VP22 of herpes simplex virus 1 promotes protein synthesis at late times in infection and accumulation of a subset of viral mRNAs at early times in infection

The precise function of alphaherpesvirus tegument proteins and their interactions during the viral life cycle

Alphaherpesvirus is a widespread pathogen that causes diverse diseases in humans and animals and can severely damage host health. Alphaherpesvirus particles comprise a DNA core, capsid, tegument and envelope; the tegument is located between the nuclear capsid and envelope. According to biochemical and proteomic analyses of alphaherpesvirus particles, the tegument contains at least 24 viral proteins and plays an important role in the alphaherpesvirus life cycle. This article reviews the important role of tegument proteins and their interactions during the viral life cycle to provide a reference and inspiration for understanding alphaherpesvirus infection pathogenesis and identifying new antiviral strategies.

Duck plague virus tegument protein vp22 plays a key role in the secondary envelopment and cell-to-cell spread

Duck plague virus (DPV) is one of the major infectious and fatal diseases of geese, ducks, and other wild waterfowl. The DPV UL49 gene product VP22 is one of the most abundant tegument proteins. However, the role of the DPV VP22 is enigmatic to be clarified. In this study, we found deletion of the UL49 gene resulted in reduced viral growth curve and smaller plaque size in duck embryo fibroblast (DEF) cells, confirming that DPV VP22 is required for efficient viral growth in vitro. In addition, deletion of the UL49 gene inhibited the secondary envelopment of the virus, the release of viral particles, and the spread of viruses between cells. Our study signified the importance of VP22 for DPV secondary envelopment, release, cell-to-cell spread, and accumulation of viral RNA. These findings provide a basis for further study of the function of VP22 in DPV or other herpesviruses.

Dysregulated metabolism of the late herpes simplex virus 1 transcriptome through the vhs-VP22 axis uncouples virus cytopathic effect and virus production

Herpes simplex virus 1 (HSV1) expresses its genes in a classical cascade culminating in the production of large amounts of structural proteins to facilitate virus assembly. HSV1 lacking the virus protein VP22 (Δ22) exhibits late translational shutoff, a phenotype that has been attributed to the unrestrained activity of the virion host shutoff (vhs) protein, a virus-encoded endoribonuclease which induces mRNA degradation during infection. We have previously shown that vhs is also involved in regulating the nuclear-cytoplasmic compartmentalisation of the virus transcriptome, and in the absence of VP22 a number of virus transcripts are sequestered in the nucleus late in infection. Here we show that despite expressing minimal amounts of structural proteins and failing to plaque on human fibroblasts, the strain 17 Δ22 virus replicates and spreads as efficiently as Wt virus, but without causing cytopathic effect (CPE). Nonetheless, CPE-causing virus spontaneously appeared on Δ22-infected human fibroblasts, and four viruses isolated in this way had all acquired point mutations in vhs which rescued late protein translation. However, unlike a virus deleted for vhs, these viruses still induced the degradation of both cellular and viral mRNA suggesting that vhs mutation in the absence of VP22 is necessary to overcome a more complex disturbance in mRNA metabolism than mRNA degradation alone. The ultimate outcome of secondary mutations in vhs is therefore the rescue of virus-induced CPE caused by late protein synthesis, and while there is a clear selective pressure on HSV1 to mutate vhs for optimal production of late structural proteins, the purpose of this is over and above that of virus production.

Herpes Simplex Virus 1 Expressing GFP-Tagged Virion Host Shutoff (vhs) Protein Uncouples the Activities of RNA Degradation and Differential Nuclear Retention of the Virus Transcriptome

Virion host shutoff (vhs) protein is an endoribonuclease encoded by herpes simplex virus 1 (HSV1). vhs causes several changes to the infected cell environment that favor the translation of late (L) virus proteins: cellular mRNAs are degraded, immediate early (IE) and early (E) viral transcripts are sequestered in the nucleus with polyA binding protein (PABPC1), and dsRNA is degraded to help dampen the PKR-dependent stress response. To further our understanding of the cell biology of vhs, we constructed a virus expressing vhs tagged at its C terminus with GFP. When first expressed, vhs-GFP localized to juxtanuclear clusters, and later it colocalized and interacted with its binding partner VP16, and was packaged into virions. Despite vhs-GFP maintaining activity when expressed in isolation, it failed to degrade mRNA or relocalise PABPC1 during infection, while viral transcript levels were similar to those seen for a vhs knockout virus. PKR phosphorylation was also enhanced in vhs-GFP infected cells, which is in line with a failure to degrade dsRNA. Nonetheless, mRNA FISH revealed that as in Wt but not Dvhs infection, IE and E, but not L transcripts were retained in the nucleus of vhs-GFP infected cells at late times. These results revealed that the vhs-induced nuclear retention of IE and E transcripts was dependent on vhs expression but not on its endoribonuclease activity, uncoupling these two functions of vhs. IMPORTANCE Like many viruses, herpes simplex virus 1 (HSV1) expresses an endoribonuclease, the virion host shutoff (vhs) protein, which regulates the RNA environment of the infected cell and facilitates the classical cascade of virus protein translation. It does this by causing the degradation of some mRNA molecules and the nuclear retention of others. Here, we describe a virus expressing vhs tagged at its C terminus with a green fluorescent protein (GFP) and show that the vhs-GFP fusion protein retains the physical properties of native vhs but does not induce the degradation of mRNA. Nonetheless, vhs-GFP maintains the ability to trap the early virus transcriptome in the nucleus to favor late protein translation, proving for the first time that mRNA degradation is not a prerequisite for vhs effects on the nuclear transcriptome. This virus, therefore, has uncoupled the nuclear retention and degradation activities of vhs, providing a new understanding of vhs during infection.

