The herpes simplex virus Us11 open reading frame encodes a sequence-specific RNA-binding protein

An Intrinsic Host Defense against HSV-1 Relies on the Activation of Xenophagy with the Active Clearance of Autophagic Receptors

Autophagy engulfs cellular components in double-membrane-bound autophagosomes for clearance and recycling after fusion with lysosomes. Thus, autophagy is a key process for maintaining proteostasis and a powerful cell-intrinsic host defense mechanism, protecting cells against pathogens by targeting them through a specific form of selective autophagy known as xenophagy. In this context, ubiquitination acts as a signal of recognition of the cargoes for autophagic receptors, which direct them towards autophagosomes for subsequent breakdown. Nevertheless, autophagy can carry out a dual role since numerous viruses including members of the Orthoherpesviridae family can either inhibit or exploit autophagy for its own benefit and to replicate within host cells. There is growing evidence that Herpes simplex virus type 1 (HSV-1), a highly prevalent human pathogen that infects epidermal keratinocytes and sensitive neurons, is capable of negatively modulating autophagy. Since the effects of HSV-1 infection on autophagic receptors have been poorly explored, this study aims to understand the consequences of HSV-1 productive infection on the levels of the major autophagic receptors involved in xenophagy, key proteins in the recruitment of intracellular pathogens into autophagosomes. We found that productive HSV-1 infection in human neuroglioma cells and keratinocytes causes a reduction in the total levels of Ub conjugates and decreases protein levels of autophagic receptors, including SQSTM1/p62, OPTN1, NBR1, and NDP52, a phenotype that is also accompanied by reduced levels of LC3-I and LC3-II, which interact directly with autophagic receptors. Mechanistically, we show these phenotypes are the result of xenophagy activation in the early stages of productive HSV-1 infection to limit virus replication, thereby reducing progeny HSV-1 yield. Additionally, we found that the removal of the tegument HSV-1 protein US11, a recognized viral factor that counteracts autophagy in host cells, enhances the clearance of autophagic receptors, with a significant reduction in the progeny HSV-1 yield. Moreover, the removal of US11 increases the ubiquitination of SQSTM1/p62, indicating that US11 slows down the autophagy turnover of autophagy receptors. Overall, our findings suggest that xenophagy is a potent host defense against HSV-1 replication and reveals the role of the autophagic receptors in the delivery of HSV-1 to clearance via xenophagy.

"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.

Disassembly of the TRIM23-TBK1 Complex by the Us11 Protein of Herpes Simplex Virus 1 Impairs Autophagy

The Us11 protein encoded by herpes simplex virus 1 (HSV-1) functions to impair autophagy; however, the molecular mechanisms of this inhibition remain to be fully established. Here, we report that the Us11 protein targets tripartite motif protein 23 (TRIM23), which is a key regulator of autophagy-mediated antiviral defense mediated by TANK-binding kinase 1 (TBK1). In virus-infected cells, the Us11 protein drastically reduces the formation of autophagosomes mediated by TRIM23 or TBK1. This autophagy-inhibitory effect is attributable to the binding of the Us11 protein to the ARF domain in TRIM23. Furthermore, such interaction spatially excludes TBK1 from the TRIM23 complex that also contains heat shock protein 90 (Hsp90). When stably expressed alone in host cells, the Us11 protein recapitulates the observed phenotypes seen in cells infected with the US11-expressing or wild-type virus. Consistent with this, expression of the Us11 protein promotes HSV-1 growth, while expression of TRIM23 restricts HSV-1 replication in the absence of US11. Together, these results suggest that disruption of the TRIM23-TBK1 complex by the Us11 protein inhibits autophagy-mediated restriction of HSV-1 infection.IMPORTANCE Autophagy is an evolutionarily conserved process that restricts certain intracellular pathogens, including HSV-1. Although HSV-1 is well known to inhibit autophagy, little is known about the precise molecular mechanisms of autophagy inhibition. We demonstrate that the Us11 protein of HSV-1 spatially disrupts the TRIM23-TBK1 complex, which subsequently suppresses autophagy and autophagy-mediated virus restriction. Thus, expression of the Us11 protein facilitates HSV-1 replication. These data unveil new insight into viral escape from autophagy-mediated host restriction mechanisms.

HSV-1\EGFP stimulates miR-146a expression in a NF-κB-dependent manner in monocytic THP-1 cells

The nuclear factor κB (NF-κB) pathway plays a key role in innate and adaptive immunity, cell proliferation and survival, inflammation and tumors development. MiR-146a is an immune system regulator that has anti-inflammatory function in multiple cell types and conditions. Here we demonstrate activation of canonical NF-κB pathway in monocytic cells upon HSV-1 replication. By constructing and using a recombinant HSV-1\EGFP virus, we monitored the capability of the virus to recruit NF-κB and we report that the phosphorylation of p65 protein correlates with an active virus replication at single-cell level. In addition, we found that upregulation of miR-146a during viral replication is strictly dependent on NF-κB activation and correlates with tight control of the interleukin-1 receptor-associate kinase 1 (IRAK1). Accordingly, THP-1 DN IκBα cells, expressing a dominant negative mIκBα, did not show upregulation of miR-146a upon HSV-1 infection. Our data suggest that the expression of miRNA-146a modulates NF-κB activation through targeting IRAK1 during HSV-1 replication in THP-1 cells.

Herpes Simplex Virus 1 Lytic Infection Blocks MicroRNA (miRNA) Biogenesis at the Stage of Nuclear Export of Pre-miRNAs

Various mechanisms have been identified by which viruses target host small RNA biogenesis pathways to achieve optimal infection outcomes. Herpes simplex virus 1 (HSV-1) is a ubiquitous human pathogen whose successful persistence in the host entails both productive (“lytic”) and latent infection. Although many HSV-1 miRNAs have been discovered and some are thought to help control the lytic/latent switch, little is known about regulation of their biogenesis. By characterizing expression of both pre-miRNAs and mature miRNAs under various conditions, this study revealed striking differences in miRNA biogenesis between lytic and latent infection and uncovered a regulatory mechanism that blocks pre-miRNA nuclear export and is dependent on viral protein ICP27 and viral DNA synthesis. This mechanism represents a new virus-host interaction that could limit the repressive effects of HSV-1 miRNAs hypothesized to promote latency and may shed light on the regulation of miRNA nuclear export, which has been relatively unexplored.

Herpes simplex virus 1 (HSV-1) switches between two infection programs, productive (“lytic”) and latent infection. Some HSV-1 microRNAs (miRNAs) have been hypothesized to help control this switch, and yet little is known about regulation of their expression. Using Northern blot analyses, we found that, despite inherent differences in biogenesis efficiency among six HSV-1 miRNAs, all six exhibited high pre-miRNA/miRNA ratios during lytic infection of different cell lines and, when detectable, in acutely infected mouse trigeminal ganglia. In contrast, considerably lower ratios were observed in latently infected ganglia and in cells transduced with lentiviral vectors expressing the miRNAs, suggesting that HSV-1 lytic infection blocks miRNA biogenesis. This phenomenon is not specific to viral miRNAs, as a host miRNA expressed from recombinant HSV-1 also exhibited high pre-miRNA/miRNA ratios late during lytic infection. The levels of most of the mature miRNAs remained stable during infection in the presence of actinomycin D, indicating that the high ratios are due to inefficient pre-miRNA conversion to miRNA. Cellular fractionation experiments showed that late (but not early) during infection, pre-miRNAs were enriched in the nucleus and depleted in the cytoplasm, indicating that nuclear export was blocked. A mutation eliminating ICP27 expression or addition of acyclovir reduced pre-miRNA/miRNA ratios, but mutations drastically reducing Us11 expression did not. Thus, HSV-1 lytic infection inhibits miRNA biogenesis at the step of nuclear export and does so in an ICP27- and viral DNA synthesis-dependent manner. This mechanism may benefit the virus by reducing expression of repressive miRNAs during lytic infection while permitting elevated expression during latency.

