Roles of the Phosphorylation of Herpes Simplex Virus 1 UL51 at a Specific Site in Viral Replication and Pathogenicity

Identification of a novel neurovirulence factor encoded by the cryptic orphan gene UL31.6 of herpes simplex virus 1

Although the herpes simplex virus type 1 (HSV-1) genome was thought to contain approximately 80 different protein coding sequences (CDSs), recent multi-omics analyses reported HSV-1 encodes more than 200 potential CDSs. However, few of the newly identified CDSs were confirmed to be expressed at the peptide or protein level in HSV-1-infected cells. Furthermore, the impact of the proteins they encode on HSV-1 infection is largely unknown. This study focused on a newly identified CDS, UL31.6. Re-analyzation of our previous chemical proteomics data verified that UL31.6 was expressed at the peptide level in HSV-1-infected cells. Antisera raised against a viral protein encoded by UL31.6 (pUL31.6) reacted with a protein with an approximate molecular mass of 37  kDa in lysates of Vero cells infected with each of three HSV-1 strains. pUL31.6 was efficiently dissociated from virions in high-salt solution. A UL31.6-null mutation had a minimal effect on HSV-1 gene expression, replication, cell-to-cell spread, and morphogenesis in Vero cells; in contrast, it significantly reduced HSV-1 cell-to-cell spread in three neural cells but not in four non-neural cells including Vero cells. The UL31.6-null mutation also significantly reduced the mortality and viral replication in the brains of mice after intracranial infection, but had minimal effects on pathogenic manifestations in and around the eyes, and viral replication detected in the tear films of mice after ocular infection. These results indicated that pUL31.6 was a tegument protein and specifically acted as a neurovirulence factor by potentially promoting viral transmission between neuronal cells in the central nervous system.IMPORTANCERecent multi-omics analyses reported the herpes simplex virus type 1 (HSV-1) genome encodes an additional number of potential coding sequences (CDSs). However, the expressions of these CDSs at the peptide or protein levels and the biological effects of these CDSs on HSV-1 infection remain largely unknown. This study annotated a cryptic orphan CDS, termed UL31.6, an HSV-1 gene that encodes a tegument protein with an approximate molecular mass of 37  kDa, which specifically acts as a neurovirulence factor. Our study indicates that HSV-1 proteins important for viral pathogenesis remain to be identified and a comprehensive understanding of the pathogenesis of HSV-1 will require not only the identification of cryptic orphan CDSs using emerging technologies but also step-by-step and in-depth analyses of each of the cryptic orphan CDSs.

Evidence of a Protein-Coding Gene Antisense to the UL5 Gene in Bovine Herpesvirus I

Bovine herpesvirus type 1 (BoHV-1) is an important agricultural pathogen that infects cattle and other ruminants worldwide. Though it was first sequenced and annotated over twenty years ago, the Cooper strain, used in this study, was sequenced as recently as 2012 and is currently said to encode 72 unique proteins. However, tandem mass spectrometry has identified several peptides produced during active infection that align with the BoHV-1 genome in unannotated regions. One of these abundant peptides, "ORF M", aligned antisense to the DNA helicase/primase protein UL5. This study characterizes the novel transcript and its protein product and provides evidence to support the existence of homolog protein-coding genes in other Herpesviruses.

Dual impacts of a glycan shield on the envelope glycoprotein B of HSV-1: evasion from human antibodies in vivo and neurovirulence

