Interactions between herpes simplex virus DNA-binding proteins

The UL12 protein of herpes simplex virus 1 is regulated by tyrosine phosphorylation

The herpes simplex virus 1 (HSV-1) UL12 protein (pUL12) is a nuclease that is critical for viral replication in vitro and neurovirulence in vivo. In this study, mass spectrometric analysis of pUL12 and phosphate-affinity SDS-polyacrylamide gel electrophoresis analysis identified tyrosine at pUL12 residue 371 (Tyr-371) as a pUL12 phosphorylation site: Tyr-371 is conserved in pUL12 homologs in herpesviruses in all Herpesviridae subfamilies. Replacement of Tyr-371 with phenylalanine (Y371F) in pUL12 (i) abolished its exonuclease activity in HSV-1-infected Vero, HEL, and A549 cells, (ii) reduced viral replication, cell-cell spread, and pUL12 expression in infected cells in a cell type-dependent manner, (iii) led to aberrant subcellular localization of pUL12 in infected cells in a cell type-dependent manner, and (iv) reduced HSV-1 neurovirulence in mice. The effects of the pUL12 Y371F mutation in cell cultures and mice were similar to those of a nuclease-dead double mutation in pUL12, although the Y371F mutation reduced viral replication severalfold more than the nuclease-dead double mutation in a cell type- and multiplicity-of-infection-dependent manner. Replacement of Tyr-371 with glutamic acid, which mimics constitutive phosphorylation, restored the wild-type phenotype in cell cultures and mice. These results suggested that phosphorylation of pUL12 Tyr-371 was essential for pUL12 to express its nuclease activity in HSV-1-infected cells and that this phosphorylation promoted viral replication and cell-cell spread in cell cultures and neurovirulence in mice mainly by upregulating pUL12 nuclease activity and, in part, by regulating the subcellular localization and expression of pUL12 in HSV-1-infected cells.

IMPORTANCE

Herpesviruses encode a considerable number of enzymes for their replication. Like cellular enzymes, the viral enzymes need to be properly regulated in infected cells. Although the functional aspects of herpesvirus enzymes have gradually been clarified, information on how most of these enzymes are regulated in infected cells is lacking. In the present study, we report that the enzymatic activity of the herpes simplex virus 1 alkaline nuclease pUL12 was regulated by phosphorylation of pUL12 Tyr-371 in infected cells and that this phosphorylation promoted viral replication and cell-cell spread in cell cultures and neurovirulence in mice, mainly by upregulating pUL12 nuclease activity. Interestingly, pUL12 and tyrosine at pUL12 residue 371 appeared to be conserved in all herpesviruses in the family Herpesviridae, raising the possibility that the herpesvirus pUL12 homologs may also be regulated by phosphorylation of the conserved tyrosine residue.

Recombination promoted by DNA viruses: phage λ to herpes simplex virus

The purpose of this review is to explore recombination strategies in DNA viruses. Homologous recombination is a universal genetic process that plays multiple roles in the biology of all organisms, including viruses. Recombination and DNA replication are interconnected, with recombination being essential for repairing DNA damage and supporting replication of the viral genome. Recombination also creates genetic diversity, and viral recombination mechanisms have important implications for understanding viral origins as well as the dynamic nature of viral-host interactions. Both bacteriophage λ and herpes simplex virus (HSV) display high rates of recombination, both utilizing their own proteins and commandeering cellular proteins to promote recombination reactions. We focus primarily on λ and HSV, as they have proven amenable to both genetic and biochemical analysis and have recently been shown to exhibit some surprising similarities that will guide future studies.

Role of the nuclease activities encoded by herpes simplex virus 1 UL12 in viral replication and neurovirulence

Enzyme-dead mutations in the herpes simplex virus 1 UL12 gene that abolished its endo- and exonuclease activities only slightly reduced viral replication in cell cultures. However, the UL12 null mutation significantly reduced viral replication, suggesting that a UL12 function(s) unrelated to its nuclease activities played a major role in viral replication. In contrast, the enzyme-dead mutations significantly reduced viral neurovirulence in mice, suggesting that UL12 nuclease activities were critical for viral pathogenesis in vivo.

The pre-NH(2)-terminal domain of the herpes simplex virus 1 DNA polymerase catalytic subunit is required for efficient viral replication

The catalytic subunit of herpes simplex virus 1 DNA polymerase (HSV-1 Pol) has been extensively studied; however, its full complement of functional domains has yet to be characterized. A crystal structure has revealed a previously uncharacterized pre-NH(2)-terminal domain (residues 1 to 140) within HSV-1 Pol. Due to the conservation of the pre-NH(2)-terminal domain within the herpesvirus Pol family and its location in the crystal structure, we hypothesized that this domain provides an important function during viral replication in the infected cell distinct from 5'-3' polymerase activity. We identified three pre-NH(2)-terminal Pol mutants that exhibited 5'-3' polymerase activity indistinguishable from that of wild-type Pol in vitro: deletion mutants PolΔN43 and PolΔN52 that lack the extreme N-terminal 42 and 51 residues, respectively, and mutant PolA(6), in which a conserved motif at residues 44 to 49 was replaced with alanines. We constructed the corresponding pol mutant viruses and found that the polΔN43 mutant displayed replication kinetics similar to those of wild-type virus, while polΔN52 and polA(6) mutant virus infection resulted in an 8-fold defect in viral yield compared to that achieved with wild type and their respective rescued derivative viruses. Additionally, both polΔN52 and polA(6) viruses exhibited defects in viral DNA synthesis that correlated with the observed reduction in viral yield. These results strongly indicate that the conserved motif within the pre-NH(2)-terminal domain is important for viral DNA synthesis and production of infectious virus and indicate a functional role for this domain.

Herpes simplex virus UL12.5 targets mitochondria through a mitochondrial localization sequence proximal to the N terminus

The herpes simplex virus type 1 (HSV-1) gene UL12 encodes a conserved alkaline DNase with orthologues in all herpesviruses. The HSV-1 UL12 gene gives rise to two separately promoted 3' coterminal mRNAs which encode distinct but related proteins: full-length UL12 and UL12.5, an amino-terminally truncated form that initiates at UL12 codon 127. Full-length UL12 localizes to the nucleus where it promotes the generation of mature viral genomes from larger precursors. In contrast, UL12.5 is predominantly mitochondrial and acts to trigger degradation of the mitochondrial genome early during infection. We examined the basis for these very different subcellular localization patterns. We confirmed an earlier report that the amino-terminal region of full-length UL12 is required for nuclear localization and provide evidence that multiple nuclear localization determinants are present in this region. In addition, we demonstrate that mitochondrial localization of UL12.5 relies largely on sequences located between UL12 residues 185 and 245 (UL12.5 residues 59 to 119). This region contains a sequence that resembles a typical mitochondrial matrix localization signal, and mutations that reduce the positive charge of this element severely impaired mitochondrial localization. Consistent with matrix localization, UL12.5 displayed a detergent extraction profile indistinguishable from that of the matrix protein cyclophilin D. Mitochondrial DNA depletion required the exonuclease activity of UL12.5, consistent with the idea that UL12.5 located within the matrix acts directly to destroy the mitochondrial genome. These results clarify how two highly related viral proteins are targeted to different subcellular locations with distinct functional consequences.

