The herpes simplex virus UL33 gene product is required for the assembly of full capsids

The antiviral agent 5-chloro-1,3-dihydroxyacridone interferes with assembly and maturation of herpes simplex virus

Antiviral drug screening and exploratory mechanistic work identified 5-chloro-1,3-dihydroxyacridone as a lead inhibitor of herpes simplex virus (HSV) replication, one without a primary effect on either HSV DNA or late viral protein synthesis (Antivir. Res. 45 (2000) 123). In this report, drug effects on viral DNA cleavage and packaging, HSV capsid production and virion morphogenesis in infected Vero cells were studied systematically in order to better localize the sensitive stage of the replication cycle. Maturation of replicating HSV DNA and virion production at late times were inhibited in the same dose-dependent fashion, suggesting that the drug might directly inhibit the cleavage and packaging processes. Based on density centrifugation analysis however, this possibility appears unlikely because overproduction of neither A- or B-capsids occurred upon drug treatment. Interestingly, similar studies coupled with either Western immunoblot or ultrastructural analysis showed that B-capsids with apparent normal protein composition accumulated at reduced levels (maximally about two- to three-fold) in drug-treated cells. Limited attempts to isolate drug-resistant viral mutants using standard approaches proved unsuccessful. In summery, 5-chloro-1,3-dihydroxyacridone inhibits one or more steps of HSV assembly since treatment results in reduced levels of capsids (particularly B-type) and reduced levels of encapsidated DNA. The action of the acridone derivative is an unusual one, with distinctive features when compared to a recently reported class of HSV encapsidation inhibitor and to the late replication defects of relevant viral mutants.

A novel nonnucleoside inhibitor specifically targets cytomegalovirus DNA maturation via the UL89 and UL56 gene products

3-Hydroxy-2,2-dimethyl-N-[4([[5-(dimethylamino)-1-naphthyl]sulfonyl]amino)-phenyl]propanamide (BAY 38-4766) is a novel selective nonnucleoside inhibitor of cytomegalovirus (CMV) replication with an excellent safety profile. This compound and structural analogues inhibit neither viral DNA synthesis nor viral transcription and translation. Accumulation of dense bodies and noninfectious enveloped particles coincides with inhibition of both concatemer processing and functional cleavage at intergenomic transitions, pointing to interference with viral DNA maturation and packaging of monomeric genome lengths. Resistant virus populations, including a murine CMV (MCMV) isolate with 566-fold-decreased drug sensitivity, were selected in vitro. Sequencing of the six open reading frames (ORFs) known to be essentially involved in viral DNA cleavage and packaging identified mutations in ORFs UL56, UL89, and UL104. Construction of MCMV recombinants expressing different combinations of murine homologues of mutant UL56, UL89, and UL104 and analysis of drug susceptibilities clearly demonstrated that mutant ORFs UL89 exon II (M360I) and M56 (P202A I208N) individually confer resistance to BAY 38-4766. A combination of both mutant proteins exhibited a strong synergistic effect on resistance, reconstituting the high-resistance phenotype of the in vitro mutant. These findings are consistent with genetic mapping of resistance to TCRB (2,5,6-trichloro-1-beta-D-ribofuranosyl benzimidazole) (P. M. Krosky et al., J. Virol. 72:4721-4728, 1998) and provide further indirect evidence that proteins encoded by UL89 and UL56 function as two subunits of the CMV terminase. While these studies also suggest that the molecular mechanism of BAY 38-4766 is distinct from that of benzimidazole ribonucleosides, they also offer an explanation for the excellent specificity and tolerability of BAY 38-4766, since mammalian DNA does not undergo comparable maturation steps.

Distinct and separate roles for herpesvirus-conserved UL97 kinase in cytomegalovirus DNA synthesis and encapsidation

The human cytomegalovirus UL97 kinase, an important target of antiviral therapy, has an impact on at least two distinct phases of viral replication. Compared with wild-type virus, the UL97 deletion mutant exhibits an early replication defect that reduces DNA accumulation by 4- to 6-fold, as well as a late capsid maturation defect responsible for most of the observed 100- to 1000-fold reduction in replication. Block-release experiments with the antiviral 2-bromo-5,6-dichloro-1-(beta-D-ribofuranosyl)-benzimidazole revealed an important role for UL97 kinase in capsid assembly. Although cleavage of concatemeric DNA intermediates to unit-length genomes remained unaffected, progeny mutant virus maturation was delayed, with accumulation of progeny at significantly reduced levels compared with wild type after release of this block. Transmission electron microscopy confirmed the aberrant accumulation of empty A-like capsids containing neither viral DNA nor an internal scaffold structure, consistent with a failure to stably package DNA in mutant virus-infected cells. The function of UL97 in DNA synthesis as well as capsid assembly suggests that protein phosphorylation mediated by this herpesvirus-conserved kinase increases the efficiency of these two distinct phases of virus replication.

The UL14 protein of herpes simplex virus type 2 translocates the minor capsid protein VP26 and the DNA cleavage and packaging UL33 protein into the nucleus of coexpressing cells

The herpes simplex virus type 2 (HSV-2) gene UL14 encodes a 32 kDa protein which is a minor component of the virion tegument and is expressed late in infection. The UL14 protein shows varied localization patterns in HSV-2-infected and singly expressing cells, suggesting the possibility that it is multifunctional. We have investigated the influence of the UL14 protein on the intracellular localization of capsid proteins and DNA cleavage and packaging proteins in coexpressing cells. VP26 is the minor capsid protein; it binds to hexons of the outer capsid shell and is predominantly cytoplasmic upon sole expression. We have found that VP26 coexpressed with the UL14 protein showed mutual and predominant relocation into the nucleus. At least seven viral genes encode proteins (UL6, UL15, UL17, UL25, UL28, UL32 and UL33) that are required for DNA cleavage and packaging. We have found that the UL33 protein, which was also cytoplasmic by sole expression, was relocated to the nucleus upon expression with the UL14 protein, which again seemed to be a result of mutual influence. Coexpression experiments also suggested the possibility of a mutual influence between the UL14 and UL17 proteins, and the UL17 protein and VP26. Our results suggest that the UL14 protein can influence the intracellular localization patterns of a number of proteins belonging to the capsid or the DNA encapsidation machinery.

Role of the UL25 gene product in packaging DNA into the herpes simplex virus capsid: location of UL25 product in the capsid and demonstration that it binds DNA

Recent studies have suggested that the herpes simplex type 1 (HSV-1) UL25 gene product, a minor capsid protein, is required for encapsidation but not cleavage of replicated viral DNA. This study set out to investigate the potential interactions of UL25 protein with other virus proteins and determine what properties it has for playing a role in DNA encapsidation. The UL25 protein is found in 42 +/- 17 copies per B capsid and is present in both pentons and hexons. We introduced green fluorescent protein (GFP) as a fluorescent tag into the N terminus of UL25 protein to identify its location in HSV-1-infected cells and demonstrated the relocation of UL25 protein from the cytoplasm into the nucleus at the late stage of HSV-1 infection. To clarify the cause of this relocation, we analyzed the interactions of UL25 protein with other virus proteins. The UL25 protein associates with VP5 and VP19C of virus capsids, especially of the penton structures, and the association with VP19C causes its relocation into the nucleus. Gel mobility shift analysis shows that UL25 protein has the potential to bind DNA. Moreover, the amino-terminal one-third of the UL25 protein is particularly important in DNA binding and forms a homo-oligomer. In conclusion, the UL25 gene product forms a tight connection with the capsid being linked with VP5 and VP19C, and it may play a role in anchoring the genomic DNA.