Identification of transcription factories in nuclei of HeLa cells transiently expressing the Us11 gene of herpes simplex virus type 1

Nuclear distribution and migration of herpes simplex virus type 1 Us11 transcripts were studied in transient expression at the ultrastructural level and compared to that of RNA polymerase II protein. Transcription was monitored by autoradiography following a short pulse with tritiated uridine. Us11 transcripts accumulated mainly over the foci of intermingled RNP fibrils as demonstrated by the presence of silver grains localizing incorporated radioactive uridine superimposed to these structures in which the presence of Us11 RNA and poly(A) tails was previously demonstrated. Silver grains were also scattered over the remaining nucleoplasm but not in the clusters of interchromatin granules, and over the dense fibrillar component of the nucleolus as in control, nontransfected HeLa cells. Pulse-chase experiments revealed the transient presence of migrating RNA in the clusters of interchromatin granules. RNA polymerase II was revealed by immunogold labeling following the use of two monoclonal antibodies: mAb H5, which recognizes the hyperphosphorylated form of the carboxy-terminal domain (CTD) of the molecule, and mAb 7C2, which recognizes both its hyperphosphorylated and unphosphorylated forms. The two mAbs bind to the newly formed Us11 transcription factories and the clusters of interchromatin granules of transfected cells. In control cells, however, clusters of interchromatin granules were labeled with mAb H5 but not with mAB 7C2. Taken together, our data demonstrate the involvement of the clusters of interchromatin granules in the intranuclear migration of Us11 RNA in transient expression. They also suggest the occurrence of changes in the accessibility of the RNA polymerase II CTD upon expression of the Us11 gene after transfection by exposing some epitopes, otherwise masked in nontransfected cells.

Herpes Simplex Virus 1 Inhibits TANK-Binding Kinase 1 through Formation of the Us11-Hsp90 Complex

The Us11 protein of herpes simplex virus 1 (HSV-1) is an accessory factor with multiple functions. In virus-infected cells, it inhibits double-stranded RNA-dependent protein kinase (PKR), 2',5'-oligoadenylate synthetase, RIG-I, and MDA-5. However, its precise role is incompletely defined. By screening a human cDNA library, we showed that the Us11 protein targets heat shock protein 90 (Hsp90), which inactivates TANK binding kinase 1 (TBK1) and antiviral immunity. When ectopically expressed, HSV-1 Us11 precludes TBK1 from access to Hsp90 and interferon (IFN) promoter activation. Consistently, the Us11 protein, upon HSV infection, suppresses the expression of beta interferon (IFN-β), RANTES, and interferon-stimulated genes. This is mirrored by a blockade in the phosphorylation of interferon regulatory factor 3. Mechanistically, the Us11 protein associates with endogenous Hsp90 to disrupt the Hsp90-TBK1 complex. Furthermore, Us11 induces destabilization of TBK1 through a proteasome-dependent pathway. Accordingly, Us11 expression facilitates HSV growth. In contrast, TBK1 expression restricts viral replication. These results suggest that control of TBK1 by Us11 promotes HSV-1 infection.IMPORTANCE TANK binding kinase 1 plays a key role in antiviral immunity. Although multiple factors are thought to participate in this process, the picture is obscure in herpes simplex virus infection. We demonstrated that the Us11 protein of HSV-1 forms a complex with heat shock protein 90, which inactivates TANK binding kinase 1 and IFN induction. As a result, expression of the Us11 protein promotes HSV replication. These experimental data provide a new insight into the molecular network of virus-host interactions.

Neurons versus herpes simplex virus: the innate immune interactions that contribute to a host-pathogen standoff

Herpes simplex virus (HSV) is a prevalent neurotropic virus, which establishes lifelong latent infections in the neurons of sensory ganglia. Despite our long-standing knowledge that HSV predominately infects sensory neurons during its life cycle, little is known about the neuronal antiviral response to HSV infection. Recent studies show that while sensory neurons have impaired intrinsic immunity to HSV infection, paracrine IFN signaling can potentiate a potent antiviral response. Additionally, antiviral autophagy plays an important role in neuronal control of HSV infection. Here we review the literature of antiviral signaling and autophagy in neurons, the mechanisms by which HSV can counteract these responses, and postulate how these two pathways may synergize to mediate neuronal control of HSV infection and yet result in lifelong persistence of the virus.

HSV-1 degrades, stabilizes, requires, or is stung by STING depending on ICP0, the US3 protein kinase, and cell derivation

STING (stimulator of IFN genes) activates the IFN pathway in response to cytosolic DNA. Knockout of STING in mice was reported to exacerbate the pathogenicity of herpes simplex virus 1 (HSV-1). Here we report the following: (i) STING is stable in cancer-derived HEp-2 or HeLa cells infected with wild-type HSV-1 but is degraded in cells infected with mutants lacking the genes encoding functional infected cell protein 0 (ICP0), ICP4, or the US3 protein kinase (US3-PK). In HEp-2 cells, depletion of STING by shRNA results in a decrease in the yields of wild-type or ΔICP0 viruses. (ii) STING is stable throughout infection with either wild-type or ICP0 mutant viruses in human embryonic lung cells (HEL) or HEK293T cells derived from normal tissues. In these cells, depletion of STING results in higher yields of both wild-type and ΔICP0 viruses. (iii) The US3-PK is also required for stabilization of IFI16, a nuclear DNA sensor. However, the stability of IFI16 does not correlate positively or negatively with that of STING. IFI16 is stable in STING-depleted HEL cells infected with wild-type virus. In contrast to HEL cells, IFI16 was undetectable in STING-depleted HEp-2 cells, and hence the role of HSV-1 in maintaining IFI16 could not be ascertained. The results indicate that in HSV-1-infected cells the stability of IFI16 and the function and stability of STING are dependent on cell derivation, the functional integrity of ICP0, and US3-PK, an indication that in wild-type virus-infected cells both proteins are actively stabilized. In HEp-2 cells, the stability of IFI16 requires STING.

Evolution and diversity in human herpes simplex virus genomes

Herpes simplex virus 1 (HSV-1) causes a chronic, lifelong infection in >60% of adults. Multiple recent vaccine trials have failed, with viral diversity likely contributing to these failures. To understand HSV-1 diversity better, we comprehensively compared 20 newly sequenced viral genomes from China, Japan, Kenya, and South Korea with six previously sequenced genomes from the United States, Europe, and Japan. In this diverse collection of passaged strains, we found that one-fifth of the newly sequenced members share a gene deletion and one-third exhibit homopolymeric frameshift mutations (HFMs). Individual strains exhibit genotypic and potential phenotypic variation via HFMs, deletions, short sequence repeats, and single-nucleotide polymorphisms, although the protein sequence identity between strains exceeds 90% on average. In the first genome-scale analysis of positive selection in HSV-1, we found signs of selection in specific proteins and residues, including the fusion protein glycoprotein H. We also confirmed previous results suggesting that recombination has occurred with high frequency throughout the HSV-1 genome. Despite this, the HSV-1 strains analyzed clustered by geographic origin during whole-genome distance analysis. These data shed light on likely routes of HSV-1 adaptation to changing environments and will aid in the selection of vaccine antigens that are invariant worldwide.

Suppression of PACT-induced type I interferon production by herpes simplex virus 1 Us11 protein

Herpes simplex virus 1 (HSV-1) Us11 protein is a double-stranded RNA-binding protein that suppresses type I interferon production through the inhibition of the cytoplasmic RNA sensor RIG-I. Whether additional cellular mediators are involved in this suppression remains to be determined. In this study, we report on the requirement of cellular double-stranded RNA-binding protein PACT for Us11-mediated perturbation of type I interferon production. Us11 associates with PACT tightly to prevent it from binding with and activating RIG-I. The Us11-deficient HSV-1 was indistinguishable from the Us11-proficient virus in the suppression of interferon production when PACT was compromised. More importantly, HSV-1-induced activation of interferon production was abrogated in PACT knockout murine embryonic fibroblasts. Our findings suggest a new mechanism for viral evasion of innate immunity through which a viral double-stranded RNA-binding protein interacts with PACT to circumvent type I interferon production. This mechanism might also be used by other PACT-binding viral interferon-antagonizing proteins such as Ebola virus VP35 and influenza A virus NS1.

