The 65,000-Mr DNA-binding and virion trans-inducing proteins of herpes simplex virus type 1

Partial complementation between the immediate early proteins ICP4 of herpes simplex virus type 1 and IE180 of pseudorabies virus

We previously described that the immediate early (IE) IE180 protein of PRV can down-regulate the transactivation of the ICP4 promoter of HSV-1, and that the d120 virus (an ICP4-deficient HSV-1 strain) can partially replicate its viral DNA in the presence of the IE180 protein. Herein, we demonstrate that this partial complementation of d120 by IE180 is sufficient for transcription of β, γ1 and γ2 products such as DNA pol, VP16 and gC, respectively. However, expression levels are low for VP16 and even lower for the gC, such that IE180 is unable to fully substitute for ICP4 functionally. Viral progeny was not detected in PK15 cells expressing PRV IE180.

HCFC2 is needed for IRF1- and IRF2-dependent Tlr3 transcription and for survival during viral infections

Sun et al. show that host cell factor C2 (HCFC2) is necessary for basal and induced Tlr3 transcription; deficiency of HCFC2 compromises survival during influenza virus and herpes simplex virus 1 infections in mice.

Transcriptional regulation of numerous interferon-regulated genes, including Toll-like receptor 3 (Tlr3), which encodes an innate immune sensor of viral double-stranded RNA, depends on the interferon regulatory factor 1 (IRF1) and IRF2 transcription factors. We detected specific abrogation of macrophage responses to polyinosinic-polycytidylic acid (poly(I:C)) resulting from three independent N-ethyl-N-nitrosourea–induced mutations in host cell factor C2 (Hcfc2). Hcfc2 mutations compromised survival during influenza virus and herpes simplex virus 1 infections. HCFC2 promoted the binding of IRF1 and IRF2 to the Tlr3 promoter, without which inflammatory cytokine and type I IFN responses to the double-stranded RNA analogue poly(I:C) are reduced in mouse macrophages. HCFC2 was also necessary for the transcription of a large subset of other IRF2-dependent interferon-regulated genes. Deleterious mutations of Hcfc2 may therefore increase susceptibility to diverse infectious diseases.

Targets for Antiviral Chemotherapy: Herpes Simplex Virus Regulatory Protein, Vmw65

The virion protein, Vmw65, of herpes simplex virus selectively induces the transcription of the virus immediate–early genes and is required for normal virus replication and for virulence in animal models. Vmw65 operates by interacting with a host cell transcription factor (Oct-1) and analysis of the structure/function relationship within Vmw65 has facilitated the design of a peptide, corresponding to a local domain of the protein, which interferes with the Vmw65–Oct-1 interaction. The selective interference of protein–protein interactions involved in gene regulation may provide a suitable target for the inhibition of virus replication.

Sites and roles of phosphorylation of the human cytomegalovirus DNA polymerase subunit UL44

The human cytomegalovirus DNA polymerase subunit UL44 is a phosphoprotein, but its sites and roles of phosphorylation have not been investigated. We compared sites of phosphorylation of UL44 in vitro by the viral protein kinase UL97 and cyclin-dependent kinase 1 with those in infected cells. Transient treatment of infected cells with a UL97 inhibitor greatly reduced labeling of two minor UL44 phosphopeptides. Viruses containing alanine substitutions of most UL44 residues that are phosphorylated in infected cells exhibited at most modest effects on viral DNA synthesis and yield. However, substitution of highly phosphorylated sites adjacent to the nuclear localization signal abolished viral replication. The results taken together are consistent with UL44 being phosphorylated directly by UL97 during infection, and a crucial role for phosphorylation-mediated nuclear localization of UL44 for viral replication, but lend little support to the widely held hypothesis that UL97-mediated phosphorylation of UL44 is crucial for viral DNA synthesis.

Replication and recombination of herpes simplex virus DNA

Replication of herpes simplex virus takes place in the cell nucleus and is carried out by a replisome composed of six viral proteins: the UL30-UL42 DNA polymerase, the UL5-UL8-UL52 helicase-primase, and the UL29 single-stranded DNA-binding protein ICP8. The replisome is loaded on origins of replication by the UL9 initiator origin-binding protein. Virus replication is intimately coupled to recombination and repair, often performed by cellular proteins. Here, we review new significant developments: the three-dimensional structures for the DNA polymerase, the polymerase accessory factor, and the single-stranded DNA-binding protein; the reconstitution of a functional replisome in vitro; the elucidation of the mechanism for activation of origins of DNA replication; the identification of cellular proteins actively involved in or responding to viral DNA replication; and the elucidation of requirements for formation of replication foci in the nucleus and effects on protein localization.

Nuclear import of HSV-1 DNA polymerase processivity factor UL42 is mediated by a C-terminally located bipartite nuclear localization signal

The polymerase accessory protein of the human herpes simplex virus type 1 (HSV-1) DNA polymerase UL42 plays an essential role in viral replication, conferring processivity to the catalytic subunit UL30. We show here that UL42 is imported to the nucleus of living cells in a Ran- and energy-dependent fashion, through a process that requires a C-terminally located bipartite nuclear localization signal (UL42-NLSbip; PTTKRGRSGGEDARADALKKPK(413)). Moreover cytoplasmic mutant derivatives of UL42 lacking UL42-NLSbip are partially relocalized into the cell nucleus upon HSV-1 infection or coexpression with UL30, implying that the HSV-1 DNA polymerase holoenzyme can assemble in the cytoplasm before nuclear translocation occurs, thus explaining why the UL42 C-terminal domain is not strictly required for viral replication in cultured cells. However, mutation of both UL30 and UL42 NLS results in retention of the DNA polymerase holoenzyme in the cytoplasm, suggesting that simultaneous inhibition of both NLSs could represent a viable strategy to hinder HSV-1 replication. Intriguingly, UL42-NLSbip is composed of two stretches of basic amino acids matching the consensus for classical monopartite NLSs (NLSA, PTTKRGR(397); NLSB, KKPK(413)), neither of which are capable of targeting GFP to the nucleus on their own, consistent with the hypothesis that P and G residues in position +3 of monopartite NLSs are not compatible with nuclear transport in the absence of additional basic sequences located in close proximity. Our results showing that substitution of G or P of the NLS with an A residue partially confers NLS function will help to redefine the consensus for monopartite NLSs.

Effect of phosphorylation on the transactivation activity of Epstein-Barr virus BMRF1, a major target of the viral BGLF4 kinase

Modification of human herpesvirus DNA polymerase processivity factors (PFs) by phosphorylation occurs frequently during viral lytic replication. However, functional regulation of the herpesvirus PFs through phosphorylation is not well understood. In addition to processivity, the PF BMRF1 of Epstein-Barr virus can function as a transactivator to activate the BHLF1 promoter within the lytic origin of replication (oriLyt), which is assumed to facilitate DNA replication through remodelling viral chromatin structure. BMRF1 is known to be phosphorylated by the viral BGLF4 kinase, but its impact on BMRF1 function is unclear. Seven candidate BGLF4 target sites were predicted within a proline-rich region between the DNA-processivity and nuclear-localization domains of BMRF1. We show that four of these residues, Ser-337, Thr-344, Ser-349 and Thr-355, are responsible for the BGLF4-induced hyperphosphorylation of BMRF1. In functional analyses, a phosphorylation-mimicking mutant of BMRF1 shows similar nuclear localization, as well as DNA-binding ability, to the wild type; however, it displays stronger synergistic activation of the BHLF1 promoter with Zta. Notably, BGLF4 downregulates BMRF1 transactivation and enhances the transactivation activity of Zta and the synergistic activation of BMRF1 and Zta on the BHLF1 promoter. Our findings suggest that BGLF4 may modulate the activation of the oriLyt BHLF1 promoter coordinately through multiple mechanisms to facilitate optimal oriLyt-dependent viral DNA replication.

