Overexpression in bacterial and identification in infected cells of the pseudorabies virus protein homologous to herpes simplex virus type 1 ICP18.5

Characterization of pseudorabies virus (PrV) cleavage-encapsidation proteins and functional complementation of PrV pUL32 by the homologous protein of herpes simplex virus type 1

Cleavage and encapsidation of newly replicated herpes simplex virus type 1 (HSV-1) DNA requires several essential viral gene products that are conserved in sequence within the Herpesviridae. However, conservation of function has not been analyzed in greater detail. For functional characterization of the UL6, UL15, UL28, UL32, and UL33 gene products of pseudorabies virus (PrV), the respective deletion mutants were generated by mutagenesis of the virus genome cloned as a bacterial artificial chromosome (BAC) in Escherichia coli and propagated in transgenic rabbit kidney cells lines expressing the deleted genes. Neither of the PrV mutants was able to produce plaques or infectious progeny in noncomplementing cells. DNA analyses revealed that the viral genomes were replicated but not cleaved into monomers. By electron microscopy, only scaffold-containing immature but not DNA-containing mature capsids were detected in the nuclei of noncomplementing cells infected with either of the mutants. Remarkably, primary envelopment of empty capsids at the nuclear membrane occasionally occurred, and enveloped tegument-containing light particles were formed in the cytoplasm and released into the extracellular space. Immunofluorescence analyses with monospecific antisera of cells transfected with the respective expression plasmids indicated that pUL6, pUL15, and pUL32 were able to enter the nucleus. In contrast, pUL28 and pUL33 were predominantly found in the cytoplasm. Only pUL6 could be unequivocally identified and localized in PrV-infected cells and in purified virions, whereas the low abundance or immunogenicity of the other proteins hampered similar studies. Yeast two-hybrid analyses revealed physical interactions between the PrV pUL15, pUL28, and pUL33 proteins, indicating that, as in HSV-1, a tripartite protein complex might catalyze cleavage and encapsidation of viral DNA. Whereas the pUL6 protein is supposed to form the portal for DNA entry into the capsid, the precise role of the UL32 gene product during this process remains to be elucidated. Interestingly, the defect of UL32-negative PrV could be completely corrected in trans by the homologous protein of HSV-1, demonstrating similar functions. However, trans-complementation of UL32-negative HSV-1 by the PrV protein was not observed.

Expression kinetics of the transcript and product of the UL28 homologue of bovine herpesvirus 1

We report that the bovine herpesvirus 1 (BHV1) UL28 ORF, a homologue of the herpes simplex virus (HSV) UL28 gene, represents a functional gene encoding a viral specific protein. The BHV1 UL28 ORF, located at positions 53058-->55538 of the viral genome, encodes a viral specific transcript of 3.4 kb detected at 6 h post-infection (p.i.) after which its levels accumulated up to 12 h p.i. and then remained constant up to 24 h p.i. Transcription of the BHV1 UL28 was determined to initiate 95 bases upstream from the ORF's initiating codon, which corresponds to 33 nucleotides downstream from a putative TATA box. A BHV1 UL28 specific antiserum, generated against a T7-Tag/UL28 fusion protein expressed in E. coli, specifically reacted with a 100 kDa protein in Western blots of BHV1-infected protein cell lysates. The expression kinetics of the protein was delayed by 6 h relative to that of its transcript suggesting that the gene is regulated at the translational level. In contrast to the HSV and pseudorabies virus UL28 genes, which belong to viral genes of the early (beta) class, that of BHV1 was unambiguously classified as a gamma2 gene. Further studies will be required to determine whether these kinetic differences have any functional implications.

