associated malignant catarrhal fever, a fatal disease characterized by lymphoproliferation and vasculitis. There is no treatment or vaccine for the disease and no cell culture system to propagate the virus. The lack of cell culture has hindered studies of OvHV-2 biology, including its entry mechanism. As an alternative method to study OvHV-2 glycoproteins responsible for membrane fusion as a part of the entry mechanism, we developed a virus-free cell-to-cell membrane fusion assay to identify the minimum required OvHV-2 glycoproteins to induce membrane fusion. OvHV-2 glycoproteins B, H, and L (gB, gH, and gL) were able to induce membrane fusion together but not when expressed individually. Additionally, open reading frame Ov8, unique to OvHV-2, was found to encode a transmembrane glycoprotein that can significantly enhance membrane fusion. Thus, OvHV-2 gB, gH, and gL are sufficient to induce membrane fusion, while glycoprotein Ov8 plays an enhancing role by an unknown mechanism.IMPORTANCE Herpesviruses enter cells via attachment of the virion to the cellular surface and fusion of the viral envelope with cellular membranes. Virus-cell membrane fusion is an important step for a successful viral infection. Elucidating the roles of viral glycoproteins responsible for membrane fusion is critical toward understanding viral entry. Entry of ovine herpesvirus 2 (OvHV-2), the causative agent of sheep associated-malignant catarrhal fever, which is one of the leading causes of death in bison and other ungulates, has not been well studied due to the lack of a cell culture system to propagate the virus. The identification of OvHV-2 glycoproteins that mediate membrane fusion may help identify viral and/or cellular factors involved in OvHV-2 cell tropism and will advance investigation of cellular factors necessary for virus-cell membrane fusion. We found that OvHV-2 glycoproteins B, H, and L are sufficient for, and viral glycoprotein Ov8 can significantly enhance, cell-cell membrane fusion.

The β-Lactamase Assay: Harnessing a FRET Biosensor to Analyse Viral Fusion Mechanisms

The β-lactamase (BlaM) assay was first revealed in 1998 and was demonstrated to be a robust Förster resonance energy transfer (FRET)-based reporter system that was compatible with a range of commonly-used cell lines. Today, the BlaM assay is available commercially as a kit and can be utilised readily and inexpensively for an array of experimental procedures that require a fluorescence-based readout. One frequent application of the BlaM assay is the measurement of viral fusion—the moment at which the genetic material harboured within virus particles is released into the cytosol following successful entry. The flexibility of the system permits evaluation of not only total fusion levels, but also the kinetics of fusion. However, significant variation exists in the scientific literature regarding the methodology by which the assay is applied to viral fusion analysis, making comparison between results difficult. In this review we draw attention to the disparity of these methodologies and examine the advantages and disadvantages of each approach. Successful strategies shown to render viruses compatible with BlaM-based analyses are also discussed.

Interplay between the Herpes Simplex Virus 1 gB Cytodomain and the gH Cytotail during Cell-Cell Fusion

Herpesvirus entry into cells is mediated by the viral fusogen gB, which is thought to refold from the prefusion to the postfusion form in a series of large conformational changes that energetically couple refolding to membrane fusion. In contrast to most viral fusogens, gB requires a conserved heterodimer, gH/gL, as well as other nonconserved proteins. In a further mechanistic twist, gB-mediated cell-cell fusion appears restricted by its intraviral or cytoplasmic domain (cytodomain) because mutations within it result in a hyperfusogenic phenotype. Here, we characterized a panel of hyperfusogenic HSV-1 gB cytodomain mutants and show that they are fully functional in cell-cell fusion at shorter coincubation times and at lower temperatures than those for wild-type (WT) gB, which suggests that these mutations reduce the kinetic energy barrier to fusion. Despite this, the mutants require both gH/gL and gD. We confirm previous observations that the gH cytotail is an essential component of the cell-cell fusion mechanism and show that the N-terminal portion of the gH cytotail is critical for this process. Moreover, the fusion levels achieved by all gB constructs, WT and mutant, were proportionate to the length of the gH cytotail. Putting these results together, we propose that the gH cytotail, in addition to the gH/gL ectodomain, plays an essential role in gB activation, potentially acting as a "wedge" to release the gB cytodomain "clamp" and enable gB activation.

IMPORTANCE

Herpesviruses infect their hosts for life and cause a substantial disease burden. Herpes simplex viruses cause oral and genital sores as well as rare yet severe encephalitis and a panoply of ocular ailments. Infection initiates when the viral envelope fuses with the host cell membrane in a process orchestrated by the viral fusogen gB, assisted by the viral glycoproteins gH, gL, and gD and a cellular gD receptor. This process is more complicated than that of most other viruses and is subject to multiple regulatory inputs. Antiviral and vaccine development would benefit from a detailed mechanistic knowledge of this process and how it is regulated.

Neuropilin 1 is an entry factor that promotes EBV infection of nasopharyngeal epithelial cells

Epstein-Barr virus (EBV) is implicated as an aetiological factor in B lymphomas and nasopharyngeal carcinoma. The mechanisms of cell-free EBV infection of nasopharyngeal epithelial cells remain elusive. EBV glycoprotein B (gB) is the critical fusion protein for infection of both B and epithelial cells, and determines EBV susceptibility of non-B cells. Here we show that neuropilin 1 (NRP1) directly interacts with EBV gB(23-431). Either knockdown of NRP1 or pretreatment of EBV with soluble NRP1 suppresses EBV infection. Upregulation of NRP1 by overexpression or EGF treatment enhances EBV infection. However, NRP2, the homologue of NRP1, impairs EBV infection. EBV enters nasopharyngeal epithelial cells through NRP1-facilitated internalization and fusion, and through macropinocytosis and lipid raft-dependent endocytosis. NRP1 partially mediates EBV-activated EGFR/RAS/ERK signalling, and NRP1-dependent receptor tyrosine kinase (RTK) signalling promotes EBV infection. Taken together, NRP1 is identified as an EBV entry factor that cooperatively activates RTK signalling, which subsequently promotes EBV infection in nasopharyngeal epithelial cells.