Curcumin inhibits herpes simplex virus immediate-early gene expression by a mechanism independent of p300/CBP histone acetyltransferase activity

Curcumin, a phenolic compound from the curry spice turmeric, exhibits a wide range of activities in eukaryotic cells, including antiviral effects that are at present incompletely characterized. Curcumin is known to inhibit the histone acetyltransferase activity of the transcriptional coactivator proteins p300 and CBP, which are recruited to the immediate early (IE) gene promoters of herpes simplex virus type 1 (HSV-1) by the viral transactivator protein VP16. We tested the hypothesis that curcumin, by inhibiting these coactivators, would block viral infection and gene expression. In cell culture assays, curcumin significantly decreased HSV-1 infectivity and IE gene expression. Entry of viral DNA to the host cell nucleus and binding of VP16 to IE gene promoters was not affected by curcumin, but recruitment of RNA polymerase II to those promoters was significantly diminished. However, these effects were observed using lower curcumin concentrations than those required to substantially inhibit global H3 acetylation. No changes were observed in histone H3 occupancy or acetylation at viral IE gene promoters. Furthermore, p300 and CBP recruitment to IE gene promoters was not affected by the presence of curcumin. Finally, disruption of p300 expression using a short hairpin RNA did not affect viral IE gene expression. These results suggest that curcumin affects VP16-mediated recruitment of RNA polymerase II to IE gene promoters by a mechanism independent of p300/CBP histone acetyltransferase activity.

An importin alpha/beta-recognized bipartite nuclear localization signal mediates targeting of the human herpes simplex virus type 1 DNA polymerase catalytic subunit pUL30 to the nucleus

Although the 1235 amino acids human herpes simplex virus type 1 (HSV-1) DNA polymerase catalytic subunit, pUL30, is essential for HSV-1 replication in the nucleus of host cells, little information is available regarding its nuclear import mechanism. The present study addresses this issue directly, characterizing pUL30's nuclear import pathway for the first time using quantitative confocal laser scanning microscopy (CLSM) on living cells, and fluorescent binding assays. In addition to a previously described nuclear localization signal (NLS) located within the pUL30 binding site for the polymerase accessory protein (PAP) pUL42, that appears to be dispensable for nuclear targeting, pUL30 possesses three putative basic NLSs. Intriguingly, the core of pUL30-NLS2 (residues 1114-1120) is highly homologous to that of the recently described NLS, similarly located upstream of the PAP binding site, of the human cytomegalovirus (HCMV) DNA polymerase catalytic subunit, pUL54. Here we show for the first time that pUL30-NLS2 itself is only partially functional in terms of nuclear import due to residue P1118 present in position 3 of the NLS core. Intriguingly, pUL30-NLS2 together with pUL30-NLS3 (residues 1133-1136) represents a fully functional bipartite NLS (pUL30-NLSbip), required for nuclear targeting of pUL30, and able to confer nuclear localization on heterologous proteins by conferring high-affinity interaction with the importin (IMP) alpha/beta heterodimer. Since nuclear targeting of HSV-1 proteins forming the replication fork is crucial for viral replication, the pUL30-NLSbip emerges for the first time as a viable therapeutic target.

Sequence and function of canine herpesvirus alpha-transinducing factor and its interaction with an immediate early promoter

The sequence of the alpha-transinducing factor (alpha-TIF) of canine herpesvirus (CHV-l) was determined. Alignment of the predicted CHV-1 alpha-TIF amino acid sequence with other alpha-TIF homologues reveals a core region of similarity with divergent amino and carboxyl termini. Analysis of the CHV-1 infected cell protein 4 promoter region identified a region containing nine copies of a 52 bp repeat that showed significant up-regulation of transcription by alpha-TIF. This region contained an imperfect 'TAATGARAT' motif, the binding site for herpes simplex virus 1 alpha-TIF, with an imperfect Oct-1 binding site immediately following. The infectious laryngotracheitis virus alpha-TIF was also shown to up-regulate transcription through this region of the promoter. Transfection of CHV-1 genomic DNA failed to yield infectious virus in canine kidney cell lines. Co-transfection of genomic DNA and an alpha-TIF expression plasmid resulted in virus plaques, indicating a potential essential role for alpha-TIF in CHV-1 infection.

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

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

Sequence analysis of the alpha trans-inducing factor of bovine herpesvirus type 5 (BHV-5)

Bovine herpesvirus (BHV), a member of the subfamily Alphaherpesvirinae, is classified into neurovirulent and non-neurovirulent subtypes on a basis of differential neuropathogenicities. Transcription of viral immediate early (IE) genes during alphaherpesvirus gene expression, is mediated by a multi-component immediate early complex (IEC) integrated by the viral tegument protein alpha trans-inducing factor (alpha-tif), a host cell protein (HCF), and a host Octamer protein (Oct). In this paper, we present a sequence analysis of the alpha-tif of the encephalitic BHV subtype, bovine herpesvirus type 5 (BHV-5). Bovine herpesvirus type 1 (BHV-1) and BHV-5 alpha-tifs share 98% amino acid sequence homology. However, BHV-5 alpha-tif is 23 residues shorter at the amino terminus than BHV-1 alpha-tif. Amino acid alignment of the alpha-tifs of BHV-1 and BHV-5 with other alphaherpesviruses indicates areas of conserved motifs but also important differences located mainly at the amino and carboxyl termini.

DNA recognition by the herpes simplex virus transactivator VP16: a novel DNA-binding structure

Upon infection, the herpes simplex virus (HSV) transcriptional activator VP16 directs the formation of a multiprotein-DNA complex-the VP16-induced complex-with two cellular proteins, the host cell factor HCF-1 and the POU domain transcription factor Oct-1, on TAATGARAT-containing sequences found in the promoters of HSV immediate-early genes. HSV VP16 contains carboxy-terminal sequences important for transcriptional activation and a central conserved core that is important for VP16-induced complex assembly. On its own, VP16 displays little, if any, sequence-specific DNA-binding activity. We show here that, within the VP16-induced complex, however, the VP16 core has an important role in DNA binding. Mutation of basic residues on the surface of the VP16 core reveals a novel DNA-binding surface with essential residues which are conserved among VP16 orthologs. These results illuminate how, through association with DNA, VP16 is able to interpret cis-regulatory signals in the DNA to direct the assembly of a multiprotein-DNA transcriptional regulatory complex.

Purification of a bacterially expressed herpes simplex virus type 1 origin binding protein for use in posttranslational processing studies

The origin binding protein (OBP) encoded by the UL9 open reading frame of herpes simplex virus type 1 (HSV-1) plays an essential role in productive infection by promoting the initiation of viral DNA synthesis. In this study, OBP was inducibly expressed in Escherichia coli and purified to homogeneity using a two-step chromatographic separation procedure. The properties of this recombinant OBP (rOBP) were found to be indistinguishable from those of the virus-encoded protein. Since rOBP was synthesized in bacterial cells, it lacked the posttranslational processing which normally occurs in OBP produced in HSV-1-infected mammalian cells and could therefore be exploited in experiments which addressed the effects of protein modification on OBP function. As an initial study, the impact of phosphorylation on enzymatic activity was examined using rOBP which had been treated with a panel of purified cellular kinases. rOBP was found to act as a substrate for nearly all of the kinases tested in (32)P-labeled phosphate transfer assays. However, only phosphorylation by protein kinase A (PKA, or cAMP-dependent protein kinase) was shown to significantly alter the enzymatic properties of rOBP, as it increased by five- to eightfold the ATPase activity associated with this protein. Activation of this critical viral DNA replication enzyme by a cAMP-dependent kinase such as PKA may be of some relevance in the natural course of HSV-1 infections, since reactivation of latent virus is thought to involve both signal transduction events and the induction of viral DNA synthesis. Thus, the expression and purification strategy outlined in this work provides an economical source of unmodified HSV-1 OBP that should prove useful in future in vitro studies.