Preventing translational inhibition from ribosomal protein insufficiency by a herpes simplex virus-encoded ribosome-associated protein

In addition to being required for protein synthesis, ribosomes and ribosomal proteins (RPs) also regulate messenger RNA translation in uninfected and virus-infected cells. By individually depleting 85 RPs using RNA interference, we found that overall protein synthesis in uninfected primary fibroblasts was more sensitive to RP depletion than those infected with herpes simplex virus-1 (HSV-1). Although representative RP depletion (uL3, uS4, uL5) inhibited protein synthesis in cells infected with two different DNA viruses (human cytomegalovirus, vaccinia virus), HSV-1-infected cell protein synthesis unexpectedly endured and required a single virus-encoded gene product, VP22. During individual RP insufficiency, VP22-expressing HSV-1 replicated better than a VP22-deficient variant. Furthermore, VP22 promotes polysome accumulation in virus-infected cells when uL3 or ribosome availability is limiting and cosediments with initiating and elongating ribosomes in infected and uninfected cells. This identifies VP22 as a virus-encoded, ribosome-associated protein that compensates for RP insufficiency to support viral protein synthesis and replication. Moreover, it reveals an unanticipated class of virus-encoded, ribosome-associated effectors that reduce the dependence of protein synthesis upon host RPs and broadly support translation during physiological stress such as infection.

"Non-Essential" Proteins of HSV-1 with Essential Roles In Vivo: A Comprehensive Review

Viruses encode for structural proteins that participate in virion formation and include capsid and envelope proteins. In addition, viruses encode for an array of non-structural accessory proteins important for replication, spread, and immune evasion in the host and are often linked to virus pathogenesis. Most virus accessory proteins are non-essential for growth in cell culture because of the simplicity of the infection barriers or because they have roles only during a state of the infection that does not exist in cell cultures (i.e., tissue-specific functions), or finally because host factors in cell culture can complement their absence. For these reasons, the study of most nonessential viral factors is more complex and requires development of suitable cell culture systems and in vivo models. Approximately half of the proteins encoded by the herpes simplex virus 1 (HSV-1) genome have been classified as non-essential. These proteins have essential roles in vivo in counteracting antiviral responses, facilitating the spread of the virus from the sites of initial infection to the peripheral nervous system, where it establishes lifelong reservoirs, virus pathogenesis, and other regulatory roles during infection. Understanding the functions of the non-essential proteins of herpesviruses is important to understand mechanisms of viral pathogenesis but also to harness properties of these viruses for therapeutic purposes. Here, we have provided a comprehensive summary of the functions of HSV-1 non-essential proteins.

Alphaherpesvirus Major Tegument Protein VP22: Its Precise Function in the Viral Life Cycle

Alphaherpesviruses are zoonotic pathogens that can cause a variety of diseases in humans and animals and severely damage health. Alphaherpesvirus infection is a slow and orderly process that can lie dormant for the lifetime of the host but may be reactivated when the immune system is compromised. All alphaherpesviruses feature a protein layer called the tegument that lies between the capsid and the envelope. Virus protein (VP) 22 is one of the most highly expressed tegument proteins; there are more than 2,000 copies of this protein in each viral particle. VP22 can interact with viral proteins, cellular proteins, and chromatin, and these interactions play important roles. This review summarizes the latest literature and discusses the roles of VP22 in viral gene transcription, protein synthesis, virion assembly, and viral cell-to-cell spread with the purpose of enhancing understanding of the life cycle of herpesviruses and other pathogens in host cells. The molecular interaction information herein provides important reference data.

