Identification of proteins directly phosphorylated by UL13 protein kinase from herpes simplex virus 1

Impact of the interaction between herpes simplex virus 1 ICP22 and FACT on viral gene expression and pathogenesis

Facilitates chromatin transcription (FACT) interacts with nucleosomes to promote gene transcription by regulating the dissociation and reassembly of nucleosomes downstream and upstream of RNA polymerase II (Pol II). A previous study reported that herpes simplex virus 1 (HSV-1) regulatory protein ICP22 interacted with FACT and was required for its recruitment to the viral DNA genome in HSV-1-infected cells. However, the biological importance of interactions between ICP22 and FACT in relation to HSV-1 infection is unclear. Here, we mapped the minimal domain of ICP22 required for its efficient interaction with FACT to a cluster of five basic amino acids in ICP22. A recombinant virus harboring alanine substitutions in this identified cluster led to the decreased accumulation of viral mRNAs from UL54, UL38, and UL44 genes, reduced Pol II occupancy of these genes in MRC-5 cells, and impaired HSV-1 virulence in mice following ocular or intracranial infection. Furthermore, the treatment of mice infected with wild-type HSV-1 with CBL0137, a FACT inhibitor currently being investigated in clinical trials, significantly improved the survival rate of mice. These results suggested that the interaction between ICP22 and FACT was required for efficient HSV-1 gene expression and pathogenicity. Therefore, FACT might be a potential therapeutic target for HSV-1 infection.IMPORTANCEICP22 is a well-known regulatory factor of HSV-1 gene expression, but its mechanism(s) are poorly understood. Although the interaction of FACT with ICP22 was reported previously, its significance in HSV-1 infection is unknown. Given that FACT is involved in gene transcription, it is of interest to investigate this interaction as it relates to HSV-1 gene expression. To determine a direct link between the interaction and HSV-1 infection, we mapped a minimal domain of ICP22 required for its efficient interaction with FACT and generated a recombinant virus carrying mutations in the identified domain. Using the recombinant virus, we obtained evidence suggesting that the interaction between ICP22 and FACT promoted Pol II transcription from HSV-1 genes and viral virulence in mice. In addition, CBL0137, an inhibitor of FACT, effectively protected mice from lethal HSV-1 infection, suggesting FACT might be a potential target for the development of novel anti-HSV drugs.

Duck plague virus Us3 regulates the expression of pUL48

Duck plague (DP) is one of the contagious diseases caused by Duck plague virus (DPV), which is a serious threat to the development of duck farming. Us3 is a PKA-like protein kinase in alphaherpesvirus, which can regulate the biological functions of many viral proteins, but whether Us3 regulates pUL48 protein has not been reported. In this paper, Western Blot, qRT-PCR, dual luciferase reporter system and Co-IP were used to investigate the relationship between pUL48 and Us3. The results showed that: 1) pUL48 interacted with Us3 at 138-256aa through its DBD region. 2) Us3 enhanced the protein expression of pUL48 in a dose-dependent manner. 3) Us3 promoted the mRNA level of pUL48 by activating its promoter activity. 4) Us3 inhibited the transcriptional activation function of pUL48. The results can provide scientific data for perfecting and supplementing the function of alpha herpesvirus Us3 and pUL48.

Mechanism of herpesvirus protein kinase UL13 in immune escape and viral replication

Upon infection, the herpes viruses create a cellular environment suitable for survival, but innate immunity plays a vital role in cellular resistance to viral infection. The UL13 protein of herpesviruses is conserved among all herpesviruses and is a serine/threonine protein kinase, which plays a vital role in escaping innate immunity and promoting viral replication. On the one hand, it can target various immune signaling pathways in vivo, such as the cGAS-STING pathway and the NF-κB pathway. On the other hand, it phosphorylates regulatory many cellular and viral proteins for promoting the lytic cycle. This paper reviews the research progress of the conserved herpesvirus protein kinase UL13 in immune escape and viral replication to provide a basis for elucidating the pathogenic mechanism of herpesviruses, as well as providing insights into the potential means of immune escape and viral replication of other herpesviruses that have not yet resolved the function of it.

Regulation of alphaherpesvirus protein via post-translational phosphorylation

An alphaherpesvirus carries dozens of viral proteins in the envelope, tegument and capsid structure, and each protein plays an indispensable role in virus adsorption, invasion, uncoating and release. After infecting the host, a virus eliminates unfavourable factors via multiple mechanisms to escape or suppress the attack of the host immune system. Post-translational modification of proteins, especially phosphorylation, regulates changes in protein conformation and biological activity through a series of complex mechanisms. Many viruses have evolved mechanisms to leverage host phosphorylation systems to regulate viral protein activity and establish a suitable cellular environment for efficient viral replication and virulence. In this paper, viral protein kinases and the regulation of viral protein function mediated via the phosphorylation of alphaherpesvirus proteins are described. In addition, this paper provides new ideas for further research into the role played by the post-translational modification of viral proteins in the virus life cycle, which will be helpful for understanding the mechanisms of viral infection of a host and may lead to new directions of antiviral treatment.

Herpes simplex virus type 2 inhibits TNF-α-induced NF-κB activation through viral protein ICP22-mediated interaction with p65

Herpes simplex virus type 2 (HSV-2) is a prevalent human pathogen and the main cause of genital herpes. After initial infection, HSV-2 can establish lifelong latency within dorsal root ganglia by evading the innate immunity of the host. NF-κB has a crucial role in regulating cell proliferation, inflammation, apoptosis, and immune responses. It is known that inhibition of NF-κB activation by a virus could facilitate it to establish infection in the host. In the current study, we found that HSV-2 inhibited TNF-α-induced activation of NF-κB-responsive promoter in a dose-dependent manner, while UV-inactivated HSV-2 did not have such capability. We further identified the immediate early protein ICP22 of HSV-2 as a vital viral element in inhibiting the activation of NF-κB-responsive promoter. The role of ICP22 was confirmed in human cervical cell line HeLa and primary cervical fibroblasts in the context of HSV-2 infection, showing that ICP22 deficient HSV-2 largely lost the capability in suppressing NF-κB activation. HSV-2 ICP22 was further shown to suppress the activity of TNF receptor-associated factor 2 (TRAF2)-, IκB kinase α (IKK α)-, IKK β-, IKK γ-, or p65-induced activation of NF-κB-responsive promoter. Mechanistically, HSV-2 ICP22 inhibited the phosphorylation and nuclear translocation of p65 by directly interacting with p65, resulting in the blockade of NF-κB activation. Furthermore, ICP22 from several alpha-herpesviruses could also inhibit NF-κB activation, suggesting the significance of ICP22 in herpesvirus immune evasion. Findings in this study highlight the importance of ICP22 in inhibiting NF-κB activation, revealing a novel mechanism by which HSV-2 evades the host antiviral responses.