Characterization of human herpesvirus 8 ORF59 protein (PF-8) and mapping of the processivity and viral DNA polymerase-interacting domains

Human herpesvirus 8 (HHV-8) or Kaposi's sarcoma-associated herpesvirus (KSHV) ORF59 protein (PF-8) is a processivity factor for HHV-8 DNA polymerase (Pol-8) and is homologous to processivity factors expressed by other herpesviruses, such as herpes simplex virus type 1 UL42 and Epstein-Barr virus BMRF1. The interaction of UL42 and BMRF1 with their corresponding DNA polymerases is essential for viral DNA replication and the subsequent production of infectious virus. Using HHV-8-specific monoclonal antibody 11D1, we have previously identified the cDNA encoding PF-8 and showed that it is an early-late gene product localized to HHV-8-infected cell nuclei (S. R. Chan, C. Bloomer, and B. Chandran, Virology 240:118-126, 1998). Here, we have further characterized PF-8. This viral protein was phosphorylated both in vitro and in vivo. PF-8 bound double-stranded DNA (dsDNA) and single-stranded DNA independent of DNA sequence; however, the affinity for dsDNA was approximately fivefold higher. In coimmunoprecipitation reactions, PF-8 also interacted with Pol-8. In in vitro processivity assays with excess poly(dA):oligo(dT) as a template, PF-8 stimulated the production of elongated DNA products by Pol-8 in a dose-dependent manner. Functional domains of PF-8 were determined using PF-8 truncation mutants. The carboxyl-terminal 95 amino acids (aa) of PF-8 were dispensable for all three functions of PF-8: enhancing processivity of Pol-8, binding dsDNA, and binding Pol-8. Residues 10 to 27 and 279 to 301 were identified as regions critical for the processivity function of PF-8. Interestingly, aa 10 to 27 were also essential for binding Pol-8, whereas aa 1 to 62 and aa 279 to 301 were involved in binding dsDNA, suggesting that the processivity function of PF-8 is correlated with both the Pol-8-binding and the dsDNA-binding activities of PF-8.

Analysis of in vitro activities of herpes simplex virus type 1 UL42 mutant proteins: correlation with in vivo function

The DNA polymerase (pol) catalytic subunit of herpes simplex virus type 1, encoded by UL30, and its accessory factor, UL42 protein, are both essential for the replication of the virus. Because the stable interaction between UL42 and pol renders the pol fully processive for replicative DNA synthesis, disruption of this interaction represents a potential goal in the development of novel antiviral compounds. To better compare the effects of mutations in UL42 protein on its known in vitro functions, mutations were expressed as glutathione-S-transferase (GST)-fusions and the fusion proteins used in affinity chromatography. In this report, we demonstrate the relationship between the abilities of mutant UL42 fusion proteins to bind pol and to stimulate pol activity in vitro, and the abilities of nonfusion mutant proteins to function in viral replication. The pol stimulation assay using GST fusion proteins was found to be a more accurate and sensitive measure of the ability of the UL42 protein to function in vitro than the pol binding assay using the fusion proteins linked to a solid matrix. We also found an excellent correlation between the ability of purified GST fusion proteins to stimulate pol activity in vitro and the ability of full-length nonfusion UL42 mutant genes to support DNA replication in infected cells. Our results demonstrate that two noncontiguous stretches of amino acids, from 137 to 142 and from 274 to 282, are essential for UL42 function in vivo and in vitro. Although mutant d241-261 exhibited close to wild-type abilities to stimulate pol activity in vitro, it was not capable of complementing the replication of a UL42 null mutant virus. The region of UL42 protein within or close to 241-261 may serve to hinge the essential regions within the N- and C-terminal portions of the protein which are thought to interdigitate. It is hypothesized that reduction in the length of the hinge region could alter the ability of UL42, and/or its complex with pol, to function with one or more of the other proteins present in the DNA replisome within infected cells.

Zhangfei: a second cellular protein interacts with herpes simplex virus accessory factor HCF in a manner similar to Luman and VP16

Host cell factor (HCF, C1, VCAF or CFF) is a cellular protein that is required for transcription activation of herpes simplex virus (HSV) immediate-early (IE) genes by the virion protein VP16. The biological function of HCF remains unclear. Recently we identified a cellular transcription activator, Luman. As with VP16, the transactivation function of Luman is also regulated by HCF. Here we report a second human protein, Zhangfei (ZF) that interacts with HCF in a fashion similar to Luman and VP16. Although ZF shares no significant sequence homology with Luman, the two proteins have some structural similarities. These include: a basic domain-leucine zipper (bZIP) region, an acidic activation domain and a consensus HCF-binding motif. Unlike Luman, or most other bZIP proteins, ZF by itself did not appear to bind consensus bZIP-binding sites. It was also unable to activate promoters containing these response elements. Although in transient expression assays ectopically expressed ZF was unable to block transactivation by VP16 of a HSV IE promoter, ZF could prevent the expression of several HSV proteins in cells infected with the virus. The ability of ZF to block the synthesis of the HSV IE protein ICP0 relied on its binding to HCF, since a mutant of ZF that was unable to bind HCF was also unable to prevent viral IE protein expression.

Potential role for luman, the cellular homologue of herpes simplex virus VP16 (alpha gene trans-inducing factor), in herpesvirus latency

The cascade of herpes simplex virus (HSV) gene expression that results in viral replication begins with the activation of viral immediate-early (IE) genes by the virion-associated protein VP16. VP16 on its own is inefficient at associating with complexes formed on IE gene promoters and depends upon the cellular factor HCF for its activity. In this respect VP16 mimics the host basic leucine zipper (bZIP) protein Luman, which also requires HCF for activating transcription. Our objective is to explore interactions between Luman and HCF and to determine if they play a role in the biology of herpesviruses. In this report we show that in cultured cells ectopically expressed Luman was retained in the cytoplasm, where it colocalized with Calnexin, a protein normally associated with the endoplasmic reticulum (ER). Retention of Luman in the ER depends on a hydrophobic segment of the protein that probably serves as a transmembrane domain. Deletion of this domain changed the intracellular location of Luman so that most of the mutant protein was in the nucleus of cells. While HCF was present in the nucleus of most cells, in cells expressing Luman it was retained in the cytoplasm where the two proteins colocalized. This cytoplasmic association of Luman and HCF could also be demonstrated in neurons in trigeminal ganglia removed from cattle soon after death. Cells in tissue culture that expressed Luman, but not a mutant form of the protein that fails to bind HCF, were resistant to a productive infection with HSV type 1 (HSV-1). We hypothesize that similar Luman-HCF interactions in sensory neurons in trigeminal ganglia result in the suppression of viral replication and the establishment of latency. Interestingly, Luman could activate the promoters of IE110 and LAT, two genes that are critical for reactivation of HSV-1 from latency. This suggests a role for Luman in the reactivation process as well.

The transcriptional activation domain of VP16 is required for efficient infection and establishment of latency by HSV-1 in the murine peripheral and central nervous systems

The herpes simplex virus (HSV) transactivator VP16 is a structural component of the virion that activates immediate-early viral gene expression. The HSV-1 mutant in1814, which contains a 12-bp insertion that compromises the transcriptional function of VP16, replicated to a low level if at all in the trigeminal ganglia of mice (I. Steiner, J. G. Spivack, S. L. Deshmane, C. I. Ace, C. M. Preston, and N. W. Fraser (1990). J. Virol. 64, 1630-1638; Valyi-Nagy et al., unpublished data). However, in1814 did establish a latent infection in the ganglia after corneal inoculation from which it could be reactivated. In this study, several HSV-1 strains were constructed with deletions in the VP16 transcriptional activation domain. These viruses were viable in cell culture, although some were significantly reduced in their ability to initiate infection. A deletion mutant completely lacking the activation domain of VP16 (RP5) was unable to replicate to any detectable level or to efficiently establish latent infections in the peripheral and central nervous systems of immunocompetent mice. However, similar to in1814, RP5 formed a slowly progressing persistent infection in immunocompromised nude mice. Thus RP5 is severely neuroattenuated in the murine model of HSV infection. However, the activation domain of VP16 is not essential for replication in the nervous system, since we observed a slow progressive infection persisting in the absence of an immune response.