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.

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.

Epstein-Barr virus transcytosis through polarized oral epithelial cells

Although Epstein-Barr virus (EBV) is an orally transmitted virus, viral transmission through the oropharyngeal mucosal epithelium is not well understood. In this study, we investigated how EBV traverses polarized human oral epithelial cells without causing productive infection. We found that EBV may be transcytosed through oral epithelial cells bidirectionally, from both the apical to the basolateral membranes and the basolateral to the apical membranes. Apical to basolateral EBV transcytosis was substantially reduced by amiloride, an inhibitor of macropinocytosis. Electron microscopy showed that virions were surrounded by apical surface protrusions and that virus was present in subapical vesicles. Inactivation of signaling molecules critical for macropinocytosis, including phosphatidylinositol 3-kinases, myosin light-chain kinase, Ras-related C3 botulinum toxin substrate 1, p21-activated kinase 1, ADP-ribosylation factor 6, and cell division control protein 42 homolog, led to significant reduction in EBV apical to basolateral transcytosis. In contrast, basolateral to apical EBV transcytosis was substantially reduced by nystatin, an inhibitor of caveolin-mediated virus entry. Caveolae were detected in the basolateral membranes of polarized human oral epithelial cells, and virions were detected in caveosome-like endosomes. Methyl β-cyclodextrin, an inhibitor of caveola formation, reduced EBV basolateral entry. EBV virions transcytosed in either direction were able to infect B lymphocytes. Together, these data show that EBV transmigrates across oral epithelial cells by (i) apical to basolateral transcytosis, potentially contributing to initial EBV penetration that leads to systemic infection, and (ii) basolateral to apical transcytosis, which may enable EBV secretion into saliva in EBV-infected individuals.

Epstein-Barr Virus (EBV)-associated gastric carcinoma

The ubiquitous Epstein-Barr virus (EBV) is associated with several human tumors, which include lymphoid and epithelial malignancies. It is known that EBV persistently infects the memory B cell pool of healthy individuals by activating growth and survival signaling pathways that can contribute to B cell lymphomagenesis. Although the monoclonal proliferation of EBV-infected cells can be observed in epithelial tumors, such as nasopharyngeal carcinoma and EBV-associated gastric carcinoma, the precise role of EBV in the carcinogenic progress is not fully understood. This review features characteristics and current understanding of EBV-associated gastric carcinoma. EBV-associated gastric carcinoma comprises almost 10% of all gastric carcinoma cases and expresses restricted EBV latent genes (Latency I). Firstly, definition, epidemiology, and clinical features are discussed. Then, the route of infection and carcinogenic role of viral genes are presented. Of particular interest, the association with frequent genomic CpG methylation and role of miRNA for carcinogenesis are topically discussed. Finally, the possibility of therapies targeting EBV-associated gastric carcinoma is proposed.

Modulation of Epstein-Barr virus glycoprotein B (gB) fusion activity by the gB cytoplasmic tail domain

Epstein-Barr virus (EBV), along with other members of the herpesvirus family, requires a set of viral glycoproteins to mediate host cell attachment and entry. Viral glycoprotein B (gB), a highly conserved glycoprotein within the herpesvirus family, is thought to be the viral fusogen based on structural comparison of EBV gB and herpes simplex virus (HSV) gB with the postfusion crystal structure of vesicular stomatitis virus fusion protein glycoprotein G (VSV-G). In addition, mutational studies indicate that gB plays an important role in fusion function. In the current study, we constructed a comprehensive library of mutants with truncations of the C-terminal cytoplasmic tail domain (CTD) of EBV gB. Our studies indicate that the gB CTD is important in the cellular localization, expression, and fusion function of EBV gB. However, in line with observations from other studies, we conclude that the degree of cell surface expression of gB is not directly proportional to observed fusion phenotypes. Rather, we conclude that other biochemical or biophysical properties of EBV gB must be altered to explain the different fusion phenotypes observed.

Epstein-Barr virus (EBV), like all enveloped viruses, fuses the virion envelope to a cellular membrane to allow release of the capsid, resulting in virus infection. To further characterize the function of EBV glycoprotein B (gB) in fusion, a comprehensive library of mutants with truncations in the gB C-terminal cytoplasmic tail domain (CTD) were made. These studies indicate that the CTD of gB is important for the cellular expression and localization of gB, as well as for the function of gB in fusion. These studies will lead to a better understanding of the mechanism of EBV-induced membrane fusion and herpesvirus-induced membrane fusion in general, which will ultimately lead to focused therapies guided at preventing viral entry into host cells.

