Epstein-Barr virus that lacks glycoprotein gN is impaired in assembly and infection

Glycoproteins gM and gN are indispensable factors for rhesus macaque rhadinovirus replication and spread but can be reconstituted by KSHV chimeras

Rhesus macaque rhadinovirus (RRV) is a primate gamma-2 herpesvirus (rhadinovirus) closely related to Kaposi sarcoma-associated herpesvirus (KSHV), the human oncovirus that causes Kaposi sarcoma. Like other herpesviruses, KSHV and RRV encode numerous envelope glycoproteins involved in cell attachment, entry, as well as assembly and release of progeny virions from infected cells. Two glycoproteins postulated to form a complex and reported to be virus-neutralizing targets are glycoproteins M (gM) and N (gN). To investigate gM and gN in rhadinovirus infection, we utilized infectious and pathogenic bacterial artificial chromosomes (BAC). RRV BACmids with nonsense mutations introduced into gM or gN did not yield an infectious virus. However, when gM or gN of RRV were exchanged for gM or gN from KSHV, each of the KSHV-chimeric RRV BACmids restored virus replication and infectious spread. Interestingly, we also discovered that the substitution of KSHVgM into the RRV BACmid was associated with attenuation in viral spread, an effect that was not countered by a double-chimeric virus. In contrast, the substitution of RRV gN into a KSHV BACmid negatively affected the assembly of KSHV, independent of gM/gN complex formation. Therefore, here, we revealed that in KSHV and RRV, gM and gN are interchangeable, contribute to crucial functions for viral assembly and spread, and have evolved in a virus-specific manner. Although more research is needed to define the roles of gM and gN, our work establishes the first glycoprotein-chimeric viruses for KSHV and RRV, which can now be used to corroborate gM/gN as targets for a cancer vaccine.IMPORTANCEKaposi sarcoma (KS) is a human cancer caused by KSHV and is one of the most frequently occurring cancers in HIV/AIDS patients, as well as in regions where KSHV is endemic. In this report, we have constructed and authenticated the first KSHV glycoprotein-encoding chimeric viruses for evaluations in the RRV/rhesus macaque model and have also uncovered fundamental roles for the glycoproteins gM and gN. Our work is significant by successfully bridging the human-specific, species barrier that has previously restricted preclinical evaluations of the KSHV glycoproteins as vaccine targets in vivo. Although there is no KSHV-specific animal model that is widely used, these KSHV-chimeric viruses may be useful as tools to guide future vaccine design and strategy as vaccine candidates progress toward clinical trials.

Research landmarks on the 60th anniversary of Epstein-Barr virus

Epstein-Barr virus (EBV), the first human oncovirus discovered in 1964, has become a focal point in virology, immunology, and oncology because of its unique biological characteristics and significant role in human diseases. As we commemorate the 60th anniversary of EBV's discovery, it is an opportune moment to reflect on the major advancements in our understanding of this complex virus. In this review, we highlight key milestones in EBV research, including its virion structure and life cycle, interactions with the host immune system, association with EBV-associated diseases, and targeted intervention strategies.

Current Insights into the Maturation of Epstein-Barr Virus Particles

The three subfamilies of herpesviruses (alphaherpesviruses, betaherpesviruses, and gammaherpesviruses) appear to share a unique mechanism for the maturation and egress of virions, mediated by several budding and fusion processes of various organelle membranes during replication, which prevents cellular membrane disruption. Newly synthesized viral DNA is packaged into capsids within the nucleus, which are subsequently released into the cytoplasm via sequential fusion (primary envelopment) and budding through the inner and outer nuclear membranes. Maturation concludes with tegumentation and the secondary envelopment of nucleocapsids, which are mediated by budding into various cell organelles. Intracellular compartments containing mature virions are transported to the plasma membrane via host vesicular trafficking machinery, where they fuse with the plasma membrane to extracellularly release mature virions. The entire process of viral maturation is orchestrated by sequential interactions between viral proteins and intracellular membranes. Compared with other herpesvirus subfamilies, the mechanisms of gammaherpesvirus maturation and egress remain poorly understood. This review summarizes the major findings, including recently updated information of the molecular mechanism underlying the maturation and egress process of the Epstein-Barr virus, a ubiquitous human gammaherpesvirus subfamily member that infects most of the population worldwide and is associated with a number of human malignancies.

Epstein-Barr Virus BBLF1 Mediates Secretory Vesicle Transport to Facilitate Mature Virion Release

Enveloped viruses undergo a complex multistep process of assembly, maturation, and release into the extracellular space utilizing host secretory machinery. Several studies of the herpesvirus subfamily have shown that secretory vesicles derived from the trans-Golgi network (TGN) or endosomes transport virions into the extracellular space. However, the regulatory mechanism underlying the release of Epstein-Barr virus, a human oncovirus, remains unclear. We demonstrate that disruption of BBLF1, a tegument component, suppressed viral release and resulted in the accumulation of viral particles on the inner side of the vesicular membrane. Organelle separation revealed the accumulation of infectious viruses in fractions containing vesicles derived from the TGN and late endosomes. Deficiency of an acidic amino acid cluster in BBLF1 reduced viral secretion. Moreover, truncational deletion of the C-terminal region of BBLF1 increased infectious virus production. These findings suggest that BBLF1 regulates the viral release pathway and reveal a new aspect of tegument protein function. IMPORTANCE Several viruses have been linked to the development of cancer in humans. Epstein-Barr virus (EBV), the first identified human oncovirus, causes a wide range of cancers. Accumulating literature has demonstrated the role of viral reactivation in tumorigenesis. Elucidating the functions of viral lytic genes induced by reactivation, and the mechanisms of lytic infection, is essential to understanding pathogenesis. Progeny viral particles synthesized during lytic infection are released outside the cell after the assembly, maturation, and release steps, leading to further infection. Through functional analysis using BBLF1-knockout viruses, we demonstrated that BBLF1 promotes viral release. The acidic amino acid cluster in BBLF1 was also important for viral release. Conversely, mutants lacking the C terminus exhibited more efficient virus production, suggesting that BBLF1 is involved in the fine-tuning of progeny release during the EBV life cycle.

EBV and Lymphomagenesis

The clinical significance of Epstein–Barr virus (EBV) cannot be understated. Not only does it infect approximately 90% of the world’s population, but it is also associated with numerous pathologies. Diseases linked to this virus include hematologic malignancies such as diffuse large B-cell lymphoma, Hodgkin lymphoma, Burkitt lymphoma, primary CNS lymphoma, and NK/T-cell lymphoma, epithelial malignancies such as nasopharyngeal carcinoma and gastric cancer, autoimmune diseases such as multiple sclerosis, Graves’ disease, and lupus. While treatment for these disease states is ever evolving, much work remains to more fully elucidate the relationship between EBV, its associated disease states, and their treatments. This paper begins with an overview of EBV latency and latency-associated proteins. It will then review EBV’s contributions to select hematologic malignancies with a focus on the contribution of latent proteins as well as their associated management.

