Herpes simplex virus specifies two subunits of ribonucleotide reductase encoded by 3'-coterminal transcripts

Manipulation of Host Cell Death Pathways by Herpes Simplex Virus

Herpes simplex virus (HSV)-1 and HSV-2 are ubiquitous human pathogens that infect keratinized epithelial surfaces and establish lifelong latent infection in sensory neurons of the peripheral nervous system. HSV-1 causes oral cold sores, and HSV-2 causes genital lesions characterized by recurrence at the site of the initial infection. In multicellular organisms, cell death plays a pivotal role in host defense by eliminating pathogen-infected cells. Apoptosis and necrosis are readily distinguished types of cell death. Apoptosis, the main form of programmed cell death, depends on the activity of certain caspases, a family of cysteine proteases. Necroptosis, a regulated form of necrosis that is unleashed when caspase activity is compromised, requires the activation of receptor-interacting protein (RIP) kinase 3 (RIPK3) through its interaction with other RIP homotypic interaction motif (RHIM)-containing proteins such as RIPK1. To ensure lifelong infection in the host, HSV carries out sophisticated molecular strategies to evade host cell death responses during viral infection. HSV-1 is a well-characterized pathogen that encodes potent viral inhibitors that modulate both caspase activation in the apoptosis pathway and RIPK3 activation in the necroptosis pathway in a dramatic, species-specific fashion. The viral UL39-encoded viral protein ICP6, the large subunit of the virus-encoded ribonucleotide reductase, functions as a suppressor of both caspase-8 and RHIM-dependent RIPK3 activities in the natural human host. In contrast, ICP6 RHIM-mediated recruitment of RIPK3 in the nonnatural mouse host drives the direct activation of necroptosis. This chapter provides an overview of the current state of the knowledge on molecular interactions between HSV-1 viral proteins and host cell death pathways and highlights how HSV-1 manipulates cell death signals for the benefit of viral propagation.

Effector-triggered immunity in mammalian antiviral defense

Effector-triggered immunity (ETI) is a common defense strategy used by mammalian host cells that is engaged upon detection of the enzymatic activities of pathogen-encoded proteins or the effects of their expression on cellular homeostasis. However, in contrast to the effector-triggered responses engaged upon bacterial infection, much less is understood about the activation and consequences of these responses following viral infection. Several recent studies have identified novel mechanisms by which viruses engage ETI, highlighting the importance of these immune responses in antiviral defense. We summarize recent advances in understanding how mammalian cells sense virus-encoded effector proteins, the downstream signaling pathways that are triggered by these sensing events, and how viruses manipulate these pathways to become more successful pathogens.

Programmed cell death: the battlefield between the host and alpha-herpesviruses and a potential avenue for cancer treatment

Programed cell death is an antiviral mechanism by which the host limits viral replication and protects uninfected cells. Many viruses encode proteins resistant to programed cell death to escape the host immune defenses, which indicates that programed cell death is more favorable for the host immune defense. Alpha-herpesviruses are pathogens that widely affect the health of humans and animals in different communities worldwide. Alpha-herpesviruses can induce apoptosis, autophagy and necroptosis through different molecular mechanisms. This review concisely illustrates the different pathways of apoptosis, autophagy, and necroptosis induced by alpha-herpesviruses. These pathways influence viral infection and replication and are a potential avenue for cancer treatment. This review will increase our understanding of the role of programed cell death in the host immune defense and provides new possibilities for cancer treatment.

Herpes Simplex Virus 1 Mutant with Point Mutations in UL39 Is Impaired for Acute Viral Replication in Mice, Establishment of Latency, and Explant-Induced Reactivation

In the process of generating herpes simplex virus 1 (HSV-1) mutations in the viral regulatory gene encoding infected cell protein 0 (ICP0), we isolated a viral mutant, termed KOS-NA, that was severely impaired for acute replication in the eyes and trigeminal ganglia (TG) of mice, defective in establishing a latent infection, and reactivated poorly from explanted TG. To identify the secondary mutation(s) responsible for the impaired phenotypes of this mutant, we sequenced the KOS-NA genome and noted that it contained two nonsynonymous mutations in UL39, which encodes the large subunit of ribonucleotide reductase, ICP6. These mutations resulted in lysine-to-proline (residue 393) and arginine-to-histidine (residue 950) substitutions in ICP6. To determine whether alteration of these amino acids was responsible for the KOS-NA phenotypes in vivo, we recombined the wild-type UL39 gene into the KOS-NA genome and rescued its acute replication phenotypes in mice. To further establish the role of UL39 in KOS-NA's decreased pathogenicity, the UL39 mutations were recombined into HSV-1 (generating UL39^mut), and this mutant virus showed reduced ocular and TG replication in mice comparable to that of KOS-NA. Interestingly, ICP6 protein levels were reduced in KOS-NA-infected cells relative to the wild-type protein. Moreover, we observed that KOS-NA does not counteract caspase 8-induced apoptosis, unlike wild-type strain KOS. Based on alignment studies with other HSV-1 ICP6 homologs, our data suggest that amino acid 950 of ICP6 likely plays an important role in ICP6 accumulation and inhibition of apoptosis, consequently impairing HSV-1 pathogenesis in a mouse model of HSV-1 infection.IMPORTANCE HSV-1 is a major human pathogen that infects ∼80% of the human population and can be life threatening to infected neonates or immunocompromised individuals. Effective therapies for treatment of recurrent HSV-1 infections are limited, which emphasizes a critical need to understand in greater detail the events that modulate HSV-1 replication and pathogenesis. In the current study, we identified a neuroattenuated HSV-1 mutant (i.e., KOS-NA) that contains novel mutations in the UL39 gene, which codes for the large subunit of ribonucleotide reductase (also known as ICP6). This mutant form of ICP6 was responsible for the attenuation of KOS-NA in vivo and resulted in diminished ICP6 protein levels and antiapoptotic effect. Thus, we have determined that subtle alteration of the UL39 gene regulates expression and functions of ICP6 and severely impacts HSV-1 pathogenesis, potentially making KOS-NA a promising vaccine candidate against HSV-1.

The interplay between human herpes simplex virus infection and the apoptosis and necroptosis cell death pathways

Human herpes simplex virus (HSV) is a ubiquitous human pathogen that establishes a lifelong latent infection and is associated with mucocutaneous lesions. In multicellular organisms, cell death is a crucial host defense mechanism that eliminates pathogen-infected cells. Apoptosis is a well-defined form of programmed cell death executed by a group of cysteine proteases, called caspases. Studies have shown that HSV has evolved strategies to counteract caspase activation and apoptosis by encoding anti-apoptotic viral proteins such as gD, gJ, Us3, LAT, and the ribonucleotide reductase large subunit (R1). Recently, necroptosis has been identified as a regulated form of necrosis that can be invoked in the absence of caspase activity. Receptor-interacting kinase 3 (RIP3 or RIPK3) has emerged as a central signaling molecule in necroptosis; it is activated via interaction with other RIP homotypic interaction motif (RHIM)-containing proteins such as RIP1 (or RIPK1). There is increasing evidence that HSV R1 manipulates necroptosis via the RHIM-dependent inactivation or activation ofRIP3 in a species-specific manner. This review summarizes the current understanding of the interplay between HSV infection and cell death pathways, with an emphasis on apoptosis and necroptosis.

