Physical and functional interactions between the herpes simplex virus UL15 and UL28 DNA cleavage and packaging proteins

Equine herpesvirus type 1 (EHV-1) replication at the upper respiratory entry site is inhibited by neutralizing EHV-1-specific IgG1 and IgG4/7 mucosal antibodies

Equine herpesvirus type 1 (EHV-1) is a contagious respiratory pathogen that infects the mucosa of the upper respiratory tract (URT). Mucosal immune responses at the URT provide the first line of defense against EHV-1 and are crucial for orchestrating immunity. To define host-pathogen interactions, we characterized B-cell responses, antibody isotype functions, and EHV-1 replication of susceptible (non-immune) and clinically protected (immune) horses after experimental EHV-1 infection. Nasal secretion and nasal wash samples were collected and used for the isolation of DNA, RNA, and mucosal antibodies. Shedding of infectious virus, EHV-1 copy numbers, viral RNA expression, and host B-cell activation in the URT were compared based on host immune status. Mucosal EHV-1-specific antibody responses were associated with EHV-1 shedding and viral RNA transcription. Finally, mucosal immunoglobulin G (IgG) and IgA isotypes were purified and tested for neutralizing capabilities. IgG1 and IgG4/7 neutralized EHV-1, while IgG3/5, IgG6, and IgA did not. Immune horses secreted high amounts of mucosal EHV-1-specific IgG4/7 antibodies and quickly upregulated B-cell pathway genes, while EHV-1 was undetected by virus isolation and PCR. RNA transcription analysis reinforced incomplete viral replication in immune horses. In contrast, complete viral replication with high viral copy numbers and shedding of infectious viruses was characteristic for non-immune horses, together with low or absent EHV-1-specific neutralizing antibodies during viral replication. These data confirm that pre-existing mucosal IgG1 and IgG4/7 and rapid B-cell activation upon EHV-1 infection are essential for virus neutralization, regulation of viral replication, and mucosal immunity against EHV-1.IMPORTANCEEquine herpesvirus type 1 (EHV-1) causes respiratory disease, abortion storms, and neurologic outbreaks known as equine herpes myeloencephalopathy (EHM). EHV-1 is transmitted with respiratory secretions by nose-to-nose contact or via fomites. The virus initially infects the epithelium of the upper respiratory tract (URT). Host-pathogen interactions and mucosal immunity at the viral entry site provide the first line of defense against the EHV-1. Robust mucosal immunity can be essential in protecting against EHV-1 and to reduce EHM outbreaks. It has previously been shown that immune horses do not establish cell-associated viremia, the prerequisite for EHM. Here, we demonstrate how mucosal antibodies can prevent the replication of EHV-1 at the epithelium of the URT and, thereby, the progression of the virus to the peripheral blood. The findings improve the mechanistic understanding of mucosal immunity against EHV-1 and can support the development of enhanced diagnostic tools, vaccines against EHM, and the management of EHV-1 outbreaks.

The Terminase Complex of Each Human Herpesvirus

The human herpesviruses (HHVs) are classified into the following three subfamilies: Alphaherpesvirinae, Betaherpesvirinae, and Gammaherpesvirinae. These HHVs have distinct pathological features, while containing a highly conserved viral replication pathway. Among HHVs, the basic viral particle structure and the sequential processes of viral replication are nearly identical. In particular, the capsid formation mechanism has been proposed to be highly similar among herpesviruses, because the viral capsid-organizing proteins are highly conserved at the structural and functional levels. Herpesviruses form capsids containing the viral genome in the nucleus of infected cells during the lytic phase, and release infectious virus (i.e., virions) to the cell exterior. In the capsid formation process, a single-unit-length viral genome is encapsidated into a preformed capsid. The single-unit-length viral genome is produced by cleavage from a viral genome precursor in which multiple unit-length viral genomes are tandemly linked. This encapsidation and cleavage is carried out by the terminase complex, which is composed of viral proteins. Since the terminase complex-mediated encapsidation and cleavage is a virus-specific mechanism that does not exist in humans, it may be an excellent inhibitory target for anti-viral drugs with high virus specificity. This review provides an overview of the functions of the terminase complexes of HHVs.

Kaposi's Sarcoma-Associated Herpesvirus ORF7 Is Essential for Virus Production

Kaposi’s sarcoma-associated herpesvirus (KSHV) causes Kaposi’s sarcoma, primary effusion lymphoma (PEL), and multicentric Castleman disease. Although capsid formation and maturation in the alpha-herpesvirus herpes simplex virus 1 are well understood, these processes in KSHV remain unknown. The KSHV ORF7, encoding the viral terminase (DNA cleavage and packaging protein), is thought to contribute to capsid formation; however, functional information is lacking. Here, we investigated the role of ORF7 during KSHV lytic replication by generating two types of ORF7 knock-out (KO) mutants (frameshift-induced and stop codon-induced ORF7 deficiency), KSHV BAC16, and its revertants. The results revealed that both ORF7-KO KSHVs showed significantly reduced viral production but there was no effect on lytic gene expression and viral genome replication. Complementation assays showed virus production from cells harboring ORF7-KO KSHV could be recovered by ORF7 overexpression. Additionally, exogenously expressed ORF7 partially induced nuclear relocalization of the other terminase components, ORF29 and ORF67.5. ORF7 interacted with both ORF29 and ORF67.5, whereas ORF29 and ORF67.5 failed to interact with each other, suggesting that ORF7 functions as a hub molecule in the KSHV terminase complex for interactions between ORF29 and ORF67.5. These findings indicate that ORF7 plays a key role in viral replication, as a component of terminase.

UL28 and UL33 homologs of Marek's disease virus terminase complex involved in the regulation of cleavage and packaging of viral DNA are indispensable for replication in cultured cells

Processing and packaging of herpesvirus genomic DNA is regulated by a packaging-associated terminase complex comprising of viral proteins pUL15, pUL28 and pUL33. Marek’s disease virus (MDV) homologs UL28 and UL33 showed conserved functional features with high sequence identity with the corresponding Herpes simplex virus 1 (HSV-1) homologs. As part of the investigations into the role of the UL28 and UL33 homologs of oncogenic MDV for DNA packaging and replication in cultured cells, we generated MDV mutant clones deficient in UL28 or UL33 of full-length MDV genomes. Transfection of UL28- or UL33-deleted BAC DNA into chicken embryo fibroblast (CEF) did not result either in the production of visible virus plaques, or detectable single cell infection after passaging onto fresh CEF cells. However, typical MDV plaques were detectable in CEF transfected with the DNA of revertant mutants where the deleted genes were precisely reinserted. Moreover, the replication defect of the UL28-deficient mutant was completely restored when fragment encoding the full UL28 gene was co-transfected into CEF cells. Viruses recovered from the revertant construct, as well as by the UL28 co-transfection, showed replication ability comparable with parental virus. Furthermore, the transmission electron microscopy study indicated that immature capsids were assembled without the UL28 expression, but with the loss of infectivity. Importantly, predicted three-dimensional structures of UL28 between MDV and HSV-1 suggests conserved function in virus replication. For the first time, these results revealed that both UL28 and UL33 are essential for MDV replication through regulating DNA cleavage and packaging.

Terminase Large Subunit Provides a New Drug Target for Herpesvirus Treatment

Herpesvirus infection is an orderly, regulated process. Among these viruses, the encapsidation of viral DNA is a noteworthy link; the entire process requires a powered motor that binds to viral DNA and carries it into the preformed capsid. Studies have shown that this power motor is a complex composed of a large subunit, a small subunit, and a third subunit, which are collectively known as terminase. The terminase large subunit is highly conserved in herpesvirus. It mainly includes two domains: the C-terminal nuclease domain, which cuts the viral concatemeric DNA into a monomeric genome, and the N-terminal ATPase domain, which hydrolyzes ATP to provide energy for the genome cutting and transfer activities. Because this process is not present in eukaryotic cells, it provides a reliable theoretical basis for the development of safe and effective anti-herpesvirus drugs. This article reviews the genetic characteristics, protein structure, and function of the herpesvirus terminase large subunit, as well as the antiviral drugs that target the terminase large subunit. We hope to provide a theoretical basis for the prevention and treatment of herpesvirus.

Human Cytomegalovirus Nuclear Capsids Associate with the Core Nuclear Egress Complex and the Viral Protein Kinase pUL97

The nuclear phase of herpesvirus replication is regulated through the formation of regulatory multi-component protein complexes. Viral genomic replication is followed by nuclear capsid assembly, DNA encapsidation and nuclear egress. The latter has been studied intensely pointing to the formation of a viral core nuclear egress complex (NEC) that recruits a multimeric assembly of viral and cellular factors for the reorganization of the nuclear envelope. To date, the mechanism of the association of human cytomegalovirus (HCMV) capsids with the NEC, which in turn initiates the specific steps of nuclear capsid budding, remains undefined. Here, we provide electron microscopy-based data demonstrating the association of both nuclear capsids and NEC proteins at nuclear lamina budding sites. Specifically, immunogold labelling of the core NEC constituent pUL53 and NEC-associated viral kinase pUL97 suggested an intranuclear NEC-capsid interaction. Staining patterns with phospho-specific lamin A/C antibodies are compatible with earlier postulates of targeted capsid egress at lamina-depleted areas. Important data were provided by co-immunoprecipitation and in vitro kinase analyses using lysates from HCMV-infected cells, nuclear fractions, or infectious virions. Data strongly suggest that nuclear capsids interact with pUL53 and pUL97. Combined, the findings support a refined concept of HCMV nuclear trafficking and NEC-capsid interaction.

