Identification of viral genes essential for replication of murine gamma-herpesvirus 68 using signature-tagged mutagenesis

Gamma-herpesvirus kinase actively initiates a DNA damage response by inducing phosphorylation of H2AX to foster viral replication

DNA virus infection can elicit the DNA damage response in host cells, including ATM kinase activation and H2AX phosphorylation. This is considered to be the host cell response to replicating viral DNA. In contrast, we show that during infection of macrophages murine gamma-herpesvirus 68 (gammaHV68) actively induces H2AX phosphorylation by expressing a viral kinase (orf36). GammaHV68-encoded orf36 kinase and its EBV homolog, BGLF4, induce H2AX phosphorylation independently of other viral genes. The process requires the kinase domain of Orf36 and is enhanced by ATM. Orf36 is important for gammaHV68 replication in infected animals, and orf36, H2AX, and ATM are all critical for efficient gammaHV68 replication in primary macrophages. Thus, activation of proximal components of the DNA damage signaling response is an active viral kinase-driven strategy required for efficient gamma-herpesvirus replication.

Transcriptional regulation of the open reading frame 35 encoded by Kaposi's sarcoma-associated herpesvirus

Kaposi's sarcoma-associated herpesvirus (KSHV) is a member of the Gammaherpesvirinae and is causally associated with Kaposi's sarcoma, primary effusion lymphoma, and multicentric Castleman's disease. The KSHV genome encodes over 85 genes; the function of some is entirely unknown. We have characterized the transcriptional regulation of a conserved and uncharacterized Gammaherpesvirinae open reading frame, orf35, which lies in a cluster of several overlapping genes, orf34 to orf38. We identified the transcription start site and analyzed upstream sequences. We found that expression of the KSHV lytic replication and transcription activator (RTA) strongly increased the orf35 promoter activity through a 46-nucleotide region which includes a conserved AP-1 binding site. Electrophoretic mobility shift assay demonstrated direct binding of cJUN and cFOS to the predicted AP-1 binding site. Finally, using a mutated promoter lacking the AP-1 site and dominant-negative cFOS, we established that the RTA-mediated orf35 transactivation is AP-1-dependent.

Structural and Functional Studies of the Abundant Tegument Protein ORF52 from Murine Gammaherpesvirus 68

The tegument is a layer of proteins between the nucleocapsid and the envelope of herpesviruses. The functions of most tegument proteins are still poorly understood. In murine gammaherpesvirus 68, ORF52 is an abundant tegument protein of 135 residues that is required for the assembly and release of infectious virus particles. To help understand the molecular basis for the function of this protein, we have determined its crystal structure at 2.1 A resolution. The structure reveals a dimeric association of this protein. Interestingly, an N-terminal alpha-helix that assumes different conformation in the two monomers of the dimer mediates the formation of an asymmetrical tetramer and contains many highly conserved residues. Structural and sequence analyses suggest that this helix is more likely involved in interactions with other components of the tegument or nucleocapsid of the virus and that ORF52 functions as a symmetrical dimer. The asymmetrical tetramer of ORF52 may be a "latent" form of the protein, when it is not involved in virion assembly. The self-association of ORF52 has been confirmed by co-immunoprecipitation and fluorescence resonance energy transfer experiments. Deletion of the N-terminal alpha-helix, as well as mutation of the conserved Arg(95) residue, abolished the function of ORF52. The results of the functional studies are fully consistent with the structural observations and indicate that the N-terminal alpha-helix is a crucial site of interaction for ORF52.

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.

A comprehensive library of mutations of Epstein Barr virus

A mutant library of 249 mutants with mutations that span the entire Epstein-Barr virus (EBV) genome was generated by transposition with EZ : : TN <KAN-2> and insertion with an apramycin resistance gene by a PCR-targeting method. This study also demonstrates the feasibility of generating deletions and site-specific mutations in the BRLF1 promoter on the EBV genome to determine the regions in the promoter that are crucial to transcription. Analysing BZLF1 and BRLF1 mutants by microarray analysis revealed that these two genes regulate the transcription of EBV lytic genes differently. A BZLF1 mutation affects global expression of EBV lytic genes; almost no lytic gene is expressed by the mutant after lytic induction. However, although a BRLF1 mutant still transcribes most lytic genes, the expression of these lytic genes is inefficient. Furthermore, this study shows that the proximal Zta-response element in the BRLF1 promoter is crucial to BRLF1 transcription from the EBV genome, despite the fact that another work demonstrated that this site was unimportant in transient transfection analysis. Furthermore, mutants with a mutation in BDLF1 and BORF1 cannot assemble viral capsids. Results of this study demonstrate the usefulness of a comprehensive mutant library in genetic analyses of EBV.

