Evaluation of colocalization interactions between the IE110, IE175, and IE63 transactivator proteins of herpes simplex virus within subcellular punctate structures

PML Body Component Sp100A Restricts Wild-Type Herpes Simplex Virus 1 Infection

Sp100 (speckled protein 100 kDa) is a constituent component of nuclear structure PML (promyelocytic leukemia) bodies, playing important roles in mediating intrinsic and innate immunity. The Sp100 gene encodes four isoforms with distinct roles in the transcriptional regulation of both cellular and viral genes. Since Sp100 is a primary intranuclear target of infected-cell protein 0 (ICP0), an immediate early E3 ligase encoded by herpes simplex virus 1 (HSV-1), previous investigations attempting to analyze the functions of individual Sp100 variants during HSV-1 infection mostly avoided using a wild-type virus. Therefore, the role of Sp100 under natural infection by HSV-1 remains to be clarified. Here, we reappraised the antiviral capacity of four Sp100 isoforms during infection by a nonmutated HSV-1, examined the molecular behavior of the Sp100 protein in detail, and revealed the following intriguing observations. First, Sp100 isoform A (Sp100A) inhibited wild-type HSV-1 propagation in HEp-2, Sp100-/-, and PML-/- cells. Second, endogenous Sp100 is located in both the nucleus and the cytoplasm. During HSV-1 infection, the nuclear Sp100 level decreased drastically upon the detection of ICP0 in the same subcellular compartment, but cytosolic Sp100 remained stable. Third, transfected Sp100A showed subcellular localizations similar to those of endogenous Sp100 and matched the protein size of endogenous cytosolic Sp100. Fourth, HSV-1 infection induced increased secretion of endogenous Sp100 and ectopically expressed Sp100A, which copurified with extracellular vesicles (EVs) but not infectious virions. Fifth, the Sp100A level in secreting cells positively correlated with its level in EVs, and EV-associated Sp100A restricted HSV-1 in recipient cells. IMPORTANCE Previous studies show that the PML body component Sp100 protein is immediately targeted by ICP0 of HSV-1 in the nucleus during productive infection. Therefore, extensive studies investigating the interplay of Sp100 isoforms with HSV-1 were conducted using a mutant virus lacking ICP0 or in the absence of infection. The role of Sp100 variants during natural HSV-1 infection remains blurry. Here, we report that Sp100A potently and independently inhibited wild-type HSV-1 and that during HSV-1 infection, cytosolic Sp100 remained stable and was increasingly secreted into the extracellular space, in association with EVs. Furthermore, the Sp100A level in secreting cells positively correlated with its level in EVs and the anti-HSV-1 potency of these EVs in recipient cells. In summary, this study implies an active antiviral role of Sp100A during wild-type HSV-1 infection and reveals a novel mechanism of Sp100A to restrict HSV-1 through extracellular communications.

ICP0 dismantles microtubule networks in herpes simplex virus-infected cells

Infected-cell protein 0 (ICP0) is a RING finger E3 ligase that regulates herpes simplex virus (HSV) mRNA synthesis, and strongly influences the balance between latency and replication of HSV. For 25 years, the nuclear functions of ICP0 have been the subject of intense scrutiny. To obtain new clues about ICP0's mechanism of action, we constructed HSV-1 viruses that expressed GFP-tagged ICP0. To our surprise, both GFP-tagged and wild-type ICP0 were predominantly observed in the cytoplasm of HSV-infected cells. Although ICP0 is exclusively nuclear during the immediate-early phase of HSV infection, further analysis revealed that ICP0 translocated to the cytoplasm during the early phase where it triggered a previously unrecognized process; ICP0 dismantled the microtubule network of the host cell. A RING finger mutant of ICP0 efficiently bundled microtubules, but failed to disperse microtubule bundles. Synthesis of ICP0 proved to be necessary and sufficient to disrupt microtubule networks in HSV-infected and transfected cells. Plant and animal viruses encode many proteins that reorganize microtubules. However, this is the first report of a viral E3 ligase that regulates microtubule stability. Intriguingly, several cellular E3 ligases orchestrate microtubule disassembly and reassembly during mitosis. Our results suggest that ICP0 serves a dual role in the HSV life cycle, acting first as a nuclear regulator of viral mRNA synthesis and acting later, in the cytoplasm, to dismantle the host cell's microtubule network in preparation for virion synthesis and/or egress.