Herpes simplex virus type 1 single-strand DNA binding protein ICP8 enhances the nuclease activity of the UL12 alkaline nuclease by increasing its processivity

UL12 is a 5'- to 3'-exonuclease encoded by herpes simplex virus type 1 (HSV-1) which degrades single- and double-stranded DNA. UL12 and the single-strand DNA binding protein ICP8 mediate a strand exchange reaction. We found that ICP8 inhibited UL12 digestion of single-stranded DNA but stimulated digestion of double-stranded DNA threefold. The stimulatory effect of ICP8 was independent of a strand exchange reaction; furthermore, the effect was specific to ICP8, as it could not be reproduced by Escherichia coli single-stranded DNA binding protein. The effect of ICP8 on the rate of UL12 double-stranded DNA digestion is attributable to an increase in processivity in the presence of ICP8.

Catalysis of strand exchange by the HSV-1 UL12 and ICP8 proteins: potent ICP8 recombinase activity is revealed upon resection of dsDNA substrate by nuclease

The replication of herpes simplex virus type 1 (HSV-1) is associated with a high degree of homologous recombination, which is likely to be mediated, in part, by HSV-1-encoded proteins. We have previously shown that the HSV-1 encoded ICP8 protein and alkaline nuclease UL12 are capable of catalyzing an in vitro strand-exchange reaction. Here, we show, by electron microscopy, that the products of the strand exchange reaction between linear double-stranded DNA and circular single-stranded DNA consist of the expected joint molecule forms: sigma, alpha, and gapped circles. Other exonucleases, such as lambda Red alpha, which, like UL12, digests 5'-3', as well as Escherichia coli exonuclease III (ExoIII), which digests 3'-5', could substitute for UL12 in the strand exchange reaction by providing a resected DNA end. ICP8 generated the same intermediates and strand exchange products when the double-stranded DNA substrate was preresected by any of the nucleases. Using substrates with large regions of non-homology we found that pairing by ICP8 could be initiated from the middle of a DNA molecule and did not require a homologous end. In this reaction, the resection of a DNA end by the nuclease is required to reveal homologous sequences capable of being paired by ICP8. This study further illustrates the complexity of the multi-functional ICP8 protein.

The UL12.5 gene product of herpes simplex virus type 1 exhibits nuclease and strand exchange activities but does not localize to the nucleus

The herpes simplex virus type 1 (HSV-1) alkaline nuclease, encoded by the UL12 gene, plays an important role in HSV-1 replication, as a null mutant of UL12 displays a severe growth defect. Although the precise in vivo role of UL12 has not yet been determined, several in vitro activities have been identified for the protein, including endo- and exonuclease activities, interaction with the HSV-1 single-stranded DNA binding protein ICP8, and an ability to promote strand exchange in conjunction with ICP8. In this study, we examined a naturally occurring N-terminally truncated version of UL12 called UL12.5. Previous studies showing that UL12.5 exhibits nuclease activity but is unable to complement a UL12 null virus posed a dilemma and suggested that UL12.5 may lack a critical activity possessed by the full-length protein, UL12. We constructed a recombinant baculovirus capable of expressing UL12.5 and purified soluble UL12.5 from infected insect cells. The purified UL12.5 exhibited both endo- and exonuclease activities but was less active than UL12. Like UL12, UL12.5 could mediate strand exchange with ICP8 and could also be coimmunoprecipitated with ICP8. The primary difference between the two proteins was in their intracellular localization, with UL12 localizing to the nucleus and UL12.5 remaining in the cytoplasm. We mapped a nuclear localization signal to the N terminus of UL12, the domain absent from UL12.5. In addition, when UL12.5 was overexpressed so that some of the enzyme leaked into the nucleus, it was able to partially complement the UL12 null mutant.

Specificity of the Endonuclease Activity of the Baculovirus Alkaline Nuclease for Single-stranded DNA

The Autographa californica multiple nucleocapsid nucleopolyhedrovirus (AcMNPV) alkaline nuclease (AN) likely participates in the maturation of virus genomes and in DNA recombination. AcMNPV AN was expressed in a recombinant baculovirus as a His -tagged fusion and obtained in pure form (*AN) or as a (6)complex with the baculoviral single-stranded DNA-binding protein LEF-3 (*AN/L3). Both AN preparations possessed potent 5' --> 3'-exonuclease and weak endonuclease activities. Mutant *AN(S146A)/L3 with a change from serine to alanine at position 146 in a conservative motif was impaired in both activities. This proved that the endonuclease is an intrinsic activity of baculovirus AN. The AN endonuclease showed specificity for single-stranded DNA and converted supercoiled plasmid DNA (replicative form I, RFI) into the open circular form (RFII) by a single strand break. Plasmid DNA relaxed with topoisomerase I was resistant to *AN/L3 indicating that the partially single-stranded regions in negatively supercoiled molecules served as targets for the endonuclease. Unwinding the supercoiled DNA with ethidium bromide also made DNA resistant to AN/L3. In reactions with nicked circular DNA (RFII), AN and AN/L3 hydrolyzed exonucleolytically the broken strand or cut endonucleolytically the intact strand at the position opposite the nick (gap). When LEF-3 was added to the assay, the balance between the exonucleolytic and endonucleolytic modes of hydrolysis shifted in favor of the exonuclease. The data suggest that the AN endonuclease may digest the intermediates in replication and recombination at positions of structural irregularities in DNA duplexes, whereas LEF-3 may further regulate processing of the intermediates by AN via the endonuclease and exonuclease pathways.

The herpes simplex virus type 1 alkaline nuclease and single-stranded DNA binding protein mediate strand exchange in vitro

The replication of herpes simplex virus type 1 (HSV-1) DNA is associated with a high degree of homologous recombination. While cellular enzymes may take part in mediating this recombination, we present evidence for an HSV-1-encoded recombinase activity. HSV-1 alkaline nuclease, encoded by the UL12 gene, is a 5'-->3' exonuclease that shares homology with Redalpha, commonly known as lambda exonuclease, an exonuclease required for homologous recombination by bacteriophage lambda. The HSV-1 single-stranded DNA binding protein ICP8 is an essential protein for HSV DNA replication and possesses single-stranded DNA annealing activities like the Redbeta synaptase component of the phage lambda recombinase. Here we show that UL12 and ICP8 work together to effect strand exchange much like the Red system of lambda. Purified UL12 protein and ICP8 mediated the complete exchange between a 7.25-kb M13mp18 linear double-stranded DNA molecule and circular single-stranded M13 DNA, forming a gapped circle and a displaced strand as final products. The optimal conditions for strand exchange were 1 mM MgCl(2), 40 mM NaCl, and pH 7.5. Stoichiometric amounts of ICP8 were required, and strand exchange did not depend on the nature of the double-stranded end. Nuclease-defective UL12 could not support this reaction. These data suggest that diverse DNA viruses appear to utilize an evolutionarily conserved recombination mechanism.

Baculovirus alkaline nuclease possesses a 5'-->3' exonuclease activity and associates with the DNA-binding protein LEF-3

Alkaline nuclease (AN) of the Autographa californica multiple-capsid nucleopolyhedrovirus (AcMNPV) (open reading frame 133) was expressed in recombinant baculovirus as a His(6)-tagged fusion and purified by sequential chromatography on Ni-NTA-agarose, DEAE-Toyopearl, and heparin-Sepharose. At all stages of purification, AcMNPV AN was found to copurify with a 44-kDa polypeptide which was identified as the baculovirus single-stranded DNA (ssDNA)-binding (SSB) protein, LEF-3. Sedimentation analysis in glycerol gradients of highly purified samples suggested that AN and LEF-3 are associated in a complex (designated *AN/L3), predominantly as heterodimers, although oligomeric forms containing both proteins were evident. In reactions with single- or double-stranded 62-mer oligonucleotides that were labeled with (32)P at the 5' or 3' ends, *AN/L3 carried out exonucleolytic hydrolysis of both substrates exclusively in a 5'-->3' direction. Saturation of ssDNA with an excess of LEF-3 prior to the addition of *AN/L3 resulted in a marked decrease in the rate of ssDNA hydrolysis. This suggests that excess LEF-3 may protect ssDNA from digestion by a AN-LEF-3 complex, thus regulating its activity in infected cells. The association of baculovirus AN with the viral SSB LEF-3 and the 5'-->3' exonuclease activity of this complex suggests that AN and LEF-3 may participate in homologous recombination of the baculovirus genome in a manner similar to that of exonuclease (Redalpha) and DNA-binding protein (Redbeta) of the Red-mediated homologous recombination system of bacteriophage lambda.

