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

Small molecule RAF265 as an antiviral therapy acts against HSV-1 by regulating cytoskeleton rearrangement and cellular translation machinery

Host-targeting antivirals (HTAs) have received increasing attention for their potential as broad-spectrum antivirals that pose relatively low risk of developing drug resistance. The repurposing of pharmaceutical drugs for use as antivirals is emerging as a cost- and time- efficient approach to developing HTAs for the treatment of a variety of viral infections. In this study, we used a virus titer method to screen 30 small molecules for antiviral activity against Herpes simplex virus-1 (HSV-1). We found that the small molecule RAF265, an anticancer drug that has been shown to be a potent inhibitor of B-RAF V600E, reduced viral loads of HSV-1 by 4 orders of magnitude in Vero cells and reduced virus proliferation in vivo. RAF265 mediated cytoskeleton rearrangement and targeted the host cell's translation machinery, which suggests that the antiviral activity of RAF265 may be attributed to a dual inhibition strategy. This study offers a starting point for further advances toward clinical development of antivirals against HSV-1.

Epstein-Barr virus genome packaging factors accumulate in BMRF1-cores within viral replication compartments

Productive replication of Epstein-Barr virus (EBV) during the lytic cycle occurs in discrete sites within nuclei, termed replication compartments. We previously proposed that replication compartments consist of two subnuclear domains: "ongoing replication foci" and "BMRF1-cores". Viral genome replication takes place in ongoing replication foci, which are enriched with viral replication proteins, such as BALF5 and BALF2. Amplified DNA and BMRF1 protein accumulate in BMRF1-cores, which are surrounded by ongoing replication foci. We here determined the locations of procapsid and genome-packaging proteins of EBV via three-dimensional (3D) surface reconstruction and correlative fluorescence microscopy-electron microscopy (FM-EM). The results revealed that viral factors required for DNA packaging, such as BGLF1, BVRF1, and BFLF1 proteins, are located in the innermost subdomains of the BMRF1-cores. In contrast, capsid structural proteins, such as BBRF1, BORF1, BDLF1, and BVRF2, were found both outside and inside the BMRF1-cores. Based on these observations, we propose a model in which viral procapsids are assembled outside the BMRF1-cores and subsequently migrate therein, where viral DNA encapsidation occurs. To our knowledge, this is the first report describing capsid assembly sites in relation to EBV replication compartments.

Experimental Dissection of the Lytic Replication Cycles of Herpes Simplex Viruses in vitro

Herpes simplex viruses type 1 and type 2 (HSV-1 and HSV-2) produce lifelong infections and are highly prevalent in the human population. Both viruses elicit numerous clinical manifestations and produce mild-to-severe diseases that affect the skin, eyes, and brain, among others. Despite the existence of numerous antivirals against HSV, such as acyclovir and acyclovir-related analogs, virus variants that are resistant to these compounds can be isolated from immunosuppressed individuals. For such isolates, second-line drugs can be used, yet they frequently produce adverse side effects. Furthermore, topical antivirals for treating cutaneous HSV infections usually display poor to moderate efficacy. Hence, better or novel anti-HSV antivirals are needed and details on their mechanisms of action would be insightful for improving their efficacy and identifying specific molecular targets. Here, we review and dissect the lytic replication cycles of herpes simplex viruses, discussing key steps involved in cell infection and the processes that yield new virions. Additionally, we review and discuss rapid, easy-to-perform and simple experimental approaches for studying key steps involved in HSV replication to facilitate the identification of the mechanisms of action of anti-HSV compounds.

Herpes Simplex Virus 1 Small Capsomere-Interacting Protein VP26 Regulates Nucleocapsid Maturation

VP26 is a herpes simplex virus 1 (HSV-1) small capsomere-interacting protein. In this study, we investigated the function of VP26 in HSV-1-infected cells with the following results. (i) The VP26 null mutation significantly impaired incorporation of minor capsid protein UL25 into nucleocapsids (type C capsids) in the nucleus. (ii) The VP26 mutation caused improper localization of UL25 in discrete punctate domains containing multiple capsid proteins (e.g., the VP5 major capsid protein) in the nucleus; these domains corresponded to capsid aggregates. (iii) The VP26 mutation significantly impaired packaging of replicated viral DNA genomes into capsids but had no effect on viral DNA concatemer cleavage. (iv) The VP26 mutation reduced the frequency of type C capsids, which contain viral DNA but not scaffolding proteins, and produced an accumulation of type A capsids, which lack both viral DNA and scaffold proteins, and had no effect on accumulation of type B capsids, which lack viral DNA but retain cleaved scaffold proteins. Collectively, these results indicated that VP26 was required for efficient viral DNA packaging and proper localization of nuclear capsids. The phenotype of the VP26 null mutation was similar to that reported previously of the UL25 null mutation and of UL25 mutations that preclude UL25 binding to capsids. Thus, VP26 appeared to regulate nucleocapsid maturation by promoting incorporation of UL25 into capsids, which is likely to be required for proper capsid nuclear localization.IMPORTANCE HSV-1 VP26 has been reported to be important for viral replication and virulence in cell cultures and/or mouse models. However, little is known about the function of VP26 during HSV-1 replication, in particular, in viral nucleocapsid maturation although HSV-1 nucleocapsids are estimated to contain 900 copies of VP26. In this study, we present data suggesting that VP26 promoted packaging of HSV-1 DNA genomes into capsids by regulating incorporation of capsid protein UL25 into capsids, which was reported to increase stability of the capsid structure. We also showed that VP26 was required for proper localization of capsids in the infected cell nucleus. This is the first report showing that HSV-1 VP26 is a regulator for nucleocapsid maturation.

Vertex-Specific Proteins pUL17 and pUL25 Mechanically Reinforce Herpes Simplex Virus Capsids

Using atomic force microscopy imaging and nanoindentation measurements, we investigated the effect of the minor capsid proteins pUL17 and pUL25 on the structural stability of icosahedral herpes simplex virus capsids. pUL17 and pUL25, which form the capsid vertex-specific component (CVSC), particularly contributed to capsid resilience along the 5-fold and 2-fold but not along the 3-fold icosahedral axes. Our detailed analyses, including quantitative mass spectrometry of the protein composition of the capsids, revealed that both pUL17 and pUL25 are required to stabilize the capsid shells at the vertices. This indicates that herpesviruses withstand the internal pressure that is generated during DNA genome packaging by locally reinforcing the mechanical sturdiness of the vertices, the most stressed part of the capsids.IMPORTANCE In this study, the structural, material properties of herpes simplex virus 1 were investigated. The capsid of herpes simplex virus is built up of a variety of proteins, and we scrutinized the influence of two of these proteins on the stability of the capsid. For this, we used a scanning force microscope that makes detailed, topographic images of the particles and that is able to perform mechanical deformation measurements. Using this approach, we revealed that both studied proteins play an essential role in viral stability. These new insights support us in forming a complete view on viral structure and furthermore could possibly help not only to develop specific antivirals but also to build protein shells with improved stability for drug delivery purposes.

Nuclear Egress of Herpesviruses: The Prototypic Vesicular Nucleocytoplasmic Transport

Herpesvirus particles mature in two different cellular compartments. While capsid assembly and packaging of the genomic linear double-stranded DNA occur in the nucleus, virion formation takes place in the cytoplasm by the addition of numerous tegument proteins as well as acquisition of the viral envelope by budding into cellular vesicles derived from the trans-Golgi network containing virally encoded glycoproteins. To gain access to the final maturation compartment, herpesvirus nucleocapsids have to cross a formidable barrier, the nuclear envelope (NE). Since the ca. 120 nm diameter capsids are unable to traverse via nuclear pores, herpesviruses employ a vesicular transport through both leaflets of the NE. This process involves proteins which support local dissolution of the nuclear lamina to allow access of capsids to the inner nuclear membrane (INM), drive vesicle formation from the INM and mediate inclusion of the capsid as well as scission of the capsid-containing vesicle (also designated as "primary virion"). Fusion of the vesicle membrane (i.e., the "primary envelope") with the outer nuclear membrane subsequently results in release of the nucleocapsid into the cytoplasm for continuing virion morphogenesis. While this process has long been thought to be unique for herpesviruses, a similar pathway for nuclear egress of macromolecular complexes has recently been observed in Drosophila. Thus, herpesviruses may have coopted a hitherto unrecognized cellular mechanism of vesicle-mediated nucleocytoplasmic transport. This could have far reaching consequences for our understanding of cellular functions as again unraveled by the study of viruses.