A Domain of Herpes Simplex Virus pUL33 Required To Release Monomeric Viral Genomes from Cleaved Concatemeric DNA

Monomeric herpesvirus DNA is cleaved from concatemers and inserted into preformed capsids through the actions of the viral terminase. The terminase of herpes simplex virus (HSV) is composed of three subunits encoded by UL15, UL28, and UL33. The UL33-encoded protein (pUL33) interacts with pUL28, but its precise role in the DNA cleavage and packaging reaction is unclear. To investigate the function of pUL33, we generated a panel of recombinant viruses with either deletions or substitutions in the most conserved regions of UL33 using a bacterial artificial chromosome system. Deletion of 11 amino acids (residues 50 to 60 or residues 110 to 120) precluded viral replication, whereas the truncation of the last 10 amino acids from the pUL33 C terminus did not affect viral replication or the interaction of pUL33 with pUL28. Mutations that replaced the lysine at codon 110 and the arginine at codon 111 with alanine codons failed to replicate, and the pUL33 mutant interacted with pUL28 less efficiently. Interestingly, genomic termini of the large (L) and small (S) components were detected readily in cells infected with these mutants, indicating that concatemeric DNA was cleaved efficiently. However, the release of monomeric genomes as assessed by pulsed-field gel electrophoresis was greatly diminished, and DNA-containing capsids were not observed. These results suggest that pUL33 is necessary for one of the two viral DNA cleavage events required to release individual genomes from concatemeric viral DNA.IMPORTANCE This paper shows a role for pUL33 in one of the two DNA cleavage events required to release monomeric genomes from concatemeric viral DNA. This is the first time that such a phenotype has been observed and is the first identification of a function of this protein relevant to DNA packaging other than its interaction with other terminase components.

Herpesvirus Capsid Assembly and DNA Packaging

Herpes simplex virus type I (HSV-1) is the causative agent of several pathologies ranging in severity from the common cold sore to life-threatening encephalitic infection. During productive lytic infection, over 80 viral proteins are expressed in a highly regulated manner, resulting in the replication of viral genomes and assembly of progeny virions. The virion of all herpesviruses consists of an external membrane envelope, a proteinaceous layer called the tegument, and an icosahedral capsid containing the double-stranded linear DNA genome. The capsid shell of HSV-1 is built from four structural proteins: a major capsid protein, VP5, which forms the capsomers (hexons and pentons), the triplex consisting of VP19C and VP23 found between the capsomers, and VP26 which binds to VP5 on hexons but not pentons. In addition, the dodecameric pUL6 portal complex occupies 1 of the 12 capsid vertices, and the capsid vertex specific component (CVSC), a heterotrimer complex of pUL17, pUL25, and pUL36, binds specifically to the triplexes adjacent to each penton. The capsid is assembled in the nucleus where the viral genome is packaged into newly assembled closed capsid shells. Cleavage and packaging of replicated, concatemeric viral DNA requires the seven viral proteins encoded by the UL6, UL15, UL17, UL25, UL28, UL32, and UL33 genes. Considerable advances have been made in understanding the structure of the herpesvirus capsid and the function of several of the DNA packaging proteins by applying biochemical, genetic, and structural techniques. This review is a summary of recent advances with respect to the structure of the HSV-1 virion capsid and what is known about the function of the seven packaging proteins and their interactions with each other and with the capsid shell.

Development of Potent Antiviral Drugs Inspired by Viral Hexameric DNA-Packaging Motors with Revolving Mechanism

The intracellular parasitic nature of viruses and the emergence of antiviral drug resistance necessitate the development of new potent antiviral drugs. Recently, a method for developing potent inhibitory drugs by targeting biological machines with high stoichiometry and a sequential-action mechanism was described. Inspired by this finding, we reviewed the development of antiviral drugs targeting viral DNA-packaging motors. Inhibiting multisubunit targets with sequential actions resembles breaking one bulb in a series of Christmas lights, which turns off the entire string. Indeed, studies on viral DNA packaging might lead to the development of new antiviral drugs. Recent elucidation of the mechanism of the viral double-stranded DNA (dsDNA)-packaging motor with sequential one-way revolving motion will promote the development of potent antiviral drugs with high specificity and efficiency. Traditionally, biomotors have been classified into two categories: linear and rotation motors. Recently discovered was a third type of biomotor, including the viral DNA-packaging motor, beside the bacterial DNA translocases, that uses a revolving mechanism without rotation. By analogy, rotation resembles the Earth's rotation on its own axis, while revolving resembles the Earth's revolving around the Sun (see animations at http://rnanano.osu.edu/movie.html). Herein, we review the structures of viral dsDNA-packaging motors, the stoichiometries of motor components, and the motion mechanisms of the motors. All viral dsDNA-packaging motors, including those of dsDNA/dsRNA bacteriophages, adenoviruses, poxviruses, herpesviruses, mimiviruses, megaviruses, pandoraviruses, and pithoviruses, contain a high-stoichiometry machine composed of multiple components that work cooperatively and sequentially. Thus, it is an ideal target for potent drug development based on the power function of the stoichiometries of target complexes that work sequentially.

Adenovirus-mediated shRNA interference against HSV-1 replication in vitro

The UL29 and UL28 proteins encoded by herpes simplex virus type 1 (HSV-1) are critical for its replication and packaging, respectively. Research has demonstrated that synthesized siRNA molecules targeting the UL29 gene are able to suppress HSV-2 replication and the UL28-null HSV-1 gene cannot form infectious viruses in vitro. Silencing the UL28 and UL29 genes by RNAi might lead to the development of novel antiviral agents for the treatment of HSV-1 infections. Two kinds of short hairpin RNAs (shRNAs) targeting the UL29 and UL28 genes were chemically synthesized and then delivered into cells by a replication-defective human adenovirus type 5 (Adv5) vector. (-) shRNAs targeting none of the genome of HSV-1 were used as the control. Vero cells were inoculated with Ad-UL28shRNA or Ad-UL29shRNA at a multiplicity of infection (MOI) of 100 and challenged 24 h later with HSV-1 at an MOI of 0.01 to inhibit HSV-1 replication, as measured by the level of the corresponding RNA and proteins. In addition, the amount of progeny virus was assessed at daily intervals. The antiviral effects of Ad-shRNAs at ongoing HSV-1 infection were explored at 12 h after inoculation of the HSV-1. The results showed that the shRNAs delivered by Adv5 significantly suppressed HSV-1 replication in vitro, as determined by the levels of viral RNA transcription, viral protein synthesis, and viral production. The Ad-UL28shRNA and Ad-UL29shRNA suppressed the replication of HSV-1, respectively, compared with the control group (P < 0.001). When Ad-UL28shRNA and Ad-UL29shRNA were combined, a synergistic effect was observed. The antiviral effects could sustain for at least 4 days after the HSV-1 infection (P < 0.001). Furthermore, antiviral effects were achieved 12 h prior to inoculation of Adv5-shRNAs (P < 0.001). Our data demonstrated comparable antiviral activities against herpes simplex virus by shRNAs targeting either UL29 or UL28 sites in vitro and the effectiveness of using the Adv5 delivery of shRNAs. Therefore, the Adv5 delivery of shRNAs targeting the UL29 and UL28 sites probably may provide an alternative strategy for controlling HSV-1 infection.

Nuclear Egress of Herpesviruses: The Prototypic Vesicular Nucleocytoplasmic Transport

Herpesvirus particles mature in two different cellular compartments. While capsid assembly and packaging of the genomic linear double-stranded DNA occur in the nucleus, virion formation takes place in the cytoplasm by the addition of numerous tegument proteins as well as acquisition of the viral envelope by budding into cellular vesicles derived from the trans-Golgi network containing virally encoded glycoproteins. To gain access to the final maturation compartment, herpesvirus nucleocapsids have to cross a formidable barrier, the nuclear envelope (NE). Since the ca. 120 nm diameter capsids are unable to traverse via nuclear pores, herpesviruses employ a vesicular transport through both leaflets of the NE. This process involves proteins which support local dissolution of the nuclear lamina to allow access of capsids to the inner nuclear membrane (INM), drive vesicle formation from the INM and mediate inclusion of the capsid as well as scission of the capsid-containing vesicle (also designated as "primary virion"). Fusion of the vesicle membrane (i.e., the "primary envelope") with the outer nuclear membrane subsequently results in release of the nucleocapsid into the cytoplasm for continuing virion morphogenesis. While this process has long been thought to be unique for herpesviruses, a similar pathway for nuclear egress of macromolecular complexes has recently been observed in Drosophila. Thus, herpesviruses may have coopted a hitherto unrecognized cellular mechanism of vesicle-mediated nucleocytoplasmic transport. This could have far reaching consequences for our understanding of cellular functions as again unraveled by the study of viruses.