The Epstein-Barr Virus Glycoprotein BDLF2 Is Essential for Efficient Viral Spread in Stratified Epithelium

Epstein-Barr virus (EBV) is a ubiquitous human pathogen that infects the majority of the adult population regardless of socioeconomic status or geographical location. EBV primarily infects B and epithelial cells and is associated with different cancers of these cell types, such as Burkitt lymphoma and nasopharyngeal carcinoma. While the life cycle of EBV in B cells is well understood, EBV infection within epithelium is not, largely due to the inability to model productive replication in epithelium in vitro. Organotypic cultures generated from primary human keratinocytes can model many aspects of EBV infection, including productive replication in the suprabasal layers. The EBV glycoprotein BDLF2 is a positional homologue of the murine gammaherpesvirus-68 protein gp48, which plays a role in intercellular spread of viral infection, though sequence homology is limited. To determine the role that BDLF2 plays in EBV infection, we generated a recombinant EBV in which the BDLF2 gene has been replaced with a puromycin resistance gene. The ΔBDLF2 recombinant virus infected both B cell and HEK293 cell lines and was able to immortalize primary B cells. However, the loss of BDLF2 resulted in substantially fewer infected cells in organotypic cultures compared to wild-type virus. While numerous clusters of infected cells representing a focus of infection are observed in wild-type-infected organotypic cultures, the majority of cells observed in the absence of BDLF2 were isolated cells, suggesting that the EBV glycoprotein BDLF2 plays a major role in intercellular viral spread in stratified epithelium. IMPORTANCE The ubiquitous herpesvirus Epstein-Barr virus (EBV) is associated with cancers of B lymphocytes and epithelial cells and is primarily transmitted in saliva. While several models exist for analyzing the life cycle of EBV in B lymphocytes, models of EBV infection in the epithelium have more recently been established. Using an organotypic culture model of epithelium that we previously determined accurately reflects EBV infection in situ, we have ascertained that the loss of the viral envelope protein BDLF2 had little effect on the EBV life cycle in B cells but severely restricted the number of infected cells in organotypic cultures. Loss of BDLF2 has a substantial impact on the size of infected areas, suggesting that BDLF2 plays a specific role in the spread of infection in stratified epithelium.

Deleting UL49.5 in duck plague virus causes attachment, entry and spread defects

Duck plague is a disease with high morbidity and mortality rates, and it causes great losses for the duck breeding industry. Duck plague virus (DPV) is the causative agent of duck plague, and DPV UL49.5 protein (pUL49.5) is homologue of glycoprotein N (gN), which is conserved in herpesviruses. UL49.5 homologues are known to be involved in processes such as immune escape, virus assembly, viral fusion, transporter associated with antigen processing (TAP) inhibition and degradation, and maturation and incorporation of glycoprotein M. However, few studies have focused on the role of gN in the early stage of virus infection cells. In this study, we determined that DPV pUL49.5 was distributed in the cytoplasm and colocalized with the endoplasmic reticulum (ER). Moreover, we found that DPV pUL49.5 was a virion component and nonglycosylated protein. To better explore its function, BAC-DPV-ΔUL49.5 was constructed, and its attachment was only approximately 25 % of the revertant virus. Additionally, the penetration ability of BAC-DPV-ΔUL49.5 has only reached 73 % of the revertant virus. The plaque sizes produced by the UL49.5-deleted virus were approximately 58 % smaller than those produced by the revertant virus. Deleting UL49.5 mainly resulted in attachment and cell-to-cell-spread defects. Taken together, these findings suggest important roles for DPV pUL49.5 in viral attachment, penetration and spread.

mRNA-Based Vaccine Designing against Epstein-Barr Virus to Induce an Immune Response Using Immunoinformatic and Molecular Modelling Approaches

Epstein-Barr Virus (EBV) is a human pathogen that has a morbidity rate of 90% in adults worldwide. Infectious mononucleosis is caused by EBV replication in B cells and epithelial cells of the host. EBV has also been related to autoimmune illnesses, including multiple sclerosis and cancers like nasopharyngeal carcinomas and Burkitt's lymphoma. Currently, no effective medications or vaccinations are available to treat or prevent EBV infection. Thus, the current study focuses on a bioinformatics approach to design an mRNA-based multi-epitope (MEV) vaccine to prevent EBV infections. For this purpose, we selected six antigenic proteins from the EBV proteome based on their role in pathogenicity to predict, extract, and analyze T and B cell epitopes using immunoinformatics tools. The epitopes were directed through filtering parameters including allergenicity, toxicity, antigenicity, solubility, and immunogenicity assessment, and finally, the most potent epitopes able to induce T and B cell immune response were selected. In silico molecular docking of prioritized T cell peptides with respective Human Leukocytes Antigens molecules, were carried out to evaluate the individual peptide's binding affinity. Six CTL, four HTL, and ten linear B cell epitopes fulfilled the set parameters and were selected for MEV-based mRNA vaccine. The prioritized epitopes were joined using suitable linkers to improve epitope presentation. The immune simulation results affirmed the designed vaccine's capacity to elicit a proper immune response. The MEV-based mRNA vaccine constructed in this study offers a promising choice for a potent vaccine against EBV.

Gastric epithelial attachment of Helicobacter pylori induces EphA2 and NMHC-IIA receptors for Epstein-Barr virus

Epstein-Barr virus (EBV)-associated gastric cancer belongs to 1 of the 4 subtypes of gastric cancer and accounts for 10% of total gastric cancers. However, most cases of gastric cancer have a history of Helicobacter pylori infection. Therefore, we investigated the possibility that H. pylori infection promotes the development of EBV-associated gastric cancer. H. pylori was exposed to principal EBV receptor, CD21, negative gastric epithelial cells, and then infected with EBV recombinant expressing enhanced green fluorescent protein. Changes in EBV infectivity due to prior H. pylori exposure were analyzed using flow cytometry. The treatment of gastric epithelial cells with H. pylori increased the efficiency of EBV infection. An increase was also observed when CagA-deficient, VacA-deficient, and FlaA-deficient H. pylori strains were used, but not when cag pathogenicity island-deficient H. pylori was used. The treatment of epithelial cells with H. pylori induced the expression of accessory EBV receptors, EphA2 and NMHC-IIA, and increased the efficiency of EBV infection depending on their expression levels. When gastric epithelial cells were treated with EPHA2 or NMHC-IIA siRNA, EBV infection via H. pylori attachment was decreased. The adhesion of H. pylori induced the expression of accessory EBV receptors in gastric epithelial cells and increased the efficiency of EBV infection.

Main Targets of Interest for the Development of a Prophylactic or Therapeutic Epstein-Barr Virus Vaccine

Epstein-Barr virus (EBV) is one of the most widespread viruses in the world; more than 90% of the planet's adult population is infected. Symptomatic primary infection by this Herpesviridae corresponds to infectious mononucleosis (IM), which is generally a benign disease. While virus persistence is often asymptomatic, it is responsible for 1.5% of cancers worldwide, mainly B cell lymphomas and carcinomas. EBV may also be associated with autoimmune and/or inflammatory diseases. However, no effective treatment or anti-EBV vaccine is currently available. Knowledge of the proteins and mechanisms involved in the different steps of the viral cycle is essential to the development of effective vaccines. The present review describes the main actors in the entry of the virus into B cells and epithelial cells, which are targets of interest in the development of prophylactic vaccines aimed at preventing viral infection. This review also summarizes the first vaccinal approaches tested in humans, all of which are based on the gp350/220 glycoprotein; while they have reduced the risk of IM, they have yet to prevent EBV infection. The main proteins involved in the EBV latency cycle and some of the proteins involved in the lytic cycle have essential roles in the oncogenesis of EBV. For that reason, these proteins are of interest for the development of therapeutic vaccines of which the objective is the stimulation of T cell immunity against EBV-associated cancers. New strategies aimed at broadening the antigenic spectrum, are currently being studied and will contribute to the targeting of the essential steps of the viral cycle, the objective being to prevent or treat the diseases associated with EBV.

Epstein-Barr Virus Exploits the Secretory Pathway to Release Virions

Herpesvirus egress mechanisms are strongly associated with intracellular compartment remodeling processes. Previously, we and other groups have described that intracellular compartments derived from the Golgi apparatus are the maturation sites of Epstein-Barr virus (EBV) virions. However, the mechanism by which these virions are released from the host cell to the extracellular milieu is poorly understood. Here, I adapted two independent induction systems of the EBV lytic cycle in vitro, in the context of Rab GTPase silencing, to characterize the EBV release pathway. Immunofluorescence staining revealed that p350/220, the major EBV glycoprotein, partially co-localized with three Rab GTPases: Rab8a, Rab10, and Rab11a. Furthermore, the knockdown of these Rab GTPases promoted the intracellular accumulation of viral structural proteins by inhibiting its distribution to the plasma membrane. Finally, the knockdown of the Rab8a, Rab10, and Rab11a proteins suppressed the release of EBV infectious virions. Taken together, these findings support the hypothesis that mature EBV virions are released from infected cells to the extracellular milieu via the secretory pathway, as well as providing new insights into the EBV life cycle.