Herpes simplex virus 1 tegument protein US11 downmodulates the RLR signaling pathway via direct interaction with RIG-I and MDA-5

The interferon (IFN)-mediated antiviral response is a major defense of the host immune system. In order to complete their life cycle, viruses must modulate host IFN-mediated immune responses. Herpes simplex virus 1 (HSV-1) is a large DNA virus containing more than 80 genes, many of which encode proteins that are involved in virus-host interactions and show immune modulatory capabilities. In this study, we demonstrate that the US11 protein, an RNA binding tegument protein of HSV-1, is a novel antagonist of the beta IFN (IFN-β) pathway. US11 significantly inhibited Sendai virus (SeV)-induced IFN-β production, and its double-stranded RNA (dsRNA) binding domain was indispensable for this inhibition activity. Additionally, wild-type HSV-1 coinfection showed stronger inhibition than US11 mutant HSV-1 in SeV-induced IFN-β production. Coimmunoprecipitation analysis demonstrated that the US11 protein in HSV-1-infected cells interacts with endogenous RIG-I and MDA-5 through its C-terminal RNA-binding domain, which was RNA independent. Expression of US11 in both transfected and HSV-1-infected cells interferes with the interaction between MAVS and RIG-I or MDA-5. Finally, US11 dampens SeV-mediated IRF3 activation. Taken together, the combined data indicate that HSV-1 US11 binds to RIG-I and MDA-5 and inhibits their downstream signaling pathway, preventing the production of IFN-β, which may contribute to the pathogenesis of HSV-1 infection.

Expression, purification of herpes simplex virus type 1 US11 protein, and production of US11 polyclonal antibody

Background

The US11 protein of herpes simplex virus type 1 (HSV-1) is a small, highly basic phosphoprotein expressed at late times during infection. To date, the function of US11 protein in cell culture and animal models is poorly understood. To further investigate the function of the US11 protein, this study was undertaken to express the US11 protein and raise a polyclonal antibody.

Results

The US11 gene was cloned into the prokaryotic expression vector pET-32a (+) to express His-tagged US11 protein in Escherichia coli. After purification by nickel affinity chromatography and refolding, the recombinant protein was used to raise the anti-US11 polyclonal antibody. Western blot analysis demonstrated that the US11 protein was specifically recognized by the polyclonal antibody, and immunofluorescent assay also showed that the antibody was able to probe the US11 protein in the cells infected with HSV-1.

Conclusions

In the present study, we obtained a high-level expression of the recombinant US11 protein as well as high titers of rabbit polyclonal antibody specially against US11 protein in HSV-1 infected cells. This special polyclonal antibody provides a good tool for further studying structural and functional characterization of HSV-1 US11 protein.

Molecular anatomy of subcellular localization of HSV-1 tegument protein US11 in living cells

The herpes simplex virus type I (HSV-1) US11 protein is an RNA-binding multifunctional regulator that specifically and stably associates with nucleoli. Although the C-terminal part of US11 was responsible for its nucleolar localization, the precise nucleolar localization signal (NoLS) and nuclear export signal (NES) of US11 and its nuclear import and export mechanisms are still elusive. In this study, fluorescence microscopy was employed to investigate the subcellular localization of US11 and characterize its transport mechanism in living cells. By constructing a series of deletion mutants fused with enhanced yellow fluorescent protein (EYFP), three novel NoLSs of US11 were for the first time mapped to amino acids 84-125, 126-152, and 89-146, respectively. Additionally, the NES was identified to locate between amino acids 89 and 119. Furthermore, the US11 protein was demonstrated to target to the cytoplasm through the NES by chromosomal region maintenance 1 (CRM1)-independent pathway, and to the nucleolus through Ran and importin beta-dependent mechanism that does not require importin alpha 5.

Mechanisms employed by herpes simplex virus 1 to inhibit the interferon response

The interferon (IFN) family of cytokines constitutes potent inducers of innate antiviral responses that also influence adaptive immune processes. Despite eliciting such formidable cellular defense responses, viruses have evolved ways to interfere with the IFN response. Herpes simplex virus 1 (HSV-1) is an enveloped, dsDNA virus and a member of the herpesvirus family. Like other herpesvirus family members, HSV-1 has become highly specialized for its host and establishes a lifelong infection by undergoing latency within neurons. A leading reason for the success of HSV-1 as a pathogen results from its ability to evade the IFN response. Specifically, HSV-1 encodes several proteins that function to inhibit both IFN production and subsequent signal transduction. This review will identify and summarize the current understanding of viral proteins encoded by HSV-1 involved in the evasion of the IFN response.

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

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

RNA binding by the herpes simplex virus type 1 nucleocytoplasmic shuttling protein UL47 is mediated by an N-terminal arginine-rich domain that also functions as its nuclear localization signal

The function of the alphaherpesvirus UL47 tegument protein has not yet been defined. Nonetheless, previous studies with transfected cells have shown that both the herpes simplex virus type 1 homologue (hUL47, or VP13/14) and the bovine herpesvirus type 1 (BHV-1) homologue (bUL47, or VP8) have the capacity to shuttle between the nucleus and the cytoplasm. Furthermore, hUL47 packaged into the virion has also been shown to bind several individual virus-specific RNA transcripts. Here, we extend these observations and show that hUL47 binds a wide range of RNA species in vitro. It has a high affinity for polyadenylated transcripts but has no apparent selectivity for virus-encoded RNA over cellular RNA. We also show that the virion population of bUL47 binds RNA in vitro. However, while purified recombinant hUL47 retains its RNA binding activity, recombinant bUL47 does not, suggesting that the BHV-1 homologue may require virus-induced modification for its activity. We identify the minimal RNA binding domain in hUL47 as a 26-residue N-terminal peptide containing an arginine-rich motif that is essential but not sufficient for optimal RNA binding, and we demonstrate that this RNA binding domain incorporates the hUL47 minimal nuclear localization signal. In addition, we show that soon after hUL47 is expressed during infection, it colocalizes in the infected cell nucleus with ICP4, the major virus transcriptional activator. Using RNA immunoprecipitations, we demonstrate that hUL47 is also bound in vivo to at least one viral transcript, the ICP0 mRNA. Taken together, these results suggest that hUL47 may play a role in RNA biogenesis in the infected cell.

Activated MEK suppresses activation of PKR and enables efficient replication and in vivo oncolysis by Deltagamma(1)34.5 mutants of herpes simplex virus 1

Herpes simplex virus mutants lacking the gamma(1)34.5 gene are not destructive to normal tissues but are potent cytolytic agents in human tumor cells in which the activation of double-stranded RNA-dependent protein kinase (PKR) is suppressed. Thus, replication of a Deltagamma(1)34.5 mutant (R3616) in 12 genetically defined cancer cell lines correlates with suppression of PKR but not with the genotype of RAS. Extensive analyses of two cell lines transduced with either dominant negative MEK (dnMEK) or constitutively active MEK (caMEK) indicated that in R3616 mutant-infected cells dnMEK enabled PKR activation and decreased virus yields, whereas caMEK suppressed PKR and enabled better viral replication and cell destruction in transduced cells in vitro or in mouse xenografts. The results indicate that activated MEK mediates the suppression of PKR and that the status of MEK predicts the ability of Deltagamma(1)34.5 mutant viruses to replicate in and destroy tumor cells.

Binding of herpes simplex virus-1 US11 to specific RNA sequences

Herpes simplex virus-1 US11 is a RNA-binding protein with a novel RNA-binding domain. US11 has been reported to exhibit sequence- and conformation-specific RNA-binding, but the sequences and conformations important for binding are not known. US11 has also been described as a double-stranded RNA (dsRNA)-binding protein. To investigate the US11-RNA interaction, we performed in vitro selection of RNA aptamers that bind US11 from a RNA library consisting of >10(14) 80 base sequences which differ in a 30 base randomized region. US11 bound specifically to selected aptamers with an affinity of 70 nM. Analysis of 23 selected sequences revealed a strong consensus sequence. The US11 RNA-binding domain and < or =46 bases of selected RNA containing the consensus sequence were each sufficient for binding. US11 binding protected the consensus motif from hydroxyl radical cleavage. RNase digestions of a selected aptamer revealed regions of both single-stranded RNA and dsRNA. We observed that US11 bound two different dsRNAs in a sequence non-specific manner, but with lower affinity than it bound selected aptamers. The results define a relatively short specific sequence that binds US11 with high affinity and indicate that dsRNA alone does not confer high-affinity binding.

Ionizing Radiation Activates Late Herpes Simplex Virus 1 Promoters via the p38 Pathway in Tumors Treated with Oncolytic Viruses

Ionizing radiation potentiates the oncolytic activity of attenuated herpes simplex viruses in tumors exposed to irradiation at specific time intervals by inducing higher virus yields. Cell culture studies have shown that an attenuated virus lacking the viral gamma(1)34.5 genes underproduces late proteins whose synthesis depends on sustained synthesis of viral DNA. Here we report that ionizing radiation enhances gene expression from late viral promoters in transduced cells in the absence of other viral gene products. Consistent with this result, we show that in tumors infected with the attenuated virus, ionizing radiation increases 13.6-fold above baseline the gene expression from a late viral promoter as early as 2 hours after virus infection, an interval too short to account for viral DNA synthesis. The radiation-dependent up-regulation of late viral genes is mediated by the p38 pathway, inasmuch as the enhancement is abolished by p38 inhibitors or a p38 dominant-negative construct. The p38 pathway is not essential for wild-type virus gene expression. The results suggest that ionizing radiation up-regulates late promoters active in the course of viral DNA synthesis and provide a rationale for use of radiation to up-regulate cytotoxic genes introduced into tumor cells by viral vectors for cytoreductive therapy.