Intermolecular Complementation between Two Varicella-Zoster Virus pORF30 Terminase Domains Essential for DNA Encapsidation

The herpesviral terminase complex is part of the intricate machinery that delivers a single viral genome into empty preformed capsids (encapsidation). The varicella-zoster virus (VZV) terminase components (pORF25, pORF30, and pORF45/42) have not been studied as extensively as those of herpes simplex virus 1 and human cytomegalovirus (HCMV). In this study, VZV bacterial artificial chromosomes (BACs) were generated with small (Δ30S), medium (Δ30M), and large (Δ30L) ORF30 internal deletions. In addition, we isolated recombinant viruses with specific alanine substitutions in the putative zinc finger motif (30-ZF3A) or in a conserved region (region IX) with predicted structural similarity to the human topoisomerase I core subdomains I and II (30-IXAla, 30-620A, and 30-622A). Recombinant viruses replicated in an ORF30-complementing cell line (ARPE30) but failed to replicate in noncomplementing ARPE19 and MeWo cells. Transmission electron microscopy of 30-IXAla-, 30-620A-, and 30-622A-infected ARPE19 cells revealed only empty VZV capsids. Southern analysis showed that cells infected with parental VZV (VZVLUC) or a repaired virus (30R) contained DNA termini, whereas cells infected with Δ30L, 30-IXAla, 30-620A, or 30-622A contained little or no processed viral DNA. These results demonstrated that pORF30, specifically amino acids 619 to 624 (region IX), was required for DNA encapsidation. A luciferase-based assay was employed to assess potential intermolecular complementation between the zinc finger domain and conserved region IX. Complementation between 30-ZF3A and 30-IXAla provided evidence that distinct pORF30 domains can function independently. The results suggest that pORF30 may exist as a multimer or participate in higher-order assemblies during viral DNA encapsidation.

IMPORTANCE

Antivirals with novel mechanisms of action are sought as additional therapeutic options to treat human herpesvirus infections. Proteins involved in the viral DNA encapsidation process have become promising antiviral targets. For example, letermovir is a small-molecule drug targeting HCMV terminase that is currently in phase III clinical trials. It is important to define the structural and functional characteristics of proteins that make up viral terminase complexes to identify or design additional terminase-specific compounds. The VZV ORF30 mutants described in this study represent the first VZV terminase mutants reported to date. Targeted mutations confirmed the importance of a conserved zinc finger domain found in all herpesvirus ORF30 terminase homologs but also identified a novel, highly conserved region (region IX) essential for terminase function. Homology modeling suggested that the structure of region IX is present in all human herpesviruses and thus represents a potential structurally conserved antiviral target.

Epstein-Barr virus BALF3 has nuclease activity and mediates mature virion production during the lytic cycle

Epstein-Barr virus (EBV) lytic replication involves complex processes, including DNA synthesis, DNA cleavage and packaging, and virion egress. These processes require many different lytic gene products, but the mechanisms of their actions remain unclear, especially for DNA cleavage and packaging. According to sequence homology analysis, EBV BALF3, encoded by the third leftward open reading frame of the BamHI-A fragment in the viral genome, is a homologue of herpes simplex virus type 1 UL28. This gene product is believed to possess the properties of a terminase, such as nucleolytic activity on newly synthesized viral DNA and translocation of unit length viral genomes into procapsids. In order to characterize EBV BALF3, the protein was produced by and purified from recombinant baculoviruses and examined in an enzymatic reaction in vitro, which determined that EBV BALF3 acts as an endonuclease and its activity is modulated by Mg(2+), Mn(2+), and ATP. Moreover, in EBV-positive epithelial cells, BALF3 was expressed and transported from the cytoplasm into the nucleus following induction of the lytic cycle, and gene silencing of BALF3 caused a reduction of DNA packaging and virion release. Interestingly, suppression of BALF3 expression also decreased the efficiency of DNA synthesis. On the basis of these results, we suggest that EBV BALF3 is involved simultaneously in DNA synthesis and packaging and is required for the production of mature virions.

IMPORTANCE

Virus lytic replication is essential to produce infectious virions, which is responsible for virus survival and spread. This work shows that an uncharacterized gene product of the human herpesvirus Epstein-Barr virus (EBV), BALF3, is expressed during the lytic cycle. In addition, BALF3 mediates an endonucleolytic reaction and is involved in viral DNA synthesis and packaging, leading to influence on the production of mature virions. According to sequence homology and physical properties, the lytic gene product BALF3 is considered a terminase in EBV. These findings identify a novel viral gene with an important role in contributing to a better understanding of the EBV life cycle.

Isolation and characterization of the herpes simplex virus 1 terminase complex

During herpes simplex virus 1 (HSV-1) infection, empty procapsids are assembled and subsequently filled with the viral genome by means of a protein complex called the terminase, which is comprised of the HSV-1 UL15, UL28, and UL33 proteins. Biochemical studies of the terminase proteins have been hampered by the inability to purify the intact terminase complex. In this study, terminase complexes were isolated by tandem-affinity purification (TAP) using recombinant viruses expressing either a full-length NTAP-UL28 fusion protein (vFH476) or a C-terminally truncated NTAP-UL28 fusion protein (vFH499). TAP of the UL28 protein from vFH476-infected cells, followed by silver staining, Western blotting, and mass spectrometry, identified the UL15, UL28, and UL33 subunits, while TAP of vFH499-infected cells confirmed previous findings that the C terminus of UL28 is required for UL28 interaction with UL33 and UL15. Analysis of the oligomeric state of the purified complexes by sucrose density gradient ultracentrifugation revealed that the three proteins formed a complex with a molecular mass that is consistent with the formation of a UL15-UL28-UL33 heterotrimer. In order to assess the importance of conserved regions of the UL15 and UL28 proteins, recombinant NTAP-UL28 viruses with mutations of the putative UL28 metal-binding domain or within the UL15 nuclease domain were generated. TAP of UL28 complexes from cells infected with each domain mutant demonstrated that the conserved cysteine residues of the putative UL28 metal-binding domain and conserved amino acids within the UL15 nuclease domain are required for the cleavage and packaging functions of the viral terminase, but not for terminase complex assembly.

Changes in subcellular localization reveal interactions between human cytomegalovirus terminase subunits

Background

During herpesvirus replication, terminase packages viral DNA into capsids. The subunits of herpes simplex virus terminase, UL15, UL28, and UL33, assemble in the cytoplasm prior to nuclear import of the complex.

Methods

To detect similar interactions between human cytomegalovirus terminase subunits, the orthologous proteins UL89, UL56, and UL51 were expressed in HEK-293 T cells (via transfection) or insect cells (via baculovirus infection) and subcellular localizations were detected by cellular fractionation and confocal microscopy.

Results

In both cell types, UL56 and UL89 expressed alone were exclusively cytoplasmic, whereas UL51 was ~50% nuclear. Both UL89 and UL56 became ~50% nuclear when expressed together, as did UL56 when expressed with UL51. Nuclear localization of each protein was greatest when all three proteins were co-expressed.

Conclusions

These results support inclusion of UL51 as an HCMV terminase subunit and suggest that nuclear import of human cytomegalovirus terminase may involve nuclear import signals that form cooperatively upon subunit associations.

An Epstein-Barr virus mutant produces immunogenic defective particles devoid of viral DNA

Virus-like particles (VLPs) from hepatitis B and human papillomaviruses have been successfully used as preventative vaccines against these infectious agents. These VLPs consist of a self-associating capsid polymer formed from a single structure protein and are devoid of viral DNA. Since virions from herpesviruses consist of a large number of molecules of viral and cellular origin, generating VLPs from a subset of these would be a particularly arduous task. Therefore, we have adopted an alternative strategy that consists of producing DNA-free defective virus particles in a cell line infected by a herpesvirus mutant incapable of packaging DNA. We previously reported that an Epstein-Barr virus (EBV) mutant devoid of the terminal repeats (ΔTR) that act as packaging signals in herpesviruses produces substantial amounts of VLPs and of light particles (LPs). However, ΔTR virions retained some infectious genomes, and although these mutants had lost their transforming abilities, this poses potential concerns for clinical applications. Therefore, we have constructed a series of mutants that lack proteins involved in maturation and assessed their ability to produce viral DNA-free VLP/LPs. Some of the introduced mutations were deleterious for capsid maturation and virus production. However, deletion of BFLF1/BFRF1A or of BBRF1 resulted in the production of DNA-free VLPs/LPs. The ΔBFLF1/BFRF1A viruses elicited a potent CD4(+) T-cell response that was indistinguishable from the one obtained with wild-type controls. In summary, the defective particles produced by the ΔBFLF1/BFRF1A mutant fulfill the criteria of efficacy and safety expected from a preventative vaccine.

Varicella zoster virus ORF25 gene product: an essential hub protein linking encapsidation proteins and the nuclear egress complex

Varicella zoster virus (VZV) ORF25 is a 156 amino acid protein belonging to the approximately 40 core proteins that are conserved throughout the Herpesviridae. By analogy to its functional orthologue UL33 in Herpes simplex virus 1 (HSV-1), ORF25 is thought to be a component of the terminase complex. To investigate how cleavage and encapsidation of viral DNA links to the nuclear egress of mature capsids in VZV, we tested 10 VZV proteins that are predicted to be involved in either of the two processes for protein interactions against each other using three independent protein-protein interaction (PPI) detection systems: the yeast-two-hybrid (Y2H) system, a luminescence based MBP pull-down interaction screening assay (LuMPIS), and a bioluminescence resonance energy transfer (BRET) assay. A set of 20 interactions was consistently detected by at least 2 methods and resulted in a dense interaction network between proteins associated in encapsidation and nuclear egress. The results indicate that the terminase complex in VZV consists of ORF25, ORF30, and ORF45/42 and support a model in which both processes are closely linked to each other. Consistent with its role as a central hub for protein interactions, ORF25 is shown to be essential for VZV replication.