ICP0 antagonizes ICP4-dependent silencing of the herpes simplex virus ICP0 gene

ICP0 is a regulatory protein that plays a critical role in the replication-latency balance of herpes simplex virus (HSV). Absence of ICP0 renders HSV prone to establish quiescent infections, and thus cellular repressor(s) are believed to silence HSV mRNA synthesis when ICP0 fails to accumulate. To date, an ICP0-antagonized repressor has not been identified that restricts HSV mRNA synthesis by more than 2-fold. We report the unexpected discovery that HSV's major transcriptional regulator, ICP4, meets the criteria of a bona fide ICP0-antagonized repressor of viral mRNA synthesis. Our study began when we noted a repressive activity that restricted ICP0 mRNA synthesis by up to 30-fold in the absence of ICP0. When ICP0 accumulated, the repressor only restricted ICP0 mRNA synthesis by 3-fold. ICP4 proved to be necessary and sufficient to repress ICP0 mRNA synthesis, and did so in an ICP4-binding-site-dependent manner. ICP4 co-immunoprecipitated with FLAG-tagged ICP0; thus, a physical interaction likely explains how ICP0 antagonizes ICP4's capacity to silence the ICP0 gene. These findings suggest that ICP0 mRNA synthesis is differentially regulated in HSV-infected cells by the virus-encoded repressor activity embedded in ICP4, and a virus-encoded antirepressor, ICP0. Bacteriophage λ relies on a similar repression-antirepression regulatory scheme to “decide” whether a given infection will be productive or silent. Therefore, our findings appear to add to the growing list of inexplicable similarities that point to a common evolutionary ancestry between the herpesviruses and tailed bacteriophage.

Mutagenic analysis of herpes simplex virus type 1 glycoprotein L reveals the importance of an arginine-rich region for function

Herpes simplex virus type 1 (HSV-1) glycoproteins H and L (gH and gL) are required for virus-induced membrane fusion. Expression of gH at the virion or infected cell surface is mediated by the chaperone-like activity of gL. We have previously shown that a region between amino acids 155 and 161 is critical for gL chaperone-like activity. Here, we conducted Ala substitution mutagenesis of residues in this region and found that substitution of Cys160, Arg156, Arg158, or Arg156/158/159 with Ala resulted in a gL mutant that bound gH but displayed a reduced ability in gH trafficking and membrane fusion. Substitution of Arg156 with another positively charged amino acid, Lys, restored function. Substitution of Arg158 with Lys restored function in gH trafficking and cell fusion but not virus entry. These results indicate that an arginine-rich region of gL is critical for function.

Translocation and colocalization of ICP4 and ICP0 in cells infected with herpes simplex virus 1 mutants lacking glycoprotein E, glycoprotein I, or the virion host shutoff product of the UL41 gene

In wild-type herpes simplex virus 1-infected cells, the major regulatory protein ICP4 resides in the nucleus whereas ICP0 becomes dynamically associated with proteasomes and late in infection is translocated and dispersed in the cytoplasm. Inhibition of proteasomal function results in retention or transport of ICP0 to the nucleus. We report that in cells infected with mutants lacking glycoprotein E (gE), glycoprotein I (gI), or the product of the U(L)41 gene, both ICP4 and ICP0 are translocated to the cytoplasm and coaggregate in small dense structures that, in the presence of proteasomal inhibitor MG132, also contain proteasomal components. Gold particle-conjugated antibody to ICP0 reacted in thin sections with dense protein aggregates in the cytoplasm of mutant virus-infected cells. Similar aggregates were present in the nuclei but not in the cytoplasm of wild-type virus-infected cells. Exposure of cells early in infection to MG132 does not result in retention of ICP0 as in wild-type virus-infected cells. The results suggest that the retention of ICP4 and ICP0 in the nucleus is a dynamic process that involves the function of other viral proteins that may include the Fc receptor formed by the gE/gI complex and is not merely the consequence of expression of a nuclear localization signal. It is noteworthy that in DeltaU(L)41-infected cells gE is retained in the trans-Golgi network and is not widely dispersed in cellular membranes.