Pseudorabies Virus Tegument Protein UL13 Suppresses RLR-Mediated Antiviral Innate Immunity through Regulating Receptor Transcription

Pseudorabies virus (PRV) has evolved various strategies to escape host antiviral immune responses. However, it remains unclear whether and how PRV-encoded proteins modulate the RIG-I-like receptor (RLR)-mediated signals for immune evasion. Here, we show that the PRV tegument protein UL13 functions as an antagonist of RLR-mediated antiviral responses via suppression of the transcription of RIG-I and MDA5, but not LGP2. UL13 overexpression significantly inhibits both the mRNA and protein levels of RIG-I and MDA5, along with RIG-I- or MDA5-mediated antiviral immune responses, whereas overexpression of RIG-I or MDA5 counteracts such UL13-induced suppression. Mechanistically, UL13 suppresses the expression of RIG-I and MDA5 by inhibiting activation of the transcription factor NF-κB. Consequently, overexpression of p65 promotes the activation of RIG-I and MDA5 promoters. Moreover, deletion of the p65-binding sites in the promoters of RIG-I or MDA5 abolishes the suppression role of UL13. As a result, mutant PRV lacking UL13 elicits stronger host antiviral immune responses than PRV-WT. Hence, our results provide a novel functional role of UL13-induced suppression of host antiviral immunity through modulating receptors' transcription.

Evasion of I Interferon-Mediated Innate Immunity by Pseudorabies Virus

Type I interferon (IFN-I) mediated innate immunity serves as the first line of host defense against viral infection, ranging from IFN-I production upon viral detection, IFN-I triggered signaling pathway that induces antiviral gene transcription the antiviral effects of IFN-I induced gene products. During coevolution, herpesviruses have developed multiple countermeasures to inhibit the various steps involved to evade the IFN response. This mini-review focuses on the strategies used by the alphaherpesvirus Pseudorabies virus (PRV) to antagonize IFN-I mediated innate immunity, with a particular emphasis on the mechanisms inhibiting IFN-I induced gene transcription through the JAK-STAT pathway. The knowledge obtained from PRV enriches the current understanding of the alphaherpesviral immune evasion mechanisms and provides insight into the vaccine development for PRV control.

Peroxiredoxin 1 Interacts with TBK1/IKKε and Negatively Regulates Pseudorabies Virus Propagation by Promoting Innate Immunity

Peroxiredoxin 1 (PRDX1) is a cellular antioxidant enzyme that is crucial for diverse fundamental biological processes, such as autophagy, inflammation, and carcinogenesis. However, molecular mechanisms underpinning its diverse roles are not well understood. Here, we report that PRDX1 positively regulates interferon (IFN) induction and that pseudorabies virus (PRV) targets PRDX1 to evade IFN induction. PRV UL13 encodes a serine/threonine kinase important for PRV infection, although its biological function remains obscure. We identified PRDX1 as a UL13-interacting protein. Virological and biochemical assays demonstrate that PRDX1 promotes IFN induction by interacting with TANK-binding kinase 1 (TBK1) and IκB kinase ε (IKKε). Conversely, UL13 accelerates PRDX1 degradation via the ubiquitin-proteosome pathway in a kinase-dependent manner. In doing so, PRV inhibits IFN induction during productive infection, which requires PRDX1 expression. This study uncovers an essential role of PRDX1 in the innate immune response and reveals a new viral immune evasion strategy to counteract cellular defenses. IMPORTANCE PRV interacts with numerous cellular proteins during productive infection. Here, we demonstrated the interaction of viral protein UL13 with the antioxidant enzyme PRDX1, which functions in multiple signal transduction pathways. We found that PRDX1 participates in the type I IFN pathway by interacting with TBK1 and IKKε, thereby negatively regulating PRV propagation. However, UL13 ubiquitinates PRDX1, which routes PRDX1 into proteasomes for degradation and effectively reduces its expression. These results illuminate the fundamental role that PRDX1 plays in the IFN pathway, and they identify a potential target for the control of PRV infection.

Multifaceted Roles of ICP22/ORF63 Proteins in the Life Cycle of Human Herpesviruses

Herpesviruses are extremely successful parasites that have evolved over millions of years to develop a variety of mechanisms to coexist with their hosts and to maintain host-to-host transmission and lifelong infection by regulating their life cycles. The life cycle of herpesviruses consists of two phases: lytic infection and latent infection. During lytic infection, active replication and the production of numerous progeny virions occur. Subsequent suppression of the host immune response leads to a lifetime latent infection of the host. During latent infection, the viral genome remains in an inactive state in the host cell to avoid host immune surveillance, but the virus can be reactivated and reenter the lytic cycle. The balance between these two phases of the herpesvirus life cycle is controlled by broad interactions among numerous viral and cellular factors. ICP22/ORF63 proteins are among these factors and are involved in transcription, nuclear budding, latency establishment, and reactivation. In this review, we summarized the various roles and complex mechanisms by which ICP22/ORF63 proteins regulate the life cycle of human herpesviruses and the complex relationships among host and viral factors. Elucidating the role and mechanism of ICP22/ORF63 in virus-host interactions will deepen our understanding of the viral life cycle. In addition, it will also help us to understand the pathogenesis of herpesvirus infections and provide new strategies for combating these infections.