Localization of a region in the fusion protein of avian metapneumovirus that modulates cell-cell fusion

The genus Metapneumovirus within the subfamily Pneumovirinae of the family Paramyxoviridae includes two members, human metapneumovirus (hMPV) and avian metapneumovirus (aMPV), causing respiratory tract infections in humans and birds, respectively. Paramyxoviruses enter host cells by fusing the viral envelope with a host cell membrane. Membrane fusion of hMPV appears to be unique, in that fusion of some hMPV strains requires low pH. Here, we show that the fusion (F) proteins of aMPV promote fusion in the absence of the attachment protein and low pH is not required. Furthermore, there are notable differences in cell-cell fusion among aMPV subtypes. Trypsin was required for cell-cell fusion induced by subtype B but not subtypes A and C. The F protein of aMPV subtype A was highly fusogenic, whereas those from subtypes B and C were not. By construction and evaluation of chimeric F proteins composed of domains from the F proteins of subtypes A and B, we localized a region composed of amino acid residues 170 to 338 in the F protein that is responsible for the hyperfusogenic phenotype of the F from subtype A. Further mutagenesis analysis revealed that residues R295, G297, and K323 in this region collectively contributed to the hyperfusogenicity. Taken together, we have identified a region in the aMPV F protein that modulates the extent of membrane fusion. A model for fusion consistent with these data is presented.

Roles of cell signaling pathways in cell-to-cell contact-mediated Epstein-Barr virus transmission

Epstein-Barr virus (EBV), a human gamma herpesvirus, establishes a life-long latent infection in B lymphocytes and epithelial cells following primary infection. Several lines of evidence indicate that the efficiency of EBV infection in epithelial cells is accelerated up to 10(4)-fold by coculturing with EBV-infected Burkitt's lymphoma (BL) cells compared to infection with cell-free virions, indicating that EBV infection into epithelial cells is mainly mediated via cell-to-cell contact. However, the molecular mechanisms involved in this pathway are poorly understood. Here, we establish a novel assay to assess cell-to-cell contact-mediated EBV transmission by coculturing an EBV-infected BL cell line with an EBV-negative epithelial cell line under stimulation for lytic cycle induction. By using this assay, we confirmed that EBV was transmitted from BL cells to epithelial cells via cell-to-cell contact but not via cell-to-cell fusion. The inhibitor treatments of extracellular signal-regulated kinase (ERK) and nuclear factor (NF)-κB pathways blocked EBV transmission in addition to lytic induction. The blockage of the phosphoinositide 3-kinase (PI3K) pathway impaired EBV transmission coupled with the inhibition of lytic induction. Knockdown of the RelA/p65 subunit of NF-κB reduced viral transmission. Moreover, these signaling pathways were activated in cocultured BL cells and in epithelial cells. Finally, we observed that viral replication was induced in cocultured BL cells. Taken together, our data suggest that cell-to-cell contact induces multiple cell signaling pathways in BL cells and epithelial cells, contributing to the induction of the viral lytic cycle in BL cells and the enhancement of viral transmission to epithelial cells.

Membrane requirement for folding of the herpes simplex virus 1 gB cytodomain suggests a unique mechanism of fusion regulation

Herpes simplex virus type 1 (HSV-1) enters cells by fusion of its envelope with a host cell membrane, which requires four viral glycoproteins and a cellular receptor. Viral fusion glycoprotein B (gB) mediates membrane fusion through the action of its ectodomain, while its cytoplasmic domain (cytodomain) regulates fusion from the opposite face of the membrane by an unknown mechanism. The gB cytodomain appears to restrict fusion, because point or truncation mutations within it increase the extent of fusion (syn mutations). Previously, we showed that the hyperfusion phenotype correlated with reduced membrane binding in gB syn truncation mutants and proposed that membrane binding was important in regulating fusion. Here, we extended our analysis to three syn point mutants: A855V, R858H, and A874P. These mutations produce local conformational changes, with some affecting membrane interaction, which suggests that while syn mutants may deregulate fusion by somewhat different mechanisms, maintaining the wild-type (WT) conformation is critical for fusion regulation. We further show that the presence of a membrane is necessary for the cytodomain to achieve its fully folded conformation and propose that the membrane-bound form of the cytodomain represents its native conformation. Taken together, our data suggest that the cytodomain of gB regulates fusion by a novel mechanism in which membrane interaction plays a key role.

Glycoprotein B of herpes simplex virus 2 has more than one intracellular conformation and is altered by low pH

The crystal structure of herpes simplex virus (HSV) gB identifies it as a class III fusion protein, and comparison with other such proteins suggests this is the postfusion rather than prefusion conformation, although this is not proven. Other class III proteins undergo a pH-dependent switch between pre- and postfusion conformations, and a low pH requirement for HSV entry into some cell types suggests that this may also be true for gB. Both gB and gH undergo structural changes at low pH, but there is debate about the extent and significance of the changes in gB, possibly due to the use of different soluble forms of the protein and different assays for antigenic changes. In this study, a complementary approach was taken, examining the conformations of full-length intracellular gB by quantitative confocal microscopy with a panel of 26 antibodies. Three conformations were distinguished, and low pH was found to be a major influence. Comparison with previous studies indicates that the intracellular conformation in low-pH environments may be the same as that of the soluble form known as s-gB at low pH. Interestingly, the antibodies whose binding was most affected by low pH both have neutralizing activity and consequently must block either the function of a neutral pH conformation or its switch from an inactive form to an activated form. If one of the intracellular conformations is the fusion-active form, another factor required for fusion is presumably absent from wherever that conformation is present in infected cells so that inappropriate fusion is avoided.