Transmembrane regions of bovine herpesvirus 1-encoded UL49.5 and glycoprotein M regulate complex maturation and ER-Golgi trafficking

Bovine herpesvirus 1 (BoHV-1)-encoded UL49.5 (a homologue of herpesvirus glycoprotein N) can combine different functions, regulated by complex formation with viral glycoprotein M (gM). We aimed to identify the mechanisms governing the immunomodulatory activity of BoHV-1 UL49.5. In this study, we addressed the impact of gM/UL49.5-specific regions on heterodimer formation, folding and trafficking from the endoplasmic reticulum (ER) to the trans-Golgi network (TGN) - events previously found to be responsible for abrogation of the UL49.5-mediated inhibition of the transporter associated with antigen processing (TAP). We first established, using viral mutants, that no other viral protein could efficiently compensate for the chaperone function of UL49.5 within the complex. The cytoplasmic tail of gM, containing putative trafficking signals, was dispensable either for ER retention of gM or for the release of the complex. We constructed cell lines with stable co-expression of BoHV-1 gM with chimeric UL49.5 variants, composed of the BoHV-1 N-terminal domain fused to the transmembrane region (TM) from UL49.5 of varicella-zoster virus or TM and the cytoplasmic tail of influenza virus haemagglutinin. Those membrane-anchored N-terminal domains of UL49.5 were sufficient to form a complex, yet gM/UL49.5 folding and ER-TGN trafficking could be affected by the UL49.5 TM sequence. Finally, we found that leucine substitutions in putative glycine zipper motifs within TM helices of gM resulted in strong reduction of complex formation and decreased ability of gM to interfere with UL49.5-mediated major histocompatibility class I downregulation. These findings highlight the importance of gM/UL49.5 transmembrane domains for the biology of this conserved herpesvirus protein complex.

Epstein-Barr Virus Acquires Its Final Envelope on Intracellular Compartments With Golgi Markers

Herpesvirus subfamilies typically acquire their final envelope in various cytoplasmic compartments such as the trans-Golgi network (TGN), and endosomes prior to their secretion into the extracellular space. However, the sites for the final envelopment of Epstein–Barr virus (EBV), a ubiquitous human gamma herpesvirus, are poorly understood. Here, we characterized the sites for the final envelopment of EBV in Burkitt’s lymphoma cell lines induced into the lytic cycle by crosslinking cell surface IgG. Electron microscopy revealed the various stages of maturation and egress of progeny virions including mature EBV in irregular cytoplasmic vesicles. Immunofluorescence staining showed that gp350/220, the major EBV glycoprotein, and the viral capsid antigen, p18, efficiently colocalized with a cis-Golgi marker, GM130. gp350/220 partly colocalized with the TGN, which was distributed in a fragmented and dispersed pattern in the cells induced into the lytic cycle. In contrast, limited colocalization was observed between gp350/220 and endosomal markers, such as a multi-vesicular bodies marker, CD63, a recycling endosome marker, Rab11, and a regulatory secretion vesicles marker, Rab27a. Finally, we observed that treatment of cells with brefeldin A, an inhibitor of vesicle trafficking between the endoplasmic reticulum and Golgi apparatus, resulted in the perinuclear accumulation of gp350/220 and inhibition of its distribution to the plasma membrane. Brefeldin A also inhibited the release of infectious EBV. Taken together, our findings support a model in which EBV acquires its final envelope in intracellular compartments containing markers of Golgi apparatus, providing new insights into how EBV matures.

Betaherpesvirus Virion Assembly and Egress

Virions are the vehicle for cell-to-cell and host-to-host transmission of viruses. Virions need to be assembled reliably and efficiently, be released from infected cells, survive in the extracellular environment during transmission, recognize and then trigger entry of appropriate target cells, and disassemble in an orderly manner during initiation of a new infection. The betaherpesvirus subfamily includes four human herpesviruses (human cytomegalovirus and human herpesviruses 6A, 6B, and 7), as well as viruses that are the basis of important animal models of infection and immunity. Similar to other herpesviruses, betaherpesvirus virions consist of four main parts (in order from the inside): the genome, capsid, tegument, and envelope. Betaherpesvirus genomes are dsDNA and range in length from ~145 to 240 kb. Virion capsids (or nucleocapsids) are geometrically well-defined vessels that contain one copy of the dsDNA viral genome. The tegument is a collection of several thousand protein and RNA molecules packed into the space between the envelope and the capsid for delivery and immediate activity upon cellular entry at the initiation of an infection. Betaherpesvirus envelopes consist of lipid bilayers studded with virus-encoded glycoproteins; they protect the virion during transmission and mediate virion entry during initiation of new infections. Here, we summarize the mechanisms of betaherpesvirus virion assembly, including how infection modifies, reprograms, hijacks, and otherwise manipulates cellular processes and pathways to produce virion components, assemble the parts into infectious virions, and then transport the nascent virions to the extracellular environment for transmission.

Genome-wide analysis of Epstein-Barr virus identifies variants and genes associated with gastric carcinoma and population structure

Epstein-Barr virus is a ubiquitous virus and is associated with several human malignances, including the significant subset of gastric carcinoma, Epstein-Barr virus-associated gastric carcinoma. Some Epstein-Barr virus-associated diseases are uniquely prevalent in populations with different geographic origins. However, the features of the disease and geographically associated Epstein-Barr virus genetic variation as well as the roles that the variation plays in carcinogenesis and evolution remain unclear. Therefore, in this study, we sequenced 95 geographically distinct Epstein-Barr virus isolates from Epstein-Barr virus-associated gastric carcinoma biopsies and saliva of healthy donors to detect variants and genes associated with gastric carcinoma and population structure from a genome-wide spectrum. We demonstrated that Epstein-Barr virus revealed the population structure between North China and South China. In addition, we observed population stratification between Epstein-Barr virus strains from gastric carcinoma and healthy controls, indicating that certain Epstein-Barr virus subtypes are associated with different gastric carcinoma risks. We identified that the BRLF1, BBRF3, and BBLF2/BBLF3 genes had significant associations with gastric carcinoma. LMP1 and BNLF2a genes were strongly geographically associated genes in Epstein-Barr virus. Our study provides insights into the genetic basis of oncogenic Epstein-Barr virus for gastric carcinoma, and the genetic variants associated with gastric carcinoma can serve as biomarkers for oncogenic Epstein-Barr virus.

Global Mapping of O-Glycosylation of Varicella Zoster Virus, Human Cytomegalovirus, and Epstein-Barr Virus

Herpesviruses are among the most complex and widespread viruses, infection and propagation of which depend on envelope proteins. These proteins serve as mediators of cell entry as well as modulators of the immune response and are attractive vaccine targets. Although envelope proteins are known to carry glycans, little is known about the distribution, nature, and functions of these modifications. This is particularly true for O-glycans; thus we have recently developed a "bottom up" mass spectrometry-based technique for mapping O-glycosylation sites on herpes simplex virus type 1. We found wide distribution of O-glycans on herpes simplex virus type 1 glycoproteins and demonstrated that elongated O-glycans were essential for the propagation of the virus. Here, we applied our proteome-wide discovery platform for mapping O-glycosites on representative and clinically significant members of the herpesvirus family: varicella zoster virus, human cytomegalovirus, and Epstein-Barr virus. We identified a large number of O-glycosites distributed on most envelope proteins in all viruses and further demonstrated conserved patterns of O-glycans on distinct homologous proteins. Because glycosylation is highly dependent on the host cell, we tested varicella zoster virus-infected cell lysates and clinically isolated virus and found evidence of consistent O-glycosites. These results present a comprehensive view of herpesvirus O-glycosylation and point to the widespread occurrence of O-glycans in regions of envelope proteins important for virus entry, formation, and recognition by the host immune system. This knowledge enables dissection of specific functional roles of individual glycosites and, moreover, provides a framework for design of glycoprotein vaccines with representative glycosylation.