Herpes simplex virus processivity factor UL42 imparts increased DNA-binding specificity to the viral DNA polymerase and decreased dissociation from primer-template without reducing the elongation rate

Herpes simplex virus DNA polymerase consists of a catalytic subunit, Pol, and a processivity subunit, UL42, that, unlike other established processivity factors, binds DNA directly. We used gel retardation and filter-binding assays to investigate how UL42 affects the polymerase-DNA interaction. The Pol/UL42 heterodimer bound more tightly to DNA in a primer-template configuration than to single-stranded DNA (ssDNA), while Pol alone bound more tightly to ssDNA than to DNA in a primer-template configuration. The affinity of Pol/UL42 for ssDNA was reduced severalfold relative to that of Pol, while the affinity of Pol/UL42 for primer-template DNA was increased approximately 15-fold relative to that of Pol. The affinity of Pol/UL42 for circular double-stranded DNA (dsDNA) was reduced drastically relative to that of UL42, but the affinity of Pol/UL42 for short primer-templates was increased modestly relative to that of UL42. Pol/UL42 associated with primer-template DNA approximately 2-fold faster than did Pol and dissociated approximately 10-fold more slowly, resulting in a half-life of 2 h and a subnanomolar Kd. Despite such stable binding, rapid-quench analysis revealed that the rates of elongation of Pol/UL42 and Pol were essentially the same, approximately 15 [corrected] nucleotides/s. Taken together, these studies indicate that (i) Pol/UL42 is more likely than its subunits to associate with DNA in a primer-template configuration rather than nonspecifically to either ssDNA or dsDNA, and (ii) UL42 reduces the rate of dissociation from primer-template DNA but not the rate of elongation. Two models of polymerase-DNA interactions during replication that may explain these findings are presented.

The herpesvirus transactivator VP16 mimics a human basic domain leucine zipper protein, luman, in its interaction with HCF

In human cells infected with herpes simplex virus (HSV), viral gene expression is initiated by the virion protein VP16. VP16 does not bind DNA directly but forms a multiprotein complex on the viral immediate-early gene promoters with two cellular proteins: the POU domain protein Oct-1 and host cell factor (HCF; also called C1, VCAF, and CFF). Despite its apparent role in stabilizing the VP16-induced transcription complex, the natural biological role of HCF is unclear. Only recently HCF has been implicated in control of the cell cycle. To determine the role of HCF in cells and answer why HSV has evolved an HCF-dependent mechanism for the initiation of the lytic cycle, we identified the first human ligand for HCF (R. Lu et al., Mol. Cell. Biol. 17:5117-5126, 1997). This protein, Luman, is a member of the CREB/ATF family of transcription factors that can activate transcription from promoters containing cyclic AMP response elements (CRE). Here we provide evidence that Luman and VP16 share two important structural features: an acidic activation domain and a common mechanism for binding HCF. We found that Luman, its homolog in Drosophila, dCREB-A (also known as BBF-2), and VP16 bind to HCF by a motif, (D/E)HXY(S/A), present in all three proteins. In addition, a mutation (P134S) in HCF that prevents VP16 binding also abolishes its binding to Luman and dCREB-A. We also show that while interaction with HCF is not required for the ability of Luman to activate transcription when tethered to the GAL4 promoter, it appears to be essential for Luman to activate transcription through CRE sites. These data suggest that the HCF-Luman interaction may represent a conserved mechanism for transcriptional regulation in metazoans, and HSV mimics this interaction with HCF to monitor the physiological state of the host cell.

Luman, a new member of the CREB/ATF family, binds to herpes simplex virus VP16-associated host cellular factor

The human host cell factor (HCF) is expressed in a variety of adult and fetal tissues, and its gene is conserved in animals as diverse as mammals and insects. However, its only known function is to stabilize the herpes simplex virus virion transactivator VP16 in a complex with the cellular POU domain protein Oct-1 and cis-acting regulatory elements in promoters of immediate-early viral genes. To identify a cellular function for HCF, we used the yeast two-hybrid system to identify a cellular ligand for HCF. This protein, Luman, appears to be a cyclic AMP response element (CRE)-binding protein/activating transcription factor 1 protein of the basic leucine zipper superfamily. It binds CREs in vitro and activates CRE-containing promoters when transfected into COS7 cells. This activation of transcription was synergistically enhanced by the presence of CCAAT/enhancer-binding protein elements and inhibited by AP-1 elements in the promoter. In addition to a basic DNA binding domain, Luman possesses an unusually long leucine zipper and an acidic amino-terminal activation domain. These features in Luman are also present in what appear to be homologs in the mouse, Drosophila melanogaster, and Caenorhabditis elegans. Luman and VP16 appear to have similar mechanisms for binding HCF, as in vitro each competitively inhibited the binding of the other to HCF. In transfected cells, however, while VP16 strongly inhibited the ability of GAL-Luman to activate transcription from a GAL4 upstream activation sequence-containing promoter, Luman was unable to inhibit the activity of GAL-VP16. Luman appears to be a ubiquitous transcription factor, and its mRNA was detected in all human adult and fetal tissues examined. The possible role of HCF in regulating the function of this ubiquitous transcription factor is discussed.

The VP16 paradox: herpes simplex virus VP16 contains a long-range activation domain but within the natural multiprotein complex activates only from promoter-proximal positions

Removal of core promoter elements like the TATA box converts several regulatory upstream regions of viral and cellular genes into classical enhancers, i.e., cis-regulatory elements capable of activating transcription over long distances in an orientation-independent manner. This is not the case with herpes simplex virus (HSV) immediate-early gene promoters, which are strongly induced by the viral transactivator VP16 (Vmw65, alphaTIF, ICP25) complexed with the cellular factors Oct-1 and HCF. Here we report that the VP16 complex can readily bring about strong activation from a promoter-proximal position but fails to induce transcription from a distal downstream enhancer position. This is in striking contrast to results obtained with GAL fusion proteins: in this context, the C-terminal "general" activation domain of VP16 activates transcription to high levels over long distances. Thus, this paradoxical behavior suggests that the VP16 activation domain is not accessible to the transcription machinery when the VP16-Oct-1-HCF complex is bound in a remote position. Only upon specific interactions in a promoter-proximal position, perhaps with the basal transcription factors, can transcription be strongly induced. In agreement with such a proposed mechanism, VP16 proteins to which a heterologous general activation domain has been added strongly activate transcription from a downstream position. The biological role of this unexpected and sophisticated mechanism is most probably a limitation of the VP16 activity to the associated immediate-early genes, without undesired long-range effects on other viral promoters within the tightly packed HSV genome.

A single serine residue at position 375 of VP16 is critical for complex assembly with Oct-1 and HCF and is a target of phosphorylation by casein kinase II

We show that VP16 is phosphorylated by cellular kinases in vivo and in vitro and map the major sites of phosphorylation to be on serines towards the C-terminus, downstream of position 370 in both cases. Deletion of the acidic activation domain had no effect on phosphorylation, refining the sites to between position 370 and 411. Within VP16, the C-terminal boundary for complex formation with Oct-1 and HCF lies at position 388, and between 370 and 388 lies one serine, at position 375. This is a consensus casein kinase II (CKII) site and, using purified wild-type and mutant proteins, we show that it is the main CKII site in the body of the N-terminal complex-forming region. This site is also phosphorylated in nuclear extracts. Although other sites, mainly Ser411, are also phosphorylated by nuclear kinase(s), the single substitution of Ser375 to alanine abolishes CKII phosphorylation in vitro and virtually eliminates complex formation. This serine lies in a surface-exposed region of VP16 and, although complex formation is disrupted, other activities of the mutant are unaffected. Ser375 is also required in vivo where substitution to alanine abolishes transactivation, while replacement with threonine restores normal levels of activity.

Purification of herpes simplex virus type 1 by density gradient centrifugation and estimation of the sedimentation coefficient of the virion

Purification of herpes simplex virus type 1 (HSV-1) is often performed by centrifugation in gradients of various materials. A major problem with such procedures is a gradual decrease in the infectivity of the virus, probably due to the influence of the gradient material. In the present work we have compared Nycodenz gradients with Ficoll gradients for HSV-1 purification. Both gradient materials have low osmolarities. The purity of the preparations recovered from the two gradients was similar, as measured by electron microscopy and two-dimensional gel electrophoresis, and the yield of infectious virus was approximately the same. Szilágyi & Cunningham (22) reported previously that Ficoll gradients separate one band of complete virions and another band containing light particles devoid of nucleocapsids. In Nycodenz gradients we observed the same bands, and an additional third band which contained aggregates and disrupted particles. In this respect, the separation in Nycodenz is slightly superior to that in Ficoll. The purified HSV-1 virions were subjected to analytical differential centrifugation and an S-value of 670S was calculated for the virus.

