Generation of an inverting herpes simplex virus 1 mutant lacking the L-S junction a sequences, an origin of DNA synthesis, and several genes including those specifying glycoprotein E and the alpha 47 gene

The US11 Gene of Herpes Simplex Virus 1 Promotes Neuroinvasion and Periocular Replication following Corneal Infection

Herpes simplex virus 1 (HSV-1) cycles between phases of latency in sensory neurons and replication in mucosal sites. HSV-1 encodes two key proteins that antagonize the shutdown of host translation, US11 through preventing PKR activation and ICP34.5 through mediating dephosphorylation of the α subunit of eukaryotic initiation factor 2 (eIF2α). While profound attenuation of ICP34.5 deletion mutants has been repeatedly demonstrated, a role for US11 in HSV-1 pathogenesis remains unclear. We therefore generated an HSV-1 strain 17 US11-null virus and examined its properties in vitro and in vivo In U373 glioblastoma cells, US11 cooperated with ICP34.5 to prevent eIF2α phosphorylation late in infection. However, the effect was muted in human corneal epithelial cells (HCLEs), which did not accumulate phosphorylated eIF2α unless both US11 and ICP34.5 were absent. Low levels of phosphorylated eIF2α correlated with continued protein synthesis and with the ability of virus lacking US11 to overcome antiviral immunity in HCLE and U373 cells. Neurovirulence following intracerebral inoculation of mice was not affected by the deletion of US11. In contrast, the time to endpoint criteria following corneal infection was greater for the US11-null virus than for the wild-type virus. Replication in trigeminal ganglia and periocular tissue was promoted by US11, as was periocular disease. The establishment of latency and the frequency of virus reactivation from trigeminal ganglia were unaffected by US11 deletion, although emergence of the US11-null virus occurred with slowed kinetics. Considered together, the data indicate that US11 facilitates the countering of antiviral response of infected cells and promotes the efficient emergence of virus following reactivation.IMPORTANCE Alphaherpesviruses are ubiquitous DNA viruses and include the human pathogens herpes simplex virus 1 (HSV-1) and HSV-2 and are significant causes of ulcerative mucosal sores, infectious blindness, encephalitis, and devastating neonatal disease. Successful primary infection and persistent coexistence with host immune defenses are dependent on the ability of these viruses to counter the antiviral response. HSV-1 and HSV-2 and other primate viruses within the Simplexvirus genus encode US11, an immune antagonist that promotes virus production by preventing shutdown of protein translation. Here we investigated the impact of US11 deletion on HSV-1 growth in vitro and pathogenesis in vivo This work supports a role for US11 in pathogenesis and emergence from latency, elucidating immunomodulation by this medically important cohort of viruses.

Roles of Us8A and Its Phosphorylation Mediated by Us3 in Herpes Simplex Virus 1 Pathogenesis

The herpes simplex virus 1 (HSV-1) Us8A gene overlaps the gene that encodes glycoprotein E (gE). Previous studies have investigated the roles of Us8A in HSV-1 infection using null mutations in Us8A and gE; therefore, the role of Us8A remains to be elucidated. In this study, we investigated the function of Us8A and its phosphorylation at serine 61 (Ser-61), which we recently identified as a phosphorylation site by mass spectrometry-based phosphoproteomic analysis of HSV-1-infected cells, in HSV-1 pathogenesis. We observed that (i) the phosphorylation of Us8A Ser-61 in infected cells was dependent on the activity of the virus-encoded Us3 protein kinase; (ii) the Us8A null mutant virus exhibited a 10-fold increase in the 50% lethal dose for virulence in the central nervous system (CNS) of mice following intracranial infection compared with a repaired virus; (iii) replacement of Ser-61 with alanine (S61A) in Us8A had little effect on virulence in the CNS of mice following intracranial infection, whereas it significantly reduced the mortality of mice following ocular infection to levels similar to the Us8A null mutant virus; (iv) the Us8A S61A mutation also significantly reduced viral yields in mice following ocular infection, mainly in the trigeminal ganglia and brains; and (v) a phosphomimetic mutation at Us8A Ser-61 restored wild-type viral yields and virulence. Collectively, these results indicate that Us8A is a novel HSV-1 virulence factor and suggest that the Us3-mediated phosphorylation of Us8A Ser-61 regulates Us8A function for viral invasion into the CNS from peripheral sites.

IMPORTANCE

The DNA genomes of viruses within the subfamily Alphaherpesvirinae are divided into unique long (UL) and unique short (Us) regions. Us regions contain alphaherpesvirus-specific genes. Recently, high-throughput sequencing of ocular isolates of HSV-1 showed that Us8A was the most highly conserved of 13 herpes simplex virus 1 (HSV-1) genes mapped to the Us region, suggesting Us8A may have an important role in the HSV-1 life cycle. However, the specific role of Us8A in HSV-1 infection remains to be elucidated. Here, we show that Us8A is a virulence factor for HSV-1 infection in mice, and the function of Us8A for viral invasion into the central nervous system from peripheral sites is regulated by Us3-mediated phosphorylation of the protein at Ser-61. This is the first study to report the significance of Us8A and its regulation in HSV-1 infection.

Exploring the Balance between DNA Pressure and Capsid Stability in Herpesviruses and Phages

We have recently shown in both herpesviruses and phages that packaged viral DNA creates a pressure of tens of atmospheres pushing against the interior capsid wall. For the first time, using differential scanning microcalorimetry, we directly measured the energy powering the release of pressurized DNA from the capsid. Furthermore, using a new calorimetric assay to accurately determine the temperature inducing DNA release, we found a direct influence of internal DNA pressure on the stability of the viral particle. We show that the balance of forces between the DNA pressure and capsid strength, required for DNA retention between rounds of infection, is conserved between evolutionarily diverse bacterial viruses (phages λ and P22), as well as a eukaryotic virus, human herpes simplex 1 (HSV-1). Our data also suggest that the portal vertex in these viruses is the weakest point in the overall capsid structure and presents the Achilles heel of the virus's stability. Comparison between these viral systems shows that viruses with higher DNA packing density (resulting in higher capsid pressure) have inherently stronger capsid structures, preventing spontaneous genome release prior to infection. This force balance is of key importance for viral survival and replication. Investigating the ways to disrupt this balance can lead to development of new mutation-resistant antivirals.

IMPORTANCE

A virus can generally be described as a nucleic acid genome contained within a protective protein shell, called the capsid. For many double-stranded DNA viruses, confinement of the large DNA molecule within the small protein capsid results in an energetically stressed DNA state exerting tens of atmospheres of pressures on the inner capsid wall. We show that stability of viral particles (which directly relates to infectivity) is strongly influenced by the state of the packaged genome. Using scanning calorimetry on a bacterial virus (phage λ) as an experimental model system, we investigated the thermodynamics of genome release associated with destabilizing the viral particle. Furthermore, we compare the influence of tight genome confinement on the relative stability for diverse bacterial and eukaryotic viruses. These comparisons reveal an evolutionarily conserved force balance between the capsid stability and the density of the packaged genome.