Identification and in vitro characterization of a Marek's disease virus-encoded Ribonucleotide reductase

Marek's disease virus (MDV) encodes a ribonucleotide reductase (RR), a key regulatory enzyme in the DNA synthesis pathway. The gene coding for the RR of MDV is located in the unique long (UL) region of the genome. The large subunit is encoded by UL39 (RR1) and is predicted to comprise 860 amino acids whereas the small subunit encoded by UL40 (RR2) is predicted to be 343 amino acids long. Immunoprecipitation analysis of MDV-1 (GA strain)-infected cells with T81, a monoclonal antibody specific for RR of MDV, identified two major proteins of 90,000 and 40,000 daltons, corresponding to RR1 and RR2, respectively. In addition, RR was abundantly expressed in the cytoplasm of cells infected with 51 strains of MDV belonging to MDV serotypes 1, 2, and 3 as demonstrated by immunofluorescence staining. Northern blot analysis of RNA extracted from MDV-infected cells showed a major band of around 4.4 kb in size corresponding to the RR1 and RR2 transcripts. In vivo, RR was abundantly expressed in lymphoid organs and feather follicle epithelium of MDV-infected chickens during early cytolytic infection, as determined by immunohistochemistry. There was, however, no expression of RR in MDV-induced tumors in lymphoid organs. The abundant expression of RR in MDV-infected chicken may suggest an important role of RR in the conversion of ribonucleotides to deoxyribonucleotides for MDV DNA synthesis.

The ribonucleotide reductase R1 subunits of herpes simplex virus types 1 and 2 protect cells against TNFα- and FasL-induced apoptosis by interacting with caspase-8

We previously reported that HSV-2 R1, the R1 subunit (ICP10; UL39) of herpes simplex virus type-2 ribonucleotide reductase, protects cells against apoptosis induced by the death receptor (DR) ligands tumor necrosis factor-alpha- (TNFα) and Fas ligand (FasL) by interrupting DR-mediated signaling at, or upstream of, caspase-8 activation. Further investigation of the molecular mechanism underlying HSV-2 R1 protection showed that extracellular-regulated kinase 1/2 (ERK1/2), phosphatidylinositol 3-kinase (PI3-K)/Akt, NF-κB and JNK survival pathways do not play a major role in this antiapoptotic function. Interaction studies revealed that HSV-2 R1 interacted constitutively with caspase-8. The HSV-2 R1 deletion mutant R1(1-834)-GFP and Epstein-Barr virus (EBV) R1, which did not protect against apoptosis induced by DR ligands, did not interact with caspase-8, indicating that interaction is required for protection. HSV-2 R1 impaired caspase-8 activation induced by caspase-8 over-expression, suggesting that interaction between the two proteins prevents caspase-8 dimerization/activation. HSV-2 R1 bound to caspase-8 directly through its prodomain but did not interact with either its caspase domain or Fas-associated death domain protein (FADD). Interaction between HSV-2 R1 and caspase-8 disrupted FADD-caspase-8 binding. We further demonstrated that individually expressed HSV-1 R1 (ICP6) shares, with HSV-2 R1, the ability to bind caspase-8 and to protect cells against DR-induced apoptosis. Finally, as the long-lived Fas protein remained stable during the early period of infection, experiments with the HSV-1 UL39 deletion mutant ICP6∆ showed that HSV-1 R1 could be essential for the protection of HSV-1-infected cells against FasL.

The role of protein kinase activity expressed by the UL13 gene of herpes simplex virus 1: the activity is not essential for optimal expression of UL41 and ICP0

Herpes simplex virus 1 (HSV-1) UL13 is a viral protein kinase that is packaged into virions and regulates optimal expression of ICP0 and a subset of late (gamma) proteins, including UL41 in infected cells. In the present study, we investigated the role(s) of the protein kinase activity of UL13 in viral replication using a recombinant virus expressing enzymatically inactive UL13 after an amino acid substitution in the invariant lysine of UL13. The recombinant virus carrying this mutation formed smaller plaques yielded 10-fold less progeny than wild-type virus but could not be differentiated from wild-type virus with respect to accumulation of UL41 and ICP0 in infected cells. These results indicate that the protein kinase activity of UL13 plays a role in viral replication in cell culture, but the activity is not essential for the optimal expression of UL41 and ICP0.

The effects of interferon-alpha and acyclovir on herpes simplex virus type-1 ribonucleotide reductase

Herpes simplex virus-type 1 (HSV-1) encodes both the small (UL40) and large (UL39) subunits of the enzyme, ribonucleotide reductase. Treatment of HSV-1-infected cells with interferon-alpha (IFN-alpha) reduced the levels of both enzyme subunits. Reduced steady state levels of the large subunit were demonstrated by immunoblot using polyclonal antibody specific for the viral enzyme. Reduction in the amount of small subunit was shown by a reduction in the electron spin resonance signal derived from the iron-containing tyrosyl free-radical present in this subunit. Treatment of cells with 100 IU/ml of IFN-alpha decreased levels of both subunits resulting in a reduction in enzyme activity as measured by conversion of CDP to dCDP. The decrease in the amount of the large subunit was not due to a reduction in the level of its mRNA. The combination of IFN-alpha and ACV treatment of human cornea stromal cells did not result in a further reduction in amounts of ribonucleotide reductase relative to that detected with IFN-alpha alone. The IFN-alpha-induced reduction in ribonucleotide reductase activity is the likely cause of decreased levels of dGTP which we have previously demonstrated in IFN-alpha-treated, infected cells.

Sequences of the ribonucleotide reductase-encoding genes of felid herpesvirus 1 and molecular phylogenetic analysis

The felid herpesvirus 1 (FHV-1) genes encoding the two ribonucleotide reductase (RR) subunits (RR1, large subunit and RR2, small subunit) were cloned and their nucleotide (nt) sequence determined. The RR1 open reading frame (ORF) is 2358 nts long and is predicted to encode a protein of 786 amino acids (aa). In common with herpesviruses in the Varicellovirus genus of the alphaherpesvirus subfamily, FHV-1 RR1 lacks the N-terminal serine threonine protein kinase region present in herpes simplex virus (HSV)-1 and -2. FHV-1 RR1 has a predicted aa identity of 47-64% with other alphaherpesvirus RR1 peptides, falling to 26-29% for gammaherpesviruses. The RR2 ORF is 996 nts long, predicted to encode a protein of 332 aa and has aa identities of 64-70% with alphaherpesviruses and 38-39% with gammaherpesviruses. Molecular phylogenetic analysis groups FHV-1 with equid herpesviruses 1 and 4 (EHV 1 and 4), pseudorabies virus (PRV) and bovid herpesvirus 1 (BHV 1) within the genus Varicellovirus.

The genome sequence of herpes simplex virus type 2

The genomic DNA sequence of herpes simplex virus type 2 (HSV-2) strain HG52 was determined as 154,746 bp with a G+C content of 70.4%. A total of 74 genes encoding distinct proteins was identified; three of these were each present in two copies, within major repeat elements of the genome. The HSV-2 gene set corresponds closely with that of HSV-1, and the HSV-2 sequence prompted several local revisions to the published HSV-1 sequence (D. J. McGeoch, M. A. Dalrymple, A. J. Davison, A. Dolan, M. C. Frame, D. McNab, L. J. Perry, J. E. Scott, and P. Taylor, J. Gen. Virol. 69:1531-1574, 1988). No compelling evidence for the existence of any additional protein-coding genes in HSV-2 was identified.