Identification of the mechanisms of viral evasion from human antibodies is crucial both for understanding viral pathogenesis and for designing effective vaccines. Here we show in cell cultures that an N-glycan shield on the herpes simplex virus 1 (HSV-1) envelope glycoprotein B (gB) mediated evasion from neutralization and antibody-dependent cellular cytotoxicity due to pooled γ-globulins derived from human blood. We also demonstrated that the presence of human γ-globulins in mice and immunity to HSV-1 induced by viral infection in mice significantly reduced replication in their eyes of a mutant virus lacking the glycosylation site but had little effect on the replication of its repaired virus. These results suggest that an N-glycan shield on a specific site of HSV-1 envelope gB mediated evasion from human antibodies in vivo and from HSV-1 immunity induced by viral infection in vivo. Notably, we also found that an N-glycan shield on a specific site of HSV-1 gB was significant for HSV-1 neurovirulence and replication in the central nervous system of naïve mice. Thus, we have identified a critical N-glycan shield on HSV-1 gB that has dual impacts, namely evasion from human antibodies in vivo and viral neurovirulence. IMPORTANCE Herpes simplex virus 1 (HSV-1) establishes lifelong latent and recurrent infections in humans. To produce recurrent infections that contribute to transmission of the virus to new human host(s), the virus must be able to evade the antibodies persisting in latently infected individuals. Here, we show that an N-glycan shield on the specific site of the envelope glycoprotein B (gB) of HSV-1 mediates evasion from pooled γ-globulins derived from human blood both in cell cultures and mice. Notably, the N-glycan shield on the specific site of gB was also significant for HSV-1 neurovirulence in naïve mice. Considering the clinical features of HSV-1 infection, these results suggest that the glycan shield not only facilitates recurrent HSV-1 infections in latently infected humans by evading antibodies but is also important for HSV-1 pathogenesis during the initial infection.

Tegument proteins of Epstein-Barr virus: Diverse functions, complex networks, and oncogenesis

The tegument is the structure between the envelope and nucleocapsid of herpesvirus particles. Viral (and cellular) proteins accumulate to create the layers of the tegument. Some Epstein-Barr virus (EBV) tegument proteins are conserved widely in Herpesviridae, but others are shared only by members of the gamma-herpesvirus subfamily. As the interface to envelope and nucleocapsid, the tegument functions in virion morphogenesis and budding of the nucleocapsid during progeny production. When a virus particle enters a cell, enzymes such as kinase and deubiquitinase, and transcriptional activators are released from the virion to promote virus infection. Moreover, some EBV tegument proteins are involved in oncogenesis. Here, we summarize the roles of EBV tegument proteins, in comparison to those of other herpesviruses.

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.

Role of VP30 Phosphorylation in Ebola Virus Nucleocapsid Assembly and Transport

Ebola virus (EBOV) VP30 regulates viral genome transcription and replication by switching its phosphorylation status. However, the importance of VP30 phosphorylation and dephosphorylation in other viral replication processes such as nucleocapsid and virion assembly is unclear. Interestingly, VP30 is predominantly dephosphorylated by cellular phosphatases in viral inclusions, while it is phosphorylated in the released virions. Thus, uncertainties regarding how VP30 phosphorylation in nucleocapsids is achieved and whether VP30 phosphorylation provides any advantages in later steps in viral replication have arisen. In the present study, to characterize the roles of VP30 phosphorylation in nucleocapsid formation, we used electron microscopic analyses and live cell imaging systems. We identified VP30 localized to the surface of protrusions surrounding nucleoprotein (NP)-forming helical structures in the nucleocapsid, suggesting the involvement in assembly and transport of nucleocapsids. Interestingly, VP30 phosphorylation facilitated its association with nucleocapsid-like structures (NCLSs). On the contrary, VP30 phosphorylation does not influence the transport characteristics and NCLS number leaving from and coming back into viral inclusions, indicating that the phosphorylation status of VP30 is not a prerequisite for NCLS departure. Moreover, the phosphorylation status of VP30 did not cause major differences in nucleocapsid transport in authentic EBOV-infected cells. In the following budding step, the association of VP30 and its phosphorylation status did not influence the budding efficiency of virus-like particles. Taken together, it is plausible that EBOV may utilize the phosphorylation of VP30 for its selective association with nucleocapsids, without affecting nucleocapsid transport and virion budding processes. IMPORTANCE Ebola virus (EBOV) causes severe fevers with unusually high case fatality rates. The nucleocapsid provides the template for viral genome transcription and replication. Thus, understanding the regulatory mechanism behind its formation is important for the development of novel therapeutic approaches. Previously, we established a live-cell imaging system based on the ectopic expression of viral fluorescent fusion proteins, allowing the visualization and characterization of intracytoplasmic transport of nucleocapsid-like structures. EBOV VP30 is an essential transcriptional factor for viral genome synthesis, and, although its role in viral genome transcription and replication is well understood, the functional importance of VP30 phosphorylation in assembly of nucleocapsids is still unclear. Our work determines the localization of VP30 at the surface of ruffled nucleocapsids, which differs from the localization of polymerase in EBOV-infected cells. This study sheds light on the novel role of VP30 phosphorylation in nucleocapsid assembly, which is an important prerequisite for virion formation.