Genetic variability in the region encompassing reiteration VII of herpes simplex virus type 1, including deletions and multiplications related to recombination between direct repeats

A number of tandemly reiterated sequences are present on the herpes simplex virus type 1 (HSV-1) DNA molecule of 152 kbp. While regions containing tandem reiterations were usually unstable, reiteration VII, which is present within the protein coding regions of gene US10 and US11, was stable; hence, reiteration VII could be used as a genetic marker. In the present study, the nucleotide sequences (159–213 bp) of a region encompassing reiteration VII of 62 HSV-1 isolates were compared with that of strain 17 as the standard strain, and the genetic variability of base substitutions, deletions, and multiplications was revealed. Base substitution was observed in nine residues on the region flanking reiteration VII and sixty-two HSV-1 isolates were classified into twelve groups based on these base substitutions. Deletions, which were present in all sixty-two isolates, were classified into six groups. Multiplications, which were present in 19 isolates having the same deletion (named del-2), were classified into four groups. The sixty-two isolates were classified into twenty patterns based on variations in the region encompassing reiteration VII, and the region encompassing reiteration VII was considered to be useful for studies on the molecular epidemiology and evolution of HSV-1. The lengths of these deletions and multiplications were multiples of 3; thus, a frame-shift mutation was not induced, and a mechanism to maintain the functions of US10 and US11 was suggested. A series of multiplications, which consisted of the duplication, triplication, and tetraplication of the same sequence, were found. Since all isolates with a multiplication had del-2, multiplications were assumed to be generated after the generation of del-2, and an isolate with del-2 was considered to have the ability to generate a multiplication. Recombination between a pair of direct repeats in and around reiteration VII was accountable for the generation of deletions and multiplications, indicating the recombinogenic property of the region encompassing reiteration VII. A correlation was revealed between a set of 20 DNA polymorphisms widely present on the HSV-1 genome and the base substitutions and deletions of the region encompassing reiteration VII, using discriminant analyses.

Nucleolin interacts with US11 protein of herpes simplex virus 1 and is involved in its trafficking

Herpes simplex virus type 1 (HSV-1) infection induces profound nucleolar modifications at the functional and organizational levels, including nucleolar invasion by several viral proteins. One of these proteins is US11, which exhibits several different functions and displays both cytoplasmic localization and clear nucleolar localization very similar to that of the major multifunctional nucleolar protein nucleolin. To determine whether US11 interacts with nucleolin, we purified US11 protein partners by coimmunoprecipitations using a tagged protein, Flag-US11. From extracts of cells expressing Flag-US11 protein, we copurified a protein of about 100 kDa that was further identified as nucleolin. In vitro studies have demonstrated that nucleolin interacts with US11 and that the C-terminal domain of US11, which is required for US11 nucleolar accumulation, is sufficient for interaction with nucleolin. This association was confirmed in HSV-1-infected cells. We found an increase in the nucleolar accumulation of US11 in nucleolin-depleted cells, thereby revealing that nucleolin could play a role in US11 nucleocytoplasmic trafficking through one-way directional transport out of the nucleolus. Since nucleolin is required for HSV-1 nuclear egress, the interaction of US11 with nucleolin may participate in the outcome of infection.

Molecular determinants responsible for the subcellular localization of HSV-1 UL4 protein

The function of the herpes simplex virus type 1 (HSV-1) UL4 protein is still elusive. Our objective is to investigate the subcellular transport mechanism of the UL4 protein. In this study, fluorescence microscopy was employed to investigate the subcellular localization of UL4 and characterize the transport mechanism in living cells. By constructing a series of deletion mutants fused with enhanced yellow fluorescent protein (EYFP), the nuclear export signals (NES) of UL4 were for the first time mapped to amino acid residues 178 to 186. In addition, the N-terminal 19 amino acids are identified to be required for the granule-like cytoplasmic pattern of UL4. Furthermore, the UL4 protein was demonstrated to be exported to the cytoplasm through the NES in a chromosomal region maintenance 1 (CRM1)-dependent manner involving RanGTP hydrolysis.

The origins of replication of granuloviruses

The genomes of eight granuloviruses (GVs), have been analyzed for the presence of homologous regions (hrs) that may act as origins of replication. Thirteen 74-76-bp palindromes within 11 hrs have previously been identified in the Cydia pomonella GV (CpGV) genome and found to replicate in an infection-dependent DNA replication assay. We report a further palindrome within one of the hrs, which was found to replicate, bringing the total to 14 palindromes. We also report imperfect palindromes, with similar 13-bp end sequences to the CpGV palindromes, within the Adoxophyes orana GV, Cryptophlebia leucotreta GV (CrleGV), Choristoneura occidentalis GV and Phthorimaea operculella GV genomes. No hrs were detected in Agrotis segetum GV, and no additional hrs or palindromes, other than those published, were detected in the Plutella xylostella GV and Xestia c-nigrum GV genomes. Several putative hrs from the GVs were tested for replication in C. pomonella cells using a CpGV-dependent replication assay. Two CrleGV hrs were found to replicate at a low level.

US11 of herpes simplex virus type 1 interacts with HIPK2 and antagonizes HIPK2-induced cell growth arrest

Homeodomain-interacting protein kinase 2 (HIPK2) is a nuclear serine/threonine kinase of the subfamily of dual-specificity Yak1-related kinase proteins. HIPK2 was first described as a homeodomain-interacting protein kinase acting as a corepressor for homeodomain transcription factors. More recently, it was reported that HIPK2 plays a role in p53-mediated cellular apoptosis and could also participate in the regulation of the cell cycle. US11 protein of herpes simplex virus type 1 is a multifunctional protein involved in the regulation of several processes related to the survival of cells submitted to environmental stresses by mechanisms that are not fully elucidated. In an attempt to better understand the multiple functions of US11, we identified cellular binding partners of this protein by using the yeast two-hybrid system. We report that US11 interacts with HIPK2 through the PEST domain of HIPK2 and that this interaction occurs also in human cells. This interaction modifies the subcellular distribution of HIPK2 and protects the cell against the HIPK2-induced cell growth arrest.

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.

Regulation of eIF2alpha phosphorylation by different functions that act during discrete phases in the herpes simplex virus type 1 life cycle

Multiple herpes simplex virus type 1 functions control translation by regulating phosphorylation of the initiation factor eIF2 on its alpha subunit. Both of the two known regulators, the gamma(1)34.5 and Us11 gene products, are produced late in the viral life cycle, although the gamma(1)34.5 gene is expressed prior to the gamma(2) Us11 gene, as gamma(2) genes require viral DNA replication for their expression while gamma(1) genes do not. The gamma(1)34.5 protein, through a GADD34-related domain, binds a cellular phosphatase (PP1alpha), maintaining pools of active, unphosphorylated eIF2. Infection of a variety of cultured cells with a gamma(1)34.5 mutant virus results in the accumulation of phosphorylated eIF2alpha and the inhibition of translation prior to the completion of the viral lytic program. Ectopic, immediate-early Us11 expression prevents eIF2alpha phosphorylation and the inhibition of translation observed in cells infected with a gamma(1)34.5 mutant by inhibiting activation of the cellular kinase PKR and the subsequent phosphorylation of eIF2alpha; however, a requirement for the Us11 protein, produced in its natural context as a gamma(2) polypeptide, remains to be demonstrated. To determine if Us11 regulates late translation, we generated two Us11 null viruses. In cells infected with a Us11 mutant, elevated levels of activated PKR and phosphorylated eIF2alpha were detected, viral translation rates were reduced 6- to 7-fold, and viral replication was reduced 13-fold compared to replication in cells infected with either wild-type virus or a virus in which the Us11 mutation was repaired. This establishes that the Us11 protein is critical for proper late translation rates. Moreover, it demonstrates that the shutoff of protein synthesis observed in cells infected with a gamma(1)34.5 mutant virus, previously ascribed solely to the gamma(1)34.5 mutation, actually results from the combined loss of gamma(1)34.5 and Us11 functions, as the gamma(2) Us11 mRNA is not translated in cells infected with a gamma(1)34.5 mutant.

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.

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.