Phosphorylation site mutations affect herpes simplex virus type 1 ICP0 function

The herpes simplex virus type 1 (HSV-1) immediate-early (IE) regulatory protein infected-cell protein 0 (ICP0) is a strong and global transactivator of both viral and cellular genes. In a previous study, we reported that ICP0 is highly phosphorylated and contains at least seven distinct phosphorylation signals as determined by phosphotryptic peptide mapping (D. J. Davido et al., J. Virol. 76:1077-1088, 2002). Since phosphorylation affects the activities of many viral regulatory proteins, we sought to determine whether the phosphorylation of ICP0 affects its functions. To address this question, it was first necessary to identify the regions of ICP0 that are phosphorylated. For this purpose, ICP0 was partially purified, and phosphorylation sites were mapped by microcapillary high-pressure liquid chromatography tandem mass spectrometry. Three phosphorylated regions containing 11 putative phosphorylation sites, all within or adjacent to domains important for the transactivating activity of ICP0, were identified. The 11 sites were mutated to alanine as clusters in each of the three regions by site-directed mutagenesis, generating plasmids expressing mutant forms of ICP0: Phos 1 (four mutated sites), Phos 2 (three mutated sites), and Phos 3 (four mutated sites). One-dimensional phosphotryptic peptide analysis confirmed that the phosphorylation state of each Phos mutant form of ICP0 is altered relative to that of wild-type ICP0. In functional assays, the ICP0 phosphorylation site mutations affected the subcellular and subnuclear localization of ICP0, its ability to alter the staining pattern of the nuclear domain 10 (ND10)-associated protein PML, and/or its transactivating activity in Vero cells. Only mutations in Phos 1, however, impaired the ability of ICP0 to complement the replication of an ICP0 null mutant in Vero cells. This study thus suggests that phosphorylation is an important regulator of ICP0 function.

Neutralizing innate host defenses to control viral translation in HSV-1 infected cells

Lytic replication of many viruses activates an innate host response designed to prevent the completion of the viral lifecycle, thus impeding the spread of the infection. One branch of the host's complex reaction functions to incapacitate the cellular translational machinery on which the synthesis of viral polypeptides completely depends. This is achieved through the activation of specific protein kinases that phosphorylate eIF2 on its alpha subunit and inactivate this critical translation initiation factor. However, as continued synthesis of viral proteins is required to assemble the viral progeny necessary to transmit the infection to neighboring cells, viruses have developed a variety of strategies to counter this cellular response. Genetic and biochemical studies with herpes simplex virus type 1 (HSV-1) have revealed that the virus produces at least two discrete products at different times during its replicative program that act to prevent the accumulation of phosphorylated eIF2alpha. The gamma(1)34.5 gene product is expressed first, encoding a regulatory subunit that binds the cellular protein phosphatase 1alpha and regenerates pools of active eIF2 by removing the inhibitory phosphate from the alpha subunit. The second function, encoded by the product of the Us11 gene, specifies a double-stranded RNA-binding protein that prevents activation of PKR, a cellular eIF2alpha kinase. Together, both proteins cooperate to overcome the antiviral response of the host and properly regulate translation in HSV-1-infected cells.

Activation of NF-kappaB in cells productively infected with HSV-1 depends on activated protein kinase R and plays no apparent role in blocking apoptosis

Microarray data reported elsewhere indicated that herpes simplex virus 1 induces the up-regulation of nuclear factor kappaB (NF-kappaB)-regulated genes, including that of its inhibitor, IkappaBalpha, consistent with the reports that wild-type virus induces the activation of NF-kappaB. In this report we show that activation of NF-kappaB in infected cells is linked to the activation of protein kinase R (PKR). Specifically: (i) PKR is activated in infected cells although the effects of the activated enzyme on protein synthesis are negated by the viral gene gamma134.5, which encodes a protein phosphatase 1alpha accessory factor that enables the dephosphorylation of the alpha subunit of eukaryotic translation initiation factor 2. NF-kappaB is activated in wild-type murine embryonic fibroblasts but not in related PKR-null cells. (ii) In cells infected with a replication-competent Deltagamma134.5 mutant (R5104), but carrying a US11 gene expressed early in infection, eukaryotic translation initiation factor 2alpha is not phosphorylated, and in in vitro assays, PKR bound to the US11 protein is not phosphorylated on subsequent addition of double-stranded RNA. Here we report that this mutant does not activate PKR, has no effect on the accumulation of IkappaBalpha, and does not cause the translocation of NF-kappaB in infected cells. (iii) One hypothesis advanced for the activation of NF-kappaB is that it blocks apoptosis induced by viral gene products. The replication-competent R5104 mutant does not induce the programmed cell's death. We conclude that in herpes simplex virus 1-infected cells, activation of NF-kappaB depends on activation of PKR and that NF-kappaB is not required to block apoptosis in productively infected cells.

A region of the Epstein-Barr virus (EBV) mRNA export factor EB2 containing an arginine-rich motif mediates direct binding to RNA

The Epstein-Barr virus (EBV) protein EB2 (also called Mta, SM, or BMLF1) has properties in common with mRNA export factors and is essential for the production of EBV infectious virions. However, to date no RNA-binding motif essential for EB2-mediated mRNA export has been located in the protein. We show here by Northwestern blot analysis that the EB2 protein purified from mammalian cells binds directly to RNA. Furthermore, using overlapping glutathione S-transferase (GST)-EB2 peptides, we have, by RNA electrophoretic mobility shift assays (REMSAs) and Northwestern blotting, located an RNA-binding motif in a 33-amino acid segment of EB2 that has structural features of the arginine-rich RNA-binding motifs (ARMs) also found in many RNA-binding proteins. A synthetic peptide (called Da), which contains this EB2 ARM, bound RNA in REMSA. A GST-Da fusion protein also bound RNA in REMSA without apparent RNA sequence specificity, because approximately 10 GST-Da molecules bound at multiple sites on a 180-nucleotide RNA fragment. Importantly, a short deletion in the ARM region impaired both EB2 binding to RNA in vivo and in vitro and EB2-mediated mRNA export without affecting the shuttling of EB2 between the nucleus and the cytoplasm. Moreover, ectopic expression of ARM-deleted EB2 did not rescue the production of infectious virions by 293 cells carrying an EBVDeltaEB2 genome, which suggests that the binding of EB2 to RNA plays an essential role in the EBV productive cycle.

Association of the herpes simplex virus type 1 Us11 gene product with the cellular kinesin light-chain-related protein PAT1 results in the redistribution of both polypeptides

The herpes simplex virus type 1 (HSV-1) Us11 gene encodes a multifunctional double-stranded RNA (dsRNA)-binding protein that is expressed late in infection and packaged into the tegument layer of the virus particle. As a tegument component, Us11 associates with nascent capsids after its synthesis late in the infectious cycle and is delivered into newly infected cells at times prior to the expression of viral genes. Us11 is also an abundant late protein that regulates translation through its association with host components and contains overlapping nucleolar retention and nuclear export signals, allowing its accumulation in both nucleoli and the cytosol. Thus, at various times during the viral life cycle and in different intracellular compartments, Us11 has the potential to execute discrete tasks. The analysis of these functions, however, is complicated by the fact that Us11 is not essential for viral replication in cultured cells. To discover new host targets for the Us11 protein, we searched for cellular proteins that interact with Us11 and have identified PAT1 as a Us11-binding protein according to multiple, independent experimental criteria. PAT1 binds microtubules, participates in amyloid precursor protein trafficking, and has homology to the kinesin light chain (KLC) in its carboxyl terminus. The carboxyl-terminal dsRNA-binding domain of Us11, which also contains the nucleolar retention and nuclear export signals, binds PAT1, whereas 149 residues derived from the KLC homology region of PAT1 are important for binding to Us11. Both PAT1 and Us11 colocalize within a perinuclear area in transiently transfected and HSV-1-infected cells. The 149 amino acids derived from the KLC homology region are required for colocalization of the two polypeptides. Furthermore, although PAT1 normally accumulates in the nuclear compartment, Us11 expression results in the exclusion of PAT1 from the nucleus and its accumulation in the perinuclear space. Similarly, Us11 does not accumulate in the nucleoli of infected cells that overexpress PAT1. These results establish that Us11 and PAT1 can associate, resulting in an altered subcellular distribution of both polypeptides. The association between PAT1, a cellular trafficking protein with homology to KLC, and Us11, along with a recent report demonstrating an interaction between Us11 and the ubiquitous kinesin heavy chain (R. J. Diefenbach et al., J. Virol. 76:3282-3291, 2002), suggests that these associations may be important for the intracellular movement of viral components.