The R1 subunit of herpes simplex virus ribonucleotide reductase is a good substrate for host cell protein kinases but is not itself a protein kinase

The N terminus of the R1 subunit of herpes simplex virus type 2 ribonucleotide reductase is believed to be a protein kinase domain mainly because the R1 protein was phosphorylated in a protein kinase assay on blot. Using Escherichia coli and adenovirus expression vectors to produce R1, we found that, whereas the reductase activity of both recombinant proteins was similar, efficient phosphorylation of R1 and casein in the presence of Mg2+ was obtained only with the R1 purified from eukaryotic cells. Phosphorylation of this R1, in solution or on blot, results mainly from the activity of casein kinase II (CKII), a co-purifying protein kinase. Labeling on blot occurs from CKII leakage off the membrane and its subsequent high affinity binding to in vivo CKII-phosphorylated R1. CKII target sites were mapped to an acidic serine-rich segment of the R1 N terminus. Improvement in purification of the R1 expressed in eukaryotic cells nearly completely abolished its phosphorylation potential. An extremely low level of phosphorylation observed in the presence of Mn2+ with the R1 produced in E. coli was probably due to an unidentified prokaryotic protein kinase. These results provide evidence that the herpes simplex virus type 2 R1 does not possess an intrinsic protein kinase activity.

AP-1 cis-response elements are involved in basal expression and Vmw110 transactivation of the large subunit of herpes simplex virus type 2 ribonucleotide reductase (ICP10)

The promoter of the large subunit of herpes simplex virus type 2 ribonucleotide reductase (ICP10) has two AP-1 cis-response elements, respectively located at positions -62 and -94 relative to the transcription start site (Wymer et al., 1989. J. Virol. 63, 2773-2784). Chloramphenicol acetyl transferase (CAT) analysis with hybrid constructions of the CAT structural gene and the ICP10 promoter or its mutants and gel retardation studies were used to examine the role of the AP-1 cis-response elements in expression from the ICP10 promoter. Basal expression from the wild-type promoter was significantly (75-90%) reduced by mutation of the upstream or downstream AP-1 element. Mutation in the upstream AP-1 element also caused a 60% reduction in c-Jun-mediated activation. Activation was decreased 40% by mutation in the downstream AP-1 element and it was abrogated by mutation of both elements. Similar results were obtained for ACT-deleted mutants and mutants in which CT was mutated to AG. The trans-activation by Vmw110 was also reduced by mutation of the AP-1 elements (10- and 2-fold for the upstream and downstream element, respectively) and it was abrogated by mutation of both AP-1 elements. Mutation of nucleotides adjacent to the AP-1 cis-response elements had no effect on trans-activation. Gel retardation assays with a DNA probe representing the wild-type ICP10 promoter and nuclear extracts from HSV-1-infected cells identified one complex that was not seen with mock-infected cells or with cells infected with a Vmw110-deleted mutant. The complex was not seen when HSV-1-infected cells were reacted with an AP-1-mutant DNA probe, and its formation was competed by an AP-1 but not a mutant AP-1 oligonucleotide. The migration of this complex was retarded by c-Fos antibody, suggesting that both AP-1 and Vmw110 are involved in its formation. A mutant deleted in all sequences upstream of the TATA box was also activated by Vmw110, but this activation was only 2-fold lower than that seen for the wild type and significantly higher (10-fold) than that seen for the double AP-1 mutants. The data suggest that AP-1 elements play a crucial role in ICP10 gene expression/activation.

ATP and SH3 binding sites in the protein kinase of the large subunit of herpes simplex virus type 2 of ribonucleotide reductase (ICP10)

The large subunit of herpes simplex virus type 2 ribonucleotide reductase (ICP10) is a multifunctional protein. It consists of a ribonucleotide reductase and a serine/threonine protein kinase (PK) domain, which has three proline-rich motifs consistent with SH3-binding sites at positions 140, 149, and 396. We used site-directed mutagenesis to identify amino acids required for kinase activity and interaction with signaling proteins. Mutation of Lys176 or Lys259 reduced PK activity (5-8-fold) and binding of the 14C-labeled ATP analog rho-fluorosulfonylbenzoyl 5'-adenosine (FSBA) but did not abrogate them. Enzymatic activity and FSBA binding were abrogated by mutation of both Lys residues, suggesting that either one can bind ATP. Mutation of Glu209 (PK catalytic motif III) virtually abrogated kinase activity in the presence of Mg2+ or Mn2+ ions, suggesting that Glu209 functions in ion-dependent PK activity. ICP10 bound the adaptor protein Grb2 in vitro. Mutation of the ICP10 proline-rich motifs at positions 396 and 149 reduced Grb2 binding 20- and 2-fold, respectively. Binding was abrogated by mutation of both motifs. Grb2 binding to wild type ICP10 was competed by a peptide for the Grb2 C-terminal SH3 motif, indicating that it involves the Grb2 C-terminal SH3.

Cervical cancer: is herpes simplex virus type II a cofactor?

In many ways, cervical cancer behaves as a sexually transmitted disease. The major risk factors are multiple sexual partners and early onset of sexual activity. Although high-risk types of human papillomaviruses (HPV) play an important role in the development of nearly all cases of cervical cancer, other sexually transmitted infectious agents may be cofactors. Herpes simplex virus type 2 (HSV-2) is transmitted primarily by sexual contact and therefore has been implicated as a risk factor. Several independent studies suggest that HSV-2 infections correlate with a higher than normal incidence of cervical cancer. In contrast, other epidemiological studies have concluded that infection with HSV-2 is not a major risk factor. Two separate transforming domains have been identified within the HSV-2 genome, but continued viral gene expression apparently is not necessary for neoplastic transformation. HSV infections lead to unscheduled cellular DNA synthesis, chromosomal amplifications, and mutations. These observations suggest that HSV-2 is not a typical DNA tumor virus. It is hypothesized that persistent or abortive infections induce permanent genetic alterations that interfere with differentiation of cervical epithelium and subsequently induce abnormal proliferation. Thus, HSV-2 may be a cofactor in some but not all cases of cervical cancer.

Fas-activated serine/threonine kinase (FAST) phosphorylates TIA-1 during Fas-mediated apoptosis

We have identified a serine/threonine kinase that is rapidly activated during Fas-mediated apoptosis. Fas-activated serine/threonine kinase (FAST) is phosphorylated on serine and threonine residues in Jurkat cells. In response to Fas ligation, it is rapidly dephosphorylated and concomitantly activated to phosphorylate TIA-1, a nuclear RNA-binding protein that has been implicated as an effector of apoptosis. Phosphorylation of TIA-1 precedes the onset of DNA fragmentation, suggesting a role in signaling downstream events in the apoptotic program. Our results introduce Fast and TIA-1 as components of a molecular cascade involved in signaling Fas-mediated apoptosis.

Characterization of the novel protein kinase activity present in the R1 subunit of herpes simplex virus ribonucleotide reductase

We have compared the protein kinase activities of the R1 subunits from herpes simplex virus types 1 (HSV-1) and 2 (HSV-2) ribonucleotide reductase following expression in Escherichia coli. Autophosphorylation activity was observed when kinase assays were performed with immunoprecipitated R1 or proteins purified to homogeneity, and the activity was stimulated by the basic protein protamine. Transphosphorylation of histones or calmodulin by purified or immunoprecipitated HSV-1 and HSV-2 R1 was not observed, and our results suggest that the activities of these two proteins are similar. We further characterized the protein kinase activity of HSV-1 R1 by producing insertion and deletion mutants constructed with a plasmid expressing R1 amino acids 1 to 449. C-terminal deletion analysis identified the catalytic core of the enzyme as comprising residues 1 to 292, and this polypeptide will be useful for structural determinations by X-ray crystallography. Insertion of a 4-amino-acid sequence at sites within the protein kinase domain identified regions essential for activity; insertions at residues 22 and 112 completely inactivated activity, and an insertion at residue 136 reduced activity sixfold. Similar insertions at residues 257, 262, 292, and 343 had no effect on activity. The ATP analog 5'-fluorosulfonylbenzoyladenosine, which covalently modifies conventional eukaryotic kinases at an essential lysine residue within the active site, did label HSV R1, but this labelling occurred outside the N-terminal domain. These data indicate that the HSV R1 kinase is novel and distinct from other eukaryotic protein kinases.