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

Epstein-Barr virus (EBV), a member of the herpesvirus family, is the causative agent of common human infections and specific malignancies. EBV entry into target cells, including B cells and epithelial cells, requires the interaction of multiple virus-encoded glycoproteins. Glycoproteins H and L (gH/gL) cooperate with glycoprotein B (gB) to mediate fusion of the viral envelope with target cell membranes. Both the gH/gL complex and gB are required for fusion, whereas glycoprotein 42 (gp42) acts as a tropism switch and is required for B cell infection and inhibits epithelial cell infection. Our previous studies identified a prominent KGD motif located on the surface of gH/gL. In the current study, we found that this motif serves as a bifunctional domain on the surface of gH/gL that directs EBV fusion of B cells and epithelial cells. Mutation of the KGD motif to AAA decreased fusion with both epithelial and B cells and reduced the binding of gH/gL to epithelial cells and to gp42. We also demonstrate that deletion of amino acids 62 to 66 of gp42 selectively reduces binding to wild-type gH/gL, but not the KGD mutant, suggesting that the KGD motif of gH/gL interacts with the N-terminal amino acids 62 to 66 of gp42.

Epithelial and B cells are the major targets of Epstein-Barr virus (EBV) infection in the human host. EBV utilizes different glycoprotein complexes to enter these cell types. For B cell fusion, EBV uses complexes containing gp42, gH/gL, and gB, whereas just gH/gL and gB are required for epithelial cell fusion. In the current study, a bifunctional domain consisting of a prominent KGD motif on the surface of the gH/gL structure was identified; this domain affects interactions with gp42 or epithelial receptors, ultimately dictating with which cell type virus-induced fusion can occur. These studies will lead to a better understanding of the mechanism of EBV-induced membrane fusion and herpesvirus-induced membrane fusion in general.

Cell fusion in tumor progression: the isolation of cell fusion products by physical methods

BACKGROUND

Cell fusion induced by polyethylene glycol (PEG) is an efficient but poorly controlled procedure for obtaining somatic cell hybrids used in gene mapping, monoclonal antibody production, and tumour immunotherapy. Genetic selection techniques and fluorescent cell sorting are usually employed to isolate cell fusion products, but both procedures have several drawbacks.

RESULTS

Here we describe a simple improvement in PEG-mediated cell fusion that was obtained by modifying the standard single-step procedure. We found that the use of two PEG undertreatments obtains a better yield of cell fusion products than the standard method, and most of these products are bi- or trinucleated polykaryocytes. Fusion rate was quantified using fluorescent cell staining microscopy. We used this improved cell fusion and cell isolation method to compare giant cells obtained in vitro and giant cells obtained in vivo from patients with Hodgkin's disease and erythroleukemia.

CONCLUSIONS

In the present study we show how to improve PEG-mediated cell fusion and that cell separation by velocity sedimentation offers a simple alternative for the efficient purification of cell fusion products and to investigate giant cell formation in tumor development.

Fusing structure and function: a structural view of the herpesvirus entry machinery

Herpesviruses are double-stranded DNA, enveloped viruses that infect host cells through fusion with either the host cell plasma membrane or endocytic vesicle membranes. Efficient infection of host cells by herpesviruses is remarkably more complex than infection by other viruses, as it requires the concerted effort of multiple glycoproteins and involves multiple host receptors. The structures of the major viral glycoproteins and a number of host receptors involved in the entry of the prototypical herpesviruses, the herpes simplex viruses (HSVs) and Epstein-Barr virus (EBV), are now known. These structural studies have accelerated our understanding of HSV and EBV binding and fusion by revealing the conformational changes that occur on virus-receptor binding, depicting potential sites of functional protein and lipid interactions, and identifying the probable viral fusogen.

Mapping regions of Epstein-Barr virus (EBV) glycoprotein B (gB) important for fusion function with gH/gL

Glycoproteins gB and gH/gL are required for entry of Epstein-Barr virus (EBV) into cells, but the role of each glycoprotein and how they function together to mediate fusion is unclear. Analysis of the functional homology of gB from the closely related primate gammaherpesvirus, rhesus lymphocryptovirus (Rh-LCV), showed that EBV gB could not complement Rh gB due to a species-specific dependence between gB and gL. To map domains of gB required for this interaction, we constructed a panel of EBV/Rh gB chimeric proteins. Analysis showed that insertion of Rh gB from residues 456 to 807 restored fusion function of EBV gB with Rh gH/gL, suggesting this region of gB is important for interaction with gH/gL. Split YFP bimolecular complementation (BiFC) provided evidence of an interaction between EBV gB and gH/gL. Together, our results suggest the importance of a gB-gH/gL interaction in EBV-mediated fusion with B cells requiring the region of EBV gB from 456 to 807.

Class III viral membrane fusion proteins

Members of class III of viral fusion proteins share common structural features and molecular architecture, although they belong to evolutionary distant viruses and carry no sequence homology. Based of the experimentally determined three-dimensional structures of their ectodomains, glycoprotein B (gB) of herpesviruses, G protein of rhabdoviruses and glycoprotein 64 (gp64) of baculoviruses have been identified as class III fusion proteins. The structures are proposed to represent post-fusion conformations, and they reveal trimeric, elongated, rod-like molecules, with each protomer being composed of five domains. Sequences which interact with target membranes and form the fusion peptides are located in two loops found at one end of the molecule. Class III fusion proteins are embedded in viral envelope with the principal function of catalyzing fusion of viral and cellular membranes, an event that is essential for infection to occur. In addition, they have been implicated in processes such as attachment to target cells and viral maturation. G protein is the only class III fusion protein for which structures of both pre- and post-fusion states have been determined, shedding light on the mechanism involved in the conformational change and membrane fusion. Whether similar structural organization of class III fusion proteins translates into a common mechanism involved in carrying out membrane fusion remains to be investigated.