Promoter- and cell-specific transcriptional transactivation by the Kaposi's sarcoma-associated herpesvirus ORF57/Mta protein

The Kaposi's sarcoma-associated herpesvirus (KSHV) Mta protein, encoded by open reading frame 57, is a transactivator of gene expression that is essential for productive viral replication. Previous studies have suggested both transcriptional and posttranscriptional roles for Mta, but little is known regarding Mta's transcriptional function. In this study, we demonstrate that Mta cooperates with the KSHV lytic switch protein, Rta, to reactivate KSHV from latency, but Mta has little effect on reactivation when expressed alone. We demonstrate that the Mta and Rta proteins are expressed with similar but distinct kinetics during KSHV reactivation. In single-cell analyses, Mta expression coincides tightly with progression to full viral reactivation. We demonstrate with promoter reporter assays that while Rta activates transcription in all cell lines tested, Mta's ability to transactivate promoters, either alone or synergistically with Rta, is cell and promoter specific. In particular, Mta robustly transactivates the nut-1/PAN promoter independently of Rta in 293 and Akata-31 cells. Using nuclear run-on assays, we demonstrate that Mta stimulates transcriptional initiation in 293 cells. Rta and Mta physically interact in infected cell extracts, and this interaction requires the intact leucine repeat and central region of Rta in vitro. We demonstrate that Mta also binds to the nut-1/PAN promoter DNA in vitro and in infected cells. An Mta mutant with a lesion in a putative A/T hook domain is altered in DNA binding and debilitated in transactivation. We propose that one molecular mechanism of Mta-mediated transactivation is a direct effect on transcription by direct and indirect promoter association.

Components of the REST/CoREST/histone deacetylase repressor complex are disrupted, modified, and translocated in HSV-1-infected cells

The infected cell protein (ICP)0 enables gene expression and the replication of herpes simplex virus (HSV)-1 in cells infected at low multiplicities and enhances the expression of genes introduced into cells by transfection or infection. We report that a short sequence of ICP0 is similar to a sequence in the amino terminus of CoREST, a corepressor that exists in complexes with the repressor REST and histone deacetylases (HDACs) 1 or 2 to repress cellular gene expression. In wild-type-virus-infected cells, HDAC1 dissociates from the CoREST/REST complex, CoREST and HDAC1 are phosphorylated by a process mediated by viral protein kinases, and CoREST and HDAC1 are partially translocated to the cytoplasm. In cells infected with a virus mutant (DeltaICP4), in which ICP0 accumulates, but post-alpha gene expression is blocked, HDAC1 is dissociated from the CoREST/REST complex, but translocation to the cytoplasm does not occur. After infection with a mutant virus from which ICP0 is deleted, the complex remains intact, but, under conditions of productive infection, the complex is partially translocated to the cytoplasm. These results suggest that, at low multiplicities of infection, ICP0 blocks CoREST-mediated silencing of viral genes by dissociation of HDAC1, whereas subsequent modifications and translocation of the components of the complex are the functions of other viral gene products made later in infection.

Regions of the varicella-zoster virus open reading frame 63 latency-associated protein important for replication in vitro are also critical for efficient establishment of latency

Varicella-zoster virus (VZV) open reading frame 63 (ORF63) is one of the most abundant transcripts expressed during VZV latency in humans, and ORF63 protein has been detected in human ganglia by several laboratories. Deletion of over 90% of the ORF63 gene showed that the protein is required for efficient establishment of latency in rodents. We have constructed viruses with a series of mutations in ORF63. While prior experiments showed that transfection of cells with a plasmid expressing ORF63 but lacking the putative nuclear localization signal of the protein resulted in increased expression of the protein in the cytoplasm, we found that ORF63 protein remained in the nucleus in cells infected with a VZV ORF63 nuclear localization signal deletion mutant. This mutant was not impaired for growth in cell culture or for latency in rodents. Replacement of five serine or threonine phosphorylation sites in ORF63 with alanines resulted in a virus that was impaired for replication in vitro and for latency. A series of ORF63 carboxy-terminal mutants showed that the last 70 amino acids do not affect replication in vitro or latency in rodents; however, the last 108 amino acids are important for replication and latency. Thus, regions of ORF63 that are important for replication in vitro are also required for efficient establishment of latency.