A mutation in UL15 of herpes simplex virus 1 that reduces packaging of cleaved genomes

Herpesvirus genomic DNA is cleaved from concatemers that accumulate in infected cell nuclei. Genomic DNA is inserted into preassembled capsids through a unique portal vertex. Extensive analyses of viral mutants have indicated that intact capsids, the portal vertex, and all components of a tripartite terminase enzyme are required to both cleave and package viral DNA, suggesting that DNA cleavage and packaging are inextricably linked. Because the processes have not been functionally separable, it has been difficult to parse the roles of individual proteins in the DNA cleavage/packaging reaction. In the present study, a virus bearing the deletion of codons 400 to 420 of U(L)15, encoding a terminase component, was analyzed. This virus, designated vJB27, failed to replicate on noncomplementing cells but cleaved concatemeric DNA to ca. 35 to 98% of wild-type levels. No DNA cleavage was detected in cells infected with a U(L)15-null virus or a virus lacking U(L)15 codons 383 to 385, comprising a motif proposed to couple ATP hydrolysis to DNA translocation. The amount of vJB27 DNA protected from DNase I digestion was reduced compared to the wild-type virus by 6.5- to 200-fold, depending on the DNA fragment analyzed, thus indicating a profound defect in DNA packaging. Capsids containing viral DNA were not detected in vJB27-infected cells, as determined by electron microscopy. These data suggest that pU(L)15 plays an essential role in DNA translocation into the capsid and indicate that this function is separable from its role in DNA cleavage.

Disulfide bond formation in the herpes simplex virus 1 UL6 protein is required for portal ring formation and genome encapsidation

The herpes simplex virus 1 (HSV-1) UL6 portal protein forms a 12-subunit ring structure at a unique capsid vertex which functions as a conduit for the encapsidation of the viral genome. We have demonstrated previously that the leucine zipper region of UL6 is important for intersubunit interactions and stable ring formation (J. K. Nellissery, R. Szczepaniak, C. Lamberti, and S. K. Weller, J. Virol. 81:8868-8877, 2007). We now demonstrate that intersubunit disulfide bonds exist between monomeric subunits and contribute to portal ring formation and/or stability. Intersubunit disulfide bonds were detected in purified portal rings by SDS-PAGE under nonreducing conditions. Furthermore, the treatment of purified portal rings with dithiothreitol (DTT) resulted in the disruption of the rings, suggesting that disulfide bonds confer stability to this complex structure. The UL6 protein contains nine cysteines that were individually mutated to alanine. Two of these mutants, C166A and C254A, failed to complement a UL6 null mutant in a transient complementation assay. Furthermore, viral mutants bearing the C166A and C254A mutations failed to produce infectious progeny and were unable to cleave or package viral DNA. In cells infected with C166A or C254A, B capsids were produced which contained UL6 at reduced levels compared to those seen in wild-type capsids. In addition, C166A and C254A mutant proteins expressed in insect cells infected with recombinant baculovirus failed to form ring structures. Cysteines at positions 166 and 254 thus appear to be required for intersubunit disulfide bond formation. Taken together, these results indicate that disulfide bond formation is required for portal ring formation and/or stability and for the production of procapsids that are capable of encapsidation.

Identification of a spliced gene from duck enteritis virus encoding a protein homologous to UL15 of herpes simplex virus 1

BACKGROUND

In herpesviruses, UL15 homologue is a subunit of terminase complex responsible for cleavage and packaging of the viral genome into pre-assembled capsids. However, for duck enteritis virus (DEV), the causative agent of duck viral enteritis (DVE), the genomic sequence was not completely determined until most recently. There is limited information of this putative spliced gene and its encoding protein.

RESULTS

DEV UL15 consists of two exons with a 3.5 kilobases (kb) inron and transcribes into two transcripts: the full-length UL15 and an N-terminally truncated UL15.5. The 2.9 kb UL15 transcript encodes a protein of 739 amino acids with an approximate molecular mass of 82 kiloDaltons (kDa), whereas the UL15.5 transcript is 1.3 kb in length, containing a putative 888 base pairs (bp) ORF that encodes a 32 kDa product. We also demonstrated that UL15 gene belonged to the late kinetic class as its expression was sensitive to cycloheximide and phosphonoacetic acid. UL15 is highly conserved within the Herpesviridae, and contains Walker A and B motifs homologous to the catalytic subunit of the bacteriophage terminase as revealed by sequence analysis. Phylogenetic tree constructed with the amino acid sequences of 23 herpesvirus UL15 homologues suggests a close relationship of DEV to the Mardivirus genus within the Alphaherpesvirinae. Further, the UL15 and UL15.5 proteins can be detected in the infected cell lysate but not in the sucrose density gradient-purified virion when reacting with the antiserum against UL15. Within the CEF cells, the UL15 and/or UL15.5 localize(s) in the cytoplasm at 6 h post infection (h p. i.) and mainly in the nucleus at 12 h p. i. and at 24 h p. i., while accumulate(s) in the cytoplasm in the absence of any other viral protein.

CONCLUSIONS

DEV UL15 is a spliced gene that encodes two products encoded by 2.9 and 1.3 kb transcripts respectively. The UL15 is expressed late during infection. The coding sequences of DEV UL15 are very similar to those of alphaherpesviruses and most similar to the genus Mardivirus. The UL15 and/or UL15.5 accumulate(s) in the cytoplasm during early times post-infection and then are translocated to the nucleus at late times.

Mutational analysis of the herpes simplex virus type 1 DNA packaging protein UL33

The UL33 protein of herpes simplex virus type 1 (HSV-1) is thought to be a component of the terminase complex that mediates the cleavage and packaging of viral DNA. In this study we describe the generation and characterization of a series of 15 UL33 mutants containing insertions of five amino acids located randomly throughout the 130-residue protein. Of these mutants, seven were unable to complement the growth of the UL33-null virus dlUL33 in transient assays and also failed to support the cleavage and packaging of replicated amplicon DNA into capsids. The insertions in these mutants were clustered between residues 51 and 74 and between 104 and 116, within the most highly conserved regions of the protein. The ability of the mutants to interact with the UL28 component of the terminase was assessed in immunoprecipitation and immunofluorescence assays. All four mutants with insertions between amino acids 51 and 74 were impaired in this interaction, whereas two of the three mutants in the second region (with insertions at positions 111 and 116) were not affected. These data indicate that the ability of UL33 to interact with UL28 is probably necessary, but not sufficient, to support viral growth and DNA packaging.

Characterization of pseudorabies virus (PrV) cleavage-encapsidation proteins and functional complementation of PrV pUL32 by the homologous protein of herpes simplex virus type 1

Cleavage and encapsidation of newly replicated herpes simplex virus type 1 (HSV-1) DNA requires several essential viral gene products that are conserved in sequence within the Herpesviridae. However, conservation of function has not been analyzed in greater detail. For functional characterization of the UL6, UL15, UL28, UL32, and UL33 gene products of pseudorabies virus (PrV), the respective deletion mutants were generated by mutagenesis of the virus genome cloned as a bacterial artificial chromosome (BAC) in Escherichia coli and propagated in transgenic rabbit kidney cells lines expressing the deleted genes. Neither of the PrV mutants was able to produce plaques or infectious progeny in noncomplementing cells. DNA analyses revealed that the viral genomes were replicated but not cleaved into monomers. By electron microscopy, only scaffold-containing immature but not DNA-containing mature capsids were detected in the nuclei of noncomplementing cells infected with either of the mutants. Remarkably, primary envelopment of empty capsids at the nuclear membrane occasionally occurred, and enveloped tegument-containing light particles were formed in the cytoplasm and released into the extracellular space. Immunofluorescence analyses with monospecific antisera of cells transfected with the respective expression plasmids indicated that pUL6, pUL15, and pUL32 were able to enter the nucleus. In contrast, pUL28 and pUL33 were predominantly found in the cytoplasm. Only pUL6 could be unequivocally identified and localized in PrV-infected cells and in purified virions, whereas the low abundance or immunogenicity of the other proteins hampered similar studies. Yeast two-hybrid analyses revealed physical interactions between the PrV pUL15, pUL28, and pUL33 proteins, indicating that, as in HSV-1, a tripartite protein complex might catalyze cleavage and encapsidation of viral DNA. Whereas the pUL6 protein is supposed to form the portal for DNA entry into the capsid, the precise role of the UL32 gene product during this process remains to be elucidated. Interestingly, the defect of UL32-negative PrV could be completely corrected in trans by the homologous protein of HSV-1, demonstrating similar functions. However, trans-complementation of UL32-negative HSV-1 by the PrV protein was not observed.

The UL15 protein of herpes simplex virus type 1 is necessary for the localization of the UL28 and UL33 proteins to viral DNA replication centres

The UL15, UL28 and UL33 proteins of herpes simplex virus type 1 (HSV-1) are thought to comprise a terminase complex responsible for cleavage and packaging of the viral genome into pre-assembled capsids. Immunofluorescence studies confirmed that shortly after infection with wild-type HSV-1 these three proteins localize to viral DNA replication compartments within the nucleus, identified by the presence of the single-stranded DNA-binding protein, ICP8. In cells infected with either UL28- or UL33-null mutants, the other two terminase proteins also co-localized with ICP8. In contrast, neither UL28 nor UL33 was detectable in replication compartments following infection with a UL15-null mutant, although Western blot analysis showed they were present in normal amounts in the infected cells. Provision of UL15 in a complementing cell line restored the ability of all three proteins to localize to replication compartments. These data indicate that UL15 plays a key role in localizing the terminase complex to DNA replication compartments, and that it can interact independently with UL28 and UL33.

