Small dense nuclear bodies are the site of localization of herpes simplex virus 1 U(L)3 and U(L)4 proteins and of ICP22 only when the latter protein is present

"Non-Essential" Proteins of HSV-1 with Essential Roles In Vivo: A Comprehensive Review

Viruses encode for structural proteins that participate in virion formation and include capsid and envelope proteins. In addition, viruses encode for an array of non-structural accessory proteins important for replication, spread, and immune evasion in the host and are often linked to virus pathogenesis. Most virus accessory proteins are non-essential for growth in cell culture because of the simplicity of the infection barriers or because they have roles only during a state of the infection that does not exist in cell cultures (i.e., tissue-specific functions), or finally because host factors in cell culture can complement their absence. For these reasons, the study of most nonessential viral factors is more complex and requires development of suitable cell culture systems and in vivo models. Approximately half of the proteins encoded by the herpes simplex virus 1 (HSV-1) genome have been classified as non-essential. These proteins have essential roles in vivo in counteracting antiviral responses, facilitating the spread of the virus from the sites of initial infection to the peripheral nervous system, where it establishes lifelong reservoirs, virus pathogenesis, and other regulatory roles during infection. Understanding the functions of the non-essential proteins of herpesviruses is important to understand mechanisms of viral pathogenesis but also to harness properties of these viruses for therapeutic purposes. Here, we have provided a comprehensive summary of the functions of HSV-1 non-essential proteins.

The Herpes Simplex Virus 1 Immediate Early Protein ICP22 Is a Functional Mimic of a Cellular J Protein

Molecular chaperones and cochaperones are the most abundant cellular effectors of protein homeostasis, assisting protein folding and preventing aggregation of misfolded proteins. We have previously shown that herpes simplex virus 1 (HSV-1) infection results in the drastic spatial reorganization of the cellular chaperone Hsc70 into nuclear domains called VICE (Virus Induced Chaperone Enriched) domains and that this recruitment is dependent on the viral immediate early protein ICP22. Here, we present several lines of evidence supporting the notion that ICP22 functions as a virally encoded cochaperone (J-protein/Hsp40) functioning together with its Hsc70 partner to recognize and manage aggregated and misfolded proteins. We show that ICP22 results in (i) nuclear sequestration of nonnative proteins, (ii) reduction of cytoplasmic aggresomes in cells expressing aggregation-prone proteins, and (iii) thermoprotection against heat inactivation of firefly luciferase, and (iv) sequence homology analysis indicated that ICP22 contains an N-terminal J domain and a C-terminal substrate binding domain, similar to type II cellular J proteins. ICP22 may thus be functionally similar to J-protein/Hsp40 cochaperones that function together with their HSP70 partners to prevent aggregation of nonnative proteins. This is not the first example of a virus hijacking a function of a cellular chaperone, since simian immunodeficiency virus T antigen was previously shown to contain a J domain; however, this the first known example of the acquisition of a functional J-like protein by a virus and suggests that HSV has taken advantage of the adaptable nature of J proteins to evolve a multifunctional cochaperone that functions with Hsc70 to promote lytic infection.IMPORTANCE Viruses have evolved a variety of strategies to succeed in a hostile environment. The herpes simplex virus 1 (HSV-1) immediate early protein ICP22 plays several roles in the virus life cycle, including downregulation of cellular gene expression, upregulation of late viral gene expression, inhibition of apoptosis, prevention of aggregation of nonnative proteins, and the recruitment of a cellular heat shock protein, Hsc70, to nuclear domains. We present evidence that ICP22 functionally resembles a cellular J-protein/HSP40 family cochaperone, interacting specifically with Hsc70. We suggest that HSV has taken advantage of the adaptable nature of J proteins to evolve a multifunctional cochaperone that functions with Hsc70 to promote lytic infection.