Importance of the N-terminal region of the phage GA-1 single-stranded DNA-binding protein for its self-interaction ability and functionality

The single-stranded DNA-binding protein (SSB) of phage GA-1 displays higher efficiency than the SSBs of the related phages phi 29 and Nf. In this work, the self-interaction ability of GA-1 SSB has been analyzed by visualization of the purified protein by electron microscopy, glycerol gradient sedimentation, and in vivo cross-linking of bacterial cultures infected with phage GA-1. GA-1 SSB contains an insert at its N-terminal region that is not present in the SSBs of phi 29 and Nf. Three deletion mutant proteins have been characterized, Delta N19, Delta N26, and Delta N33, which lack the 19, 26 or 33 amino acids, respectively, that follow the initial methionine of GA-1 SSB. Mutant protein Delta N19 retains the structural and functional behavior of GA-1 SSB, whereas mutant proteins Delta N26 and Delta N33 no longer stimulate viral DNA replication or display helix-destabilizing activity. Analysis of the mutant proteins by ultracentrifugation in glycerol gradients and electron microscopy indicates that deletion of 26 or 33 but not of 19 amino acids of the N-terminal region of GA-1 SSB results in the loss of the oligomerization ability of this protein. Our data support the importance of the N-terminal region of GA-1 SSB for the differential self-interaction ability and functional behavior of this protein.

Herpesvirus alkaline deoxyribonuclease; a possible candidate as a novel target for anti-herpesvirus therapy

Herpesvirus alkaline deoxyribonucrease (DNase) is coded in the genome of all herpesvirus species determined total sequence and is conserved in structure. In order to determine whether the enzyme could be a target for a novel antiherpesvirus therapy, the anti-herpes simplex virus type 1 (HSV-1) activity of antisense oligonucleotide for HSV-1 alkaline DNase was studied. Six antisense phosphorothioate oligonucleotides, targeted to an internal AUG start codon, were designed and evaluated. One of the oligonucleotides, UL12-4, inhibited wild type and thymidine kinase-deficient HSV-1 replication to 21.5 and 19.5% at 40 microM, respectively. The quantity of alkaline DNase mRNA and DNase activity in HSV-1-infected Vero cells was reduced to one eighth and 66.9% of control, respectively, by treatment with 40 microM of UL12-4, but no effect was observed on the quantity of HSV-1 glycoprotein H mRNA (gamma2 gene) or on the replication of Vero cells. These results indicate that UL12-4 inhibits HSV-1 replication by decreasing the amount of alkaline DNase mRNA. The herpesvirus alkaline DNase could be a novel target for anti-herpesvirus drug.

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.

Structural organization of a conserved gene cluster of Tupaia herpesvirus encoding the DNA polymerase, glycoprotein B, a probable processing and transport protein, and the major DNA binding protein

The Tupaia herpesviruses (THVs) have been isolated from malignant lymphoma tissue cultures and from degenerating lung and spleen cell cultures of tree shrews (Tupaia spp.). Recently we succeeded in the localization of the gene locus of the THV DNA polymerase (DPOL) gene within the viral genome. Based on these results the highly conserved gene cluster of herpesviruses encoding the DPOL, the glycoprotein B (gB), a probable processing and transport protein (PRTP), and the major DNA binding protein (DNBI) was characterized in the genome of THV strain 2 (THV-2) in its entirety. The complete nucleotide sequence of the gene cluster was determined and it was discovered that the THV-2 gene products are most closely related to the corresponding proteins of mammalian cytomegaloviruses. The transcriptional activity of the four genes was confirmed by amplification of a part of the corresponding mRNAs obtained from infected cell RNA by RT-PCR. The homology values and the overall structure of the gene cluster, that shows specific colinearity with the corresponding clusters of the mammalian cytomegaloviruses, is further evidence that THV-2 is a member of the subfamily Betaherpesvirinae.

Soluble expression and complex formation of proteins required for HCMV DNA replication using the SFV expression system

Several of the viral proteins required for human cytomegalovirus (HCMV) DNA replication have been difficult to study due to their low abundance in infected cells and low solubility in bacterial or insect-cell expression systems. Therefore we used the Semliki Forest virus expression system to express these proteins in mammalian cells. All of the recombinant proteins were soluble, on the basis of ultracentrifugation properties and their ability to be immunoprecipitated from solution with specific antibodies. Pulse-chase analysis of the 86-kDa major immediate-early protein (IE86) revealed two expressed forms-a precursor and a product-indicating that this recombinant protein, like the native HCMV protein, is posttranslationally processed. The recombinant proteins (polymerase core and accessory as well as the IE86 and pUL84) formed stable complexes similar to those known to form in HCMV-infected cells. The recombinant DNA polymerase holoenzyme also exhibited enzyme activity that was phosphonoformic acid sensitive, as is the infected-cell DNA polymerase activity. This expression system offers many advantages for the expression and study of the HCMV replication proteins, including the expression of soluble, active proteins that are able to interact to form complexes. Additionally, the relative ease with which SFV recombinants can be made lends itself to the construction and evaluation of mutants.

Herpes simplex virus type-1 single-strand DNA-binding protein (ICP8) enhances the ability of the viral DNA helicase-primase to unwind cisplatin-modified DNA

The herpes simplex virus type-1 UL5, UL8, and UL52 genes encode an essential heterotrimeric DNA helicase-primase that is responsible for concomitant DNA unwinding and primer synthesis at the viral DNA replication fork. The viral single-strand DNA-binding protein (ICP8) can stimulate DNA unwinding by the helicase-primase as a result of a physical interaction that is mediated by the UL8 subunit. In this study, we investigated the ability of the helicase-primase to unwind a fork-like substrate that contains an intrastrand d(GpG) DNA cross-link produced by the antitumor drug cisplatin. We also examined the ability of ICP8 to modulate the effect of the cisplatin lesion. The data show that the lesion inhibited the helicase-primase when located on the DNA strand along which it translocates. However, the lesion did not represent a permanent obstacle to its progression. In contrast, the adduct did not affect the helicase-primase when located on the opposite DNA strand. ICP8 specifically stimulated DNA unwinding by the helicase-primase. Coating concentrations of ICP8 were necessary for optimal unwinding of damaged DNA. Addition of competitor DNA to helicase reactions led to substantial reduction of DNA unwinding by the helicase-primase, suggesting that the enzyme is distributive. ICP8 did not abolish the competition, indicating that it did not stimulate the helicase by increasing its processivity. Rather, ICP8 may stimulate DNA unwinding and enable bypass of cisplatin damaged DNA by recruiting the helicase-primase to the DNA.