Alphaherpesvirus glycoprotein E: A review of its interactions with other proteins of the virus and its application in vaccinology

The viral envelope glycoprotein E (gE) is required for cell-to-cell transmission, anterograde and retrograde neurotransmission, and immune evasion of alphaherpesviruses. gE can also interact with other proteins of the virus and perform various functions in the virus life cycle. In addition, the gE gene is often the target gene for the construction of gene-deleted attenuated marker vaccines. In recent years, new progress has been made in the research and vaccine application of gE with other proteins of the virus. This article reviews the structure of gE, the relationship between gE and other proteins of the virus, and the application of gE in vaccinology, which provides useful information for further research on gE.

SUN2 Modulates the Propagation of HSV-1

Herpesviruses assemble new viral particles in the nucleus. These nucleocapsids bud through the inner nuclear membrane to produce enveloped viral particles in the perinuclear space before fusing with the outer nuclear membrane to reach the cytoplasm. This unusual route is necessary since viral capsids are too large to pass through nuclear pores. However, the transient perinuclear nucleocapsids (250 nm in diameter) are also larger than the width of the perinuclear space (30 to 50 nm). Interestingly, linker of the nucleoskeleton and cytoskeleton (LINC) components SUN and KASH connect the inner and outer nuclear membranes and regulate their spacing. Previous work by others on the related pseudorabies virus and human cytomegalovirus showed that they functionally interact with SUN proteins. To clarify the role of SUN proteins, we explored their impact on herpes simplex virus 1 (HSV-1), another herpesvirus. Using dominant negative SUN mutants and RNA interference, we show that HSV-1 propagation is dependent on the LINC complex. In contrast to pseudorabies virus, SUN2 disruption by either approach led to increased HSV-1 extracellular viral yields. This SUN2 dependency may be linked to its greater impact on perinuclear spacing in infected cells compared to SUN1. Finally, the virus itself seems to modulate perinuclear spacing. IMPORTANCE The large size of herpesviruses prevents them from travelling across the nuclear pores, and they instead egress across the two nuclear membranes, generating short-lived enveloped perinuclear virions. This poses a challenge as the perinuclear space is smaller than the virions. This implies the separation (unzipping) of the two nuclear membranes to accommodate the viral particles. The LINC complex bridges the two nuclear membranes and is an important regulator of perinuclear spacing. Work by others hint at its functional implication during pseudorabies virus and cytomegalovirus propagation. The present study probes the importance for HSV-1 of the SUN proteins, the LINC components found in the inner nuclear membrane. Using dominant negative constructs and RNA interference (RNAi), the data reveal that SUN2 exhibits antiviral propriety toward HSV-1, as disrupting the protein leads to increased viral yields. This is in contrast with that reported for pseudorabies and suggests that differences among herpesviruses may, once again, prevail.

Role of the arginine cluster in the disordered domain of Herpes Simplex Virus 1 UL34 for the recruitment of ESCRT-III for viral primary envelopment

During the nuclear export of nascent nucleocapsids of herpesviruses, the nucleocapsids bud through the inner nuclear membrane (INM) by acquiring the INM as a primary envelope (primary envelopment). We recently reported that herpes simplex virus 1 (HSV-1) nuclear egress complex (NEC), which consists of UL34 and UL31, interacts with an ESCRT-III adaptor ALIX and recruits ESCRT-III machinery to the INM for efficient primary envelopment. In this study, we identified a cluster of six arginine residues in the disordered domain of UL34 as a minimal region required for the interaction with ALIX as well as the recruitment of ALIX and an ESCRT-III protein CHMP4B to the INM in HSV-1-infected cells. Mutations in the arginine cluster exhibited phenotypes similar to those with ESCRT-III inhibition reported previously, including the mis-localization of NEC, induction of membranous invagination structures containing enveloped virions, aberrant accumulation of enveloped virions in the invaginations and perinuclear space, and reduction of viral replication. We also showed that the effect of the arginine cluster in UL34 on HSV-1 replication was dependent primarily on ALIX. These results indicated that the arginine cluster in the disordered domain of UL34 was required for the interaction with ALIX and the recruitment of ESCRT-III machinery to the INM to promote primary envelopment. IMPORTANCE Herpesvirus UL34 homologs contain conserved amino-terminal domains that mediate vesicle formation through interactions with UL31 homologs during primary envelopment. UL34 homologs also comprise other domains adjacent to their membrane-anchoring regions, which differ in length, are variable in herpesviruses and do not form distinguished secondary structures. However, the role of these disordered domains in infected cells remains to be elucidated. In this study, we present data suggesting that the arginine cluster in the disordered domain of HSV-1 UL34 mediates the interaction with ALIX, thereby leading to the recruitment of ESCRT-III machinery to the INM for efficient primary envelopment. This is the first study to report the role of the disordered domain of a UL34 homolog in herpesvirus infections.