Epstein-Barr virus glycoprotein gM can interact with the cellular protein p32 and knockdown of p32 impairs virus

The Epstein-Barr virus glycoprotein complex gMgN has been implicated in assembly and release of fully enveloped virus, although the precise role that it plays has not been elucidated. We report here that the long predicted cytoplasmic tail of gM is not required for complex formation and that it interacts with the cellular protein p32, which has been reported to be involved in nuclear egress of human cytomegalovirus and herpes simplex virus. Although redistribution of p32 and colocalization with gM was not observed in virus infected cells, knockdown of p32 expression by siRNA or lentivirus-delivered shRNA recapitulated the phenotype of a virus lacking expression of gNgM. A proportion of virus released from cells sedimented with characteristics of virus lacking an intact envelope and there was an increase in virus trapped in nuclear condensed chromatin. The observations suggest the possibility that p32 may also be involved in nuclear egress of Epstein-Barr virus.

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.

Human herpesvirus 6 glycoprotein M is essential for virus growth and requires glycoprotein N for its maturation

Human herpesvirus 6 (HHV-6) is a T-lymphotropic virus belonging to the betaherpesvirus family. Several HHV-6-encoded glycoproteins are required for cell entry and virion maturation. Glycoprotein M (gM) is conserved among all herpesviruses, and therefore thought to have important functions; however, the HHV-6 g has not been characterized. Here, we examined the expression of HHV-6 g, and examined its function in viral replication, using a mutant and revertant gM. HHV-6 g was expressed on virions as a glycoprotein modified with complex N-linked oligosaccharides. As in other herpesviruses, HHV-6 g formed a complex with glycoprotein N (gN), and was transported from the endoplasmic reticulum to the trans-Golgi network only when part of this complex. Finally, a gM mutant virus in which the gM start codon was destroyed was not reconstituted, although its revertant was, indicating that HHV-6 g is essential for virus production, unlike the gM of alphaherpesviruses.

HCMV-encoded glycoprotein M (UL100) interacts with Rab11 effector protein FIP4

The envelope of human cytomegalovirus (HCMV) consists of a large number of glycoproteins. The most abundant glycoprotein in the HCMV envelope is the glycoprotein M (UL100), which together with glycoprotein N (UL73) form the gM/gN protein complex. Using yeast two-hybrid screening, we found that the gM carboxy-terminal cytoplasmic tail (gM-CT) interacts with FIP4, a Rab11-GTPase effector protein. Depletion of FIP4 expression in HCMV-infected cells resulted in a decrease in infectious virus production that was also associated with an alteration of the HCMV assembly compartment (AC) phenotype. A similar phenotype was also observed in HCMV-infected cells that expressed dominant negative Rab11(S25N). Recently, it has been shown that FIP4 interactions with Rab11 and additionally with Arf6/Arf5 are important for the vesicular transport of proteins in the endosomal recycling compartment (ERC) and during cytokinesis. Surprisingly, FIP4 interaction with gM-CT limited binding of FIP4 with Arf5/Arf6; however, FIP4 interaction with gM-CT did not prevent recruitment of Rab11 into the ternary complex. These data argued for a contribution of the ERC during cytoplasmic envelopment of HCMV and showed a novel FIP4 function independent of Arf5 or Arf6 activity.

Detection of EBV infection and gene expression in oral cancer from patients in Taiwan by microarray analysis

Epstein-Barr virus is known to cause nasopharyngeal carcinoma. Although oral cavity is located close to the nasal pharynx, the pathogenetic role of Epstein-Barr virus (EBV) in oral cancers is unclear. This molecular epidemiology study uses EBV genomic microarray (EBV-chip) to simultaneously detect the prevalent rate and viral gene expression patterns in 57 oral squamous cell carcinoma biopsies (OSCC) collected from patients in Taiwan. The majority of the specimens (82.5%) were EBV-positive that probably expressed coincidently the genes for EBNAs, LMP2A and 2B, and certain structural proteins. Importantly, the genes fabricated at the spots 61 (BBRF1, BBRF2, and BBRF3) and 68 (BDLF4 and BDRF1) on EBV-chip were actively expressed in a significantly greater number of OSCC exhibiting exophytic morphology or ulceration than those tissues with deep invasive lesions (P = .0265 and .0141, resp.). The results may thus provide the lead information for understanding the role of EBV in oral cancer pathogenesis.

Kaposi's sarcoma-associated herpesvirus glycoproteins B and K8.1 regulate virion egress and synthesis of vascular endothelial growth factor and viral interleukin-6 in BCBL-1 cells

Kaposi's sarcoma-associated herpesvirus (KSHV) viral glycoproteins play important roles in the infectious life cycle and have been implicated in KSHV-associated endothelial cell transformation, angiogenesis, and KS-induced malignancies. KSHV-associated primary effusion lymphomas (PELs) secrete high levels of vascular endothelial growth factor (VEGF) and viral interleukin-6 (vIL-6) in vitro and VEGF, vIL-6, and basic-fibroblast growth factor (b-FGF) in mouse xenografts. KSHV-encoded glycoproteins B (gB) and K8.1 stimulate VEGF secretion, most likely mediated by direct or indirect binding to cell surface receptors, including the gB-specific alphaVbeta3 and alpha3beta1 integrins. In this study, the short interfering RNA (siRNA)-mediated inhibition of either gB or K8.1 transcription by anti-gB or -K8.1 siRNAs caused a substantial reduction in virion egress and a decrease in both vIL-6 and VEGF production. Similarly, the treatment of BCBL-1 cells with anti-gB or anti-K8.1 antibodies caused a substantial reduction in vIL-6 and VEGF production. Codon-optimized versions of either wild-type gB, mutant gB having the RGD amino acid motif changed to RAA, or K8.1 efficiently rescued virion egress and VEGF and vIL-6 production. These results suggest that the binding of gB via its RGD motif to integrin receptors was not responsible for the observed gB-associated regulation of VEGF and vIL-6 transcription. Conditioned medium collected from BCBL-1 cells transfected with anti-gB and anti-K8.1 siRNAs or treated with anti-gB and anti-K8.1 antibodies exhibited a significantly reduced ability to induce the formation of the capillary network of endothelial cells compared to the ability of medium from mock-infected BCBl-1 cells. Furthermore, medium obtained from BCBL-1 cells expressing smaller amounts of gB and K8.1 produced a substantial reduction in endothelial cell migration in a vertical migration assay compared to that of control medium containing wild-type levels of gB and K8.1. These results suggest a functional linkage between gB/K8.1 synthesis and VEGF/vIL-6 transcriptional regulation via paracrine and/or autocrine signaling pathways.