Crystal structure of the Epstein-Barr virus (EBV) glycoprotein H/glycoprotein L (gH/gL) complex

The Epstein-Barr virus (EBV) is a γ-herpesvirus that infects B cells and epithelial cells and that has been linked to malignancies in both cell types in vivo. EBV, like other herpesviruses, has three glycoproteins, glycoprotein B (gB), gH, and gL, that form the core membrane fusion machinery mediating viral penetration into the cell. The gH and gL proteins associate to form a heterodimeric complex, which is necessary for efficient membrane fusion and also implicated in direct binding to epithelial cell receptors required for viral entry. To gain insight into the mechanistic role of gH/gL, we determined the crystal structure of the EBV gH/gL complex. The structure is comprised of four domains organized along the longest axis of the molecule. Comparisons with homologous HSV-2 gH/gL and partial pseudorabies virus gH structures support the domain boundaries determined for the EBV gH/gL structure and illustrate significant differences in interdomain packing angles. The gL subunit and N-terminal residues of gH form a globular domain at one end of the structure, implicated in interactions with gB and activation of membrane fusion. The C-terminal domain of gH, proximal to the viral membrane, is also implicated in membrane fusion. The gH/gL structure locates an integrin binding motif, implicated in epithelial cell entry, on a prominent loop in the central region of the structure. Multiple regions of gH/gL, including its two extreme ends, are functionally important, consistent with the multiple roles of gH/gL in EBV entry.

Burkitt lymphoma: pathogenesis and immune evasion

B-cell lymphomas arise at distinct stages of cellular development and maturation, potentially influencing antigen (Ag) presentation and T-cell recognition. Burkitt lymphoma (BL) is a highly malignant B-cell tumor associated with Epstein-Barr Virus (EBV) infection. Although BL can be effectively treated in adults and children, leading to high survival rates, its ability to mask itself from the immune system makes BL an intriguing disease to study. In this paper, we will provide an overview of BL and its association with EBV and the c-myc oncogene. The contributions of EBV and c-myc to B-cell transformation, proliferation, or attenuation of cellular network and immune recognition or evasion will be summarized. We will also discuss the various pathways by which BL escapes immune detection by inhibiting both HLA class I- and II-mediated Ag presentation to T cells. Finally, we will provide an overview of recent developments suggesting the existence of BL-associated inhibitory molecules that may block HLA class II-mediated Ag presentation to CD4+ T cells, facilitating immune escape of BL.

Mapping the N-terminal residues of Epstein-Barr virus gp42 that bind gH/gL by using fluorescence polarization and cell-based fusion assays

Epstein-Barr virus (EBV) requires at a minimum membrane-associated glycoproteins gB, gH, and gL for entry into host cells. B-cell entry additionally requires gp42, which binds to gH/gL and triggers viral entry into B cells. The presence of soluble gp42 inhibits membrane fusion with epithelial cells by forming a stable heterotrimer of gH/gL/gp42. The interaction of gp42 with gH/gL has been previously mapped to residues 36 to 81 at the N-terminal region of gp42. In this study, we further mapped this region to identify essential features for binding to gH/gL by use of synthetic peptides. Data from fluorescence polarization, cell-cell fusion, and viral infection assays demonstrated that 33 residues corresponding to 44 to 61 and 67 to 81 of gp42 were indispensable for maintaining low-nanomolar-concentration gH/gL binding affinity and inhibiting B-cell fusion and epithelial cell fusion as well as viral infection. Overall, specific, large hydrophobic side chain residues of gp42 appeared to provide critical interactions, determining the binding strength. Mutations of these residues also diminished the inhibition of B-cell and epithelial cell fusions as well as EBV infection. A linker region (residues 62 to 66) between two gH/gL binding regions served as an important spacer, but individual amino acids were not critical for gH/gL binding. Probing the binding site of gH/gL and gp42 with gp42 peptides is critical for a better understanding of the interaction of gH/gL with gp42 as well as for the design of novel entry inhibitors of EBV and related human herpesviruses.

Crystal structure of the conserved herpesvirus fusion regulator complex gH-gL

Herpesviruses, which cause many incurable diseases, infect cells by fusing viral and cellular membranes. Whereas most other enveloped viruses use a single viral catalyst called a fusogen, herpesviruses, inexplicably, require two conserved fusion-machinery components, gB and the heterodimer gH-gL, plus other nonconserved components. gB is a class III viral fusogen, but unlike other members of its class, it does not function alone. We determined the crystal structure of the gH ectodomain bound to gL from herpes simplex virus 2. gH-gL is an unusually tight complex with a unique architecture that, unexpectedly, does not resemble any known viral fusogen. Instead, we propose that gH-gL activates gB for fusion, possibly through direct binding. Formation of a gB-gH-gL complex is critical for fusion and is inhibited by a neutralizing antibody, making the gB-gH-gL interface a promising antiviral target.

Bimolecular complementation defines functional regions of Herpes simplex virus gB that are involved with gH/gL as a necessary step leading to cell fusion

Herpes simplex virus (HSV) entry into cells requires four membrane glycoproteins: gD is the receptor binding protein, and gB and gH/gL constitute the core fusion machinery. Crystal structures of gD and its receptors have provided a basis for understanding the initial triggering steps, but how the core fusion proteins function remains unknown. The gB crystal structure shows that it is a class III fusion protein, yet unlike other class members, gB itself does not cause fusion. Bimolecular complementation (BiMC) studies have shown that gD-receptor binding triggers an interaction between gB and gH/gL and concurrently triggers fusion. Left unanswered was whether BiMC led to fusion or was a by-product of it. We used gB monoclonal antibodies (MAbs) to block different aspects of these events. Non-virus-neutralizing MAbs to gB failed to block BiMC or fusion. In contrast, gB MAbs that neutralize virus blocked fusion. These MAbs map to three functional regions (FR) of gB. MAbs to FR1, which contains the fusion loops, and FR2 blocked both BiMC and fusion. In contrast, MAbs to FR3, a region involved in receptor binding, blocked fusion but not BiMC. Thus, FR3 MAbs separate the BiMC interaction from fusion, suggesting that BiMC occurs prior to fusion. When substituted for wild-type (wt) gB, fusion loop mutants blocked fusion and BiMC, suggesting that loop insertion precedes BiMC. Thus, we postulate that each of the gB FRs are involved in different aspects of the path leading to fusion. Upon triggering by gD, gB fusion loops are inserted into target lipid membranes. gB then interacts with gH/gL, and this interaction is eventually followed by fusion.