CRISPR/Cas9-Constructed Pseudorabies Virus Mutants Reveal the Importance of UL13 in Alphaherpesvirus Escape from Genome Silencing

Latent and recurrent productive infection of long-living cells, such as neurons, enables alphaherpesviruses to persist in their host populations. Still, the viral factors involved in these events remain largely obscure. Using a complementation assay in compartmented primary peripheral nervous system (PNS) neuronal cultures, we previously reported that productive replication of axonally delivered genomes is facilitated by pseudorabies virus (PRV) tegument proteins. Here, we sought to unravel the role of tegument protein UL13 in this escape from silencing. We first constructed four new PRV mutants in the virulent Becker strain using CRISPR/Cas9-mediated gene replacement: (i) PRV Becker defective for UL13 expression (PRV ΔUL13), (ii) PRV where UL13 is fused to eGFP (PRV UL13-eGFP), and two control viruses (iii and iv) PRV where VP16 is fused with mTurquoise at either the N terminus (PRV mTurq-VP16) or the C terminus (PRV VP16-mTurq). Live-cell imaging of PRV capsids showed efficient retrograde transport after axonal infection with PRV UL13-eGFP, although we did not detect dual-color particles. However, immunofluorescence staining of particles in mid-axons indicated that UL13 might be cotransported with PRV capsids in PNS axons. Superinfecting nerve cell bodies with UV-inactivated PRV ΔUL13 failed to efficiently promote escape from genome silencing compared to UV-PRV wild type and UV-PRV UL13-eGFP superinfection. However, UL13 does not act directly in the escape from genome silencing, as adeno-associated virus (AAV)-mediated UL13 expression in neuronal cell bodies was not sufficient to provoke escape from genome silencing. Based on this, we suggest that UL13 may contribute to initiation of productive infection through phosphorylation of other tegument proteins.IMPORTANCE Alphaherpesviruses have mastered various strategies to persist in an immunocompetent host, including the induction of latency and reactivation in peripheral nervous system (PNS) ganglia. We recently discovered that the molecular mechanism underlying escape from latency by the alphaherpesvirus pseudorabies virus (PRV) relies on a structural viral tegument protein. This study aimed at unravelling the role of tegument protein UL13 in PRV escape from latency. First, we confirmed the use of CRISPR/Cas9-mediated gene replacement as a versatile tool to modify the PRV genome. Next, we used our new set of viral mutants and AAV vectors to conclude the indirect role of UL13 in PRV escape from latency in primary neurons, along with its spatial localization during retrograde capsid transport in axons. Based on these findings, we speculate that UL13 phosphorylates one or more tegument proteins, thereby priming these putative proteins to induce escape from genome silencing.

Role of the DNA Binding Activity of Herpes Simplex Virus 1 VP22 in Evading AIM2-Dependent Inflammasome Activation Induced by the Virus

AIM2 is a cytosolic DNA sensor of the inflammasome, which induces critical innate immune responses against various invading pathogens. Earlier biochemical studies showed that the binding of AIM2 to DNA triggered the self-oligomerization of AIM2, which is essential for AIM2 inflammasome activation. We recently reported that VP22, a virion tegument protein of herpes simplex virus 1 (HSV-1), inhibited activation of the AIM2 inflammasome in HSV-1-infected cells by preventing AIM2 oligomerization. VP22 binds non-specifically to DNA; however, its role in HSV-1 replication is unclear. We investigated the role of VP22 DNA binding activity in the VP22-mediated inhibition of AIM2 inflammasome activation. We identified a VP22 domain encoded by amino acids 227 to 258 as the minimal domain required for its binding to DNA in vitro Consecutive alanine substitutions in this domain substantially impaired the DNA binding activity of VP22 in vitro and attenuated the inhibitory effect of VP22 on AIM2 inflammasome activation in an AIM2 inflammasome reconstitution system. The inhibitory effect of VP22 on AIM2 inflammasome activation was completely abolished in macrophages infected with a recombinant virus harboring VP22 with one of the consecutive alanine substitutions, similar to the effect of a VP22-null mutant virus. These results suggested that the DNA binding activity of VP22 is critical for VP22-mediated AIM2 inflammasome activation in HSV1-infected cells.IMPORTANCE VP22, a major component of the HSV-1 virion tegument, is conserved in alphaherpesviruses and has structural similarity to ORF52, a component of the virion tegument that is well-conserved in gammaherpesviruses. Although the potential DNA binding activity of VP22 was discovered decades ago, its significance in the HSV-1 life cycle is poorly understood. Here, we show that the DNA binding activity of VP22 is critical for the inhibition of AIM2 inflammasome activation induced in HSV-1-infected cells. This is the first report to show a role for the DNA binding activity of VP22 in the HSV-1 life cycle, allowing the virus to evade AIM2 inflammasome activation, which is critical for its replication in vivo.

"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.

PRV-encoded UL13 protein kinase acts as an antagonist of innate immunity by targeting IRF3-signaling pathways

Pseudorabies virus (PRV), a porcine alphaherpesvirus, causes neurological disorders and reproductive failure in swine. It is capable of avoiding host antiviral responses, resulting in viral latency in infected animals. The mechanisms by which many PRV proteins help the virus to evade immune surveillance are poorly understood. In this study, we found that the PRV protein kinase, UL13, inhibits the IFN-β signaling pathway by targeting interferon regulatory factor 3 (IRF3) for ubiquitination and degradation. PRV with mutant of UL13 is impaired in its ability to hinder IRF3 and interferon-β (IFN-β) activation, and has significantly less pathogenesis in mice that wild-type PRV. Our findings reveal an as yet undescribed mechanism utilized by PRV to evade host immune responses. PRV UL13 is a potential target for attenuated vaccines and antiviral drugs.

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.