Bovine herpesvirus 1 U(S) ORF8 protein induces apoptosis in infected cells and facilitates virus egress

The bovine herpesvirus 1 (BHV-1) U(S) ORF8 protein with homology to the Us9 protein of other alphaherpesviruses induces apoptosis in rabbit kidney (RK13) cells without the presence of other BHV-1-encoded proteins. In this article, we have characterized the cytotoxicity and growth behavior of a BHV-1 recombinant, BHV-1/D8, which fails to express the U(S) ORF8 protein in infected cells. BHV-1/D8 exhibited a reduced cytotoxicity to RK13 cells when compared to the cytotoxicity of control BHV-1 strains. In RK13 cells, the onset of apoptosis was not observed during the infection with BHV-1/D8, and the virus multiplication of BHV-1/D8 was markedly greater than that of control viruses. However, virus release of progeny viruses from the infected RK13 cells into culture supernatant was significantly decreased by the loss of the U(S) ORF8 protein. These data demonstrate that the U(S) ORF8 protein activates the apoptotic process and facilitates virus release from the BHV-1-infected cells.

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.

Comparison of the complete DNA sequences of the Oka varicella vaccine and its parental virus

The DNA sequences of the Oka varicella vaccine virus (V-Oka) and its parental virus (P-Oka) were completed. Comparison of the sequences revealed 42 base substitutions, which led to 20 amino acid conversions and length differences in tandem repeat regions (R1, R3, and R4) and in an origin of DNA replication. Amino acid substitutions existed in open reading frames (ORFs) 6, 9A, 10, 21, 31, 39, 50, 52, 55, 59, 62, and 64. Of these, 15 base substitutions, leading to eight amino acid substitutions, were in the gene 62 region alone. Further DNA sequence analysis showed that these substitutions were specific for V-Oka and were not present in nine clinical isolates. The immediate-early gene 62 product (IE62) of P-Oka had stronger transactivational activity than the mutant IE62 contained in V-Oka in 293 and CV-1 cells. An infectious center assay of a plaque-purified clone (S7-01) from the V-Oka with 8 amino acid substitutions in ORF 62 showed smaller plaque formation and less-efficient virus-spreading activity than did P-Oka in human embryonic lung cells. Another clone (S-13) with only five substitutions in ORF 62 spread slightly faster than S7-01 but not as effectively as P-Oka. Moreover, transient luciferase assay in 293 cells showed that transactivational activities of IE62s of S7-01 and S7-13 were lower than that of P-Oka. Based on these results, it appears that amino acid substitutions in ORF 62 are responsible for virus growth and spreading from infected to uninfected cells. Furthermore, the Oka vaccine virus was completely distinguishable from P-Oka and 54 clinical isolates by seven restriction-enzyme fragment length polymorphisms that detected differences in the DNA sequence.

Of the three tegument proteins that package mRNA in herpes simplex virions, one (VP22) transports the mRNA to uninfected cells for expression prior to viral infection

An earlier report has shown that herpes simplex virus 1 virions package RNA. Experiments designed to reveal the identity of the virion proteins capable of binding the RNA and to show whether the mRNA carried in the newly infected cells was expressed showed the following: (i) (32)P-labeled riboprobe generated by in vitro transcription of the U(S)8.5 ORF bound three proteins identified as the products of U(S)11, U(L)47, and U(L)49 (VP22) genes. (ii) Viral RNA was bound to U(L)47 or U(S)11 proteins immune precipitated from cells transduced with baculoviruses expressing U(L)47 or U(S)11 and then superinfected with HSV-1 under conditions that blocked DNA synthesis and assembly of virions. (iii) Virions were purified from cells transduced with a baculovirus encoding a U(S)8.5 protein fused to green fluorescent protein and superinfected with an HSV-1 mutant lacking the U(S)8-12 genes. HEp-2 cells infected with these virions expressed the chimeric protein in approximately 1% of infected cells. (iv) In mixed cultures, untreated Vero cells acquired the mRNA encoding the green fluorescent-U(S)8.5 chimeric protein from HEp-2 cells doubly transduced with the genes encoding VP22 and the chimeric protein. The transfer was RNase sensitive and VP22 dependent, indicating that the RNA encoded by the chimeric gene was transferred to Vero cells as mRNA. We conclude that (i) three virion proteins are capable of binding RNA; (ii) the packaged RNA can be expressed in newly infected cells; and (iii) the U(L)47 protein was earlier reported to shuttle from nucleus to the cytoplasm and may transport RNA. VP22 thus appears to be a member of a new class of viral proteins whose major function is to bind and transport infected cell mRNA to uninfected cells to create the environment for effective initiation of infection.

Identification of two nuclear import signals in the alpha-gene product ICP22 of herpes simplex virus 1

The herpes simplex virus 1 (HSV-1) infected cell protein 22 (ICP22) is a multifunctional viral regulator that localizes in the nucleus of infected cells. ICP22 is required for optimal virus replication in certain cell types and is subject to extensive posttranslational modification. To map the signals in ICP22 which mediate its efficient nuclear localization, we investigated the nuclear import of fusion proteins comprising various fragments of ICP22 fused to green fluorescent protein (GFP) or beta-galactosidase (beta-Gal). These data demonstrated that ICP22 contains two independent regions with nuclear localization signal (NLS) activity. NLS1 maps to ICP22 amino acid position 16-31 and closely resembles the classical bipartite NLS of the type originally identified in nucleoplasmin. In contrast, NLS2 maps to ICP22 amino acid position 118-131 and contains multiple critical basic residues. Furthermore, fusion of both NLSs to chimeric glutathione-S-transferase (GST)-GFP protein and subsequent cytoplasmic microinjection of the respective transport substrates allowed us to monitor nuclear import in real-time. These data demonstrated that both ICP22-derived NLSs mediated efficient nuclear import with identical kinetics, resulting in complete nuclear accumulation of the chimeric transport cargoes at approximately 30 min postinjection. Finally, our data provide new insights into the domain structure of the multifunctional alpha-gene product ICP22 of HSV-1.

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.

Cleavage in and around the DR1 element of the A sequence of herpes simplex virus type 1 relevant to the excision of DNA fragments with length corresponding to one and two units of the A sequence

The A sequence of herpes simplex virus type 1 (HSV-1) is a region bracketed by two direct repeats named DR1. Concatemeric HSV-1 DNA, the product of DNA replication, is cleaved at a specific site on the second DR1 distal from the S component (authentic cleavage) to yield unit-length linear HSV-1 DNA prior to or during packaging of HSV-1 DNA. The presence of two DNA bands, of 0.25 kb (shorter band) and 0.5 kb (longer band), the lengths of which correspond to one and two units of the A sequence, was identified using acrylamide gel electrophoresis of HSV-1 DNA preparations extracted by the method of Hirt. Twelve DNA fragments from each band were molecularly cloned, and nucleotide sequences were determined. Both termini of eight (67%) DNA clones from the shorter band corresponded to the specific cleavage site on DR1. Five (41%) DNA clones from the longer band had a terminus corresponding to the specific cleavage site on DR1 on one side, but not on the opposite side. Thirteen (54%) of 24 termini of 12 analyzed DNA clones from the longer band were in and around DR1. Thus, cleavage events of DR1 can be classified into three categories: (i) authentic cleavage; (ii) site-specific cleavage on the third DR1 distal from the S component (secondary site-specific cleavage), which is related to the generation of the shorter DNA band in combination with authentic cleavage; and (iii) less-specific cleavage events in and around other DR1 elements which relate to the generation of the longer DNA band.