Human Cytomegalovirus pUL93 Is Required for Viral Genome Cleavage and Packaging

Human cytomegalovirus (HCMV) pUL93 is essential for virus growth, but its precise function in the virus life cycle is unknown. Here, we characterize a UL93 stop mutant virus (UL93st-TB40/E-BAC) to demonstrate that the absence of this protein does not restrict viral gene expression; however, cleavage of viral DNA into unit-length genomes as well as genome packaging is abolished. Thus, pUL93 is required for viral genome cleavage and packaging.

The putative herpes simplex virus 1 chaperone protein UL32 modulates disulfide bond formation during infection

During DNA encapsidation, herpes simplex virus 1 (HSV-1) procapsids are converted to DNA-containing capsids by a process involving activation of the viral protease, expulsion of the scaffold proteins, and the uptake of viral DNA. Encapsidation requires six minor capsid proteins (UL6, UL15, UL17, UL25, UL28, and UL33) and one viral protein, UL32, not found to be associated with capsids. Although functions have been assigned to each of the minor capsid proteins, the role of UL32 in encapsidation has remained a mystery. Using an HSV-1 variant containing a functional hemagglutinin-tagged UL32, we demonstrated that UL32 was synthesized with true late kinetics and that it exhibited a previously unrecognized localization pattern. At 6 to 9 h postinfection (hpi), UL32 accumulated in viral replication compartments in the nucleus of the host cell, while at 24 hpi, it was additionally found in the cytoplasm. A newly generated UL32-null mutant was used to confirm that although B capsids containing wild-type levels of capsid proteins were synthesized, these procapsids were unable to initiate the encapsidation process. Furthermore, we showed that UL32 is redox sensitive and identified two highly conserved oxidoreductase-like C-X-X-C motifs that are essential for protein function. In addition, the disulfide bond profiles of the viral proteins UL6, UL25, and VP19C and the viral protease, VP24, were altered in the absence of UL32, suggesting that UL32 may act to modulate disulfide bond formation during procapsid assembly and maturation.

IMPORTANCE

Although functions have been assigned to six of the seven required packaging proteins of HSV, the role of UL32 in encapsidation has remained a mystery. UL32 is a cysteine-rich viral protein that contains C-X-X-C motifs reminiscent of those in proteins that participate in the regulation of disulfide bond formation. We have previously demonstrated that disulfide bonds are required for the formation and stability of the viral capsids and are also important for the formation and stability of the UL6 portal ring. In this report, we demonstrate that the disulfide bond profiles of the viral proteins UL6, UL25, and VP19C and the viral protease, VP24, are altered in cells infected with a newly isolated UL32-null mutant virus, suggesting that UL32 acts as a chaperone capable of modulating disulfide bond formation. Furthermore, these results suggest that proper regulation of disulfide bonds is essential for initiating encapsidation.

The varicella-zoster virus portal protein is essential for cleavage and packaging of viral DNA

The varicella-zoster virus (VZV) open reading frame 54 (ORF54) gene encodes an 87-kDa monomer that oligomerizes to form the VZV portal protein, pORF54. pORF54 was hypothesized to perform a function similar to that of a previously described herpes simplex virus 1 (HSV-1) homolog, pUL6. pUL6 and the associated viral terminase are required for processing of concatemeric viral DNA and packaging of individual viral genomes into preformed capsids. In this report, we describe two VZV bacterial artificial chromosome (BAC) constructs with ORF54 gene deletions, Δ54L (full ORF deletion) and Δ54S (partial internal deletion). The full deletion of ORF54 likely disrupted essential adjacent genes (ORF53 and ORF55) and therefore could not be complemented on an ORF54-expressing cell line (ARPE54). In contrast, Δ54S was successfully propagated in ARPE54 cells but failed to replicate in parental, noncomplementing ARPE19 cells. Transmission electron microscopy confirmed the presence of only empty VZV capsids in Δ54S-infected ARPE19 cell nuclei. Similar to the HSV-1 genome, the VZV genome is composed of a unique long region (UL) and a unique short region (US) flanked by inverted repeats. DNA from cells infected with parental VZV (VZVLUC strain) contained the predicted UL and US termini, whereas cells infected with Δ54S contained neither. This result demonstrates that Δ54S is not able to process and package viral DNA, thus making pORF54 an excellent chemotherapeutic target. In addition, the utility of BAC constructs Δ54L and Δ54S as tools for the isolation of site-directed ORF54 mutants was demonstrated by recombineering single-nucleotide changes within ORF54 that conferred resistance to VZV-specific portal protein inhibitors. Importance: Antivirals with novel mechanisms of action would provide additional therapeutic options to treat human herpesvirus infections. Proteins involved in the herpesviral DNA encapsidation process have become promising antiviral targets. Previously, we described a series of N-α-methylbenzyl-N'-aryl thiourea analogs that target the VZV portal protein (pORF54) and prevent viral replication in vitro. To better understand the mechanism of action of these compounds, it is important to define the structural and functional characteristics of the VZV portal protein. In contrast to HSV, no VZV mutants have been described for any of the seven essential DNA encapsidation genes. The VZV ORF54 deletion mutant described in this study represents the first VZV encapsidation mutant reported to date. We demonstrate that the deletion mutant can serve as a platform for the isolation of portal mutants via recombineering and provide a strategy for more in-depth studies of VZV portal structure and function.

Varicella-zoster virus induces the formation of dynamic nuclear capsid aggregates

The first step of herpesviruses virion assembly occurs in the nucleus. However, the exact site where nucleocapsids are assembled, where the genome and the inner tegument are acquired, remains controversial. We created a recombinant VZV expressing ORF23 (homologous to HSV-1 VP26) fused to the eGFP and dually fluorescent viruses with a tegument protein additionally fused to a red tag (ORF9, ORF21 and ORF22 corresponding to HSV-1 UL49, UL37 and UL36). We identified nuclear dense structures containing the major capsid protein, the scaffold protein and maturing protease, as well as ORF21 and ORF22. Correlative microscopy demonstrated that the structures correspond to capsid aggregates and time-lapse video imaging showed that they appear prior to the accumulation of cytoplasmic capsids, presumably undergoing the secondary egress, and are highly dynamic. Our observations suggest that these structures might represent a nuclear area important for capsid assembly and/or maturation before the budding at the inner nuclear membrane.

Analysis of the early steps of herpes simplex virus 1 capsid tegumentation

Herpes simplex virus type 1 particles are multilayered structures with a DNA genome surrounded by a capsid, tegument, and envelope. While the protein content of mature virions is known, the sequence of addition of the tegument and the intracellular compartments where this occurs are intensely debated. To probe this process during the initial stages of egress, we used two approaches: an in vitro nuclear egress assay, which reconstitutes the exit of nuclear capsids to the cytoplasm, and a classical nuclear capsid sedimentation assay. As anticipated, in vitro cytoplasmic capsids did not harbor UL34, UL31, or viral glycoproteins but contained US3. In agreement with previous findings, both nuclear and in vitro capsids were positive for ICP0 and ICP4. Unexpectedly, nuclear C capsids and cytoplasmic capsids produced in vitro without any cytosolic viral proteins also scored positive for UL36 and UL37. Immunoelectron microscopy confirmed that these tegument proteins were closely associated with nuclear capsids. When cytosolic viral proteins were present in the in vitro assay, no additional tegument proteins were detected on the capsids. As previously reported, the tegument was sensitive to high-salt extraction but, surprisingly, was stabilized by exogenous proteins. Finally, some tegument proteins seemed partially lost during egress, while others possibly were added at multiple steps or modified along the way. Overall, an emerging picture hints at the early coating of capsids with up to 5 tegument proteins at the nuclear stage, the shedding of some viral proteins during nuclear egress, and the acquisition of others tegument proteins during reenvelopment.

Release of the herpes simplex virus 1 protease by self cleavage is required for proper conformation of the portal vertex

We identify an NLS within herpes simplex virus scaffold proteins that is required for optimal nuclear import of these proteins into infected or uninfected nuclei, and is sufficient to mediate nuclear import of GFP. A virus lacking this NLS replicated to titers reduced by 1000-fold, but was able to make capsids containing both scaffold and portal proteins suggesting that other functions can complement the NLS in infected cells. We also show that Vp22a, the major scaffold protein, is sufficient to mediate the incorporation of portal protein into capsids, whereas proper portal immunoreactivity in the capsid requires the larger scaffold protein pU(L)26. Finally, capsid angularization in infected cells did not require the HSV-1 protease unless full length pU(L)26 was expressed. These data suggest that the HSV-1 portal undergoes conformational changes during capsid maturation, and reveal that full length pU(L)26 is required for this conformational change.