VHS, US3 and UL13 viral tegument proteins are required for Herpes Simplex Virus-Induced modification of protein kinase R

To replicate, spread and persist in the host environment, viruses have evolved several immunological escape mechanisms via the action of specific viral proteins. The model "host shut off" adopted by virion host shut off (VHS) protein of Herpes simplex type 1 (HSV-1) represents an immune evasion mechanism which affects the best-characterized component of the innate immunological response, protein kinase R (PKR). However, up to now, the real mechanism employed by VHS to control PKR is still unknown. In this paper, we implement and extend our previous findings reporting that wild-type HSV-1 is able to control PKR, whereas a VHS mutant virus (R2621) clearly induces an accumulation of phosphorylated PKR in several cell types in a VHS-RNase activity-dependent manner. Furthermore, we demonstrate for the first time a new PKR-regulatory mechanism based on the involvement of Us3 and UL13 tegument viral proteins. The combined approach of transfection and infection assay was useful to discover the new role of both viral proteins in the immunological escape and demonstrate that Us3 and UL13 control the accumulation of the phosphorylated form (ph-PKR). Lastly, since protein kinases are tightly regulated by phosphorylation events and, at the same time, phosphorylate other proteins by inducing post-translational modifications, the interplay between Us3 and VHS during HSV-1 infection has been investigated. Interestingly, we found that VHS protein accumulates at higher molecular weight following Us3 transfection, suggesting an Us3-mediated phosphorylation of VHS. These findings reveal a new intriguing interplay between viral proteins during HSV-1 infection involved in the regulation of the PKR-mediated immune response.

Identification of Marek's Disease Virus VP22 Tegument Protein Domains Essential for Virus Cell-to-Cell Spread, Nuclear Localization, Histone Association and Cell-Cycle Arrest

VP22 is a major tegument protein of alphaherpesviruses encoded by the UL49 gene. Two properties of VP22 were discovered by studying Marek's disease virus (MDV), the Mardivirus prototype; it has a major role in virus cell-to-cell spread and in cell cycle modulation. This 249 AA-long protein contains three regions including a conserved central domain. To decipher the functional VP22 domains and their relationships, we generated three series of recombinant MDV genomes harboring a modified UL49 gene and assessed their effect on virus spread. Mutated VP22 were also tested for their ability to arrest the cell cycle, subcellular location and histones copurification after overexpression in cells. We demonstrated that the N-terminus of VP22 associated with its central domain is essential for virus spread and cell cycle modulation. Strikingly, we demonstrated that AAs 174-190 of MDV VP22 containing the end of a putative extended alpha-3 helix are essential for both functions and that AAs 159-162 located in the putative beta-strand of the central domain are mandatory for cell cycle modulation. Despite being non-essential, the 59 C-terminal AAs play a role in virus spread efficiency. Interestingly, a positive correlation was observed between cell cycle modulation and VP22 histones association, but none with MDV spread.

Nuclear-cytoplasmic compartmentalization of the herpes simplex virus 1 infected cell transcriptome is co-ordinated by the viral endoribonuclease vhs and cofactors to facilitate the translation of late proteins

HSV1 encodes an endoribonuclease termed virion host shutoff (vhs) that is produced late in infection and packaged into virions. Paradoxically, vhs is active against not only host but also virus transcripts, and is involved in host shutoff and the temporal expression of the virus transcriptome. Two other virus proteins-VP22 and VP16 -are proposed to regulate vhs to prevent uncontrolled and lethal mRNA degradation but their mechanism of action is unknown. We have performed dual transcriptomic analysis and single-cell mRNA FISH of human fibroblasts, a cell type where in the absence of VP22, HSV1 infection results in extreme translational shutoff. In Wt infection, host mRNAs exhibited a wide range of susceptibility to vhs ranging from resistance to 1000-fold reduction, a variation that was independent of their relative abundance or transcription rate. However, vhs endoribonuclease activity was not found to be overactive against any of the cell transcriptome in Δ22-infected cells but rather was delayed, while its activity against the virus transcriptome and in particular late mRNA was minimally enhanced. Intriguingly, immediate-early and early transcripts exhibited vhs-dependent nuclear retention later in Wt infection but late transcripts were cytoplasmic. However, in the absence of VP22, not only early but also late transcripts were retained in the nucleus by a vhs-dependent mechanism, a characteristic that extended to cellular transcripts that were not efficiently degraded by vhs. Moreover, the ability of VP22 to bind VP16 enhanced but was not fundamental to the rescue of vhs-induced nuclear retention of late transcripts. Hence, translational shutoff in HSV1 infection is primarily a result of vhs-induced nuclear retention and not degradation of infected cell mRNA. We have therefore revealed a new mechanism whereby vhs and its co-factors including VP22 elicit a temporal and spatial regulation of the infected cell transcriptome, thus co-ordinating efficient late protein production.

