Point mutations in EBV gH that abrogate or differentially affect B cell and epithelial cell fusion

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.

Epstein-Barr virus (EBV) hyperimmune globulin isolated from donors with high gp350 antibody titers protect humanized mice from challenge with EBV

Primary infection with Epstein-Barr virus (EBV) is associated with post-transplant lymphoproliferative disease and severe disease in patients with X-linked lymphoproliferative disease; no therapies are approved to prevent EBV infection in these patients. Hyperimmune globulin has been used to prevent some virus infections in immunocompromised persons. Here, we identified plasma donors with high titers of EBV gp350 and EBV B cell neutralizing antibodies. Pooled IgG isolated from these donors was compared to intravenous immunoglobulin (IVIG) for its ability to reduce viral load in the blood in humanized mice challenged with EBV. Mice that received EBV hyperimmune globulin had significantly reduced EBV DNA copy numbers compared to animals that received saline control; however, while animals that received EBV hyperimmune globulin had lower EBV DNA copies than those that received IVIG, the difference was not significant. Thus, while EBV hyperimmune globulin reduced viral load compared to IVIG, the effect was modest.

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

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

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.

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

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

The 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.

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.

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.

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.

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.

Structure-function analysis of varicella-zoster virus glycoprotein H identifies domain-specific roles for fusion and skin tropism

Enveloped viruses require membrane fusion for cell entry and replication. For herpesviruses, this event is governed by the multiprotein core complex of conserved glycoproteins (g)B and gH/gL. The recent crystal structures of gH/gL from herpes simplex virus 2, pseudorabies virus, and Epstein-Barr virus revealed distinct domains that, surprisingly, do not resemble known viral fusogens. Varicella-zoster virus (VZV) causes chicken pox and shingles. VZV is an α-herpesvirus closely related to herpes simplex virus 2, enabling prediction of the VZV gH structure by homology modeling. We have defined specific roles for each gH domain in VZV replication and pathogenesis using structure-based site-directed mutagenesis of gH. The distal tip of domain (D)I was important for skin tropism, entry, and fusion. DII helices and a conserved disulfide bond were essential for gH structure and VZV replication. An essential (724)CXXC(727) motif was critical for DIII structural stability and membrane fusion. This assignment of domain-dependent mechanisms to VZV gH links elements of the glycoprotein structure to function in herpesvirus replication and virulence.

Fusion of Epstein-Barr virus with epithelial cells can be triggered by αvβ5 in addition to αvβ6 and αvβ8, and integrin binding triggers a conformational change in glycoproteins gHgL

Fusion of herpesviruses with their target cells requires a minimum of three glycoproteins, namely, gB and a complex of gH and gL. Epstein-Barr virus (EBV) fusion with an epithelial cell requires no additional virus glycoproteins, and we have shown previously that it can be initiated by an interaction between integrin αvβ6 or αvβ8 and gHgL. We now report that integrin αvβ5 can also bind to gHgL and trigger fusion. Binding of gHgL to integrins is a two-step reaction. The first step, analyzed by surface plasmon resonance, was fast, with high association and low dissociation rate constants. The second step, detected by fluorescence spectroscopy of gHgL labeled at cysteine 153 at the domain I-domain II interface with the environmentally sensitive probes acrylodan and IANBD, involved a slower conformational change. Interaction of gHgL with neutralizing monoclonal antibodies or Fab' fragments was also consistent with a two-step reaction involving fast high-affinity binding and a subsequent slower conformational change. None of the antibodies bound to the same epitope, and none completely inhibited integrin binding. However, binding of each decreased the rate of conformational change induced by integrin binding, suggesting that neutralization might involve a conformational change that precludes fusion. Overall, the data are consistent with the interaction of gHgL with an integrin inducing a functionally important rearrangement at the domain I-domain II interface.