Polymorphism within the herpes simplex virus (HSV) ribonucleotide reductase large subunit (ICP6) confers type specificity for recognition by HSV type 1-specific cytotoxic T lymphocytes

A panel of herpes simplex virus type 1 (HSV-1)-specific, CD8+, major histocompatibility complex class I (H-2Kb)-restricted cytotoxic T-lymphocyte (CTL) clones was derived from HSV-1-immunized C57BL/6 (H-2b) mice in order to identify the HSV-1 CTL recognition epitope(s) which confers type specificity. HSV-1 x HSV-2 intertypic recombinants were used to narrow the region encoding potential CTL recognition epitopes to within 0.51 to 0.58 map units of the HSV-1 genome. Using an inhibitor of viral DNA synthesis and an ICP6 deletion mutant, the large subunit of ribonucleotide reductase (ICP6, RR1) was identified as a target protein for these type-specific CTL. Potential CTL recognition epitopes within RR1 were located on the basis of the peptide motif predicted to bind to the MHC class I H-2Kb molecule. A peptide corresponding to residues 822 to 829 of RR1 was shown to confer susceptibility on H-2Kb-expressing target cells to lysis by the type 1-specific CTL. On the basis of a comparison of the HSV-1 RR1 epitope (residues 822 to 829) with the homologous sequence of HSV-2 RR1 (residues 828 to 836) and by the use of amino acid substitutions within synthetic peptides, we identified HSV-1 residue 828 as being largely responsible for the type specificity exhibited by HSV-1-specific CTL. This HSV-1 RR1 epitope, when expressed in recombinant simian virus 40 large T antigen in primary C57BL/6 cells, was recognized by the HSV-1 RR1-specific CTL clones. These results indicate that an early HSV protein with enzymatic activity provides a target for HSV-specific CTL and that type specificity is dictated largely by a single amino acid.

The herpes simplex virus type 2 gene which encodes the large subunit of ribonucleotide reductase has unusual regulatory properties

Expression of herpes simplex virus type 2 (HSV-2) encoded ribonucleotide reductase (RR) is required for growth of the virus in non-dividing cells. The functional enzyme is composed of a large (RRA) and small (RRB) subunit and the enzyme is expressed as a delayed early activity. The promoter of RRA contains a cis-acting motif (TAATGARAT) which resembles those found in immediate early (IE) genes suggesting RRA is an IE gene. When primate cells were infected with HSV-2, low levels of RRA transcripts were expressed in the presence of cycloheximide indicating RRA is not a true IE gene. Conditions which allow for efficient RRA RNA expression in the presence of cycloheximide were identified in human cells. A phorbol ester, 12-O-tetradecanoyl phorbol-13- acetate (TPA), and hydroxyurea increased the level of RRA RNA expression in the presence of cycloheximide. Hydroxyurea and TPA also stimulated RRA promoter activity in transient assays suggesting these agents induced factors which transactivated the RRA promoter. Expression of an intact c-myc gene transactivated the RRA promoter more than 30-fold in transient assays. Although expression of an intact retinoblastoma gene (Rb) had a slight stimulatory effect on the RRA promoter, mutant Rb proteins also stimulated the RRA promoter. These studies demonstrated that inducible factors in permissive cells increase the steady state levels of RRA RNA in the presence of cycloheximide.

The RR1 gene of herpes simplex virus type 1 is uniquely trans activated by ICP0 during infection

As has been demonstrated for herpes simplex virus type 2, we show in this report that the herpes simplex virus type 1 ribonucleotide reductase large subunit (RR1) gene is trans activated in transient transfection assays by VP16 and ICP0 but not by ICP4. Deletion analysis demonstrated that responsiveness to induction to VP16 resides in an octamer/TAATGARAT sequence of the RR1 promoter and that the TATA box alone is sufficient to provide induction by ICP0. The induction of the RR1 gene by ICP0 but not by ICP4 suggested that it might be possible to identify the cis-acting element(s) responsive to ICP4 in an ICP4-inducible promoter. To this end, a series of chimeric promoters containing various portions of the regulatory sequences of the RR1 promoter and thymidine kinase (TK) promoter were constructed. The TK promoter is trans activated by both ICP0 and ICP4 in transient transfection assays and by ICP4 in infection. The data show that replacing the RR1 TATA region with the TK TATA region permits ICP4 inducibility even if the rest of the RR1 promoter elements remain intact. To test whether the RR1 gene is induced by ICP0 during infection, four mutant viruses were constructed. (i) TAATGARAT+ has the wild-type RR1 promoter driving chloramphenicol acetyltransferase (CAT) and the RR2 promoter driving the lacZ gene. The RR2 gene codes for the small subunit of the ribonucleotide reductase and is expressed as a beta gene. (ii) TAATGARAT- has a triple-base change in the octamer/TAATGARAT element which renders it unresponsive to VP16 trans activation, eliminating that portion of the activation of the RR1 gene. (iii) TAATGARAT- delta alpha 0 has a deletion of the alpha 0 gene. (iv) TAATGARAT- delta alpha 4 has a deletion of the alpha 4 gene. Infections were carried out in Vero cells at a multiplicity of infection of 10 per cell; cells were assayed for CAT and beta-galactosidase (beta-Gal) activities and for virus yields. The first two infections gave strong CAT and beta-Gal activities and high yields of progeny virus. Infection with the third virus showed no CAT activity but did produce high levels of beta-Gal activity and virus progeny. The fourth infection resulted in strong CAT activity but no beta-Gal activity or progeny virus. The data demonstrated that the RR1 promoter was activated in the absence of ICP4 but not in the absence of ICP0 in these infections.(ABSTRACT TRUNCATED AT 400 WORDS)

The herpes simplex virus UL37 protein is phosphorylated in infected cells

The herpes simplex virus type 1 (HSV-1) UL37 open reading frame encodes a 120-kDa late (gamma 1), nonstructural protein in infected cells. Recent studies in our laboratory have demonstrated that the UL37 protein interacts in the cytoplasm of infected cells with ICP8, the major HSV-1 DNA-binding protein. As a result of this interaction, the UL37 protein is transported to the nucleus and can be coeluted with ICP8 from single-stranded DNA columns. Pulse-labeling and pulse-chase studies of HSV-1-infected cells with [35S]methionine and 32Pi demonstrated that UL37 was a phosphoprotein which did not have a detectable rate of turnover. The protein was phosphorylated soon after translation and remained phosphorylated throughout the viral replicative cycle. UL37 protein expressed from a vaccinia virus recombinant was also phosphorylated during infection, suggesting that the UL37 protein was phosphorylated by a cellular kinase and that interaction with the ICP8 protein was not a prerequisite for UL37 phosphorylation.