Herpes Simplex Virus Type 2 Inhibits Type I IFN Signaling Mediated by the Novel E3 Ubiquitin Protein Ligase Activity of Viral Protein ICP22

Type I IFNs play an important role in innate immunity against viral infections by inducing the expression of IFN-stimulated genes (ISGs), which encode effectors with various antiviral functions. We and others previously reported that HSV type 2 (HSV-2) inhibits the synthesis of type I IFNs, but how HSV-2 suppresses IFN-mediated signaling is less understood. In the current study, after the demonstration of HSV-2 replication resistance to IFN-β treatment in human epithelial cells, we reveal that HSV-2 and the viral protein ICP22 significantly decrease the expression of ISG54 at both mRNA and protein levels. Likewise, us1 del HSV-2 (ICP22-deficient HSV-2) replication is more sensitive to IFN-β treatment, indicating that ICP22 is a vital viral protein responsible for the inhibition of type I IFN-mediated signaling. In addition, overexpression of HSV-2 ICP22 inhibits the expression of STAT1, STAT2, and IFN regulatory factor 9 (IRF9), resulting in the blockade of ISG factor 3 (ISGF3) nuclear translocation, and mechanistically, this is due to ICP22-induced ubiquitination of STAT1, STAT2, and IRF9. HSV-2 ICP22 appears to interact with STAT1, STAT2, IRF9, and several other ubiquitinated proteins. Following further biochemical study, we show that HSV-2 ICP22 functions as an E3 ubiquitin protein ligase to induce the formation of polyubiquitin chains. Taken together, we demonstrate that HSV-2 interferes with type I IFN-mediated signaling by degrading the proteins of ISGF3, and we identify HSV-2 ICP22 as a novel E3 ubiquitin protein ligase to induce the degradation of ISGF3. Findings in this study highlight a new mechanism by which HSV-2 circumvents the host antiviral responses through a viral E3 ubiquitin protein ligase.

Herpes simplex virus-1 evasion of CD8+ T cell accumulation contributes to viral encephalitis

Herpes simplex virus-1 (HSV-1) is the most common cause of sporadic viral encephalitis, which can be lethal or result in severe neurological defects even with antiviral therapy. While HSV-1 causes encephalitis in spite of HSV-1-specific humoral and cellular immunity, the mechanism by which HSV-1 evades the immune system in the central nervous system (CNS) remains unknown. Here we describe a strategy by which HSV-1 avoids immune targeting in the CNS. The HSV-1 UL13 kinase promotes evasion of HSV-1-specific CD8+ T cell accumulation in infection sites by downregulating expression of the CD8+ T cell attractant chemokine CXCL9 in the CNS of infected mice, leading to increased HSV-1 mortality due to encephalitis. Direct injection of CXCL9 into the CNS infection site enhanced HSV-1-specific CD8+ T cell accumulation, leading to marked improvements in the survival of infected mice. This previously uncharacterized strategy for HSV-1 evasion of CD8+ T cell accumulation in the CNS has important implications for understanding the pathogenesis and clinical treatment of HSV-1 encephalitis.

A Herpesviral Immediate Early Protein Promotes Transcription Elongation of Viral Transcripts

Herpes simplex virus 1 (HSV-1) genes are transcribed by cellular RNA polymerase II (RNA Pol II). While four viral immediate early proteins (ICP4, ICP0, ICP27, and ICP22) function in some capacity in viral transcription, the mechanism by which ICP22 functions remains unclear. We observed that the FACT complex (comprised of SSRP1 and Spt16) was relocalized in infected cells as a function of ICP22. ICP22 was also required for the association of FACT and the transcription elongation factors SPT5 and SPT6 with viral genomes. We further demonstrated that the FACT complex interacts with ICP22 throughout infection. We therefore hypothesized that ICP22 recruits cellular transcription elongation factors to viral genomes for efficient transcription elongation of viral genes. We reevaluated the phenotype of an ICP22 mutant virus by determining the abundance of all viral mRNAs throughout infection by transcriptome sequencing (RNA-seq). The accumulation of almost all viral mRNAs late in infection was reduced compared to the wild type, regardless of kinetic class. Using chromatin immunoprecipitation sequencing (ChIP-seq), we mapped the location of RNA Pol II on viral genes and found that RNA Pol II levels on the bodies of viral genes were reduced in the ICP22 mutant compared to wild-type virus. In contrast, the association of RNA Pol II with transcription start sites in the mutant was not reduced. Taken together, our results indicate that ICP22 plays a role in recruiting elongation factors like the FACT complex to the HSV-1 genome to allow for efficient viral transcription elongation late in viral infection and ultimately infectious virion production.

HSV-1 interacts with many cellular proteins throughout productive infection. Here, we demonstrate the interaction of a viral protein, ICP22, with a subset of cellular proteins known to be involved in transcription elongation. We determined that ICP22 is required to recruit the FACT complex and other transcription elongation factors to viral genomes and that in the absence of ICP22 viral transcription is globally reduced late in productive infection, due to an elongation defect. This insight defines a fundamental role of ICP22 in HSV-1 infection and elucidates the involvement of cellular factors in HSV-1 transcription.

Involvement of herpes simplex virus type 1 UL13 protein kinase in induction of SOCS genes, the negative regulators of cytokine signaling

The suppressor of cytokine signaling (SOCS) family has eight members and suppresses various cytokine signaling pathways, including IFN signaling. Therefore, some viruses have evolved molecular mechanisms for inducing SOCS proteins and thus escaping host immunity. Herpes simplex virus type 1 (HSV-1) has a mechanism for escaping from type I IFN by induction of both SOCS1 and SOCS3. In this study, expression of the eight members of the SOCS family stimulated by HSV-1 infection was comparatively analyzed by qRT-PCR. It was found that SOCS1 and SOCS3 are induced by HSV-1-infection at 4 hr post infection. However, such induction was not observed in UL13 deficient virus-infected cells, suggesting that UL13 protein kinase participates in induction of both genes. The transcription factor Sp1-binding sites of SOCS3 promoter/enhancer region were identified as the regulatory elements for induction of SOCS3 in HSV-1 infected cells. Accumulation of activated Sp1 was detectable in the nuclei of HSV-1-infected cells before induction of SOCS3. Taken together, these results suggest that HSV-1 has a potent mechanism for escaping from the IFN system.

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.