The potential terminase subunit of human cytomegalovirus, pUL56, is translocated into the nucleus by its own nuclear localization signal and interacts with importin alpha

Human cytomegalovirus (HCMV) DNA-binding protein pUL56 is thought to be involved in the cleavage/packaging process of viral DNA and therefore needs to be transported into the nucleus. By using indirect immunofluorescence analysis, HCMV pUL56 (p130) was found to be localized predominantly in the nucleus of infected cells. Solitary expression of wild-type as well as epitope-tagged pUL56 also resulted in nuclear distribution after transfection, suggesting the presence of an endogenous nuclear localization signal (NLS). Deletion of a carboxy-terminal stretch of basic amino acids (aa 816-827) prevented nuclear translocation, indicating that the sequence RRVRATRKRPRR of HCMV pUL56 mediates nuclear targetting. The signal character of the NLS sequence was demonstrated by successful transfer of the NLS to a reporter protein chimera. Furthermore, sequential substitutions of pairs of amino acids by alanine in the context of the reporter protein as well as substitutions within the full-length pUL56 sequence indicated that residues at positions 7 and 8 of the NLS (R and K at positions 822 and 823 of pUL56) were essential for nuclear translocation. In order to identify the transport machinery involved, the potential of pUL56 to bind importin alpha (hSRP1 alpha) was examined. Clear evidence of a direct interaction of a carboxy-terminal portion as well as the NLS of pUL56 with hSRP1 alpha was provided by in vitro binding assays. In view of these findings, it is suggested that nuclear translocation of HCMV pUL56 is mediated by the importin-dependent pathway.

Regulation of herpesvirus replication by subcellular compartmentalization

The transcriptional regulation of herpesvirus gene expression has been well documented. A second model is proposed that is superimposed on regulation at the transcriptional level. The regulation is post-translational in nature. Three examples of the model are found in viral DNA replication, capsid assembly, and the cleavage and packaging of DNA into capsids. For each example, at least one viral protein depends upon an interaction with a second viral protein for transport into the nucleus. A model is proposed whereby these protein-protein interactions control the efficiency of these processes by the formation of the appropriate protein complexes in the cytoplasm. The model predicts that these interactions impose a necessary control and that mechanisms to bypass this control would deleteriously affect virus replication. It is probable that level of regulation extends for each of these processes among other herpesviruses.

Identification of the Marek's disease virus serotype 2 genes homologous to the glycoprotein B (UL27), ICP18.5 (UL28) and major DNA-binding protein (UL29) genes of herpes simplex virus type 1

We determined the nucleotide sequence of non-pathogenic Marek's disease virus serotype 2 (MDV2) strain HPRS24 glycoprotein B (gB) (UL27), ICP18.5 (UL28) and major DNA-binding protein (MDBP) (UL29) genes homologous to herpes simplex virus type 1 (HSV-1). The sequence data revealed that important motives in the proteins are conserved in MDV2 ICP18.5 and MDBP, however the sequence of viral DNA replication origin which exists in the regions between the UL29 and UL30 genes of other alphaherpesviruses was not found in the regions of the MDV2 genome. By northern blot analyses, we also demonstrated that 8.9, 5.0 and 2.6 kb transcripts were actually transcribed from the sequenced region in MDV2-infected cells. The MDV2 UL28 and UL29 genes have not been reported in other serotypes of MDV.

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.

Physical and functional interactions between the herpes simplex virus UL15 and UL28 DNA cleavage and packaging proteins

Herpes simplex virus (HSV) DNA is cleaved from concatemers and packaged into capsids in infected cell nuclei. This process requires seven viral proteins, including UL15 and UL28. UL15 expressed alone displays a nuclear localization, while UL28 remains cytoplasmic. Coexpression with UL15 enables UL28 to enter nuclei, suggesting an interaction between the two proteins. Additionally, UL28 copurified with UL15 from HSV-infected cells after ion-exchange and DNA affinity chromatography, and the complex sedimented as a 1:1 heterodimer upon sucrose gradient centrifugation. These findings are evidence of a physical interaction of UL15 and UL28 and a functional role for UL15 in directing UL28 to the nucleus.