Novel class of thiourea compounds that inhibit herpes simplex virus type 1 DNA cleavage and encapsidation: resistance maps to the UL6 gene

In our search for novel inhibitors of herpes simplex virus type 1 (HSV-1), a new class of thiourea inhibitors was discovered. N-(4-[3-(5-Chloro-2,4-dimethoxyphenyl)-thioureido]-phenyl)-acetamide and its 2-fluoro-benzamide derivative inhibited HSV-1 replication. HSV-2, human cytomegalovirus, and varicella-zoster virus were inhibited to a lesser extent. The compounds acted late in the replication cycle by impairing both the cleavage of concatameric viral DNA into progeny genome length and the packaging of the DNA into capsids, indicative of a defect in the encapsidation process. To uncover the molecular target of the inhibition, resistant HSV-1 isolates were generated, and the mutation responsible for the resistance was mapped using marker transfer techniques. Each of three independent isolates had point mutations in the UL6 gene which resulted in independent single-amino-acid changes. One mutation was located in the N terminus of the protein (E121D), while two were located close together in the C terminus (A618V and Q621R). Each of these point mutations was sufficient to confer drug resistance when introduced into wild-type virus. The UL6 gene is one of the seven HSV-1 genes known to play a role in DNA packaging. This novel class of inhibitors has provided a new tool for dissection of HSV-1 encapsidation mechanisms and has uncovered a new viable target for the treatment of herpesviral diseases.

Rat cytomegalovirus R89 is a highly conserved gene which expresses a spliced transcript

In all sequenced herpesvirus genomes, a homolog of the herpes simplex virus type 1 UL15 gene has been identified. This gene encodes a protein that is involved in viral genome maturation. Although transcription of the alphaherpesvirus UL15 gene has been analyzed in detail, not much is known about the expression of its betaherpesvirus homologs. We therefore set out to characterize transcription of the rat cytomegalovirus counterpart of UL15, R89. Here we report that R89 consists of two exons separated by a 4.7-kb intron. The spliced R89 transcript, which is expressed at late times postinfection (p.i.), has the capacity to encode a protein of 670 amino acids with a calculated molecular mass of 77.1 kDa. The predicted amino acid sequence of this protein is highly similar to that of the proteins predicted to be encoded by the human cytomegalovirus UL89 and murine cytomegalovirus M89 genes (64.3 and 84.5% overall identity, respectively). The region between R89 exon 1 and exon 2 was found to contain five additional genes, r90, R91, R92, R93 and R94, the latter two of which are conserved among all herpesviruses. We show that these genes are transcribed in a highly complex fashion, resulting in numerous mono- and polycistronic mRNAs.

Characterization of the U(L)33 gene product of herpes simplex virus 1

The U(L)33 protein is one of six genes (including U(L)6, U(L)15, U(L)17, U(L)28, and U(L)32) required for cleavage of viral concatemeric DNA into unit-length genomes and packaging of the virus genomes into preformed capsids. The U(L)25 gene product is dispensable for cleavage of viral DNA but essential for packaging of DNA into capsids. A polyclonal antiserum was produced against an affinity-purified protein containing the full-length U(L)33 gene product of herpes simplex virus 1 fused to glutathione-S-transferase. A protein of approximate M(r) 19,000 that reacted with the antiserum was detected in immunoblots of herpes simplex virus 1-infected cellular lysates. This protein was not detected in lysates of mock-infected cells or cells infected with a mutant virus containing a stop codon in U(L)33, indicating that the 19,000 M(r) protein is the product of the U(L)33 open reading frame. The U(L)33 gene product was not detected in purified virions or capsids. Accumulation of the U(L)33 protein to detectable levels required viral DNA synthesis, indicating that the protein was regulated as a late gene. Indirect immunofluorescence analysis demonstrated that U(L)33 protein accumulated predominantly within replication compartments in the central domains of infected cell nuclei and within the cytoplasm. Localization of the U(L)33 gene product in replication compartments was maintained in cells infected with a variety of cleavage/packaging mutants.

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.

Herpes simplex virus type 1 U(L)34 gene product is required for viral envelopment

The herpes simplex virus type 1 U(L)34 gene encodes a protein that is conserved in all human herpesviruses. The association of the U(L)34 protein with membranes in the infected cell and its expression as a gamma-1 gene suggest a role in maturation or egress of the virus particle from the cell. To determine the function of this gene product, we have constructed a recombinant virus that fails to express the U(L)34 protein. This recombinant virus, in which the U(L)34 protein coding sequence has been replaced by green fluorescent protein, forms minute plaques and replicates in single-step growth experiments to titers 3 to 5 log orders of magnitude lower than wild-type or repair viruses. On Vero cells, the deletion virus synthesizes proteins of all kinetic classes in normal amounts. Electron microscopic and biochemical analyses show that morphogenesis of the deletion virus proceeds normally to the point of formation of DNA-containing nuclear capsids, but electron micrographs show no enveloped virus particles in the cytoplasm or at the surface of infected cells, suggesting that the U(L)34 protein is essential for efficient envelopment of capsids.

Genetic analysis of the bovine herpesvirus type 4 gene locus for the putative terminase

The complete DNA sequence of the 10-45 kbp HindIII B fragment of bovine herpesvirus type 4 (BoHV-4) was determined. This fragment contains nine complete and two incomplete open reading frames (ORFs), all of which are homologous to herpesvirus saimiri (HVS), Kaposi's Sarcoma-associated herpesvirus (HHV-8) and Epstein-Barr virus (EBV). Particularly, the arrangement of the gene for the terminase-related protein with the two coding exons 29a/29b is conserved among all herpesviruses sequenced to date. The intron carries the ORFs 30 to 33 in the opposite direction. Analysis by reverse transcription and polymerase chain reaction (PCR) of the transcript across the proposed splice junction of the ORF 29a/29b and subsequent sequence determination of the amplified product revealed the precise structure of the splice junction. Furthermore, the phylogenetic analysis of the 29a/29b protein and its counterparts in other herpesviruses revealed that BoHV-4 clustered in the genus Rhadinovirus of the subfamily Gammaherpesvirinae.