Spatial and Temporal Resolution of Global Protein Synthesis during HSV Infection Using Bioorthogonal Precursors and Click Chemistry

We used pulse-labeling with the methionine analogue homopropargylglycine (HPG) to investigate spatiotemporal aspects of protein synthesis during herpes simplex virus (HSV) infection. In vivo incorporation of HPG enables subsequent selective coupling of fluorochrome-capture reagents to newly synthesised proteins. We demonstrate that HPG labeling had no effect on cell viability, on accumulation of test early or late viral proteins, or on overall virus yields. HPG pulse-labeling followed by SDS-PAGE analysis confirmed incorporation into newly synthesised proteins, while parallel processing by in situ cycloaddition revealed new insight into spatiotemporal aspects of protein localisation during infection. A striking feature was the rapid accumulation of newly synthesised proteins not only in a general nuclear pattern but additionally in newly forming sub-compartments represented by small discrete foci. These newly synthesised protein domains (NPDs) were similar in size and morphology to PML domains but were more numerous, and whereas PML domains were progressively disrupted, NPDs were progressively induced and persisted. Immediate-early proteins ICP4 and ICP0 were excluded from NPDs, but using an ICP0 mutant defective in PML disruption, we show a clear spatial relationship between NPDs and PML domains with NPDs frequently forming immediately adjacent and co-joining persisting PML domains. Further analysis of location of the chaperone Hsc70 demonstrated that while NPDs formed early in infection without overt Hsc70 recruitment, later in infection Hsc70 showed pronounced recruitment frequently in a coat-like fashion around NPDs. Moreover, while ICP4 and ICP0 were excluded from NPDs, ICP22 showed selective recruitment. Our data indicate that NPDs represent early recruitment of host and viral de novo translated protein to distinct structural entities which are precursors to the previously described VICE domains involved in protein quality control in the nucleus, and reveal new features from which we propose spatially linked platforms of newly synthesised protein processing after nuclear import.

The first genome sequence of a metatherian herpesvirus: Macropodid herpesvirus 1

Background

While many placental herpesvirus genomes have been fully sequenced, the complete genome of a marsupial herpesvirus has not been described. Here we present the first genome sequence of a metatherian herpesvirus, Macropodid herpesvirus 1 (MaHV-1).

Results

The MaHV-1 viral genome was sequenced using an Illumina MiSeq sequencer, de novo assembly was performed and the genome was annotated. The MaHV-1 genome was 140 kbp in length and clustered phylogenetically with the primate simplexviruses, sharing 67 % nucleotide sequence identity with Human herpesviruses 1 and 2. The MaHV-1 genome contained 66 predicted open reading frames (ORFs) homologous to those in other herpesvirus genomes, but lacked homologues of UL3, UL4, UL56 and glycoprotein J. This is the first alphaherpesvirus genome that has been found to lack the UL3 and UL4 homologues. We identified six novel ORFs and confirmed their transcription by RT-PCR.

Conclusions

This is the first genome sequence of a herpesvirus that infects metatherians, a taxonomically unique mammalian clade. Members of the Simplexvirus genus are remarkably conserved, so the absence of ORFs otherwise retained in eutherian and avian alphaherpesviruses contributes to our understanding of the Alphaherpesvirinae. Further study of metatherian herpesvirus genetics and pathogenesis provides a unique approach to understanding herpesvirus-mammalian interactions.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-2390-2) contains supplementary material, which is available to authorized users.

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.

HSV-1 ICP22: hijacking host nuclear functions to enhance viral infection

During its productive infection, HSV-1 dramatically remodels the architecture and physiology of the host cell nucleus. The immediate-early proteins, the first viral proteins to be expressed during infection, are key players in this process. Here, we review the known properties and functions of immediate-early protein ICP22. Although this polypeptide has received less attention than other immediate-early proteins, the published evidence indicates that it mediates several striking changes to important host nuclear systems, including those involved in RNA polymerase II transcription, cell cycle regulation and protein quality control. Recent genetic analyses suggest that these alterations can promote HSV-1 productive infection. Thus, future work on ICP22 is likely to reveal novel mechanisms by which herpesviruses, and possibly other DNA viruses, manipulate the host cell nucleus to enhance their replication.