The herpes simplex virus (HSV) protein ICP34.5 is a virion component that forms a DNA-binding complex with proliferating cell nuclear antigen and HSV replication proteins

The replicative ability of ICP34.5-null herpes simplex virus (HSV) is cell type and state dependent. In certain cells, ICP34.5 interacts with protein phosphatase 1 to preclude host cell protein synthesis shutoff by dephosphorylation of the eukaryotic initiation factor eIF-2alpha. However, host cell shutoff is not induced by ICP34.5-null HSV in most cells, irrespective of type and state. In general, dividing cells support replication of ICP34.5-null HSV; nondividing cells cannot. Previously the authors showed that ICP34.5 binds to proliferating cell nuclear antigen (PCNA), a protein necessary for cellular DNA replication and repair. Here the authors demonstrate that (1) the interaction between ICP34.5 and PCNA involves two regions of the virus protein; (2) ICP34.5 forms a complex with HSV replication proteins that is DNA binding; (3) at early times in infection, ICP34.5 colocalizes with PCNA and HSV replication proteins in cell nuclei, before accumulating in the cytoplasm; and (4) ICP34.5 is a virion protein. In light of ongoing clinical trials assessing the safety and efficacy of ICP34.5-null HSV, it is vital that the roles of ICP34.5 in HSV replication are understood. The authors propose that in nondividing cells, ICP34.5 is required to switch PCNA from repair to replication mode, a prerequisite for the initiation of HSV replication.

In vivo replication of an ICP34.5 second-site suppressor mutant following corneal infection correlates with in vitro regulation of eIF2 alpha phosphorylation

In animal models of herpes simplex virus type 1 (HSV-1) infection, ICP34.5-null viruses are avirulent and also fail to grow in a variety of cultured cells due to their inability to prevent RNA-dependent protein kinase (PKR)-mediated inhibition of protein synthesis. We show here that the inability of ICP34.5 mutants to grow in vitro is due specifically to the accumulation of phosphorylated eIF2 alpha. Mutations suppressing the in vitro phenotype of ICP34.5-null mutants have been described which map to the unique short region of the HSV-1 genome, resulting in dysregulated expression of the US11 gene. Despite the inability of the suppressor mutation to suppress the avirulent phenotype of the ICP34.5-null parental virus following intracranial inoculation, the suppressor mutation enhanced virus growth in the cornea, trigeminal ganglia, and periocular skin following corneal infection compared to that with the ICP34.5-null virus. The phosphorylation state of eIF2 alpha following in vitro infection with the suppressor virus was examined to determine if in vivo differences could be attributed to differential regulation of eIF2 alpha phosphorylation. The suppressor virus prevented accumulation of phosphorylated eIF2 alpha, while the wild-type virus substantially reduced eIF2 alpha phosphorylation levels. These data suggest that US11 functions as a PKR antagonist in vivo, although its activity may be modulated by tissue-specific differences in translation regulation.

Counteraction of interferon-induced antiviral responses by herpes simplex viruses

The outcome of a viral infection of a host involves the complex interplay of viral determinants of virulence and host resistance factors. Among the first lines of defense for the host in attempts to control viral infection are the interferons (IFNs). A large body of work has now shown that the IFNs are a family of soluble proteins that serve to mediate antiviral effects, to regulate cell growth, and to modulate the activation of immune responses. The innate antiviral activities of IFNs are exceedingly potent and rapid. It is, therefore, not surprising that so many viruses have evolved ways to either preclude the synthesis of IFNs or evade downstream antiviral events. Such evasion allows for the virus to spread before the development of a specific adaptive immune response and likely represents a pivotal determinant of virulence for the invading virus. This review describes some of the research on herpes simplex virus (HSV) that has elucidated genes involved in evasion of the IFN response. In particular, the roles of specific viral genes in resistance to the antiviral effects of PKR and RNaseL are described, along with other HSV genes and loci associated with resistance to IFN for which mechanisms have yet to be described.

Characterization of RNA determinants recognized by the arginine- and proline-rich region of Us11, a herpes simplex virus type 1-encoded double-stranded RNA binding protein that prevents PKR activation

The herpes simplex virus Us11 gene product inhibits activation of the cellular PKR kinase and associates with a limited number of unrelated viral and cellular RNA molecules via a carboxyl-terminal 68-amino-acid segment rich in arginine and proline. To characterize the determinants underlying the recognition of an RNA target by Us11, we employed an in vitro selection technique to isolate RNA ligands that bind Us11 with high affinity from a population of molecules containing an internal randomized segment. Binding of Us11 to these RNA ligands is specific and appears to occur preferentially on conformational isoforms that possess a higher-order structure. While the addition of unlabeled poly(I. C) reduced binding of Us11 to a selected radiolabeled RNA, single-stranded homopolymers were not effective competitors. Us11 directly associates with poly(I. C), and inclusion of an unlabeled selected RNA in the reaction reduces poly(I. C) binding, while single-stranded RNA homopolymers have no effect. Finally, Us11 binds to defined, double-stranded RNA (dsRNA) molecules that exhibit greater sequence complexity. Binding to these dsRNA perfect duplexes displays a striking dependence on length, as 39-bp or shorter duplexes do not bind efficiently. Furthermore, this interaction is specific for dsRNA as opposed to dsDNA, implying that the Us11 RNA binding domain can distinguish nucleic acid duplexes containing 2' hydroxyl groups from those that do not. These results establish that Us11 is a dsRNA binding protein. The arginine- and proline-rich Us11 RNA binding domain is unrelated to known dsRNA binding elements and thus constitutes a unique recognition motif that interacts with dsRNA. The ability of Us11 to bind dsRNA may be important for inhibiting activation of the cellular PKR kinase in response to dsRNA.

Epstein-Barr virus mRNA export factor EB2 is essential for production of infectious virus

The splicing machinery which positions a protein export complex near the exon-exon junction mediates nuclear export of mRNAs generated from intron-containing genes. Many Epstein-Barr virus (EBV) early and late genes are intronless, and an alternative pathway, independent of splicing, must export the corresponding mRNAs. Since the EBV EB2 protein induces the cytoplasmic accumulation of intronless mRNA, it is tempting to speculate that EB2 is a viral adapter involved in the export of intronless viral mRNA. If this is true, then the EB2 protein is essential for the production of EBV infectious virions. To test this hypothesis, we generated an EBV mutant in which the BMLF1 gene, encoding the EB2 protein, has been deleted (EBV(BMLF1-KO)). Our studies show that EB2 is necessary for the production of infectious EBV and that its function cannot be transcomplemented by a cellular factor. In the EBV(BMLF1-KO) 293 cells, oriLyt-dependent DNA replication was greatly enhanced by EB2. Accordingly, EB2 induced the cytoplasmic accumulation of a subset of EBV early mRNAs coding for essential proteins implicated in EBV DNA replication during the productive cycle. Two herpesvirus homologs of the EB2 protein, the herpes simplex virus type 1 protein ICP27 and, the human cytomegalovirus protein UL69, only partly rescued the phenotype of the EBV(BMLF1-KO) mutant, indicating that some EB2 functions in virus production cannot be transcomplemented by ICP27 and UL69.

The herpes simplex virus type 1 US11 protein binds the coterminal UL12, UL13, and UL14 RNAs and regulates UL13 expression in vivo

The US11 protein of herpes simplex virus type 1 (HSV-1) is a small, highly basic phosphoprotein expressed at late times during infection. US11 localizes to the nucleolus in infected cells, can associate with ribosomes, and has been shown to bind RNA. The RNA substrates of US11 identified thus far have no apparent role in the virus lytic cycle, so we set out to identify a novel, biologically relevant RNA substrate(s) for this protein in HSV-1-infected cells. We designed a reverse transcriptase PCR-based protocol that allowed specific selection of a 600-bp RNA binding partner for US11. This RNA sequence, designated 12/14, is present in the coterminal HSV-1 mRNAs UL12, UL13, and UL14. We show that the binding of US11 to 12/14 is sequence-specific and mediated by the C-terminal domain of the protein. To elucidate the role of US11 in the virus life cycle, we infected cells with wild-type virus, a cosmid-reconstructed US11 HSV-1 null mutant, and a cosmid-reconstructed wild-type virus and analyzed expression of UL12, -13, and -14 during a time course of infection. These experiments revealed that this interaction has biological activity; at early times of infection, US11 down-regulates UL13 protein kinase mRNA and protein.