Herpes simplex virus requires poly(ADP-ribose) polymerase activity for efficient replication and induces extracellular signal-related kinase-dependent phosphorylation and ICP0-dependent nuclear localization of tankyrase 1

Tankyrase 1 is a poly(ADP-ribose) polymerase (PARP) which localizes to multiple subcellular sites, including telomeres and mitotic centrosomes. Poly(ADP-ribosyl)ation of the nuclear mitotic apparatus (NuMA) protein by tankyrase 1 during mitosis is essential for sister telomere resolution and mitotic spindle pole formation. In interphase cells, tankyrase 1 resides in the cytoplasm, and its role therein is not well understood. In this study, we found that herpes simplex virus (HSV) infection induced extensive modification of tankyrase 1 but not tankyrase 2. This modification was dependent on extracellular signal-regulated kinase (ERK) activity triggered by HSV infection. Following HSV-1 infection, tankyrase 1 was recruited to the nucleus. In the early phase of infection, tankyrase 1 colocalized with ICP0 and thereafter localized within the HSV replication compartment, which was blocked in cells infected with the HSV-1 ICP0-null mutant R7910. In the absence of infection, ICP0 interacted with tankyrase 1 and efficiently promoted its nuclear localization. HSV did not replicate efficiently in cells depleted of both tankyrases 1 and 2. Moreover, XAV939, an inhibitor of tankyrase PARP activity, decreased viral titers to 2 to 5% of control values. We concluded that HSV targets tankyrase 1 in an ICP0- and ERK-dependent manner to facilitate its replication.

A mutation in UL15 of herpes simplex virus 1 that reduces packaging of cleaved genomes

Herpesvirus genomic DNA is cleaved from concatemers that accumulate in infected cell nuclei. Genomic DNA is inserted into preassembled capsids through a unique portal vertex. Extensive analyses of viral mutants have indicated that intact capsids, the portal vertex, and all components of a tripartite terminase enzyme are required to both cleave and package viral DNA, suggesting that DNA cleavage and packaging are inextricably linked. Because the processes have not been functionally separable, it has been difficult to parse the roles of individual proteins in the DNA cleavage/packaging reaction. In the present study, a virus bearing the deletion of codons 400 to 420 of U(L)15, encoding a terminase component, was analyzed. This virus, designated vJB27, failed to replicate on noncomplementing cells but cleaved concatemeric DNA to ca. 35 to 98% of wild-type levels. No DNA cleavage was detected in cells infected with a U(L)15-null virus or a virus lacking U(L)15 codons 383 to 385, comprising a motif proposed to couple ATP hydrolysis to DNA translocation. The amount of vJB27 DNA protected from DNase I digestion was reduced compared to the wild-type virus by 6.5- to 200-fold, depending on the DNA fragment analyzed, thus indicating a profound defect in DNA packaging. Capsids containing viral DNA were not detected in vJB27-infected cells, as determined by electron microscopy. These data suggest that pU(L)15 plays an essential role in DNA translocation into the capsid and indicate that this function is separable from its role in DNA cleavage.

Characterization of the subcellular localization of herpes simplex virus type 1 proteins in living cells

In this study, we presented the construction of a library of expression clones for the herpes simplex virus type 1 (HSV-1) proteome and subcellular localization map of HSV-1 proteins in living cells using yellow fluorescent protein (YFP) fusion proteins. As a result, 21 proteins showed cytoplasmic or subcytoplasmic localization, 16 proteins showed nuclear or subnuclear localization, and others were present both in the nucleus and cytoplasm. Interestingly, most capsid proteins showed enriched or exclusive localization in the nucleus, and most of the envelope proteins showed cytoplasmic localization, suggesting that subcellular localization of the proteins correlated with their functions during virus replication. These results present a subcellular localization map of HSV-1 proteins in living cells, which provide useful information to further characterize the functions of these proteins.

Plus- and minus-end directed microtubule motors bind simultaneously to herpes simplex virus capsids using different inner tegument structures

Many viruses depend on host microtubule motors to reach their destined intracellular location. Viral particles of neurotropic alphaherpesviruses such as herpes simplex virus 1 (HSV1) show bidirectional transport towards the cell center as well as the periphery, indicating that they utilize microtubule motors of opposing directionality. To understand the mechanisms of specific motor recruitment, it is necessary to characterize the molecular composition of such motile viral structures. We have generated HSV1 capsids with different surface features without impairing their overall architecture, and show that in a mammalian cell-free system the microtubule motors dynein and kinesin-1 and the dynein cofactor dynactin could interact directly with capsids independent of other host factors. The capsid composition and surface was analyzed with respect to 23 structural proteins that are potentially exposed to the cytosol during virus assembly or cell entry. Many of these proteins belong to the tegument, the hallmark of all herpesviruses located between the capsid and the viral envelope. Using immunoblots, quantitative mass spectrometry and quantitative immunoelectron microscopy, we show that capsids exposing inner tegument proteins such as pUS3, pUL36, pUL37, ICP0, pUL14, pUL16, and pUL21 recruited dynein, dynactin, kinesin-1 and kinesin-2. In contrast, neither untegumented capsids exposing VP5, VP26, pUL17 and pUL25 nor capsids covered by outer tegument proteins such as vhs, pUL11, ICP4, ICP34.5, VP11/12, VP13/14, VP16, VP22 or pUS11 bound microtubule motors. Our data suggest that HSV1 uses different structural features of the inner tegument to recruit dynein or kinesin-1. Individual capsids simultaneously accommodated motors of opposing directionality as well as several copies of the same motor. Thus, these associated motors either engage in a tug-of-war or their activities are coordinately regulated to achieve net transport either to the nucleus during cell entry or to cytoplasmic membranes for envelopment during assembly.

Many viruses, particularly neurotropic alphaherpesviruses such as herpes simplex virus (HSV), require an intact microtubule network for efficient replication and pathogenesis. In living cells, host and viral cargo show rapid reversals in transport direction, suggesting that they can recruit motors of opposing directionality simultaneously. To elucidate the molecular mechanisms for specific motor-cargo recognition, it is necessary to characterize the surface of such cargos. We established a cell-free system that reconstitutes the binding of native, mammalian microtubule motors to intact tegumented HSV capsids. Our data suggest that the inbound motor dynein and the outbound motor kinesin-1 bind directly and independently of other host factors to the inner tegument that coats the capsids during cytosolic transport. Identifying viral receptors for the hosts' transport machinery will provide us on the one hand with new potential targets for antiviral therapy. On the other hand, such viral protein domains could be added to viral vectors or even to artificial nano carriers designed to deliver therapeutic genes or molecules to the nucleus or other subcellular destinations.

The capsid protein encoded by U(L)17 of herpes simplex virus 1 interacts with tegument protein VP13/14

The U(L)17 protein (pU(L)17) of herpes simplex virus 1 (HSV-1) likely associates with the surfaces of DNA-containing capsids in a heterodimer with pU(L)25. pU(L)17 is also associated with viral light particles that lack capsid proteins, suggesting its presence in the tegument of the HSV-1 virion. To help determine how pU(L)17 becomes incorporated into virions and its functions therein, we identified pU(L)17-interacting proteins by immunoprecipitation with pU(L)17-specific IgY at 16 h postinfection, followed by mass spectrometry. Coimmunoprecipitated proteins included cellular histone proteins H2A, H3, and H4; the intermediate filament protein vimentin; the major HSV-1 capsid protein VP5; and the HSV tegument proteins VP11/12 (pU(L)46) and VP13/14 (pU(L)47). The pU(L)17-VP13/14 interaction was confirmed by coimmunoprecipitation in the presence and absence of intact capsids and by affinity copurification of pU(L)17 and VP13/14 from lysates of cells infected with a recombinant virus encoding His-tagged pU(L)17. pU(L)17 and VP13/14-HA colocalized in the nuclear replication compartment, in the cytoplasm, and at the plasma membrane between 9 and 18 h postinfection. One possible explanation of these data is that pU(L)17 links the external face of the capsid to VP13/14 and associated tegument components.