Herpes simplex virus 1 ICP22 inhibits the transcription of viral gene promoters by binding to and blocking the recruitment of P-TEFb

ICP22 is a multifunctional herpes simplex virus 1 (HSV-1) immediate early protein that functions as a general repressor of a subset of cellular and viral promoters in transient expression systems. Although the exact mechanism of repression remains unclear, this protein induces a decrease in RNA polymerase II Serine 2 (RNAPII Ser-2) phosphorylation, which is critical for transcription elongation. To characterize the mechanism of transcriptional repression by ICP22, we established an in vivo transient expression reporter system. We found that ICP22 inhibits transcription of the HSV-1 α, β and γ gene promoters. The viral tegument protein VP16, which plays vital roles in initiation of viral gene expression and viral proliferation, can overcome the inhibitory effect of ICP22 on α-gene transcription. Further immunoprecipitation studies indicated that both ICP22 and VP16 bind to positive transcription elongation factor b (P-TEFb) and form a complex with it in vivo. We extended this to show that P-TEFb regulates transcription of the viral α-gene promoters and affects transcriptional regulation of ICP22 and VP16 on the α-genes. Additionally, ChIP assays demonstrated that ICP22 blocks the recruitment of P-TEFb to the viral promoters, while VP16 reverses this blocking effect by recruiting P-TEFb to the viral α-gene promoters through recognition of the TAATGARAT motif. Taken together, our results suggest that ICP22 interacts with and blocks the recruitment of P-TEFb to viral promoter regions, which inhibits transcription of the viral gene promoters. The transactivator VP16 binds to and induces the recruitment of P-TEFb to viral α-gene promoters, which counteracts the transcriptional repression of ICP22 on α-genes by recruiting p-TEFb to the promoter region.

A novel virus-encoded nucleocytoplasmic shuttling protein: the UL3 protein of herpes simplex virus type 1

Herpes simplex virus type 1 (HSV-1) UL3 protein is a nuclear protein. In this study, the molecular mechanism of the subcellular localization of UL3 was characterized by fluorescence microscopy in living cells. A nuclear localization signal (NLS) and a nuclear export signal (NES) were also identified. UL3 was demonstrated to target to the cytoplasm through the NES via chromosomal region maintenance 1 (CRM-1) dependent pathway, and to the nucleus through RanGTP-dependent mechanism. Heterokaryon assays confirmed that UL3 was capable of shuttling between the nucleus and the cytoplasm. These results demonstrate that the UL3 protein is a novel HSV-1 encoded nucleocytoplasmic shuttling protein.

Molecular determinants responsible for the subcellular localization of HSV-1 UL4 protein

The function of the herpes simplex virus type 1 (HSV-1) UL4 protein is still elusive. Our objective is to investigate the subcellular transport mechanism of the UL4 protein. In this study, fluorescence microscopy was employed to investigate the subcellular localization of UL4 and characterize the transport mechanism in living cells. By constructing a series of deletion mutants fused with enhanced yellow fluorescent protein (EYFP), the nuclear export signals (NES) of UL4 were for the first time mapped to amino acid residues 178 to 186. In addition, the N-terminal 19 amino acids are identified to be required for the granule-like cytoplasmic pattern of UL4. Furthermore, the UL4 protein was demonstrated to be exported to the cytoplasm through the NES in a chromosomal region maintenance 1 (CRM1)-dependent manner involving RanGTP hydrolysis.

Sequence variation in the herpes simplex virus U(S)1 ocular virulence determinant

PURPOSE

The herpes simplex virus type 1 (HSV-1) U(S)1 gene encodes host-range and ocular virulence determinants. Mutations in U(S)1 affecting virulence are known in strain OD4, but the genomic variation across several strains is not known. The goal was to determine the degree of sequence variation in the gene from several ocular HSV isolates.

METHODS

The U(S)1 gene from six ocular HSV-1 isolates, as well as strains KOS and F, were sequenced, and bioinformatics analyses were applied to the data.

RESULTS

Strains 17, F, CJ394, and CJ311 had identical amino acid sequences. With the other strains, most of the variability was concentrated in the amino-terminal third of the protein. MEME analysis identified a 63-residue core sequence (motif 1) present in all α-herpesvirus U(S)1 homologs that were located in a region identified as structured. Ten amino acids were absolutely conserved in all the α-herpesvirus U(S)1 homologs and were all located in the central core. Consensus-binding motifs for cyclin-dependent kinases and pocket proteins were also identified.