HSV-1-based vectors for gene therapy of neurological diseases and brain tumors: part I. HSV-1 structure, replication and pathogenesis

The design of effective gene therapy strategies for brain tumors and other neurological disorders relies on the understanding of genetic and pathophysiological alterations associated with the disease, on the biological characteristics of the target tissue, and on the development of safe vectors and expression systems to achieve efficient, targeted and regulated, therapeutic gene expression. The herpes simplex virus type 1 (HSV-1) virion is one of the most efficient of all current gene transfer vehicles with regard to nuclear gene delivery in central nervous system-derived cells including brain tumors. HSV-1-related research over the past decades has provided excellent insight into the structure and function of this virus, which, in turn, facilitated the design of innovative vector systems. Here, we review aspects of HSV-1 structure, replication and pathogenesis, which are relevant for the engineering of HSV-1-based vectors.

Proteolytic cleavage of the amino terminus of the U(L)15 gene product of herpes simplex virus type 1 is coupled with maturation of viral DNA into unit-length genomes

The U(L)15 gene of herpes simplex virus type 1 (HSV-1), like U(L)6, U(L)17, U(L)28, U(L)32, and U(L)33, is required for cleavage of concatameric DNA into genomic lengths and for packaging of cleaved genomes into preformed capsids. A previous study indicated that the U(L)15 gene encodes minor capsid proteins. In the present study, we have shown that the amino-terminal 509 amino acids of the U(L)15-encoded protein are sufficient to confer capsid association inasmuch as a carboxyl-terminally truncated form of the U(L)15-encoded protein with an M(r) of approximately 55,000 readily associated with capsids. This and previous studies have shown that, whereas three U(L)15-encoded proteins with apparent M(r)s of 83,000, 80,000, and 79,000 associated with wild-type B capsids, only the full-length 83,000-M(r) protein associated with B capsids purified from cells infected with viruses lacking functional U(L)6, U(L)17, U(L)28, U(L)32, and U(L)33 genes (B. Salmon and J. D. Baines, J. Virol. 72:3045-3050, 1998). Thus, all viral mutants that fail to cleave viral DNA into genomic-length molecules also fail to produce capsid-associated U(L)15 80,000- and 79,000-M(r) proteins. In contrast, the 80,000- and 79,000-M(r) proteins were readily detected in capsids purified from cells infected with a U(L)25 null virus that cleaves, but does not package, DNA. The conclusion that the amino terminus of the 83,000-M(r) protein is truncated to produce the 80,000- and/or 79,000-M(r) protein was supported by the following observations. (i) Whereas the C termini of the 83,000-, 80, 000-, and 79,000-M(r) proteins are identical, immunoreactivity dependent on the first 35 amino acids of the U(L)15 83,000-M(r) protein was absent from the 80,000- and 79,000-M(r) proteins. (ii) The 79,000- and 80,000-M(r) proteins were detected in capsids from cells infected with HSV-1(U(L)15M36V), an engineered virus encoding valine rather than methionine at codon 36. Thus, initiation at codon 36 is unlikely to account for production of the 80,000- and/or 79, 000-M(r) protein. Taken together, these data strongly suggest that capsid-associated U(L)15-encoded protein is proteolytically cleaved near the N terminus and indicate that this modification is tightly linked to maturation of genomic DNA.

Identification and characterization of the UL14 gene product of herpes simplex virus type 2

The UL14 gene of herpes simplex virus type 2 (HSV-2) is predicted to encode a 219 amino acid protein with a molecular mass of 23 kDa. In this study, the HSV-2 UL14 gene product has been identified by using a rabbit polyclonal antiserum raised against a recombinant 6 x His-UL14 fusion protein expressed in E. coli. The antiserum reacted specifically with 34, 33 and 28 kDa proteins in HSV-2-infected cell lysates and also with a 34 kDa protein produced by in vitro transcription and translation reactions, suggesting that the 34 kDa protein is the primary translation product of the UL14 gene. The protein was synthesized at late times post-infection (p.i.) and was not detectable in the presence of the viral DNA synthesis inhibitor acycloguanosine. Indirect immunofluorescence studies localized the UL14 protein both to the nucleus and to perinuclear regions of the cytoplasm, and the nuclear UL14 protein was found to co-localize with the scaffolding protein ICP35 at 9 h p.i. However, the protein accumulated in a perinuclear region of the cytoplasm at 12 h p.i., while most of the ICP35 protein localized within assemblons in the nucleus. Although no detectable UL14 protein was associated with intracellular capsids isolated in the presence of 0.5 M NaCl, it was detected in purified virions. Furthermore, the UL14 protein expressed alone was detected both in the nucleus and in the cytoplasm at 24 h after transfection, but was mainly localized to the cytoplasm at later times.

ATP depletion blocks herpes simplex virus DNA packaging and capsid maturation

During herpes simplex virus (HSV) assembly, immature procapsids must expel their internal scaffold proteins, transform their outer shell to form mature polyhedrons, and become packaged with the viral double-stranded (ds) DNA genome. A large number of virally encoded proteins are required for successful completion of these events, but their molecular roles are poorly understood. By analogy with the dsDNA bacteriophage we reasoned that HSV DNA packaging might be an ATP-requiring process and tested this hypothesis by adding an ATP depletion cocktail to cells accumulating unpackaged procapsids due to the presence of a temperature-sensitive lesion in the HSV maturational protease UL26. Following return to permissive temperature, HSV capsids were found to be unable to package DNA, suggesting that this process is indeed ATP dependent. Surprisingly, however, the display of epitopes indicative of capsid maturation was also inhibited. We conclude that either formation of these epitopes directly requires ATP or capsid maturation is normally arrested by a proofreading mechanism until DNA packaging has been successfully completed.

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.

Herpes simplex virus 1 DNA cleavage/packaging: the UL28 gene encodes a minor component of B capsids

An antiserum directed against a bacterial fusion protein containing UL28 protein sequences specifically recognized an 86,000 apparent Mr protein in immunoblots of wild-type capsids. This protein was not detected in immunoblots of capsids purified from cells infected with a UL28 deletion virus, indicating that the protein was a product of UL28. The 86,000 Mr protein was also detected in capsids purified from cells infected with mutant viruses lacking the UL6, UL15, and UL25 genes, indicating that the UL28 protein can associate with capsids independently of successful DNA packaging and other minor capsid components. The UL6 protein, full-length UL15 protein, and UL25-encoded proteins were also detected in capsids purified from cells infected with the UL28 deletion virus. The UL28 and UL6 proteins remained associated with capsids treated with 1.0 M guanidine-HCl, indicating that, like the UL6 protein, the UL28 protein was an integral component of capsids. Amounts of UL28 protein were reduced in DNA-containing capsids and UL28 protein was not detected in virions, suggesting that some UL28 protein is lost during the cleavage-packaging reaction.