The UL13 and US3 Protein Kinases of Herpes Simplex Virus 1 Cooperate to Promote the Assembly and Release of Mature, Infectious Virions

Herpes simplex virus type 1 (HSV-1) encodes two bona fide serine/threonine protein kinases, the US3 and UL13 gene products. HSV-1 ΔUS3 mutants replicate with wild-type efficiency in cultured cells, and HSV-1 ΔUL13 mutants exhibit <10-fold reduction in infectious viral titers. Given these modest phenotypes, it remains unclear how the US3 and UL13 protein kinases contribute to HSV-1 replication. In the current study, we designed a panel of HSV-1 mutants, in which portions of UL13 and US3 genes were replaced by expression cassettes encoding mCherry protein or green fluorescent protein (GFP), respectively, and analyzed DNA replication, protein expression, and spread of these mutants in several cell types. Loss of US3 function alone had largely negligible effect on viral DNA accumulation, gene expression, virion release, and spread. Loss of UL13 function alone also had no appreciable effects on viral DNA levels. However, loss of UL13 function did result in a measurable decrease in the steady-state levels of two viral glycoproteins (gC and gD), release of total and infectious virions, and viral spread. Disruption of both genes did not affect the accumulation of viral DNA, but resulted in further reduction in gC and gD steady-state levels, and attenuation of viral spread and infectious virion release. These data show that the UL13 kinase plays an important role in the late phase of HSV-1 infection, likely by affecting virion assembly and/or release. Moreover, the data suggest that the combined activities of the US3 and UL13 protein kinases are critical to the efficient assembly and release of infectious virions from HSV-1-infected cells.

DNA methyltransferase DNMT3A associates with viral proteins and impacts HSV-1 infection

Viral infections can alter the cellular epigenetic landscape, through modulation of either DNA methylation profiles or chromatin remodeling enzymes and histone modifications. These changes can act to promote viral replication or host defense. Herpes simplex virus type 1 (HSV-1) is a prominent human pathogen, which relies on interactions with host factors for efficient replication and spread. Nevertheless, the knowledge regarding its modulation of epigenetic factors remains limited. Here, we used fluorescently-labeled viruses in conjunction with immunoaffinity purification and MS to study virus-virus and virus-host protein interactions during HSV-1 infection in primary human fibroblasts. We identified interactions among viral capsid and tegument proteins, detecting phosphorylation of the capsid protein VP26 at sites within its UL37-binding domain, and an acetylation within the major capsid protein VP5. Interestingly, we found a nuclear association between viral capsid proteins and the de novo DNA methyltransferase DNA (cytosine-5)-methyltransferase 3A (DNMT3A), which we confirmed by reciprocal isolations and microscopy. We show that drug-induced inhibition of DNA methyltransferase activity, as well as siRNA- and shRNA-mediated DNMT3A knockdowns trigger reductions in virus titers. Altogether, our results highlight a functional association of viral proteins with the mammalian DNA methyltransferase machinery, pointing to DNMT3A as a host factor required for effective HSV-1 infection.

HSV-2 immediate-early protein US1 inhibits IFN-β production by suppressing association of IRF-3 with IFN-β promoter

HSV-2 is the major cause of genital herpes, and its infection increases the risk of HIV-1 acquisition and transmission. After initial infection, HSV-2 can establish latency within the nervous system and thus maintains lifelong infection in humans. It has been suggested that HSV-2 can inhibit type I IFN signaling, but the underlying mechanism has yet to be determined. In this study, we demonstrate that productive HSV-2 infection suppresses Sendai virus (SeV) or polyinosinic-polycytidylic acid-induced IFN-β production. We further reveal that US1, an immediate-early protein of HSV-2, contributes to such suppression, showing that US1 inhibits IFN-β promoter activity and IFN-β production at both mRNA and protein levels, whereas US1 knockout significantly impairs such capability in the context of HSV-2 infection. US1 directly interacts with DNA binding domain of IRF-3, and such interaction suppresses the association of nuclear IRF-3 with the IRF-3 responsive domain of IFN-β promoter, resulting in the suppression of IFN-β promoter activation. Additional studies demonstrate that the 217-414 aa domain of US1 is critical for the suppression of IFN-β production. Our results indicate that HSV-2 US1 downmodulates IFN-β production by suppressing the association of IRF-3 with the IRF-3 responsive domain of IFN-β promoter. Our findings highlight the significance of HSV-2 US1 in inhibiting IFN-β production and provide insights into the molecular mechanism by which HSV-2 evades the host innate immunity, representing an unconventional strategy exploited by a dsDNA virus to interrupt type I IFN signaling pathway.

Herpes simplex virus protein kinases US3 and UL13 modulate VP11/12 phosphorylation, virion packaging, and phosphatidylinositol 3-kinase/Akt signaling activity

The phosphatidylinositol 3-kinase (PI3K)/Akt signaling pathway plays key roles in diverse cellular activities and promotes cell growth and survival. It is therefore unsurprising that most viruses modify this pathway in order to facilitate their replication and spread. Previous work has suggested that the herpes simplex virus 1 (HSV-1) tegument proteins VP11/12 and US3 protein kinase modulate the PI3K/Akt pathway, albeit in opposing ways: VP11/12 binds and activates Src family kinases (SFKs), is tyrosine phosphorylated, recruits PI3K in an SFK-dependent fashion, and is required for HSV-induced phosphorylation of Akt on its activating residues; in contrast, US3 inhibits Akt activation and directly phosphorylates downstream Akt targets. We examined if US3 negatively regulates Akt by dampening the signaling activity of VP11/12. Consistent with this hypothesis, the enhanced Akt activation that occurs during US3-null infection requires VP11/12 and correlates with an increase in SFK-dependent VP11/12 tyrosine phosphorylation. In addition, deleting US3 leads to a striking increase in the relative abundances of several VP11/12 species that migrate with reduced mobility during SDS-PAGE. These forms arise through phosphorylation, strictly require the viral UL13 protein kinase, and are excluded from virions. Taken in combination, these data indicate that US3 dampens SFK-dependent tyrosine and UL13-dependent serine/threonine phosphorylation of VP11/12, thereby inhibiting VP11/12 signaling and promoting virion packaging of VP11/12. These results illustrate that protein phosphorylation events mediated by viral protein kinases serve to coordinate the roles of VP11/12 as a virion component and intracellular signaling molecule.

IMPORTANCE

Herpesvirus tegument proteins play dual roles during the viral life cycle, serving both as structural components of the virus particle and as modulators of cellular and viral functions in infected cells. How these two roles are coordinated during infection and virion assembly is a fundamental and largely unanswered question. Here we addressed this issue with herpes simplex virus VP11/12, a tegument protein that activates the cellular PI3K/Akt signaling pathway. We showed that protein phosphorylation mediated by the viral US3 and UL13 kinases serves to orchestrate its functions: UL13 appears to inhibit VP11/12 virion packaging, while US3 antagonizes UL13 action and independently dampens VP11/12 signaling activity.