Structure-function analysis of the herpes simplex virus type 1 UL12 gene: correlation of deoxyribonuclease activity in vitro with replication function

Although the product of the UL12 gene of herpes simplex virus type 1 (HSV-1) has been shown to possess both exonuclease and endonuclease activities in vitro, and deletion of most of the gene within the viral genome results in inefficient production and maturation of infectious virions, the function of the deoxyribonuclease (DNase) activity per se in virus replication remains unclear. In order to correlate the in vitro and in vivo activities of the protein encoded by UL12, mutant proteins were tested for nuclease activity in vitro by a novel hypersensitivity cleavage assay and for their ability to complement the replication of a DNase null mutant, AN-1. Rabbit reticulocyte lysates programmed with wild-type UL12 RNA cleaved at the same sites cleaved by purified HSV-1 DNase, but distinct from those cleaved by DNase 1 or micrococcal nuclease. All mutants which lacked DNase activity in vitro also failed to complement the replication of AN-1 in nonpermissive cells. Likewise, all mutants which contained HSV-1 DNase activity, as detected by the hypersensitivity cleavage assay, were capable of complementing the replication of the DNase null mutant, though to varying extents. Of particular note was the d1-126 mutant protein, which, despite having the same specific activity as the wild-type enzyme in vitro, complemented the replication of AN-1 significantly less than the wild-type protein. The results suggest that DNase activity per se is required for efficient replication of HSV-1 in vivo. However, residues, including the N-terminal 126 amino acids, which are dispensable for enzymatic activity in vitro may facilitate the accessibility or activity of the protein in vivo.

The herpes simplex virus type-1 single-strand DNA-binding protein, ICP8, increases the processivity of the UL9 protein DNA helicase

Herpes simplex virus type-1 UL9 protein is a sequence-specific DNA-binding protein that recognizes elements in the viral origins of DNA replication and possesses DNA helicase activity. It forms an essential complex with its cognate single-strand DNA-binding protein, ICP8. The DNA helicase activity of the UL9 protein is greatly stimulated as a consequence of this interaction. A complex of these two proteins is thought to be responsible for unwinding the viral origins of DNA replication. The aim of this study was to identify the mechanism by which ICP8 stimulates the translocation of the UL9 protein along DNA. The data show that the association of the UL9 protein with DNA substrate is slow and that its dissociation from the DNA substrate is fast, suggesting that it is nonprocessive. ICP8 caused maximal stimulation of DNA unwinding activity at equimolar UL9 protein concentrations, indicating that the active species is a complex that contains UL9 protein and ICP8 in 1:1 ratio. ICP8 prevented dissociation of UL9 protein from the DNA substrate, suggesting that it increases its processivity. ICP8 specifically stimulated the DNA-dependent ATPase activity of the UL9 protein with DNA cofactors that allow translocation of UL9 protein and those with secondary structure. These data suggest that UL9 protein and ICP8 form a specific complex that translocates along DNA. Within this complex, ICP8 tethers the UL9 protein to the DNA substrate, thereby preventing its dissociation, and participates directly in the assimilation and stabilization of the unwound DNA strand, thus facilitating translocation of the complex through regions of duplex DNA.

A major DNA binding protein encoded by BALF2 open reading frame of Epstein-Barr virus (EBV) forms a complex with other EBV DNA-binding proteins: DNAase, EA-D, and DNA polymerase

A major 135-kDa DNA binding protein (mDBP) encoded by the BALF2 open reading frame of Epstein-Barr Virus (EBV) is known to be an essential protein for the induction of the lytic cycle. The present investigation was carried out to know whether this protein forms a complex in vivo with other viral DNA binding proteins (DBP) involved in DNA replication: DNA polymerase, EA-D (diffused early antigen), and DNAase. Immunoprecipitation assays followed by mono- and two-dimensional electrophoresis showed that mDBP forms a complex with these three DBP. Other complexes were also found such as EA-D/DNAase, DNA polymerase/DNAase, and DNA polymerase/EA-D. The complexed forms already exist in the early stage of EBV cycle before DNA synthesis is induced in the EBV producer P3HR-1 cell line. The exonuclease activity encoded by DNAase was found to be inhibited when this enzyme complexed with mDBP, while the EBV DNA polymerase retained its activity in the complexed form with mDBP. Our results suggest that these complexes already present before DNA synthesis are necessary for EBV DNA synthesis.

The product of the UL12.5 gene of herpes simplex virus type 1 is a capsid-associated nuclease

The UL12 open reading frame of herpes simplex virus type 1 (HSV-1) encodes a deoxyribonuclease that is frequently referred to as alkaline nuclease (AN) because of its high pH optimum. Recently, an alternate open reading frame designated UL12.5 was identified within the UL12 gene. UL12.5 and UL12 have the same translational stop codon, but the former utilizes an internal methionine codon of the latter gene to initiate translation of a 60-kDa amino-terminal truncated form of AN. Since the role of the UL12.5 protein in the HSV-1 life cycle has not yet been determined, its properties were investigated in this study. Unlike AN, which can be readily solubilized from infected cell lysates, the UL12.5 protein was found to be a highly insoluble species, even when isolated by high-salt detergent lysis. Since many of the structural polypeptides which constitute the HSV-1 virion are similarly insoluble, a potential association of UL12.5 protein with virus particles was examined. By using Western blot analysis, the UL12.5 protein could be readily detected in preparations of intact virions, isolated capsid classes, and even capsids that had been extracted with 2 M guanidine-HCl. In contrast, AN was either missing or present at only low levels in each of these structures. Since the inherent insolubility of the UL12.5 protein prevented its potential deoxyribonuclease activity from being assayed in infected-cell lysates, partially purified fractions of soluble UL12.5 protein were generated by selectively solubilizing either insoluble infected-cell proteins or isolated capsid proteins with urea and renaturing them by stepwise dialysis. Initial analysis of these preparations revealed that they did contain an enzymatic activity that was not present in comparable fractions from cells infected with a UL12.5 null mutant of HSV-1. Additional biochemical characterization revealed that UL12.5 protein was similar to AN with respect to pH optimum, ionic strength, and divalent cation requirements and possessed both exonucleolytic and endonucleolytic functions. The finding that the UL12.5 protein represents a capsid-associated form of AN which exhibits nucleolytic activity suggests that it may play some role in the processing of genomic DNA during encapsidation.

Herpes simplex virus DNA replication

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

Cloning, sequencing, and functional characterization of the two subunits of the pseudorabies virus DNA polymerase holoenzyme: evidence for specificity of interaction

The pseudorabies virus (PRV) genes encoding the two subunits of the DNA polymerase were located on the genome by hybridization to their herpes simplex virus type 1 (HSV-1) homologs, pol and UL42, and subsequently were sequenced. Like the HSV-1 homologs, in vitro translation products of the PRV gene encoding the catalytic subunit (pol) possessed activity in the absence of the Pol accessory protein (PAP). However, the PRV PAP stimulated the activity of Pol fourfold in the presence of 150 mM KCl, using an activated calf thymus DNA template. The stimulation of Pol activity by PAP under high-salt conditions and the inhibition of Pol activity by PAP when assayed in low salt (0 mM KCl) together were used to determine the specificity with which PAP interacted with Pol. Despite functional similarity, HSV-1 UL42 and PRV PAP could neither stimulate the noncognate Pols at high salt nor inhibit them at low salt. Furthermore, a PRV Pol mutant lacking the 30 C-terminal amino acids retained basal Pol activity but could be neither stimulated nor inhibited by the PRV PAP. Sequence comparisons of the Pol proteins of the alphaherpesviruses reveal a conserved domain in the C terminus which terminates immediately before the last 41 residues of both PRV and HSV-1 proteins. These results indicate that the ability and specificity for interaction of the PRV Pol with PAP most likely resides predominantly in the extreme Pol C terminus.