Heterogeneous pathways of maternal-fetal transmission of human viruses (review)

Several viruses can pass the maternal-fetal barrier, and cause diseases of the fetus or the newborn. Recently, however, it became obvious, that viruses may invade fetal cells and organs through different routes without acute consequences. Spermatozoa, seminal fluid and lymphocytes in the sperm may transfer viruses into the human zygotes. Viruses were shown to be integrated into human chromosomes and transferred into fetal tissues. The regular maternal-fetal transport of maternal cells has also been discovered. This transport might implicate that lymphotropic viruses can be released into the fetal organs following cellular invasion. It has been shown that many viruses may replicate in human trophoblasts and syncytiotrophoblast cells thus passing the barrier of the maternal-fetal interface. The transport of viral immunocomplexes had also been suggested, and the possibility has been put forward that even anti-idiotypes mimicking viral epitopes might be transferred by natural mechanisms into the fetal plasma, in spite of the selective mechanisms of apical to basolateral transcytosis in syncytiotrophoblast and basolateral to apical transcytosis in fetal capillary endothelium. The mechanisms of maternal-fetal transcytosis seem to be different of those observed in differentiated cells and tissue cultures. Membrane fusion and lipid rafts of high cholesterol content are probably the main requirements of fetal transcytosis. The long term presence of viruses in fetal tissues and their interactions with the fetal immune system might result in post partum consequences as far as increased risk of the development of malignancies and chronic pathologic conditions are discussed.

Functional analysis of glycoprotein L (gL) from rhesus lymphocryptovirus in Epstein-Barr virus-mediated cell fusion indicates a direct role of gL in gB-induced membrane fusion

Glycoprotein L (gL), which complexes with gH, is a conserved herpesvirus protein that is essential for Epstein-Barr virus (EBV) entry into host cells. The gH/gL complex has a conserved role in entry among herpesviruses, yet the mechanism is not clear. To gain a better understanding of the role of gL in EBV-mediated fusion, chimeric proteins were made using rhesus lymphocryptovirus (Rh-LCV) gL (Rh gL), which shares a high sequence homology with EBV gL but does not complement EBV gL in mediating fusion with B cells. A reduction in fusion activity was observed with chimeric gL proteins that contained the amino terminus of Rh gL, although they retained their ability to process and transport gH/gL to the cell surface. Amino acids not conserved within this region in EBV gL when compared to Rh gL were further analyzed, with the results mapping residues 54 and 94 as being functionally important for EBV-mediated fusion. All chimeras and mutants displayed levels of cell surface expression similar to that of wild-type gL and interacted with gH and gp42. Our data also suggest that the role of gL involves the activation or recruitment of gB with the gH/gL complex, as we found that reduced fusion of Rh gL, EBV/Rh-LCV chimeras, and gL point mutants could be restored by replacing EBV gB with Rh gB. These observations demonstrate a distinction between the role of gL in the processing and trafficking of gH to the cell surface and a posttrafficking role in cell-cell fusion.

Structure of Epstein-Barr virus glycoprotein 42 suggests a mechanism for triggering receptor-activated virus entry

Epstein-Barr virus requires glycoproteins gH/gL, gB, and gp42 to fuse its lipid envelope with B cells. Gp42 is a type II membrane protein consisting of a flexible N-terminal region, which binds gH/gL, and a C-terminal lectin-like domain that binds to the B-cell entry receptor human leukocyte antigen (HLA) class II. Gp42 triggers membrane fusion after HLA binding, a process that requires simultaneous binding to gH/gL and a functional hydrophobic pocket in the lectin domain adjacent to the HLA binding site. Here we present the structure of gp42 in its unbound form. Comparisons to the previously determined structure of a gp42:HLA complex reveals additional N-terminal residues forming part of the gH/gL binding site and structural changes in the receptor binding domain. Although the core of the lectin domain remains similar, significant shifts in two loops and an alpha helix bordering the essential hydrophobic pocket suggest a structural mechanism for triggering fusion.

Cleavage and secretion of Epstein-Barr virus glycoprotein 42 promote membrane fusion with B lymphocytes

Epstein-Barr virus (EBV) membrane glycoprotein 42 (gp42) is required for viral entry into B lymphocytes through binding to human leukocyte antigen (HLA) class II on the B-cell surface. EBV gp42 plays multiple roles during infection, including acting as a coreceptor for viral entry into B cells, binding to EBV glycoprotein H (gH) and gL during the process of membrane fusion, and blocking T-cell recognition of HLA class II-peptide complexes through steric hindrance. EBV gp42 occurs in two forms in infected cells, a full-length membrane-bound form and a soluble form generated by proteolytic cleavage that is secreted from infected cells due to loss of the N-terminal transmembrane domain. Both the full-length and the secreted gp42 forms bind to gH/gL and HLA class II, and the functional significance of gp42 cleavage is currently unclear. We found that in a virus-free cell-cell fusion assay, enhanced secretion of gp42 promoted fusion with B lymphocytes, and mutation of the site of gp42 cleavage inhibited membrane fusion activity. The site of gp42 cleavage was found to be physically distinct from the residues of gp42 necessary for binding to gH/gL. These results suggest that cleavage and secretion of gp42 are necessary for the process of membrane fusion with B lymphocytes, providing the first indicated functional difference between full-length and cleaved, secreted gp42.