Interaction domains of the UL16 and UL21 tegument proteins of herpes simplex virus

The UL16 protein of herpes simplex virus is capsid associated and was previously identified as a binding partner of the membrane-associated UL11 tegument protein (J. S. Loomis, R. J. Courtney, and J. W. Wills, J. Virol. 77:11417-11424, 2003). In those studies, a less-prominent, approximately 65-kDa binding partner of unknown identity was also observed. Mass spectrometry studies have now revealed this species to be UL21, a tegument protein that has been implicated in the transport of capsids in the cytoplasm. The validity of the mass spectrometry results was tested in a variety of coimmunoprecipitation and glutathione S-transferase pull-down experiments. The data revealed that UL21 and UL16 can form a complex in the absence of other viral proteins, even when the assays used proteins purified from Escherichia coli. Moreover, UL11 was able to pull down UL21 only when UL16 was present, suggesting that all three proteins can form a complex. Deletion analyses revealed that the second half of UL21 (residues 268 to 535) is sufficient for the UL16 interaction and packaging into virions; however, attempts to map a subdomain of UL16 were largely unsuccessful, with only the first 40 (of 373) residues being found to be dispensable. Nevertheless, it is clear that UL16 must have two distinct binding sites, because covalent modification of its free cysteines with N-ethylmaleimide blocked binding to UL11 but not UL21. These findings should prove useful for elucidating the molecular machinery used to transmit a signal into a virion when it attaches to cells, a recently discovered mechanism in which UL16 is a central player.

Effects of major capsid proteins, capsid assembly, and DNA cleavage/packaging on the pUL17/pUL25 complex of herpes simplex virus 1

The U(L)17 and U(L)25 proteins (pU(L)17 and pU(L)25, respectively) of herpes simplex virus 1 are located at the external surface of capsids and are essential for DNA packaging and DNA retention in the capsid, respectively. The current studies were undertaken to determine whether DNA packaging or capsid assembly affected the pU(L)17/pU(L)25 interaction. We found that pU(L)17 and pU(L)25 coimmunoprecipitated from cells infected with wild-type virus, whereas the major capsid protein VP5 (encoded by the U(L)19 gene) did not coimmunoprecipitate with these proteins under stringent conditions. In addition, pU(L)17 (i) coimmunoprecipitated with pU(L)25 in the absence of other viral proteins, (ii) coimmunoprecipitated with pU(L)25 from lysates of infected cells in the presence or absence of VP5, (iii) did not coimmunoprecipitate efficiently with pU(L)25 in the absence of the triplex protein VP23 (encoded by the U(L)18 gene), (iv) required pU(L)25 for proper solubilization and localization within the viral replication compartment, (v) was essential for the sole nuclear localization of pU(L)25, and (vi) required capsid proteins VP5 and VP23 for nuclear localization and normal levels of immunoreactivity in an indirect immunofluorescence assay. Proper localization of pU(L)25 in infected cell nuclei required pU(L)17, pU(L)32, and the major capsid proteins VP5 and VP23, but not the DNA packaging protein pU(L)15. The data suggest that VP23 or triplexes augment the pU(L)17/pU(L)25 interaction and that VP23 and VP5 induce conformational changes in pU(L)17 and pU(L)25, exposing epitopes that are otherwise partially masked in infected cells. These conformational changes can occur in the absence of DNA packaging. The data indicate that the pU(L)17/pU(L)25 complex requires multiple viral proteins and functions for proper localization and biochemical behavior in the infected cell.

The Marek's disease virus (MDV) protein encoded by the UL17 ortholog is essential for virus growth

Marek’s disease virus type 1 (MDV-1) shows a strict dependency on the direct cell-to-cell spread for its propagation in cell culture. As MDV-1 shows an impaired nuclear egress in cell culture, we wished to address the characterization of capsid/tegument genes which may intervene in the maturation of intranuclear capsids. Orthologs of UL17 are present in all herpesviruses and, in all reported case, were shown to be essential for viral growth, playing a role in capsid maturation and DNA packaging. As only HSV-1 and PrV UL17 proteins have been characterized so far, we wished to examine the role of MDV-1 pUL17 in virus replication. To analyze MDV-1 UL17 gene function, we created deletion mutants or point mutated the open reading frame (ORF) to interrupt its coding phase. We established that a functional ORF UL17 is indispensable for MDV-1 growth. We chose to characterize the virally encoded protein by tagging the 729 amino-acid long protein with a repeat of the HA peptide that was fused to its C-terminus. Protein pUL17 was identified in infected cell extracts as an 82 kDa protein which localized to the nucleus, colocalizing with VP5, the major capsid protein, and VP13/14, a major tegument protein. By using green fluorescent protein fusion and HA tagged proteins expressed under the cytomegalovirus IE gene enhancer/promoter (P_{CMV IE}), we showed that MDV-1 pUL17 nuclear distribution in infected cells is not an intrinsic property. Although our results strongly suggest that another viral protein retains (or relocate) pUL17 to the nucleus, we report that none of the tegument protein tested so far were able to mediate pUL17 relocation to the nucleus.

The putative leucine zipper of the UL6-encoded portal protein of herpes simplex virus 1 is necessary for interaction with pUL15 and pUL28 and their association with capsids

Herpes simplex virus (HSV) type 1 capsids contain a single portal vertex that is composed of 12 copies of the U(L)6 gene product (pU(L)6), which forms a pore through which DNA is inserted during packaging. This unique vertex is also believed to comprise the site with which a molecular motor, termed the terminase, associates during the DNA packaging reaction. In HSV, the terminase likely comprises the U(L)15, U(L)28, and U(L)33 proteins (pU(L)15, pU(L)28, and pU(L)33, respectively). The current study was undertaken to identify portal domains required for interaction with the terminase. Both the amino and carboxyl termini, as well as amino acids 422 to 443 of pU(L)6 forming a putative leucine zipper motif, were critical for coimmunoprecipitation with pU(L)15 in the absence of other viral proteins. Amino acids 422 to 443 were also necessary for interaction with pU(L)28 in the absence of other viral proteins. By using an engineered recombinant virus, it was further determined that although amino acids 422 to 443 were dispensable for interaction with scaffold protein and incorporation of portal protein into capsids, they were necessary for coimmunoprecipitation of pU(L)6 and pU(L)15 from infected cell lysates, association of optimal levels of pU(L)15, pU(L)28, and pU(L)33 with capsids, and DNA cleavage and packaging. These data identify a portal protein domain critical for terminase association with the capsid and suggest that both the pU(L)15- and pU(L)28-bearing terminase subunits mediate docking of the terminase with the portal vertex.

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.

The major tegument structural protein VP22 targets areas of dispersed nucleolin and marginalized chromatin during productive herpes simplex virus 1 infection

The herpes simplex virus (HSV) major tegument structural protein VP22 resides in multiple subcellular regions during productive infection. During an analysis of the molecular determinants of these localizations, we observed that a transfected fusion of the C-terminal portion of VP22, containing its pat4 nuclear localization signal, with GFP lacked nucleolar sparing compared to GFP alone. Thus, the initial goal was to determine whether VP22 associates with nucleoli. Using an optimized indirect immunofluorescence system to visualize nucleolin and viral proteins, we observed that VP22 present in VP22-expressing Vero (V49) cells "surrounded" nucleolin. These two initial findings implied that VP22 might associate directly with nucleoli. We next analyzed HSV-infected cells and observed that at late times, anti-nucleolin immune reactivity was dispersed throughout the nuclei while it retained uniform, circular staining in mock-infected cells. Time course infection experiments indicated that nucleolin initiated its transition from uniform to dispersed structures between 2 and 4 hpi. Comparison of Hoechst stained nuclei showed bright anti-nucleolin staining localized to regions of marginalized chromatin. These effects required de novo infected cell protein synthesis. A portion of VP22 detected in nuclei at 4 and 6 hpi localized to these areas of altered nucleolin and marginalized chromatin. VP22 was excluded from viral replication compartments containing the viral regulatory protein ICP22. Finally, altered nucleolin and marginalized chromatin were detected with a VP22-null virus, indicating that VP22 was not responsible for these nuclear architecture alterations. Thus, we conclude that nuclear VP22 targets unique subnuclear structures early (<6hpi) during herpes simplex virus 1 (HSV-1) infection.

The UL15 protein of herpes simplex virus type 1 is necessary for the localization of the UL28 and UL33 proteins to viral DNA replication centres

The UL15, UL28 and UL33 proteins of herpes simplex virus type 1 (HSV-1) are thought to comprise a terminase complex responsible for cleavage and packaging of the viral genome into pre-assembled capsids. Immunofluorescence studies confirmed that shortly after infection with wild-type HSV-1 these three proteins localize to viral DNA replication compartments within the nucleus, identified by the presence of the single-stranded DNA-binding protein, ICP8. In cells infected with either UL28- or UL33-null mutants, the other two terminase proteins also co-localized with ICP8. In contrast, neither UL28 nor UL33 was detectable in replication compartments following infection with a UL15-null mutant, although Western blot analysis showed they were present in normal amounts in the infected cells. Provision of UL15 in a complementing cell line restored the ability of all three proteins to localize to replication compartments. These data indicate that UL15 plays a key role in localizing the terminase complex to DNA replication compartments, and that it can interact independently with UL28 and UL33.