A herpes simplex virus type 1 gamma34.5 second-site suppressor mutant that exhibits enhanced growth in cultured glioblastoma cells is severely attenuated in animals

We describe here the neurovirulence properties of a herpes simplex virus type 1 gamma34.5 second-site suppressor mutant. gamma34.5 mutants are nonneurovirulent in animals and fail to grow in a variety of cultured cells due to a block at the level of protein synthesis. Extragenic suppressors with restored capacity to replicate in cells that normally do not support the growth of the parental gamma34.5 deletion mutant have been isolated. Although the suppressor virus reacquires the ability to grow in nonpermissive cultured cells, it remains severely attenuated in mice and is indistinguishable from the mutant gamma34.5 parent virus at the doses investigated. Repairing the gamma34.5 mutation in the suppressor mutant restores neurovirulence to wild-type levels. These studies illustrate that (i) the protein synthesis and neurovirulence defects observed in gamma34.5 mutant viruses can be genetically separated by an extragenic mutation at another site in the viral chromosome; (ii) the extragenic suppressor mutation does not affect neurovirulence; and (iii) the attenuated gamma34.5 mutant, which replicates poorly in many cell types, can be modified by genetic selection to generate a nonpathogenic variant that regains the ability to grow robustly in a nonpermissive glioblastoma cell line. As this gamma34.5 second-site suppressor variant is attenuated and replicates vigorously in neoplastic cells, it may have potential as a replication-competent, viral antitumor agent.

Varicella-zoster virus (VZV) ORF65 virion protein is dispensable for replication in cell culture and is phosphorylated by casein kinase II, but not by the VZV protein kinases

The unique short region of varicella zoster virus (VZV) encodes four genes. One of these, ORF65, is predicted to encode an 11-kDa protein. Antibody to ORF65 protein immunoprecipitated a 16-kDa protein from the membrane fraction of VZV-infected cells. ORF65 protein was shown to be phosphorylated by casein kinase II. The VZV ORF47 or ORF66 protein kinases were not required for phosphorylation of ORF65. VZV with a large deletion in ORF65 was constructed and was shown to be dispensable for replication of virus in cell culture. The herpes simplex virus homolog of VZV ORF65 has been reported to be located in the nucleus of infected cells and in virions as a tegument protein, whereas the pseudorabies virus homolog is located in the Golgi apparatus of infected cells and in virions as a type II membrane protein. The ORF65 protein localized to the Golgi apparatus in virus-infected cells and was located in virions, most likely as a type II membrane protein. Thus, VZV ORF65 more closely resembles its pseudorabies virus homolog in its localization in infected cells and virions.

Machinery to support genome segment inversion exists in a herpesvirus which does not naturally contain invertible elements

In many herpesviruses, genome segments flanked by inverted repeats invert during DNA replication. It is not known whether this inversion is a consequence of an inherently recombinagenic replicative mechanism common to all herpesviruses or whether the replication enzymes of viruses with invertible segments have specifically evolved additional enzymatic activities to drive inversion. By artificially inserting a fusion of terminal sequences into the genome of a virus which normally lacks invertible elements (murine cytomegalovirus), we created a genome composed of long and short segments flanked by 1,359- and 543-bp inverted repeats. Analysis of genomic DNA from this virus revealed that inversion of both segments generates equimolar amounts of four isomers during the viral propagation necessary to produce DNA for analysis from a single viral particle. We conclude that a herpesvirus which naturally lacks invertible elements is able to support efficient segment inversion. Thus, the potential to invert is probably inherent in the replication machinery of all herpesviruses, irrespective of genome structure, and therefore genomes with invertible elements could have evolved simply by acquisition of inverted repeats and without concomitant evolution of enzymatic activities to mediate inversion. Furthermore, the recombinagenicity of herpesvirus DNA replication must have some importance independent of genome segment inversion.

Identification of a novel expressed open reading frame situated between genes U(L)20 and U(L)21 of the herpes simplex virus 1 genome

An open reading frame (ORF) situated between the U(L)20 and U(L)21 genes encodes a protein designated as U(L)20.5. The U(L)20.5 ORF lies 5' and in the same orientation as the U(L)20 ORF. The expression of the U(L)20.5 ORF was verified by RNase protection assays and by in-frame insertion of an amino acid sequence encoding an epitope of an available monoclonal antibody. The tagged U(L)20.5 protein colocalized in small dense nuclear structures with products of the alpha22/U(S)1.5, U(L)3, and U(L)4 genes. Expression of the U(L)20.5 gene was blocked in cells infected and maintained in the presence of phosphonoacetate, indicating that it belongs to the late, or gamma(2), kinetic class. U(L)20.5 is not essential for viral replication inasmuch as a recombinant virus made by insertion of the thymidine kinase gene into the U(L)20.5 ORF replicates in all cell lines tested [J. D. Baines, P. L. Ward, G. Campadelli-Fiume, and B. Roizman (1991) J. Virol. 65, 6414-6424]. The genomic location of the recently discovered genes illustrates the compact nature of the viral genome.

Herpes simplex virus type 2 glycoprotein G-negative clinical isolates are generated by single frameshift mutations

Herpes simplex virus (HSV) codes for several envelope glycoproteins, including glycoprotein G-2 (gG-2) of HSV type 2 (HSV-2), which are dispensable for replication in cell culture. However, clinical isolates which are deficient in such proteins occur rarely. We describe here five clinical HSV-2 isolates which were found to be unreactive to a panel of anti-gG-2 monoclonal antibodies and therefore considered phenotypically gG-2 negative. These isolates were further examined for expression of the secreted amino-terminal and cell-associated carboxy-terminal portions of gG-2 by immunoblotting and radioimmunoprecipitation. The gG-2 gene was completely inactivated in four isolates, with no expression of the two protein products. For one isolate a normally produced secreted portion and a truncated carboxy-terminal portion of gG-2 were detected in virus-infected cell medium. Sequencing of the complete gG-2 gene identified a single insertion or deletion of guanine or cytosine nucleotides in all five strains, resulting in a premature termination codon. The frameshift mutations were localized within runs of five or more guanine or cytosine nucleotides and were dispersed throughout the gene. For the isolate for which a partially inactivated gG-2 gene was detected, the frameshift mutation was localized upstream of but adjacent to the nucleotides coding for the transmembranous region. Thus, this study demonstrates the existence of clinical HSV-2 isolates which do not express an envelope glycoprotein and identifies the underlying molecular mechanism to be a single frameshift mutation.

Mechanism and application of genetic recombination in herpesviruses

Herpes simplex virus type 1 (HSV-1) is a ubiquitous human pathogen that latently infects sensory ganglia and encodes over 80 genes in a 152 kbp DNA genome. This well characterised virus provides a model for analysing genetic recombination in herpesviruses, a fundamental biological process by which new combinations of genetic materials are generated. The frequency of homologous recombination was estimated to be 0.0048-0.007 (0.48%-0.7%)/kb of the HSV-1 genome, determined using physical markers. The double-strand break repair model, the current model of homologous recombination, adequately explains L-S inversion of herpesvirus genomes and the recombinogenicity of the a sequence. Several herpesvirus genomes, including HSV-1 consist of a unique sequence bracketed by a pair of inverted repeat sequences. This arrangement is attributed to illegitimate recombination between molecules arranged in an inverse orientation. Junctions of unique and repeated sequences that correspond to the crossover site of illegitimate recombination are recombinogenic. Recombination is important for virus evolution, construction of mutated virus, gene therapy and vaccination in which the potential for recombination between engineered input virus and wild type virus has to be considered.