Human herpesvirus 6 glycoprotein complex formation is required for folding and trafficking of the gH/gL/gQ1/gQ2 complex and its cellular receptor binding

Human herpesvirus 6 (HHV-6) is a T-cell-tropic betaherpesvirus. A glycoprotein (g) complex that is unique to HHV-6, gH/gL/gQ1/gQ2, is a viral ligand for its cellular receptor, human CD46. However, whether complex formation or one component of the complex is required for CD46 binding and how the complex is transported in cells are open questions. Furthermore, in HHV-6-infected cells the gQ1 protein modified with N-linked glycans is expressed in two forms with different molecular masses: an 80-kDa form (gQ1-80K) and a 74-kDa form (gQ1-74K). Only gQ1-80K, but not gQ1-74K, forms the complex with gQ2, gH, and gL, and this four-component complex is incorporated into mature virions. Here, we characterized the molecular context leading to the maturation of gQ1 by expressing combinations of the individual gH/gL/gQ1/gQ2 components in 293T cells. Surprisingly, only when all four molecules were expressed was a substantial amount of gQ1-80K detected, indicating that all three of the other molecules (gQ2, gH, and gL) were necessary and sufficient for gQ1 maturation. We also found that only the tetrameric complex, and not its subsets, binds to CD46. Finally, a gQ2-null virus constructed in the BAC (bacterial artificial chromosome) system could not be reconstituted, indicating that gQ2 is essential for virus growth. These results show that gH, gL, gQ1, and gQ2 are all essential for the trafficking and proper folding of the gH/gL/gQ1/gQ2 complex and, thus, for HHV-6 infection.

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.

Integrins as triggers of Epstein-Barr virus fusion and epithelial cell infection

Epstein-Barr virus is a ubiquitous orally-transmitted human herpesvirus that is carried by most of the adult population. It establishes latent infections in B lymphocytes, reactivates periodically from latency and can be amplified in epithelial cells where it is thought more commonly to undergo lytic replication. Entry into either cell involves fusion of the virus envelope with a cell membrane. Fusion with a B cell requires four envelope glycoproteins, gB and a ternary complex of gHgLgp42. Fusion is triggered by an interaction between gp42 and HLA class II. Fusion with an epithelial cell requires three envelope glycoproteins, gB and a binary complex of gHgL. The presence of gp42 blocks infection and blocks the interaction of gHgL with a specific receptor on the epithelial cell surface. We recently demonstrated that both integrins αvβ6 and αvβ8 can serve as specific receptors for gHgL and that on binding to gHgL, even in a soluble form, can provide the trigger for direct virus fusion with the epithelial cell plasma membrane. It reveals yet another way in which an integrin can be used by a pathogen to invade a cell.

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.

Structure of a core fragment of glycoprotein H from pseudorabies virus in complex with antibody

Compared with many well-studied enveloped viruses, herpesviruses use a more sophisticated molecular machinery to induce fusion of viral and cellular membranes during cell invasion. This essential function is carried out by glycoprotein B (gB), a class III viral fusion protein, together with the heterodimer of glycoproteins H and L (gH/gL). In pseudorabies virus (PrV), a porcine herpesvirus, it was shown that gH/gL can be substituted by a chimeric fusion protein gDgH, containing the receptor binding domain (RBD) of glycoprotein D fused to a truncated version of gH lacking its N-terminal domain. We report here the 2.1-Å resolution structure of the core fragment of gH present in this chimera, bound to the Fab fragment of a PrV gH-specific monoclonal antibody. The structure strongly complements the information derived from the recently reported structure of gH/gL from herpes simplex virus type 2 (HSV-2). Together with the structure of Epstein-Barr virus (EBV) gH/gL reported in parallel, it provides insight into potentially functional conserved structural features. One feature is the presence of a syntaxin motif, and the other is an extended "flap" masking a conserved hydrophobic patch in the C-terminal domain, which is closest to the viral membrane. The negative electrostatic surface potential of this domain suggests repulsive interactions with the lipid heads. The structure indicates the possible unmasking of an extended hydrophobic patch by movement of the flap during a receptor-triggered conformational change of gH, exposing a hydrophobic surface to interact with the viral membrane during the fusion process.

Characteristics of Epstein-Barr virus envelope protein gp42

Epstein-Barr virus (EBV) glycoprotein 42 (gp42) is a membrane protein essential for fusion and entry of EBV into host B-lymphocytes. Gp42 is a member of the protein-fold family C-type lectin or lectin-like domains (CLECT or CTLD) and specifically is classified as a natural-killer receptor (NKR)-like CLECT. Literature review and phylogenetic comparison show that EBV gp42 shares a common structure with other NKR-like CLECTs and possibly with many viral CTLDs, but does not appear to exhibit some common binding characteristics of many CTLDs, such as features required for calcium binding. The flexible N-terminal region adjacent to the CTLD fold is important for binding to other EBV glycoproteins and for a cleavage site that is necessary for infection of host cells. From structural studies of gp42 unbound and bound to receptor and extensive mutational analysis, a general model of how gp42 triggers membrane fusion utilizing both the flexible N-terminal region and the CTLD domain has emerged.