Domain within herpes simplex virus 1 scaffold proteins required for interaction with portal protein in infected cells and incorporation of the portal vertex into capsids

The portal vertex of herpesvirus capsids serves as the conduit through which DNA is inserted during the assembly process. In herpes simplex virus (HSV), the portal is composed of 12 copies of the U(L)6 gene product, pU(L)6. Previous results identified a domain in the major capsid scaffold protein, ICP35, required for interaction with pU(L)6 and its incorporation into capsids formed in vitro (G. P. Singer et al., J. Virol. 74:6838-6848, 2005). In the current studies, pU(L)6 and scaffold proteins were found to coimmunoprecipitate from lysates of both HSV-infected cells and mammalian cells expressing scaffold proteins and pU(L)6. The coimmunoprecipitation of pU(L)6 and scaffold proteins was precluded upon deletion of codons 143 to 151 within U(L)26.5, encoding ICP35. While wild-type scaffold proteins colocalized with pU(L)6 when transiently coexpressed as viewed by indirect immunofluorescence, deletion of U(L)26.5 codons 143 to 151 precluded this colocalization. A recombinant herpes simplex virus, vJB11, was generated that lacked U(L)26.5 codons 143 to 151. A virus derived from this mutant but bearing a restored U(L)26.5 was also generated. vJB11 was unable to cleave or package viral DNA, whereas the restored virus packaged DNA normally. vJB11 produced ample numbers of B capsids in infected cells, but these lacked normal levels of pU(L)6. The deletion in U(L)26.5 also rendered pU(L)6 resistant to detergent extraction from vJB11-infected cells. These data indicate that, as was observed in vitro, amino acids 143 to 151 of ICP35 are critical for (i) interaction between scaffold proteins and pU(L)6 and (ii) incorporation of the HSV portal into capsids.

Characterization of mre11 loss following HSV-1 infection

Herpes simplex virus induces the activation of the cellular DNA double strand break response pathway dependent upon initiation of viral DNA replication. The MRN complex, consisting of Mre11, Rad50 and Nbs1, is an essential component of the DNA double strand break response and other reports have documented its presence at sites of viral DNA replication, interaction with ICP8 and its contribution to efficient viral DNA replication. During our characterization of the DSB response following infection of normal human fibroblasts and telomerase-immortalized keratinocytes, we observed the loss of Mre11 protein at late times following infection. The loss was not dependent upon ICP0, the proteasome or lysosomal protease activity. Like activation of the DSB response pathway, Mre11 loss was prevented under conditions which inhibited viral DNA replication. Analysis of a series of mutant viruses with defects in cleavage and packaging (UL6, UL15, UL17, UL25, UL28, UL32) of viral DNA or in the maturational protease (UL26) failed to identify a viral gene product necessary for Mre11 loss. Inactivation of ATM, a key effector kinase in the DNA double strand break response, had no effect on Mre11 loss and only a moderate effect on HSV yield. Finally, treatment of uninfected cells with the topoisomerase I inhibitor camptothecin, to induce generation of free DNA ends, also resulted in Mre11 loss. These results suggest that Mre11 loss following infection is caused by the generation of free DNA ends during or following viral DNA replication.

Aggresomes and pericentriolar sites of virus assembly: cellular defense or viral design?

Virus replication and virus assembly often occur in virus inclusions or virus factories that form at pericentriolar sites close to the microtubule organizing center or in specialized nuclear domains called ND10/PML bodies. Similar inclusions called aggresomes form in response to protein aggregation. Protein aggregates are toxic to cells and are transported along microtubules to aggresomes for immobilization and subsequent degradation by proteasomes and/or autophagy. The similarity between aggresomes and virus inclusions raises the possibility that viruses use aggresome pathways to concentrate cellular and viral proteins to facilitate replication and assembly. Alternatively, aggresomes may be part of an innate cellular response that recognizes virus components as foreign or misfolded and targets them for storage and degradation. Insights into the possible roles played by aggresomes during virus assembly are emerging from an understanding of how virus inclusions form and how viral proteins are targeted to them.

A guide to viral inclusions, membrane rearrangements, factories, and viroplasm produced during virus replication

Virus replication can cause extensive rearrangement of host cell cytoskeletal and membrane compartments leading to the “cytopathic effect” that has been the hallmark of virus infection in tissue culture for many years. Recent studies are beginning to redefine these signs of viral infection in terms of specific effects of viruses on cellular processes. In this chapter, these concepts have been illustrated by describing the replication sites produced by many different viruses. In many cases, the cellular rearrangements caused during virus infection lead to the construction of sophisticated platforms in the cell that concentrate replicase proteins, virus genomes, and host proteins required for replication, and thereby increase the efficiency of replication. Interestingly, these same structures, called virus factories, virus inclusions, or virosomes, can recruit host components that are associated with cellular defences against infection and cell stress. It is possible that cellular defence pathways can be subverted by viruses to generate sites of replication. The recruitment of cellular membranes and cytoskeleton to generate virus replication sites can also benefit viruses in other ways. Disruption of cellular membranes can, for example, slow the transport of immunomodulatory proteins to the surface of infected cells and protect against innate and acquired immune responses, and rearrangements to cytoskeleton can facilitate virus release.

Evolutionary sequence comparison of the Mitf gene reveals novel conserved domains

The microphthalmia-associated transcription factor (MITF) is a member of the MYC family of basic helix-loop-helix leucine zipper transcription factors. The corresponding gene was initially discovered in the mouse based on mutations which affect the development of several different cell types, including melanocytes and retinal pigment epithelium cells. Subsequently, it was shown to be associated with deafness and hypo-pigmentation disorders in humans. More recently, the gene has been shown to be critical in melanoma formation and to play a role in melanocyte stem cell maintenance. Thus, the mouse Mitf gene represents an important model system for the study of human disease as well as an interesting model for the study of transcription factor function in the organism. Here we use the evolutionary relationship of Mitf genes from numerous distantly related species, including vertebrates and invertebrates, to identify novel conserved domains in the Mitf protein and regions of possible functional importance in the 3' untranslated region. We also characterize the nine different 5' exons of the Mitf gene and identify a new 5' exon in the Drosophila Mitf gene. Our analysis sheds new light on the conservation of the Mitf gene and protein and opens the door for further functional analysis.

Putative terminase subunits of herpes simplex virus 1 form a complex in the cytoplasm and interact with portal protein in the nucleus

Herpes simplex virus (HSV) terminase is an essential component of the molecular motor that translocates DNA through the portal vertex in the capsid during DNA packaging. The HSV terminase is believed to consist of the UL15, UL28, and UL33 gene products (pUL15, pUL28, and pUL33, respectively), whereas the HSV type 1 portal vertex is encoded by UL6. Immunoprecipitation reactions revealed that pUL15, pUL28, and pUL33 interact in cytoplasmic and nuclear lysates. Deletion of a canonical nuclear localization signal (NLS) from pUL15 generated a dominant-negative protein that, when expressed in an engineered cell line, decreased the replication of wild-type virus up to 80-fold. When engineered into the genome of recombinant HSV, this mutation did not interfere with the coimmunoprecipitation of pUL15, pUL28, and pUL33 from cytoplasmic lysates of infected cells but prevented viral replication, most nuclear import of both pUL15 and pUL28, and coimmunoprecipitation of pUL15, pUL28, and pUL33 from nuclear lysates. When the pUL15/pUL28 interaction was reduced in infected cells by the truncation of the C terminus of pUL28, pUL28 remained in the cytoplasm. Whether putative terminase components localized in the nucleus or cytoplasm, pUL6 localized in infected cell nuclei, as viewed by indirect immunofluorescence. The finding that the portal and terminase do eventually interact was supported by the observation that pUL6 coimmunoprecipitated strongly with pUL15 and weakly with pUL28 from extracts of infected cells in 1.0 M NaCl. These data are consistent with the hypothesis that the pUL15/pUL28/pUL33 complex forms in the cytoplasm and that an NLS in pUL15 is used to import the complex into the nucleus where at least pUL15 and pUL28 interact with the portal to mediate DNA packaging.