ORF73-null murine gammaherpesvirus 68 reveals roles for mLANA and p53 in virus replication

Gammaherpesviruses establish lifelong, latent infections in host lymphocytes, during which a limited subset of viral gene products facilitates maintenance of the viral episome. Among the gamma-2-herpesvirus (rhadinovirus) subfamily, this includes expression of the conserved ORF73-encoded LANA proteins. We previously demonstrated by loss-of-function mutagenesis that the murine gammaherpesvirus 68 (MHV68) ORF73 gene product, mLANA, is required for the establishment of latency following intranasal inoculation of mice (N. J. Moorman, D. O. Willer, and S. H. Speck, J. Virol. 77:10295-10303, 2003). mLANA-deficient viruses also exhibited a defect in acute virus replication in the lungs of infected mice. The latter observation led us to examine the role of mLANA in productive viral replication. We assessed the capacity of mLANA-deficient virus (73.Stop) to replicate in cell culture at low multiplicities of infection (MOIs) and found that 73.Stop growth was impaired in murine fibroblasts but not in Vero cells. A recombinant virus expressing an mLANA-green fluorescent protein (GFP) fusion revealed that mLANA is expressed throughout the virus replication cycle. In addition, 73.Stop infection of murine fibroblasts at high MOIs was substantially more cytotoxic than infection with a genetically repaired marker rescue virus (73.MR), a phenotype that correlated with enhanced kinetics of viral gene expression and increased activation of p53. Notably, augmented cell death, viral gene expression, and p53 induction were independent of viral DNA replication. Expression of a mLANA-GFP fusion protein in fibroblasts correlated with both reduced p53 stabilization and reduced cell death following treatment with p53-inducing agonists. In agreement, accentuated cell death associated with 73.Stop infection was reduced in p53-deficient murine embryonic fibroblasts. Additionally, replication of 73.Stop in p53-deficient cells was restored to levels comparable to those of 73.MR. More remarkably, the absence of p53 led to an overall delay in replication for both 73.Stop and 73.MR viruses, which correlated with delayed viral gene expression, indicating a role for p53 in MHV68 replication. Consistent with these findings, the expression of replication-promoting viral genes was positively influenced by p53 overexpression or treatment with the p53 agonist etoposide. Overall, these data demonstrate the importance of mLANA in MHV68 replication and suggest that LANA proteins limit the induction of cellular stress responses to regulate the viral gene expression cascade and limit host cell injury.

The ORF49 protein of murine gammaherpesvirus 68 cooperates with RTA in regulating virus replication

Our functional mapping study of murine gammaherpesvirus 68 (MHV-68, or gammaHV-68) revealed that a mutant harboring a transposon at the ORF49 locus (ORF49(null)) evidenced a highly attenuated in vitro growth. ORF49 resides adjacent to and in an opposite direction from RTA, the primary switch of the gammaherpesvirus life cycle. A FLAG-tagged ORF49 protein was able to transcomplement ORF49(null), and a revertant of ORF49(null) restored its attenuated growth to a level comparable to that of the wild type. The FLAG-tagged ORF49 protein promoted the ability of RTA to activate downstream target promoters and enhanced virus replication from the ORF50(null) virus in the presence of RTA. Furthermore, ORF49 enhanced wild-type virus replication by increasing the RTA transcript levels. Our data indicate that ORF49 may perform an important function in MHV-68 replication in cooperation with RTA.