A herpesvirus ribosome-associated, RNA-binding protein confers a growth advantage upon mutants deficient in a GADD34-related function

The herpes simplex virus type 1 gamma34.5 gene product and the cellular GADD34 protein both contain similar domains that can regulate the activity of eukaryotic initiation factor 2 (eIF2), a critical translation initiation factor. Viral mutants that lack the GADD34-related function grow poorly on a variety of malignant human cells, as activation of the cellular PKR kinase leads to the accumulation of inactive, phosphorylated eIF2 at late times postinfection. Termination of translation prior to the completion of the viral reproductive cycle leads to impaired growth. Extragenic suppressors that regain the ability to synthesize proteins efficiently in the absence of the viral GADD34-related function have been isolated. These suppressor alleles are dominant in trans and affect the steady-state accumulation of several viral mRNA species. We demonstrate that deregulated expression of Us11, a virus-encoded RNA-binding, ribosome-associated protein is necessary and sufficient to confer a growth advantage upon viral mutants that lack a GADD34-related function. Ectopic expression of Us11 reduces the accumulation of the activated cellular PKR kinase and allows for sustained protein synthesis. Thus, an RNA-binding, ribosome-associated protein (Us11) and a GADD34-related protein (gamma34.5) both function in a signal pathway that regulates translation by modulating eIF2 phosphorylation.

Herpes simplex virus genome isomerization: origins of adjacent long segments in concatemeric viral DNA

Herpes simplex virus type 1 DNA isomerization was studied by using a viral mutant, 5B8, lacking the unique SpeI site of its parent, SC16. In coinfected cells, SC16 genomic long segments flanked 5B8 genomes in all possible orientations with similar frequencies. Thus, recombination between progeny of different replication templates is sufficient to explain genomic isomerization.

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.

The second-site mutation in the herpes simplex virus recombinants lacking the gamma134.5 genes precludes shutoff of protein synthesis by blocking the phosphorylation of eIF-2alpha

In cells infected with the herpes simplex virus 1 (HSV-1) recombinant R3616 lacking both copies of the gamma134.5 gene, the double-stranded protein kinase R (PKR) is activated, eIF-2alpha is phosphorylated, and protein synthesis is shut off. Although PKR is also activated in cells infected with the wild-type virus, the product of the gamma134.5 gene, infected-cell protein 34.5 (ICP34.5), binds protein phosphatase 1alpha and redirects it to dephosphorylate eIF-2alpha, thus enabling sustained protein synthesis. Serial passage in human cells of a mutant lacking the gamma134.5 gene yields second-site, compensatory mutants lacking various domains of the alpha47 gene situated next to the US11 gene (I. Mohr and Y. Gluzman, EMBO J. 15:4759-4766, 1996). We report the construction of two recombinant viruses: R5103, lacking the gamma134. 5, US8, -9, -10, and -11, and alpha47 (US12) genes; and R5104, derived from R5103 and carrying a chimeric DNA fragment containing the US10 gene and the promoter of the alpha47 gene fused to the coding domain of the US11 gene. R5104 exhibited a protein synthesis profile similar to that of wild-type virus, whereas protein synthesis was shut off in cells infected with R5103 virus. Studies on the wild-type parent and mutant viruses showed the following: (i) PKR was activated in cells infected with parent or mutant virus but not in mock-infected cells, consistent with earlier studies; (ii) lysates of R3616, R5103, and R5104 virus-infected cells lacked the phosphatase activity specific for eIF-2alpha characteristic of wild-type virus-infected cells; and (iii) lysates of R3616 and R5103, which lacked the second-site compensatory mutation, contained an activity which phosphorylated eIF-2alpha in vitro, whereas lysates of mock-infected cells or cells infected with HSV-1(F) or R5104 did not phosphorylate eIF-2alpha. We conclude that in contrast to wild-type virus-infected cells, which preclude the shutoff of protein synthesis by causing rapid dephosphorylation of eIF-2alpha, in cells infected with gamma134.5(-) virus carrying the compensatory mutation, eIF-2alpha is not phosphorylated. The activity made apparent by the second-site mutation may represent a more ancient mechanism evolved to preclude the shutoff of protein synthesis.

UL13 protein kinase of herpes simplex virus 1 complexes with glycoprotein E and mediates the phosphorylation of the viral Fc receptor: glycoproteins E and I

Herpes simplex virus 1 encodes a Fc receptor consisting of glycoproteins E (gE) and I (gI) and two protein kinases specified by UL13 and US3, respectively. We report the following: (i) Antibody to UL13 formed immune complexes containing gE and gI in addition to UL13 protein. Immune complexes formed by monoclonal antibody to gE, but not those formed by monoclonal antibody to gI, also contained the UL13 protein. This association may reflect direct interaction between gE and UL13 inasmuch as IgG in preimmune rabbit serum and an antiserum made against another viral protein which does not react with the UL13 protein directly also bound gE and UL13. (ii) In cells infected with the wild-type virus, gE formed two sharp bands and a diffuse, slower migrating band. The slower sharp band was undetectable, and the diffuse slower migrating forms of gE were diminished in lysates of cells infected with a mutant virus lacking the UL13 gene (DeltaUL13). (iii) Both gE and gI were labeled with 32Pi in cells infected with wild-type or the DeltaUL13 virus, but the labeling was significantly stronger in cells infected with the wild-type virus than in those infected with the DeltaUL13 virus. (iv) In an in vitro protein kinase assay, UL13 immunoprecipitated from cells infected with wild-type virus labeled gE in the presence of [gamma-32P]ATP. This activity was absent in precipitates from cells infected with DeltaUL13 virus. The labeled gE comigrated with the slower, sharp band of gE. (v) gI present in the UL13 immune complex was also phosphorylated in the in vitro kinase assay. (vi) The cytoplasmic domain of gE contains recognition sequences for phosphorylation by casein kinase II (CKII). Exogenous CKII phosphorylated gE in immune complexes from lysates of cells infected with the DeltaUL13 mutant or in immune complexes from lysates of cells infected with wild-type virus that had been heated to inactivate all endogenous kinase activity including that of UL13. In both instances, CKII phosphorylated gE in both the slow and fast migrating sharp bands. We conclude that UL13 physically associates with gE and mediates the phosphorylation of gE and gI. UL13 may also be a determinant in posttranslational processing of gE.

Us9, a stable lysine-less herpes simplex virus 1 protein, is ubiquitinated before packaging into virions and associates with proteasomes

The US9 gene of herpes simplex virus 1 encodes a virion tegument protein with a predicted Mr of 10,000. Earlier studies have shown that the gene is not essential for viral replication in cells in culture. We report that (i) US9 forms in denaturing polyacrylamide gels multiple overlapping bands ranging in Mr from 12,000 to 25,000; (ii) the protein recovered from infected cells or purified virions reacts with anti-ubiquitin antibodies; (iii) autoradiographic images of US9 protein immunoprecipitated from cells infected with [35S]methionine-labeled virus indicate that the protein is stable for at least 4 h after entry into cells (the protein was also stable for at least 4 h after a 1-h labeling interval 12 h after infection); (iv) antibody to subunit 12 of proteasomes pulls down US9 protein from herpes simplex virus-infected cell lysates; and (v) the US9 gene is highly conserved among the members of the alpha subfamily of herpes viruses, and the US9 gene product lacks lysines. We conclude that US9 is a lysine-less, ubiquitinated protein that interacts with the ubiquitin-dependent pathway for degradation of proteins and that this function may be initiated at the time of entry of the virus into the cell.