The herpes simplex virus 1 UL 17 gene is required for localization of capsids and major and minor capsid proteins to intranuclear sites where viral DNA is cleaved and packaged

In nuclei of cells infected with herpes simplex virus (HSV), synthesized viral DNA accumulates as concatamers that are cleaved into genomic lengths and inserted into preformed capsids. Whereas newly replicated DNA and enzymes required for DNA synthesis accumulate in sites of infected cell nuclei termed replication compartments, the intranuclear site of DNA cleavage and packaging is currently controversial. DNA packaging requires the UL6, UL15, UL17, UL25, UL28, UL32, and UL33 genes in addition to the major capsid proteins. Using confocal immunofluorescence microscopy, it was observed that in > 95% of HEp-2 cells fixed at late times after infection with wild-type HSV-1, capsids, major capsid proteins ICP5 and ICP35, and the UL6-encoded minor capsid protein localized in DNA replication compartments. These data support the hypothesis that capsid assembly and DNA cleavage/packaging normally occur in HEp-2 cell replication compartments. In contrast, cells infected with a viral mutant lacking functional UL17 contained antigenically dense nuclear aggregates that stained with ICP35, ICP5, and capsid specific antibodies. Cells infected with the UL17 mutant virus also displayed UL6-specific fluorescence in a diffuse pattern at the nuclear periphery in regions not containing ICP35 and ICP5. Displacement of ICP35 from replication compartments was not observed in cells infected with cleavage/packaging mutants lacking UL28 and UL33. We conclude that the UL17 gene is required for correct targeting of capsids and major and minor capsid proteins to the DNA replication compartment of HEp-2 cells and deduce that this targeting reflects one functional role of UL17 in viral DNA cleavage and packaging.

Herpes simplex virus type 1 cleavage and packaging proteins UL15 and UL28 are associated with B but not C capsids during packaging

At least seven viral genes encode proteins (UL6, UL15, UL17, UL25, UL28, UL32, and UL33) that are required for DNA cleavage and packaging of herpes simplex virus type 1 (HSV-1) DNA. Sequence analysis reveals that UL15 shares homology with gp17, the large catalytic subunit of the bacteriophage T4 terminase. Thus, UL15 may play a direct role in the cleavage of viral DNA replication intermediates into monomers. In this study, we asked whether UL15 and other cleavage and packaging proteins could be detected in capsids isolated from infected cells. Consistent with previous studies showing that UL6 and UL25 are minor protein constituents of the capsids, we detected these proteins in both B and C capsids. In contrast, the previously identified full-length version (81 kDa) of UL15 was found predominantly in B capsids and in much smaller amounts in C capsids. In addition, the UL28 protein was found predominantly in B but not C capsids in a distribution similar to that of the 81-kDa version of UL15. These results suggest that UL28 and the 81-kDa form of UL15 are transiently associated with capsid intermediates during the packaging process. Surprisingly, however, a previously unidentified 87-kDa form of UL15 was found in the B and C capsids and in virions. Analysis of cells infected with mutants individually lacking UL6, UL15, UL25, UL28, or UL32 demonstrates that the lack of one cleavage and packaging protein does not affect the expression of the others. Furthermore, this analysis, together with guanidine HCl extraction analysis of purified capsids, indicates that UL6, UL25, and UL28 are able to associate with B capsids in the absence of other DNA cleavage and packaging proteins. On the other hand, the two UL15-related proteins (81 and 87 kDa) do not associate efficiently with B capsids in cells infected with UL6 and UL28 mutants. These results suggest that the ability of the UL15-related proteins to bind to B capsids may be mediated through interactions with UL6 and UL28.

HSV molecular biology: general aspects of herpes simplex virus molecular biology

Comparison of the herpes simplex virus type 1 (HSV-1) DNA sequence with that of other alpha, beta and gamma-herpesviruses, allied with molecular genetic studies have greatly increased understanding of the HSV genome and the functions encoded by individual virus genes and has facilitated the development of rational antiviral strategies. Here we review the coding content of the HSV-1 genome and identify: genes encoding structural components of the capsid, tegument or envelope; genes whose products are essential for growth in tissue culture; and genes that are conserved between members of the alpha, beta and gamma-herpesvirinae. The HSV lifecycle and the main regulation cascade is discussed and genes that present targets for antiviral intervention identified. The protein content of the infectious virion particle is reviewed and compared with that of two additional non-infectious HSV-related particles species (L-particles and pre-DNA replication particles (PREPs)). The potential of HSV-1 L particles and PREP particles as DNA-free HSV-1 vaccine candidates and the desirability of deleting specific gene products from live HSV vaccines is discussed.

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 herpes simplex virus type 1 U(L)17 gene encodes virion tegument proteins that are required for cleavage and packaging of viral DNA

Previous studies have suggested that the U(L)17 gene of herpes simplex virus type 1 (HSV-1) is essential for virus replication. In this study, viral mutants incorporating either a lacZ expression cassette in place of 1,490 bp of the 2,109-bp U(L)17 open reading frame [HSV-1(deltaU(L)17)] or a DNA oligomer containing an in-frame stop codon inserted 778 bp from the 5' end of the U(L)17 open reading frame [HSV-1(U(L)17-stop)] were plaque purified on engineered cell lines containing the U(L)17 gene. A virus derived from HSV-1(U(L)17-stop) but containing a restored U(L)17 gene was also constructed and was designated HSV-1(U(L)17-restored). The latter virus formed plaques and cleaved genomic viral DNA in a manner indistinguishable from wild-type virus. Neither HSV-1(deltaU(L)17) nor HSV-1(U(L)17-stop) formed plaques or produced infectious progeny when propagated on noncomplementing Vero cells. Furthermore, genomic end-specific restriction fragments were not detected in DNA purified from noncomplementing cells infected with HSV-1(deltaU(L)17) or HSV-1(U(L)17-stop), whereas end-specific fragments were readily detected when the viruses were propagated on complementing cells. Electron micrographs of thin sections of cells infected with HSV-1(deltaU(L)17) or HSV-1(U(L)17-stop) illustrated that empty capsids accumulated in the nuclei of Vero cells, whereas DNA-containing capsids accumulated in the nuclei of complementing cells and enveloped virions were found in the cytoplasm and extracellular space. Additionally, protein profiles of capsids purified from cells infected with HSV-1(deltaU(L)17) compared to wild-type virus show no detectable differences. These data indicate that the U(L)17 gene is essential for virus replication and is required for cleavage and packaging of viral DNA. To characterize the U(L)17 gene product, an anti-U(L)17 rabbit polyclonal antiserum was produced. The antiserum reacted strongly with a major protein of apparent Mr 77,000 and weakly with a protein of apparent Mr 72,000 in wild-type infected cell lysates and in virions. Bands of similar sizes were also detected in electrophoretically separated tegument fractions of virions and light particles and yielded tryptic peptides of masses characteristic of the predicted U(L)17 protein. We therefore conclude that the U(L)17 gene products are associated with the virion tegument and note that they are the first tegument-associated proteins shown to be required for cleavage and packaging of viral DNA.