Fusion of epithelial cells by Epstein-Barr virus proteins is triggered by binding of viral glycoproteins gHgL to integrins alphavbeta6 or alphavbeta8

Epstein-Barr virus (EBV) is a ubiquitous human herpesvirus that is causally implicated in the development of lymphoid and epithelial tumors. Entry of virus requires fusion of virus envelopes and cell membranes. Fusion with B lymphocytes requires virus glycoprotein gB and a 3-part complex of glycoproteins, gHgLgp42. It is triggered by interactions between glycoprotein 42 (gp42) and HLA class II. However, fusion with epithelial cells is impeded by gp42 and instead is triggered by interactions between an unknown epithelial protein and a 2-part complex of gHgL. We report here that gHgL binds with high affinity to epithelial cells and that affinity of binding is increased by 3 orders of magnitude in the presence of Mn(2+). Binding and infection can be reduced by fibronectin and vitronectin, by down-regulation of integrin alphav, or by a peptide corresponding to 13 aa of gH which include a KGDE motif. Fusion of cells expressing gB and gHgL can be blocked by vitronectin or triggered by addition of soluble truncated integrins alphavbeta6 and alphavbeta8. We conclude that the direct interaction between EBV gHgL and integrins alphavbeta6 and alphavbeta8 can provide the trigger for fusion of EBV with an epithelial cell.

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.

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.

Binding-site interactions between Epstein-Barr virus fusion proteins gp42 and gH/gL reveal a peptide that inhibits both epithelial and B-cell membrane fusion

Herpesviruses require membrane-associated glycoproteins gB, gH, and gL for entry into host cells. Epstein-Barr virus (EBV) gp42 is a unique protein also required for viral entry into B cells. Key interactions between EBV gp42 and the EBV gH/gL complex were investigated to further elucidate their roles in membrane fusion. Deletion and point mutants within the N-terminal region of gp42 revealed residues important for gH/gL binding and membrane fusion. Many five-residue deletion mutants in the N-terminal region of gp42 that exhibit reduced membrane fusion activity retain binding with gH/gL but map out two functional stretches between residues 36 and 96. Synthetic peptides derived from the gp42 N-terminal region were studied in in vitro binding experiments with purified gH/gL and in cell-cell fusion assays. A peptide spanning gp42 residues 36 to 81 (peptide 36-81) binds gH/gL with nanomolar affinity, comparable to full-length gp42. Peptide 36-81 efficiently inhibits epithelial cell membrane fusion and competes with soluble gp42 to inhibit B-cell fusion. Additionally, this peptide at low nanomolar concentrations inhibits epithelial cell infection by intact virus. Shorter gp42 peptides spanning the two functional regions identified by deletion mutagenesis had little or no binding to soluble gH/gL and were also unable to inhibit epithelial cell fusion, nor could they complement gp42 deletion mutants in B-cell fusion. These studies identify key residues of gp42 that are essential for gH/gL binding and membrane fusion activation, providing a nanomolar inhibitor of EBV-mediated membrane fusion.

Functional homology of gHs and gLs from EBV-related gamma-herpesviruses for EBV-induced membrane fusion

Epstein-Barr virus (EBV) is a human gamma-herpesvirus that primarily infects B lymphocytes and epithelial cells. Entry of EBV into B cells requires the viral glycoproteins gp42, gH/gL and gB, while gp42 is not necessary for infection of epithelial cells. In EBV, gH and gL form two distinct complexes, a bipartite complex that contains only gH and gL, used for infection of epithelial cells, and a tripartite complex that additionally includes gp42, used for infection of B cells. The gH/gL complex is conserved within the herpesvirus family, but its exact role in entry and mechanism of fusion is not yet known. To understand more about the functionality of EBVgH/gL, we investigated the functional homology of gHs and gLs from human herpesvirus 8 (HHV8) and two primate (rhesus and marmoset) gamma-herpesviruses in EBV-mediated virus-free cell fusion assay. Overall, gHs and gLs from the more homologous primate herpesviruses were better at complementing EBV gH and gL in fusion than HHV8 gH and gL. Interestingly, marmoset gH was able to complement fusion with epithelial cells, but not B cells. Further investigation of this led to the discovery that EBVgH is the binding partner of gp42 in the tripartite complex and the absence of fusion with B cells in the presence of marmoset gH/gL is due to its inability to bind gp42.