Analysis of a herpes simplex virus 2 fragment from the open reading frame of the large subunit of ribonucleotide reductase with transcriptional regulatory activity

Expression of herpes simplex virus 2 (HSV-2)-encoded ribonucleotide reductase (RR) is required for growth in nondividing cells. The functional enzyme is composed of a large and a small subunit. In virus-infected cells, RR is expressed temporally as a delayed early protein. However, the promoter regulatory region of the large subunit can function as an immediate early promoter in transient transfection assays, suggesting that expression may be quite complex. In this study, a 95-bp fragment derived from the open reading frame of the large subunit of RR (RR-A) functioned as a silencer when placed adjacent to a heterologous promoter. If the fragment was placed distal to the promoter, repression was relieved and in human keratinocytes promoter activity was consistently higher than control constructs. Exonuclease III protection assays revealed that nuclear factors from human keratinocytes as well as other primate cells specifically bind to this fragment. A 30-bp motif containing a consensus SP-1 binding site and an alternating Pu/Py element was protected in all cell lines. These results suggest that a 95-bp fragment in the open reading frame of HSV-2 RR-A plays a role in regulating viral gene expression.

An autophosphorylating but not transphosphorylating activity is associated with the unique N terminus of the herpes simplex virus type 1 ribonucleotide reductase large subunit

We report on a protein kinase function encoded by the unique N terminus of the herpes simplex virus type 1 (HSV-1) ribonucleotide reductase large subunit (R1). R1 expressed in Escherichia coli exhibited autophosphorylation activity in a reaction which depended on the presence of the unique N terminus. When the N terminus was separately expressed in E. coli and partially purified, a similar autophosphorylation reaction was observed. Importantly, transphosphorylation of histones and of proteins in HSV-1-infected cell extracts was also observed with purified R1 and with truncated R1 mutants in which most of the N terminus was deleted. Ion-exchange chromatography was used to separate the autophosphorylating activity of the N terminus from the transphosphorylating activity of an E. coli contaminant protein kinase. We propose a putative function for this activity of the HSV-1 R1 N terminus during the immediate-early phase of virus replication.

Sequencing and 5'- and 3'-end transcript mapping of the gene encoding the small subunit of ribonucleotide reductase from bovine herpesvirus type-1

The complete nucleotide sequence of the gene encoding the small subunit of ribonucleotide reductase (RNR) from bovine herpesvirus type-1 (BHV-1) was determined. The genomic DNA fragment sequenced also represented regions corresponding to the carboxy termini of RNR large subunit and of a virion protein causing host shut-off. The small subunit polypeptide was constituted of 314 amino acid residues totalling 35.25 kDa. The major transcription initiation and termination sites of the small subunit mRNA were located 95 bases upstream and 88 nucleotides downstream from the coding region, respectively. These findings indicate that the mRNA was 1128 bases long which correlated well with the size of the polyadenylated transcript detected in Northern blot analysis (1.3 kb). Within the RNR large subunit coding region, a TATA box and two CAAT box motifs were found 26, 104, and 190 nucleotides, respectively, upstream from the transcription initiation site of the small subunit mRNA. In contrast to previous studies (Slabaugh et al., J. Virol. 1988, 62, 519-527; Boursnell et al., Virology 1991, 184, 411-416), our comparative analysis of five herpesviruses, one iridovirus, and one poxvirus small subunit protein sequences suggested that the seven viruses arose from a common lineage.

Localization of the antigenic sites and intrinsic protein kinase domain within a 300 amino acid segment of the ribonucleotide reductase large subunit from herpes simplex virus type 2

The 140-kDa ribonucleotide reductase (RR1) protein of herpes simplex virus type 2 (HSV-2) functions as the large subunit of virus-specified RR1 and exhibits an intrinsic protein kinase (PK) activity at its unique NH2-terminal region. The N-terminal half of RR1 contains the protein and DNA functions of the morphological transforming region III (mtrIII) of HSV-2. In the present study, we have expressed a number of truncated RR1 derivatives in a mammalian expression vector containing NH2-terminal RR1 gene fragments and amber mutants generated by site-specific mutagenesis. These derivatives, synthesized in transient expression assays, were used as test antigens to localize the epitopes of a panel of HSV-2 RR1-reactive monoclonal antibodies and to fine-map the PK catalytic domain. Our data show that the epitope for HSV-2-specific monoclonal antibody 6A-6 is located in a region of RR1 protein spanning aa 72-350. The epitopes for cross-reactive antibodies to HSV RR1, i.e., 48S and 51S, are formed predominantly by a stretch of amino acid residues specified by aa 350-376 of the RR1 molecule. The 6A-6 antibody utilized in conjunction with the RR1 amber mutants has allowed us to define a 278 aa domain within the NH2-terminal half of the 140-kDa RR1 (aa 72-350) that is sufficient for PK activity.

Enhanced malignant transformation induced by expression of a distinct protein domain of ribonucleotide reductase large subunit from herpes simplex virus type 2

The 1.3-kilobase (kb) Pst I DNA fragment C (Pst I-C) of herpes simplex virus type 2 (HSV-2) morphological transforming region III (mtrIII; map unit 0.562-0.570) encodes part of the N-terminal half of the large subunit of ribonucleotide reductase (RR1; amino acid residues 71-502) and induces the neoplastic transformation of immortalized cell lines. To assess directly the role of these RR1 protein sequences in cell transformation, the Pst I-C fragment was cloned in an expression vector (p91023) containing an adenovirus-simian virus 40 promoter-enhancer to generate recombinant plasmid p9-C. Expression of a protein domain (approximately 65 kDa) was observed in p9-C-transfected COS-7 and Rat2 cells but not in those transfected with plasmid pHC-14 (Pst I-C in a promoterless vector). In Rat2 cells, p9-C induced highly transformed foci at an elevated frequency compared with that of pHC-14. Introduction of translation termination (TAG) condons within the RR1 coding sequence and within all three reading frames inactivated RR1 protein expression from p9-C and reduced its transforming activity to the level seen with the standard pHC-14 construct. Wild-type p9-C specified a protein kinase capable of autophosphorylation. Computer-assisted analysis further revealed significant similarity between regions of mtrIII-specific RR1 and amino acid patterns conserved within the proinsulin precursor family and DNA transposition proteins. These results identify a distinct domain of the HSV-2 RR1 protein involved in the induction of enhanced malignant transformation. In addition, the data indicate that the mtrIII DNA itself can induce basal-level transformation in the absence of protein expression.

The large subunit of herpes simplex virus type 1 ribonucleotide reductase: expression in Escherichia coli and purification

The open reading frame of the large subunit (R1) of herpes simplex virus type 1 (HSV-1) ribonucleotide reductase has been positioned downstream of the phage T7 gene 10 promoter in the expression vector, pET. Transformation of this recombinant plasmid into Escherichia coli BL21 DE3 cells containing the T7 RNA polymerase, under the control of the lac UV5 promoter, allows expression of the subunit on induction of the T7 RNA polymerase by isopropyl thiodigalactoside. The expressed protein is soluble and can be purified with yields up to 0.5 mg of R1 per litre of bacterial culture. The subunit can complement R2 produced in BHK cells or E. coli to give specific activities comparable to that produced in BHK cells infected with HSV-1. Enzyme activity reconstituted from E. coli-expressed R1 and R2 is inhibited by the nonapeptide YAGAVVNDL with an IC50 comparable to that obtained with enzyme extracted from BHK cells infected with HSV-1. Results suggest that the E. coli produced enzyme is a good source of protein for further structural and functional studies.