Herpes simplex virus DNA cleavage and packaging: association of multiple forms of U(L)15-encoded proteins with B capsids requires at least the U(L)6, U(L)17, and U(L)28 genes

The U(L)15 gene of herpes simplex virus (HSV) is one of several genes required for the packaging of viral DNA into intranuclear B capsids to produce C capsids that become enveloped at the inner nuclear membrane. A rabbit antiserum directed against U(L)15-encoded protein recognized three proteins with apparent Mrs of 79,000, 80,000, and 83,000 in highly purified B capsids. The 83,000-Mr protein was detected in type C capsids and comigrated with the product of a U(L)15 cDNA transcribed and translated in vitro. The 83,000- and 80,000-Mr proteins were readily detected in purified virions. Inasmuch as (i) none of these proteins were detectable in capsids purified from cells infected with HSV-1(deltaU(L)15), a virus lacking an intact U(L)15 gene, and (ii) corresponding proteins in capsids purified from cells infected with a recombinant virus [HSV-1(R7244), containing a 20-codon tag at the 3' end of U(L)15] were decreased in electrophoretic mobility relative to the wild-type proteins, we conclude that the proteins with apparent Mrs of 83,000, 80,000, and 79,000 are products of U(L)15 with identical C termini. The 79,000-, 80,000-, and 83,000-Mr proteins remained associated with B capsids in the presence of 0.5 M guanidine HCl and remained detectable in capsids treated with 2.0 M guanidine HCl and lacking proteins associated with the capsid core. These data, therefore, indicate that U(L)15-encoded proteins are integral components of B capsids. Only the 83,000-Mr protein was detected in B capsids purified from cells infected with viruses lacking the U(L)6, U(L)17, or U(L)28 genes, which are required for DNA cleavage and packaging, suggesting that capsid association of the 80,000- and 79,000-Mr proteins requires intact cleavage and packaging machinery. These data, therefore, indicate that capsid association of the 80,000- and 79,000-Mr U(L)15-encoded proteins reflects a previously unrecognized step in the DNA cleavage and packaging reaction.

Genetic analysis of the UL 15 gene locus for the putative terminase of herpes simplex virus type 1

The herpes simplex virus (HSV-1) UL15 gene encodes one of the six viral gene products required for viral DNA cleavage and packaging. UL15 is a spliced gene and encodes two separately translated proteins, UL15 and UL15.5. Sequence analysis reveals that UL15 shares homology with gp 17, the large catalytic subunit of the bacteriophage T4 terminase, a protein which cleaves the polymeric T4 DNA into monomers. Both proteins contain a putative ATP binding motif known as the Walker A and B boxes. In this report, immunofluorescence was used to show that UL15 localizes to the nucleus in the absence of any other viral proteins; this indicates that UL15 contains its own nuclear localization signal. In addition, we found that UL15 colocalizes with replication compartments at early times (6 h postinfection). Since, at this time, preformed capsids as well as other cleavage and packaging proteins are also recruited to replication compartments, it seems likely that cleavage and packaging occurs in the same compartments in which DNA synthesis occurs. Also in this report, we have investigated UL15.5, the N-terminally truncated gene product of the UL15 open reading frame (ORF). The start codon has been mapped to Met443 within the UL15 ORF. Furthermore, we have shown that plasmids containing a UL15.5 knockout mutation still complement the growth of UL15 insertion mutant viruses, indicating that UL15.5 is not required for viral growth in cell culture. Last, we constructed a UL15 mutant, UL15C(G263A), in which the invariant Gly263 in the Walker box A of the ATP binding motif (GKT) was substituted with an alanine. We show that the mutant gene fails to support the growth of UL15 insertion mutant viruses, indicating that the putative ATP binding motif of UL15 is indispensable for its function.

The herpes simplex virus type 1 cleavage/packaging protein, UL32, is involved in efficient localization of capsids to replication compartments

Six genes, including UL32, have been implicated in the cleavage and packaging of herpesvirus DNA into preassembled capsids. We have isolated a UL32 insertion mutant which is capable of near-wild-type levels of viral DNA synthesis; however, the mutant virus is unable to cleave and package viral DNA, consistent with the phenotype of a previously isolated temperature-sensitive herpes simplex virus type 1 mutant, tsN20 (P. A. Schaffer, G. M. Aron, N. Biswal, and M. Benyesh-Melnick, Virology 52:57-71, 1973). A polyclonal antibody which recognizes UL32 was previously used by Chang et al. (Y. E. Chang, A. P. Poon, and B. Roizman, J. Virol. 70:3938-3946, 1996) to demonstrate that UL32 accumulates predominantly in the cytoplasm of infected cells. In this report, a functional epitope-tagged version of UL32 showed that while UL32 is predominantly cytoplasmic, some nuclear staining which colocalizes with the major DNA binding protein (ICP8, UL29) in replication compartments can be detected. We have also used a monoclonal antibody (5C) specific for the hexon form of major capsid protein VP5 to study the distribution of capsids during infection. In cells infected with wild-type KOS (6 and 8 h postinfection), 5C staining patterns indicate that capsids are present in nuclei within replication compartments. These results suggest that cleavage and packaging occur in replication compartments at least at 6 and 8 h postinfection. Cells infected with the UL32 mutant exhibit a hexon staining pattern which is more diffusely distributed throughout the nucleus and which is not restricted to replication compartments. We propose that UL32 may play a role in "bringing" preassembled capsids to the sites of DNA packaging and that the failure to localize to replication compartments may explain the cleavage/packaging defect exhibited by this mutant. These results suggest that the UL32 protein is required at a step distinct from those at which other cleavage and packaging proteins are required and may be involved in the correct localization of capsids within infected cells.

The product of the herpes simplex virus type 1 UL25 gene is required for encapsidation but not for cleavage of replicated viral DNA

The herpes simplex virus type 1 (HSV-1) UL25 gene contains a 580-amino-acid open reading frame that codes for an essential protein. Previous studies have shown that the UL25 gene product is a virion component (M. A. Ali et al., Virology 216:278-283, 1996) involved in virus penetration and capsid assembly (C. Addison et al., Virology 138:246-259, 1984). In this study, we describe the isolation of a UL25 mutant (KUL25NS) that was constructed by insertion of an in-frame stop codon in the UL25 open reading frame and propagated on a complementing cell line. Although the mutant was capable of synthesis of viral DNA, it did not form plaques or produce infectious virus in noncomplementing cells. Antibodies specific for the UL25 protein were used to demonstrate that KUL25NS-infected Vero cells did not express the UL25 protein. Western immunoblotting showed that the UL25 protein was associated with purified, wild-type HSV A, B, and C capsids. Transmission electron microscopy indicated that the nucleus of Vero cells infected with KUL25NS contained large numbers of both A and B capsids but no C capsids. Analysis of infected cells by sucrose gradient sedimentation analysis confirmed that the ratio of A to B capsids was elevated in KUL25NS-infected Vero cells. Following restriction enzyme digestion, specific terminal fragments were observed in DNA isolated from KUL25NS-infected Vero cells, indicating that the UL25 gene was not required for cleavage of replicated viral DNA. The latter result was confirmed by pulsed-field gel electrophoresis (PFGE), which showed the presence of genome-size viral DNA in KUL25NS-infected Vero cells. DNase I treatment prior to PFGE demonstrated that monomeric HSV DNA was not packaged in the absence of the UL25 protein. Our results indicate that the product of the UL25 gene is required for packaging but not cleavage of replicated viral DNA.