Two-dimensional gel analysis of [35S]methionine labelled and phosphorylated proteins present in virions and light particles of herpes simplex virus type 1, and detection of potentially new structural proteins

Cells infected with herpes simplex virus (HSV) synthesize both infectious viruses and non-infectious light particles (L-particles). The latter contain the envelope and tegument components of the virions, but lack virus capsid and DNA. Electrophoresis in SDS-polyacrylamide gels (SDS-PAGE) has been used extensively for analysis of structural proteins in virions and L-particles. Two-dimensional (2-D) gel electrophoresis, however has a markedly higher resolution, and in the present work we have used this technique to study both [35S]methionine labelled and phosphorylated structural proteins in virions and L-particles. Proteins were assigned to the tegument or the envelope by the analysis of L-particles. Localization of structural proteins was also determined by stepwise solubilization in the presence of the neutral detergent NP-40 and NaCl, and by isolation of capsids from nuclei of infected cells. Different steps in posttranslational modification can be detected by 2-D gel electrophoresis such that a single polypeptide may appear as several spots. This was most clearly observed for some of the HSV-encoded glycoproteins which were shown to exist in multiple forms in the virion. Some polypeptides apparently not identified previously were either capsid associated, or localized in the tegument or envelope. The degrees of phosphorylation in L-particles and virions are almost identical for some proteins, but markedly different for others. Thus, glycoprotein E of HSV-1 is for the first time shown to be phosphorylated, and most heavily so in virions. The IE VMW)110 protein represents a group of proteins which are more phosphorylated in L-particles than in virions. Attempts are made to correlate the proteins detected by 2-D analysis with those previously separated by SDS-PAGE.

Determination and analysis of the complete nucleotide sequence of human herpesvirus

Human herpesvirus 7 (HHV-7) is a recently isolated betaherpesvirus that is prevalent in the human population, with primary infection usually occurring in early childhood. HHV-7 is related to human herpesvirus 6 (HHV-6) in terms of both biological and, from limited prior DNA sequence analysis, genetic criteria. However, extensive analysis of the HHV-7 genome has not been reported, and the precise phylogenetic relationship of HHV-7 to the other human betaherpesviruses HHV-6 and human cytomegalovirus has not been determined. Here I report on the determination and analysis of the complete DNA sequence of HHV-7 strain JI. The data establish that the close biological relationship of HHV-6 and HHV-7 is reflected at the genetic level, where there is a very high degree of conservation of genetic content and encoded amino acid sequences. The data also delineate loci of divergence between the HHV-6 and HHV-7 genomes, which occur at the genome terminal in the region of the terminal direct-repeat elements and within limited regions of the unique component. Of potential significance with respect to biological and evolutionary divergence of HHV-6 and HHV-7 are notable structural differences in putative transcriptional regulatory genes specified by the direct-repeat and immediate-early region A loci of these viruses and the absence of an equivalent of the HHV-6 adeno-associated virus type 2 rep gene homolog in HHV-7.

Human interferon reduces surface expression but not total production of herpes simplex virus type 1 glycoproteins gC and gE in heterologous hamster cells

The effect of interferon (IFN) on herpes simplex virus type 1 (HSV-1)-induced glycoproteins gC and gE was investigated in a heterologous IFN/cell model. In this model, the effect on surface expression of the glycoproteins could be studied separately from the effect on virus multiplication. Pretreatment of baby hamster kidney cells (BHK) with heterologous human leukocyte IFN suppressed surface expression of HSV-1-encoded gC and gE but had no influence on total production of the glycoproteins. This was in contrast to the effect on human embryonic fibroblast cells (HE) (homologous IFN and cells), where surface expression as well as total production of glycoproteins were reduced. The surface expression was demonstrated by antibody-sensitized monodisperse polystyrene beads, and immunoblotting and two-dimensional electrophoretic analysis of radioisotope-labeled proteins were used to study the total production.

Conformational alteration of Oct-1 upon DNA binding dictates selectivity in differential interactions with related transcriptional coactivators

VP16 (termed VP16-H here) of herpes simplex virus (HSV) belongs to a family of related regulatory proteins which includes VP16-B of bovine herpesvirus (BHV). We show that VP16-B, while also being a powerful transactivator of transcription dependent on Oct-1 binding sites in its target promoters, has virtually no activity on a defined VP16-H-responsive, octamer-containing target promoter. While Oct-1 binds equally well to the VP16-B-responsive and -nonresponsive sites, VP16-B interacts with Oct-1 only when Oct-1 is bound to the BHV octamer site and not when it is bound to the HSV site. We show from the analysis of chimeric proteins that the ability of VP16-B to discriminate between the Oct-1 forms depends on features of its N-terminal region. We also show from an analysis of chimeric DNA motifs that sequences that lie 3' to the POU domain-contacting region of the HSV octamer site play a role in making it unresponsive to VP16-B. Finally, we show by high-resolution hydroxyl radical footprint analysis that the conformation of Oct-l is different on the two sites. These results augment our previous report on an allosteric effect of DNA signals on the conformation of bound proteins and indicate that different conformations of the same DNA binding protein can be recognized selectively by related members of interacting regulatory proteins. The possible implications of our observations for selective gene regulation by Oct-1, a ubiquitous transcription factor, and other multimember transcription families are discussed.

Characterization of monoclonal antibodies recognizing amino- and carboxy-terminal epitopes of the herpes simplex virus UL42 protein

A panel of monoclonal antibodies (MAbs) directed against the herpes simplex virus type 1 (HSV-1) DNA polymerase (Pol) accessory protein, UL42, was developed and characterized. Thirteen different MAbs were isolated which exhibited varied affinities for the protein. All MAbs reacted with UL42 in ELISA, Western blot and immunoprecipitation analyses. Competitive ELISA was used to show that 6 different epitopes within UL42 were recognized by the MAbs. Immunoprecipitation of amino- and carboxy-terminal truncations of UL42 mapped the epitopes to regions containing amino acids 1-10, 10-108, 338-402, 402-460, and 460-477. All but one of these epitopes were outside the minimal active portion of the protein previously mapped to amino acids 20-315. None of these MAbs, alone or in combination, specifically neutralized the ability of UL42 to stimulate Pol activity in vitro. These results are consistent with structure-function studies that showed that N- and C-terminal regions of the UL42 protein, those recognized by the MAbs, are not involved in UL42 function in vitro.

The bovine herpesvirus alpha gene trans-inducing factor activates transcription by mechanisms different from those of its herpes simplex virus type 1 counterpart VP16

In herpes simplex virus (HSV)-infected cells, viral gene expression is initiated when the immediate-early, or alpha, genes are transactivated by the alpha gene trans-inducing factor (alpha TIF), a component of the infecting virion. The protein binds to one or more recognition elements (TAATGARAT) in the promoters of alpha genes via interaction with the cellular proteins Oct-1 and CFF. The alpha TIF of HSV (HSV-alpha TIF) is believed to subsequently accelerate the assembly of the transcription complex by direct contact between its carboxyl-terminal acidic activation domain and at least two components of the transcription apparatus, TAFII40 and TFIIB. Like its HSV counterpart, the alpha TIF of bovine herpesvirus (BHV) (designated BHV-alpha TIF) also transactivates alpha gene promoters and for full activity exhibits a requirement for its extended carboxyl-terminal region. Despite this requirement, there is a notable lack of homology to the carboxyl-terminal acidic activation domain of HSV-alpha TIF. We swapped the amino- and carboxyl-terminal domains of HSV-alpha TIF and BHV-alpha TIF to make chimeric proteins. Using these chimeras, we show that the carboxyl terminus of BHV-alpha TIF is insufficient for transactivation, which requires cooperative determinants in both the amino-terminal and carboxyl-terminal regions of the protein. We have previously shown that the amino-terminal determinant in BHV-alpha TIF displays reduced but significant independent transactivation potential. Interestingly, this amino-terminal determinant appears not to reside in the HSV-alpha TIF, which displays no independent amino-terminal activity. Furthermore, we show that the amino-terminal activation domain of BHV-alpha TIF may be able to act synergistically with the carboxyl-terminal activation domain of HSV-alpha TIF, since a chimeric protein containing both domains appeared to be more efficient at activating transcription than either alpha TIF. In addition, the amino terminus of HSV-alpha TIF could not restore activity when linked to the carboxyl terminus of BHV-alpha TIF, while the amino terminus of BHV-alpha TIF reconstituted an intact protein with potent activation potential. We also show that in fusions with the DNA binding domain of GAL4, full activity requires the entire BHV-alpha TIF, although both amino and carboxyl termini display some activity on their own. In contrast, for HSV-alpha TIF, the carboxyl terminus is sufficient and possibly even more potent than the entire protein, while the amino-terminus is devoid of activity.(ABSTRACT TRUNCATED AT 400 WORDS)