Multiple Posttranscriptional Strategies To Regulate the Herpes Simplex Virus 1 vhs Endoribonuclease

The herpes simplex virus 1 (HSV-1) virion host shutoff (vhs) protein is an endoribonuclease that binds to the cellular translation initiation machinery and degrades associated mRNAs, resulting in the shutoff of host protein synthesis. Hence, its unrestrained activity is considered lethal, and it has been proposed that vhs is regulated by two other virus proteins, VP22 and VP16. We have found that during infection, translation of vhs requires VP22 but not the VP22-VP16 complex. Moreover, in the absence of VP22, vhs is not overactive against cellular or viral transcripts. In transfected cells, vhs was also poorly translated, correlating with the aberrant localization of its mRNA. Counterintuitively, vhs mRNA was predominantly nuclear in cells where vhs protein was detected. Likewise, transcripts from cotransfected plasmids were also retained in the same nuclei where vhs mRNA was located, while poly(A) binding protein (PABP) was relocalized to the nucleus in a vhs-dependent manner, implying a general block to mRNA export. Coexpression of VP16 and VP22 rescued the cytoplasmic localization of vhs mRNA but failed to rescue vhs translation. We identified a 230-nucleotide sequence in the 5' region of vhs that blocked its translation and, when transferred to a heterologous green fluorescent protein transcript, reduced translation without altering mRNA levels or localization. We propose that expression of vhs is tightly regulated by a combination of inherent untranslatability and autoinduced nuclear retention of its mRNA that results in a negative feedback loop, with nuclear retention but not translation of vhs mRNA being the target of rescue by the vhs-VP16-VP22 complex.IMPORTANCE A myriad of gene expression strategies has been discovered through studies carried out on viruses. This report concerns the regulation of the HSV-1 vhs endoribonuclease, a virus factor that is important for counteracting host antiviral responses by degrading their mRNAs but that must be regulated during infection to ensure that it does not act against and inhibit the virus itself. We show that regulation of vhs involves multifaceted posttranscriptional cellular and viral processes, including aberrant mRNA localization and a novel, autoregulated negative feedback loop to target its own and coexpressed mRNAs for nuclear retention, an activity that is relieved by coexpression of two other virus proteins, VP22 and VP16. These studies reveal the interplay of strategies by which multiple virus-encoded factors coordinate gene expression at the time that they are needed. These findings are broadly relevant to both virus and cellular gene expression.

Herpes Simplex Virus 1 Tegument Protein VP22 Abrogates cGAS/STING-Mediated Antiviral Innate Immunity

Cytosolic DNA arising from intracellular pathogens is sensed by cyclic GMP-AMP synthase (cGAS) and triggers a powerful innate immune response. However, herpes simplex virus 1 (HSV-1), a double-stranded DNA virus, has developed multiple mechanisms to attenuate host antiviral machinery and facilitate viral infection and replication. In the present study, we found that HSV-1 tegument protein VP22 acts as an inhibitor of cGAS/stimulator of interferon genes (cGAS/STING)-mediated production of interferon (IFN) and its downstream antiviral genes. Our results showed that ectopic expression of VP22 decreased cGAS/STING-mediated IFN-β promoter activation and IFN-β production. Infection with wild-type (WT) HSV-1, but not VP22-deficient virus (ΔVP22), inhibited immunostimulatory DNA (ISD)-induced activation of the IFN signaling pathway. Further study showed that VP22 interacted with cGAS and inhibited the enzymatic activity of cGAS. In addition, stable knockdown of cGAS facilitated the replication of ΔVP22 virus but not the WT. In summary, our findings indicate that HSV-1 VP22 acts as an antagonist of IFN signaling to persistently evade host innate antiviral responses.IMPORTANCE cGAS is very important for host defense against viral infection, and many viruses have evolved ways to target cGAS and successfully evade the attack by the immune system of their susceptible host. This study demonstrated that HSV-1 tegument protein VP22 counteracts the cGAS/STING-mediated DNA-sensing antiviral innate immunity signaling pathway by inhibiting the enzymatic activity of cGAS. The findings in this study will expand our understanding of the interaction between HSV-1 replication and the host DNA-sensing signaling pathway.

Varicella-Zoster Virus ORF9p Binding to Cellular Adaptor Protein Complex 1 Is Important for Viral Infectivity

ORF9p (homologous to herpes simplex virus 1 [HSV-1] VP22) is a varicella-zoster virus (VZV) tegument protein essential for viral replication. Even though its precise functions are far from being fully described, a role in the secondary envelopment of the virus has long been suggested. We performed a yeast two-hybrid screen to identify cellular proteins interacting with ORF9p that might be important for this function. We found 31 ORF9p interaction partners, among which was AP1M1, the μ subunit of the adaptor protein complex 1 (AP-1). AP-1 is a heterotetramer involved in intracellular vesicle-mediated transport and regulates the shuttling of cargo proteins between endosomes and the trans-Golgi network via clathrin-coated vesicles. We confirmed that AP-1 interacts with ORF9p in infected cells and mapped potential interaction motifs within ORF9p. We generated VZV mutants in which each of these motifs was individually impaired and identified leucine 231 in ORF9p to be critical for the interaction with AP-1. Disrupting ORF9p binding to AP-1 by mutating leucine 231 to alanine in ORF9p strongly impaired viral growth, most likely by preventing efficient secondary envelopment of the virus. Leucine 231 is part of a dileucine motif conserved among alphaherpesviruses, and we showed that VP22 of Marek's disease virus and HSV-2 also interacts with AP-1. This indicates that the function of this interaction in secondary envelopment might be conserved as well.IMPORTANCE Herpesviruses are responsible for infections that, especially in immunocompromised patients, can lead to severe complications, including neurological symptoms and strokes. The constant emergence of viral strains resistant to classical antivirals (mainly acyclovir and its derivatives) pleads for the identification of new targets for future antiviral treatments. Cellular adaptor protein (AP) complexes have been implicated in the correct addressing of herpesvirus glycoproteins in infected cells, and the discovery that a major constituent of the varicella-zoster virus tegument interacts with AP-1 reveals a previously unsuspected role of this tegument protein. Unraveling the complex mechanisms leading to virion production will certainly be an important step in the discovery of future therapeutic targets.