Amplification by host cell factors of a sequence contained within the herpes simplex virus 1 genome

We report that a cloned 1620-base-pair (bp) DNA fragment mapping in the BamHI O fragment of herpes simplex virus 1 DNA is amplified after transfection into uninfected cells. The DNA fragment maps entirely within a portion of the open reading frame encoding the large subunit of the viral ribonucleotide reductase and does not contain any of the known lytic origins of viral DNA synthesis. Amplification of this sequence in transfected cells results in accumulation of full-sized Dpn I-resistant plasmids containing the sequence in Hirt extracts of low molecular weight DNA. Subfragments of the 1620-bp fragment were not amplified, whereas larger fragments containing the intact 1620-bp fragment were amplified. The amplification of the fragment in MCF7 cells, which express steroid receptors, was stimulated by the addition of estrogen to the medium. Addition of progesterone, dexamethasone, or testosterone was ineffective. The viral genome therefore contains at least one origin of DNA synthesis capable of supporting replication of viral DNA by cellular factors. The existence of such a host origin of DNA replication in the viral genome was predicted by the hypothesis that viral DNA is amplified by cellular enzymes in sensory neurons harboring latent virus; the link between these sequences and amplification of viral DNA during latency remains to be proven.

Myristylation and polylysine-mediated activation of the protein kinase domain of the large subunit of herpes simplex virus type 2 ribonucleotide reductase (ICP10)

The amino-terminal domain of the large subunit of herpes simplex virus type 2 (HSV-2) ribonucleotide reductase (ICP10) was previously shown to possess protein kinase (PK) activity that localizes to the cytosolic, cytoskeletal, and plasma membrane fractions. Further studies of the PK domain using computer-assisted sequence analysis have identified a single transmembrane segment and fatty acid incorporation findings indicate that ICP10 is myristylated. Myristylation does not depend on a viral enzyme, since myristic acid is incorporated into ICP10 precipitated from cells transfected with an ICP10 expression vector. It is also incorporated into the 57-kDa protein expressed by the amino-terminal PK expression vector. The myristyl moiety is linked through an amide bond. The basic protein polylysine stimulates the kinase activity and alters its divalent cation requirements resulting in 20- to 40-fold stimulation in the presence of 0.1 mM Mn2+. The PK activity is inhibited by antibody to synthetic peptides corresponding to residues 355-369 and 13-26, respectively, located within, and amino-terminal to, the predicted PK catalytic domain.

Three-dimensional structure of the free radical protein of ribonucleotide reductase

The enzyme ribonucleotide reductase furnishes precursors for the DNA synthesis of all living cells. One of its constituents, the free radical protein, has an unusual alpha-helical structure. There are two iron centres that are about 25 A apart in the dimeric molecule. Tyrosine 122, which harbours the stable free radical necessary for the activity of ribonucleotide reductase, is buried inside the protein and is located 5 A from the closest iron atom.

Identification and characterization of the herpes simplex virus type 2 gene encoding the essential capsid protein ICP32/VP19c

We describe the characterization of the herpes simplex virus type 2 (HSV-2) gene encoding infected cell protein 32 (ICP32) and virion protein 19c (VP19c). We also demonstrate that the HSV-1 UL38/ORF.553 open reading frame (ORF), which has been shown to specify a viral protein essential for capsid formation (B. Pertuiset, M. Boccara, J. Cebrian, N. Berthelot, S. Chousterman, F. Puvian-Dutilleul, J. Sisman, and P. Sheldrick, J. Virol. 63: 2169-2179, 1989), must encode the cognate HSV type 1 (HSV-1) ICP32/VP19c protein. The region of the HSV-2 genome deduced to contain the gene specifying ICP32/VP19c was isolated and subcloned, and the nucleotide sequence of 2,158 base pairs of HSV-2 DNA mapping immediately upstream of the gene encoding the large subunit of the viral ribonucleotide reductase was determined. This region of the HSV-2 genome contains a large ORF capable of encoding two related 50,538- and 49,472-molecular-weight polypeptides. Direct evidence that this ORF encodes HSV-2 ICP32/VP19c was provided by immunoblotting experiments that utilized antisera directed against synthetic oligopeptides corresponding to internal portions of the predicted polypeptides encoded by the HSV-2 ORF or antisera directed against a TrpE/HSV-2 ORF fusion protein. The type-common immunoreactivity of the two antisera and comparison of the primary amino acid sequences of the predicted products of the HSV-2 ORF and the equivalent genomic region of HSV-1 provided evidence that the HSV-1 UL38 ORF encodes the HSV-1 ICP32/VP19c. Analysis of the expression of the HSV-1 and HSV-2 ICP32/VP19c cognate proteins indicated that there may be differences in their modes of synthesis. Comparison of the predicted structure of the HSV-2 ICP32/VP19c protein with the structures of related proteins encoded by other herpes viruses suggested that the internal capsid architecture of the herpes family of viruses varies substantially.

Protein kinase activity associated with the large subunit of herpes simplex virus type 2 ribonucleotide reductase (ICP10)

The large subunit of the herpes simplex virus type 2 (HSV-2) ribonucleotide reductase (RR1) is demonstrated to possess serine/threonine-specific kinase activity. Computer-assisted sequence analysis identified regions within the amino terminus of ICP10 that are homologous to the catalytic domain of known protein kinases (PKs). An in vitro kinase assay confirmed the ability of ICP10, immunoprecipitated from either HSV-2-infected cells or from cells transfected with an ICP10 expression vector, to autophosphorylate and transphosphorylate exogenous substrates in the presence of ATP and Mg2+. The HSV-1 RR1 was shown to be negative for PK activity under these conditions. PK activity was localized to a 57-kDa amino-terminal region within ICP10 that lacked RR activity.

Identification of immediate-early-type cis-response elements in the promoter for the ribonucleotide reductase large subunit from herpes simplex virus type 2

Regulation of the expression of the herpes simplex virus (HSV) type 2 large subunit of ribonucleotide reductase (ICP10) gene was studied directly by immunofluorescence or by chloramphenicol acetyltransferase analysis with hybrid ICP10 promoter constructions. In Vero cells, cotransfection with DNA encoding HSV IE110 or Vmw65 proteins or HCMV IE2 enhanced expression at least 10-fold. In contrast, expression was minimally enhanced by DNA encoding IE175 at low doses and slightly reduced at high doses. IE110-mediated trans-activation was minimal in primary astrocytes and cells from line 293. However, Vmw65 enhanced expression 20-fold in all cell types. cis-Response elements in the ICP10 promoter include a TAATGARAT-like element and other sequences associated with regulation of IE gene expression and potential SP-1, consensus AP-1, and octamer transcription factor 1 binding elements. Factors that bind to the ICP10 promoter were identified in mock and HSV-infected cell extracts. DNA-protein complex formation, presumably involving Vmw65, was demonstrated by gel retardation analysis with mixtures of uninfected cell nuclear extracts and virion lysates. The octamer transcription factor 1 motif (ATGCAAAT) was necessary for optimal Vmw65 binding to the ICP10 promoter as evidenced by competition experiments with oligonucleotides overlapping the consensus IE110 promoter virion response element. The data suggest that ICP10 can be regulated as an immediate-early gene.