Host and Viral Factors Involved in Nuclear Egress of Herpes Simplex Virus 1

Herpes simplex virus 1 (HSV-1) replicates its genome and packages it into capsids within the nucleus. HSV-1 has evolved a complex mechanism of nuclear egress whereby nascent capsids bud on the inner nuclear membrane to form perinuclear virions that subsequently fuse with the outer nuclear membrane, releasing capsids into the cytosol. The viral-encoded nuclear egress complex (NEC) plays a crucial role in this vesicle-mediated nucleocytoplasmic transport. Nevertheless, similar system mediates the movement of other cellular macromolecular complexes in normal cells. Therefore, HSV-1 may utilize viral proteins to hijack the cellular machinery in order to facilitate capsid transport. However, little is known about the molecular mechanisms underlying this phenomenon. This review summarizes our current understanding of the cellular and viral factors involved in the nuclear egress of HSV-1 capsids.

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.

Prohibitin-1 Contributes to Cell-to-Cell Transmission of Herpes Simplex Virus 1 via the MAPK/ERK Signaling Pathway

Viral cell-to-cell spread, a method employed by several viral families for entrance via cell junctions, is highly relevant to the pathogenesis of various viral infections. Cell-to-cell spread of herpes simplex virus 1 (HSV-1) is known to depend greatly on envelope glycoprotein E (gE). However, the molecular mechanism by which gE acts in HSV-1 cell-to-cell spread and the mechanisms of cell-to-cell spread by other herpesviruses remain poorly understood. Here, we describe our identification of prohibitin-1 as a novel gE-interacting host cell protein. Ectopic expression of prohibitin-1 increased gE-dependent HSV-1 cell-to-cell spread. As observed with the gE-null mutation, decreased expression or pharmacological inhibition of prohibitin-1 reduced HSV-1 cell-to-cell spread without affecting the yield of virus progeny. Similar effects were produced by pharmacological inhibition of the mitogen-activated protein kinase/extracellular signal-regulated kinase (MAPK/ERK) pathway, wherein prohibitin-1 acts as a protein scaffold and is required for induction of this pathway. Furthermore, artificial activation of the MAPK/ERK pathway restored HSV-1 cell-to-cell spread impaired by the gE-null mutation. Notably, pharmacological inhibition of prohibitins or the MAPK/ERK pathway reduced viral cell-to-cell spread of representative members in all herpesvirus subfamilies. Our results suggest that prohibitin-1 contributes to gE-dependent HSV-1 cell-to-cell spread via the MAPK/ERK pathway and that this mechanism is conserved throughout the Herpesviridae, whereas gE is conserved only in the Alphaherpesvirinae subfamily.IMPORTANCE Herpesviruses are ubiquitous pathogens of various animals, including humans. These viruses primarily pass through cell junctions to spread to uninfected cells. This method of cell-to-cell spread is an important pathogenic characteristic of these viruses. Here, we show that the host cell protein prohibitin-1 contributes to HSV-1 cell-to-cell spread via a downstream intracellular signaling cascade, the MAPK/ERK pathway. We also demonstrate that the role of the prohibitin-1-mediated MAPK/ERK pathway in viral cell-to-cell spread is conserved in representative members of every herpesvirus subfamily. This study has revealed a common molecular mechanism of the cell-to-cell spread of herpesviruses.

"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.