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.

HSV-1 ICP22: hijacking host nuclear functions to enhance viral infection

During its productive infection, HSV-1 dramatically remodels the architecture and physiology of the host cell nucleus. The immediate-early proteins, the first viral proteins to be expressed during infection, are key players in this process. Here, we review the known properties and functions of immediate-early protein ICP22. Although this polypeptide has received less attention than other immediate-early proteins, the published evidence indicates that it mediates several striking changes to important host nuclear systems, including those involved in RNA polymerase II transcription, cell cycle regulation and protein quality control. Recent genetic analyses suggest that these alterations can promote HSV-1 productive infection. Thus, future work on ICP22 is likely to reveal novel mechanisms by which herpesviruses, and possibly other DNA viruses, manipulate the host cell nucleus to enhance their replication.

Proteomics analysis of herpes simplex virus type 1-infected cells reveals dynamic changes of viral protein expression, ubiquitylation, and phosphorylation

Herpesviruses are among the most complex and widespread human viruses and cause a number of diseases ranging from cold sores to genital infections and encephalitis. While the composition of viral particles has been studied, less is known about the expression of the whole viral proteome in infected cells. Here, we analyzed the proteome of the prototypical Herpes Simplex Virus type 1 (HSV1) in infected cells by mass spectrometry. Using a high sensitivity LTQ-Orbitrap, we achieved a very high level of protein coverage and identified a total of 67 structural and nonstructural viral proteins. We also identified 90 novel phosphorylation sites and 10 novel ubiquitylation sites on different viral proteins. Ubiquitylation was observed on nine HSV1 proteins. We identified phosphorylation sites on about half of the detected viral proteins; many of the highly phosphorylated ones are known to regulate gene expression. Treatment with inhibitors of DNA replication induced changes of both viral protein abundance and modifications, highlighting the interdependence of viral proteins during the life cycle. Given the importance of expression dynamics, ubiquitylation, and phosphorylation for protein function, these findings will serve as important tools for future studies on herpesvirus biology.

Intracerebral propagation of Alzheimer's disease: strengthening evidence of a herpes simplex virus etiology

BACKGROUND

A faulty human protein, abnormally phosphorylated tau, was recently publicized to spread "like a virus" from neuron to neuron in Alzheimer's patients' brains. For several decades, we have been amassing arguments showing that herpes simplex virus type 1 (HSV-1), not p-tau, propagates this interneuronal, transsynaptic pathologic cascade.

METHODS

We reiterate convincing data from our own (and other) laboratories, reviewing the first anatomic foothold neurofibrillary tangles gain in brainstem and/or entorhinal cortex; the chronic immunosurveillance cellularity of the trigeminal ganglia wherein HSV-1 awakens from latency to reactivate; the inabilities of p-tau protein's physical properties to promote it to jump synapses; the amino acid homology between human p-tau and VP22, a key target for phosphorylation by HSV serine/threonine-protein kinase UL13; and the exosomic secretion of HSV-1-infected cells' L-particles, attesting to the cell-to-cell passage of microRNAs of herpesviruses.

RESULTS

The now-maturing construct that reactivated HSV-1 best accounts for the intracerebral propagation of AD changes in the human brain should at last seem highly attractive. This hypothesis might even explain statins' apparent mechanism in some studies for lowering AD incidence.

CONCLUSION

Provided that funding agencies will quickly ignite a new realm of investigation, the rejuvenated enthusiasm for testing this optimistic construct holds incalculable potential for rapid, efficacious clinical application, through already available and relatively safe antiviral therapeutics.

ORF9p phosphorylation by ORF47p is crucial for the formation and egress of varicella-zoster virus viral particles

The role of the tegument during the herpesvirus lytic cycle is still not clearly established, particularly at the late phase of infection, when the newly produced viral particles need to be fully assembled before being released from the infected cell. The varicella-zoster virus (VZV) protein coded by open reading frame (ORF) 9 (ORF9p) is an essential tegument protein, and, even though its mRNA is the most expressed during the productive infection, little is known about its functions. Using a GalK positive/negative selection technique, we modified a bacterial artificial chromosome (BAC) containing the complete VZV genome to create viruses expressing mutant versions of ORF9p. We showed that ORF9p is hyperphosphorylated during the infection, especially through its interaction with the viral Ser/Thr kinase ORF47p; we identified a consensus site within ORF9p recognized by ORF47p and demonstrated its importance for ORF9p phosphorylation. Strikingly, an ultrastructural analysis revealed that the mutation of this consensus site (glutamate 85 to arginine) strongly affects viral assembly and release, reproducing the ORF47 kinase-dead VZV phenotype. It also slightly diminishes the infectivity toward immature dendritic cells. Taken together, our results identify ORF9p as a new viral substrate of ORF47p and suggest a determinant role of this phosphorylation for viral infectivity, especially during the process of viral particle formation and egress.

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.

Multiplex sequencing of seven ocular herpes simplex virus type-1 genomes: phylogeny, sequence variability, and SNP distribution

PURPOSE

Little is known about the role of sequence variation in the pathology of HSV-1 keratitis virus. The goal was to show that a multiplex, high-throughput genome-sequencing approach is feasible for simultaneously sequencing seven HSV-1 ocular strains.

METHODS

A genome sequencer was used to sequence the HSV-1 ocular isolates TFT401, 134, CJ311, CJ360, CJ394, CJ970, and OD4, in a single lane. Reads were mapped to the HSV-1 strain 17 reference genome by high-speed sequencing. ClustalW was used for alignment, and the Mega 4 package was used for phylogenetic analysis (www.megasoftware.net). Simplot was used to compare genetic variability and high-speed sequencing was used to identify SNPs (developed by Stuart Ray, Johns Hopkins University School of Medicine, Baltimore, MD, http://sray.med.som.jhml.edu/SCRoftware/simplot).