Herpes simplex virus tegument protein US11 interacts with conventional kinesin heavy chain

Little is known about the mechanisms of transport of neurotropic herpesviruses, such as herpes simplex virus (HSV), varicella-zoster virus, and pseudorabies virus, within neurons. For these viruses, which replicate in the nucleus, anterograde transport from the cell body of dorsal root ganglion (DRG) neurons to the axon terminus occurs over long distances. In the case of HSV, unenveloped nucleocapsids in human DRG neurons cocultured with autologous skin were observed by immunoelectron microscopy to colocalize with conventional ubiquitous kinesin, a microtubule-dependent motor protein, in the cell body and axon during anterograde axonal transport. Subsequently, four candidate kinesin-binding structural HSV proteins were identified (VP5, VP16, VP22, and US11) using oligohistidine-tagged human ubiquitous kinesin heavy chain (uKHC) as bait. Of these viral proteins, a direct interaction between uKHC and US11 was identified. In vitro studies identified residues 867 to 894 as the US11-binding site in uKHC located within the proposed heptad repeat cargo-binding domain of uKHC. In addition, the uKHC-binding site in US11 maps to the C-terminal RNA-binding domain. US11 is consistently cotransported with kinetics similar to those of the capsid protein VP5 into the axons of dissociated rat neurons, unlike the other tegument proteins VP16 and VP22. These observations suggest a major role for the uKHC-US11 interaction in anterograde transport of unenveloped HSV nucleocapsids in axons.

The US11 gene product of herpes simplex virus has intercellular trafficking activity

The US11 gene product of herpes simplex virus is an abundant virion structural protein with RNA-binding regulatory activity. Its carboxyl-terminal half consists of tandem tripeptide repeats of the sequence RXP. We demonstrate that the US11 protein has intercellular trafficking activity and accumulates in the nucleolus when singly expressed in cultured cells, and that the RXP repeats are responsible for this activity. These same properties were also observed in cells expressing a fusion protein linking US11 to the green fluorescent protein. Furthermore, exogenous US11 protein was internalized by cells at 4 degrees C, which suggests that US11 protein uptake occurs primarily through an energy-independent pathway.

RNAs extracted from herpes simplex virus 1 virions: apparent selectivity of viral but not cellular RNAs packaged in virions

Following the lead of recent studies on the presence of RNA in virions of human cytomegalovirus, we investigated the presence and identity of RNAs from purified virions of herpes simple virus 1. To facilitate these studies, we designed primers for all known open reading frames (ORFs) and also constructed cDNA arrays containing probes designed to detect all known transcripts. In the first series of experiments, labeled DNA made by reverse transcription of poly(A)(+) RNA extracted from infected HEp-2 or rabbit skin cells hybridized to all but two of the probes in the cDNA array. A similar analysis of the RNA extracted from purified extracellular virions derived from infected HEp-2 cells hybridized to probes representing 24 of the ORFs. In the second series of analyses, we reverse transcribed and amplified by PCR RNAs from purified intracellular or extracellular virions derived from infected HEp-2 or Vero cell lines. The positive RNAs were retested by PCR with and without prior reverse transcription to ensure that the samples tested were free of contaminating DNA. The results were as follows. (i) Only a fraction of viral ORF transcripts were represented in virion RNA, and only nine RNAs (U(L)10, U(L)34/U(L)35, U(L)36, U(L)42, U(L)48, U(L)51, U(S)1/U(S)1.5, U(S)8.5, and U(S)10/U(S)11) were positive in all RT PCR assays. Of these, seven were positive by hybridization to cDNA arrays. (ii) RNA extracted from cells infected with a mutant virus lacking the U(S)8 to U(S)12 genes yielded results similar to those described above, indicating that U(S)11, a known RNA binding protein, does not play a role in packaging RNA in virions. (iii) Cellular RNAs detected in virions were representative of the abundant cellular RNAs. Last, RNA extracted from virions was translated in vitro and the translation products were reacted with antibody to alphaTIF (VIP16). The immune precipitate contained a labeled protein with the apparent molecular weight of alphaTIF, indicating that at least one mRNA packaged in virions was intact and capable of being translated. The basis for the apparent selectivity in the packaging of the viral RNAs packaged in virions is unknown.

Subversion and piracy: DNA viruses and immune evasion

During the co-evolution of viruses with their vertebrate hosts, the DNA viruses have acquired an impressive array of immunomodulatory genes to combat host immune responses and their hosts have developed a sophisticated immune system to contain virus infections. In order to replicate, the viruses have evolved mechanisms to inhibit key host anti-virus responses that include apoptosis, interferon production, chemokine production, inflammatory cytokine production, and the activity of cytotoxic T-cells, natural killer cells and antibody. In addition, some of the viruses encode cytokine or chemokine homologues that recruit or expand cell numbers for infection or that subvert the host cellular response from a protective response to a benign one. The specificity of the viral immunomodulatory molecules reflects the life cycle and the pathogenesis of the viruses. Herpesviruses achieve latency in host cells by inducing cell survival and protecting infected cells from immune recognition. This involves interference with cell signal transduction pathways. Many of the viral immunomodulatory proteins are homologues of host proteins that appear to have been pirated from the host and reassorted in the virus genomes. Some of these have unique functions and indicate novel or important aspects of both viral pathogenesis and host immunity to viruses. The specific example of orf virus infection of sheep is described.

A subset of viral transcripts packaged within human cytomegalovirus particles

A human cytomegalovirus gene array was used to identify a previously unidentified class of viral transcripts. These transcripts, termed virion RNAs, were packaged within infectious virions and were delivered to the host cell on infection. This mechanism of herpesvirus gene expression allows for viral genes to be expressed within an infected cell immediately after virus entry and in the absence of transcription from the viral genome.

Posttranscriptional regulation of US11 in cells infected with a herpes simplex virus 1 recombinant lacking both 222-bp domains containing S-component origins of DNA synthesis

The US11 gene of herpes simplex virus 1 maps in the unique sequences of the short component of the HSV-1(F) genome approximately 775 bp from the center of the DNA replication origin (OriS) and encodes a virion protein which binds RNA in sequence- and conformation-specific fashion, negatively regulates the accumulation of a prematurely terminated transcript of UL34, associates in the infected cell with the 60S ribosomal subunit, and, late in infection, accumulates in nucleoli. We report the following: (i) Deletion of a 222-bp sequence including OriS (DeltaOriS) negatively affected the accumulation of the US11 protein without decreasing the accumulation of the US11 transcript. (ii) The defect, observed at all times after infection, was multiplicity independent, was unrelated to US11 protein stability, and apparently resulted from a cis-acting element since a coinfecting virus was unable to complement the DeltaOriS virus. (iii) Transcription from the US11 promoter initiated from three sites on the DeltaOriS virus. Transcripts initiated from two of the three initation sites accumulated similarly in cells infected with the DeltaOriS virus or wild-type parent virus. The low-abundance transcript initiating from the third site was apparently unique to the DeltaOriS virus but was not expected to alter the coding capacity of the mRNA. (iv) Infected cells accumulated RNA derived by antisense transcription of the genome domain containing the US11 gene. One transcript accumulated in larger amounts in cells infected with the DeltaOriS virus than in cells infected with parent or repaired virus.