Evidence for a direct interaction between HSV-1 ICP27 and ICP8 proteins

Herpes simplex virus 1 (HSV-1) ICP27 and ICP8 proteins have both been implicated in the transcription of late genes and regulation of viral gene expression. We showed previously that ICP27 and ICP8 associate with the RNAP II holoenzyme (Zhou and Knipe, J. Virol. 76, 5893-5904). Here, we demonstrate that ICP27 and ICP8 coprecipitate from lysates of HSV-1-infected HEp2 cells and from lysates of insect cells expressing ICP27 and ICP8, the latter being in the absence of other HSV-1 proteins. By expressing and purifying hexahistidine-tagged ICP8 (His-ICP8) and maltose binding protein (MBP)-tagged ICP27 (MBP-27) proteins and performing in vitro immunoprecipitation and pull-down assays, we also demonstrate that ICP27 and ICP8 coprecipitate in the absence of other viral or cellular proteins. Taken together, these data provide evidence that ICP27 and ICP8 interact directly in vitro and in infected cells. We hypothesize that the ICP27-ICP8 interaction plays a role in the stimulation of late gene transcription.

Characterization of nuclear localization and export signals of the major tegument protein VP8 of bovine herpesvirus-1

Bovine herpesvirus-1 (BHV-1) VP8 is found in the nucleus immediately after infection. Transient expression of VP8 fused to yellow fluorescent protein (YFP) in COS-7 cells confirmed the nuclear localization of VP8 in the absence of other viral proteins. VP8 has four putative nuclear localization signals (NLS). Deletion of pat4 ((51)RRPR(54)) or pat7 ((48)PRVRRPR(54)) NLS2 abrogated nuclear accumulation, whereas deletion of (48)PRV(50) did not, so pat4 NLS2 is critical for nuclear localization of VP8. Furthermore, NLS1 ((11)RRPRR(15)), pat4 NLS2, and pat7 NLS2 were all capable of transporting the majority of YFP to the nucleus. Finally, a 12-amino-acid peptide with the sequence RRPRRPRVRRPR directed all of YFP into the nucleus, suggesting that reiteration of the RRPR motif makes the nuclear localization more efficient. Heterokaryon assays demonstrated that VP8 is also capable of shuttling between the nucleus and cytoplasm of the cell. Deletion mutant analysis revealed that this property is attributed to a leucine-rich nuclear export sequence (NES) consisting of amino acids (485)LSAYLTLFVAL(495). This leucine-rich NES caused transport of YFP to the cytoplasm. These results demonstrate that VP8 shuttles between the nucleus and cytoplasm.

The promyelocytic leukemia nuclear body: sites of activity?

The promyelocytic leukemia (PML) nuclear body is one of many subnuclear domains in the eukaryotic cell nucleus. It has received much attention in the past few years because it accumulates the promyelocytic leukemia protein called PML. This protein is implicated in many nuclear events and is found as a fusion with the retinoic acid receptor RARalpha in leukemic cells. The importance of PML bodies in cell differentiation and growth is implicated in acute promyelocitic leukemia cells, which do not contain PML bodies. Treatment of patients with drugs that reverse the disease phenotype also causes PML bodies to reform. In this review, we discuss the structure, composition, and dynamics that may provide insights into the function of PML bodies. We also discuss the repsonse of PML bodies to cellular stresses, such as virus infection and heat shock. We interpret the changes that occur as evidence for a role of these structures in gene transcription. We also examine the role of the posttranslational modification. SUMO-1 addition, in directing proteins to this nuclear body. Characterization of the mobility of PML body associated proteins further supports a role in specific nuclear events, rather than the bodies resulting from random accumulations of proteins.

The interaction between dendritic cells and herpes simplex virus-1

Dendritic cells (DCs) are the most potent antigen-presenting cells, because they are also able to induce native T cells. Thus they are crucial in the induction of antiviral immune responses. Several viral immune escape mechanisms have been described; here we concentrate on the interaction between DCs and herpes simplex virus type 1 (HSV-1). DCs can be infected by HSV-1; however, only immature DCs generate infectious viral particles, whereas mature DCs do not support virus production and only immediate-early and early viral transcripts are generated. To induce potent immune responses DCs must mature. Interestingly, HSV-1 interferes with this maturation process, thus inhibiting antiviral T cell stimulation. Furthermore, HSV-1 strongly interferes with DC-mediated T cell proliferation. A striking finding was the complete degradation of CD83, the best-known marker for mature DC, after HSV-1 infection in lysosomal compartments. This CD83 degradation coincided with a clearly reduced T cell stimulation representing an additional new escape strategy. The functional role and the importance of CD83 are discussed in detail.