RESULTS

Approximately 95% to 99% of the seven genomes were sequenced in a single lane with average coverage ranging from 224 to 1345. Phylogenetic analysis of the sequenced genome regions revealed at least three clades. Each strain had approximately 200 coding SNPs compared to strain 17, and these were evenly spaced along the genomes. Four genes were highly conserved, and six were more variable. Reduced coverage was obtained in the highly GC-rich terminal repeat regions.

CONCLUSIONS

Multiplex sequencing is a cost-effective way to obtain the genomic sequences of ocular HSV-1 isolates with sufficient coverage of the unique regions for genomic analysis. The number of SNPs and their distribution will be useful for analyzing the genetics of virulence, and the sequence data will be useful for studying HSV-1 evolution and for the design of structure-function studies.

Complete genome sequence of virulent duck enteritis virus (DEV) strain 2085 and comparison with genome sequences of virulent and attenuated DEV strains

We here report the complete genome sequence of the duck enteritis virus (DEV) wild-type strain 2085, an avian herpesvirus (GenBank ID: JF999965). The nucleotide sequence was derived from the 2085 genome cloned as an infectious bacterial artificial chromosome (BAC) clone. The DEV 2085 genome is 160,649-bp in length and encodes 78 predicted open reading frames (ORFs), a number identical to that identified for the attenuated DEV VAC strain (GenBank ID: EU082088.2). Comparison of the genome sequences DEV 2085 and VAC with partial sequences of the virulent CHv strain and the attenuated strain Clone-03 was carried out to identify nucleotide or amino acid polymorphisms that potentially contribute to DEV virulence. No amino acid changes were identified in 24 of the 78 ORFs, a result indicating high conservation in DEV independently of strain origin or virulence. In addition, 39 ORFs contain non-synonymous nucleotide substitutions, while 15 ORFs had nucleotide insertions or deletions, frame-shift mutations and/or non-synonymous nucleotide substitutions with an effect on ORF initiation or termination. In 7 of the 15 ORFs with high and 27 of the 39 ORFs with low variability, polymorphisms were exclusively found in DEV 2085, a finding that likely is a result of a different origin of 2085 (Europe) or VAC, Clone-03 and CHv (Eastern Asia). Five ORFs (UL2, UL12, US10, UL47 and UL41) with polymorphisms were identical between the virulent DEV 2085 and CHv but different from VAC or Clone-03. They, individually or in combination, may therefore represent DEV virulence factors. Our comparative analysis of four DEV sequences provides a comprehensive overview of DEV genome structure and identifies ORFs that are changed during serial virus passage.

Viral serine/threonine protein kinases

Phosphorylation represents one the most abundant and important posttranslational modifications of proteins, including viral proteins. Virus-encoded serine/threonine protein kinases appear to be a feature that is unique to large DNA viruses. Although the importance of these kinases for virus replication in cell culture is variable, they invariably play important roles in virus virulence. The current review provides an overview of the different viral serine/threonine protein kinases of several large DNA viruses and discusses their function, importance, and potential as antiviral drug targets.

Escape of herpesviruses from the nucleus

The nuclear envelope of eukaryotic cells is composed of double lipid-bilayer membranes, the membrane-connected nuclear pore complexes and an underlying nuclear lamina network. The nuclear pore complexes serve as gates for regulating the transport of macromolecules between cytoplasm and nucleus. The nuclear lamina not only provides an intact meshwork for maintaining the nuclear stiffness but also presents a natural barrier against most DNA viruses. Herpesviruses are large DNA viruses associated with multiple human and animal diseases. The complex herpesviral virion contains more than 30 viral proteins. After viral DNA replication, the newly synthesised genome is packaged into the pre-assembled intranuclear capsid. The nucleocapsid must then transverse through the nuclear envelope to the cytoplasm for the subsequent maturation process. Information regarding how nucleocapsid breaches the rigid nuclear lamina barrier and accesses the inner nuclear membrane for primary envelopment has emerged recently. From the point of view of both viral components and nuclear structure, this review summarises recent advances in the complicated protein-protein interactions and the phosphorylation regulations involved in the nuclear egress of herpesviral nucleocapsids.

The herpes simplex virus type 1 infected cell protein 22

As one of the immediate-early (IE) proteins of herpes simplex virus type 1 (HSV-1), ICP22 is a multifunctional viral regulator that localizes in the nucleus of infected cells. It is required in experimental animal systems and some nonhuman cell lines, but not in Vero or HEp-2 cells. ICP22 is extensively phosphorylated by viral and cellular kinases and nucleotidylylated by casein kinase II. It has been shown to be required for efficient expression of early (E) genes and a subset of late (L) genes. ICP22, in conjunction with the UL13 kinase, mediates the phosphorylation of RNA polymerase II. Both ICP22 and UL13 are required for the activation of cdc2, the degradation of cyclins A and B and the acquisition of a new cdc2 partner, the UL42 DNA polymerase processivity factor. The cdc2-UL42 complex mediates postranscriptional modification of topoisomerase IIα in an ICP22-dependent manner to promote L gene expression. In addition, ICP22 interacts with cdk9 in a Us3 kinase dependent fashion to phosphorylate RNA polymerase II.

Bovine herpesvirus-1 US3 protein kinase: critical residues and involvement in the phosphorylation of VP22

The US3 gene product of bovine herpesvirus-1 (BoHV-1) is a protein kinase that is expressed early during infection and capable of autophosphorylation. By examining differentially labelled US3 moieties by co-immunoprecipitation, we demonstrated that the protein kinase interacts with itself in vitro, which supports autophosphorylation by US3. Based on its homology to other serine/threonine protein kinases, we defined two highly conserved lysines in US3, at position 195 within the ATP-binding pocket and at position 282 within the catalytic loop; altering either residue resulted in kinase-dead mutants, demonstrating that these two residues are critical for the catalytic activity of BoHV-1 US3. During immunoprecipitation experiments, US3 interacted weakly with VP22, another tegument protein of BoHV-1. Furthermore, VP22 co-localized with US3 inside the nucleus in BoHV-1-infected cells. In vitro kinase assays demonstrated that VP22 is phosphorylated not only by US3, but also by the cellular casein kinase 2 (CK2) protein. The selective CK2 protein kinase inhibitor, 2-dimethylamino-4,5,6,7-tetrabromo-1H-benzimidazole (DMAT) and the less specific CK2 inhibitor Kenpaullone reduced VP22 phosphorylation, while CK1, protein kinase C or protein kinase A inhibitors did not affect phosphorylation. When US3 was included with VP22 in the kinase assay in the presence of DMAT, a low level of VP22 phosphorylation was observed. These data demonstrate that BoHV-1 VP22 interacts with both CK2 and US3, and that CK2 is the major kinase phosphorylating VP22, with US3 playing a minor role.