Viral nanomotors for packaging of dsDNA and dsRNA

While capsid proteins are assembled around single-stranded genomic DNA or RNA in rod-shaped viruses, the lengthy double-stranded genome of other viruses is packaged forcefully within a preformed protein shell. This entropically unfavourable DNA or RNA packaging is accomplished by an ATP-driven viral nanomotor, which is mainly composed of two components, the oligomerized channel and the packaging enzymes. This intriguing DNA or RNA packaging process has provoked interest among virologists, bacteriologists, biochemists, biophysicists, chemists, structural biologists and computational scientists alike, especially those interested in nanotechnology, nanomedicine, AAA+ family proteins, energy conversion, cell membrane transport, DNA or RNA replication and antiviral therapy. This review mainly focuses on the motors of double-stranded DNA viruses, but double-stranded RNA viral motors are also discussed due to interesting similarities. The novel and ingenious configuration of these nanomotors has inspired the development of biomimetics for nanodevices. Advances in structural and functional studies have increased our understanding of the molecular basis of biological movement to the point where we can begin thinking about possible applications of the viral DNA packaging motor in nanotechnology and medical applications.

The Varicella-zoster virus DNA encapsidation genes: Identification and characterization of the putative terminase subunits

The putative DNA encapsidation genes encoded by open reading frames (ORFs) 25, 26, 30, 34, 43, 45/42 and 54 were cloned from Varicella-zoster virus (VZV) strain Ellen. Sequencing revealed that the Ellen ORFs were highly conserved at the amino acid level when compared to those of 19 previously published VZV isolates. Additionally, RT-PCR provided the first evidence that ORF45/42 was expressed as a spliced transcript in VZV-infected cells. All seven ORFs were expressed in vitro and full length products were identified using a C-terminal V5 epitope tag. The in vitro products of the putative VZV terminase subunits encoded by ORFs 30 and 45/42 proved useful in protein-protein interaction assays. Previous studies have reported the formation of a heterodimeric terminase complex involved in DNA encapsidation for both herpes simplex virus-type 1 (HSV-1) and human cytomegalovirus (HCMV). Here we report that the C-terminal portion of exon II of ORF45/42 (ORF42-C269) interacted in GST-pull down experiments with in vitro synthesized ORF30 and ORF45/42. The interactions were maintained in the presence of anionic detergents and in buffers of increasing ionic strength. Cells transiently transfected with epitope tagged ORF45/42 or ORF30 showed primarily cytoplasmic staining. In contrast, an antiserum directed to the N-terminal portion of ORF45 showed nearly exclusive nuclear localization of the ORF45/42 gene product in infected cells. An ORF30 specific antiserum detected an 87 kDa protein in both the cytoplasmic and nuclear fractions of VZV infected cells. The results were consistent with the localization and function of herpesviral terminase subunits. This is the first study aimed at the identification and characterization of the VZV DNA encapsidation gene products.

Putative terminase subunits of herpes simplex virus 1 form a complex in the cytoplasm and interact with portal protein in the nucleus

Herpes simplex virus (HSV) terminase is an essential component of the molecular motor that translocates DNA through the portal vertex in the capsid during DNA packaging. The HSV terminase is believed to consist of the UL15, UL28, and UL33 gene products (pUL15, pUL28, and pUL33, respectively), whereas the HSV type 1 portal vertex is encoded by UL6. Immunoprecipitation reactions revealed that pUL15, pUL28, and pUL33 interact in cytoplasmic and nuclear lysates. Deletion of a canonical nuclear localization signal (NLS) from pUL15 generated a dominant-negative protein that, when expressed in an engineered cell line, decreased the replication of wild-type virus up to 80-fold. When engineered into the genome of recombinant HSV, this mutation did not interfere with the coimmunoprecipitation of pUL15, pUL28, and pUL33 from cytoplasmic lysates of infected cells but prevented viral replication, most nuclear import of both pUL15 and pUL28, and coimmunoprecipitation of pUL15, pUL28, and pUL33 from nuclear lysates. When the pUL15/pUL28 interaction was reduced in infected cells by the truncation of the C terminus of pUL28, pUL28 remained in the cytoplasm. Whether putative terminase components localized in the nucleus or cytoplasm, pUL6 localized in infected cell nuclei, as viewed by indirect immunofluorescence. The finding that the portal and terminase do eventually interact was supported by the observation that pUL6 coimmunoprecipitated strongly with pUL15 and weakly with pUL28 from extracts of infected cells in 1.0 M NaCl. These data are consistent with the hypothesis that the pUL15/pUL28/pUL33 complex forms in the cytoplasm and that an NLS in pUL15 is used to import the complex into the nucleus where at least pUL15 and pUL28 interact with the portal to mediate DNA packaging.

Direct visualization of the putative portal in the Kaposi's sarcoma-associated herpesvirus capsid by cryoelectron tomography

Genetic and biochemical studies have suggested the existence of a bacteriophage-like, DNA-packaging/ejecting portal complex in herpesviruses capsids, but its arrangement remained unknown. Here, we report the first visualization of a unique vertex in the Kaposi's sarcoma-associated herpesvirus (KSHV) capsid by cryoelectron tomography, thus providing direct structural evidence for the existence of a portal complex in a gammaherpesvirus. This putative KSHV portal is an internally localized, umbilicated structure and lacks all of the external machineries characteristic of portals in DNA bacteriophages.

Linker insertion mutations in the herpes simplex virus type 1 UL28 gene: effects on UL28 interaction with UL15 and UL33 and identification of a second-site mutation in the UL15 gene that suppresses a lethal UL28 mutation

The UL28 protein of herpes simplex virus type 1 (HSV-1) is one of seven viral proteins required for the cleavage and packaging of viral DNA. Previous results indicated that UL28 interacts with UL15 and UL33 to form a protein complex (terminase) that is presumed to cleave concatemeric DNA into genome lengths. In order to define the functional domains of UL28 that are important for DNA cleavage/packaging, we constructed a series of HSV-1 mutants with linker insertion and nonsense mutations in UL28. Insertions that blocked DNA cleavage and packaging were found to be located in two regions of UL28: the first between amino acids 200 to 400 and the second between amino acids 600 to 740. Insertions located in the N terminus or in a region located between amino acids 400 and 600 did not affect virus replication. Insertions in the carboxyl terminus of the UL28 protein were found to interfere with the interaction of UL28 with UL33. In contrast, all of the UL28 insertion mutants were found to interact with UL15 but the interaction was reduced with mutants that failed to react with UL33. Together, these observations were consistent with previous conclusions that UL15 and UL33 interact directly with UL28 but interact only indirectly with each other. Revertant viruses that formed plaques on Vero cells were detected for one of the lethal UL28 insertion mutants. DNA sequence analysis, in combination with genetic complementation assays, demonstrated that a second-site mutation in the UL15 gene restored the ability of the revertant to cleave and package viral DNA. The isolation of an intergenic suppressor mutant provides direct genetic evidence of an association between the UL28 and UL15 proteins and demonstrates that this association is essential for DNA cleavage and packaging.

Identification of the interaction domain of the small terminase subunit pUL89 with the large subunit pUL56 of human cytomegalovirus

The small terminase subunit pUL89 of human cytomegalovirus (HCMV) is thought to be required for cleavage of viral DNA into unit-length genomes in the cleavage/packaging process. Immunoprecipitations with a UL89-specific antibody demonstrated that pUL89 occurs predominantly as a monomer of approximate M(r) 75.000 together with a dimer of approximate 150.000. This was confirmed by gel permeation chromatography. In view of its putative function, pUL89 needs to be transported into the nucleus. By use of laser scanning confocal microscopy, pUL89 was found to be predominantly localized throughout the nucleus and in particular in viral replication centers of infected cells. By immunofluorescence, we demonstrated that both terminase subunits co-localized in viral replication centers. Furthermore, analysis with pUL89 GST-fusion protein mutants showed that amino acids 580-600 may represent the interaction domain with pUL56. To verify this result, a recombinant HCMV genome was constructed in which the UL89 open reading frame was disrupted. By transfection of the deletion BACmid alone, we showed that it has a lethal phenotype. Cotransfection assays demonstrated that, in contrast to pUL89 wild-type, a plasmid construct encoding a pUL89 variant without aa 580-590 as well as one encoding a variant without aa 590-600 could not complement the HCMV-pUL89 null genome, thus, suggesting that the 20 aa sequence GRDKALAVEQFISRFNSGYIK is sufficient for the interaction with pUL56 and in conclusion required for DNA packaging.

Herpes simplex virus 1 DNA packaging proteins encoded by UL6, UL15, UL17, UL28, and UL33 are located on the external surface of the viral capsid

Studies to localize the herpes simplex virus 1 portal protein encoded by UL6, the putative terminase components encoded by UL15, UL 28, and UL33, the minor capsid proteins encoded by UL17, and the major scaffold protein ICP35 were conducted. ICP35 in B capsids was more resistant to trypsin digestion of intact capsids than pUL6, pUL15, pUL17, pUL28, or pUL33. ICP35 required sectioning of otherwise intact embedded capsids for immunoreactivity, whereas embedding and/or sectioning decreased the immunoreactivities of pUL6, pUL17, pUL28, and pUL33. Epitopes of pUL15 were recognized roughly equally well in both sectioned and unsectioned capsids. These data indicate that pUL6, pUL17, pUL28, pUL33, and at least some portion of pUL15 are located at the external surface of the capsid.