Association of herpes simplex virus type 1 ICP8 and ICP27 proteins with cellular RNA polymerase II holoenzyme

Herpes simplex virus 1 (HSV-1) infection causes the shutoff of host gene transcription and the induction of a transcriptional program of viral gene expression. Cellular RNA polymerase II is responsible for transcription of all the viral genes, but several viral proteins stimulate viral gene transcription. ICP4 is required for all delayed-early and late gene transcription, ICP0 stimulates transcription of viral genes, and ICP27 stimulates expression of some early genes and transcription of at least some late viral genes. The early DNA-binding protein, ICP8, also stimulates late gene transcription. We therefore investigated which HSV proteins interact with RNA polymerase II. Using immunoprecipitation and Western blotting methods, we observed the coprecipitation of ICP27 and ICP8 with RNA polymerase II holoenzyme. The association of ICP27 with RNA polymerase II was detectable as early as 3 h postinfection, while ICP8 association became evident by 5 h postinfection, and the association of both was independent of viral DNA synthesis. Infections with ICP27 gene mutant viruses revealed that ICP27 is required for the association of ICP8 with RNA polymerase II, while studies with ICP8 gene deletion mutants showed no apparent role for ICP8 in the association of ICP27 with RNA polymerase II. The association of ICP27 and ICP8 with RNA polymerase II holoenzyme appeared to be independent of nucleic acids. We hypothesize that the interaction of ICP27 with RNA polymerase II holoenzyme reflects its role in stimulating early and late gene expression and/or its role in inhibiting host transcription and that the interaction of ICP8 with RNA polymerase II holoenzyme reflects its role in stimulating late gene transcription.

Patterns of gene expression and a transactivation function exhibited by the vGCR (ORF74) chemokine receptor protein of Kaposi's sarcoma-associated herpesvirus

The ORF74 or vGCR gene encoded by Kaposi's sarcoma-associated herpesvirus (KSHV; also called human herpesvirus 8) has properties of a ligand-independent membrane receptor signaling protein with angiogenic properties that is predicted to play a key role in the biology of the virus. We have examined the expression of vGCR mRNA and protein in primary effusion lymphoma (PEL) cell lines, PEL and multicentric Castleman's disease (MCD) tumors, Kaposi's sarcoma lesions and infected endothelial cell cultures. The vGCR gene proved to be expressed in PEL cell lines as a large spliced bicistronic mRNA of 3.2 kb that also encompasses the upstream vOX2 (K14) gene. This mRNA species was induced strongly by phorbol ester (TPA) and sodium butyrate treatment in the BCBL-1 cell line, but only weakly in the HBL6 cell line, and was classified as a relatively late and low-abundance delayed early class lytic cycle gene product. A complex bipartite upstream lytic cycle promoter for this mRNA was nestled within the intron of the 5'-overlapping but oppositely oriented latent-state transcription unit for LANA1/vCYC-D/vFLIP and responded strongly to both TPA induction and cotransfection with the KSHV RNA transactivator protein (RTA or ORF50) in transient reporter gene assays. A vGCR protein product of 45 kDa that readily dimerized was detected by Western blotting and in vitro translation and was localized in a cytoplasmic and membrane pattern in DNA-transfected Vero and 293T cells or adenovirus vGCR-transduced dermal microvascular endothelial cells (DMVEC) as detected by indirect immunofluorescence assay (IFA) and immunohistochemistry with a specific rabbit anti-vGCR antibody. Similarly, a subfraction of KSHV-positive cultured PEL cells and of KSHV (JSC-1) persistently infected DMVEC cells displayed cytoplasmic vGCR protein expression, but only after TPA or spontaneous lytic cycle induction, respectively. The vGCR protein was also detectable by immunohistochemical staining in a small fraction (0.5 to 3%) of the cells in PEL and MCD tumor and nodular Kaposi's sarcoma lesion specimens that were apparently undergoing lytic cycle expression. These properties are difficult to reconcile with the vGCR protein's playing a direct role in spindle cell proliferation, transformation, or latency, but could be compatible with proposed contributions to angiogenesis via downstream paracrine effects. The ability of vGCR to transactivate expression of both several KSHV promoter-driven luciferase (LUC) reporter genes and an NFkappaB motif containing the chloramphenicol acetyltransferase (CAT) reporter gene may also suggest an unexpected regulatory role in viral gene expression.