Involvement of UL24 in herpes-simplex-virus-1-induced dispersal of nucleolin

UL24 of herpes simplex virus 1 is important for efficient viral replication, but its function is unknown. We generated a recombinant virus, vHA-UL24, encoding UL24 with an N-terminal hemagglutinin tag. By indirect immunofluorescence at 9 h post-infection (hpi), we detected HA-UL24 in nuclear foci and in cytoplasmic speckles. HA-UL24 partially co-localized with nucleolin, but not with ICP8 or coilin, markers for nucleoli, viral replication compartments, and Cajal bodies respectively. HA-UL24 staining was often juxtaposed to that of another nucleolar protein, fibrillarin. Analysis of HSV-1-induced nucleolar modifications revealed that by 18 hpi, nucleolin staining had dispersed, and fibrillarin staining went from clusters of small spots to a few separate but prominent spots. Fibrillarin redistribution appeared to be independent of UL24. In contrast, cells infected with a UL24-deficient virus retained foci of nucleolin staining. Our results demonstrate involvement of UL24 in dispersal of nucleolin during infection.

Herpes simplex virus 1 DNA packaging proteins encoded by UL6, UL15, UL17, UL28, and UL33 are located on the external surface of the viral capsid

Studies to localize the herpes simplex virus 1 portal protein encoded by UL6, the putative terminase components encoded by UL15, UL 28, and UL33, the minor capsid proteins encoded by UL17, and the major scaffold protein ICP35 were conducted. ICP35 in B capsids was more resistant to trypsin digestion of intact capsids than pUL6, pUL15, pUL17, pUL28, or pUL33. ICP35 required sectioning of otherwise intact embedded capsids for immunoreactivity, whereas embedding and/or sectioning decreased the immunoreactivities of pUL6, pUL17, pUL28, and pUL33. Epitopes of pUL15 were recognized roughly equally well in both sectioned and unsectioned capsids. These data indicate that pUL6, pUL17, pUL28, pUL33, and at least some portion of pUL15 are located at the external surface of the capsid.

Structural characterization of the UL25 DNA-packaging protein from herpes simplex virus type 1

Herpesviruses replicate their double stranded DNA genomes as high-molecular-weight concatemers which are subsequently cleaved into unit-length genomes by a complex mechanism that is tightly coupled to DNA insertion into a preformed capsid structure, the procapsid. The herpes simplex virus type 1 UL25 protein is incorporated into the capsid during DNA packaging, and previous studies of a null mutant have demonstrated that its function is essential at the late stages of the head-filling process, either to allow packaging to proceed to completion or for retention of the viral genome within the capsid. We have expressed and purified an N-terminally truncated form of the 580-residue UL25 protein and have determined the crystallographic structure of the region corresponding to amino acids 134 to 580 at 2.1-Angstroms resolution. This structure, the first for any herpesvirus protein involved in processing and packaging of viral DNA, reveals a novel fold, a distinctive electrostatic distribution, and a unique "flexible" architecture in which numerous flexible loops emanate from a stable core. Evolutionary trace analysis of UL25 and its homologues in other herpesviruses was used to locate potentially important amino acids on the surface of the protein, leading to the identification of four putative docking regions for protein partners.

Identification, subviral localization, and functional characterization of the pseudorabies virus UL17 protein

Homologs of the UL17 gene of the alphaherpesvirus herpes simplex virus 1 (HSV-1) are conserved in all three subfamilies of herpesviruses. However, only the HSV-1 protein has so far been characterized in any detail. To analyze UL17 of pseudorabies virus (PrV) the complete 597-amino-acid protein was expressed in Escherichia coli and used for rabbit immunization. The antiserum recognized a 64-kDa protein in PrV-infected cell lysates and purified virions, identifying PrV UL17 as a structural virion component. In indirect immunofluorescence analyses of PrV-infected cells the protein was predominantly found in the nucleus. In electron microscopic studies after immunogold labeling of negatively stained purified virion preparations, UL17-specific label was detected on single, mostly damaged capsids, whereas complete virions and the majority of capsids were free of label. In ultrathin sections of infected cells, label was primarily found dispersed around scaffold-containing B-capsids, whereas on DNA-filled C-capsids it was located in the center. Empty intranuclear A-capsids were free of label, as were extracellular capsid-less L-particles. Functional characterization of PrV-DeltaUL17F, a deletion mutant lacking codons 23 to 444, demonstrated that cleavage of viral DNA into unit-length genomes was inhibited in the absence of UL17. In electron microscopic analyses of PrV-DeltaUL17F-infected RK13 cells, DNA-containing capsids were not detected, while numerous capsidless L-particles were observed. In summary, our data indicate that the PrV UL17 protein is an internal nucleocapsid protein necessary for DNA cleavage and packaging but suggest that the protein is not a prominent part of the tegument.

Genetic and molecular in vivo analysis of herpes simplex virus assembly in murine visual system neurons

Herpes simplex virus (HSV) infects both epithelial cells and neuronal cells of the human host. Although HSV assembly has been studied extensively for cultured epithelial and neuronal cells, cultured neurons are biochemically, physiologically, and anatomically significantly different than mature neurons in vivo. Therefore, it is imperative that viral maturation and assembly be studied in vivo. To study viral assembly in vivo, we inoculated wild-type and replication-defective viruses into the posterior chamber of mouse eyes and followed infection in retinal ganglion cell bodies and axons. We used PCR techniques to detect viral DNA and RNA and electron microscopy immunohistochemistry and Western blotting to detect viral proteins in specific portions of the optic tract. This approach has shown that viral DNA replication is necessary for viral DNA movement into axons. Movement of viral DNA along ganglion cell axons occurs within capsid-like structures at the speed of fast axonal transport. These studies show that the combined use of intravitreal injections of replication-defective viruses and molecular probes allows the genetic analysis of essential viral replication and maturation processes in neurons in vivo. The studies also provide novel direct evidence for the axonal transport of viral DNA and support for the subassembly hypothesis of viral maturation in situ.

Involvement of the portal at an early step in herpes simplex virus capsid assembly

DNA enters the herpes simplex virus capsid by way of a ring-shaped structure called the portal. Each capsid contains a single portal, located at a unique capsid vertex, that is composed of 12 UL6 protein molecules. The position of the portal requires that capsid formation take place in such a way that a portal is incorporated into one of the 12 capsid vertices and excluded from all other locations, including the remaining 11 vertices. Since initiation or nucleation of capsid formation is a unique step in the overall assembly process, involvement of the portal in initiation has the potential to cause its incorporation into a unique vertex. In such a mode of assembly, the portal would need to be involved in initiation but not able to be inserted in subsequent assembly steps. We have used an in vitro capsid assembly system to test whether the portal is involved selectively in initiation. Portal incorporation was compared in capsids assembled from reactions in which (i) portals were present at the beginning of the assembly process and (ii) portals were added after assembly was under way. The results showed that portal-containing capsids were formed only if portals were present at the outset of assembly. A delay caused formation of capsids lacking portals. The findings indicate that if portals are present in reaction mixtures, a portal is incorporated during initiation or another early step in assembly. If no portals are present, assembly is initiated in another, possibly related, way that does not involve a portal.

The DNA cleavage and packaging protein encoded by the UL33 gene of herpes simplex virus 1 associates with capsids

The U(L)33 gene of herpes simplex virus 1 (HSV-1) encodes a protein (pU(L)33) that is essential for the cleavage and packaging of concatameric herpesvirus DNA into preformed capsids. Previous data have suggested that the U(L)33 protein interacts with the cleavage and packaging proteins encoded by U(L)15 and U(L)28 that are known to associate with capsids. Examination of purified A capsids that lack DNA and are derived from aborted packaging events, B capsids that lack DNA, and C capsids that contain DNA revealed an association of the U(L)33 protein with all three capsid types. More U(L)33 protein was detected in A capsids than was present in B capsids. Capsid association was susceptible to guanidine-HCl treatment and independent of the presence of U(L)15 or U(L)28. Capsid association of pU(L)33 was also independent of U(L)6, which is believed to encode the portal into which DNA is inserted. These data suggest that pU(L)33 may act as part of the capsid-associated molecular machinery that translocates cleaved genomic DNA into the capsid interior.