Protein and DNA elements involved in transactivation of the promoter of the bovine herpesvirus (BHV) 1 IE-1 transcription unit by the BHV alpha gene trans-inducing factor

In herpes simplex virus (HSV)-infected cells, the transcription of immediate-early (alpha) genes is regulated by a virion component, the alpha gene trans-inducing factor (alpha TIF). This protein forms a complex with cellular factors and TAATGARAT motifs present in one or more copies in the promoters of all alpha genes. We have characterized the bovine herpesvirus 1 (BHV-1) homolog of this protein. Like its HSV counterpart, the BHV alpha TIF was synthesized in the later stages of infection and could be demonstrated to be a component of purified virions. In transient expression assays, BHV alpha TIF was a strong transactivator and stimulated the activity of IE-1, the major BHV-1 alpha gene promoter, with an efficiency comparable to that of HSV alpha TIF. This stimulation was largely dependent on a TAATGAGCT sequence present in a single copy in IE-1, and BHV alpha TIF, in conjunction with cellular factors, formed a complex with oligonucleotides containing this sequence. Despite these similarities between the two alpha TIFs, our preliminary observations suggest that the proteins may activate transcription by different mechanisms. Although BHV alpha TIF strongly transactivated IE-1, it differed from its HSV counterpart in that the carboxyl terminus of BHV alpha TIF, when fused to the DNA-binding domain of GAL4, was a relatively poor stimulator of a promoter containing GAL4-binding sites. Also unlike HSV alpha TIF, removal of the carboxyl terminus of BHV alpha TIF reduced but did not eliminate the ability of the protein to transactivate IE-1. These results are discussed in view of the structural similarities and differences among the alpha TIFs of alphaherpes-viruses.

Transcriptional mapping of the varicella-zoster virus regulatory genes encoding open reading frames 4 and 63

Four of the 68 varicella-zoster virus (VZV) unique open reading frames (ORFs), i.e., ORFs 4, 61, 62, and 63, encode proteins that influence viral transcription and are considered to be positional homologs of herpes simplex virus type 1 (HSV-1) immediate-early (IE) proteins. In order to identify the elements that regulate transcription of VZV ORFs 4 and 63, the encoded mRNAs were mapped in detail. For ORF 4, a major 1.8-kb and a minor 3.0-kb polyadenylated [poly(A)+] RNA were identified, whereas ORF 63-specific probes recognized 1.3- and 1.9-kb poly(A)+ RNAs. Probes specific for sequences adjacent to the ORFs and mapping of the RNA 3' ends indicated that the ORF 4 RNAs were 3' coterminal, whereas the RNAs for ORF 63 represented two different termination sites. S1 nuclease mapping and primer extension analyses indicated a single transcription initiation site for ORF 4 at 38 bp upstream of the ORF start codon. For ORF 63, multiple transcriptional start sites at 87 to 95, 151 to 153, and (tentatively) 238 to 243 bp upstream of the ORF start codon were identified. TATA box motifs at good positional locations were found upstream of all mapped transcription initiation sites. However, no sequences resembling the TAATGARAT motif, which confers IE regulation upon HSV-1 IE genes, were found. The finding of the absence of this motif was supported through analyses of the regulatory sequences of ORFs 4 and 63 in transient transfection assays alongside those of ORFs 61 and 62. Sequences representing the promoters for ORFs 4, 61, and 63 were all stimulated by VZV infection but failed to be stimulated by coexpression with the HSV-1 transactivator Vmw65. In contrast, the promoter for ORF 62, which contains TAATGARAT motifs, was activated by VZV infection and coexpression with Vmw65. These results extend the transcriptional knowledge for VZV and suggest that ORFs 4 and 63 contain regulatory signals different from those of the ORF 62 and HSV-1 IE genes.

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

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

Nucleotide sequence analysis of a 38.5-kilobase-pair region of the genome of human herpesvirus 6 encoding human cytomegalovirus immediate-early gene homologs and transactivating functions

Human herpesvirus 6 (HHV-6) is prevalent in the human population, with primary infection occurring early in life. Its predominant CD4+ T-lymphocyte tropism, its ability to activate human immunodeficiency virus type 1 (HIV-1) gene expression in vitro, and its upregulation of CD4 expression has led to speculation that HHV-6 may act as a positive cofactor in the progression of HIV infection to AIDS in individuals infected with both viruses. Previous sequencing studies of restricted regions of the 161.5-kbp genome of HHV-6 have demonstrated unequivocally that it is a member of the betaherpesvirus subgroup and have indicated that the HHV-6 genome is generally collinear with the unique long (UL) component of human cytomegalovirus (HCMV). In the work described in this report we have extended these sequencing studies by determining the primary structure of 38.5-kbp of the HHV-6 genome (genomic position 21.0 to 59.5 kbp). Within the sequenced region lie 31 open reading frames, 20 of which are homologous to positional counterparts in HCMV. Of particular significance is the identification of homologs of the HCMV UL36-38 and US22-type genes, which have been shown to encode transactivating proteins. We show that DNA sequences encoding these HHV-6 homologs were able to transactivate HIV-1 long terminal repeat-directed chloramphenicol acetyltransferase expression in cotransfection assays, thus demonstrating functional as well as structural conservation of these betaherpesvirus-specific gene products. Our data therefore confirm the close relationship between HHV-6 and HCMV and identify putative immediate-early regulatory genes of HHV-6 likely to play key roles in lytic replication and possibly also in the interactions between HHV-6 and HIV in dually infected cells.

Two regions of the herpes simplex virus type 1 UL42 protein are required for its functional interaction with the viral DNA polymerase

Two essential gene products of herpes simplex virus type 1, the viral DNA polymerase (pol) and UL42, its accessory protein, physically and functionally interact to form the core of the viral DNA replication complex. Understanding this essential interaction would provide a basis from which to develop novel anti-herpesvirus agents. We previously have shown that when coexpressed in an in vitro transcription-translation system, UL42 stimulates pol activity (M. L. Gallo, D. I. Dorsky, C. S. Crumpacker, and D. S. Parris, J. Virol. 63:5023-5029, 1989). By analyzing various insertion, deletion, and frameshift mutations of UL42 in this system, we found the C-terminal 149 amino acids to be dispensable for the ability of the protein to stimulate pol activity. In addition, two distinct internal regions of UL42 were found to be required for pol stimulation. Regions I and II were defined to lie between amino acid residues 129 and 163 and between residues 202 and 337, respectively. When physical association was examined with antibody to UL42, pol was found to coimmunoprecipitate to the same level when expressed with a UL42 mutant protein lacking region I as that with wild-type UL42. Thus, mere physical association is insufficient for stimulation of pol activity. Deletion of region II reduced or eliminated coimmunoprecipitation with pol. Interestingly, an antibody to pol specific for residues 1216 to 1224 coimmunoprecipitated UL42 when both proteins were synthesized in a baculovirus expression system but not in rabbit reticulocyte lysates. These results indicate that (i) at least a portion of the region recognized by the pol antiserum may be accessible in the pol-UL42 heterodimer and (ii) immunoprecipitation results for products made in different expression systems may vary. Thus, at least two distinct regions of UL42 are essential for functional interaction with pol. Moreover, these results point to a UL42 region I function other than physical association with pol.

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.