Renaturation of complementary DNA strands by herpes simplex virus type 1 ICP8

ICP8, the major single-stranded DNA-binding protein of herpes simplex virus type 1, promotes renaturation of complementary single strands of DNA. This reaction is ATP independent but requires Mg2+. The activity is maximal at pH 7.6 and 80 mM NaCl. The major product of the reaction is double-stranded DNA, and no evidence of large DNA networks is seen. The reaction occurs at subsaturating concentrations of ICP8 but reaches maximal levels with saturating concentrations of ICP8. Finally, the renaturation reaction is second order with respect to DNA concentration. The ability of ICP8 to promote the renaturation of complementary single strands suggests a role for ICP8 in the high level of recombination seen in cells infected with herpes simplex virus type 1.

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.

Herpesvirus ICP18.5 and DNA-binding protein genes are conserved in equine herpesvirus-1

The genome of equine herpesvirus-1 (EHV-1) contained three open reading frames (ORFs) in a 3.9 kbp BamHI-SmaI fragment at 0.38-0.41 map units in the long unique region. The most 5' ORF encoded the carboxy terminus of a protein with 45-55 percent amino acid homology to the DNA-binding proteins (ICP8-DBP) of four other alpha-herpesviruses. The middle ORF translated to a polypeptide of 775 residues with 43-55% homology to the ICP18.5 proteins. The most 3' ORF encoded the EHV-1 glycoprotein B (gB) gene. Three mRNAs of 4.3, 4.4-4.8, and 3.5-3.9 kb (corresponding to the three sequenced ORFs) were all transcribed from the same strand. The gene order of this group was conserved in all herpesviruses examined.

Herpes simplex virus type 1 ICP8: helix-destabilizing properties

The major single-stranded DNA-binding protein, ICP8, of herpes simplex virus type 1 (HSV-1) is one of seven virus-encoded polypeptides required for HSV-1 DNA replication. To investigate the role of ICP8 in viral DNA replication, we have examined the interaction of ICP8 with partial DNA duplexes and found that it can displace oligonucleotides annealed to single-stranded M13 DNA. In addition, ICP8 can melt small fragments of fully duplex DNA. Unlike a DNA helicase, ICP8-promoted strand displacement is ATP and Mg2+ independent and exhibits no directionality. It requires saturating amounts of ICP8 and is both efficient and highly cooperative. These properties make ICP8 suitable for a role in DNA replication in which ICP8 destabilizes duplex DNA during origin unwinding and replication fork movement.

Surface lysine and tyrosine residues are required for interaction of the major herpes simplex virus type 1 DNA-binding protein with single-stranded DNA

Modification of the herpes simplex virus type 1 major DNA-binding protein (ICP8) with reagents and conditions specific for arginine, lysine, and tyrosine residues indicates that surface lysine and tyrosine residues are required for the interaction of this protein with single-stranded DNA. Modification of either of these two amino acids resulted in a loss and/or modification of binding activity as judged by nitrocellulose filter assays and gel shift. Modification specific for arginine residues did not affect binding within the limits of the assays used. Finally, quenching of the intrinsic tryptophan fluorescence of ICP8 in the presence of single-stranded DNA either suggests involvement of this amino acid in the binding reaction or reflects a conformational change in the protein upon binding.

Herpes simplex virus induces unscheduled DNA synthesis in virus-infected cervical cancer cell lines

We evaluated herpes-simplex-virus-type-2(HSV2)-induced unscheduled DNA synthesis in virus-infected cervical cancer (HeLa, CaSki, C-33A, and SiHa) cells. HSV2 replication was approximately 100-fold more efficient in the HeLa cells than in less susceptible C-33A and SiHa cells. In dual parameter flow cytometric analysis of bromodeoxyuridine (BrdU) incorporation, HSV2-infected HeLa cells showed a rapid increase in the proportions of DNA-synthesizing G1- and S-phase cells, whereas in C-33A and SiHa cells, the proportions of DNA-synthesizing G1- and early S-phase cells were increased late in the infection. Blocking of HSV2 replication by phosphonoformate inhibited virus-induced changes in HeLa cells, but not in C-33A and SiHa cells. Anti-BrdU antibodies exhibited a coarse globular nuclear staining pattern in the C-33A cells, while the other cells showed speckled and/or fine globular nuclear fluorescence. Anti-ICP8 (HSV-specified major DNA-binding protein) antibodies revealed that, in C-33A cells, ICP8 remained in the cytoplasm, whereas in the other cells, speckled or globular nuclear fluorescence was found. Our results showed that HSV2 induced the unscheduled synthesis of cellular DNA, which was host-cell-dependent, and in virus infected C-33A cells, it may be attributable to both viral and cellular proteins.

Association between the herpes simplex virus major DNA-binding protein and alkaline nuclease

Herpes simplex virus encodes seven proteins which have been shown to be both necessary and sufficient for in vitro replication of origin-containing plasmids. We have shown previously that one of these proteins, the major DNA-binding protein mDBP, forms a complex with alkaline nuclease, which is not one of the seven essential proteins. In this study, we have employed immunological reagents and a series of deletion mutants to investigate this complex further. We have determined the regions of mDBP which are important in the formation of this complex, and we have shown that the intranuclear locations of alkaline nuclease and major DNA-binding protein overlap.

Localization of the herpes simplex virus type 1 65-kilodalton DNA-binding protein and DNA polymerase in the presence and absence of viral DNA synthesis

Using indirect immunofluorescence, well-characterized monoclonal and polyclonal antibodies, and temperature-sensitive (ts) mutants of herpes simplex virus type 1, we demonstrated that the 65-kilodalton DNA-binding protein (65KDBP), the major DNA-binding protein (infected cell polypeptide 8 [ICP8]), and the viral DNA polymerase (Pol) colocalize to replication compartments in the nuclei of infected cells under conditions which permit viral DNA synthesis. When viral DNA synthesis was blocked by incubation of the wild-type virus with phosphonoacetic acid, the 65KDBP, Pol, and ICP8 failed to localize to replication compartments. Instead, ICP8 accumulated nearly exclusively to prereplication sites, while the 65KDBP was only diffusely localized within the nuclei. Although some of the Pol accumulated in prereplication sites occupied by ICP8 in the presence of phosphonoacetic acid, a significant amount of Pol also was distributed throughout the nuclei. Examination by double-labeling immunofluorescence of DNA- ts mutant virus-infected cells revealed that the 65KDBP also did not colocalize with ICP8 to prereplication sites at temperatures nonpermissive for virus replication. These results are in disagreement with the hypothesis that ICP8 is the major organizational protein responsible for attracting other replication protein to prereplication sites in preparation for viral DNA synthesis (A. de Bruyn Kops and D. M. Knipe, Cell 55:857-868, 1988), and they suggest that other viral proteins, perhaps in addition to ICP8, or replication fork progression per se are required to organize the 65KDBP.

Rapid detection of herpes simplex virus in clinical samples by flow cytometry after amplification in tissue culture

Murine monoclonal antibodies (MAbs) against herpes simplex virus type 1 and 2 (HSV-1 and -2, respectively) nuclear antigens were used to identify cells infected with HSV-1 or -2 by indirect immunofluorescence in conjunction with flow cytometry after virus amplification of MRC-5 cell monolayers. The results indicate that MAbs Q1, Q2, and H-640 detect HSV-1- and HSV-2-infected cells, MAb SD-1 detects HSV-2- but not HSV-1-infected cells, and MAb 58-S detects HSV-1- but not HSV-2-infected cells. MAb Q1, which detects HSV-1- as well as HSV-2-infected cells, was used to detect HSV-infected cells after inoculation and overnight (16- to 20-h) incubation of MRC-5 monolayers with clinical samples suspected of containing HSV. In comparing the efficiency of flow cytometry with cytopathic effect (CPE) in tissue culture for detecting HSV in clinical samples, HSV was detected by flow cytometry in 77% of the cases where HSV was detected by CPE in tissue culture. In many cases, flow cytometry detected HSV from 1 to 3 days before HSV was detected by CPE.