Identification of a motif in the C terminus of herpes simplex virus regulatory protein ICP4 that contributes to activation of transcription

Expression of most viral genes during productive infection by herpes simplex virus is regulated by the viral protein ICP4 (also called IE175 or Vmw175). The N-terminal portion of ICP4 contains well-defined transactivation, DNA binding, and dimerization domains that contribute to promoter regulation. The C-terminal half of ICP4 contributes to the activity of ICP4, but the functional motifs have not been well mapped. To localize functional motifs in the C-terminal half of ICP4, we have compared the relative specific activities of ICP4 variants in transient-transfection assays. Deletion of the C-terminal 56 residues reduces the specific activity more than 10-fold. Mutational analysis identified three consecutive residues (1252 to 1254) that are conserved in ICP4 orthologs and are essential for full activity, especially in the context of ICP4 variants with a deletion in the N-terminal transactivation domain. Recombinant viruses that encode variants of ICP4 with mutations in the N-terminal transactivation domain and/or the extreme C terminus were constructed. The phenotypes of these recombinant viruses support the hypothesis that efficient promoter activation by ICP4 requires motifs at both the N and C termini. The data suggest that the C terminus of ICP4 functions not as an independent transactivation domain but as an enhancer of the ICP4 N-terminal transactivation domain. The data provide further support for the hypothesis that some ICP4 motifs required for promoter activation are not required for promoter repression and suggest that ICP4 utilizes different cellular factors for activation or repression of viral promoters.

Proteasome-independent disruption of PML oncogenic domains (PODs), but not covalent modification by SUMO-1, is required for human cytomegalovirus immediate-early protein IE1 to inhibit PML-mediated transcriptional repression

Human cytomegalovirus (HCMV) major immediate-early protein IE1 is an abundant 72-kDa nuclear phosphoprotein that is thought to play an important role in efficient triggering of the lytic cycle, especially at low multiplicity of infection. The best-known properties of IE1 at present are its transient targeting to punctate promyelocytic leukemia protein (PML)-associated nuclear bodies (PML oncogenic domains [PODs] or nuclear domain 10 [ND10]), with associated displacement of the cellular PML tumor suppressor protein into a diffuse nucleoplasmic form and its association with metaphase chromosomes. Recent studies have shown that the targeting of PML (and associated proteins such as hDaxx) to PODs is dependent on modification of PML by ubiquitin-like protein SUMO-1. In this study, we provide direct evidence that IE1 is also covalently modified by SUMO-1 in both infected and cotransfected cells, as well as in in vitro assays, with up to 30% of the protein representing the covalently conjugated 90-kDa form in stable U373/IE1 cell lines. Lysine 450 was mapped as the major SUMO-1 conjugation site, but a point mutation of this lysine residue in IE1 did not interfere with its targeting to and disruption of the PODs. Surprisingly, unlike PML or IE2, IE1 did not interact with either Ubc9 or SUMO-1 in yeast two-hybrid assays, suggesting that some additional unknown intranuclear cofactors must play a role in IE1 sumoylation. Interestingly, stable expression of either exogenous PML or exogenous Flag-SUMO-1 in U373 cell lines greatly enhanced both the levels and rate of in vivo IE1 sumoylation during HCMV infection. Unlike the disruption of PODs by the herpes simplex virus type 1 IE110(ICP0) protein, the disruption of PODs by HCMV IE1 proved not to involve proteasome-dependent degradation of PML. We also demonstrate here that the 560-amino-acid PML1 isoform functions as a transcriptional repressor when fused to the GAL4 DNA-binding domain and that wild-type IE1 inhibits the repressor function of PML1 in transient cotransfection assays. Furthermore, both IE1(1-346) and IE1(L174P) mutants, which are defective in displacing PML from PODs, failed to inhibit the repression activity of PML1, whereas the sumoylation-negative IE1(K450R) mutant derepressed as efficiently as wild-type IE1. Taken together, our results suggest that proteasome-independent disruption of PODs, but not IE1 sumoylation, is required for efficient IE1 inhibition of PML-mediated transcriptional repression.