Inhibition of human cytomegalovirus DNA maturation by a benzimidazole ribonucleoside is mediated through the UL89 gene product

2-Bromo-5,6-dichloro-1-beta-D-ribofuranosyl benzimidazole (BDCRB) is a member of a new class of benzimidazole ribonucleosides which inhibit human cytomegalovirus (HCMV) late in the replication cycle without inhibiting viral DNA synthesis. We show here that polygenomic concatemeric HCMV DNA does not mature to unit genome length in the presence of BDCRB. To discover the locus of action, virus resistant to BDCRB was selected by serial passage in the presence of the compound. Genetic mapping experiments with BDCRB-resistant virus demonstrated that the resistance phenotype mapped to one amino acid (Asp344Glu; low resistance) or two amino acids (Asp344Glu and Ala355Thr; high resistance) within the product of exon 2 of the HCMV U(L)89 open reading frame. The HCMV U(L)89 open reading frame and its homologs are among the most conserved open reading frames in the herpesviruses, and their products have sequence similarities to a known ATP-dependent endonuclease from the double-stranded DNA bacteriophage T4. These findings strongly suggest that BDCRB prevents viral DNA maturation by interacting with a U(L)89 gene product and that the U(L)89 open reading frame may encode an endonucleolytic subunit of the putative HCMV terminase. Further, since mammalian cell DNA replication does not involve a DNA maturation step, compounds which inhibit viral DNA maturation should be selective and safe.

Sequences within the herpesvirus-conserved pac1 and pac2 motifs are required for cleavage and packaging of the murine cytomegalovirus genome

The DNA sequence motifs pac1 [an A-rich region flanked by poly(C) runs] and pac2 (CGCGGCG near an A-rich region) are conserved near herpesvirus genomic termini and are believed to mediate cleavage of genomes from replicative concatemers. To determine their importance in the cleavage process, we constructed a number of recombinant murine cytomegaloviruses with a second cleavage site inserted at an ectopic location within the viral genome. Cleavage at a wild-type ectopic site occurred as frequently as at the natural cleavage site, whereas mutation of this ectopic site revealed that some of the conserved motifs of pac1 and pac2 were essential for cleavage whereas others were not. Within pac1, the left poly(C) region was very important for cleavage and packaging but the A-rich region was not. Within pac2, the A-rich region and adjacent sequences were essential for cleavage and packaging and the CGCGGCG region contributed to, but was not strictly essential for, efficient cleavage and packaging. A second A-rich region was not important at all. Furthermore, mutations that prevented cleavage also blocked duplication and deletion of the murine cytomegalovirus 30-bp terminal repeat at the ectopic site, suggesting that repeat duplication and deletion are consequences of cleavage. Given that the processes of genome cleavage and packaging appear to be highly conserved among herpesviruses, these findings should be relevant to other members of this family.

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.

Capsid assembly and DNA packaging in herpes simplex virus

The genome of HSV-1 contains 80-85 open reading frames. Genetic and biochemical evidence suggests that at least 39 of these genes encode proteins that are components of the HSV-1 virion. The architecture of the HSV-1 virion consists of a trilaminar lipid envelope, an amorphous layer known as the tegument, a capsid shell, and a DNA-containing core. The capsid is an icosahedral shell whose major morphological features are 162 capsomers. It is composed of a major capsid protein called VP5 and three less abundant proteins, VP19C, VP23 and VP26. VP5 is the structural subunit of all 162 capsomers while VP19C and VP23 are located in the space between the capsomers. In addition to the structural proteins, capsid assembly involves participation of the HSV-1-encoded protease and the scaffolding protein, preVP22a. DNA packaging involves participation of DNA, empty capsids, and at least seven additional HSV-1-encoded proteins. Considerable advances have been made in understanding the structure of the capsid shell, largely as the result of applying cryoelectron microscopy techniques. Use of recombinant baculoviruses has allowed for a detailed analysis of the proteins required for capsid assembly. More recently, an in vitro system has been developed which has aided in defining the assembly pathway by identifying intermediates in the assembly of intact capsids. The in vitro system has identified a fragile roundish procapsid which matures into the polyhedral capsid in a transition similar to that undergone by bacteriophage proheads. This review is a summary of our present knowledge with respect to the structure and assembly of the HSV-1 capsid and what is known about the seven genes involved in DNA packaging. Copyright 1997 John Wiley & Sons Ltd.

Study of herpes simplex virus maturation during a synchronous wave of assembly

Production of an infectious herpes simplex virus (HSV) particle requires sequential progression of maturing virions through a series of complex assembly events. Capsids must be constructed in the nucleus, packaged with the viral genome, and transported to the nuclear periphery. They then bud into the nuclear membrane to acquire an envelope, traffic through the cytoplasm, and are released from the cell. Most of these phenomena are very poorly defined, and no suitable model system has previously been available to facilitate molecular analyses of genomic DNA packaging, capsid envelopment, and intracellular virion trafficking. We report the development of such an assay system for HSV type 1 (HSV-1). Using a reversible temperature-sensitive mutation in capsid assembly, we have developed conditions in which an accumulated population of immature capsids can be rapidly, efficiently, and synchronously chased to maturity. By assaying synchronized scaffold cleavage, DNA packaging, and acquisition of infectivity, we have demonstrated the kinetics with which these events occur. Kinetic and morphological features of intranuclear and extranuclear virion trafficking have similarly been examined by indirect immunofluorescence microscopy and electron microscopy. This system should prove a generally useful tool for the molecular dissection of many late events in HSV-1 biogenesis.

The U(L)15 gene of herpes simplex virus type 1 contains within its second exon a novel open reading frame that is translated in frame with the U(L)15 gene product

The U(L)15 gene of herpes simplex virus type 1 is composed of two exons. A mutation previously shown to preclude viral DNA cleavage and packaging at the nonpermissive temperature was identified as a change from a highly conserved serine to proline at codon 653. Separate viral mutants that contained stop codons inserted into exon I of U(L)15 (designated S648) or an insertion of the Escherichia coli lacZ gene into a truncated U(L)15 exon II [designated HSV-1(delta U(L)15ExII)] were constructed. Recombinant viruses derived from S648 and HSV-1(delta U(L)15ExII) and containing restored U(L)15 genes were constructed and designated S648R and HSV-1(delta U(L)15ExIIR), respectively. Unlike HSV-1(delta U(L)15ExIIR) and S648R, the viruses containing mutant U(L)15 genes failed to cleave and package viral DNA when propagated on noncomplementing cells. As revealed by electron microscopy, large numbers of enveloped capsids lacking viral DNA accumulated within the cytoplasm of cells infected with either S648 or HSV-1(delta U(L)15ExII) but not in cells infected with HSV-1(delta U(L)15ExIIR) or S648R. Thus, one function of the U(L)15 gene is to effectively prevent immature particles lacking DNA from exiting the nucleus by envelopment at the inner lamella of the nuclear membrane. Cells infected with HSV-1(delta U(L)15ExII) did not express the 75,000- or 35,000-apparent-Mr proteins previously shown to be products of the U(L)15 open reading frame, whereas the 35,000-apparent-Mr protein was readily detectable in cells infected with S648. We conclude that at least the 75,000-Mr protein is required for viral DNA cleavage and packaging and hypothesize that the 35,000-Mr protein is derived from translation of a novel mRNA located partially or completely within the second exon of U(L)15.