Herpes simplex virus 1 U(L)31 and U(L)34 gene products promote the late maturation of viral replication compartments to the nuclear periphery

Herpes simplex virus 1 (HSV-1) forms replication compartments (RCs), domains in which viral DNA replication, late-gene transcription, and encapsidation take place, in the host cell nucleus. The formation of these domains leads to compression and marginalization of host cell chromatin, which forms a dense layer surrounding the viral RCs and constitutes a potential barrier to viral nuclear egress or primary envelopment at the inner nuclear membrane. Surrounding the chromatin layer is the nuclear lamina, a further host cell barrier to egress. In this study, we describe an additional phase in RC maturation that involves disruption of the host chromatin and nuclear lamina so that the RC can approach the nuclear envelope. During this phase, the structure of the chromatin layer is altered so that it no longer forms a continuous layer around the RCs but instead is fragmented, forming islands between which RCs extend to reach the nuclear periphery. Coincident with these changes, the nuclear lamina components lamin A/C and lamin-associated protein 2 appear to be redistributed via a mechanism involving the U(L)31 and U(L)34 gene products. Viruses in which the U(L)31 or U(L)34 gene has been deleted are unable to undergo this phase of chromatin reorganization and lamina alterations and instead form RCs which are bounded by an intact host cell chromatin layer and nuclear lamina. We postulate that these defects in chromatin restructuring and lamina reorganization explain the previously documented growth defects of these mutant viruses.

Quantification of the DNA cleavage and packaging proteins U(L)15 and U(L)28 in A and B capsids of herpes simplex virus type 1

The proteins produced by the herpes simplex virus type 1 (HSV-1) genes U(L)15 and U(L)28 are believed to form part of the terminase enzyme, a protein complex essential for the cleavage of newly synthesized, concatameric herpesvirus DNA and the packaging of the resultant genome lengths into preformed capsids. This work describes the purification of recombinant forms of pU(L)15 and pU(L)28, which allowed the calculation of the average number of copies of each protein in A and B capsids and in capsids lacking the putative portal encoded by U(L)6. On average, 1.0 (+/-0.29 [standard deviation]) copies of pU(L)15 and 2.4 (+/-0.97) copies of pU(L)28 were present in B capsids, 1.2 (+/-0.72) copies of pU(L)15 and 1.5 (+/-0.86) copies of pU(L)28 were found in mutant capsids lacking the putative portal protein pU(L)6, and approximately 12.0 (+/-5.63) copies of pU(L)15 and 0.6 (+/-0.32) copies of pU(L)28 were present in each A capsid. These results suggest that the packaging machine is partly comprised of approximately 12 copies of pU(L)15, as found in A capsids, with wild-type B and mutant U(L)6(-) capsids containing an incomplete complement of cleavage and packaging proteins. These results are consistent with observations that B capsids form by default in the absence of packaging machinery in vitro and in vivo. In contrast, A capsids may be the result of initiated but aborted attempts at DNA packaging, resulting in the retention of at least part of the DNA packaging machinery.

Point mutations in exon I of the herpes simplex virus putative terminase subunit, UL15, indicate that the most conserved residues are essential for cleavage and packaging

The herpes simplex virus UL15 and UL28 genes are believed to encode two subunits of the terminase involved in cleavage and packaging of viral genomes. Analysis of the UL15 protein sequence and its herpesvirus homologues revealed the presence of 20 conserved regions. Twelve of the twenty regions conserved among herpesviruses are also conserved in terminases from DNA bacteriophage. Point mutations in UL15 were designed in four conserved regions: L120N (CR1), Q205E (CR2), Q251E (CR3), G263A (CR3), and Y285S (CR4). Transfection experiments indicated that each mutant gene could produce stable UL15 protein at wild-type levels; however, only one mutant (Q251E) was able to complement the UL15-null virus. Each mutation was introduced into the viral genome by marker transfer, and all mutants except Q251E were unable to form plaques on Vero cells. Furthermore, failure to form plaques on Vero cells correlated with a defect in cleavage and packaging. Immunofluorescence experiments indicated that in cells infected with all mutant viruses the UL15 protein could be detected and was found to localize to replication compartments. Although wild-type and mutant Q251E were able to produce A, B, and C capsids, the rest of the mutants were only able to produce B capsids, a finding consistent with their defects in cleavage and packaging. In addition, all mutant UL15 proteins retained their ability to interact with B capsids. Therefore, amino acid residues 120, 205, 263, and 285 are essential for the cleavage and packaging process rather than for association with capsids or localization to replication compartments.

Implication of the product of the bovine herpesvirus type 1 UL25 gene in capsid assembly

The UL25 ORF of bovine herpesvirus type 1 (BHV-1) was demonstrated recently to represent a gene encoding a 63 kDa viral protein. To investigate the role of this gene in virus replication, a BHV-1 UL25 deletion mutant was constructed. Although the UL25 mutant synthesizes late viral proteins and viral DNA, it fails to produce virus progeny in cells that do not express the UL25 gene, demonstrating that the UL25 protein is essential for the replicative cycle of BHV-1. Moreover, Southern blotting analyses of HindIII-digested DNA from infected non-complementing cells probed with the leftward terminal fragment of the BHV-1 linear genome revealed that the cleavage of the viral DNA produced is not impaired. However, the packaging of this cleaved DNA is compromised severely, since only few full C capsids were observed in infected non-complementing cells by transmission electron microscopy.

Inhibition of herpes simplex virus replication by WAY-150138: assembly of capsids depleted of the portal and terminase proteins involved in DNA encapsidation

Studies were carried out to examine the mechanism of action of WAY-150138, a member of a novel group of thiourea compounds recently shown to inhibit replication of herpes simplex virus type 1 (HSV-1). Previous studies have shown that the drug acts by preventing DNA encapsidation and that resistant mutants map to U(L)6, the gene encoding the protein subunit of the portal complex through which DNA enters the capsid. We tested the idea that WAY-150138 acts by preventing the incorporation of DNA-packaging proteins into capsids as they are assembled. Capsids were isolated from HSV-1-infected, drug-treated cells and examined by Western immunoblotting for the presence of two packaging proteins, the portal subunit (U(L)6) and a candidate terminase subunit (U(L)15). The results showed that both proteins were depleted in the capsids, suggesting that WAY-150138 antagonizes DNA encapsidation by depriving capsids of packaging proteins during the assembly process.

Ultrastructural localization of the herpes simplex virus type 1 UL31, UL34, and US3 proteins suggests specific roles in primary envelopment and egress of nucleocapsids

The wild-type UL31, UL34, and US3 proteins localized on nuclear membranes and perinuclear virions; the US3 protein was also on cytoplasmic membranes and extranuclear virions. The UL31 and UL34 proteins were not detected in extracellular virions. US3 deletion caused (i) virion accumulation in nuclear membrane invaginations, (ii) delayed virus production onset, and (iii) reduced peak virus titers. These data support the herpes simplex virus type 1 deenvelopment-reenvelopment model of virion egress and suggest that the US3 protein plays an important, but nonessential, role in the egress pathway.

DNA cleavage and packaging proteins encoded by genes U(L)28, U(L)15, and U(L)33 of herpes simplex virus type 1 form a complex in infected cells

Previous studies have indicated that the U(L)6, U(L)15, U(L)17, U(L)28, U(L)32, and U(L)33 genes are required for the cleavage and packaging of herpes simplex viral DNA. To identify proteins that interact with the U(L)28-encoded DNA binding protein of herpes simplex virus type 1 (HSV-1), a previously undescribed rabbit polyclonal antibody directed against the U(L)28 protein fused to glutathione S-transferase was used to immunopurify U(L)28 and the proteins with which it associated. It was found that the antibody specifically coimmunoprecipitated proteins encoded by the genes U(L)28, U(L)15, and U(L)33 from lysates of both HEp-2 cells infected with HSV-1(F) and insect cells infected with recombinant baculoviruses expressing these three proteins. In reciprocal reactions, antibodies directed against the U(L)15- or U(L)33-encoded proteins also coimmunoprecipitated the U(L)28 protein. The coimmunoprecipitation of the three proteins from HSV-infected cells confirms earlier reports of an association between the U(L)28 and U(L)15 proteins and represents the first evidence of the involvement of the U(L)33 protein in this complex.