A self-inducing runaway-replication plasmid expression system utilizing the Rop protein

A highly efficient prokaryotic expression system has been developed that produces proteins at levels exceeding 150 micrograms/ml of culture medium. The system consists of a temperature-sensitive-copy-number plasmid that carries the rop gene and promoter downstream from the trp promoter. Any sequence cloned into the PvuII site of the rop gene alters Rop protein activity and causes lethal runaway plasmid DNA replication. This plasmid replication can be suppressed in trans by complementation with a similar wild-type plasmid. Cells harboring both plasmids are quite stable, and induction of plasmid DNA synthesis occurs only after cells are grown for several generations under conditions that lead to the loss of the trans-acting repressor. Large amounts of Rop fusion proteins accumulate in the cell as the trp operon is gradually induced via repressor titration. All chimeric proteins accumulate as insoluble aggregates, and are therefore easily purified. They can be solubilized using relatively mild conditions, and the partially purified proteins are highly amenable to cleavage by chemical methods. Using this system we have made Rop fusions with the HIV Tat protein, the herpes simplex virus type-2 38K protein, and Chinese hamster metallothionin.

The minimal transforming fragment of HSV-2 mtrIII can function as a complex promoter element

The minimal transforming fragment (486 TF) of HSV-2 mtrIII (0.567-0.570 map units) is composed of two distinct and non-overlapping promoter elements when linked to bacterial CAT genes. A 230-nucleotide fragment of 486 TF, Sal1-Hpa1, was active as a promoter element in primate cells but not rodent cells. A 173-nucleotide fragment, Sma1-Pst1, was active in both primate and rodent cells. The 486 TF did not compete for limiting cellular factors required to drive the CAT gene under control of the SV40 early promoter/enhancer. However, gel-retardation assays suggest that unique factors exist in cells transformed by HSV-2 which specifically recognized regions of 486 TF. These results are discussed with respect to HSV-2-mediated transformation.

2-Acetylpyridine 5-[(dimethylamino)thiocarbonyl]-thiocarbonohydrazone (A1110U), a potent inactivator of ribonucleotide reductases of herpes simplex and varicella-zoster viruses and a potentiator of acyclovir

2-Acetylpyridine 5-[(dimethylamino)thiocarbonyl]thiocarbonohydrazone (A1110U) was found to be a potent inactivator of the ribonucleotide reductases (EC 1.17.4.1) encoded by herpes simplex virus types 1 and 2 and by varicella-zoster virus and to be a weaker inactivator of human ribonucleotide reductase. It also markedly potentiated the antiherpetic activity of acyclovir against these viruses in tissue culture. A1110U both decreased the dGTP pool that builds up when infected cells are treated with acyclovir and induced a large increase in the pool of acyclovir triphosphate. The resultant 100-fold increase in the ratio of the concentrations of acyclovir triphosphate to dGTP should facilitate the binding of the fraudulent nucleotide to its target enzyme, herpes virus-encoded DNA polymerase, and could account for the synergy between A1110U and acyclovir. A similar change in the acyclovir triphosphate-to-dGTP ratio was previously reported to be induced by another ribonucleotide reductase inhibitor, 2-acetylpyridine 4-(2-morpholinoethyl)thiosemicarbazone (A723U). However, A1110U is considerably more potent and may have better clinical potential. Synergistic toxic interactions between A1110U and acyclovir were not detected in uninfected cells.

Factor(s) present in herpes simplex virus type 1-infected cells can compensate for the loss of the large subunit of the viral ribonucleotide reductase: characterization of an ICP6 deletion mutant

Herpes simplex virus type 1 encodes a ribonucleotide reductase (RR) consisting of two subunits (140 and 38 kDa) whose genes map to coordinates 0.56 to 0.60 on the viral genome. We previously reported the isolation and characterization of a mutant with a lacZ insertion into the large subunit (ICP6) gene (Goldstein and Weller, 1988). Studies with this blue-plaque mutant, hrR3, showed that the viral RR activity is not essential in dividing cells in culture. This mutant, however, synthesizes the N-terminal one-third (434 amino acids) of ICP6 which may have an additional, required function. To test this possibility, a deletion of the ICP6 gene was created by introducing a deleted ICP6 gene into infectious hrR3 DNA and screening for white plaques from a background of blue plaques. Studies with this mutant, ICP6 delta, demonstrated that ICP6 is not required for virus growth and DNA synthesis in dividing cells in culture. However, we show that the ability of ICP6 delta to grow and induce viral DNA synthesis is dependent on the state of the infected cells; ICP6 delta is severely compromised in nondividing cells or in cells at 39.5 degrees. We propose that an alternate pathway(s) for obtaining deoxyribonucleotides is operating in infected cells and can compensate for defects in viral RR. In addition, our experiments suggest that these alternate sources are not available either in nondividing cells or in cells at 39.5 degrees.

Herpes simplex virus immunoglobulin G Fc receptor activity depends on a complex of two viral glycoproteins, gE and gI

Evidence was recently presented that herpes simplex virus type 1 (HSV-1) immunoglobulin G (IgG) Fc receptors are composed of a complex containing a previously described glycoprotein, gE, and a novel virus-induced polypeptide, provisionally named g70 (D. C. Johnson and V. Feenstra, J. Virol. 61:2208-2216, 1987). Using a monoclonal antibody designated 3104, which recognizes g70, in conjunction with antipeptide sera and virus mutants unable to express g70 or gE, we have mapped the gene encoding g70 to the US7 open reading frame of HSV-1 adjacent to the gE gene. Therefore, g70 appears to be identical to a recently described polypeptide which was named gI (R. Longnecker, S. Chatterjee, R. J. Whitley, and B. Roizman, Proc. Natl. Acad. Sci. USA 84:147-151, 1987). Under mildly denaturing conditions, monoclonal antibody 3104 precipitated both gI and gE from extracts of HSV-1-infected cells. In addition, rabbit IgG precipitated the gE-gI complex from extracts of cells transfected with a fragment of HSV-1 DNA containing the gI, gE, and US9 genes. Cells infected with mutant viruses which were unable to express gE or gI did not bind radiolabeled IgG; however, cells coinfected with two viruses, one unable to express gE and the other unable to express gI, bound levels of IgG approaching those observed with wild-type viruses. These results further support the hypothesis that gE and gI form a complex which binds IgG by the Fc domain and that neither polypeptide alone can bind IgG.

Sequence analyses of herpesviral enzymes suggest an ancient origin for human sexual behavior

Comparison of the amino acid sequences of the deoxythymidine kinases of herpes simplex (HSV) and of marmoset herpes viruses (MHV) suggests a divergence time of 8 to 10 million years ago for HSV-1 and -2. Like MHV, HSV-1 and -2 cause local infections in their natural hosts, and direct contact between two individuals during the brief period of infectivity is needed for transmission. Because B virus, a nearer relative of HSV, depends on both oral and genital routes of transmission, we postulate that ancestral HSV (aHSV) was similar, and that for HSV-1 and -2 to diverge, genital and oral sites had to become microbiologically somewhat isolated from each other, while oral--oral and genital--genital contact had to be facilitated to maintain both aHSV strains. We propose that acquisition of continual sexual attractiveness by the ancestral human female and the adoption of close face-to-face mating, two hallmarks of human sexual behavior, provided the conditions for the divergence.

Expression of the HSV-2 ribonucleotide reductase subunits in adenovirus vectors or stably transformed cells: restoration of enzymatic activity by reassociation of enzyme subunits in the absence of other HSV proteins

We have cloned the large subunit (RR1) of the HSV-2 ribonucleotide reductase into a helper-independent adenovirus 5 vector under control of the viral major late promoter. Infection of 293 cells with the AdRed-1 recombinant virus resulted in the expression of the HSV-2 RR1 protein. We have also produced cells which constitutively express the small (RR2) subunit of the HSV-2 enzyme by transfecting 293 cells with a plasmid encoding this protein and the neo resistance marker (pSV2neo-RR2). Infection of the A439-14 producer cells with AdRed-1 resulted in the expression of enzymatically active HSV-2 ribonucleotide reductase. HSV-2 reductase activity could also be detected upon mixing of extracts from cells expressing either subunit. Our results indicate that the HSV-2 holoenzyme can be reconstituted in vivo and in vitro and that no HSV-2 proteins, beyond the enzyme subunits, are required for the formation and activity of the viral reductase.