CONCLUSIONS

These results suggest that significant sequence variation exists in the U(S)1 gene, that the α22 protein contains a conserved central core region with structurally variable regions at the amino- and carboxyl termini, that 10 amino acids are conserved in α-herpes U(S)1 homologs, and that additional host proteins may interact with the HSV-1 U(S)1 and U(S)1.5 proteins. This information will be valuable in designing further studies on structure-function relationships and on the role these play in host-range determination and keratitis.

Comprehensive characterization of interaction complexes of herpes simplex virus type 1 ICP22, UL3, UL4, and UL20.5

It has been reported that herpes simplex virus type 1 UL3, UL4, and UL20.5 proteins are localized to small, dense nuclear bodies together with ICP22 in infected cells. In the present study, we comprehensively characterized these interactions by subcellular colocalization, coimmunoprecipitation, and bimolecular fluorescence complementation assays. For the first time, it was demonstrated that both UL3 and UL20.5 are targeted to small, dense nuclear bodies by a direct interaction with ICP22, whereas UL4 colocalizes with ICP22 through its interaction with UL3 but not UL20.5 or ICP22. There was no detectable interaction between UL3 and UL20.5.

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.

Expression, purification of the UL3 protein of herpes simplex virus type 1, and production of UL3 polyclonal antibody

Herpes simplex virus type 1 (HSV-1) is a common pathogen which causes infections of the mucocutaneous membranes. The UL3 protein belongs to a group of HSV-1 late proteins. To date, the function of the UL3 protein in cell culture, animal models, and natural infection is unknown. To investigate further the function of the UL3 protein, this study was undertaken to express the UL3 protein and raise a polyclonal antibody. The UL3 gene was cloned in the prokaryotic expression vector pET-28a (+) to yield pET-28a (+)-UL3. The His6-tagged UL3 protein was expressed in Escherichia coli (E. coli) BL21 (DE3) cells and analyzed by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE). After purification by nickel affinity chromatography and refolding, the recombinant protein was used to raise the anti-UL3 polyclonal antibody. Western blot analysis demonstrated that the UL3 protein was recognized by the polyclonal antibody, and immunofluorescent assay also showed that the antibody was able to recognize the UL3 protein in the cells infected with HSV-1.

The herpes simplex virus type 1 infected cell protein 22

As one of the immediate-early (IE) proteins of herpes simplex virus type 1 (HSV-1), ICP22 is a multifunctional viral regulator that localizes in the nucleus of infected cells. It is required in experimental animal systems and some nonhuman cell lines, but not in Vero or HEp-2 cells. ICP22 is extensively phosphorylated by viral and cellular kinases and nucleotidylylated by casein kinase II. It has been shown to be required for efficient expression of early (E) genes and a subset of late (L) genes. ICP22, in conjunction with the UL13 kinase, mediates the phosphorylation of RNA polymerase II. Both ICP22 and UL13 are required for the activation of cdc2, the degradation of cyclins A and B and the acquisition of a new cdc2 partner, the UL42 DNA polymerase processivity factor. The cdc2-UL42 complex mediates postranscriptional modification of topoisomerase IIα in an ICP22-dependent manner to promote L gene expression. In addition, ICP22 interacts with cdk9 in a Us3 kinase dependent fashion to phosphorylate RNA polymerase II.

Herpes simplex virus type 1 immediate-early protein ICP22 is required for VICE domain formation during productive viral infection

During productive infection, herpes simplex virus type 1 (HSV-1) induces the formation of discrete nuclear foci containing cellular chaperone proteins, proteasomal components, and ubiquitinated proteins. These structures are known as VICE domains and are hypothesized to play an important role in protein turnover and nuclear remodeling in HSV-1-infected cells. Here we show that VICE domain formation in Vero and other cells requires the HSV-1 immediate-early protein ICP22. Since ICP22 null mutants replicate efficiently in Vero cells despite being unable to induce VICE domain formation, it can be concluded that VICE domain formation is not essential for HSV-1 productive infection. However, our findings do not exclude the possibility that VICE domain formation is required for viral replication in cells that are nonpermissive for ICP22 mutants. Our studies also show that ICP22 itself localizes to VICE domains, suggesting that it could play a role in forming these structures. Consistent with this, we found that ICP22 expression in transfected cells is sufficient to reorganize the VICE domain component Hsc70 into nuclear inclusion bodies that resemble VICE domains. An N-terminal segment of ICP22, corresponding to residues 1 to 146, is critical for VICE domain formation in infected cells and Hsc70 reorganization in transfected cells. We previously found that this portion of the protein is dispensable for ICP22's effects on RNA polymerase II phosphorylation. Thus, ICP22 mediates two distinct regulatory activities that both modify important components of the host cell nucleus.