Identification of a herpes simplex virus 1 gene encoding neurovirulence factor by chemical proteomics

Identification of the complete set of translated genes of viruses is important to understand viral replication and pathogenesis as well as for therapeutic approaches to control viral infection. Here, we use chemical proteomics, integrating bio-orthogonal non-canonical amino acid tagging and high-resolution mass spectrometry, to characterize the newly synthesized herpes simplex virus 1 (HSV-1) proteome in infected cells. In these infected cells, host cellular protein synthesis is shut-off, increasing the chance to preferentially detect viral proteomes. We identify nine previously cryptic orphan protein coding sequences whose translated products are expressed in HSV-1-infected cells. Functional characterization of one identified protein, designated piUL49, shows that it is critical for HSV-1 neurovirulence in vivo by regulating the activity of virally encoded dUTPase, a key enzyme that maintains accurate DNA replication. Our results demonstrate that cryptic orphan protein coding genes of HSV-1, and probably other large DNA viruses, remain to be identified.

Here the authors use chemical proteomics to identify the herpes simplex virus 1 encoded proteome in infected cells. Functional characterization of one of the nine identified proteins, designated piUL49, shows that it acts as neurovirulence factor in mice by regulating a virally encoded dUTPase.

Phosphoregulation of a Conserved Herpesvirus Tegument Protein by a Virally Encoded Protein Kinase in Viral Pathogenicity and Potential Linkage between Its Evolution and Viral Phylogeny

Us3 proteins of herpes simplex virus 1 (HSV-1) and HSV-2 are multifunctional serine-threonine protein kinases. Here, we identified an HSV-2 tegument protein, UL7, as a novel physiological substrate of HSV-2 Us3. Mutations in HSV-2 UL7, which precluded Us3 phosphorylation of the viral protein, significantly reduced mortality, viral replication in the vagina, and development of vaginal disease in mice following vaginal infection. These results indicated that Us3 phosphorylation of UL7 in HSV-2 was required for efficient viral replication and pathogenicity in vivo Of note, this phosphorylation was conserved in UL7 of chimpanzee herpesvirus (ChHV), which phylogenetically forms a monophyletic group with HSV-2 and the resurrected last common ancestral UL7 for HSV-2 and ChHV. In contrast, the phosphorylation was not conserved in UL7s of HSV-1, which belongs to a sister clade of the monophyletic group, the resurrected last common ancestor for HSV-1, HSV-2, and ChHV, and other members of the genus Simplexvirus that are phylogenetically close to these viruses. Thus, evolution of Us3 phosphorylation of UL7 coincided with the phylogeny of simplex viruses. Furthermore, artificially induced Us3 phosphorylation of UL7 in HSV-1, in contrast to phosphorylation in HSV-2, had no effect on viral replication and pathogenicity in mice. Our results suggest that HSV-2 and ChHV have acquired and maintained Us3 phosphoregulation of UL7 during their evolution because the phosphoregulation had an impact on viral fitness in vivo, whereas most other simplex viruses have not because the phosphorylation was not necessary for efficient fitness of the viruses in vivo IMPORTANCE It has been hypothesized that the evolution of protein phosphoregulation drives phenotypic diversity across species of organisms, which impacts fitness during their evolution. However, there is a lack of information regarding linkage between the evolution of viral phosphoregulation and the phylogeny of virus species. In this study, we clarified the novel HSV-2 Us3 phosphoregulation of UL7 in infected cells, which is important for viral replication and pathogenicity in vivo We also showed that the evolution of Us3 phosphoregulation of UL7 was linked to the phylogeny of viruses that are phylogenetically close to HSV-2 and to the phosphorylation requirements for the efficient in vivo viral fitness of HSV-2 and HSV-1, which are representative of viruses that have and have not evolved phosphoregulation, respectively. This study reports the first evidence showing that evolution of viral phosphoregulation coincides with phylogeny of virus species and supports the hypothesis regarding the evolution of viral phosphoregulation during viral evolution.