Herpes simplex virus type 1-induced ribonucleotide reductase activity is dispensable for virus growth and DNA synthesis: isolation and characterization of an ICP6 lacZ insertion mutant

Herpes simplex virus (HSV) encodes a ribonucleotide reductase consisting of two subunits (140 and 38 kilodaltons) whose genes map to coordinates 0.56 to 0.60 on the viral genome. Host cell lines containing the HpaI F fragment which includes the reductase subunit genes of HSV type 1 strain KOS (coordinates 0.535 to 0.620) were generated. Transfection of these cells with a plasmid containing the immediate-early ICP0 gene resulted in the expression of ICP6; interestingly, ICP4 plasmids failed to induce expression, indicating an unusual pattern of ICP6 regulation. One such cell line (D14) was used to isolate a mutant with the structural gene of lacZ inserted into the ICP6 gene such that the lacZ gene is read in frame with the N-terminal region of ICP6. This mutant generated a protein containing 434 amino acids (38%) of the N terminus of ICP6 fused to beta-galactosidase under control of the endogenous ICP6 promoter. Screening for virus recombinants was greatly facilitated by staining virus plaques with 5-bromo-4-chloro-3-indoyl-beta-D-galactoside (X-gal). Enzyme assays of infected BHK cells indicated that the mutant is incapable of inducing viral ribonucleotide reductase activity. Surprisingly, although plaque size was greatly reduced, mutant virus yield was reduced only four- to fivefold compared with that of the wild type grown in exponentially growing Vero cells. Mutant virus plaque size, yields, and ability to synthesize viral DNA were more severely compromised in serum-starved cells as compared with the wild type grown under the same condition. Although our evidence suggests that the HSV type 1 ribonucleotide reductase is not required for virus growth and DNA replication in dividing cells, it may be required for growth in nondividing cells.

Ribonucleotide reductase induced by varicella zoster virus. Characterization, and potentiation of acyclovir by its inhibition

An enzyme that catalyzes the conversion of CDP to 2'-dCDP in the presence of dithiothreitol (DTT) was detected in ammonium sulfate fractionated-extracts of varicella zoster virus (VZV)-infected cells. This ribonucleotide reductase was antigenically distinguishable from the isofunctional eucaryotic enzyme as well as the ribonucleotide reductases induced by herpes simplex virus types 1 and 2 (HSV-1 and HSV-2). The VZV-induced enzyme was purified to the extent that most of the contaminating enzymes, which would significantly deplete the substrate, were removed. The VZV-induced ribonucleotide reductase exhibited maximum activity in the absence of ATP and/or magnesium and was only weakly inhibited by 2'-deoxynucleoside triphosphates. Furthermore, ADP, UDP and GDP competitively inhibited CDP reduction with Ki (Km) values of 15, 20, 1.8 and 0.88 microM, respectively. These kinetic properties were very similar to those of the correspondingly purified ribonucleotide reductases induced by HSV-1 [Averett et al., J. biol. Chem. 258, 9831 (1983)] and HSV-2 [Averett et al., J. Virol. 52, 981 (1984)] and were dissimilar to the allosterically regulated mammalian enzyme. A723U, an inactivator of HSV-1 ribonucleotide reductase that potentiates the anti-HSV-1 activity of acyclovir [Spector et al., Proc. natn. Acad. Sci. U.S.A. 82, 4254 (1985)], also appeared to inactivate this VZV-induced ribonucleotide reductase and to potentiate the anti-VZV activity of acyclovir.

Study of ribonucleotide reductase in cells infected with six clinical isolates of herpes simplex virus type 2 (HSV-2) with mutations in its larger subunit

Herpes simplex virus type 2 (HSV-2) induces a novel ribonucleotide reductase (RR) composed of two subunits (140 and 38 kDa) in infected cells. Other investigators have developed a monoclonal antibody, A6, against the 140-kDa subunit of RR and have found, in about 1% of the cases, an inability to detect this protein in cells infected with clinical isolates of HSV-2. We therefore investigated whether in such cases the clinical isolates were capable of inducing viral RR activity and whether the lack of detection of the 140-kDa protein by the monoclonal antibody was due to an alteration in the antigenic site of this protein. Six such isolates were examined and were found to induce RR activity, similar to HSV-2 (strain 333) RR, which did not require ATP for CDP reduction. Western blot analyses using A6 failed to detect the protein. However, R1, a polyclonal antibody raised against viral RR was capable of detecting this subunit. In addition, R1 was also capable of neutralizing RR activity induced by all the isolates and HSV-2 (strain 333). In conclusion, the lack of detection of the large subunit of RR was not due to the lack of induction but was due to an alteration in the antigenic site recognized by A6; this alteration did not appear to affect the properties of the induced RR activity.

Identification of viral polypeptides involved in pseudorabies virus ribonucleotide reductase activity

We studied pseudorabies virus-induced ribonucleotide reductase and found that it exhibited biochemical properties very similar to those of herpes simplex virus reductase. A polyclonal rabbit antiserum (P9) directed against the carboxy terminus of subunit H2 polypeptide (38,000 daltons) of herpes simplex virus reductase neutralized the pseudorabies virus reductase, as well as the herpes simplex virus isozyme. This serum recognized two pseudorabies virus-specified polypeptides of 34,000 and 110,000 daltons, which may represent the two subunits of the enzyme. Furthermore, as already shown for herpes simplex virus reductase (E. A. Cohen, P. Gaudreau, P. Brazeau, and Y. Langelier, Nature [London] 321:441-443, 1986), we show that the nonapeptide itself specifically inhibited pseudorabies reductase activity.

The herpes simplex virus type 1 ribonucleotide reductase is a tight complex of the type alpha 2 beta 2 composed of 40K and 140K proteins, of which the latter shows multiple forms due to proteolysis

We have isolated two monospecific monoclonal mouse antibodies directed against the HSV-1 ribonucleotide reductase. When immobilized to Sepharose, both antibodies remove enzyme activity from solution. However, on immunoblots of crude extracts of HSV-1-infected cells, one antibody only detects a 140K protein and the other antibody only a 40K protein. Neither antibody recognizes the cellular ribonucleotide reductase or the related pseudorabies virus-induced enzyme. Therefore, our data strongly suggest that the HSV-1 ribonucleotide reductase consists of a 140K and a 40K protein. The 140K protein is sequentially degraded to 110K, 93K, and 81K proteins by a Vero cell-specific, N alpha-p-tosyl-L-lysine chloromethyl ketone-sensitive protease. Of the different proteolytic products, at least the 93K species seems to be enzymatically active, suggesting that part of the 140K protein may have functions not related to ribonucleotide reduction. There is a very high affinity between the 140K and 40K proteins as evident from affinity chromatography on antibody-Sepharose and sedimentation velocity centrifugation in a glycerol gradient. The 140K and 40K proteins cosediment with the HSV-1 ribonucleotide reductase activity at 17 S. This indicates that the active form of the HSV-1 reductase consists of the 140K and 40K proteins forming a tight complex of the alpha 2 beta 2 type.