Expression, purification of herpes simplex virus type 1 UL4 protein, and production and characterization of UL4 polyclonal antibody

The herpes simplex virus type 1 (HSV-1) UL4 protein is a late protein encoded by the UL4 gene. To date, the function of this protein is poorly understood. To aid further investigation of the function of this protein, the UL4 gene was cloned into the vector pET28a (+) to express His-tagged UL4 protein in Escherichia coli. The recombinant fusion protein was purified from inclusion body by histidine selected nickel affinity chromatography under denaturing conditions. After refolding, the purified recombinant protein was used to produce anti-UL4 polyclonal antibody. Western blot analysis demonstrated that the polyclonal sera could recognize the purified UL4 protein specifically, and in the immunofluorescence assay, the antibody was able to probe the UL4 protein with a punctate staining in HSV-1 infected cells.

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.

The herpes simplex virus type 1 UL3 transcript starts within the UL3 open reading frame and encodes a 224-amino-acid protein

Several different herpes simplex viruses (HSVs) and vectors are being explored as therapeutic products for use in the treatment of cancer and neurological disorders. The viral strain and the combination of mutant viral genes that ultimately may serve as a safe and optimal backbone for such products are still being explored. The large genome size and complexity of the viral life cycle make such determinations difficult, because the significance of differences between proposed products is difficult to evaluate. For example, we previously reported that two lineages of gamma34.5-deleted HSVs used in clinical studies differ from each other in the size of the UL3 protein expressed (M. J. Dambach et al., Mol. Ther. 13:891-898, 2006). Because the function of UL3 is not known and UL3 gene expression is poorly understood, the significance of such a difference cannot be predicted. Here, I begin to address the function of UL3 by investigating UL3 gene expression. I report that the transcript start site of UL3 mRNA isolated from HSV type 1 (HSV-1)-infected cells maps to a position downstream of the predicted translation start site. By constructing and characterizing the recombinant virus CB8116, which has a mutation in the first in-frame start codon of this UL3 transcript, I demonstrated that UL3 protein translation initiates at the second in-frame start codon of the UL3 open reading frame. This information adds to the body of basic knowledge of HSV-1 biology that forms the foundation for our current understanding of HSV-based products. Future research on HSV-1 biology will facilitate the rational design and evaluation of future generations of therapeutic viruses.

Oncolytic viruses derived from the gamma34.5-deleted herpes simplex virus recombinant R3616 encode a truncated UL3 protein

Replication-competent herpes simplex virus (HSV-1) mutants are used in clinical trials in the experimental treatment of cancer. Mutants G207, HSV1716, NV1020, and Oncovex GM-CSF share in common a defect in one or both copies of the gene encoding the neurovirulence factor, ICP34.5, and are thus neuroattenuated. These viruses are acknowledged to differ from one another (a) in the specific types of mutations intentionally introduced during their derivation and (b) in the inherent genetic differences retained from the different parent strains used in their construction. Unintended mutations are expected to emerge at some low frequency during the selection for and passage of mutant viruses. Here we demonstrate that during the construction of the oncolytic virus R3616, a nonsense mutation arose in an untargeted region of the HSV-1 genome that resulted in a substantial truncation of the viral protein known as UL3. This report is the first published documentation that oncolytic herpesviruses developed and used in clinical trials contain adventitious mutations. The implications of these findings for the characterization and development of vectors proposed for use in clinical trials are discussed.

Quantitative comparison of the HSV-1 and HSV-2 transcriptomes using DNA microarray analysis

The genomes of human herpes virus type-1 and type-2 share a high degree of sequence identity; yet, they exhibit important differences in pathology in their natural human host as well as in animal host and cell cultures. Here, we report the comparative analysis of the time and relative abundance profiles of the transcription of each virus type (their transcriptomes) using parallel infections and microarray analysis using HSV-1 probes which hybridize with high efficiency to orthologous HSV-2 transcripts. We have confirmed that orthologous transcripts belong to the same kinetic class; however, the temporal pattern of accumulation of 4 transcripts (U(L)4, U(L)29, U(L)30, and U(L)31) differs in infections between the two virus types. Interestingly, the protein products of these transcripts are all involved in nuclear organization and viral DNA localization. We discuss the relevance of these findings and whether they may have potential roles in the pathological differences of HSV-1 and HSV-2.