Murine gammaherpesvirus 68 ORF52 encodes a tegument protein required for virion morphogenesis in the cytoplasm

The tegument, a semiordered matrix of proteins overlying the nucleocapsid and underlying the virion envelope, in viruses in the gamma subfamily of Herpesviridae is poorly understood. Murine gammaherpesvirus 68 (MHV-68) is a robust model for studying gammaherpesvirus virion structure, assembly, and composition, as MHV-68 efficiently completes the lytic phase and productively infects cultured cells. We have found that MHV-68 ORF52 encodes an abundant tegument protein conserved among gammaherpesviruses. Detergent sensitivity experiments revealed that the MHV-68 ORF52 protein is more tightly bound to the virion nucleocapsid than the ORF45 tegument protein but could be dissociated from particles that retained the ORF65 small capsomer protein. ORF52, tagged with enhanced green fluorescent protein or FLAG epitope, localized to the cytoplasm. A recombinant MHV-68 bacterial artificial chromosome mutant with a nonsense mutation incorporated into ORF52 exhibited viral DNA replication, expression of late lytic genes, and capsid assembly and packaging at levels near those of the wild type. However, the MHV-68 ORF52-null virus was deficient in the assembly and release of infectious virion particles. Instead, partially tegumented capsids produced by the ORF52-null mutant accumulated in the cytoplasm, containing conserved capsid proteins, the ORF64 and ORF67 tegument proteins, but virtually no ORF45 tegument protein. Thus, ORF52 is essential for the tegumentation and egress of infectious MHV-68 particles in the cytoplasm, suggesting an important conserved function in gammaherpesvirus virion morphogenesis.

Murine gammaherpesvirus 68 open reading frame 24 is required for late gene expression after DNA replication

Open reading frame 24 (ORF24) of murine gammaherpesvirus 68 (MHV-68) is conserved among beta- and gammaherpesviruses; however, its function in viral replication has not been defined. Using MHV-68 as a model, we have identified ORF24 as being essential for viral replication. An ORF24-null virus was generated and shown to be defective in late gene expression. Expression of early genes, as well as viral genome replication, was not affected. Furthermore, the defect in late gene expression was likely due to a deficiency in transcription. Thus, we have identified an MHV-68 protein, ORF24, that is essential for the expression of viral late proteins yet dispensable for viral DNA replication.

Signature-tagged mutagenesis: barcoding mutants for genome-wide screens

DNA signature tags (molecular barcodes) facilitate functional screens by identifying mutants in mixed populations that have a reduced or increased adaptation to a particular environment. Many innovative adaptations and refinements in the technology have been described since its original use with Salmonella; they have yielded a wealth of information on a broad range of biological processes--mainly in bacteria, but also in yeast and other fungi, viruses, parasites and, most recently, in mammalian cells. By combining whole-genome microarrays and comprehensive ordered libraries of mutants, high-throughput functional screens can now be achieved on a genomic scale.

Murine gammaherpesvirus-68 glycoprotein B presents a difficult neutralization target to monoclonal antibodies derived from infected mice

Persistent viruses disseminate from immune hosts. They must therefore resist neutralization by antibody. Murine gammaherpesvirus-68 (MHV-68) represents an accessible model with which to address how resistance to neutralization is achieved and how overcoming it might improve infection control. The MHV-68 glycoprotein B (gB), like that of other herpesviruses, is a virion protein that is essential for infectivity. As such, it presents a potential neutralization target. In order to test whether virus-induced antibodies reduce virion infectivity by binding to gB, monoclonal antibodies (mAbs) were derived from MHV-68-infected mice. gB-specific mAbs were common, but only an IgM specific for the gB N terminus reduced virion infectivity significantly. It inhibited MHV-68 entry into BHK-21 cells at a post-binding step that was linked closely to membrane fusion. Reducing the mAb to IgM monomers compromised neutralization severely, suggesting that a pentameric structure was crucial to its function. Antibody treatment never blocked BHK-21 cell infection completely and blocked the infection of NMuMG epithelial cells hardly at all. Virions saturated with antibody also remained infectious to mice. Thus, the MHV-68 gB presents at best a very difficult target for antibody-mediated neutralization.

Glycoprotein L disruption reveals two functional forms of the murine gammaherpesvirus 68 glycoprotein H

The herpesvirus glycoprotein H (gH) and gL associate to form a heterodimer that plays a central role in virus-driven membrane fusion. When archetypal alpha- or betaherpesviruses lack gL, gH misfolds and progeny virions are noninfectious. In order to define the role that gL plays in gamma-2 herpesvirus infections, we disrupted its coding sequence in murine gammaherpesvirus-68 (MHV-68). MHV-68 lacking gL folded gH into a conformation antigenically distinct from the form that normally predominates on infected cells. gL-deficient virions bound less well than the wild type to epithelial cells and fibroblasts. However, they still incorporated gH and remained infectious. The cell-to-cell spread of gL-deficient viruses was remarkably normal, as was infection, dissemination, and latency establishment in vivo. Viral membrane fusion was therefore gL independent. The major function of gL appeared to be allowing gH to participate in cell binding prior to membrane fusion. This function was most important for the entry of MHV-68 virions into fibroblasts and epithelial cells.