Class III viral membrane fusion proteins

Accumulating structural studies of viral fusion glycoproteins have revealed unanticipated structural relationships between unrelated virus families and allowed the grouping of these membrane fusogens into three distinct classes. Here we review the newly identified group of class III viral fusion proteins, whose members include fusion proteins from rhabdoviruses, herpesviruses, and baculoviruses. While clearly related in structure, the class III viral fusion proteins exhibit distinct structural features in their architectures as well as in their membrane interacting fusion loops, which are likely related to their virus-specific differences in cellular entry. Further study of the similarities and differences in the class III viral fusion glycoproteins may provide greater insights into protein:membrane interactions that are key to promoting efficient bilayer fusion during virus entry.

Cleavage of Epstein-Barr virus glycoprotein B is required for full function in cell-cell fusion with both epithelial and B cells

Glycoprotein B (gB) homologues within the herpesvirus family display high sequence conservation, and a number of gB homologues contain a cleavage motif R-X-K/R-R recognized by the cellular protease furin. Epstein-Barr virus (EBV) gB contains this motif and cleaved gB is found in EBV virions. To determine the functional significance of this cleavage motif in EBV gB, a deletion mutant (gB Deltafurin) was created lacking the motif. This cleavage mutant was expressed well in cell culture but was not cleaved. Experiments examining gB Deltafurin in a cell-fusion assay revealed that fusion was reduced by 52 % in epithelial and 28 % in B cells when compared with wild-type EBV gB. This decrease in cell-cell fusion is similar to that observed with multiple alphaherpesvirus gB cleavage mutants and supports a conserved function for cleaved gB.

The BDLF2 protein of Epstein-Barr virus is a type II glycosylated envelope protein whose processing is dependent on coexpression with the BMRF2 protein

Epstein-Barr virus has been documented to encode for ten envelope glycoproteins, gB, gH, gL, gM, gN, gp350, gp42, gp78, gp150 and BMRF2. The BDLF2 open reading frame is also predicted to encode a type II membrane protein but, although found in the virion, it has been described as a component of the tegument. We show here that, as predicted, it is the eleventh envelope glycoprotein of the virus. The full length 65 kDa glycoprotein formed a complex with BMRF2 and, as its homologs in other gammaherpesviruses, was dependent on BMRF2, for authentic processing and transport. Two cleavage products of BDLF2 were also identified in cells and in purified virion particles, one corresponding approximately to the aminoterminal half of the protein, that remained associated with the full length form, and one corresponding to the carboxyterminal glycosylated portion of the protein which did not.

Analysis of Epstein-Barr virus glycoprotein B functional domains via linker insertion mutagenesis

Epstein-Barr Virus (EBV) glycoprotein B (gB) is essential for viral fusion events with epithelial and B cells. This glycoprotein has been studied extensively in other herpesvirus family members, but functional domains outside of the cytoplasmic tail have not been characterized in EBV gB. In this study, a total of 28 linker insertion mutations were generated throughout the length of gB. In general, the linker insertions did not disrupt intracellular expression and variably altered cell surface expression. Oligomerization was disrupted by insertions located between residues 561 and 620, indicating the location of a potential site of oligomer contacts between EBV gB monomers. In addition, a novel N-glycosylated form of wild-type gB was identified under nonreducing Western blot conditions that likely represents a mature form of the protein. Fusion activity was abolished in all but three variants containing mutations in the N-terminal region (gB30), within the ectodomain (gB421), and in the intracellular C-terminal domain (gB832) of the protein. Fusion activity with variants gB421 and gB832 was comparable to that of the wild type with epithelial and B cells, and only these two mutants, but not gB30, were able to complement gB-null virus and subsequently function in virus entry. The mutant gB30 exhibited a low level of fusion activity with B cells and was unable to complement gB-null virus. The mutations generated here indicate important structural domains, as well as regions important for function in fusion, within EBV gB.

Membrane binding properties of EBV gp110 C-terminal domain; evidences for structural transition in the membrane environment

Gp110 of Epstein-Barr virus (EBV) mainly localizes on nuclear/ER membranes and plays a role in the assembly of EBV nucleocapsid. The C-terminal tail domain (gp110 CTD) is essential for the function of gp110 and the nuclear/ER membranes localization of gp110 is ruled by its C-terminal unique nuclear localization signal (NLS), consecutive four arginines. In the present study, the structural properties of gp110 CTD in membrane mimics were investigated using CD, size-exclusion chromatography, and NMR, to elucidate the effect of membrane environment on the structural transition and to compare the structural feature of the protein in the solution state with that of the membrane-bound form. CD and NMR analysis showed that gp110 CTD in a buffer solution appears to adopt a stable folding intermediate which lacks compactness, and a highly helical structure is formed only in membrane environments. The helical content of gp110 CTD was significantly affected by the negative charge as well as the size of membrane mimics. Based on the elution profiles of the size-exclusion chromatography, we found that gp110 CTD intrinsically forms a trimer, revealing that a trimerization region may exist in the C-terminal domain of gp110 like the ectodomain of gp110. The mutation of NLS (RRRR) to RTTR does not affect the overall structure of gp110 CTD in membrane mimics, while the helical propensity in a buffer solution was slightly different between the wild-type and the mutant proteins. This result suggests that not only the helicity induced in membrane environment but also the local structure around NLS may be related to trafficking to the nuclear membrane. More detailed structural difference between the wild-type and the mutant in membrane environment was examined using synthetic two peptides including the wild-type NLS and the mutant NLS.