Epstein-Barr Virus BBRF2 Is Required for Maximum Infectivity

Epstein-Barr virus (EBV) is a member of the gammaherpesvirinae, which causes infectious mononucleosis and several types of cancer. BBRF2 is an uncharacterized gene of EBV and is expressed during the lytic phase. To evaluate its function, BBRF2-knockout EBV was prepared using bacterial artificial chromosome (BAC) technology and the CRISPR/Cas9 system. Although viral gene expression, DNA synthesis, and progeny secretion were not affected, the infectivity of progeny viruses was significantly reduced by the disruption of BBRF2. When expressed alone, BBRF2 protein localized to the nucleus and cytoplasm, while the coexpression of an interacting partner, BSRF1, resulted in its relocalization to the cytoplasm. Interestingly, the coexpression of BBRF2 protected BSRF1 from proteasome/ubiquitin-dependent degradation. Therefore, BBRF2, together with BSRF1, augments viral infectivity.

Identification of the Capsid Binding Site in the Herpes Simplex Virus 1 Nuclear Egress Complex and Its Role in Viral Primary Envelopment and Replication

During nuclear egress of nascent progeny herpesvirus nucleocapsids, the nucleocapsids acquire a primary envelope by budding through the inner nuclear membrane of infected cells into the perinuclear space between the inner and outer nuclear membranes. Herpes simplex virus 1 (HSV-1) U_L34 and U_L31 proteins form a nuclear egress complex (NEC) and play critical roles in this budding process, designated primary envelopment. To clarify the role of NEC binding to progeny nucleocapsids in HSV-1 primary envelopment, we established an assay system for HSV-1 NEC binding to nucleocapsids and capsid proteins in vitro Using this assay system, we showed that HSV-1 NEC bound to nucleocapsids and to capsid protein U_L25 but not to the other capsid proteins tested (i.e., VP5, VP23, and U_L17) and that HSV-1 NEC binding of nucleocapsids was mediated by the interaction of NEC with U_L25. U_L31 residues arginine-281 (R281) and aspartic acid-282 (D282) were required for efficient NEC binding to nucleocapsids and U_L25. We also showed that alanine substitution of U_L31 R281 and D282 reduced HSV-1 replication, caused aberrant accumulation of capsids in the nucleus, and induced an accumulation of empty vesicles that were similar in size and morphology to primary envelopes in the perinuclear space. These results suggested that NEC binding via U_L31 R281 and D282 to nucleocapsids, and probably to U_L25 in the nucleocapsids, has an important role in HSV-1 replication by promoting the incorporation of nucleocapsids into vesicles during primary envelopment.IMPORTANCE Binding of HSV-1 NEC to nucleocapsids has been thought to promote nucleocapsid budding at the inner nuclear membrane and subsequent incorporation of nucleocapsids into vesicles during nuclear egress of nucleocapsids. However, data to directly support this hypothesis have not been reported thus far. In this study, we have present data showing that two amino acids in the membrane-distal face of the HSV-1 NEC, which contains the putative capsid binding site based on the solved NEC structure, were in fact required for efficient NEC binding to nucleocapsids and for efficient incorporation of nucleocapsids into vesicles during primary envelopment. This is the first report showing direct linkage between NEC binding to nucleocapsids and an increase in nucleocapsid incorporation into vesicles during herpesvirus primary envelopment.

Functional interactions between herpes simplex virus pUL51, pUL7 and gE reveal cell-specific mechanisms for epithelial cell-to-cell spread

Herpes simplex virus spread between epithelial cells is mediated by virus tegument and envelope protein complexes including gE/gI and pUL51/pUL7. pUL51 interacts with both pUL7 and gE/gI in infected cells. We show that amino acids 30-90 of pUL51 mediate interaction with pUL7. We also show that deletion of amino acids 167-244 of pUL51, or ablation of pUL7 expression both result in failure of gE to concentrate at junctional surfaces of Vero cells. We also tested the hypothesis that gE and pUL51 function on the same pathway for cell-to-cell spread by analyzing the phenotype of a double gE/UL51 mutant. In HaCaT cells, pUL51 and gE function on the same spread pathway, whereas in Vero cells they function on different pathways. Deletion of the gE gene strongly enhanced virus release to the medium in Vero cells, suggesting that the gE-dependent spread pathway may compete with virion release to the medium.