ORF18 is a transfactor that is essential for late gene transcription of a gammaherpesvirus

Lytic replication of the tumor-associated human gammaherpesviruses Epstein-Barr virus and Kaposi's sarcoma-associated herpesvirus has important implications in pathogenesis and tumorigenesis. Herpesvirus lytic genes have been temporally classified as exhibiting immediate-early (IE), early, and late expression kinetics. Though the regulation of IE and early gene expression has been studied extensively, very little is known regarding the regulation of late gene expression. Late genes, which primarily encode virion structural proteins, require viral DNA replication for their expression. We have identified a murine gammaherpesvirus 68 (MHV-68) early lytic gene, ORF18, essential for viral replication. ORF18 is conserved in both beta- and gammaherpesviruses. By generating an MHV-68 ORF18-null virus, we characterized the stage of the virus lytic cascade that requires the function of ORF18. Gene expression profiling and quantitation of viral DNA synthesis of the ORF18-null virus revealed that the expression of early genes and viral DNA replication were not affected; however, the transcription of late genes was abolished. Hence, we have identified a gammaherpesvirus-encoded factor essential for the expression of late genes independently of viral DNA synthesis.

Murine Gammaherpesvirus (MHV) MK3 Gene Sequence Diversity among 72, 4556, and 68 Strains

Murid herpesvirus 4 (MuHV-4) currently serves as a model for study of human gamma-herpesvirus pathogenesis. It codes for MK3 protein that similarly as K5 protein of Kaposi's sarcoma-associated herpesvirus are members of a family of structurally related viral immune evasion molecules possessing RING-CH finger domain with ubiquitin ligase activity. Murine herpesvirus 72 (MHV-72) isolated from the same species of free-living small rodent is considered as closely related to Murine herpesvirus 68 (MHV-68). Studies on MHV-72, identified dissimilarity from MHV-68 in the sequence of glycoprotein 150 [K. Macáková, J. Matis, I. Rezuchová, O. Kúdela, H. Raslová, M. Kúdelová, Virus Genes 26, 89-95 (2003)]. Murine herpesvirus 4556 (MHV-4556) is relatively new, till now, uncharacterised strain isolated from different murid species Apodemus flavicollis. We have therefore sequenced the MK3 gene of MHW-72 as well as of MHV-4556 to find out the evidence of their difference from that of MHV-68. We show here the unique nucleotide mutation in MHV-72 MK3 gene changing the codon at C-end of MK3 protein that was earlier predicted to function in interaction with TAP1/2. Furthermore, one from two nucleotide mutations found for MHV-4556 MK3 gene changed the codon that is localized at N-terminus of MK3 protein. MHV-4556-specific mutation was found within MK3 RING-CH finger domain known to be necessary for the ubiquitination of MHC class I proteins. Moreover, the latter established the new restriction site specific for MHV-4556.

In vitro transposon mutagenesis of an equine herpesvirus 1 genome cloned as a bacterial artificial chromosome

The 150-kbp genome of the alphaherpesvirus equine herpesvirus 1 (EHV-1) strain HVS25A was cloned as a bacterial artificial chromosome (EHV-1 BAC), with mini F plasmid sequences inserted between genes 62 and 63. Transfection of EHV-1 BAC DNA purified from E. coli gave rise to progeny virus that had a similar growth rate and yield in mammalian cell culture to those of parental wild-type EHV-1. Using in vitro mutagenesis with a Mu transposon, a large library of EHV-1 BAC mutants was generated, and sequence analysis indicated that insertions were dispersed randomly across the EHV-1 genome. Following transfections of a pilot sample of mutant EHV-1 BAC DNAs into mammalian cells, no CPE was observable by light microscopy for mutants carrying insertions in genes for the major capsid protein, large tegument protein, glycoprotein K, catalytic subunit of DNA polymerase, or single-stranded DNA-binding protein. Mutants that were able to produce CPE similar to wild-type EHV-1 included those with interruptions in ORFs of several tegument proteins. Analysis of several glycoprotein gene mutants indicated that those carrying insertions near the start of genes encoding glycoproteins E and I were viable, but showed markedly diminished plaque areas. These results were supported by confocal microscopy of transfected or infected cultures. Electron microscopy of cells infected with a gE mutant revealed accumulations of particles within cytoplasmic vesicles, consistent with a partial obstruction of maturation. The transposon library is a resource for comprehensive functional analysis of the HVS25A genome, with multiple mutants available in any of the predicted genes of EHV-1.