Deletions of the carboxy terminus of herpes simplex virus type 1 UL42 define a conserved amino-terminal functional domain

The herpes simplex virus type 1 UL42 protein was synthesized in reticulocyte lysates and assayed for activity in vitro. Three functional assays were used to examine the properties of in vitro-synthesized UL42: (i) coimmunoprecipitation to detect stable complex formation with purified herpes simplex virus type 1 DNA polymerase (Pol), (ii) a simple gel-based assay for DNA binding, and (iii) a sensitive assay for the stimulation of Pol activity. UL42 synthesized in reticulocyte lysates formed a stable coimmunoprecipitable complex with Pol, bound to double-stranded DNA, and stimulated the activity of Pol in vitro. Carboxy-terminal truncations of the UL42 protein were synthesized from restriction enzyme-digested UL42 gene templates and gene templates made by polymerase chain reaction and assayed for in vitro activity. Truncations of the 488-amino-acid (aa) UL42 protein to aa 315 did not abolish its ability to bind to Pol and DNA or to stimulate Pol activity. Proteins terminating at aas 314 and 313 showed reduced levels of binding to Pol, but these and shorter proteins were unable to bind to DNA or to stimulate Pol activity. These results suggest that all three of the biochemical functions of UL42 colocalize entirely within the N-terminal 315 aas of the UL42 protein. Amino acid sequence alignment of alpha herpesvirus UL42 homologs revealed that the N-terminal functional domain corresponds to the most highly conserved region of the protein, while the dispensable C terminus is not conserved. Conservative aa changes at the C terminus of the 315-aa truncated protein were used to show that conserved residues were important for activity. These results suggest that 173 aa of UL42 can be deleted without a loss of activity and that DNA-binding and Pol-binding activities are correlated with the ability of UL42 to stimulate Pol activity.

Functional analysis of the herpes simplex virus UL42 protein

The herpes simplex virus UL42 gene encodes a multifunctional polypeptide (UL42) that is essential for virus DNA replication. To further understand the relationship between the structure of UL42 and the role that it plays during virus replication, we analyzed an extensive set of mutant UL42 proteins for the ability to perform the three major biochemical functions ascribed to the protein:binding to DNA, stably associating with the virus DNA polymerase (Pol), and acting to increase the length of DNA chains synthesized by Pol. Selected mutants were also assayed for their ability to complement the replication of a UL42 null virus. The results indicated that the N-terminal 340 amino acids of UL42 were sufficient for all three biochemical activities and could also support virus replication. Progressive C-terminal truncation resulted in the loss of detectable DNA-binding activity before Pol binding, while several mutations near the N terminus of the polypeptide resulted in an altered interaction with DNA but had no apparent affect on Pol binding. More dramatically, an insertion mutation at residue 160 destroyed the ability to bind Pol but had no effect on DNA binding. This altered polypeptide also failed to increase the length of DNA product synthesized by Pol, and the mutant gene could not complement the growth of a UL42 null virus, indicating that the specific interaction between Pol and UL42 is necessary for full Pol function and for virus replication. This study confirms the validity of the Pol-UL42 interaction as a target for the design of novel therapeutic agents.

The structure and function of the HSV DNA replication proteins: defining novel antiviral targets

The absolute dependence of herpes simplex virus (HSV) replication on HSV DNA polymerase and six other viral-encoded replication proteins implies that specific inhibitors of these proteins' functions would be potent antiviral agents. The only currently licensed anti-herpes simplex drug, acyclovir, is an inhibitor of HSV DNA polymerase and is widely held to block viral replication primarily by specifically inhibiting viral DNA replication. In spite of the substantial advance in HSV therapy in recent years through the introduction of acyclovir, this anti-HSV compound and most of the other compounds under pharmaceutical development are substrate analogs. Since antiviral drug resistance has become an issue of increasing clinical importance, the need for structurally unrelated agents which incorporate novel mechanisms of viral inhibition is apparent. Understanding the structure and function of herpesvirus DNA polymerase and its interaction with the other six essential replication proteins at the replication origin should assist us in designing the next generation of therapeutic agents. The sequences of these proteins have been deduced and the proteins themselves have been expressed and purified in a variety of systems. The current challenge, therefore, is to use the available information about these proteins to identify and develop new, exquisitely specific antiviral therapeutics. In this review, we have summarized the current approaches and the results of structure/function studies of the herpes virus proteins essential for DNA replication, with the goal of more precisely defining novel antiviral targets.

Mapping of a major surface-exposed site in herpes simplex virus protein Vmw65 to a region of direct interaction in a transcription complex assembly

The cellular factor Oct-1 is selectively recruited, together with at least one other cellular protein (CFF), into a multicomponent transcription complex whose assembly is directed by the herpes simplex virus regulatory protein Vmw65 (VP16). The acidic carboxy terminus of Vmw65 is not involved in assembly of the complex but is absolutely required for subsequent transcriptional activation. Elucidation of the mechanism of action of Vmw65 is important for an understanding not only of combinatorial control of gene expression by POU- and homeodomain proteins but also of the interaction(s) between activation domains of regulatory proteins and components of the basal transcriptional apparatus. We used a combination of limited proteolysis with a number of site-specific proteases and immunological detection to demonstrate the presence of two main surface-exposed regions in Vmw65. We mapped these sites to within a few amino acids at positions 365-370 408/409. The site at 408/409 is indicative of a flexible exposed linker region between the acidic carboxy-terminal activation domain (residues 430-480) and an N-terminal domain involved in complex formation with the two cellular factors. The site around residues 365-370 is precisely within a region that results from this and other laboratories have shown to be critical for complex formation. Furthermore, we show that this site is selectively protected from proteolysis after complex assembly. Finally, using a series of overlapping peptide encompassing this region, we show that the eight amino acids, R-E-H-A-Y-S-R-A, from positions 360 through 367 are sufficient to inhibit complex formation by intact Vmw65. We propose that these residues contain sufficient information to selectively bind one of the cellular partners involved in complex assembly and that these residues are located in a physical surface-exposed domain of the protein.

Mutations in the C terminus of herpes simplex virus type 1 DNA polymerase can affect binding and stimulation by its accessory protein UL42 without affecting basal polymerase activity

We have analyzed the effects of mutations in the herpes simplex virus type 1 DNA polymerase (Pol) C-terminal UL42 binding domain on the activity of Pol and its ability to form complexes with and be stimulated by UL42 in vitro. Wild-type Pol expressed in Saccharomyces cerevisiae was both bound and stimulated by UL42 in vitro. C-terminal truncations of 19 and 40 amino acids (aa) did not affect the ability of Pol to be stimulated by UL42 in vitro. This stimulation as well as basal Pol activity in the presence of UL42 was inhibited by polyclonal anti-UL42 antiserum, thus indicating a physical interaction between Pol and UL42. Removal of the C-terminal 59 aa of Pol and internal deletions of 72 aa within the Pol C terminus eliminated stimulation by UL42. None of the truncations or deletions within Pol affected basal polymerase activity. In contrast with their ability to be stimulated by UL42, only wild-type Pol and Pol lacking the C-terminal 19 aa bound UL42 in a coimmunoprecipitation assay. These results demonstrate that a functional UL42 binding domain of Pol is separable from sequences necessary for basal polymerase activity and that the C-terminal 40 aa of Pol appear to contain a region which modulates the stability of the Pol-UL42 interaction.

The extreme C terminus of herpes simplex virus DNA polymerase is crucial for functional interaction with processivity factor UL42 and for viral replication

The herpes simplex virus DNA polymerase is composed of two subunits, a large catalytic subunit (Pol) and a smaller subunit (UL42) that increases the processivity of the holoenzyme. The interaction between the two polypeptides is of interest both for the mechanism by which it enables the enzyme to synthesize long stretches of DNA processively and as a possible target for the rational design of novel antiviral drugs. Here, we demonstrate through a combination of insertion and deletion mutagenesis that the carboxy-terminal 35 amino acids of Pol are crucial for binding UL42. The functional importance of the interaction was confirmed by the finding that a pol mutant defective for UL42 binding retained polymerase activity, but did not synthesize longer DNA products in the presence of UL42. Moreover, several association-incompetent mutants failed to complement the replication of a pol null mutant in a transient transfection assay, confirming that the Pol-UL42 interaction is necessary for virus replication in vivo and therefore a valid target for directed drug design.