The Kaposi's Sarcoma-Associated Herpesvirus Protein ORF42 Is Required for Efficient Virion Production and Expression of Viral Proteins

Kaposi’s sarcoma-associated herpesvirus (KSHV), the etiologic agent of Kaposi’s sarcoma and other aggressive AIDS-associated malignancies, encodes over 90 genes, most of which are expressed only during the lytic replication cycle. The role of many of the KSHV lytic proteins in the KSHV replication cycle remains unknown, and many proteins are annotated based on known functions of homologs in other herpesviruses. Here we investigate the role of the previously uncharacterized KSHV lytic protein ORF42, a presumed tegument protein. We find that ORF42 is dispensable for reactivation from latency but is required for efficient production of viral particles. Like its alpha- and beta-herpesviral homologs, ORF42 is a late protein that accumulates in the viral particles. However, unlike its homologs, ORF42 appears to be required for efficient expression of at least some viral proteins and may potentiate post-transcriptional stages of gene expression. These results demonstrate that ORF42 has an important role in KSHV replication and may contribute to shaping viral gene expression.

Regulation of Human Cytomegalovirus Secondary Envelopment by a C-Terminal Tetralysine Motif in pUL71

Human cytomegalovirus (HCMV) secondary envelopment requires the viral tegument protein pUL71. The lack of pUL71 results in a complex ultrastructural phenotype with increased numbers of viral capsids undergoing envelopment at the cytoplasmic virus assembly complex. Here, we report a role of the pUL71 C terminus in secondary envelopment. Mutant viruses expressing C-terminally truncated pUL71 (TB71del327-361 and TB71del348-351) exhibited an impaired secondary envelopment in transmission electron microscopy (TEM) studies. Further mutational analyses of the C terminus revealed a tetralysine motif whose mutation (TB71mutK348-351A) resulted in an envelopment defect that was undistinguishable from the defect caused by truncation of the pUL71 C terminus. Interestingly, not all morphological alterations that define the ultrastructural phenotype of a TB71stop virus were found in cells infected with the C-terminally mutated viruses. This suggests that pUL71 provides additional functions that modulate HCMV morphogenesis and are harbored elsewhere in pUL71. This is also reflected by an intermediate growth defect of the C-terminally mutated viruses compared to the growth of the TB71stop virus. Electron tomography and three-dimensional visualization of different stages of secondary envelopment in TB71mutK348-351A-infected cells showed unambiguously the formation of a bud neck. Furthermore, we provide evidence for progressive tegument formation linked to advancing grades of capsid envelopment, suggesting that tegumentation and envelopment are intertwined processes. Altogether, we identified the importance of the pUL71 C terminus and, specifically, of a positively charged tetralysine motif for HCMV secondary envelopment.IMPORTANCE Human cytomegalovirus (HCMV) is an important human pathogen that causes severe symptoms, especially in immunocompromised hosts. Furthermore, congenital HCMV infection is the leading viral cause of severe birth defects. Development of antiviral drugs to prevent the production of infectious virus progeny is challenging due to a complex and multistep virion morphogenesis. The mechanism of secondary envelopment is still not fully understood; nevertheless, it represents a potential target for antiviral drugs. Our identification of the role of a positively charged motif in the pUL71 C terminus for efficient HCMV secondary envelopment underlines the importance of pUL71 and, especially, its C terminus for this process. It furthermore shows how cell-associated spread and virion release depend on secondary envelopment. Ultrastructural analyses of different stages of envelopment contribute to a better understanding of the mechanisms underlying the process of secondary envelopment. This may bring us closer to the development of novel concepts to treat HCMV infections.

Roles of the Interhexamer Contact Site for Hexagonal Lattice Formation of the Herpes Simplex Virus 1 Nuclear Egress Complex in Viral Primary Envelopment and Replication