Purification and properties of Epstein-Barr virus DNase expressed in Escherichia coli

A cDNA corresponding to the BGLF5 open reading frame of the Epstein-Barr virus (EBV) genome and coding for an early DNase was inserted into the procaryotic expression vector pKK223-3. One bacterial clone producing the expected 52-kilodalton DNase was used as a source of EBV DNase. The 52-kilodalton Dnase was purified in the active form to near homogeneity by ammonium sulfate precipitation and successive chromatographies on phosphocellulose, DNA-cellulose, and gel filtration columns. The purified enzyme exhibited both exonuclease and endonuclease activities, an absolute requirement for divalent cations, an alkaline pH preference, and a typical residual activity in presence of 300 mM KCl. Moreover, the enzyme was specifically inhibited by human sera with high antibody titers to EBV early antigens. These properties are similar to those observed for EBV-induced DNase from lymphoblastoid cell extracts. In addition, the enzyme was recognized by both immunoglobulin G and A serum fractions from patients with nasopharyngeal carcinoma (NPC). From these results and previous studies which demonstrated the value of antibody titers to this viral DNase as an NPC marker, it appears that EBV-encoded DNase produced in a heterologous expression system could be used in the development of a specific and early NPC diagnosis test.

The essential 65-kilodalton DNA-binding protein of herpes simplex virus stimulates the virus-encoded DNA polymerase

The 65-kilodalton DNA-binding protein (65KDBP) of herpes simplex virus type 1 (HSV-1), the product of the UL42 gene, is required for DNA replication both in vitro and in vivo, yet its actual function is unknown. By two independent methods, it was shown that the 65KDBP stimulates the activity of the HSV-1-encoded DNA polymerase (Pol). When Pol, purified from HSV-1-infected cells, was separated from the 65KDBP, much of its activity was lost. However, addition of the 65KDBP, purified from infected cells, stimulated the activity of Pol 4- to 10-fold. The ability of a monoclonal antibody to the 65KDBP to remove the Pol-stimulating activity from preparations of the 65KDBP confirmed that the activity was not due to a trace contaminant. Furthermore, the 65KDBP did not stimulate the activity of other DNA polymerases derived from T4, T7, or Escherichia coli. The 65KDBP gene transcribed in vitro from cloned DNA and translated in vitro in rabbit reticulocyte lysates also was capable of stimulating the product of the pol gene when the RNAs were cotranslated. The product of a mutant 65KDBP gene missing the carboxy-terminal 28 amino acids exhibited wild-type levels of Pol stimulation, while the products of two large deletion mutants of the gene could not stimulate Pol activity. These experiments suggest that the 65KDBP may be an accessory protein for the HSV-1 Pol.

Kinetics of expression of the gene encoding the 65-kilodalton DNA-binding protein of herpes simplex virus type 1

The 65-kilodalton DNA-binding protein (65KDBP) of herpes simplex virus type 1, encoded by gene UL42, is required for herpes simplex virus origin-dependent DNA replication (C.A. Wu, N.J. Nelson, D.J. McGeoch, and M.D. Challberg, J. Virol. 62:435-443, 1988). We found by indirect immunofluorescence with monoclonal antibody to 65KDBP that the protein was first detectable at 3 h postinfection. It localized first to the inner periphery of the nucleus, but accumulated in large globular compartments within the nucleus by 6 h postinfection in a pattern similar to that displayed by the major DNA-binding protein ICP8. Immune electron microscopy revealed that 65KDBP was associated with the marginated heterochromatin at the early times, but migrated further into the nucleus at late times when the only discernible areas devoid of 65KDBP were the nucleoli and heterochromatin. The 65KDBP gene is a member of the beta kinetic class as determined by the ability of the mRNA to be expressed at significant levels even in the absence of viral DNA synthesis. Furthermore, in the presence or absence of the DNA polymerase inhibitor phosphonoacetic acid, the patterns of accumulation of protein as well as mRNA were virtually indistinguishable from those displayed by the model beta genes encoding ICP8 and thymidine kinase. Nuclear run-on experiments demonstrated that maximum rates of 65KDBP gene transcription occurred prior to the maximum rate of progeny viral DNA synthesis and confirmed that the expression of the 65KDBP gene is regulated at the level of transcriptional initiation.

The role of viral and cellular nuclear proteins in herpes simplex virus replication

Following infection of cells by herpes simplex virus, the cell nucleus is subverted for transcription and replication of the viral genome and assembly of progeny nucleocapsids. The transition from host to viral transcription involves viral proteins that influence the ability of the cellular RNA polymerase II to transcribe a series of viral genes. The regulation of RNA polymerase II activity by viral gene products seems to occur by several different mechanisms: (1) viral proteins complex with cellular proteins and alter their transcription-promoting activity (e.g., alpha TIF), (2) viral proteins bind to specific DNA sequences and alter transcription (e.g., ICP4), and (3) viral proteins affect the posttranslational modification of viral or cellular transcriptional regulatory proteins (e.g., possibly ICP27). Thus, HSV may utilize several different approaches to influence the ability of host-cell RNA polymerase II to transcribe viral genes. Although it is known that viral transcription uses the host-cell polymerase II, it is not known whether viral infection causes a change in the structural elements of the nucleus that promote transcription. In contrast, HSV encodes a new DNA polymerase and accessory proteins that complex with and reorganize cellular proteins to form new structures where viral DNA replication takes place. HSV may encode a large number of DNA replication proteins, including a new polymerase, because it replicates in resting cells where these cellular gene products would never be expressed. However, it imitates the host cell in that it localizes viral DNA replication proteins to discrete compartments of the nucleus where viral DNA synthesis takes place. Furthermore, there is evidence that at least one specific viral gene protein can play a role in organizing the assembly of the DNA replication structures. Further work in this system may determine whether assembly of these structures is essential for efficient viral DNA replication and if so, why assembly of these structures is necessary. Thus, the study of the localization and assembly of HSV DNA replication proteins provides a system to examine the mechanisms involved in morphogenesis of the cell nucleus. Therefore, several critical principles are apparent from these discussions of the metabolism of HSV transcription and DNA replication. First, there are many ways in which the activity of RNA polymerase II can be regulated, and HSV proteins exploit several of these in controlling the transcription of a single DNA molecule. Second, the interplay of these multiple regulatory pathways is likely to control the progress of the lytic cycle and may play a role in determining the lytic versus latent infection decision.(ABSTRACT TRUNCATED AT 400 WORDS)

Herpes simplex virus replication in the presence of DNA polymerase alpha inhibitors

2-(p-n-butylanilino)deoxyadenosine (BuAdA), and N-2-(p-n-butylphenyl)deoxyguanosine (BuPdG), selective inhibitors of mammalian DNA polymerase alpha, were added to BHK-21(C13) cell cultures infected with herpes simplex virus type 1 (HSV-1) strain 17 syn +. Infectious virus production decreased significantly in the presence of the inhibitor at concentrations varying from 1 nM to 100 microM. BuPdG was more effective than BuAdA at all concentrations tested, while it inhibited virus yield as much as BuAdA when CVG2, a thymidine kinase deficient (TK-) HSV-1, was employed. HSV DNA synthesis, determined by quantitation of CsCl separated DNA peaks, was inhibited by each compound. BuPdG inhibited viral DNA replication more than BuAdA, while the effect on cell DNA synthesis was the same as that of BuAdA. CVG2 DNA replication was inhibited to the same level by BuAdA as by BuPdG. These results indicate that HSV DNA replication is partially dependent on cell DNA polymerase alpha activity, and that the greater effect of BuPdG on viral replication may be ascribed to its action on HSV thymidine kinase.