The immediate-early 63 protein of Varicella-Zoster virus: analysis of functional domains required for replication in vitro and for T-cell and skin tropism in the SCIDhu model in vivo

The immediate-early 63-kDa (IE63) protein of varicella-zoster virus (VZV) is a phosphoprotein encoded by open reading frame (ORF) ORF63/ORF70. To identify functional domains, 22 ORF63 mutations were evaluated for effects on IE63 binding to the major VZV transactivator, IE62, and on IE63 phosphorylation and nuclear localization in transient transfections, and after insertion into the viral genome with VZV cosmids. The IE62 binding site was mapped to IE63 amino acids 55 to 67, with R59/L60 being critical residues. Alanine substitutions within the IE63 center region showed that S165, S173, and S185 were phosphorylated by cellular kinases. Four mutations that changed two putative nuclear localization signal (NLS) sequences altered IE63 distribution to a cytoplasmic/nuclear pattern. Only three of 22 mutations in ORF63 were compatible with recovery of infectious VZV from our cosmids, but infectivity was restored by inserting intact ORF63 into each mutated cosmid. The viable IE63 mutants had a single alanine substitution, altering T171, S181, or S185. These mutants, rOKA/ORF63rev[T171], rOKA/ORF63rev[S181], and rOKA/ORF63rev[S185], produced less infectious virus and had a decreased plaque phenotype in vitro. ORF47 kinase protein and glycoprotein E (gE) synthesis was reduced, indicating that IE63 contributed to optimal expression of early and late gene products. The three IE63 mutants replicated in skin xenografts in the SCIDhu mouse model, but virulence was markedly attenuated. In contrast, infectivity in T-cell xenografts was not altered. Comparative analysis suggested that IE63 resembled the herpes simplex virus type 1 U(S)1.5 protein, which is expressed colinearly with ICP22 (U(S)1). In summary, most mutations of ORF63 made with our VZV cosmid system were lethal for infectivity. The few IE63 changes that were tolerated resulted in VZV mutants with an impaired capacity to replicate in vitro. However, the IE63 mutants were attenuated in skin but not T cells in vivo, indicating that the contribution of the IE63 tegument/regulatory protein to VZV pathogenesis depends upon the differentiated human cell type which is targeted for infection within the intact tissue microenvironment.

Pseudorabies virus UL3 gene codes for a nuclear protein which is dispensable for viral replication

Many of the products of the ca. 80 genes encoded by alphaherpesviruses have already been identified and, at least tentatively, functionally characterized. Among the least characterized proteins are the products of the genes homologous to herpes simplex virus UL3, which are present only in the subfamily Alphaherpesvirinae: To identify the UL3 protein of the porcine alphaherpesvirus pseudorabies virus (PrV), the complete PrV UL3 open reading frame was cloned, expressed in Escherichia coli as a glutathione S-transferase fusion protein, and used for immunization of a rabbit. In Western blots, the generated antiserum specifically detected a 34-kDa protein in PrV-infected cells, which was absent from purified virus preparations, indicating that PrV UL3 encodes a nonstructural protein. In indirect immunofluorescence analysis, the anti-UL3 serum produced predominantly nuclear staining in transfected as well as in infected cells, which was not altered in the absence of other virus-encoded nuclear proteins such as the UL31 and UL34 gene products. To investigate UL3 function, a deletion mutant, PrV-DeltaUL3F2, was constructed and characterized. This mutant replicated and formed plaques on noncomplementing cells indistinguishable from wild-type PrV, demonstrating that PrV UL3 is not required for virus propagation in cultured cells. Moreover, ultrastructural examinations revealed no impairment of capsid formation in the nucleus, nuclear egress of capsids, virion maturation in the cytoplasm, or virus release. Thus, the overall properties of PrV UL3 are similar to those described for the homologous herpes simplex virus proteins which may be indicative of a common, yet hitherto unknown, function in alphaherpesvirus replication. However, based on our studies, an involvement of the UL3 homologs in virion formation appears unlikely.