Characterization of ICP6::lacZ insertion mutants of the UL15 gene of herpes simplex virus type 1 reveals the translation of two proteins

The herpes simplex virus type 1 (HSV-1) UL15 gene is a spliced gene composed of two exons and is predicted to encode an 81-kDa protein of 735 amino acids (aa). Two UL15 gene products with molecular masses of 75 and 35 kDa have been observed (J. Baines, A. Poon, J. Rovnak, and B. Roizman, J. Virol. 68:8118-8124, 1994); however, it is not clear whether the smaller form represents a proteolytic cleavage product of the larger form or whether it is separately translated. In addition, an HSV-1 temperature-sensitive mutant in the UL15 gene (ts66.4) is defective in both cleavage of viral DNA concatemers into unit-length monomers and packaging of viral DNA into capsids (A. Poon and B. Roizman, J. Virol. 67:4497-4503, 1993; J. Baines et al., J. Virol. 68:8118-8124, 1994). In this study, we detected two UL15 gene products of 81 and 30 kDa in HSV-1-infected cells, using a polyclonal antibody raised against a maltose binding protein fusion construct containing UL15 exon 2. In addition, we report the isolation of two HSV-1 insertion mutants, hr81-1 and hr81-2, which contain an ICP6::lacZ insertion in UL15 exon 1 and exon 2 and thus would be predicted to encode C-terminally truncated peptides of 153 and 509 aa long, respectively. hr81-1 and hr81-2 are defective in DNA cleavage and packaging and accumulate only B capsids. However, both mutants are able to undergo wild-type levels of DNA replication and genomic inversion, suggesting that genomic inversion is a result of DNA replication rather than of DNA cleavage and packaging. We also provide evidence that the 81- and 30-kDa proteins are the products of separate in-frame translation events from the UL15 gene and that the 81-kDa full-length UL15 protein is required for DNA cleavage and packaging.

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.

Inhibition of generation of authentic genomic termini of herpes simplex virus type 1 DNA in temperature-sensitive mutant BHK-21 cells with a mutated CCG1/TAF(II)250 gene

A temperature-sensitive (ts) mutant from the BHK-21 hamster cell line, tsBN462, has a defect in progression of the G1 phase at the nonpermissive temperature of 39.5 degrees C. The ts mutation in tsBN462 is located in the CCG1 gene, encoding the general transcription factor TAF(II)250. In tsBN462 at 39.5 degrees C, infectious progeny of herpes simplex virus type 1 (HSV-1) was not produced and generation of authentic genomic termini of HSV-1 was inhibited. HSV-1 concatemers containing L components in two possible orientations were produced in tsBN462 at 39.5 degrees C; hence, the generation of authentic genomic termini seemed to be dispensable for inversion of the L component. As production of mRNAs of HSV-1 genes of three kinetic classes in the tsBN462 at 39.5 degrees C was comparable to findings under permissive conditions, the sequential and regulated manner in which HSV-1 gene expression is processed is likely to be maintained in the nonpermissive condition.

Branched structures in the intracellular DNA of herpes simplex virus type 1

Herpes simplex virus type 1 (HSV-1) replication produces large intracellular DNA molecules that appear to be in a head-to-tail concatemeric arrangement. We have previously suggested (A. Severini, A.R. Morgan, D.R. Tovell, and D.L.J. Tyrrell, Virology 200:428-435, 1994) that these DNA species may have a complex branched structure. We now provide direct evidence for the presence of branches in the high-molecular-weight DNA produced during HSV-1 replication. On neutral agarose two-dimensional gel electrophoresis, a technique that allows separation of branched restriction fragments from linear fragments, intracellular HSV-1 DNA produces arches characteristic of Y junctions (such as replication forks) and X junctions (such as merging replication forks or recombination intermediates). Branched structures were resolved by T7 phage endonuclease I (gene 3 endonuclease), an enzyme that specifically linearizes Y and X structures. Resolution was detected by the disappearance of the arches on two-dimensional gel electrophoresis. Branched structures were also visualized by electron microscopy. Molecules with a single Y junction were observed, as well as large tangles containing two or more consecutive Y junctions. We had previously shown that a restriction enzyme which cuts the HSV-1 genome once does not resolve the large structure of HSV-1 intracellular DNA on pulsed-field gel electrophoresis. We have confirmed that result by using sucrose gradient sedimentation, in which both undigested and digested replicative intermediates sediment to the bottom of the gradient. Taken together, our experiments show that the intracellular HSV-1 DNA is held together in a large complex by frequent branches that create a network of replicating molecules. The fact that most of these branches are Y structures suggests that the network is held together by frequent replication forks and that it resembles the replicative intermediates of bacteriophage T4. Our findings add complexity to the simple model of rolling-circle DNA replication, and they pose interesting questions as to how the network is formed and how it is resolved for packaging into progeny virions.

The product of the UL6 gene of herpes simplex virus type 1 is associated with virus capsids

We report on the analysis of the UL6 and UL7 open reading frames of the herpes simplex virus type 1 (HSV-1) genome. The UL6 and UL7 transcripts were identified in HSV-1-infected cells by Northern blotting and shown to be coterminal at their 3' ends. Both transcripts were synthesized in the presence of phosphonoacetic acid, although in reduced amounts, indicating that UL6 and UL7 are expressed as delayed-early or gamma-1 genes. The 5' ends of the two transcripts were mapped by S1 nuclease and primer extension analysis. A polyclonal antiserum directed against an Escherichia coli-expressed 6 x His-UL6 fusion protein identified a protein of approximate M(r) 75,000 in cells infected with either HSV-1 or with a vaccinia virus recombinant expressing the HSV-1 UL6 protein. As with the transcript, the UL6 protein was synthesized at reduced levels in the absence of viral DNA replication. Western immunoblotting showed that the UL6 protein was present in purified virions but not in L-particles of HSV-1, and that it was located exclusively in the tegument/capsid fraction of virion. Further analysis of the UL6 protein revealed that this protein was associated with virus capsids.