Characterization of major recognition sequences for a herpes simplex virus type 1 origin-binding protein

To investigate early initiation events in the replication of herpes simplex virus type 1, we analyzed interactions of proteins from infected cell extracts with the small origin of herpes simplex virus type 1 (oris1). Using the mobility shift assay, we detected two origin-specific binding interactions. We characterized the more prominent interaction on both strands of the DNA duplex with DNase I protection and methylation interference assays. Protein binding protects 17 bases of DNA on each strand from DNase I. These sequences are located at the left end of the central palindrome and are shifted four bases relative to one another. On the basis of the DNase protection pattern, we believe this protein to be related to the origin-binding protein defined by Elias et al. (P. Elias, M.E. O'Donnell, E.S. Mocarski, and I.R. Lehman, Proc. Natl. Acad. Sci. 83:6322-6326, 1986). Our DNase I footprint shows both strong and weak areas of protection. The regions strongly protected from DNase I align with the essential contact residues identified by interference footprinting. Methylation interference defines a small binding domain of 8 base pairs: 5'-GTTCGCAC-3'/3'-CAAGCGTG-5'. This recognition sequence contains two inverted 5'-GT(T/G)CG-3' repeats which share a 2-base overlap; thus, the origin-binding protein probably binds to the inverted repeats as a dimer.

Purification of the herpes simplex virus type 1 65-kilodalton DNA-binding protein: properties of the protein and evidence of its association with the virus-encoded DNA polymerase

Using a combination of conventional column chromatography and velocity sedimentation, we have purified the 65-kilodalton DNA-binding protein (65KDBP) encoded by herpes simplex virus (HSV) greater than 625-fold. The HSV type 1 (HSV-1)-encoded DNA polymerase (pol) cofractionated with 65KDBP through DEAE-Sephacel, Blue Sepharose, and Mono Q columns and was only separated from 65KDBP by sedimentation through a glycerol gradient. Immunoaffinity columns containing monoclonal antibody (MAb) 6898 immunoglobulin effectively bound most of the HSV-1 pol activity which coeluted with 65KDBP. The pattern of reactivities of HSV-1/HSV-2 recombinants with MAbs specific for HSV-1 65KDBP or the HSV-2-infected cell-specific protein ICSP34,35 strongly suggests that these two species are serotype equivalents of the same protein. Taken together, all these data indicate that 65KDBP is a pol-associated protein and the HSV-1 counterpart of HSV-2 ICSP34,35 previously reported to have similar properties (P. J. Vaughan, D. J. M. Purifoy, and K. L. Powell, J. Virol. 53:501-508, 1985). Purified preparations of 65KDBP were capable of binding to double-stranded DNA, as determined by filter retention and mobility shift assays. The protein-DNA complex formed with 65KDBP was distinct from that produced by pol and could be further shifted by the addition of immunoglobulin specific for 65KDBP. These results demonstrate that 65KDBP has been purified substantially free from pol and indicate that DNA binding is an inherent property of the protein.

The herpes simplex virus type I DNA polymerase. Polypeptide structure and antigenic domains

Polyclonal antibodies responding specifically to the N-terminal, central and C-terminal polypeptide domains of the herpes simplex virus type I (HSV-1) DNA polymerase of strain Angelotti were generated. Each of the five different rabbit antisera reacted specifically with a viral 132 +/- 5-kDa polypeptide as shown by immunoblot analysis. Enzyme binding and inhibition studies revealed that antibodies raised to the central and the C-terminal domains of the protein inhibited the polymerizing activity by 70-90%, respectively, which is well in line with the proposed site of the catalytic center of the enzyme and with the possible involvement of these polypeptide chains in DNA-protein interactions. In agreement with this, antibodies directed towards the N-terminal domain bound to the enzyme without effecting the enzymatic activity. The strong binding but low inhibitory properties of antibodies directed to the polypeptide region between residues 1072 and 1146 confirms previous suggestions that these C-terminal sequences, which share no homology to the Epstein-Barr virus DNA polymerase, are less likely involved in the building of the polymerase catalytic site. Antibodies, raised to the very C terminus of the polymerase (EX3), were successfully used to identify a single 132 +/- 5-kDa polypeptide, which coeluted with the HSV DNA polymerase activity during DEAE-cellulose chromatography, and were further shown to precipitate a major viral polypeptide of identical size. From the presented data it can be concluded that the native enzyme consists of a single polypeptide with a size predicted from the long open reading frame of the HSV-1 DNA polymerase gene.

Altered expression of two Epstein-Barr virus early genes localized in BamHI-A in nonproducer Raji cells

The Epstein-Barr virus-carrying lymphoblastoid cell line Raji has two major genomic deletions and is incapable of virus production. Two cDNA clones, c70 and c55, were constructed from early mRNA of P3HR-1 cells and localized, respectively, in BALF-2 and BARF-1 open reading frames where one of the major genomic deletion in Raji cells is situated. These were used to search the different early viral transcripts in producer P3HR-1 and nonproducer Raji lines. c70 and c55 hybridized with their corresponding mRNAs only in producer lines. Analysis with in vitro-synthesized RNA probes showed quite a different transcriptional profile in Raji cells than in P3HR-1 cells. In the P3HR-1 line, BALF-2 encodes a 3.4-kilobase (kb) mRNA during the early phase and a 3.3-kb mRNA during the late phase, and in the Raji line, the probe corresponding to BALF-2 hybridized with three mRNAs of 5.0, 3.1, and 2.4 kb; in P3HR-1 cells, BARF-1 encodes a group of 3'-conterminal transcripts (0.8, 1.2, 1.7, 2.7, 3.2, and 5.0 kb) during both the early and late stages; in Raji cells, however, 0.8-, 1.2-, and 1.7-kb mRNAs are absent, the only mRNAs transcribed being upstream of the deletion and of 5.0, 2.6, and 2.0 kb in size. In vivo and in vitro experiments demonstrated that the BALF-2 open reading frame encodes an early 135-kilodalton (kDa) protein which possesses DNA-binding ability and can be recognized by a herpes simplex virus ICP-8 antiserum. The BARF-1 open reading frame encodes in vitro a 26- to 33-kDa early protein recognized by anti-EA serum. The proteins of both two genes expressed in psi AM 22b cells were localized in nuclei. According to their properties, both proteins, particularly the BALF-2-encoded 135-kDa DNA-binding protein, could play a role in virus replication.

Production of antibodies of predetermined specificity against herpes simplex virus DNA polymerase and their use in characterization of the enzyme

Peptides from preselected regions of the herpes simplex virus DNA polymerase were used to generate monospecific antisera to defined regions of the enzyme. The antisera were used to localize the polymerase within the infected cell and to determine the time of synthesis during productive infection. Comparison with a neutralizing polyclonal antiserum was used to show the specificity of the peptide antisera. By using the antisera the stabilities of the DNA polymerase, the alkaline nuclease, and the major DNA-binding protein were determined, and the state of phosphorylation of the DNA polymerase was compared with each of these proteins.