Sequential localization of two herpes simplex virus tegument proteins to punctate nuclear dots adjacent to ICP0 domains

The subcellular localization of herpes simplex virus tegument proteins during infection is varied and complex. By using viruses expressing tegument proteins tagged with fluorescent proteins, we previously demonstrated that the major tegument protein VP22 exhibits a cytoplasmic localization, whereas the major tegument protein VP13/14 localizes to nuclear replication compartments and punctate domains. Here, we demonstrate the presence of a second minor population of VP22 in nuclear dots similar in appearance to those formed by VP13/14. We have constructed the first-described doubly fluorescence-tagged virus expressing VP22 and VP13/14 as fusion proteins with cyan fluorescent protein and yellow fluorescent protein, respectively. Visualization of both proteins within the same live infected cells has indicated that these two tegument proteins localize to the same nuclear dots but that VP22 appears there earlier than VP13/14. Further studies have shown that these tegument-specific dots are detectable as phase-dense bodies as early as 2 h after infection and that they are different from the previously described nuclear domains that contain capsid proteins. They are also different from the ICP0 domains formed at cellular nuclear domain 10 sites early in infection but, in almost all cases, are located in juxtaposition to these ICP0 domains. Hence, these tegument proteins join a growing number of proteins that are targeted to discrete nuclear domains in the herpesvirus-infected cell nucleus.

Identification of two nuclear import signals in the alpha-gene product ICP22 of herpes simplex virus 1

The herpes simplex virus 1 (HSV-1) infected cell protein 22 (ICP22) is a multifunctional viral regulator that localizes in the nucleus of infected cells. ICP22 is required for optimal virus replication in certain cell types and is subject to extensive posttranslational modification. To map the signals in ICP22 which mediate its efficient nuclear localization, we investigated the nuclear import of fusion proteins comprising various fragments of ICP22 fused to green fluorescent protein (GFP) or beta-galactosidase (beta-Gal). These data demonstrated that ICP22 contains two independent regions with nuclear localization signal (NLS) activity. NLS1 maps to ICP22 amino acid position 16-31 and closely resembles the classical bipartite NLS of the type originally identified in nucleoplasmin. In contrast, NLS2 maps to ICP22 amino acid position 118-131 and contains multiple critical basic residues. Furthermore, fusion of both NLSs to chimeric glutathione-S-transferase (GST)-GFP protein and subsequent cytoplasmic microinjection of the respective transport substrates allowed us to monitor nuclear import in real-time. These data demonstrated that both ICP22-derived NLSs mediated efficient nuclear import with identical kinetics, resulting in complete nuclear accumulation of the chimeric transport cargoes at approximately 30 min postinjection. Finally, our data provide new insights into the domain structure of the multifunctional alpha-gene product ICP22 of HSV-1.

Identification of a novel expressed open reading frame situated between genes U(L)20 and U(L)21 of the herpes simplex virus 1 genome

An open reading frame (ORF) situated between the U(L)20 and U(L)21 genes encodes a protein designated as U(L)20.5. The U(L)20.5 ORF lies 5' and in the same orientation as the U(L)20 ORF. The expression of the U(L)20.5 ORF was verified by RNase protection assays and by in-frame insertion of an amino acid sequence encoding an epitope of an available monoclonal antibody. The tagged U(L)20.5 protein colocalized in small dense nuclear structures with products of the alpha22/U(S)1.5, U(L)3, and U(L)4 genes. Expression of the U(L)20.5 gene was blocked in cells infected and maintained in the presence of phosphonoacetate, indicating that it belongs to the late, or gamma(2), kinetic class. U(L)20.5 is not essential for viral replication inasmuch as a recombinant virus made by insertion of the thymidine kinase gene into the U(L)20.5 ORF replicates in all cell lines tested [J. D. Baines, P. L. Ward, G. Campadelli-Fiume, and B. Roizman (1991) J. Virol. 65, 6414-6424]. The genomic location of the recently discovered genes illustrates the compact nature of the viral genome.