The herpes simplex virus 1 UL15 gene encodes two proteins and is required for cleavage of genomic viral DNA

Previous studies have shown that a ts mutant [herpes simplex virus 1 (mP)ts66.4] in the UL15 gene fails to package viral DNA into capsids (A. P. W. Poon and B. Roizman, J. Virol. 67:4497-4503, 1993) and that although the intron separating the first and second exons of the UL15 gene contains UL16 and UL17 open reading frames, replacement of the first exon with a cDNA copy of the entire gene does not affect viral replication (J.D. Baines, and B. Roizman, J. Virol. 66:5621-5626, 1992). We report that (i) a polyclonal rabbit antiserum generated against a chimeric protein consisting of the bacterial maltose-binding protein fused in frame to the majority of sequences contained in the second exon of the UL15 gene reacted with two proteins with M(r) of 35,000 and 75,000, respectively, in cells infected with a virus containing the authentic gene yielding a spliced mRNA or with a virus in which the authentic UL15 gene was replaced with a cDNA copy. (ii) Insertion of 20 additional codons into the C terminus of UL15 exon II caused a reduction in the electrophoretic mobility of both the apparently 35,000- and 75,000-M(r) proteins, unambiguously demonstrating that both share the carboxyl terminus of the UL15 exon II. (iii) Accumulation of the 35,000-M(r) protein was reduced in cells infected and maintained in the presence of phosphonoacetate, an inhibitor of viral DNA synthesis. (iv) The UL15 proteins were localized in the perinuclear space at 6 h after infection and largely in the nucleus at 12 h after infection. (v) Viral DNA accumulating in cells infected with herpes simplex virus 1(mP)ts66.4 and maintained at the nonpermissive temperature was in an endless (concatemeric) form, and therefore UL15 is required for the cleavage of mature, unit-length molecules for packaging into capsids.

Cell-free assembly of the herpes simplex virus capsid

Herpes simplex virus type 1 (HSV-1) capsids were found to assemble spontaneously in a cell-free system consisting of extracts prepared from insect cells that had been infected with recombinant baculoviruses coding for HSV-1 capsid proteins. The capsids formed in this system resembled native HSV-1 capsids in morphology as judged by electron microscopy, in sedimentation rate on sucrose density gradients, in protein composition, and in their ability to react with antibodies specific for the HSV-1 major capsid protein, VP5. Optimal capsid assembly required the presence of extracts containing capsid proteins VP5, VP19, VP23, VP22a, and the maturational protease (product of the UL26 gene). Assembly was more efficient at 27 degrees C than at 4 degrees C. The availability of a cell-free assay for HSV-1 capsid formation will be of help in identifying the morphogenetic steps that occur during capsid assembly in vivo and in evaluating candidate antiherpes therapeutics directed at capsid assembly.

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

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

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.

Mutations in herpes simplex virus type 1 genes encoding VP5 and VP23 abrogate capsid formation and cleavage of replicated DNA

The herpes simplex virus type 1 capsid is composed of seven capsid proteins which are termed VP5, VP19c, VP21, VP22a, VP23, VP24, and VP26. Major capsid protein VP5 is encoded by the gene UL19. UL18, whose transcript is 3' coterminal with that of VP5, specifies capsid protein VP23. Vero cell lines have been isolated that are transformed with either the BglII N (UL19) or EcoRI G (UL16 to UL21) fragment of KOS. These cell lines, selected for the ability to support the replication of a temperature-sensitive VP5 mutant, were used to isolate VP5 and VP23 null mutants. The mutations in VP5 (K5 delta Z) and VP23 (K23Z) were generated by insertion of the lacZ gene at the beginning of the coding sequences of the genes. Both mutants failed to form plaques on the nonpermissive cell line, and therefore, VP23, like VP5, is an essential gene product for virus replication. Both mutants expressed wild-type levels of infected-cell proteins upon infection of permissive and nonpermissive cell lines. However, the VP5 (150-kDa) and VP23 (33-kDa) polypeptides were absent in lysates prepared from K5 delta Z- and K23Z-infected Vero cells, respectively. No capsid structures were observed by electron microscopic analysis of thin sections of K5 delta Z- and K23Z-infected Vero cells. Following sedimentation of lysates from cells infected by the mutants, capsid proteins were not observed in the fractions where capsids normally sediment. The amounts of DNA replicated in the VP5 and VP23 mutant and in KOS-infected Vero cells were the same as in permissive cells. However, genomic ends were not evident in Vero cells infected with the mutants, suggesting that the DNA remains in concatemers and is not processed into unit length genomes.

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.

Deletion of the VP16 open reading frame of herpes simplex virus type 1

VP16 (also called Vmw65 and alpha TIF) is a structural protein of herpes simplex virus type 1 (HSV-1) that trans-induces HSV-1 immediate-early gene transcription. This report describes an HSV-1 VP16 deletion mutant that was constructed and propagated in a cell line transformed with a VP16 expression vector. The VP16 deletion mutant replicated like wild-type HSV-1 during infection of the VP16-expressing cell line. Deletion mutant virions propagated in this cell line contained wild-type, cell-derived VP16 protein that was recruited during virion assembly and was functional for immediate-early gene trans-induction. The mutant failed to replicate during subsequent infection of cells that do not express VP16, as determined in plaque assays and single-step replication assays. The deletion mutant induced nearly normal levels of viral DNA synthesis and capsid production during these infections, but it induced slightly lower levels of viral DNA encapsidation and appeared by transmission electron microscopy to be defective in further steps of virion maturation. A genetic revertant of the deletion mutant that was restored for VP16-coding sequences exhibited fully wild-type replication properties in both VP16-expressing and nonexpressing cells. The absence of VP16 protein synthesis at late times of HSV-1 infection prevents the production of infectious progeny virus and correlates with a profound defect in HSV-1 particle assembly.

Processing of the herpes simplex virus assembly protein ICP35 near its carboxy terminal end requires the product of the whole of the UL26 reading frame

The herpes simplex virus (HSV) type 1 assembly protein ICP35 consists of a family of polypeptides, ranging in molecular weight from about 45,000-39,000. The lower molecular weight forms of ICP35 are derived from the higher molecular weight species by slow post-translational modification. The reading frame of gene UL26 and the region within this gene which exhibited homology to the cytomegalovirus assembly protein, the analogous protein to ICP35, were expressed separately under immediate-early (IE) gene regulation in a HSV vector containing a temperature-sensitive mutation in the major transcriptional regulator Vmw175. Monoclonal antibody specific for ICP35 immunoprecipitated several polypeptides with molecular weights around 75,000 from extracts of cells infected with a recombinant expressing the IE gene UL26 at the nonpermissive temperature (NPT). These results suggested that the UL26 gene specified a protein distinct from ICP35 but which had some antigenic sites in common with ICP35. In extracts of cells infected at the NPT with a recombinant expressing only the carboxy terminal half of UL26 coding sequences, the monoclonal antibody immunoprecipitated large amounts of the high molecular weight forms of ICP35. The lower molecular weight processed forms of ICP35, however, were not detectable. When cells were coinfected with both recombinants ICP35 was processed to its lower molecular weight forms. This processing step, which occurred near the carboxy terminus of ICP35, was not dependent on capsid formation. The work, together with previous information on the processing of the CMV assembly protein, suggests that UL26 product may be a protease.