Function of human cytomegalovirus UL97 kinase in viral infection and its inhibition by maribavir

The serine/threonine kinase expressed by human cytomegalovirus from gene UL97 phosphorylates the antiviral drug ganciclovir, but its biological function is the phosphorylation of its natural viral and cellular protein substrates which affect viral replication at many levels. The UL97 kinase null phenotype is therefore complex, as is the mechanism of action of maribavir, a highly specific inhibitor of its enzymatic activity. Studies that utilise the drug corroborate results from genetic approaches and together have elucidated many functions of the UL97 kinase that are critical for viral replication. The kinase phosphorylates eukaryotic elongation factor 1delta, the carboxyl terminal domain of the large subunit of RNA polymerase II, the retinoblastoma tumour suppressor and lamins A and C. Each of these is also phosphorylated and regulated by cdc2/cyclin-dependent kinase 1, suggesting that the viral kinase may perform a similar function. These and other activities of the UL97 kinase appear to stimulate the cell cycle to support viral DNA synthesis, enhance the expression of viral genes, promote virion morphogenesis and facilitate the egress of mature capsids from the nucleus. In the absence of UL97 kinase activity, viral DNA synthesis is inefficient and structural proteins are sequestered in nuclear aggresomes, reducing the efficiency of virion morphogenesis. Mature capsids that do form fail to egress the nucleus as the nuclear lamina are not dispersed by the kinase. The critical functions performed by the UL97 kinase illustrate its importance in viral replication and confirm that the kinase is a target for the development of antiviral therapies.

Functional roles of the tegument proteins of herpes simplex virus type 1

Herpes virions consist of four morphologically distinct structures, a DNA core, capsid, tegument, and envelope. Tegument occupies the space between the nucleocapsid (capsid containing DNA core) and the envelope. A combination of genetic, biochemical and proteomic analysis of alphaherpes virions suggest the tegument contains in the order of 20 viral proteins. Historically the tegument has been described as amorphous but increasing evidence suggests there is an ordered addition of tegument during assembly. This review highlights the diverse roles, in addition to structural, that tegument plays during herpes viral replication using as an example herpes simplex virus type 1. Such diverse roles include: capsid transport during entry and egress; targeting of the capsid to the nucleus; regulation of transcription, translation and apoptosis; DNA replication; immune modulation; cytoskeletal assembly; nuclear egress of capsid; and viral assembly and final egress.

Origin of expression of the herpes simplex virus type 1 protein U(S)1.5

ICP22, an immediate-early protein of herpes simplex virus type 1 (HSV-1), is required for viral replication in nonpermissive cell types and for expression of a class of late viral proteins which includes glycoprotein C. An understanding of the mechanism of ICP22 function has been complicated by the coexpression of the full-length protein with an in-frame, C-terminus-specific protein, U(S)1.5. In this report, we confirm that the U(S)1.5 protein is a bona fide translation product since it is detected during infections with three laboratory strains and two low-passage clinical isolates of HSV-1. To clarify the expression patterns of the ICP22 and U(S)1.5 proteins, we examined their synthesis from plasmids in transient expression assays. Because previous studies had identified two different U(S)1.5 translational start sites, we attempted to determine which is correct by studying the effects of a series of deletion, nonsense, and methionine substitutions on U(S)1.5 expression. First, amino acids 90 to 420 encoded by the ICP22 open reading frame (ORF) migrated at the mobility of U(S)1.5 in sodium dodecyl sulfate-polyacrylamide gels. Second, introduction of a stop codon downstream of M90 ablated expression of both ICP22 and U(S)1.5. Finally, mutation of M90 to alanine (M90A) allowed expression of full-length ICP22 while dramatically reducing expression of U(S)1.5. Levels of U(S)1.5 but not ICP22 protein expression were also reduced in cells infected with an M90A mutant virus. Thus, we conclude that expression of IC22 and that of U(S)1.5 can occur independently of each other and that U(S)1.5 translation initiates at M90 of the ICP22 ORF.

Conserved herpesviral kinase promotes viral persistence by inhibiting the IRF-3-mediated type I interferon response

A conserved herpesviral kinase, designated ORF36 in murine gamma-herpesvirus 68 (MHV-68), plays multiple vital roles in the viral life cycle. Here, we show by screening mutant viruses that ORF36 counteracts the antiviral type I interferon (IFN) response. ORF36 specifically binds to the activated form of interferon regulatory factor 3 (IRF-3) in the nucleus, inhibiting IRF-3 interaction with the cotranscriptional activator CBP and thereby suppressing the recruitment of RNA polymerase II to the interferon beta promoter. The anti-IFN function of ORF36 is conserved among herpesvirus subfamilies, although the conserved kinase activity is not absolutely required for this function. MHV-68 lacking ORF36 induces a greater interferon response and is attenuated in vitro and in vivo, where acute viral infection in the lung and latency in the spleen are compromised. Our data suggest that herpesviruses have evolved within their conserved kinase an anti-IFN activity critical for evasion of host immunity and for persistence.

Two viral kinases are required for sustained long distance axon transport of a neuroinvasive herpesvirus

Axonal transport is essential for the successful establishment of neuroinvasive herpesvirus infections in peripheral ganglia (retrograde transport) and the subsequent spread to exposed body surfaces following reactivation from latency (anterograde transport). We examined two components of pseudorabies virus (US3 and UL13), both of which are protein kinases, as potential regulators of axon transport. Following replication of mutant viruses lacking kinase activity, newly assembled capsids displayed an increase in retrograde motion that prevented efficient delivery of capsids to the distal axon. The aberrant increase in retrograde motion was accompanied by loss of a viral membrane marker from the transported capsids, indicating that the viral kinases allow for efficient anterograde transport by stabilizing membrane-capsid interactions during the long transit from the neuron cell body to the distal axon.