Decreased expression of the immediate early protein, ICP4, by deletion of the tegument protein VP22 of equine herpesvirus type 1

VP22 is a major tegument protein of equine herpesvirus type 1 (EHV-1). In the present study, we examined functions of VP22 in EHV-1 replication by viral protein expression analyses in cells infected with the VP22-deficient virus. The expressions of several viral proteins in the cells infected with the VP22-deficient virus were lower than those in the cells infected with the parent virus. One of the weakly expressed proteins was identified as ICP4, which is a major regulatory protein encoded by an immediate early gene of EHV-1. A real-time PCR analysis showed that the mRNA expression of ICP4 was the same in cells infected with the parent and VP22-deficient viruses. Hence, VP22 appears to promote synthesis of ICP4 post-transcriptionally.

A VP22-Null HSV-1 Is Impaired in Inhibiting CD1d-Mediated Antigen Presentation

CD1d-restricted T (natural killer T [NKT]) cells are important for controlling a herpes simplex virus (HSV) infection. One of the mechanisms of immune evasion by HSV is to downregulate CD1d-mediated activation of NKT cells. VP22 is an HSV-1-encoded protein responsible for reorganizing the host cell's cytoskeletal network and viral spreading. We have previously shown that modification of the cytoskeleton can alter CD1d-mediated antigen presentation. In this study, we found that an HSV-1 lacking VP22 (ΔUL49) was impaired in its ability to inhibit CD1d-mediated antigen presentation compared with the wild-type (WT) virus; this was reversed by a repair virus (UL49R) in CD1d-expressing cells. We further demonstrated that CD1d recycling was inhibited by infection with WT and UL49R, but not the ΔUL49 virus. Ectopic expression of VP22 in CD1d-expressing cells complemented the VP22-deficient virus in inhibiting antigen presentation. Moreover, inhibiting viral protein synthesis rescued VP22-dependent inhibition of CD1d antigen presentation. In conclusion, our findings suggest that VP22 is required (but not sufficient) for the inhibition of CD1d-mediated antigen presentation by an HSV-1 infection.

Insights into the function of tegument proteins from the varicella zoster virus

Chickenpox (varicella) is caused by primary infection with varicella zoster virus (VZV), which can establish long-term latency in the host ganglion. Once reactivated, the virus can cause shingles (zoster) in the host. VZV has a typical herpesvirus virion structure consisting of an inner DNA core, a capsid, a tegument, and an outer envelope. The tegument is an amorphous layer enclosed between the nucleocapsid and the envelope, which contains a variety of proteins. However, the types and functions of VZV tegument proteins have not yet been completely determined. In this review, we describe the current knowledge on the multiple roles played by VZV tegument proteins during viral infection. Moreover, we discuss the VZV tegument protein-protein interactions and their impact on viral tissue tropism in SCID-hu mice. This will help us develop a better understanding of how the tegument proteins aid viral DNA replication, evasion of host immune response, and pathogenesis.

Elucidation of the block to herpes simplex virus egress in the absence of tegument protein UL16 reveals a novel interaction with VP22

UL16 is a tegument protein of herpes simplex virus (HSV) that is conserved among all members of the Herpesviridae, but its function is poorly understood. Previous studies revealed that UL16 is associated with capsids in the cytoplasm and interacts with the membrane protein UL11, which suggested a "bridging" function during cytoplasmic envelopment, but this conjecture has not been tested. To gain further insight, cells infected with UL16-null mutants were examined by electron microscopy. No defects in the transport of capsids to cytoplasmic membranes were observed, but the wrapping of capsids with membranes was delayed. Moreover, clusters of cytoplasmic capsids were often observed, but only near membranes, where they were wrapped to produce multiple capsids within a single envelope. Normal virion production was restored when UL16 was expressed either by complementing cells or from a novel position in the HSV genome. When the composition of the UL16-null viruses was analyzed, a reduction in the packaging of glycoprotein E (gE) was observed, which was not surprising, since it has been reported that UL16 interacts with this glycoprotein. However, levels of the tegument protein VP22 were also dramatically reduced in virions, even though this gE-binding protein has been shown not to depend on its membrane partner for packaging. Cotransfection experiments revealed that UL16 and VP22 can interact in the absence of other viral proteins. These results extend the UL16 interaction network beyond its previously identified binding partners to include VP22 and provide evidence that UL16 plays an important function at the membrane during virion production.