Identification and characterization of a varicella-zoster virus DNA-binding protein by using antisera directed against a predicted synthetic oligopeptide

We have identified, in varicella-zoster virus (VZV)-infected cells, the product of the gene predicted to code for the VZV analog of the herpes simplex virus major DNA-binding protein. The open reading frame of the VZV gene has the potential to code for a protein with a predicted molecular weight of 132,000 (a 132K protein). To detect the protein, a 12-amino-acid oligopeptide corresponding to the carboxyl terminus of the putative open reading frame was synthesized and used to prepare antisera in rabbits. The resulting antibodies reacted specifically in Western immunoblot analysis and immunoprecipitation with a single 130K polypeptide found in VZV-infected cells. The specific reactivity of the antisera with the 130K polypeptide was inhibited by the addition of synthetic peptide. Immunofluorescence studies with the antisera as probe for the 130K polypeptide suggested that this peptide is located predominantly within the nuclei of infected cells. Analysis of proteins that bind to single-stranded DNA immobilized on cellulose matrices indicated that 30 to 50% of the 130K polypeptide is capable of interacting with single-stranded DNA and that this interaction is overcome with 0.5 M NaCl. Thus, we have prepared a specific polyclonal antiserum that identifies a VZV DNA-binding protein whose properties are similar to those of the herpes simplex virus ICP8 (Vmw130) DNA-binding protein.

N-ethylmaleimide inhibition of the DNA-binding activity of the herpes simplex virus type 1 major DNA-binding protein

The major herpes simplex virus DNA-binding protein, designated ICP8, binds tightly to single-stranded DNA and is required for replication of viral DNA. The sensitivity of the DNA-binding activity of ICP8 to the action of the sulfhydryl reagent N-ethylmaleimide has been examined by using nitrocellulose filter-binding and agarose gel electrophoresis assays. Incubation of ICP8 with N-ethylmaleimide results in a rapid loss of DNA-binding activity. Preincubation of ICP8 with single-stranded DNA markedly inhibits this loss of binding activity. These results imply that a free sulfhydryl group is involved in the interaction of ICP8 with single-stranded DNA and that this sulfhydryl group becomes less accessible to the environment upon binding. Agarose gel electrophoretic analysis of the binding interaction in the presence and absence of N-ethylmaleimide indicates that the cooperative binding exhibited by ICP8 is lost upon treatment with this reagent but that some residual noncooperative binding may remain. This last result was confirmed by equilibrium dialysis experiments with the 32P-labeled oligonucleotide dT10 and native and N-ethylmaleimide-treated ICP8.

Identification of the gene encoding the 65-kilodalton DNA-binding protein of herpes simplex virus type 1

Hybrid arrest of in vitro translation was used to localize the region of the herpes simplex virus type 1 genome encoding the 65-kilodalton DNA-binding protein (65KDBP) to between genome coordinates 0.592 and 0.649. Knowledge of the DNA sequence of this region allowed us to identify three open reading frames as likely candidates for the gene encoding 65KDBP. Two independent approaches were used to determine which of these three open reading frames encoded the protein. For the first approach a monoclonal antibody, MAb 6898, which reacted specifically with 65KDBP, was isolated. This antibody was used, with the techniques of hybrid arrest of in vitro translation and in vitro translation of selected mRNA, to identify the gene encoding 65KDBP. The second approach involved preparation of antisera directed against oligopeptides corresponding to regions of the predicted amino acid sequence of this gene. These antisera reacted specifically with 65KDBP, thus confirming the gene assignment.

Immunological characterization of an early cytomegalovirus single-strand DNA-binding protein with similarities to the HSV major DNA-binding protein

Monospecific polyclonal antisera were prepared against the 129-kDa, early, single-strand DNA-binding protein (DB129) of strain Colburn cytomegalovirus (CMV), and used to study its distribution in infected cells and its relatedness to a proposed human CMV (HCMV) counterpart (DB140). Indirect immunofluorescence of fixed, infected human fibroblasts showed DB129 to be localized within the intranuclear inclusions characteristic of replicating CMV. Treatment of infected cells with 50 to 100 micrograms phosphonoformic acid per milliliter resulted in the overproduction of DB129 and its accumulation within nuclei, both inside the inclusions and in surrounding areas of the nucleoplasm, whereas treatment with 500 micrograms/ml prevented inclusion formation, and DB129 was localized at discrete points throughout the infected-cell nuclei. The sera cross-reacted an estimated 10% with HCMV DB140 in an indirect immunoassay, and their use in immunofluorescence localized DB140 to the nuclear inclusions of HCMV-infected cells. Their immunological cross-reactivity, as well as their similar biochemical properties and intracellular distribution, support the likelihood that DB129 and DB140 are the protein products of homologous genes. The relationship of these proteins to the herpes simplex major DNA-binding protein is discussed.

Alkaline nuclease activity in cells infected with herpes simplex virus type 1 (HSV-1) and HSV-1 temperature-sensitive mutants

An in situ assay has been adapted to the herpes simplex virus type 1 (HSV-1) system which can detect alkaline nuclease activity in infected cell lysates following sodium dodecylsulfate polyacrylamide gel electrophoresis. Lysates of cells infected with HSV-1 temperature-sensitive (ts) mutants possessing mutations in the genes for an immediate-early transcriptional regulatory protein (ICP4), viral DNA polymerase (pol), and the major HSV-1 DNA binding protein (ICP8) exhibited altered alkaline nuclease profiles relative to that of wild-type virus-infected cells at 39 degrees C. Infections with a control mutant defective in the gene for glycoprotein B yielded wild-type nuclease profiles. The diverse effects on alkaline nuclease expression of mutants with lesions in different viral proteins involved directly in viral DNA synthesis provides evidence for the cooperative interaction between HSV-encoded viral DNA replication components.

Column liquid chromatography of integral membrane proteins

Biological membranes have as a major function the compartmentation of biological processes in cells and organelles. They consist of a bilayer of phospholipid molecules in which proteins are embedded. These integral membrane proteins, which cross the bilayer once or several times, generally have a higher than average hydrophobicity and tend to aggregate. Detergents are needed to remove integral membrane proteins from the lipid bilayer and they have to be present during further chromatographic purification. Predominantly, four modes of HPLC have been used alone or in combination for the purification of integral membrane proteins. These are based on differences of proteins in size (size-exclusion chromatography, SEC), electrostatic interaction (ion-exchange chromatography, IEC), bioaffinity (bioaffinity chromatography, BAC) and hydrophobic interaction (reversed-phase chromatography, RPC, and hydrophobic-interaction chromatography, HIC). SEC, IEC, BAC and HIC are used under relatively mild conditions, and buffer systems generally contain a non-ionic detergent. RPC generally has a denaturing effect on the protein and should preferably be used for the purification of integral membrane proteins smaller than 50 kD.

Antigenic cross-reactions among herpes simplex virus types 1 and 2, Epstein-Barr virus, and cytomegalovirus

Polyvalent rabbit antisera against herpes simplex virus type 1 and 2 (HSV-1 and HSV-2), cytomegalovirus (CMV), and Epstein-Barr virus (EBV), monospecific antisera against affinity-purified HSV-2 glycoproteins gB and gG, and a panel of monoclonal antibodies against HSV and EBV proteins were used to analyze cross-reactive molecules in cells infected with the four herpesviruses. A combination of immunoprecipitation and Western blotting with these reagents was used to determine that all four viruses coded for a glycoprotein that cross-reacted with HSV-1 gB. CMV coded for proteins that cross-reacted with HSV-2 gC, gD, and gE. Both CMV and EBV coded for proteins that cross-reacted with HSV-2 gG. Antigenic counterparts to the p45 nucleocapsid protein of HSV-2 were present in HSV-1 and CMV, and counterparts of the major DNA-binding protein and the ribonucleotide reductase of HSV-1 were present in all the viruses. The EBV virion glycoprotein gp85 was immunoprecipitated by antisera to HSV-1, HSV-2, and CMV. Antisera to CMV and EBV neutralized the infectivity of both HSV-1 and HSV-2 at high concentrations. This suggests that cross-reactivity between these four human herpesviruses may have pathogenic as well as evolutionary significance.