Resistance of human cytomegalovirus to benzimidazole ribonucleosides maps to two open reading frames: UL89 and UL56

2,5,6-Trichloro-1-beta-D-ribofuranosyl benzimidazole (TCRB) is a potent and selective inhibitor of human cytomegalovirus (HCMV) replication. TCRB acts via a novel mechanism involving inhibition of viral DNA processing and packaging. Resistance to the 2-bromo analog (BDCRB) has been mapped to the UL89 open reading frame (ORF), and this gene product was proposed as the viral target of the benzimidazole nucleosides. In this study, we report the independent isolation of virus that is 20- to 30-fold resistant to TCRB (isolate C4) and the characterization of the virus. The six ORFs known to be essential for viral DNA cleavage and packaging (UL51, UL52, UL56, UL77, UL89, and UL104) were sequenced from wild-type HCMV, strain Towne, and from isolate C4. Mutations were identified in UL89 (D344E) and in UL56 (Q204R). The mutation in UL89 was identical to that previously reported for virus resistant to BDCRB, but the mutation in UL56 is novel. Marker transfer analysis demonstrated that each of these mutations individually caused approximately 10-fold resistance to the benzimidazoles and that the combination of both mutations caused approximately 30-fold resistance. The rate and extent of replication of the mutants was the same as for wild-type virus, but the viruses were less sensitive to inhibition of DNA cleavage by TCRB. Mapping of resistance to UL56 supports and extends recent work showing that UL56 codes for a packaging motif binding protein which also has specific nuclease activity (E. Bogner et al., J. Virol. 72:2259-2264, 1998). Resistance which maps to two different genes suggests that their putative proteins interact and/or that either or both have a benzimidazole ribonucleoside binding site. The results also suggest that the gene products of UL89 and UL56 may be antiviral drug targets.

The gene product of human cytomegalovirus open reading frame UL56 binds the pac motif and has specific nuclease activity

Using the cis-acting human cytomegalovirus (HCMV) packaging elements (pac 1 and pac 2) as DNA probes, specific DNA-protein complexes were detected by electrophoretic mobility shift assay (EMSA) in both HCMV-infected cell nuclear extracts and recombinant baculovirus-infected cell extracts containing the HCMV p130 (pUL56) protein. DNA-binding proteins, which were common in uninfected and infected cell extracts, were also detected. Mutational analysis showed that only the AT-rich core sequences in these cis-acting motifs, 5'-TAAAAA-3' (pac 1) and 5'-TTTTAT-3' (pac 2), were required for specific DNA-protein complex formation. The specificity of the DNA-protein complexes was confirmed by EMSA competition. Furthermore, a specific endonuclease activity was found to be associated with lysates of baculovirus-infected cells expressing recombinant p130 (rp130). This nuclease activity was time dependent, related to the amount of rp130 in the assay, and ATP independent. Nuclease activity remained associated with rp130 after partial purification by sucrose gradient centrifugation, suggesting that this activity is a property of HCMV p130. We propose a possible involvement of p130 in HCMV DNA packaging.

The pseudorabies virus UL28 protein enters the nucleus after coexpression with the herpes simplex virus UL15 protein

Herpesvirus DNA is packaged into capsids in the nuclei of infected cells in a process requiring at least six viral proteins. Of the proteins required for encapsidation of viral DNA, UL15 and UL28 are the most conserved among herpes simplex virus type 1 (HSV), varicella-zoster virus, and equine herpesvirus 1. The subcellular distribution of the pseudorabies virus (PRV) UL28 protein was examined by in situ immunofluorescence. UL28 was present in the nuclei of infected cells; however, UL28 was limited to the cytoplasm in the absence of other viral proteins. When cells expressing variant forms of UL28 were infected with a PRV UL28-null mutant, UL28 entered the nucleus, provided the carboxyl-terminal 155 amino acids were present. Additionally, PRV UL28 entered the nucleus in cells infected with HSV. Two HSV packaging proteins were tested for the ability to affect the subcellular distribution of UL28. Coexpression of HSV UL15 enabled PRV UL28 to enter the nucleus in a manner that required the carboxyl-terminal 155 amino acids of UL28. Coexpression of HSV UL25 did not affect the distribution of UL28. We propose that an interaction between UL15 and UL28 facilitates the transport of a UL15-UL28 complex to the infected-cell nucleus.