Effects of simultaneous deletion of pUL11 and glycoprotein M on virion maturation of herpes simplex virus type 1

The conserved membrane-associated tegument protein pUL11 and envelope glycoprotein M (gM) are involved in secondary envelopment of herpesvirus nucleocapsids in the cytoplasm. Although deletion of either gene had only moderate effects on replication of the related alphaherpesviruses herpes simplex virus type 1 (HSV-1) and pseudorabies virus (PrV) in cell culture, simultaneous deletion of both genes resulted in a severe impairment in virion morphogenesis of PrV coinciding with the formation of huge inclusions in the cytoplasm containing nucleocapsids embedded in tegument (M. Kopp, H. Granzow, W. Fuchs, B. G. Klupp, and T. C. Mettenleiter, J. Virol. 78:3024-3034, 2004). To test whether a similar phenotype occurs in HSV-1, a gM and pUL11 double deletion mutant was generated based on a newly established bacterial artificial chromosome clone of HSV-1 strain KOS. Since gM-negative HSV-1 has not been thoroughly investigated ultrastructurally and different phenotypes have been ascribed to pUL11-negative HSV-1, single gene deletion mutants were also constructed and analyzed. On monkey kidney (Vero) cells, deletion of either pUL11 or gM resulted in ca.-fivefold-reduced titers and 40- to 50%-reduced plaque diameters compared to those of wild-type HSV-1 KOS, while on rabbit kidney (RK13) cells the defects were more pronounced, resulting in ca.-50-fold titer and 70% plaque size reduction for either mutant. Electron microscopy revealed that in the absence of either pUL11 or gM virion formation in the cytoplasm was inhibited, whereas nuclear stages were not visibly affected, which is in line with the phenotypes of corresponding PrV mutants. Simultaneous deletion of pUL11 and gM led to additive growth defects and, in RK13 cells, to the formation of large intracytoplasmic inclusions of capsids and tegument material, comparable to those in PrV-DeltaUL11/gM-infected RK13 cells. The defects of HSV-1DeltaUL11 and HSV-1DeltaUL11/gM could be partially corrected in trans by pUL11 of PrV. Thus, our data indicate that PrV and HSV-1 pUL11 and gM exhibit similar functions in cytoplasmic steps of virion assembly.

The cytoplasmic terminus of Kaposi's sarcoma-associated herpesvirus glycoprotein B is not essential for virion egress and infectivity

Kaposi's sarcoma-associated herpesvirus (KSHV)-encoded glycoprotein B (gB) is an important determinant of viral infectivity and virion egress. A small interfering RNA (siRNA)-based strategy was devised to inhibit KSHV gB gene expression. Transient cotransfection of plasmids constitutively expressing gB and anti-gB siRNAs in 293 cells substantially inhibited gB mRNA levels and protein production. Similarly, transient expression of siRNAs into the primary effusion lymphoma cell line BCBL-1 caused a substantial reduction of gB transcripts and protein synthesis. TaqMan real-time PCR assays against the lytic KSHV gene ORF59 and infectivity assays on 293 cells were employed to assess the effect of inhibiting gB synthesis on virion egress from BCBL-1 cells and infectivity on 293 cells, respectively. These experiments showed that gB was essential for virion egress and infectivity. Transfection of a codon-optimized gB gene with the first 540 nucleotides altered, and therefore not recognized by anti-gB siRNAs that target the native but not the codon-optimized sequence, efficiently rescued virion egress and infectivity in BCBL-1 cells in the presence of siRNAs inhibiting wild-type gB expression. To assess the role of the cytoplasmic domain of gB in virion egress, mutant gB genes were generated specifying carboxyl terminal truncations of 25 and 58 amino acids disrupting two prominent predicted alpha-helical domains associated with virus-induced cell fusion. A third truncation removed the entire predicted cytoplasmic terminus of 84 amino acids, while a fourth truncation removed 110 amino acids, including the terminal most hydrophobic, intramembrane anchoring sequence. Virion egress experiments revealed that all truncated gBs facilitated virion egress from BCBL-1 cells, with the exception of the largest 110-amino-acid truncation, which removed the gB anchoring sequence. Importantly, the gB truncation that removed the entire predicted cytoplasmic domain increased virion egress, suggesting the presence of a egress regulation domain located proximal to the intramembrane sequence within the cytoplasmic domain of gB. All supernatant virions were infectious on 293 cells, indicating that the carboxyl terminus of gB is not essential for either virion egress or virus infectivity.

An essential role for the proximal but not the distal cytoplasmic tail of glycoprotein M in murid herpesvirus 4 infection

Murid herpesvirus-4 (MuHV-4) provides a tractable model with which to define common, conserved features of gamma-herpesvirus biology. The multi-membrane spanning glycoprotein M (gM) is one of only 4 glycoproteins that are essential for MuHV-4 lytic replication. gM binds to gN and is thought to function mainly secondary envelopment and virion egress, for which several predicted trafficking motifs in its C-terminal cytoplasmic tail could be important. We tested the contribution of the gM cytoplasmic tail to MuHV-4 lytic replication by making recombinant viruses with varying C-terminal deletions. Removing an acidic cluster and a distal YXXΦ motif altered the capsid distribution somewhat in infected cells but had little effect on virus replication, either in vitro or in vivo. In contrast, removing a proximal YXXΦ motif as well completely prevented productive replication. gM was still expressed, but unlike its longer forms showed only limited colocalization with co-transfected gN, and in the context of whole virus appeared to support gN expression less well. We conclude that some elements of the gM cytoplasmic tail are dispensible for MuHV-4 replication, but the tail as a whole is not.

Evidence for a multiprotein gamma-2 herpesvirus entry complex

Herpesviruses use multiple virion glycoproteins to enter cells. How these work together is not well understood: some may act separately or they may form a single complex. Murine gammaherpesvirus 68 (MHV-68) gB, gH, gL, and gp150 all participate in entry. gB and gL are involved in binding, gB and gH are conserved fusion proteins, and gp150 inhibits cell binding until glycosaminoglycans are engaged. Here we show that a gH-specific antibody coprecipitates gB and thus that gH and gB are associated in the virion membrane. A gH/gL-specific antibody also coprecipitated gB, implying a tripartite complex of gL/gH/gB, although the gH/gB association did not require gL. The association was also independent of gp150, and gp150 was not demonstrably bound to gB or gH. However, gp150 incorporation into virions was partly gL dependent, suggesting that it too contributes to a single entry complex. gp150- and gL- gp150- mutants bound better than the wild type to B cells and readily colonized B cells in vivo. Thus, gp150 and gL appear to be epithelial cell-adapted accessories of a core gB/gH entry complex. The cell binding revealed by gp150 disruption did not require gL and therefore seemed most likely to involve gB.

The cytoplasmic tail of glycoprotein M (gpUL100) expresses trafficking signals required for human cytomegalovirus assembly and replication

The virion envelope of human cytomegalovirus (HCMV) is complex and consists of an incompletely defined number of glycoproteins. The gM/gN protein complex is the most abundant protein component of the envelope. Studies have indicated that deletion of the viral gene encoding either gM or gN is a lethal mutation. Analysis of the amino acid sequence of gM disclosed a C-terminal acidic cluster of amino acids and a tyrosine-containing trafficking motif, both of which are well-described trafficking/sorting signals in the cellular secretory pathway. To investigate the roles of these signals in the trafficking of the gM/gN complex during virus assembly, we made a series of gM (UL100 open reading frame) mutants in the AD169 strain of HCMV. Mutant viruses that lacked the entire C-terminal cytoplasmic tail of gM were not viable, suggesting that the cytoplasmic tail of gM is essential for virus replication. In addition, the gM mutant protein lacking the cytoplasmic domain exhibited decreased protein stability. Mutant viruses with a deletion of the acidic cluster or alanine substitutions in tyrosine-based motifs were viable but exhibited a replication-impaired phenotype suggestive of a defect in virion assembly. Analysis of these mutant gMs using static immunofluorescence and fluorescence recovery after photobleaching demonstrated delayed kinetics of intracellular localization of the gM/gN protein to the virus assembly compartment compared to the wild-type protein. These data suggest an important role of the glycoprotein gM during virus assembly, particularly in the dynamics of gM trafficking during viral-particle assembly.