Virus-encoded endonucleases: expected and novel functions

Endonucleases catalyze critical steps in the processing, function, and turnover of many cellular RNAs. It is, therefore, not surprising that a number of viruses encode endonucleases that play important roles in viral gene expression. The virion host shutoff (Vhs) endonuclease of herpes simplex virus, the SOX protein of Kaposi Sarcoma Herpesvirus (KSHV), and the influenza virus PB1 endonuclease have well-characterized functions that stem from their abilities to cleave RNA. Vhs accelerates turnover of many cellular and viral mRNAs, redirecting the cell from host to viral gene expression, counteracting key elements of the innate immune response, and facilitating sequential expression of different classes of viral genes. SOX reduces synthesis of many host proteins during the lytic phase of KSHV infections. PB1 is a component of the influenza RNA polymerase that snatches capped oligonucleotides from cellular pre-mRNAs to serve as primers during viral mRNA synthesis. However, all three proteins have important second functions. Vhs stimulates translation of the 3' cistron of bicistronic mRNAs that have selected cellular internal ribosome entry sites, and stimulates polysome loading and translation of selected viral mRNAs at late times during productive infections. SOX has an alkaline exonuclease activity that is important for processing and maturation of newly synthesized copies of the KSHV genome. The influenza RNA polymerase binds the cap and 5' region of viral mRNAs and recruits eIF4G and other factors to viral mRNAs, allowing them to be translated under conditions of reduced eIF4E functionality. This review will discuss the novel and expected functions of these viral endonucleases.

Mode of virus rescue determines the acquisition of VHS mutations in VP22-negative herpes simplex virus 1

It has been proposed that herpes simplex virus 1 with VP22 deleted requires secondary mutation of VHS for viability. Here we show that a replication-competent Δ22 virus constructed by homologous recombination maintains a wild-type (Wt) VHS gene and has no other gross mutations. By contrast, Δ22 viruses recovered from a bacterial artificial chromosome contain multiple amino acid changes within a conserved region of VHS. Hence, the mode of virus rescue influences the acquisition of secondary mutations.

Deletion of the herpes simplex virus 1 UL49 gene results in mRNA and protein translation defects that are complemented by secondary mutations in UL41

Herpes simplex virus 1 (HSV-1) virions, like those of all herpesviruses, contain a protein layer termed the tegument localized between the capsid and the envelope. VP22, encoded by the U(L)49 gene, is one of the most abundant tegument proteins in HSV-1 virions. Studies with a U(L)49-null mutant showed that the absence of VP22 resulted in decreased protein synthesis at late times in infection. VP22 is known to form a tripartite complex with VP16 and vhs through direct interactions with VP16. Given that U(L)49-null mutants have been shown to acquire spontaneous secondary mutations in the U(L)41 gene, which encodes vhs, we hypothesized that VP22 and vhs may play antagonistic roles during HSV-1 infections. In the present study, we show that the protein synthesis defect observed in U(L)49-null virus infections was rescued by a secondary, compensatory frameshift mutation in U(L)41. A double mutant bearing a deletion of U(L)49 and a point mutation in vhs previously shown to specifically abrogate vhs's RNase activity also resulted in a rescue of protein synthesis. To determine whether the U(L)49(-) protein synthesis defect, and the rescue by secondary mutations in vhs, occurred at the mRNA and/or translational levels, quantitative reverse transcriptase PCR (qRT-PCR) and polysome analyses were performed. We found that the absence of VP22 caused a small decrease in mRNA levels as well as a defect in polysome assembly that was independent of mRNA abundance. Both defects were complemented by the secondary mutations in vhs, indicating functional interplay between VP22 and vhs in both accumulation and translation of viral mRNAs.

Purification of full-length VP22 from cells infected with HSV-1: A two-pronged approach for the solubilization and purification of viral proteins for use in biochemical studies

VP22, encoded by the U(L)49 gene, is one of the most abundant proteins of the herpes simplex virus type 1 (HSV-1) tegument and has been shown to be important for virus replication and spread. However, the exact role(s) played by VP22 in the HSV-1 replication cycle have yet to be delineated. The lack of a procedure to purify full-length VP22 has limited molecular studies on VP22 function. A procedure was developed for the purification of soluble, full-length VP22 from cells infected with HSV-1. A recombinant virus encoding His-tagged VP22 was generated and found to express VP22 at levels comparable to the wild type virus upon infection of Vero cells. By experimenting with a wide variety of cell lysis buffer conditions, several buffers that promote the solubility of full-length VP22 were identified. Buffers that gave the highest levels of solubility were then used in immobilized metal ion affinity chromatography experiments to identify conditions that provided the greatest level of VP22 binding and recovery from cobalt and nickel affinity resins. Using this strategy soluble, full-length VP22 was purified from cells infected with HSV-1.