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 DNA cleavage and encapsidation require the product of the UL28 gene: isolation and characterization of two UL28 deletion mutants

The herpes simplex virus type 1 UL28 gene contains a 785-amino-acid open reading frame that codes for an essential protein. Studies with temperature-sensitive mutants which map to the UL28 gene indicate that the UL28 gene product (ICP18.5) is required for packaging of viral DNA and for expression of viral glycoproteins on the surface of infected cells (C. Addison, F. J. Rixon, and V. G. Preston, J. Gen. Virol. 71:2377-2384, 1990; B. A. Pancake, D. P. Aschman, and P. A. Schaffer, J. Virol. 47:568-585, 1983). In this study, we describe the isolation of two UL28 deletion mutants that were constructed and propagated in Vero cells transformed with the UL28 gene. The mutants, gCB and gC delta 7B, contained deletions of 1,881 and 537 bp, respectively, in the UL28 gene. Although the mutants synthesize viral DNA, they fail to form plaques or produce infectious virus in cells that do not express the UL28 gene. Transmission electron microscopy and Southern blot analysis demonstrated that both mutants are defective in cleavage and encapsidation of viral DNA. Analysis by cell surface immunofluorescence showed that the UL28 gene is not required for expression of viral glycoproteins on the surface of infected cells. A rabbit polyclonal antiserum was made against an Escherichia coli-expressed Cro-UL28 fusion protein. This antibody reacted with an infected-cell protein having an apparent molecular mass of 87 kDa. The 87-kDa protein was first detected at 6 h postinfection and was expressed as late as 24 h postinfection. No detectable UL28 protein was synthesized in gCB- or gC delta 7B-infected Vero cells.

Nucleotide sequence of the major DNA-binding protein gene of herpes simplex virus type 2 and a comparison with the type 1

The nucleotide sequence of a region encompassing about 5,200 base pairs (bp) of the left side of the origin of replication in the long unique region of the herpes simplex virus type 2 (HSV-2) has been determined. This region contained the major DNA-binding protein or the infected-cell protein 8 (ICP 8) gene and 5'-part of the counterpart of HSV-1 ICP 18.5 gene. A comparison of the nucleotide sequence of the ICP8 gene between HSV-1 and HSV-2 showed an 89.8% homology. A primer extension analysis for the HSV-2 ICP 8 mRNA showed that the major transcriptional start site was mapped at 315 bp upstream of the initiation codon. A comparison of the predicted functional amino acid sequence of the ICP 8 between HSV-1 and HSV-2 revealed a striking homology (97.2%), the value of which was the highest among those of the other polypeptides encoded by HSV-1 and HSV-2. Some domains, which were shown to be required for the nuclear function, the binding to single-stranded DNA and the nuclear localization were well conserved. In addition, the nucleotide and the functional amino acid sequences of a part of the HSV-2 counterpart of the HSV-1 ICP 18.5 gene were also compared, demonstrating an 88.4% and 95.9% homology, respectively.

Pseudorabies virus protein homologous to herpes simplex virus type 1 ICP18.5 is necessary for capsid maturation

In pseudorabies virus (PrV), an open reading frame that partially overlaps the gene for the essential glycoprotein gII has been shown to encode a protein homologous to the ICP18.5 polypeptide of herpes simplex virus type 1 (N. Pederson and L. Enquist, Nucleic Acids Res. 17:3597, 1989). To study the function of this protein during the viral replicative cycle, a PrV mutant which carries a beta-galactosidase expression cassette interrupting the ICP18.5(PrV) gene was constructed. This mutant could be propagated only on cell lines that were able to provide ICP18.5(PrV) in trans after transformation with a corresponding genomic PrV DNA fragment. Detailed analysis showed that inactivation of the ICP18.5(PrV) gene did not impair infection of noncomplementing cells, nor did it impair early or late gene expression, as shown by immunoprecipitation of glycoproteins gII, gIII, and gp50. Surface localization of glycoproteins as demonstrated by fluorescence-activated cell sorting analyses was also not affected. Southern blot hybridizations, however, showed that cleavage of replicative concatemeric viral DNA did not occur in noncomplementing cells infected by the ICP18.5 mutant PrV. In addition, electron microscopic analysis revealed an accumulation of empty capsids in the nucleus of mutant-infected noncomplementing cells. We conclude that the ICP18.5(PrV) protein is necessary for viral replication and plays an essential role in the process of mature capsid formation.