Characterization of the Epstein–Barr virus glycoprotein BMRF-2

Epstein-Barr virus (EBV) BMRF-2 protein interaction with the beta1 family of integrins plays an important role in EBV infection of polarized oral epithelial cells. In this work, we characterized BMRF-2 protein expression in EBV-infected B lymphoblastoid and polarized oral epithelial cells, and in hairy leukoplakia (HL) epithelium. BMRF-2 expression in B cells and polarized oral epithelial cells was associated with the EBV lytic infection. In these cells, BMRF-2 is efficiently transported to the cell membrane and its integrin binding Arg-Gly-Asp (RGD) motif is exposed on the cell surface. BMRF-2 is highly expressed in HL epithelium and accumulates at the lateral border of oral keratinocytes. In EBV-infected polarized oral epithelial cells, this protein is transported to the basolateral membranes and co-localized with beta1 integrin. These data suggest that BMRF-2 may play an important role in cell-to-cell spread of EBV within the oral epithelium. BMRF-2 is glycosylated through O-linked oligosaccharides; it forms oligomers and is associated with the virion envelope. Its C-terminal tail is localized in the cytoplasm. We found that beta1, alpha5, and alpha3 integrins are present in purified EBV virions. We show that BMRF-2 is a ligand for beta1, alpha5, alpha3, and alphav integrins and our data are consistent with a role for BMRF-2 in viral lytic infection.

The carboxy-terminal domain of glycoprotein N of human cytomegalovirus is required for virion morphogenesis

Glycoproteins M and N (gM and gN, respectively) are among the few proteins that are conserved across the herpesvirus family. The function of the complex is largely unknown. Whereas deletion from most alphaherpesviruses has marginal effects on the replication of the respective viruses, both proteins are essential for replication of human cytomegalovirus (HCMV). We have constructed a series of mutants in gN to study the function of this protein. gN of HCMV is a type I glycoprotein containing a short carboxy-terminal domain of 14 amino acids, including two cysteine residues directly adjacent to the predicted transmembrane anchor at positions 125 and 126. Deletion of the entire carboxy-terminal domain as well as substitution with the corresponding region from alpha herpesviruses or mutations of both cysteine residues resulted in a replication-incompetent virus. Recombinant viruses containing point mutations of either cysteine residue could be generated. These viruses were profoundly defective for replication. Complex formation of the mutant gNs with gM and transport of the complex to the viral assembly compartment appeared unaltered compared to the wild type. However, in infected cells, large numbers of capsids accumulated in the cytoplasm that failed to acquire an envelope. Transiently expressed gN was shown to be modified by palmitic acid at both cysteine residues. In summary, our data suggest that the carboxy-terminal domain of gN plays a critical role in secondary envelopment of HCMV and that palmitoylation of gN appears to be essential for function in secondary envelopment of HCMV and virus replication.

Glycoprotein M of herpes simplex virus 1 is incorporated into virions during budding at the inner nuclear membrane

It is widely accepted that nucleocapsids of herpesviruses bud through the inner nuclear membrane (INM), but few studies have been undertaken to characterize the composition of these nascent virions. Such knowledge would shed light on the budding reaction at the INM and subsequent steps in the egress pathway. The present study focuses on glycoprotein M (gM), a type III integral membrane protein of herpes simplex virus 1 (HSV-1) that likely contains eight transmembrane domains. The results indicated that gM localized primarily at the perinuclear region, with especially bright staining near the nuclear membrane (NM). Immunogold electron microscopic analysis indicated that, like gB and gD (M. R. Torrisi et al., J. Virol. 66:554-561, 1992), gM localized within both leaflets of the NM, the envelopes of nascent virions that accumulate in the perinuclear space, and the envelopes of cytoplasmic and mature extracellular virus particles. Indirect immunofluorescence studies revealed that gM colocalized almost completely with a marker of the Golgi apparatus and partially with a marker of the trans-Golgi network (TGN), whether or not these markers were displaced to the perinuclear region during infection. gM was also located in punctate extensions and invaginations of the NM induced by the absence of a viral kinase encoded by HSV-1 U(S)3 and within virions located in these extensions. Our findings therefore support the proposition that gM, like gB and gD, becomes incorporated into the virion envelope upon budding through the INM. The localization of viral glycoproteins and Golgi and TGN markers to a perinuclear region may represent a mechanism to facilitate the production of infectious nascent virions, thereby increasing the amount of infectivity released upon cellular lysis.

Epstein-Barr virus shed in saliva is high in B-cell-tropic glycoprotein gp42

Epstein-Barr virus is an orally transmitted human herpesvirus that infects epithelial cells and establishes latency in memory B lymphocytes. Movement of virus between the two cell types is facilitated by changes in amounts of an envelope glycoprotein, gp42, which are effected by interaction of gp42 with HLA class II in a B cell. Here we used the differential ability of virus to bind to CD21-positive B cells and CD21-negative epithelial cells, which is also influenced by levels of gp42, to determine that the majority of virus shed in saliva is derived from an HLA class II-negative cell.

Murine gammaherpesvirus-68 glycoprotein H-glycoprotein L complex is a major target for neutralizing monoclonal antibodies

Herpesviruses characteristically persist in immune hosts as latent genomes, but to transmit infection they must reactivate and replicate lytically. The interaction between newly formed virions and pre-existing antibody is therefore likely to be a crucial determinant of viral fitness. Murine gammaherpesvirus-68 (MHV-68) behaves as a natural pathogen of conventional, inbred mice and consequently allows such interactions to be analysed experimentally in a relatively realistic setting. Here, monoclonal antibodies (mAbs) were derived from MHV-68-infected mice and all those recognizing infected-cell surfaces were tested for their capacity to neutralize MHV-68 virions. All of the neutralizing mAbs identified were specific for the viral glycoprotein H (gH)-gL heterodimer and required both gH and gL to reproduce their cognate epitopes. Based on antibody interference, there appeared to be two major neutralization epitopes on gH-gL. Analysis of a representative mAb indicated that it blocked infection at a post-binding step--either virion endocytosis or membrane fusion.

Intercellular gamma-herpesvirus dissemination involves co-ordinated intracellular membrane protein transport

The murine gamma-herpesvirus-68 (MHV-68) ORF27 encodes gp48, a type 2 transmembrane glycoprotein that contributes to intercellular viral spread. Gp48 is expressed on the surface of infected cells but is retained intracellularly after transfection. In this study, we show that the multimembrane spanning ORF58 gene product is both necessary and sufficient for gp48 to reach the cell surface. ORF58-deficient MHV-68 expressed ORF27 in normal amounts, but retained it in the endoplasmic reticulum (ER). Transfected ORF27 also remained in ER, whereas green fluorescent protein-tagged ORF58 localized to the ER and trans-Golgi network. When ORF27 and ORF58 were co-transfected, they formed a protein complex and reached the cell surface. Surprisingly, ORF58 rather than ORF27 mediated cell binding via a small extracellular loop. The heavily glycosylated ORF27 component of the complex may, therefore, act mainly to protect this loop against antibody. The interdependent transport of ORF27 and ORF58 transport ensures that such protection is always present.

Envelope glycoprotein gB of Kaposi's sarcoma-associated herpesvirus is essential for egress from infected cells

Kaposi's sarcoma-associated herpesvirus (KSHV) envelope glycoprotein gB interacts with cell surface heparan sulfate (HS) and alpha3beta1 integrin and plays roles in the initial binding and entry into the target cells and in the induction of preexisting host cell signal pathways. To define gB function further, using a bacterial artificial chromosome (BAC) system carrying the KSHV genome (BAC36wt-KSHV), we constructed a recombinant virus genome with the gB open reading frame (ORF) deleted by replacing a 2-kb gB ORF with a 1.3-kb Kan(r) gene. Stable 293T cells carrying BAC36wt-KSHV and DeltagBBAC36-KSHV genomes were generated. Transcript analyses and immunoprecipitation reactions confirmed the absence of gB in the 293T-DeltagBBAC36 cells. When monolayers of 293T-BAC36wt and 293T-DeltagBBAC36 cells were induced with tetradecanoylphorbol-13-acetate, infectious virus was detected only from the 293T-BAC36wt cell supernatants. No significant amount of DNase I-resistant viral DNA was detected in the supernatants of 293T-DeltagBBAC36 cells. BAC36wt-KSHV infected the target cells, and in contrast, no viral DNA and transcripts could be detected in cells infected with DeltagBBAC36-KSHV. Electron microscopy of 293T-DeltagBBAC36 cells revealed capsids in the nuclei, cytoplasmic vesicles with core-containing capsids, and occasional enveloped virions in the cytoplasm. However, enveloped virus particles were observed in the extracellular compartments of 293T-BAC36wt cells only and not in 293T-DeltagBBAC36 cells. Transfection of 293T-DeltagBBAC36 cells with plasmid expressing full-length gB restored the recovery of infectious KSHV in the supernatant. These results suggest that, besides its role in virus binding and entry into the target cells, KSHV gB also plays a role in the maturation and egress of virus from the infected cells.