Mutational analysis of the role of glycoprotein I in varicella-zoster virus replication and its effects on glycoprotein E conformation and trafficking

The contributions of the glycoproteins gI (ORF67) and gE (ORF68) to varicella-zoster virus (VZV) replication were investigated in deletion mutants made by using cosmids with VZV DNA derived from the Oka strain. Deletion of both gI and gE prevented virus replication. Complete deletion of gI or deletions of 60% of the N terminus or 40% of the C terminus of gI resulted in a small plaque phenotype as well as reduced yields of infectious virus. Melanoma cells infected with gI deletion mutants formed abnormal polykaryocytes with a disrupted organization of nuclei. In the absence of intact gI, gE became localized in patches on the cell membrane, as demonstrated by confocal microscopy. A truncated N-terminal form of gI was transported to the cell surface, but its expression did not restore plaque morphology or infectivity. The fusogenic function of gH did not compensate for gI deletion or the associated disruption of the gE-gI complex. These experiments demonstrated that gI was dispensable for VZV replication in vitro, whereas gE appeared to be required. Although VZV gI was dispensable, its deletion or mutation resulted in a significant decrease in infectious virus yields, disrupted syncytium formation, and altered the conformation and distribution of gE in infected cells. Normal cell-to-cell spread and replication kinetics were restored when gI was expressed from a nonnative locus in the VZV genome. The expression of intact gI, the ORF67 gene product, is required for efficient membrane fusion during VZV replication.

Herpes simplex virus DNA replication

The Herpesviridae comprise a large class of animal viruses of considerable public health importance. Of the Herpesviridae, replication of herpes simplex virustype-1 (HSV-1) has been the most extensively studied. The linear 152-kbp HSV-1 genome contains three origins of DNA replication and approximately 75 open-reading frames. Of these frames, seven encode proteins that are required for originspecific DNA replication. These proteins include a processive heterodimeric DNA polymerase, a single-strand DNA-binding protein, a heterotrimeric primosome with 5'-3' DNA helicase and primase activities, and an origin-binding protein with 3'-5' DNA helicase activity. HSV-1 also encodes a set of enzymes involved in nucleotide metabolism that are not required for viral replication in cultured cells. These enzymes include a deoxyuridine triphosphatase, a ribonucleotide reductase, a thymidine kinase, an alkaline endo-exonuclease, and a uracil-DNA glycosylase. Host enzymes, notably DNA polymerase alpha-primase, DNA ligase I, and topoisomerase II, are probably also required. Following circularization of the linear viral genome, DNA replication very likely proceeds in two phases: an initial phase of theta replication, initiated at one or more of the origins, followed by a rolling-circle mode of replication. The latter generates concatemers that are cleaved and packaged into infectious viral particles. The rolling-circle phase of HSV-1 DNA replication has been reconstituted in vitro by a complex containing several of the HSV-1 encoded DNA replication enzymes. Reconstitution of the theta phase has thus far eluded workers in the field and remains a challenge for the future.

The a sequence is dispensable for isomerization of the herpes simplex virus type 1 genome

The herpes simplex virus type 1 (HSV-1) genome consists of two components, L (long) and S (short), that invert relative to each other during productive infection to generate four equimolar isomeric forms of viral DNA. Recent studies have indicated that this genome isomerization is the result of DNA replication-mediated homologous recombination between the large inverted repeat sequences that exist in the genome, rather than site-specific recombination through the terminal repeat a sequences present at the L-S junctions. However, there has never been an unequivocal demonstration of the dispensability of the latter element for this process using a recombinant virus whose genome lacks a sequences at its L-S junctions. This is because the genetic manipulations required to generate such a viral mutant are not possible using simple marker transfer, since the cleavage and encapsidation signals of the a sequence represent essential cis-acting elements which cannot be deleted outright from the viral DNA. To circumvent this problem, a simple two-step strategy was devised by which essential cis-acting sites like the a sequence can be readily deleted from their natural loci in large viral DNA genomes. This method involved initial duplication of the element at a neutral site in the viral DNA and subsequent deletion of the element from its native site. By using this approach, the a sequence at the L-S junction was rendered dispensable for virus replication through the insertion of a second copy into the thymidine kinase (TK) gene of the viral DNA; the original copies at the L-S junctions were then successfully deleted from this virus by conventional marker transfer. The final recombinant virus, HSV-1::L-S(delta)a, was found to be capable of undergoing normal levels of genome isomerization on the basis of the presence of equimolar concentrations of restriction fragments unique to each of the four isomeric forms of the viral DNA. Interestingly, only two of these genomic isomers could be packaged into virions. This restriction was the result of inversion of the L component during isomerization, which prevented two of the four isomers from having the cleavage and encapsidation signals of the a sequence in the TK gene in a packageable orientation. This phenomenon was exploited as a means of directly measuring the kinetics of HSV-1::L-S(delta)a genome isomerization. Following infection with virions containing just the two packaged genomic isomers, all four isomers were readily detected at a stage in infection coincident with the onset of DNA replication, indicating that the loss of the a sequence at the L-S junction had no adverse effect on the frequency of isomerization events in this virus. These results therefore validate the homologous recombination model of HSV-1 genome isomerization by directly demonstrating that the a sequence at the L-S junction is dispensable for this process. The strategy used to remove the a sequence from the HSV-1 genome in this work should be broadly applicable to studies of essential cis-acting elements in other large viral DNA molecules.

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.

A novel herpes simplex virus 1 gene, UL43.5, maps antisense to the UL43 gene and encodes a protein which colocalizes in nuclear structures with capsid proteins

An open reading frame mapping antisense to the UL43 gene of herpes simplex virus 1 encodes a protein with an apparent Mr of 38,000. The protein was detected in wild-type-infected cells with rabbit monospecific polyclonal antibody directed against a fusion protein containing all of the sequences encoded by the open reading frame. The antibody did not react with mutants from which the open reading frame was deleted. Expression of this gene, designated UL43.5, was grossly decreased or abolished in infected cells incubated in medium containing inhibitory concentrations of phosphonoacetic acid, suggesting that it is regulated as a gamma gene. UL43.5 is dispensable in cell culture. UL43.5 protein colocalized with the major capsid protein (infected cell protein 5) and the capsid scaffolding proteins (infected cell protein 35) in nuclear structures situated at the periphery of the nucleus. The predicted amino acid sequence indicates that the UL43.5 protein is a highly hydrophilic protein. The colocalization of UL43.5 protein with capsid proteins in discrete nuclear structures suggests that the former may be involved in assembly of viral particles in an accessory role in cells in culture.

Bovine herpesvirus 1 UL49.5 homolog gene encodes a novel viral envelope protein that forms a disulfide-linked complex with a second virion structural protein

We previously reported that the genome of bovine herpesvirus 1 (BHV-1) contains an open reading frame (ORF) homologous to the herpes simplex virus UL49.5 ORF, and as with the herpes simplex virus UL49.5 ORF, the deduced amino acid sequence of the BHV-1 UL49.5 homolog (UL49.5h) contains features characteristic of an integral membrane protein, implying that it may constitute a functional gene encoding a novel viral envelope protein. This communication reports on the identification of the BHV-1 UL49.5h gene product. By employing an antibody against a synthetic BHV-1 UL49.5h peptide and an UL49.5h gene deletion mutant, the primary product of BHV-UL49.5h gene was identified as a polypeptide with a size of approximately 9 kDa; in both infected cells and isolated virions, the UL49.5h products were found to exist in three forms; monomer, disulfide-linked homodimer, and disulfide-linked heterodimer containing a second viral protein with a size of about 39 kDa. O-Glycosidase digestion and [3H]glucosamine labelling experiments showed that the UL49.5h protein is not glycosylated. Although the deduced amino acid sequence contains putative sites for myristylation and phosphorylation, we were unable to detect either modification. Surface labelling and trypsin digestion protection experiments showed that the BHV-1 UL49.5h protein was present on the surface of infected cells and on the surface of mature virions. Nonionic detergent partition of isolated virions revealed that the UL49.5h protein is more tightly associated with the virion tegument-nucleocapsid structure than envelope protein gD. The results from this study demonstrate that the BHV-1 UL49.5h gene encodes a nonglycosylated virion envelope protein which may associate with virion internal structures by forming a complex with the 39-kDa virion structural protein.