The putative terminase subunit of herpes simplex virus 1 encoded by UL28 is necessary and sufficient to mediate interaction between pUL15 and pUL33

Viral terminases play essential roles as components of molecular motors that package viral DNA into capsids. Previous results indicated that the putative terminase subunits of herpes simplex virus 1 (HSV-1) encoded by U(L)15 and U(L)28 (designated pU(L)15 and pU(L)28, respectively) coimmunoprecipitate with the U(L)33 protein from lysates of infected cells. All three proteins are among six required for HSV-1 DNA packaging but dispensable for assembly of immature capsids. The current results show that in both infected- and uninfected-cell lysates, pU(L)28 coimmunoprecipitates with either pU(L)33 or pU(L)15, whereas pU(L)15 and pU(L)33 do not coimmunoprecipitate unless pU(L)28 is present. The U(L)28 protein was sufficient to stabilize pU(L)33 from proteasomal degradation in an engineered cell line and was necessary to stabilize pU(L)33 in infected cells, whereas pU(L)15 had no such effects. The presence of pU(L)33 was dispensable for the pU(L)15/pU(L)28 interaction in lysates of both infected and uninfected cells but augmented the tendency for pU(L)15 and pU(L)28 to coimmunoprecipitate. These data suggest that pU(L)28 and pU(L)33 interact directly and that pU(L)15 interacts directly with pU(L)28 but only indirectly with pU(L)33. It is logical to propose that the indirect interaction of pU(L)15 and pU(L)33 is mediated through the interaction of both proteins with pU(L)28. The data also suggest that one function of pU(L)33 is to optimize the pU(L)15/pU(L)28 interaction.

Impact of 2-bromo-5,6-dichloro-1-beta-D-ribofuranosyl benzimidazole riboside and inhibitors of DNA, RNA, and protein synthesis on human cytomegalovirus genome maturation

Herpesvirus genome maturation is a complex process in which concatemeric DNA molecules are translocated into capsids and cleaved at specific sequences to produce encapsidated-unit genomes. Bacteriophage studies further suggest that important ancillary processes, such as RNA transcription and DNA synthesis, concerned with repeat duplication, recombination, branch resolution, or damage repair may also be involved with the genome maturation process. To gain insight into the biochemical activities needed for herpesvirus genome maturation, 2-bromo-5,6-dichloro-1-beta-d-ribofuranosyl benzimidazole riboside (BDCRB) was used to allow the accumulation of human cytomegalovirus concatemeric DNA while the formation of new genomes was being blocked. Genome formation was restored upon BDCRB removal, and addition of various inhibitors during this time window permitted evaluation of their effects on genome maturation. Inhibitors of protein synthesis, RNA transcription, and the viral DNA polymerase only modestly reduced genome formation, demonstrating that these activities are not required for genome maturation. In contrast, drugs that inhibit both viral and host DNA polymerases potently blocked genome formation. Radioisotope incorporation in the presence of a viral DNA polymerase inhibitor further suggested that significant host-mediated DNA synthesis occurs throughout the viral genome. These results indicate a role for host DNA polymerases in genome maturation and are consistent with a need for terminal repeat duplication, debranching, or damage repair concomitant with DNA packaging or cleavage. Similarities to previously reported effects of BDCRB on guinea pig cytomegalovirus were also noted; however, BDCRB induced low-level formation of a supergenomic species called monomer+ DNA that is unique to human cytomegalovirus. Analysis of monomer+ DNA suggested a model for its formation in which BDCRB permits limited packaging of concatemeric DNA but induces skipping of cleavage sites.

Nucleotide sequence of an ICP18.5 assembly protein (UL28) gene of green turtle herpesvirus pathogenically associated with green turtle fibropapilloma

Because newly identified green turtle herpesvirus (GTHV) is associated pathogenically with marine turtle fibropapillomatosis (FP) and it has not been isolated in vitro, molecular sequencing and analysis of the genomic DNA of this putative reptilian herpesvirus will enhance the current understanding of GTHV in causing the FP disease. An inverse polymerase chain reaction (IPCR) genomic walking technique was developed to obtain new DNA sequences based on a portion of known genomic sequence. Through two genomic walks, a 2169 bp DNA fragment of GTHV was cloned and sequenced. Sequence analysis shows that this DNA fragment contains the entire gene of the UL28, as well as the partial genomic sequence of the UL27 gene. The UL28 gene is 2250 bp long and encodes a 750-amino acid peptide known as ICP18.5 assembly protein of herpesviruses. Phylogenetic analysis of the GHTV UL28 gene showed a high sequence homology with the UL28 homologs of other herpesviruses and supports the current classification of GTHV to be a member of Alphaherpesvirinae. Identification of the genomic sequences of GTHV provides a molecular base for the development of diagnostic immunoassay and also for the determination of the pathogenic role of GTHV infection.

Human cytomegalovirus TRS1 protein is required for efficient assembly of DNA-containing capsids

The human cytomegalovirus tegument protein, pTRS1, appears to function at several discrete stages of the virus replication cycle. We previously demonstrated that pTRS1 acts during the late phase of infection to facilitate the production of infectious virions. We now have more precisely identified the late pTRS1 function by further study of a mutant virus lacking the TRS1 region, ADsubTRS1. We observed a significant reduction in the production of capsids, especially DNA-containing C-capsids, in mutant virus-infected cells. ADsubTRS1 exhibited normal cleavage of DNA concatemers, so the defect in C-capsid production must occur after DNA cleavage and before DNA is stably inserted into a capsid. Further, the normal virus-induced morphological reorganization of the nucleus did not occur after infection with the pTRS1-deficient mutant.

The DNA cleavage and packaging protein encoded by the UL33 gene of herpes simplex virus 1 associates with capsids

The U(L)33 gene of herpes simplex virus 1 (HSV-1) encodes a protein (pU(L)33) that is essential for the cleavage and packaging of concatameric herpesvirus DNA into preformed capsids. Previous data have suggested that the U(L)33 protein interacts with the cleavage and packaging proteins encoded by U(L)15 and U(L)28 that are known to associate with capsids. Examination of purified A capsids that lack DNA and are derived from aborted packaging events, B capsids that lack DNA, and C capsids that contain DNA revealed an association of the U(L)33 protein with all three capsid types. More U(L)33 protein was detected in A capsids than was present in B capsids. Capsid association was susceptible to guanidine-HCl treatment and independent of the presence of U(L)15 or U(L)28. Capsid association of pU(L)33 was also independent of U(L)6, which is believed to encode the portal into which DNA is inserted. These data suggest that pU(L)33 may act as part of the capsid-associated molecular machinery that translocates cleaved genomic DNA into the capsid interior.

Dramatic effects of 2-bromo-5,6-dichloro-1-beta-D-ribofuranosyl benzimidazole riboside on the genome structure, packaging, and egress of guinea pig cytomegalovirus

The halogenated benzimidazoles BDCRB (2-bromo-5,6-dichloro-1-beta-D-riborfuranosyl benzimidazole riboside) and TCRB (2,5,6-trichloro-1-beta-D-riborfuranosyl benzimidazole riboside) were the first compounds shown to inhibit cleavage and packaging of herpesvirus genomes. Both inhibit the formation of unit length human cytomegalovirus (HCMV) genomes by a poorly understood mechanism (M. R. Underwood et al., J. Virol. 72:717-715, 1998; P. M. Krosky et al., J. Virol. 72:4721-4728, 1998). Because the simple genome structure of guinea pig cytomegalovirus (GPCMV) provides a useful model for the study of herpesvirus DNA packaging, we investigated the effects of BDCRB on GPCMV. GPCMV proved to be sensitive to BDCRB (50% inhibitory concentration = 4.7 microM), although somewhat less so than HCMV. In striking contrast to HCMV, however, a dose of BDCRB sufficient to reduce GPCMV titers by 3 logs (50 microM) had no effect on the quantity of GPCMV genomic DNA that was formed in infected cells. Electron microscopy revealed that this DNA was in fact packaged within intranuclear capsids, but these capsids failed to egress from the nucleus and failed to protect the DNA from nuclease digestion. The terminal structure of genomes formed in the presence of BDCRB was also altered. Genomes with ends lacking a terminal repeat at the right end, which normally exist in an equimolar ratio with those having one copy of the repeat at the right end, were selectively eliminated by BDCRB treatment. At the left end, BDCRB treatment appeared to induce heterogeneous truncations such that 2.7 to 4.9 kb of left-end-terminal sequences were missing. These findings suggest that BDCRB induces premature cleavage events that result in truncated genomes packaged within capsids that are permeable to nuclease. Based on these and other observations, we propose a model for duplication of herpesvirus terminal repeats during the cleavage and packaging process that is similar to one proposed for bacteriophage T7 (Y. B. Chung, C. Nardone, and D. C. Hinkle, J. Mol. Biol. 216:939-948, 1990).