The nonessential UL49.5 gene of infectious laryngotracheitis virus encodes an O-glycosylated protein which forms a complex with the non-glycosylated UL10 gene product

The UL10 and UL49.5 genes of avian infectious laryngotracheitis virus (ILTV) encode putative envelope proteins which are conserved in Alpha, Beta, and Gammaherpesvirinae. Many of the corresponding gene products have been shown to be glycosylated and to form heterodimeric protein complexes with each other. Unlike the homologous gM proteins of other herpesviruses, the UL10 protein of ILTV is not detectably glycosylated [Fuchs, W., Mettenleiter, T.C., 1999. DNA sequence of the UL6 to UL20 genes of infectious laryngotracheitis virus and characterization of the UL10 gene product as a nonglycosylated and nonessential virion protein. J. Gen. Virol. 80, 2173-2182]. Using a monospecific antiserum, we now identified the UL49.5 gene product of ILTV as an O-glycosylated membrane protein (gN). Correct processing of gN was shown to depend on the presence of the UL10 protein. Both gN and UL10 could be co-immunoprecipitated from ILTV-infected cell lysates with antisera against either of the proteins, indicating stable protein-protein interactions. For functional analysis parts of the UL10 and UL49.5 open reading frames were deleted from the ILTV genome, and replaced by a beta-galactosidase expression cassette. The resulting virus mutants were isolated and propagated in non-complementing chicken cells, which demonstrated that the UL10 and UL49.5 genes are not essential for in vitro replication of ILTV.

Complex formation by glycoproteins M and N of human cytomegalovirus: structural and functional aspects

The genomes of herpesviruses contain a number of genes which are conserved throughout the family of Herpesviridae, indicating that the proteins may serve important functions in the replication of these viruses. Among these are several envelope glycoproteins, including glycoprotein M (gM) and gN, which form a complex that is covalently linked via disulfide bonds in some herpesviruses. However, deletion of gM and/or gN from most alphaherpesviruses has limited effects on replication of the respective viruses in vitro. In contrast, insertional inactivation of the gM gene of the betaherpesvirus human cytomegalovirus (HCMV) results in a replication-incompetent virus. We have started to analyze the structural and functional aspects of the interaction between gM and gN of HCMV. We show that large parts of gM are dispensable for the formation of a gM/gN complex that is transported to distal parts of the cellular secretory pathway. In addition, we demonstrate that the disulfide bond is between the cysteine at position 44 in gM and cysteine 90 in gN. However, disulfide linkage is not a prerequisite for modification and transport of the gM/gN complex. Moreover, mutant viruses that lack a disulfide bridge between gM and gN replicate with efficiencies similar to that of wild-type viruses.

Salivary lactoferrin is recognized by the human herpesvirus-8

Human herpesvirus-8 (HHV-8) is commonly detected in all epidemiologic forms of Kaposi's sarcoma. Despite the broad cellular tropism of HHV-8, studies on mucosal shedding of HHV-8 have shown that infectious particles are restricted to saliva isolated from the oropharynx. We used biotinylated purified HHV-8 particles in a direct binding assay to whole clarified human salivary samples isolated from HHV-8-infected and uninfected individuals. We found that the major binding activity was carried out by a protein of 78-kDa size, which was further characterized as human lactoferrin (hLf) using 2-D electrophoresis and MALDI-ToF analysis. Preliminary comparison of HHV-8 binding activity of 76 salivary samples from HHV-8-infected and uninfected individuals showed that 7.8% of the uninfected population exhibited a form of Lf not recognized by HHV-8. Deglycosylation of hLf by PNGase F did not reduce HHV-8 binding activity, whereas endoproteinase cleavage of native hLf generated a non-glycosylated 8-kDa peptide recognized by HHV-8 particles and was located at the position Ala606-Tyr679 in the native hLf amino acid sequence, corresponding to the C-terminal region of the glycoprotein. This work identify the lactoferrin in saliva as a ligand for HHV-8 and suggests that this glycoprotein could be used as a carrier for the viral particles.

The murine gammaherpesvirus 68 ORF27 gene product contributes to intercellular viral spread

Herpesviruses remain predominantly cell associated within their hosts, implying that they spread between cells by a mechanism distinct from free virion release. We previously identified the efficient release of murine gammaherpesvirus 68 (MHV-68) virions as a function of the viral gp150 protein. Here we show that the MHV-68 ORF27 gene product, gp48, contributes to the direct spread of viruses from lytically infected to uninfected cells. Monoclonal antibodies to gp48 identified it on infected cell surfaces and in virions. gp48-deficient viruses showed no obvious deficit in virion cell binding, single-cycle replication, or virion release but had reduced lytic propagation between cells. After intranasal infection of mice, ORF27-deficient viruses were impaired predominantly in lytic replication in the lungs. There was a small deficit in latency establishment, but long-term latency appeared normal. Since ORF27 has homologs in both Epstein-Barr virus and Kaposi's sarcoma-associated herpesvirus, it is likely part of a conserved mechanism employed by gammaherpesviruses to disseminate lytically in their hosts.

Glycoprotein M is an essential lytic replication protein of the murine gammaherpesvirus 68

All herpesviruses encode a homolog of glycoprotein M (gM), which appears to function in virion morphogenesis. Despite its conservation, gM is inessential for the lytic replication of alphaherpesviruses. In order to address the importance of gM in gammaherpesviruses, we disrupted it in the murine gammaherpesvirus 68 (MHV-68). The mutant virus completely failed to propagate in normally permissive fibroblasts. The defective genome was rescued by either homologous recombination to restore the wild-type gM in situ or the insertion of an ectopic, intergenic expression cassette encoding gM into the viral genome. Thus, gM was essential for the lytic replication of MHV-68.

BFRF1 of Epstein-Barr virus is essential for efficient primary viral envelopment and egress

The molecular mechanisms that underlie maturation and egress of Epstein-Barr virus (EBV) virions are only partially characterized. We have recently shown that the BFRF1 gene, the EBV positional homolog of herpes simplex virus type 1 and pseudorabies virus UL34, is expressed early during EBV lytic replication and that it is found predominantly on the nuclear membrane (A. Farina, R. Santarelli, R. Gonnella, R. Bei, R. Muraro, G. Cardinali, S. Uccini, G. Ragona, L. Frati, A. Faggioni, and A. Angeloni, J. Virol. 74:3235-3244, 2000). These data suggest that the BFRF1 protein might be involved in viral primary envelopment. To precisely determine the function of this protein, we have constructed an EBV mutant devoid of the BFRF1 gene (BFRF1-KO). 293 cells carrying BFRF1-KO showed no differences in comparison with wild-type EBV in terms of DNA lytic replication or expression of late viral proteins upon induction of the lytic cycle. However, binding assays and infection experiments using cell lines or human cord blood lymphocytes showed a clear reduction in the viral mutant titers. Complementation experiments with BFRF1-KO and a BFRF1 expression vector restored viral titers to levels similar to those for the wild-type control, showing that the modifications that we introduced were limited to the BFRF1 gene. Electron microscopic observations showed that the reduction in viral titers was due to sequestration of EBV nucleocapsids in the nuclei of lytically induced cells. This suggests that BFRF1 is involved in transport of the maturing virion across the nuclear membrane. This hypothesis was further supported by the observation that BFRF1 is present in maturing intracellular virions but not in their extracellular counterparts. This implies that BFRF1 is a key protein for EBV maturation.