The carboxyl terminus of the murine MyD116 gene substitutes for the corresponding domain of the gamma(1)34.5 gene of herpes simplex virus to preclude the premature shutoff of total protein synthesis in infected human cells

The herpes simplex virus 1 mutants from which both copies of the gamma(1)34.5 gene had been deleted trigger total shutoff of protein synthesis in human neuroblastoma cells and human foreskin fibroblasts but not in African green monkey (Vero) cells. The carboxyl-terminal 64 amino acids of gamma(1)34.5 are homologous to the corresponding domain of MyD116, a murine myeloid differentiation primary responsive gene. The carboxyl-terminal domain of gamma(1)34.5 is required to preclude the shutoff of protein synthesis (J. Chou and B. Roizman, Proc. Natl. Acad. Sci. USA 91:5247-5251, 1994). We report that in-frame substitution of the carboxyl terminus of gamma(1)34.5 with the corresponding domain of MyD116 in the context of the viral genome restored the ability of gamma(1)34.5 to preclude premature shutoff of protein synthesis in both neuroblastoma cells and in human foreskin fibroblasts. The results suggest that (i) in the course of its evolution, the virus "borrowed" a gene fragment to preclude a cell response to infection and (ii) the carboxyl terminus of MyD116 and its family of genes known as GADD34 may have a similar function(s) in cells stressed by growth arrest, DNA damage, and differentiation and in herpes simplex virus infection.

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.

Glycoproteins E and I facilitate neuron-to-neuron spread of herpes simplex virus

Two herpes simplex virus (HSV) glycoproteins E and I (gE and gI) form a heterooligomer which acts as an Fc receptor and also facilitates cell-to-cell spread of virus in epithelial tissues and between certain cultured cells. By contrast, gE-gI is not required for infection of cells by extracellular virus. HSV glycoproteins gD and gJ are encoded by neighboring genes, and gD is required for both virus entry into cells and cell-to-cell spread, whereas gJ has not been shown to influence these processes. Since HSV infects neurons and apparently spreads across synaptic junctions, it was of interest to determine whether gD, gE, gI and gJ are also important for interneuronal transfer of virus. We tested the roles of these glycoproteins in neuron-to-neuron transmission of HSV type 1 (HSV-1) by injecting mutant viruses unable to express these glycoproteins into the vitreous body of the rat eye. The spread of virus infection was measured in neuron-rich layers of the retina and in the major retinorecipient areas of the brain. Wild-type HSV-1 and a gJ- mutant spread rapidly between synaptically linked retinal neurons and efficiently infected major retinorecipient areas of the brain. gD mutants, derived from complementing cells, infected only a few neurons and did not spread in the retina or brain. Mutants unable to express gE or gI were markedly restricted in their ability to spread within the retina, produced 10-fold-less virus in the retina, and spread inefficiently to the brain. Furthermore, when compared with wild-type HSV-1, gE- and gI- mutants spread inefficiently from cell to cell in cultures of neurons derived from rat trigeminal ganglia. Together, our results suggest that the gE-gI heterooligomer is required for efficient neuron-to-neuron transmission through synaptically linked neuronal pathways.

Herpes simplex virus type 1 DNA replication is specifically required for high-frequency homologous recombination between repeated sequences

Using an assay for recombination that measures deletion of a beta-galactosidase gene positioned between two directly repeated 350-bp sequences in plasmids transiently maintained in COS cells, we have found that replication from a simian virus 40 origin produces a high frequency of nonhomologous recombination. In contrast, plasmids replicating from a herpesvirus origin (oris) in COS cells superinfected with herpes simplex virus type 1 (HSV-1) show high levels of homologous recombination between the repeats and an enhanced recombinogenicity of the HSV-1 a sequence that is not seen during simian virus 40 replication. When the same assay was used to study recombination between 120- to 150-bp repeats in uninfected Vero cells, the level of recombination was extremely low or undetectable (< 0.03%), consistent with the fact that these repeats are smaller than the minimal efficient processing sequence for homologous recombination in mammalian cells. Recombination between these short repeats was easily measurable (0.5 to 0.8%) following HSV-1 infection, suggesting that there is an alteration of the recombination machinery. The frequency of recombination between repeats of the Uc-DR1 region, previously identified as the only segment of the HSV-1 a sequence indispensable for enhanced a-sequence recombination, was not significantly higher than that measured for other short sequences.

Mutational analysis of varicella-zoster virus major immediate-early protein IE62

The varicella-zoster virus (VZV) open reading frame 62 encodes an immediate-early protein (IE62) that transactivates expression of various VZV promoters and autoregulates its own expression in transient expression assays. In Vero cells, IE62 was shown to transactivate the expression of all putative immediate-early (IE) and early (E) genes of VZV with an up-regulating effect at low intracellular concentrations. To define the functional domains involved in the regulatory properties of IE62, a large number of in-frame insertions and deletions were introduced into a plasmid-borne copy of the gene encoding IE62. Studies of the regulatory activities of the resultant mutant polypeptides in transient expression assays allowed to delineate protein regions important for repression of its own promoter and for transactivation of a VZV putative immediate-early gene (ORF61) promoter and an early gene (ORF29) promoter. This mutational analysis resulted in the identification of a new functional domain situated at the border between regions 4 and 5 which plays a crucial role in the IE62 regulatory functions. This domain turned out to be very well conserved amongst homologous alphaherpesvirus regulatory proteins and appeared to be rich in bulky hydrophobic and proline residues, similar to the proline-rich region of the CAAT box binding protein CTF-1. By immunofluorescence, a nuclear localization signal has been mapped in region 3.

Transcriptional analyses of the unique short segment of EHV-1 strain Kentucky A

The unique short (Us) segment of the genome of equine herpesvirus type 1 (EHV-1) strain KyA is comprised of six open reading frames (ORFs) that encode: a) a homolog of the Us2 protein of herpes simplex virus type 1 (HSV-1); b) a serine threonine protein kinase that is a homolog of the HSV-1 Us3 protein; c) a homolog of pseudorabies virus glycoprotein gX and HSV-2 gG; d) a novel glycoprotein, EUS4, not encoded by other herpesviruses sequenced to date; e) a homolog of HSV-1 gD; and f) a homolog of HSV-1 Us9. The KyA strain is a deletion mutant that lacks Us sequences encoding gI, gE, and a potential 10 kD polypeptide, and thus may be useful as a parent virus for the generation of live virus vaccines. To complete the elucidation of the transcriptional program of the Us segment, Northern blot hybridization and S1 nuclease analyses were performed on poly(A)(+)-selected RNA isolated from infected cells maintained under early (phosphonoacetic acid-block) and late conditions. The findings revealed that the gene (EUS2 ORF) encoding the protein kinase is expressed as an early 2.9 kb transcript that overlaps and is 3' coterminal with a 1.6 kb early transcript that encodes the gG/gX homolog (EUS3 ORF). Two transcripts of 1.6 kb and 5.8 kb are 5' coterminal and may both encode the novel glycoprotein gene EUS4. The 1.6 kb transcript terminates at a poly(A) signal site downstream of the EUS4 ORF, and the 5.8 kb transcript terminates within the inverted repeat (IR) segment. Overall, the transcriptional program of the EHV-1 KyA Us segment is complex and exhibits similarities to that of HSV-1 Us segment: a) transcripts arise from both DNA strands; b) some transcripts, including those mapping at the termini of the Us segment, extend into the IR segments and are 3' coterminal with the 1.2 kb IR6 transcript; c) at least one transcript reads through a functional polyadenylation signal; d) some transcripts encoding genes that lie in different reading frames exist as a family of overlapping mRNAs, some in an anti-sense manner. Lastly, of the six Us genes of the EHV-1 KyA strain, only those encoding the EHV-1 protein kinase and the HSV-2 gG/gX homolog are members of the early kinetic class.