Herpes simplex virus 1 VP22 regulates translocation of multiple viral and cellular proteins and promotes neurovirulence

Herpes simplex virus 1 (HSV-1) protein VP22, encoded by the UL49 gene, is a major virion tegument protein. In the present study, we showed that VP22 was required for efficient redistribution of viral proteins VP16, VP26, ICP0, ICP4, and ICP27 and of cellular protein Hsc-70 to the cytoplasm of infected cells. We found that two dileucine motifs in VP22, at amino acids 235 and 236 and amino acids 251 and 252, were necessary for VP22 regulation of the proper cytoplasmic localization of these viral and cellular proteins. The dileucine motifs were also required for proper cytoplasmic localization of VP22 itself and for optimal expression of viral proteins VP16, VP22, ICP0, UL41, and glycoprotein B. Interestingly, a recombinant mutant virus with alanines substituted for the dileucines at amino acids 251 and 252 had a 50% lethal dose (LD(50)) for neurovirulence in mice following intracerebral inoculation about 10(3)-fold lower than the LD(50) of the repaired virus. Furthermore, the replication and spread of this mutant virus in the brains of mice following intracerebral inoculation were significantly impaired relative to those of the repaired virus. The ability of VP22 to regulate the localization and expression of various viral and cellular proteins, as shown in this study, was correlated with an increase in viral replication and neurovirulence in the experimental murine model. Thus, HSV-1 VP22 is a significant neurovirulence factor in vivo.

Herpes virus infection can cause intervertebral disc degeneration

It has been proposed that intervertebral disc degeneration might be caused by low-grade infection. The purpose of the present study was to assess the incidence of herpes viruses in intervertebral disc specimens from patients with lumbar disc herniation. A polymerase chain reaction based assay was applied to screen for the DNA of eight different herpes viruses in 16 patients and two controls. DNA of at least one herpes virus was detected in 13 specimens (81.25%). Herpes Simplex Virus type-1 (HSV-1) was the most frequently detected virus (56.25%), followed by Cytomegalovirus (CMV) (37.5%). In two patients, co-infection by both HSV-1 and CMV was detected. All samples, including the control specimens, were negative for Herpes Simplex Virus type-2, Varicella Zoster Virus, Epstein Barr Virus, Human Herpes Viruses 6, 7 and 8. The absence of an acute infection was confirmed both at the serological and mRNA level.

To our knowledge this is the first unequivocal evidence of the presence of herpes virus DNA in intervertebral disc specimens of patients with lumbar disc herniation suggesting the potential role of herpes viruses as a contributing factor to the pathogenesis of degenerative disc disease.

Deletion of the virion host shut: off gene of pseudorabies virus results in selective upregulation of the expression of early viral genes in the late stage of infection

A real-time RT-PCR technique was applied to evaluate the impact of deletion of the virion host shut-off (VHS) gene on the kinetics of pseudorabies virus gene expression. Selective suppression of early gene transcripts by the viral ribonuclease occurs after 4h of infection; while VHS protein appears to act non-selectively on the transcripts belonging in different kinetic classes in the first 2h of infection. VHS protein disrupts the close correlation between the transcription kinetics of the immediate-early 180 protein and the other pseudorabies virus transcripts. The typical pattern of early gene expression was found to be altered in the VHS gene-deleted virus in that the production rates of their transcripts did not decline from 4h post-infection. This observation led us to put forward the hypothesis that the VHS protein may play a pivotal role in the switch from the early to the late stage of infection.

Characterization of the nuclear import and export signals, and subcellular transport mechanism of varicella-zoster virus ORF9

Varicella-zoster virus (VZV) open reading frame 9 (ORF9) mRNA is one of the most abundantly expressed messages during VZV infection. However, little is known concerning the function of ORF9 protein. Here, we found that transient expression of ORF9 fused to enhanced yellow fluorescent protein (EYFP) in COS-7 cells showed a predominantly cytoplasmic localization in the absence of other viral proteins. By constructing a series of ORF9 variants fused to EYFP, a bona fide bipartite nuclear localization signal of ORF9 was, for the first time, determined and mapped to aa 16-32 (RRKTTPSYSGQYRTARR). Additionally, the nuclear export signal (NES) was identified and found to be in a leucine-rich region at aa 103-117 (LRHELVEDAVYENPL). Finally, ORF9 was demonstrated to be targeted to the cytoplasm through the functional NES by Ran and the chromosomal region maintenance 1-dependent pathway, and to the nucleus via an importin β-dependent pathway that does not require importin α5.