During the nuclear export of nascent nucleocapsids of herpes simplex virus 1 (HSV-1), the nucleocapsids acquire a primary envelope by budding through the inner nuclear membrane into the perinuclear space between the inner and outer nuclear membranes. This unique budding process, termed primary envelopment, is initiated by the nuclear egress complex (NEC), composed of the HSV-1 UL31 and UL34 proteins. Earlier biochemical approaches have shown that the NEC has an intrinsic ability to vesiculate membranes through the formation of a hexagonal lattice structure. The significance of intrahexamer interactions of the NEC in the primary envelopment of HSV-1-infected cells has been reported. In contrast, the contribution of lattice formation of the NEC hexamer to primary envelopment in HSV-1-infected cells remains to be elucidated. Therefore, we constructed and characterized a recombinant HSV-1 strain carrying an amino acid substitution in a UL31 residue that is an interhexamer contact site for the lattice formation of the NEC hexamer. This mutation was reported to destabilize the interhexamer interactions of the HSV-1 NEC. Here, we demonstrate that the mutation causes the aberrant accumulation of nucleocapsids in the nucleus and reduces viral replication in Vero and HeLa cells. Thus, the ability of HSV-1 to form the hexagonal lattice structure of the NEC was linked to an increase in primary envelopment and viral replication. Our results suggest that the lattice formation of the NEC hexamer has an important role in HSV-1 replication by regulating primary envelopment.IMPORTANCE The scaffolding proteins of several envelope viruses required for virion assembly form high-order lattice structures. However, information on the significance of their lattice formation in infected cells is limited. Herpesviruses acquire envelopes twice during their viral replication. The first envelop acquisition (primary envelopment) is one of the steps in the vesicle-mediated nucleocytoplasmic transport of nascent nucleocapsids, which is unique in biology. HSV-1 NEC, thought to be conserved in all members of the Herpesviridae family, is critical for primary envelopment and was shown to form a hexagonal lattice structure. Here, we investigated the significance of the interhexamer contact site for hexagonal lattice formation of the NEC in HSV-1-infected cells and present evidence suggesting that the lattice formation of the NEC hexamer has an important role in HSV-1 replication by regulating primary envelopment. Our results provide insights into the mechanisms of the envelopment of herpesviruses and other envelope viruses.

Initial Characterization of the Epstein⁻Barr Virus BSRF1 Gene Product

Epstein⁻Barr virus (EBV) is a ubiquitous virus that causes infectious mononucleosis and several types of cancer, such as Burkitt lymphoma, T/NK-cell lymphoma, and nasopharyngeal carcinoma. As a herpesvirus, it encodes more than 80 genes, many of which have not been characterized. EBV BamHI S rightward reading frame 1 (BSRF1) encodes a tegument protein that, unlike its homologs herpes simplex virus unique long 51 (UL51) and human cytomegalovirus UL71, has not been extensively investigated. To examine the role of BSRF1, we prepared knockout and revertant strains using the bacterial artificial chromosome system. Unexpectedly, the disruption of the gene had little or no effect on EBV lytic replication and the transformation of primary B cells. However, the knockdown of BSRF1 in B95-8 cells decreased progeny production. An immunofluorescence assay revealed that BSRF1 localized to the Golgi apparatus in the cytoplasm, as did its homologs. BSRF1 also associated with BamHI G leftward reading frame 3.5 (BGLF3.5), BamHI B rightward reading frame 2 (BBRF2), and BamHI A leftward reading frame 1 (BALF1), and BALF1 was incorporated into the tegument fraction with BSRF1. Taken together, our results indicate that BSRF1 plays a role in secondary envelopment or virion egress in the cytoplasm, as do its homolog genes.

The phosphorylation of the N protein could affect PRRSV virulence in vivo

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We confirmed that phosphorylation modification of the N protein could impair PRRSV growth efficiency in PAMs.

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This study is the first to report that phosphorylation modification of the N protein could regulate PRRSV pathogenicity in piglets.

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Mutation of PTM sites could be a novel way to attenuate the virus. This strategy will be beneficial to distinguish the vaccine from a wild-type virus.

The porcine respiratory and reproductive syndrome virus (PRRSV) nucleocapsid (N) protein is a multiphosphorylated protein.It has been proved that the phosphorylation of N protein could regulate the growth ability of PRRSV in Marc-145 cells. However, further investigation is needed to determine whether phosphorylation of the N protein could affect PRRSV virulence in piglets. In this study, we confirmed that the mutations could impair PRRSV replication ability in porcine primary macrophages (PAMs) as they did in Marc-145 cells. The animal experiments suggested that the pathogenicity of the mutated virus (A105-120) was significantly reduced compared with parent strain (XH-GD). Our results suggested that the phosphorylation of the N protein contributes to virus replication and virulence. This study is the first to identify a specific modification involved in PRRSV pathogenicity. Mutation of PTMs sites is also a novel way to attenuate PRRSV virulence. The mutations could be a marker in a vaccine. In conclusion, our study will improve our understanding of the molecular mechanisms of PRRSV pathogenicity.