Characterization of Marek's disease virus insertion and deletion mutants that lack US1 (ICP22 homolog), US10, and/or US2 and neighboring short-component open reading frames

We report the characterization of Marek's disease virus (MDV) strains having mutations in various genes that map to the unique short (US) region of the viral genome. A deletion mutant (GA delta 4.8lac) lacks 4.8 kbp of US region DNA, the deleted segment having been replaced by the lacZ gene of Escherichia coli. This deletion results in the loss of the MDV-encoded US1, US10, and US2 homologs of herpes simplex virus type 1, as well as three putative MDV-specific genes, Sorf1, Sorf2, and Sorf3. Two mutants containing lacZ insertions in the US1 and US10 genes have been constructed, and we have previously reported a US2lac insertion mutant (J. L. Cantello, A. S. Anderson, A. Francesconi, and R. W. Morgan, J. Virol. 65:1584-1588, 1991). The isolation of these mutants indicates that the relevant genes are not required for growth of MDV in chicken embryo fibroblasts. The mutants had early growth kinetics indistinguishable from those of their parent viruses; however, 5 to 7 days after being plated, the US1 insertion mutant (US1lac) and the GA delta 4.8lac deletion mutant showed a 5- to 10-fold decrease in virus growth. This decrease in virus accumulation correlated with a 30 to 50% decrease in plaquing efficiency when these viruses were plated onto established versus fresh chicken embryo fibroblast monolayers compared with a 10 to 15% decrease seen for the parent viruses and for the US10lac or US2lac insertion mutants. Finally, GA delta 4.8lac could be reisolated from chickens, indicating that the deleted genes are not required for the infection of chickens following intra-abdominal inoculation of an attenuated serotype 1 MDV.

Localization and putative function of the UL20 membrane protein in cells infected with herpes simplex virus 1

The UL20 protein of herpes simplex virus 1, an intrinsic membrane protein, is required in infected Vero cells in which the Golgi apparatus is fragmented for the transport of virions from the space between the inner and outer nuclear membranes and for the transport of fully processed cell membrane-associated glycoproteins from the trans-Golgi to the plasma membrane. It is not required in the human 143TK- cell line, in which the Golgi apparatus remains intact. We report the following. (i) The UL20 protein was detected in infected cells beginning at 6 h postinfection and was regulated as a gamma 1 gene. (ii) Pulse-chase experiments revealed no detectable alteration in the mobility of the UL20 protein in polyacrylamide gels. (iii) In both infected Vero and infected 143TK- cells, the UL20 protein was detected by immunofluorescence in association with nuclear membranes and in the cytoplasm. Some of the cytoplasmic fluorescence colocalized with beta-COP, a protein associated with Golgi-derived transport vesicles. UL20 protein was present in virions purified from the extracellular space but could not be detected in the plasma membrane. These results are consistent with the hypothesis that UL20 is a component of virion envelopes and membranes of virion transport vesicles and is selectively retained from the latter in a Golgi compartment.

Glycoprotein E of pseudorabies virus and homologous proteins in other alphaherpesvirinae

This paper reviews biological properties of glycoprotein E (gE) of pseudorabies virus (Aujeszky's disease virus) and homologous proteins in other alphaherpesvirinae. It focuses on the gene encoding gE, conserved regions in the gE protein and its homologs, the complex of gE and gI, biological functions of gE in vitro and in vivo, the role of gE in latency and the role of gE in the induction of humoral and cellular immune responses. Special emphasis is placed on the use of gE as a marker protein in the control and eradication of pseudorabies virus.

Construction of recombinant Marek's disease virus type 1 (MDV1) expressing the Escherichia coli lacZ gene as a possible live vaccine vector: the US10 gene of MDV1 as a stable insertion site

This paper describes the construction of a recombinant Marek's disease virus serotype 1 (MDV1) in which the Escherichia coli lacZ gene was inserted into the open reading frame homologous to the US10 gene of herpes simplex virus 1 (HSV1). The recombinant virus replicated as well in cell culture as the parental MDV1 K-554 strain. Chickens immunized with the virus were protected against challenge with virulent MDV1, and produced a high level of antibodies against beta-galactosidase as well as against MDV1 antigens. The antibody titres persisted for at least 16 weeks. These results demonstrate that the US10 gene of MDV1 is an effective site for the insertion of foreign genes from which to construct a polyvalent live vaccine for poultry.

Herpes simplex virus type 1 recombination: the Uc-DR1 region is required for high-level a-sequence-mediated recombination

The a sequences of herpes simplex virus type 1 are believed to be the cis sites for inversion events that generate four isomeric forms of the viral genome. Using an assay that measures deletion of a beta-galactosidase gene positioned between two directly repeated sequences in plasmids transiently maintained in Vero cells, we had found that the a sequence is more recombinogenic than another sequence of similar size. To investigate the basis for the enhanced recombination mediated by the a sequence, we examined plasmids containing direct repeats of approximately 350 bp from a variety of sources and with a wide range of G+C content. We observed that all of these plasmids show similar recombination frequencies (3 to 4%) in herpes simplex virus type 1-infected cells. However, recombination between directly repeated a sequences occurs at twice this frequency (6 to 10%). In addition, we find that insertion of a cleavage site for an a-sequence-specific endonuclease into the repeated sequences does not appreciably increase the frequency of recombination, indicating that the presence of endonuclease cleavage sites within the a sequence does not account for its recombinogenicity. Finally, by replacing segments of the a sequence with DNA fragments of similar length, we have determined that only the 95-bp Uc-DR1 segment is indispensable for high-level a-sequence-mediated recombination.

The UL21 gene products of herpes simplex virus 1 are dispensable for growth in cultured cells

A viral deletion mutant (delta UL21) that lacked the sequences encoding 484 of the predicted first 535 amino acids of the UL21 open reading frame was genetically engineered and studied with respect to its phenotype in cells in culture. We report the following. (i) The replication of delta UL21 was identical to that of the parent herpes simplex virus 1 (HSV-1) strain F in Vero cells, but the yields were three- to fivefold lower than those of the parent virus in human embryonic lung cells. (ii) To characterize the UL21 protein, we immunized rabbits against a purified bacterial fusion protein consisting of glutathione S-transferase fused to the majority of the coding domain of the UL21 gene. Rabbit antiserum directed against the fusion protein recognized a broad band with an apparent M(r) of 62,000 to 64,000 in lysates of cells infected with HSV-1 strain F and in virions purified from the infected cell cytoplasm. This band was absent from lysates of mock-infected cells or cells infected with the delta UL21 virus. The band was significantly reduced in intensity in lysates of cells infected in the presence of phosphonoacetic acid, indicating that it is expressed as a late (gamma 1) gene. (iii) Immunofluorescence studies localized the UL21 antigen primarily in brightly staining granules in the cytoplasms of infected cells. Taken together, the data indicate that the UL21 protein is a virion component dispensable for all aspects of replication of HSV-1 in the cells tested. The electrophoretic mobility of the UL21 protein suggests that it is extensively modified posttranslationally.