The UL6 gene product forms the portal for entry of DNA into the herpes simplex virus capsid

During replication of herpes simplex virus type 1 (HSV-1), viral DNA is synthesized in the infected cell nucleus, where DNA-free capsids are also assembled. Genome-length DNA molecules are then cut out of a larger, multigenome concatemer and packaged into capsids. Here we report the results of experiments carried out to test the idea that the HSV-1 UL6 gene product (pUL6) forms the portal through which viral DNA passes as it enters the capsid. Since DNA must enter at a unique site, immunoelectron microscopy experiments were undertaken to determine the location of pUL6. After specific immunogold staining of HSV-1 B capsids, pUL6 was found, by its attached gold label, at one of the 12 capsid vertices. Label was not observed at multiple vertices, at nonvertex sites, or in capsids lacking pUL6. In immunoblot experiments, the pUL6 copy number in purified B capsids was found to be 14.8 +/- 2.6. Biochemical experiments to isolate pUL6 were carried out, beginning with insect cells infected with a recombinant baculovirus expressing the UL6 gene. After purification, pUL6 was found in the form of rings, which were observed in electron micrographs to have outside and inside diameters of 16.4 +/- 1.1 and 5.0 +/- 0.7 nm, respectively, and a height of 19.5 +/- 1.9 nm. The particle weights of individual rings as determined by scanning transmission electron microscopy showed a majority population with a mass corresponding to an oligomeric state of 12. The results are interpreted to support the view that pUL6 forms the DNA entry portal, since it exists at a unique site in the capsid and forms a channel through which DNA can pass. The HSV-1 portal is the first identified in a virus infecting a eukaryote. In its dimensions and oligomeric state, the pUL6 portal resembles the connector or portal complexes employed for DNA encapsidation in double-stranded DNA bacteriophages such as phi29, T4, and P22. This similarity supports the proposed evolutionary relationship between herpesviruses and double-stranded DNA phages and suggests the basic mechanism of DNA packaging is conserved.

U(L)31 and U(L)34 proteins of herpes simplex virus type 1 form a complex that accumulates at the nuclear rim and is required for envelopment of nucleocapsids

The herpes simplex virus type 1 (HSV-1) U(L)34 protein is likely a type II membrane protein that localizes within the nuclear membrane and is required for efficient envelopment of progeny virions at the nuclear envelope, whereas the U(L)31 gene product of HSV-1 is a nuclear matrix-associated phosphoprotein previously shown to interact with U(L)34 protein in HSV-1-infected cell lysates. For these studies, polyclonal antisera directed against purified fusion proteins containing U(L)31 protein fused to glutathione-S-transferase (U(L)31-GST) and U(L)34 protein fused to GST (U(L)34-GST) were demonstrated to specifically recognize the U(L)31 and U(L)34 proteins of approximately 34,000 and 30,000 Da, respectively. The U(L)31 and U(L)34 gene products colocalized in a smooth pattern throughout the nuclear rim of infected cells by 10 h postinfection. U(L)34 protein also accumulated in pleiomorphic cytoplasmic structures at early times and associated with an altered nuclear envelope late in infection. Localization of U(L)31 protein at the nuclear rim required the presence of U(L)34 protein, inasmuch as cells infected with a U(L)34 null mutant virus contained U(L)31 protein primarily in central intranuclear domains separate from the nuclear rim, and to a lesser extent in the cytoplasm. Conversely, localization of U(L)34 protein exclusively at the nuclear rim required the presence of the U(L)31 gene product, inasmuch as U(L)34 protein was detectable at the nuclear rim, in replication compartments, and in the cytoplasm of cells infected with a U(L)31 null virus. When transiently expressed in the absence of other viral factors, U(L)31 protein localized diffusely in the nucleoplasm, whereas U(L)34 protein localized primarily in the cytoplasm and at the nuclear rim. In contrast, coexpression of the U(L)31 and U(L)34 proteins was sufficient to target both proteins exclusively to the nuclear rim. The proteins were also shown to directly interact in vitro in the absence of other viral proteins. In cells infected with a virus lacking the U(S)3-encoded protein kinase, previously shown to phosphorylate the U(L)34 gene product, U(L)31 and U(L)34 proteins colocalized in small punctate areas that accumulated on the nuclear rim. Thus, U(S)3 kinase is required for even distribution of U(L)31 and U(L)34 proteins throughout the nuclear rim. Taken together with the similar phenotypes of the U(L)31 and U(L)34 deletion mutants, these data strongly suggest that the U(L)31 and U(L)34 proteins form a complex that accumulates at the nuclear membrane and plays an important role in nucleocapsid envelopment at the inner nuclear membrane.

Microtubule reorganization during herpes simplex virus type 1 infection facilitates the nuclear localization of VP22, a major virion tegument protein

Full-length VP22 is necessary for efficient spread of herpes simplex virus type 1 (HSV-1) from cell to cell during the course of productive infection. VP22 is a virion phosphoprotein, and its nuclear localization initiates between 5 and 7 h postinfection (hpi) during the course of synchronized infection. The goal of this study was to determine which features of HSV-1 infection function to regulate the translocation of VP22 into the nucleus. We report the following. (i) HSV-1(F)-induced microtubule rearrangement occurred in infected Vero cells by 13 hpi and was characterized by the loss of obvious microtubule organizing centers (MtOCs). Reformed MtOCs were detected at 25 hpi. (ii) VP22 was observed in the cytoplasm of cells prior to microtubule rearrangement and localized in the nucleus following the process. (iii) Stabilization of microtubules by the addition of taxol increased the accumulation of VP22 in the cytoplasm either during infection or in cells expressing VP22 in the absence of other viral proteins. (iv) While VP22 localized to the nuclei of cells treated with the microtubule depolymerizing agent nocodazole, either taxol or nocodazole treatment prevented optimal HSV-1(F) replication in Vero cells. (v) VP22 migration to the nucleus occurred in the presence of phosphonoacetic acid, indicating that viral DNA and true late protein synthesis were not required for its translocation. Based on these results, we conclude that (iv) microtubule reorganization during HSV-1 infection facilitates the nuclear localization of VP22.

Structural organization and analysis of the viral terminase gene locus of Tupaia herpesvirus

Tupaia herpesvirus (THV) was isolated from spontaneously degenerating tissue cultures of malignant lymphoma, lung, and spleen cell cultures of tree shrews (Tupaia spp.). In order to determine the phylogenetic relatedness of THV the complete nucleotide sequence of the viral terminase (VTER) gene locus (6223 bp) of Tupaia herpesvirus strain 2 (THV-2) was elucidated and analysed. The VTER gene locus, encoding one of the most highly conserved herpes viral proteins is composed of two exons. The intron contains five potential open reading frames (ORFs). The arrangement of these ORFs is colinear with the corresponding regions in the genomes of the mammalian cytomegaloviruses. The precise primary structure of the THV-2 VTER splice junction was determined using RT-PCR and was found to be in agreement with the corresponding splice donor and acceptor sites of the mammalian cytomegaloviruses. The comparison of all six putative THV-2 proteins with the corresponding counterparts in other herpesviruses revealed that THV resides between the Human and the Murine cytomegalovirus (HCMV, MCMV). These results are in agreement with our previous statement, that THV and the known cytomegaloviruses are closely related to each other and should be classified into one taxonomic group. The genetic data presented here and in previous studies are based on the detailed comparison of highly conserved viral genes. Consequently, the classification of the Human and the cytomegaloviruses into the two genera Cyto- and Muromegalovirus, that is mainly based on overall genome structure, should be reconsidered.

Herpes simplex virus-induced keratitis: evaluation of the role of molecular mimicry in lesion pathogenesis

Viruses are suspected but usually unproven triggering factors in autoimmunity. One favored mechanism to explain the role of viruses in the genesis of autoimmunity is molecular mimicry. An immunoinflammatory blinding lesion called herpetic stromal keratitis (HSK) that follows ocular infection with herpes simplex virus (HSV) is suggested to result from a CD4(+) T-cell response to a UL6 peptide of HSV that cross-reacts with a corneal autopeptide shared with the immunoglobulin G2a(b) (IgG2a(b)) isotype. The present report reevaluates the molecular mimicry hypothesis to explain HSK pathogenesis. Our results failed to reveal cross-reactivity between the UL6 and IgG2a(b) peptides or between peptide reactive T cells and HSV antigens. More importantly, animals infected with HSV failed to develop responses that reacted with either peptide, and infection with a recombinant vaccinia UL6 vector failed to cause HSK, in spite of generating UL6 reactivity. Other lines of evidence also failed to support the molecular mimicry hypothesis, such as the failure to affect HSK severity upon tolerization of susceptible BALB/c and B-cell-deficient mice with IgG2a(b) or UL6 peptides. An additional study system revealed that HSK could be induced in mouse strains, such as the OT2 x RAG1(-/-) mice (T cell receptor transgenic recognizing OVA(323-339)) that were unable to produce CD4(+) T-cell responses to any detectable HSV antigens. Our results cast doubt on the molecular mimicry hypothesis as an explanation for the pathogenesis of HSK and indicate that if autoimmunity is involved its likely proceeds via a bystander activation mechanism.