Structures and mechanisms of viral membrane fusion proteins: multiple variations on a common theme

Recent work has identified three distinct classes of viral membrane fusion proteins based on structural criteria. In addition, there are at least four distinct mechanisms by which viral fusion proteins can be triggered to undergo fusion-inducing conformational changes. Viral fusion proteins also contain different types of fusion peptides and vary in their reliance on accessory proteins. These differing features combine to yield a rich diversity of fusion proteins. Yet despite this staggering diversity, all characterized viral fusion proteins convert from a fusion-competent state (dimers or trimers, depending on the class) to a membrane-embedded homotrimeric prehairpin, and then to a trimer-of-hairpins that brings the fusion peptide, attached to the target membrane, and the transmembrane domain, attached to the viral membrane, into close proximity thereby facilitating the union of viral and target membranes. During these conformational conversions, the fusion proteins induce membranes to progress through stages of close apposition, hemifusion, and then the formation of small, and finally large, fusion pores. Clearly, highly divergent proteins have converged on the same overall strategy to mediate fusion, an essential step in the life cycle of every enveloped virus.

Epstein-Barr virus-associated epithelial and mesenchymal neoplasms

Epstein-Barr virus (EBV) is a ubiquitous human pathogen that usually maintains a harmonious relationship with its host. Rarely, this host-virus balance is perturbed, causing a diverse group of malignancies in both immunocompetent and immunosuppressed patients. In addition to its role in hematologic malignancies (Burkitt lymphoma, subsets of Hodgkin and T-cell lymphomas, posttransplant lymphomas), EBV has been implicated in both epithelial (undifferentiated nasopharyngeal carcinoma, a subset of gastric adenocarcinomas) and mesenchymal (EBV-associated smooth muscle tumor, inflammatory pseudotumor-like follicular dendritic cell tumor) neoplasms. This review will focus on EBV-associated epithelial and mesenchymal neoplasms.

The Epstein-Barr virus BMRF-2 protein facilitates virus attachment to oral epithelial cells

We previously reported that BMRF-2, an Epstein-Barr virus (EBV) glycoprotein, binds to beta1 family integrins and is important for EBV infection of polarized oral epithelial cells. To further study the functions of BMRF-2, we constructed a recombinant EBV that lacks BMRF-2 expression by homologous recombination in B95-8 cells. We found that lack of BMRF-2 resulted in about 50% reduction of EBV attachment to oral epithelial cells, but not to B lymphocytes, suggesting that BMRF-2 is critical for EBV infection in oral epithelial cells, but not in B lymphocytes. In polarized oral epithelial cells, infection rate of the recombinant EBV virus was about 4- to 8-fold lower than the wild-type B95-8 virus. Cell adhesion assays using the BMRF-2 RGD peptide and its RGE and AAA mutants showed that the RGD motif is critical for BMRF-2 binding to integrins. These data are consistent with our previous observation that interactions between EBV BMRF-2 and integrins are critical for infection of oral epithelial cells with EBV.

Compatibility of the gH homologues of Epstein-Barr virus and related lymphocryptoviruses

Glycoprotein gH, together with its chaperone gL and a third glycoprotein gB, is essential for cell-cell fusion and virus-cell fusion mediated by herpesviruses. Epstein-Barr virus (EBV), the prototype human lymphocryptovirus, requires a fourth glycoprotein gp42 to support fusion with B cells in addition to epithelial cells. Two other lymphocryptoviruses, the rhesus lymphocryptovirus (Rh-LCV) and the common marmoset lymphocryptovirus (CalHV3), have been sequenced in their entirety and each has a gp42 homologue. Combinations of proteins from EBV, Rh-LCV and CalHV3 were able to mediate fusion of epithelial cells, but, even when complexed with EBV gp42, only Rh-LCV and not CalHV3 proteins were able to mediate fusion with human B cells. CalHV3 gL was also unable to function effectively as a chaperone for EBV or Rh-LCV gH. The Rh-LCV gH homologue supported more fusion than EBV gH with an epithelial cell and supported the highest levels of fusion with a B cell. Chimeric constructs made from Rh-LCV gH and EBV gH that have 85.4 % sequence identity should prove useful for mapping the regions of gH that are of importance to fusion as a whole and to B-cell fusion in particular.

Characterization of EBV gB indicates properties of both class I and class II viral fusion proteins

To gain insight into Epstein-Barr virus (EBV) glycoprotein B (gB), recombinant, secreted variants were generated. The role of putative transmembrane regions, the proteolytic processing and the oligomerization state of the gB variants were investigated. Constructs containing 2 of 3 C-terminal hydrophobic regions were secreted, indicating that these do not act as transmembrane anchors. The efficiency of cleavage of the gB furin site was found to depend on the nature of C-terminus. All of the gB constructs formed rosette structures reminiscent of the postfusion aggregates formed by other viral fusion proteins. However, substitution of putative fusion loop residues, WY(112-113) and WLIY(193-196), with less hydrophobic amino acids from HSV-1 gB, produced trimeric protein and abrogated the ability of the EBV gB ectodomains to form rosettes. These data demonstrate biochemical features of EBV gB that are characteristic of other class I and class II viral fusion proteins, but not of HSV-1 gB.