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.

A gamma-herpesvirus deficient in replication establishes chronic infection in vivo and is impervious to restriction by adaptive immune cells

Chronic gamma-herpesvirus infection is a dynamic process involving latent infection, reactivation from latency, and low level persistent replication. The gamma-herpesviruses maintain latent infection in restricted subsets of hematopoietic cells as a result of an intricate balance between host factors that suppress infection and viral factors that facilitate evasion of the immune response. Immune effectors limit reactivation and subsequent replication events, and the adaptive immune response ultimately restricts infection to a level compatible with life-long infection. However, it has not been possible to determine whether the immune system constrains chronic infection by directly targeting latently infected cells in vivo due to the complex nature of chronic infection. To begin to address this issue, we generated a murine gamma-herpesvirus 68 (gammaHV68) deficient in its ability to replicate or undergo reactivation from latency via a mutation in the single-stranded DNA binding protein encoded by ORF6. Even in the absence of lytic replication, this virus established long-term infection in peritoneal cells of wild-type mice at levels identical to that of wild-type gammaHV68, and generated an immune response that was sufficient to protect against secondary challenge with wild-type gammaHV68. Nevertheless, the number of latently infected cells was not significantly altered in mice deficient in T cells or both T cells and B cells, demonstrating that the adaptive immune system is incapable of altering infection with a virus lacking the capacity for lytic replication and reactivation from latency. Thus, these data support the conclusion that latency is immunologically silent.

Mass spectrometric analyses of purified rhesus monkey rhadinovirus reveal 33 virion-associated proteins

The repertoire of proteins that comprise intact gammaherpesviruses, including the human pathogen Kaposi's sarcoma-associated herpesvirus (KSHV), is likely to have critical functions not only in viral structure and assembly but also in the early stages of infection and evasion of the host's rapidly deployed antiviral defenses. To develop a better understanding of these proteins, we analyzed the composition of rhesus monkey rhadinovirus (RRV), a close phylogenetic relative of KSHV. Unlike KSHV, RRV replicates to high titer in cell culture and thus serves as an effective model for studying primate gammaherpesvirus structure and virion proteomics. We employed two complementary mass spectrometric approaches and found that RRV contains at least 33 distinct virally encoded proteins. We have assigned 7 of these proteins to the capsid, 17 to the tegument, and 9 to the envelope. Of the five gammaherpesvirus-specific tegument proteins, three have no known function. We also found three proteins not previously associated with a purified herpesvirus and an additional seven that represent new findings for a member of the gamma-2 herpesviruses. Detergent extraction resulted in particles that contained six distinct tegument proteins in addition to the expected capsid structural proteins, suggesting that this subset of tegument components may interact more directly with or with higher affinity for the underlying capsid and, in turn, may play a role in assembly or transport of viral or subviral particles during entry or egress.

New genes from old: redeployment of dUTPase by herpesviruses

Published work (D. J. McGeoch, Nucleic Acids Res. 18:4105-4110, 1990; J. E. McGeehan, N. W. Depledge, and D. J. McGeoch, Curr. Protein Peptide Sci. 2:325-333, 2001) has indicated that evolution of dUTPase in the class of herpesviruses that infect mammals and birds involved capture of a host gene followed by a duplication event that resulted in a coding region comprising two fused dUTPase domains. Some of the conserved residues required for enzyme activity were then lost, resulting in a dUTPase containing a single active site with different elements contributed by each half of the protein. Further conserved residues were lost in one subfamily (the Betaherpesvirinae), yielding a protein that is related to herpesvirus dUTPases but has a different and as yet unrecognized function. Evidence from sequence similarities and structural predictions now indicates that several additional genes were derived from the herpesvirus dUTPase gene, probably by duplication. These are UL31, UL82, UL83, and UL84 in human cytomegalovirus (and counterparts in other members of the Betaherpesvirinae) and ORF10 and ORF11 in human herpesvirus 8 (and counterparts in other members of the Gammaherpesvirinae). The findings clarify the evolutionary history of these genes and provide novel insights for structural and functional studies.