Herpes simplex virus immediate early gene expression in the absence of transinduction by Vmw65 varies during the cell cycle

The requirement for the herpes simplex virus type 1 (HSV-1) protein Vmw65 (VP16) for activation of immediate early (IE) gene expression was examined in synchronized HeLa cells. Analyses of IE RNA levels were conducted during infection with a viral Vmw65 mutant, in1814. The results revealed an increased requirement for Vmw65 when cultures reached G2 phase of the cell cycle. The levels of IE RNAs 1, 2, and 4 were reduced 5-10 times more in G2 than G1/S for in1814-infected cells when compared to cells infected with wild-type virus or 1814R (a rescued virus), and similar but smaller effects were observed on IE RNA 3 levels. The relative decrease at G2 was reversed by resynchronization of cells to G1/S. Mutant in1814 formed plaques less efficiently on cells at G2 than on cells synchronized at G1/S. The results show that, in the absence of functional Vmw65, HSV-1 IE gene expression and replication vary during the cell cycle.

Immediate early and functional AP-1 cis-response elements are involved in the transcriptional regulation of the large subunit of herpes simplex virus type 2 ribonucleotide reductase (ICP10)

Expression from the promoter of the herpes simplex virus type 2 (HSV-2) large subunit of ribonucleotide reductase (ICP10) is stimulated by co-transfection with DNA that encodes the virion protein Vmw65 previously shown to activate in trans the transcription of all IE genes (Wymer et al., 1989). Specific cis response elements involved in ICP10 transcriptional regulation were studied by chloramphenicol acetyltransferase analysis with hybrid ICP10 promoter/CAT structural gene constructions containing wild type or site-directed mutations of the promoter sequences. The data indicate that Vmw65 activation requires an intact TAAT-GARAT motif while complex formation requires an intact Oct-1 element, and the AP-1 consensus elements in the ICP10 promoter are functional in vitro. Thus, expression from the wild type and GA-rich mutant constructions was enhanced 10-20-fold by co-transfection with DNA encoding Vmw65. The GARAT and POU homeobox (PHB) binding motifs were required for Vmw65 mediated activation but the mutant in the POU specific box (PSB) binding motif was activated at higher concentrations of Vmw65 DNA (1.0-3.0 micrograms). The PHB and PSB binding motifs were necessary for complex formation as determined by gel retardation analysis with in vitro synthesized OTF-1 and Vmw65 proteins. The GARAT and GA-rich elements were not required. CAT expression from pICP10-cat was enhanced by co-transfection with jun and fos encoding DNA, and the ICP10 promoter complexed with in vitro synthesized jun protein.

Analysis of nucleotide sequence of the rightmost 43 kbp of herpesvirus saimiri (HVS) L-DNA: general conservation of genetic organization between HVS and Epstein-Barr virus

We present an analysis of 43,658 bp of contiguous nucleotide sequence comprising the right terminal region (conventional orientation) of the unique protein-coding component (L-DNA) of the herpesvirus saimiri (HVS) genome. Within this region lie the genes encoding the 160-kDa virion protein, which is homologous to the 140-kDa membrane antigen of Epstein-Barr virus (EBV), thymidylate synthase (TS), and the immediate-early (IE) 52-kDa protein which is homologous to the EBV BMLF1 product. The 160-kDa gene of HVS lies at the right terminus of HVS L-DNA, its homologue in EBV occurring at the left terminus of the EBV genome (conventional orientation). The TS gene of HVS occurs within a group of 5 genes that have no homologues in EBV. The translation product of one of these genes, ECRF3, shows amino acid sequence and hydrophobicity pattern similarities to the HCMV and cellular G-protein-coupled receptor family of proteins. Another, ECLF2, is homologous to the cyclin family of cellular proteins. The 5 nonconserved genes lie adjacent to the 160-kDa gene. In EBV, the region to the right of the 140-kDa gene (BNRF1) contains the latent replication origin (OriP) and the open reading frames BCRF1, BWRF1 (repeated 12 times), BYRF1, BHLF1, and BHRF1, counterparts of which are not present in this position in HVS. The subsequent 18 genes in EBV (BFLF2 to BLRF2, approximate positions 56,000-89,500) are represented in HVS, and the relative positions and orientations of these genes are directly comparable between the two viruses. There then occurs a nonhomologous gene in HVS, and genes BLLF2 to BZLF1 (positions 89,500 to 103,200) in EBV which are not present in this region of HVS, before collinearity resumes. Thus, the HVS sequence presented here shows general collinearity between conserved genes in the right terminal region of HVS and the left terminal region of EBV and reveals the presence of two sets of unique genes which occur in exactly analogous positions in HVS and EBV.

Differential dependence of herpes simplex virus immediate-early gene expression on de novo-infected cell protein synthesis

The time course of accumulation of herpes simplex virus immediate-early (IE) mRNA and the requirement for infected cell protein synthesis for mRNA transcription and accumulation were compared. Measurements of transcription in nuclear run-on assays, accumulation of cytoplasmic mRNA by Northern (RNA) blot hybridization, and rates of infected cell protein synthesis by pulse-labeling did not indicate differences among the five IE gene, consistent with previous studies. However, as a result of varying the amount of de novo protein synthesis after infection, at least three patterns of maximal expression of the IE genes were revealed. Addition of the protein synthesis inhibitor anisomycin to cells coincident with infection resulted in maximal rates of transcription and accumulation of functional ICP0 mRNA, while 0.5 h of infected cell protein synthesis prior to addition of the drug was required for maximal expression of ICP22/47 and ICP27 mRNAs. Maximal expression of ICP4 mRNA occurred only when 1 h of de novo protein synthesis occurred prior to the addition of the drug. These results are discussed in the context of alternative mechanisms for regulating IE gene expression.

Sequence, function, and regulation of the Vmw65 gene of herpes simplex virus type 2

We determined the sequence of the gene for the virion transactivator protein Vmw65 of herpes simplex virus type 2 (HSV-2), strain 333. An analysis of the coding sequence revealed an overall high degree of primary sequence conservation (86%) relative to the HSV-1 protein, although the carboxy-terminal region which encompasses the powerful acidic transactivation domain of the HSV-1 protein was slightly less well conserved (70%). One important change in this region was the presence of a proline residue in a region of the HSV-2 protein which is thought to form an amphipathic alpha-helix in the HSV-1 homolog. Despite the occurrence of this helix-disrupting residue, the HSV-2 protein exhibited powerful transactivation properties for immediate-early target promoters. We also demonstrated that the HSV-2 protein forms a transcriptional complex (TRF.C) with the cellular Oct-1 protein and target TAATGARAT elements from immediate-early promoters. A comparison of upstream sequences from the two Vmw65 genes revealed good conservation of proximal promoter elements but considerable divergence elsewhere. Specifically, the HSV-2 promoter alone carries 9.5 copies of a 9-bp direct repeat (GGGGCGGGA) ending 85 bp upstream of the conserved TTAAAT element. An analysis of transcription factor binding sites in vitro revealed that cellular factor Sp1 bound to the direct repeat sequence of the HSV-2 promoter and that cellular factor USF bound to a proximal element present in both HSV-1 and HSV-2 promoters. Mutational analysis of the HSV-2 promoter demonstrated that the integrity of both of these binding sites was important for the full activity of the promoter.

Polymerization activity of an alpha-like DNA polymerase requires a conserved 3'-5' exonuclease active site

Most DNA polymerases are multifunctional proteins that possess both polymerizing and exonucleolytic activities. For Escherichia coli DNA polymerase I and its relatives, polymerase and exonuclease activities reside on distinct, separable domains of the same polypeptide. The catalytic subunits of the alpha-like DNA polymerase family share regions of sequence homology with the 3'-5' exonuclease active site of DNA polymerase I; in certain alpha-like DNA polymerases, these regions of homology have been shown to be important for exonuclease activity. This finding has led to the hypothesis that alpha-like DNA polymerases also contain a distinct 3'-5' exonuclease domain. We have introduced conservative substitutions into a 3'-5' exonuclease active site homology in the gene encoding herpes simplex virus DNA polymerase, an alpha-like polymerase. Two mutants were severely impaired for viral DNA replication and polymerase activity. The mutants were not detectably affected in the ability of the polymerase to interact with its accessory protein, UL42, or to colocalize in infected cell nuclei with the major viral DNA-binding protein, ICP8, suggesting that the mutation did not exert global effects on protein folding. The results raise the possibility that there is a fundamental difference between alpha-like DNA polymerases and E. coli DNA polymerase I, with less distinction between 3'-5' exonuclease and polymerase functions in alpha-like DNA polymerases.