The pseudorabies virus homology of the herpes simplex virus UL21 gene product is a capsid protein which is involved in capsid maturation

We mutagenized, mapped, and sequenced the pseudorabies virus (PRV) homology of gene UL21 of herpes simplex virus type 1. A polyclonal mouse antiserum against the protein encoded by the UL21 homolog was generated and used to monitor the expression and subcellular localization of the UL21-encoded protein. We found that the protein is identical to a previously detected PRV capsid protein. We analyzed viable PRV strains encoding mutant UL21 homologys, truncated by insertion of an oligonucleotide that contains stop codons in all reading frames. In two PRV mutants carrying the oligonucleotide at two sites within the gene, processing of newly replicated viral DNA was impaired. In addition, we show that one of the UL21 mutants has strongly reduced virulence for mice.

Characterization of the murine cytomegalovirus genes encoding the major DNA binding protein and the ICP18.5 homolog

In several herpesviruses the genes for the major DNA binding protein (MDBP), a putative assembly protein, the glycoprotein B (gB), and the viral DNA polymerase (pol) collocate. In murine cytomegalovirus (MCMV), two members of this gene block, pol (Elliott, Clark, Jaquish, and Spector, 1991, Virology 185, 169-186) and gB (Rapp, Messerle, Bühler, Tannheimer, Keil, and Koszinowski, 1992, J. Virol., 66, 4399-4406) have been characterized. Here the two other MCMV genes are characterized, the gene encoding the MDBP and the ICP18.5 homolog encoding a putative assembly protein. Like in human cytomegalovirus (HCMV) the genes order is pol, gB, ICP18.5, and MDBP. The 4.2-kb MDBP mRNA is expressed first in the early phase, whereas the 3.0-kb ICP18.5 mRNA is a late transcript. The open reading frame of the MDBP gene has the capacity of encoding a protein of 1191 amino acids with a predicted molecular mass of 131.7 kDa. The MCMV ICP18.5 ORF is translated into a polypeptide of 798 amino acids with a calculated molecular mass of 89.1 kDa. Comparison of the amino acid sequences of the predicted proteins of MCMV with the respective proteins of HCMV, Epstein-Barr virus (EBV), and herpes simplex virus type-1 (HSV-1) reveals a striking homology ranging from 72% (HCMV), 50% (EBV), to 45% (HSV-1) for the MDBP sequence and from 74% (HCMV), 51% (EBV), to 49% (HSV-1) for the ICP18.5 sequence. These results establish the close relationship of the two cytomegaloviruses, and underline the usefulness of the murine model for studies on the biology of the CMV infection.

Identification and nucleotide sequence of a gene in feline herpesvirus type 1 homologous to the herpes simplex virus gene encoding the glycoprotein B

The nucleotide sequence of the glycoprotein B (gB) homologous gene of feline herpesvirus type 1 (FHV-1) was determined. The gene was found to be located within a 9.6 kbp SalI fragment by Southern-blot hybridization with a probe derived from the herpes simplex virus type 1 (HSV-1) gB DNA sequence. Furthermore, the predominant portion of the coding sequences was mapped to a 1.9 kbp Hin cII-EcoRI and its flanking 2.7 kbp Eco RI-Eco RI subfragments in the 9.6 kbp SalI fragment. The entire nucleotide sequence revealed that the FHV-1 gB homologous gene is capable of encoding a polypeptide of 948 amino acids. The predicted precursor polypeptide derived from this open reading frame could have a calculated M(r) of 106 kDa in unglycosylated form and contains ten potential N-linked glycosylation sites and a probable internal proteolytic cleavage site. By Northern-blot analysis using portions of the open reading frame as a probe, 3.9 and 3.3 kb RNA transcripts were identified in FHV-1 infected cells. The alignment of the amino acid sequence of the FHV-1 gB homologue with those of 14 other herpesviruses revealed that 10 cysteine residues were completely conserved. Meanwhile, when evolutionary trees were generated among these herpesvirus gB counterparts, the FHV-1 gB homologous nucleotide sequence seems to be closely related to equine herpesvirus type 4 and its amino acid sequence to pseudorabies virus.