Quantification of the DNA cleavage and packaging proteins U(L)15 and U(L)28 in A and B capsids of herpes simplex virus type 1

The proteins produced by the herpes simplex virus type 1 (HSV-1) genes U(L)15 and U(L)28 are believed to form part of the terminase enzyme, a protein complex essential for the cleavage of newly synthesized, concatameric herpesvirus DNA and the packaging of the resultant genome lengths into preformed capsids. This work describes the purification of recombinant forms of pU(L)15 and pU(L)28, which allowed the calculation of the average number of copies of each protein in A and B capsids and in capsids lacking the putative portal encoded by U(L)6. On average, 1.0 (+/-0.29 [standard deviation]) copies of pU(L)15 and 2.4 (+/-0.97) copies of pU(L)28 were present in B capsids, 1.2 (+/-0.72) copies of pU(L)15 and 1.5 (+/-0.86) copies of pU(L)28 were found in mutant capsids lacking the putative portal protein pU(L)6, and approximately 12.0 (+/-5.63) copies of pU(L)15 and 0.6 (+/-0.32) copies of pU(L)28 were present in each A capsid. These results suggest that the packaging machine is partly comprised of approximately 12 copies of pU(L)15, as found in A capsids, with wild-type B and mutant U(L)6(-) capsids containing an incomplete complement of cleavage and packaging proteins. These results are consistent with observations that B capsids form by default in the absence of packaging machinery in vitro and in vivo. In contrast, A capsids may be the result of initiated but aborted attempts at DNA packaging, resulting in the retention of at least part of the DNA packaging machinery.

Genetic analysis of the vaccinia virus I6 telomere-binding protein uncovers a key role in genome encapsidation

The linear, double-stranded DNA genome of vaccinia virus contains covalently closed hairpin termini. These hairpin termini comprise a terminal loop and an A+T-rich duplex stem that has 12 extrahelical bases. DeMasi et al. have shown previously that proteins present in infected cells and in virions form distinct complexes with the telomeric hairpins and that these interactions require the extrahelical bases. The vaccinia virus I6 protein was identified as the protein showing the greatest specificity and affinity for interaction with the viral hairpins (J. DeMasi, S. Du, D. Lennon, and P. Traktman, J. Virol. 75:10090-10105, 2001). To gain insight into the role of I6 in vivo, we generated eight recombinant viruses bearing altered alleles of I6 in which clusters of charged amino acids were changed to alanine residues. One allele (temperature-sensitive I6-12 [tsI6-12]) conferred a tight ts phenotype and was used to examine the stage(s) of the viral life cycle that was affected at the nonpermissive temperature. Gene expression, DNA replication, and genome resolution proceeded normally in this mutant. However, proteolytic processing of structural proteins, which accompanies virus maturation, was incomplete. Electron microscopic studies confirmed a severe block in morphogenesis in which immature, but no mature, virions were observed. Instead, aberrant spherical virions and large crystalloids were seen. When purified, these aberrant virions were found to have normal protein content but to be devoid of viral DNA. We propose that the binding of I6 to viral telomeres directs genome encapsidation into the virus particle.

Point mutations in exon I of the herpes simplex virus putative terminase subunit, UL15, indicate that the most conserved residues are essential for cleavage and packaging

The herpes simplex virus UL15 and UL28 genes are believed to encode two subunits of the terminase involved in cleavage and packaging of viral genomes. Analysis of the UL15 protein sequence and its herpesvirus homologues revealed the presence of 20 conserved regions. Twelve of the twenty regions conserved among herpesviruses are also conserved in terminases from DNA bacteriophage. Point mutations in UL15 were designed in four conserved regions: L120N (CR1), Q205E (CR2), Q251E (CR3), G263A (CR3), and Y285S (CR4). Transfection experiments indicated that each mutant gene could produce stable UL15 protein at wild-type levels; however, only one mutant (Q251E) was able to complement the UL15-null virus. Each mutation was introduced into the viral genome by marker transfer, and all mutants except Q251E were unable to form plaques on Vero cells. Furthermore, failure to form plaques on Vero cells correlated with a defect in cleavage and packaging. Immunofluorescence experiments indicated that in cells infected with all mutant viruses the UL15 protein could be detected and was found to localize to replication compartments. Although wild-type and mutant Q251E were able to produce A, B, and C capsids, the rest of the mutants were only able to produce B capsids, a finding consistent with their defects in cleavage and packaging. In addition, all mutant UL15 proteins retained their ability to interact with B capsids. Therefore, amino acid residues 120, 205, 263, and 285 are essential for the cleavage and packaging process rather than for association with capsids or localization to replication compartments.

Herpes simplex virus type 1 portal protein UL6 interacts with the putative terminase subunits UL15 and UL28

The herpes simplex virus type 1 (HSV-1) UL6, UL15, and UL28 proteins are essential for cleavage of replicated concatemeric viral DNA into unit length genomes and their packaging into a preformed icosahedral capsid known as the procapsid. The capsid-associated UL6 DNA-packaging protein is located at a single vertex and is thought to form the portal through which the genome enters the procapsid. The UL15 protein interacts with the UL28 protein, and both are strong candidates for subunits of the viral terminase, a key component of the molecular motor that drives the DNA into the capsid. To investigate the association of the UL6 protein with the UL15 and UL28 proteins, the three proteins were produced in large amounts in insect cells with the baculovirus expression system. Interactions between UL6 and UL28 and between UL6 and UL15 were identified by an immunoprecipitation assay. These results were confirmed by transiently expressing wild-type and mutant proteins in mammalian cells and monitoring their distribution by immunofluorescence. In cells expressing the single proteins, UL6 and UL15 were concentrated in the nuclei whereas UL28 was found in the cytoplasm. When the UL6 and UL28 proteins were coexpressed, UL28 was redistributed to the nuclei, where it colocalized with UL6. In cells producing either of two cytoplasmic UL6 mutant proteins and a functional epitope-tagged form of UL15, the UL15 protein was concentrated with the mutant UL6 protein in the cytoplasm. These observed interactions of UL6 with UL15 and UL28 are likely to be of major importance in establishing a functional DNA-packaging complex at the portal vertex of the HSV-1 capsid.

Herpes simplex virus type-1: a model for genome transactions

In many respects, HSV-1 is the prototypic herpes virus. However, HSV-1 also serves as an excellent model system to study genome transactions, including DNA replication, homologous recombination, and the interaction of DNA replication enzymes with DNA damage. Like eukaryotic chromosomes, the HSV-1 genome contains multiple origins of replication. Replication of the HSV-1 genome is mediated by the concerted action of several virus-encoded proteins that are thought to assemble into a multiprotein complex. Several host-encoded factors have also been implicated in viral DNA replication. Furthermore, replication of the HSV-1 genome is known to be closely associated with homologous recombination that, like in many cellular organisms, may function in recombinational repair. Finally, recent data have shed some light on the interaction of essential HSV-1 replication proteins, specifically its DNA polymerase and DNA helicases, with damaged DNA.

Interactions among antiviral drugs acting late in the replication cycle of human cytomegalovirus

This study describes the extent of cross-resistance and interactions for selected inhibitors of human cytomegalovirus (HCMV) DNA synthesis and DNA processing. HCMV isolates resistant to the benzimidazole D-ribonucleoside viral DNA processing inhibitors TCRB and BDCRB were sensitive to BAY 38-4766, a non-nucleoside inhibitor of viral DNA processing. This indicates that these two drug types have distinct interactions with the products of HCMV genes UL56 and UL89 required for viral DNA cleavage and packaging. These virus isolates also were sensitive to ganciclovir (GCV) but slightly resistant to the L-benzimidazole ribonucleoside viral DNA synthesis inhibitor 1263W94. Virus resistant to 1263W94 remained sensitive to BDCRB, GCV, and BAY 38-4766. Examination of drug-drug interactions in cell culture assays measuring inhibition of HCMV replication revealed strong synergism for the combination of BDCRB with 1263W94, and for combinations of 1263W94 with cidofovir (CDV) and foscarnet (PFA), but not with GCV. Combinations of GCV with CDV and PFA were synergistic as well. The combination of GCV with 1263W94 showed additive antiviral interactions, whereas, a combination of BAY 38-4766 with GCV showed antagonism. Interaction of BDCRB with BAY 38-4766 showed a mixed pattern of synergy and antagonism. The antiviral synergy observed between GCV and PFA or CDV serves to validate clinical combination therapies for these drugs. Antagonism seen for BAY 38-4766 with GCV indicates that these two drugs are unlikely to be useful for combination therapies. Notably, 1263W94 demonstrated greater synergy in combination with PFA or CDV than did GCV, suggesting some promise for this benzimidazole L-riboside in such combination therapies.

Fundamental and accessory systems in herpesviruses

Evolutionary studies have a large theoretical component and will not directly provide therapies for herpesvirus infections. However, they do provide a conceptual framework within which we can evaluate the origins of the various systems that contribute to viral lifestyle. An evolutionary context allows ancient systems that are fundamental to the replication of all herpesviruses to be distinguished from those that have developed relatively recently in order to tailor viruses to particular biological niches. Both categories are in principle accessible to intervention, either to prevent basic replicative capabilities or to reduce the advantages that the virus has in its interactions with the host. Phylogenetic data provide estimates of evolutionary rate for herpesviruses that are only between one and two orders of magnitude greater than those of their hosts. However, it is becoming apparent that certain genes have evolved much faster under selection pressures and by mechanisms that are not well understood. Nonetheless, the mutation rates of even the most highly conserved genes are sufficient to permit herpesviruses to escape from antiviral therapy. Greater understanding of the origins and functions of herpesvirus genes may lead to new insights into the determinants of pathogenesis and hence to new diagnostic and therapeutic targets.