Alphaherpesvirus glycoprotein M causes the relocalization of plasma membrane proteins

Herpesvirus glycoprotein M (gM) is a multiple-spanning integral membrane protein found within the envelope of mature herpesviruses and is conserved throughout the Herpesviridae. gM is defined as a non-essential glycoprotein in alphaherpesviruses and has been proposed as playing a role in controlling final envelopment in a late secretory-pathway compartment such as the trans-Golgi network (TGN). Additionally, gM proteins have been shown to inhibit cell-cell fusion in transfection-based assays by an as yet unclear mechanism. Here, the effect of pseudorabies virus (PRV) gM and the herpes simplex virus type 1 (HSV-1) gM/UL49A complex on the fusion events caused by the HSV-1 glycoproteins gB, gD, gH and gL was investigated. Fusion of cells expressing HSV-1 gB, gD, gH and gL was efficiently inhibited by both PRV gM and HSV-1 gM/UL49A. Furthermore, expression of PRV gM or HSV-1 gM/UL49A, which are themselves localized to the TGN, caused both gD and gH/L to be relocalized from the plasma membrane to a juxtanuclear compartment, suggesting that fusion inhibition is caused by the removal of 'fusion' proteins from the cell surface. The ability of gM to cause the relocalization of plasma membrane proteins was not restricted to HSV-1 glycoproteins, as other viral and non-viral proteins were also affected. These data suggest that herpesvirus gM (gM/N) can alter the membrane trafficking itineraries of a broad range of proteins and this may have multiple functions.

Identification of candidate gammaherpesvirus 68 genes required for virus replication by signature-tagged transposon mutagenesis

Current methods for determining the role of a given gene product in the gammaherpesvirus 68 (gammaHV68) life cycle require generation of a specific mutation by either homologous recombination in mammalian cells or bacterial artificial chromosome-mediated mutagenesis in Escherichia coli. The mutant virus is then compared to wild-type virus, and the role of the gene in the viral life cycle is deduced from its phenotype. This process is both time-consuming and labor intensive. Here we present the use of random, transposon-mediated signature-tagged mutagenesis for the identification of candidate viral genes involved in virus replication. Pools of viral mutants, each containing a random insertion of a transposon, were generated with a transposon donor library in which each transposon contains a unique sequence identifier. These pools were transfected into mammalian cells, and the ability of each mutant to replicate was assessed by comparing the presence of virus in the output pool to that present in the input pool of viral genomes. With this approach we could rapidly screen up to 96 individual mutants simultaneously. The location of the transposon insertion was determined by sequencing individual clones with a common primer specific for the transposon end. Here we present the characterization of 53 distinct viral mutants that correspond to insertions in 29 open reading frames within the gammaHV68 genome. To confirm the results of the signature-tagged mutagenesis screen, we quantitated the ability of each mutant to replicate compared to wild-type gammaHV68. From these analyses we identified 16 gammaHV68 open reading frames that, when disrupted by transposon insertions, score as essential for virus replication, and six other open reading frames whose disruption led to significant attenuation of virus replication. In addition, transposon insertion in five other gammaHV68 open reading frames did not affect virus replication. Notably, all but one of the candidate essential replication genes identified in this screen have been shown to be essential for the replication of at least one other herpesvirus.

The Epstein-Barr virus BFRF1 and BFLF2 proteins interact and coexpression alters their cellular localization

The BFRF1 protein of Epstein-Barr virus (EBV) is a recently identified membrane protein that is the homolog of the alphaherpesvirus UL34 gene product. We report here that a yeast two-hybrid screen identified the BFLF2 gene product, a homolog of alphaherpesvirus UL31, as a protein that interacts with BFRF1. Expression of BFLF2 in mammalian cells revealed a protein of approximately 28 kDa that associated with BFRF1 in a noncovalently linked complex. When expressed alone, the BFRF1 protein was found in the cytoplasm and perinuclear region. BFLF2 was found diffusely in the nucleus in the absence of BFRF1, but coexpression of BFRF1 and BFLF2 resulted in colocalization of the two proteins at the nuclear rim. These data recapitulate the behavior of the alphaherpesvirus homologs of BFRF1 and BFLF2 and suggest that functional as well as structural and positional homology may be conserved.

Simultaneous deletion of pseudorabies virus tegument protein UL11 and glycoprotein M severely impairs secondary envelopment

The pseudorabies virus (PrV) proteins UL11, glycoprotein E (gE), and gM are involved in secondary envelopment of tegumented nucleocapsids in the cytoplasm. To assess the relative contributions of these proteins to the envelopment process, virus mutants with deletions of either UL11, gM, or gE as well as two newly constructed mutant viruses with simultaneous deletions of UL11 and gE or of UL11 and gM were analyzed in cell culture for their growth phenotype. We show here that simultaneous deletion of UL11 and gE reduced plaque size in an additive manner over the reduction observed by deletion of only UL11 or gE. However, one-step growth was not further impaired beyond the level of the UL11 deletion mutant. Moreover, in electron microscopic analyses PrV-DeltaUL11/gE exhibited a phenotype similar to that of the UL11 mutant virus. In contrast, plaque formation was virtually abolished by the simultaneous absence of UL11 and gM, and one-step growth was significantly reduced. Electron microscopy showed the presence of huge intracytoplasmic inclusions in PrV-DeltaUL11/gM-infected cells, with a size reaching 3 micro m and containing nucleocapsids embedded in tegument. We hypothesize that UL11 and gM are involved in different steps during secondary envelopment and that simultaneous deletion of both interrupts both processes, resulting in the observed drastic impairment of secondary envelopment.

Intracellular localization of the Epstein-Barr virus BFRF1 gene product in lymphoid cell lines and oral hairy leukoplakia lesions

A novel protein encoded by the BFRF1 gene of the Epstein-Barr virus was identified recently [Farina et al. (2000) J Virol 74:3235-3244], which is antigenic "in vivo" and expressed early in the viral replicative cycle. In the present study, its subcellular localization was examined in greater detail comparing Epstein-Barr virus (EBV) induced producing and nonproducing cell lines by immunofluorescence: in 12-0-tetradecanoyl phorbol-13-acetate (TPA)-induced Raji and B95-8 cells, as well as in anti-IgG-stimulated Akata cells, the protein appeared to be localized over the cell nuclear membrane. A similar nuclear membrane localization was observed in epithelial cells of oral hairy leukoplakia, a pathological manifestation of permissive EBV infection. In contrast, upon transfection of BFRF1 in the EBV-negative Burkitt's lymphoma cell line DG75, the protein was localized predominantly over the plasma membrane. The membrane localization was abolished when DG75 cells were transfected with a C-terminal deletion mutant of BFRF1 lacking the transmembrane domain. Because induced Raji cells do not produce virus, the above observations indicate that the nuclear membrane localization is not associated with viral production, but requires the expression of EBV genes, and suggest that additional proteins, expressed early during viral lytic infection, might be necessary to target the protein to the nuclear membrane. Immunogold electron microscopy on ultrathin cryosections of induced B95-8 cells showed that BFRF1 on the nuclear membranes was concentrated over multilayered domains representing areas of active viral replication or at the sites of viral budding, suggesting that BFRF1 is involved in the process of viral assembly.

Identification of proteins associated with murine gammaherpesvirus 68 virions

Murine gammaherpesvirus 68 (MHV68 [also known as gammaHV-68]) is distinguished by its ability to replicate to high titers in cultured cells, making it an excellent candidate for studying gammaherpesvirus virion composition. Extracellular MHV68 virions were isolated, and abundant virion-associated proteins were identified by mass spectrometry. Five nucleocapsid protein homologues, the tegument protein homologue encoded by open reading frame (ORF) 75c, and envelope glycoproteins B and H were detected. In addition, gene products from MHV68 ORF20, ORF24, ORF28, ORF45, ORF48, and ORF52 were identified in association with virions, suggesting that these gammaherpesvirus genes are involved in the early phase of infection or virion assembly and egress.