The C-terminal region but not the Arg-X-Pro repeat of Epstein-Barr virus protein EB2 is required for its effect on RNA splicing and transport

The Epstein-Barr virus BMLF1 gene product EB2 has been shown to efficiently transform immortalized Rat1 and NIH 3T3 cells, to bind RNA, and to shuttle from the nucleus to the cytoplasm. In transient-expression assays EB2 seems to affect mRNA nuclear export of intronless RNAs and pre-mRNA 3' processing, but no direct proof of EB2 being involved in RNA processing and transport has been provided, and no specific functional domain of EB2 has been mapped. Here we significantly extend these findings and directly demonstrate that (i) EB2 inhibits the cytoplasmic accumulation of mRNAs, but only if they are generated from precursors containing weak (cryptic) 5' splice sites, (ii) EB2 has no effect on the cytoplasmic accumulation of mRNA generated from precursors containing constitutive splice sites, and (iii) EB2 has no effect on the 3' processing of precursor RNAs containing canonical and noncanonical cleavage-polyadenylation signals. We also show that in the presence of EB2, intron-containing and intronless RNAs accumulate in the cytoplasm. EB2 contains an Arg-X-Pro tripeptide repeated eight times, similar to that described as an RNA-binding domain in the herpes simplex virus type 1 protein US11. As glutathione S-transferase fusion proteins, both EB2 and the Arg-X-Pro repeat bound RNA in vitro. However, by using EB2 deletion mutants, we demonstrated that the effect of EB2 on splicing and RNA transport requires the C-terminal half of the protein but not the Arg-X-Pro repeat.

The herpes simplex virus US11 protein effectively compensates for the gamma1(34.5) gene if present before activation of protein kinase R by precluding its phosphorylation and that of the alpha subunit of eukaryotic translation initiation factor 2

In herpes simplex virus-infected cells, viral gamma134.5 protein blocks the shutoff of protein synthesis by activated protein kinase R (PKR) by directing the protein phosphatase 1alpha to dephosphorylate the alpha subunit of eukaryotic translation initiation factor 2 (eIF-2alpha). The amino acid sequence of the gamma134.5 protein which interacts with the phosphatase has high homology to a domain of the eukaryotic protein GADD34. A class of compensatory mutants characterized by a deletion which results in the juxtaposition of the alpha47 promoter next to US11, a gamma2 (late) gene in wild-type virus-infected cells, has been described. In cells infected with these mutants, protein synthesis continues even in the absence of the gamma134.5 gene. In these cells, PKR is activated but eIF-2alpha is not phosphorylated, and the phosphatase is not redirected to dephosphorylate eIF-2alpha. We report the following: (i) in cells infected with these mutants, US11 protein was made early in infection; (ii) US11 protein bound PKR and was phosphorylated; (iii) in in vitro assays, US11 blocked the phosphorylation of eIF-2alpha by PKR activated by poly(I-C); and (iv) US11 was more effective if present in the reaction mixture during the activation of PKR than if added after PKR had been activated by poly(I-C). We conclude the following: (i) in cells infected with the compensatory mutants, US11 made early in infection binds to PKR and precludes the phosphorylation of eIF-2alpha, whereas US11 driven by its natural promoter and expressed late in infection is ineffective; and (ii) activation of PKR by double-stranded RNA is a common impediment countered by most viruses by different mechanisms. The gamma134.5 gene is not highly conserved among herpesviruses. A likely scenario is that acquisition by a progenitor of herpes simplex virus of a portion of the cellular GADD34 gene resulted in a more potent and reliable means of curbing the effects of activated PKR. US11 was retained as a gamma2 gene because, like many viral proteins, it has multiple functions.

Suppression of the phenotype of gamma(1)34.5- herpes simplex virus 1: failure of activated RNA-dependent protein kinase to shut off protein synthesis is associated with a deletion in the domain of the alpha47 gene

Earlier studies have shown that infection of human cells by herpes simplex virus 1 (HSV-1) results in the activation of RNA-dependent protein kinase (PKR) but that the alpha subunit of eIF-2 is not phosphorylated and that protein synthesis is unaffected. In the absence of the viral gamma(1)34.5 gene, eIF-2alpha is phosphorylated and protein synthesis is prematurely shut off (J. Chou, J. J. Chen, M. Gross, and B. Roizman, Proc. Natl. Acad. Sci. USA 92:10516-10520, 1995). A second recent paper reported the selection of second-site suppressor mutants characterized by near-wild-type protein synthesis in cells infected with gamma(1)34.5- mutants (I. Mohr and Y. Gluzman, EMBO J. 15:4759-4766, 1996). Here, we report the properties of the spontaneous HSV-1 suppressor mutant Sup-1, which is characterized by spontaneous deletion of 503 bp encompassing the domain of the alpha47 gene and junction with the inverted repeats flanking the unique short (U(S)) sequence of the HSV-1 DNA resulting in the juxtaposition of the alpha47 promoter to the coding domain of the U(S)11 gene. This mutant does not exhibit the shutoff of protein synthesis characteristic of the gamma(1)34.5- virus. Specifically, Sup-1 in SK-N-SH human neuroblastoma cells (i) did not exhibit the function of the alpha47 gene characterized by a reduction in the transport of peptides across the endoplasmic reticulum of permealized cells consistent with the absence of alpha47 gene sequences, (ii) accumulated U(S)11 protein at levels analogous to those of the wild-type parent but the protein was made at earlier times after infection, as would be expected from a change in the promoter, and (iii) activated PKR like that of the parent, gamma(1)34.5- virus, but (iv) did not cause premature shutoff of protein synthesis and therefore was similar to the wild-type parent virus rather than the gamma(1)34.5- virus from which it was derived. We conclude that the mechanism by which Sup-1 blocks the shutoff of protein synthesis associated with phosphorylation of eIF-2alpha by the activated PKR is not readily explainable by a secondary mutation characterized by a deletion.

Characterization of the products of the U(L)43 gene of herpes simplex virus 1: potential implications for regulation of gene expression by antisense transcription

The products, RNA or proteins, of the herpes simplex virus 1 open reading frame U(L)43 have not been previously identified. The expression of an open reading frame antisense to U(L)43, U(L)43.5 (P. L. Ward, D. E. Barker, and B. Roizman, J. Virol. 70:2684-2690, 1996), has been reported. We report the existence of a transcript corresponding to the domain of the U(L)43 open reading frame extending approximately 30 bp from the predicted TATA box to the predicted polyadenylation signal. The RNA of the U(L)43 open reading frame accumulates to higher levels in the presence of phosphonoacetic acid, an inhibitor of viral DNA synthesis, than in its absence, whereas the U(L)43.5 transcript accumulates in larger amounts in the absence of phosphonoacetic acid. The open reading frame tagged with a sequence encoding a 20-amino-acid epitope yielded a protein with an apparent Mr of 32,000, i.e., considerably lower than that predicted from the size of the open reading frame. The discovery of a pair of antisense genes expressed during productive infection raises the possibilities that additional antisense genes exist and that the antisense arrangement provides still another mechanism for regulation of gene expression.

Alternatively spliced mRNAs predicted to yield frame-shift proteins and stable intron 1 RNAs of the herpes simplex virus 1 regulatory gene alpha 0 accumulate in the cytoplasm of infected cells

The infected cell protein no. 0 (ICP0), the product of the alpha 0 gene, and an important herpes simplex virus 1 regulatory protein is encoded by three exons. We report that intron 1 forms a family of four stable nonpolyadenylylated cytoplasmic RNAs sharing a common 5' end but differing in 3' ends. The 5' and 3' ends correspond to the accepted splice donor and four splice acceptor sites within the mapped intron domain. The most distant splice acceptor site yields the mRNA encoding the 775-aa protein known as ICP0. The mRNAs resulting from the use of alternative splice acceptor sites were also present in the cytoplasm of infected cells and would be predicted to encode proteins of 152 (ICP0-B), 87 (ICP0-C), and 90 (ICP0-D) amino acids, respectively. Both the stability of the alpha 0 mRNA and the utilization of at least one splice acceptor site was regulated by ICP22 and or US1.5 protein inasmuch as cells infected with a mutant from which these genes had been deleted accumulated smaller amounts of alpha 0 mRNA than would be predicted from the amounts of accumulated intron RNAs. In addition, one splice acceptor site was at best underutilized. These results indicate that both the splicing pattern and longevity of alpha 0 mRNA are regulated. These and other recent examples indicate that herpes simplex virus 1 regulates its own gene expression and that of the infected cells through control of mRNA splicing and longevity.