DNA cleavage and packaging proteins encoded by genes U(L)28, U(L)15, and U(L)33 of herpes simplex virus type 1 form a complex in infected cells

Previous studies have indicated that the U(L)6, U(L)15, U(L)17, U(L)28, U(L)32, and U(L)33 genes are required for the cleavage and packaging of herpes simplex viral DNA. To identify proteins that interact with the U(L)28-encoded DNA binding protein of herpes simplex virus type 1 (HSV-1), a previously undescribed rabbit polyclonal antibody directed against the U(L)28 protein fused to glutathione S-transferase was used to immunopurify U(L)28 and the proteins with which it associated. It was found that the antibody specifically coimmunoprecipitated proteins encoded by the genes U(L)28, U(L)15, and U(L)33 from lysates of both HEp-2 cells infected with HSV-1(F) and insect cells infected with recombinant baculoviruses expressing these three proteins. In reciprocal reactions, antibodies directed against the U(L)15- or U(L)33-encoded proteins also coimmunoprecipitated the U(L)28 protein. The coimmunoprecipitation of the three proteins from HSV-infected cells confirms earlier reports of an association between the U(L)28 and U(L)15 proteins and represents the first evidence of the involvement of the U(L)33 protein in this complex.

Epstein-Barr virus complementary strand transcripts (CSTs/BARTs) and cancer

Epstein-Barr virus (EBV) encodes a family of related transcripts, the complementary strand transcripts (CSTs) or BARTs (Bam A rightward transcripts). These are present in all types of EBV infection but are expressed to particularly high levels in nasopharyngeal carcinomas. Although convincing demonstration of protein expression from these transcripts is still subject to some debate, potential proteins encoded by them have been shown to modify Notch signalling pathways.

The UL6 gene product forms the portal for entry of DNA into the herpes simplex virus capsid

During replication of herpes simplex virus type 1 (HSV-1), viral DNA is synthesized in the infected cell nucleus, where DNA-free capsids are also assembled. Genome-length DNA molecules are then cut out of a larger, multigenome concatemer and packaged into capsids. Here we report the results of experiments carried out to test the idea that the HSV-1 UL6 gene product (pUL6) forms the portal through which viral DNA passes as it enters the capsid. Since DNA must enter at a unique site, immunoelectron microscopy experiments were undertaken to determine the location of pUL6. After specific immunogold staining of HSV-1 B capsids, pUL6 was found, by its attached gold label, at one of the 12 capsid vertices. Label was not observed at multiple vertices, at nonvertex sites, or in capsids lacking pUL6. In immunoblot experiments, the pUL6 copy number in purified B capsids was found to be 14.8 +/- 2.6. Biochemical experiments to isolate pUL6 were carried out, beginning with insect cells infected with a recombinant baculovirus expressing the UL6 gene. After purification, pUL6 was found in the form of rings, which were observed in electron micrographs to have outside and inside diameters of 16.4 +/- 1.1 and 5.0 +/- 0.7 nm, respectively, and a height of 19.5 +/- 1.9 nm. The particle weights of individual rings as determined by scanning transmission electron microscopy showed a majority population with a mass corresponding to an oligomeric state of 12. The results are interpreted to support the view that pUL6 forms the DNA entry portal, since it exists at a unique site in the capsid and forms a channel through which DNA can pass. The HSV-1 portal is the first identified in a virus infecting a eukaryote. In its dimensions and oligomeric state, the pUL6 portal resembles the connector or portal complexes employed for DNA encapsidation in double-stranded DNA bacteriophages such as phi29, T4, and P22. This similarity supports the proposed evolutionary relationship between herpesviruses and double-stranded DNA phages and suggests the basic mechanism of DNA packaging is conserved.

A novel nonnucleoside inhibitor specifically targets cytomegalovirus DNA maturation via the UL89 and UL56 gene products

3-Hydroxy-2,2-dimethyl-N-[4([[5-(dimethylamino)-1-naphthyl]sulfonyl]amino)-phenyl]propanamide (BAY 38-4766) is a novel selective nonnucleoside inhibitor of cytomegalovirus (CMV) replication with an excellent safety profile. This compound and structural analogues inhibit neither viral DNA synthesis nor viral transcription and translation. Accumulation of dense bodies and noninfectious enveloped particles coincides with inhibition of both concatemer processing and functional cleavage at intergenomic transitions, pointing to interference with viral DNA maturation and packaging of monomeric genome lengths. Resistant virus populations, including a murine CMV (MCMV) isolate with 566-fold-decreased drug sensitivity, were selected in vitro. Sequencing of the six open reading frames (ORFs) known to be essentially involved in viral DNA cleavage and packaging identified mutations in ORFs UL56, UL89, and UL104. Construction of MCMV recombinants expressing different combinations of murine homologues of mutant UL56, UL89, and UL104 and analysis of drug susceptibilities clearly demonstrated that mutant ORFs UL89 exon II (M360I) and M56 (P202A I208N) individually confer resistance to BAY 38-4766. A combination of both mutant proteins exhibited a strong synergistic effect on resistance, reconstituting the high-resistance phenotype of the in vitro mutant. These findings are consistent with genetic mapping of resistance to TCRB (2,5,6-trichloro-1-beta-D-ribofuranosyl benzimidazole) (P. M. Krosky et al., J. Virol. 72:4721-4728, 1998) and provide further indirect evidence that proteins encoded by UL89 and UL56 function as two subunits of the CMV terminase. While these studies also suggest that the molecular mechanism of BAY 38-4766 is distinct from that of benzimidazole ribonucleosides, they also offer an explanation for the excellent specificity and tolerability of BAY 38-4766, since mammalian DNA does not undergo comparable maturation steps.

Herpes simplex virus DNA cleavage and packaging proteins associate with the procapsid prior to its maturation

Packaging of DNA into preformed capsids is a fundamental early event in the assembly of herpes simplex virus type 1 (HSV-1) virions. Replicated viral DNA genomes, in the form of complex branched concatemers, and unstable spherical precursor capsids termed procapsids are thought to be the substrates for the DNA-packaging reaction. In addition, seven viral proteins are required for packaging, although their individual functions are undefined. By analogy to well-characterized bacteriophage systems, the association of these proteins with various forms of capsids, including procapsids, might be expected to clarify their roles in the packaging process. While the HSV-1 UL6, UL15, UL25, and UL28 packaging proteins are known to associate with different forms of stable capsids, their association with procapsids has not been tested. Therefore, we isolated HSV-1 procapsids from infected cells and used Western blotting to identify the packaging proteins present. Procapsids contained UL15 and UL28 proteins; the levels of both proteins are diminished in more mature DNA-containing C-capsids. In contrast, UL6 protein levels were approximately the same in procapsids, B-capsids, and C-capsids. The amount of UL25 protein was reduced in procapsids relative to that in more mature B-capsids. Moreover, C-capsids contained the highest level of UL25 protein, 15-fold higher than that in procapsids. Our results support current hypotheses on HSV DNA packaging: (i) transient association of UL15 and UL28 proteins with maturing capsids is consistent with their proposed involvement in site-specific cleavage of the viral DNA (terminase activity); (ii) the UL6 protein may be an integral component of the capsid shell; and (iii) the UL25 protein may associate with capsids after scaffold loss and DNA packaging, sealing the DNA within capsids.

Interaction of the herpes simplex virus type 1 packaging protein UL15 with full-length and deleted forms of the UL28 protein

The UL15 and UL28 proteins of herpes simplex virus type 1 are both required for the packaging of replicated viral DNA into the viral capsid. We have expressed UL28 and a functional epitope-tagged form of UL15 in mammalian and insect cells. Immunoprecipitation experiments confirmed that the two proteins can interact. In agreement with previous results, UL15, when expressed alone, entered the nucleus but UL28 remained cytoplasmic. When co-expressed the two proteins co-localized in the nucleus. Six UL28 deletion mutants were constructed and similarly analysed. The results obtained by immunoprecipitation and immunofluorescence were consistent and demonstrate that at least two separate regions of the UL28 polypeptide chain have the ability to interact with UL15. Surprisingly, three of the mutants prevented the UL15 protein from localizing to the cell nucleus, and these were not functional in a transient DNA packaging assay. Of the three UL28 mutant proteins that entered the nucleus with UL15, one containing an internal deletion of 13 amino acids was able to complement a UL28 null mutant in both DNA packaging and virus yield assays, demonstrating that this region of the protein is not essential for function. In addition to interacting with the UL28 protein we also demonstrated that UL15 molecules can interact with each other, and that sequences within the second exon contribute to this interaction.

Novel class of thiourea compounds that inhibit herpes simplex virus type 1 DNA cleavage and encapsidation: resistance maps to the UL6 gene

In our search for novel inhibitors of herpes simplex virus type 1 (HSV-1), a new class of thiourea inhibitors was discovered. N-(4-[3-(5-Chloro-2,4-dimethoxyphenyl)-thioureido]-phenyl)-acetamide and its 2-fluoro-benzamide derivative inhibited HSV-1 replication. HSV-2, human cytomegalovirus, and varicella-zoster virus were inhibited to a lesser extent. The compounds acted late in the replication cycle by impairing both the cleavage of concatameric viral DNA into progeny genome length and the packaging of the DNA into capsids, indicative of a defect in the encapsidation process. To uncover the molecular target of the inhibition, resistant HSV-1 isolates were generated, and the mutation responsible for the resistance was mapped using marker transfer techniques. Each of three independent isolates had point mutations in the UL6 gene which resulted in independent single-amino-acid changes. One mutation was located in the N terminus of the protein (E121D), while two were located close together in the C terminus (A618V and Q621R). Each of these point mutations was sufficient to confer drug resistance when introduced into wild-type virus. The UL6 gene is one of the seven HSV-1 genes known to play a role in DNA packaging. This novel class of inhibitors has provided a new tool for dissection of HSV-1 encapsidation mechanisms and has uncovered a new viable target for the treatment of herpesviral diseases.