Herpes simplex virus type 1/adeno-associated virus hybrid vectors

Herpes simplex virus type 1 (HSV-1) amplicons can accommodate foreign DNA of any size up to 150 kbp and, therefore, allow extensive combinations of genetic elements. Genomic sequences as well as cDNA, large transcriptional regulatory sequences for cell type-specific expression, multiple transgenes, and genetic elements from other viruses to create hybrid vectors may be inserted in a modular fashion. Hybrid amplicons use genetic elements from HSV-1 that allow replication and packaging of the vector DNA into HSV-1 virions, and genetic elements from other viruses that either direct integration of transgene sequences into the host genome or allow episomal maintenance of the vector. Thus, the advantages of the HSV-1 amplicon system, including large transgene capacity, broad host range, strong nuclear localization, and availability of helper virus-free packaging systems are retained and combined with those of heterologous viral elements that confer genetic stability to the vector DNA. Adeno-associated virus (AAV) has the unique capability of integrating its genome into a specific site, designated AAVS1, on human chromosome 19. The AAV rep gene and the inverted terminal repeats (ITRs) that flank the AAV genome are sufficient for this process. HSV-1 amplicons have thus been designed that contain the rep gene and a transgene cassette flanked by AAV ITRs. These HSV/AAV hybrid vectors direct site-specific integration of transgene sequences into AAVS1 and support long-term transgene expression.

Herpes simplex virus type 1 tegument protein VP22 is capable of modulating the transcription of viral TK and gC genes via interaction with viral ICP0

VP22, a tegument protein of herpes simplex virus type 1 (HSV-1), is present in many copies in one virion and undergoes different types of post-translational modification. VP22 is believed to have certain functions in viral infection apart from virus assembly. Here we show that VP22 physically interacted with infected cell polypeptide 0 (ICP0) and colocalized in the nucleus, indicating that VP22 could be functionally involved in the modulation of viral transcription through interaction with ICP0. In the HSV-1 infection system and chloramphenicol acetyltransferase (CAT) transcriptional system, VP22-ICP0 interaction was confirmed to play a role in modulating the transcription of some viral genes and could be a factor in viral transcription, which is probably required in the transcriptional control of latent infection.

Recruitment of herpes simplex virus type 1 immediate-early protein ICP0 to the virus particle

Although the herpes simplex virus type 1 (HSV-1) tegument is comprised of a large number of viral and cellular proteins, how and where in the cell these proteins are recruited into the virus structure is poorly understood. We have shown previously that the immediate-early gene product ICP0 is packaged by a mechanism dependent on the major tegument protein VP22, while others have shown a requirement for ICP27. We now extend our studies to show that ICP0 packaging correlates directly with the ability of ICP0 to complex with VP22 in infected cells. ICP27 is not, however, present in this VP22-ICP0 complex but is packaged into the virion in a VP22- and ICP0-independent manner. Biochemical fractionation of virions indicated that ICP0 associates tightly with the virus capsid, but intranuclear capsids contained no detectable ICP0. The RING finger domain of ICP0 and the N terminus of VP22 were both shown to be essential but not sufficient for ICP0 packaging and complex formation. Strikingly, however, the N-terminal region of VP22, while unable to form a complex with ICP0, inhibited its translocation from the nucleus to the cytoplasm. PML degradation by ICP0 was efficient in cells infected with this VP22 mutant virus, confirming that ICP0 retains activity. Hence, we would suggest that VP22 is an important molecular partner of ICP0 that controls at least one of its activities: its assembly into the virion. Moreover, we propose that the pathway by which VP22 recruits ICP0 to the virion may begin in the nucleus prior to ICP0 translocation to its final site of assembly in the cytoplasm.

Herpes Virus Amplicon Vectors

Since its emergence onto the gene therapy scene nearly 25 years ago, the replication-defective Herpes Simplex Virus Type-1 (HSV-1) amplicon has gained significance as a versatile gene transfer platform due to its extensive transgene capacity, widespread cellular tropism, minimal immunogenicity, and its amenability to genetic manipulation. Herein, we detail the recent advances made with respect to the design of the HSV amplicon, its numerous in vitro and in vivo applications, and the current impediments this virus-based gene transfer platform faces as it navigates a challenging path towards future clinical testing.

A conserved carboxy-terminal domain in the major tegument structural protein VP22 facilitates virion packaging of a chimeric protein during productive herpes simplex virus 1 infection

Recombinant virus HSV-1(RF177) was previously generated to examine tegument protein VP22 function by inserting the GFP gene into the gene encoding VP22. During a detailed analysis of this virus, we discovered that RF177 produces a novel fusion protein between the last 15 amino acids of VP22 and GFP, termed GCT-VP22. Thus, the VP22 carboxy-terminal specific antibody 22-3 and two anti-GFP antibodies reacted with an approximately 28 kDa protein from RF177-infected Vero cells. GCT-VP22 was detected at 1 and 3 hpi. Examination of purified virions indicated that GCT-VP22 was incorporated into RF177 virus particles. These observations imply that at least a portion of the information required for virion targeting is located in this domain of VP22. Indirect immunofluorescence analyses showed that GCT-VP22 also localized to areas of marginalized chromatin during RF177 infection. These results indicate that the last fifteen amino acids of VP22 participate in virion targeting during HSV-1 infection.