A herpesvirus genetic element which affects translation in the absence of the viral GADD34 function

Novel suppressor variants of conditionally lethal HSV-1 gamma34.5 deletion mutants have been isolated which exhibit restored ability to grow on neoplastic neuronal cells. Deletion of the viral gamma34.5 genes, whose products share functional similarity with the cellular GADD34 gene, renders the virus non-neurovirulent and imposes a block to viral replication in neuronal cells. Protein synthesis ceases at late times post-infection and the translation initiation factor eIF2alpha is phosphorylated by the cellular PKR kinase [Chou et al. (1990) Science, 252, 1262-1266; (1995) Proc. Natl Acad. Sci. USA, 92, 10516-10520]. The suppressor mutants have overcome the translational block imposed by PKR. Multiple, independent isolates all contain rearrangements within a 595 bp element in the HSV-1 genome where the unique short component joins the terminal repeats. This alteration, which affects the production of the viral mRNA and protein from the Us11 and Us12 genes, is both necessary and sufficient to confer the suppressor phenotype on gamma34.5 mutant viruses. HSV-1 thus encodes a specific element which inhibits ongoing protein synthesis in the absence of the viral GADD34-like function. Since this inhibition involves the accumulation of phosphorylated eIF2alpha, the element identified by the suppressor mutations may be a discrete PKR activator. Activation of the PKR kinase thus does not proceed through a general, cellular 'antiviral' sensing mechanism. Instead, the virus deliberately activates PKR and encodes a separate function which selectively prevents the phosphorylation of at least one PKR target, eIF2alpha. The nature of this potential activator element, and how analogous cellular elements could affect PKR pathways which affect growth arrest and differentiation are discussed.

Assemblons: nuclear structures defined by aggregation of immature capsids and some tegument proteins of herpes simplex virus 1

In cells infected with herpes simplex virus 1 (HSV-1), the viral proteins ICP5 (infected-cell protein 5) and VP19c (the product of UL38) are associated with mature capsids, whereas the same proteins, along with ICP35, are components of immature capsids. Here we report that ICP35, ICP5, and UL38 (VP19c) coalesce at late times postinfection and form antigenically dense structures located at the periphery of nuclei, close to but not abutting nuclear membranes. These structures were formed in cells infected with a virus carrying a temperature-sensitive mutation in the UL15 gene at nonpermissive temperatures. Since at these temperatures viral DNA is made but not packaged, these structures must contain the proteins for immature-capsid assembly and were therefore designated assemblons. These assemblons are located at the periphery of a diffuse structure composed of proteins involved in DNA synthesis. This structure overlaps only minimally with the assemblons. In contrast, tegument proteins were located in asymmetrically distributed structures also partially overlapping with assemblons but frequently located nearer to nuclear membranes. Of particular interest is the finding that the UL15 protein colocalized with the proteins associated with viral DNA synthesis rather than with assemblons, suggesting that the association with DNA may take place during its synthesis and precedes the involvement of this protein in packaging of the viral DNA into capsids. The formation of three different compartments consisting of proteins involved in viral DNA synthesis, the capsid proteins, and tegument proteins suggests that there exists a viral machinery which enables aggregation and coalescence of specific viral protein groups on the basis of their function.

Structure and function in the herpes simplex virus 1 RNA-binding protein U(s)11: mapping of the domain required for ribosomal and nucleolar association and RNA binding in vitro

The herpes simplex virus 1 US11 protein is an RNA-binding regulatory protein that specifically and stably associates with 60S ribosomal subunits and nucleoli and is incorporated into virions. We report that US11/ beta-galactosidase fusion protein expressed in bacteria bound to rRNA from the 60S subunit and not the 40S subunit. This binding reflects the specificity of ribosomal subunit association. Analyses of deletion mutants of the US11 gene showed that specific RNA binding activity, nucleolar localization, and association with 60S ribosomal subunits were found to map to the amino acid sequences of the carboxyl terminus of US11 protein, suggesting that these activities all reflect specific binding of US11 to large subunit rRNA. The carboxyl-terminal half of the protein consists of a regular tripeptide repeat of the sequence RXP and constitutes a completely novel RNA-binding domain. All of the mutant US11 proteins could be incorporated into virus particles, suggesting that the signal for virion incorporation either is at the amino-terminal four amino acids or is redundant in the protein.

Post-transcriptional transactivation of human retroviral envelope glycoprotein expression by herpes simplex virus Us11 protein

Herpes simplex virus type 1 (HSV-1) Us11 protein, a true late gene product packaged within the virion, is delivered into cells after infection, exhibits a nucleocytoplasmic localization at early times, and later accumulates in the nucleoli. This RNA-binding basic phosphoprotein, capable of oligomerization, is supposed to be involved in post-transcriptional regulation of gene expression after HSV-1 infection. Expression of human T-cell leukaemia/lymphoma virus type-I (HTLV-I) and of human immunodeficiency virus type 1 (HIV-1) is post-transcriptionally regulated by Rex and Rev, respectively. These proteins are required for the cytoplasmic expression of unspliced gag-pol and singly spliced env transcripts. Here we show that HSV-1 Us11 protein is able to bind Rex- and Rev-responsive elements and to transactivate envelope retroviral glycoprotein expression.

Characterization of the US8.5 protein of herpes simplex virus

In a previous study a novel gene designated as US8.5 was identified in the unique short component of the herpes simplex virus type 1 (HSV1) genome. Epitope tagging experiments suggested the existence of a protein encoded by this gene in HSV1 infected cells. To further analyze the US8.5 gene product and function, a rabbit polyclonal antiserum was raised against a recombinant beta-galactosidase-US8.5 fusion protein expressed in E. coli. This antiserum reacted specifically with a 19 kDa protein in HSV1(F) infected cells as shown by immunoblotting and immunoprecipitation experiments. In addition, using the same antiserum a 21 kDa protein was detected in lysates from cells infected with HSV2(G), which was most likely the HSV2 homolog of US8.5. Kinetic studies indicated that US8.5 is expressed as gamma 1 gene. In addition, the US8.5 protein was immunoprecipitated with the anti-US8.5 serum from 32P-labeled lysates of Vero cells infected with HSV1, demonstrating that the protein is phosphorylated. Immunofluorescence studies localized the US8.5 protein to the nucleoli of HSV1 infected cells.

The herpes simplex virus type 1 particle: structure and molecular functions. Review article

This review is a summary of our present knowledge with respect to the structure of the virion of herpes simplex virus type 1. The virion consists of a capsid into which the DNA is packaged, a tegument and an external envelope. The protein compositions of the structures outside the genome are described as well as the functions of individual proteins. Seven capsid proteins are identified, and two of them are mainly present in precursors of mature DNA-containing capsids. The protein components of the 150 hexamers and 12 pentamers in the icosahedral capsid are known. These capsomers all have a central channel and are connected by Y-shaped triplexes. In contrast to the capsid, the tegument has a less defined structure in which 11 proteins have been identified so far. Most of them are phosphorylated. Eleven virus-encoded glycoproteins are present in the envelope, and there may be a few more membrane proteins not yet identified. Functions of these glycoproteins include attachment to and penetration of the cellular membrane. The structural proteins, their functions, coding genes and localizations are listed in table form.

A herpes simplex virus 1 US11-expressing cell line is resistant to herpes simplex virus infection at a step in viral entry mediated by glycoprotein D

A baby hamster kidney [BHK(tk-)] cell line (US11cl19) which stably expresses the US11 and alpha 4 genes of herpes simplex virus 1 strain F [HSV-1(F)] was found to be resistant to infection with HSV-1. Although wild-type HSV-1(F) attached with normal kinetics to the surface of US11cl19 cells, most cells showed no evidence of infection and failed to accumulate detectable amounts of alpha mRNAs. The relationship between the expression of UL11 and resistance to HSV infection in US11cl19 cells has not been defined, but the block to infection with wild-type HSV-1 was overcome by exposing cells with attached virus on their surface to the fusogen polyethylene glycol, suggesting that the block to infection preceded the fusion of viral and cellular membranes. An escape mutant of HSV-1(F), designated R5000, that forms plaques on US11cl19 cells was selected. This mutant was found to contain a mutation in the glycoprotein D (gD) coding sequence that results in the substitution of the serine at position 140 in the mature protein to asparagine. A recombinant virus, designated R5001, was constructed in which the wild-type gD gene was replaced with the R5000 gD gene. The recombinant formed plaques on US11cl19 cells with an efficiency comparable to that of the escape mutant R5000, suggesting that the mutation in gD determines the ability of the mutant R5000 to grow on US11cl19 cells. The observation that the US11cl19 cells were slightly more resistant to fusion by polyethylene glycol than parental BHK(tk-) cells led to the selection and testing of clonal lines from unselected and polyethylene glycol-selected BHK(tk-) cells. The results were that 16% of unselected to as much as 36% of the clones selected for relative resistance to polyethylene glycol fusion exhibited various degrees of resistance to infection. The exact step at which the infection was blocked is not known, but the results illustrate the ease of selection of cell clones with one or more sites at which infection could be blocked.