Origin-independent assembly of Kaposi's sarcoma-associated herpesvirus DNA replication compartments in transient cotransfection assays and association with the ORF-K8 protein and cellular PML

Six predicted Kaposi's sarcoma virus herpesvirus (KSHV) proteins have homology with other well-characterized herpesvirus core DNA replication proteins and are expected to be essential for viral DNA synthesis. Intact Flag-tagged protein products from all six were produced from genomic expression vectors, although the ORF40/41 transcript encoding a primase-helicase component proved to be spliced with a 127-bp intron. The intracellular localization of these six KSHV replication proteins and the mechanism of their nuclear translocation were investigated. SSB (single-stranded DNA binding protein, ORF6) and PPF (polymerase processivity factor, ORF59) were found to be intrinsic nuclear proteins, whereas POL (polymerase, ORF9), which localized in the cytoplasm on its own, was translocated to the nucleus when cotransfected with PPF. PAF (primase-associated factor, ORF40/41), a component of the primase-helicase tripartite subcomplex together with PRI (primase, ORF56) and HEL (helicase, ORF44), required the presence of all five other replication proteins for efficient nuclear translocation. Surprisingly, even in the absence of a lytic cycle replication origin (ori-Lyt) and any known initiator or origin binding protein, the protein products of all six KSHV core replication genes cooperated in a transient cotransfection assay to form large globular shaped pseudo-replication compartments (pseudo-RC), which excluded cellular DNA. These pseudo-RC structures were confirmed to include POL, SSB, PRI, and PAF but did not contain any newly synthesized DNA. Similar to the human cytomegalovirus system, the peripheries of these KSHV pre-RC were also found to be surrounded by punctate PML oncogenic domains (PODs). Furthermore, by transient cotransfection, the six KSHV core replication machinery proteins successfully replicated a plasmid containing EBV ori-Lyt in the presence of the Epstein-Barr virus-encoded DNA binding initiator protein, ZTA. The KSHV-encoded K8 (ORF-K8) protein, which is a distant evolutionary homologue to ZTA, was incorporated into pseudo-RC structures formed by transient cotransfection with the six core KSHV replication genes. However, unlike ZTA, K8 displayed a punctate nuclear pattern both in transfected cells and at early stages of lytic infection and colocalized with the cellular PML proteins in PODs. Finally, K8 was also found to accumulate in functional viral RC, detected by incorporation of pulse-labeled bromodeoxyuridine into newly synthesized DNA in both tetradecanoyl phorbol acetate-induced JSC-1 primary effusion lymphoblasts and in KSHV lytically infected endothelial cells.

A single amino acid substitution in the ICP27 protein of herpes simplex virus type 1 is responsible for its resistance to leptomycin B

Leptomycin B (LMB) is a specific inhibitor of Crm1-dependent nuclear export of proteins. The replication of herpes simplex virus (HSV) is normally highly sensitive to LMB; a resistant HSV variant, however, was isolated by serial passages of the virus. Analysis of marker transfer and viral DNA sequences revealed that a single amino acid substitution within the ICP27 gene is responsible for conferring this resistance.

Interferon coordinately inhibits the disruption of PML-positive ND10 and immediate-early gene expression by herpes simplex virus

Interferons (IFNs) are important components of the innate immune response, limiting herpes simplex virus (HSV) infection. In recombinant HSV-infected cells, IFN inhibited expression of beta-galactosidase from the immediate-early gene, ICP4, promoter. The extent of inhibition was dependent on IFN dose, IFN type, cell type, and multiplicity of infection (moi). IFN inhibited gene transcription, leading to a complete block in ICP4 promoter-driven gene expression in 90% of cells. The same IFN treatments resulted in an increase in the size and number of nuclear domain 10 (ND10) structures that stained positive by immunofluorescence for the promyelocytic leukemia (PML) protein. In cultures infected at low moi with a recombinant HSV producing ICP4 as a fusion protein with green fluorescence protein, the appearance of green fluorescence in the nucleus coincided with loss of PML-positive ND10 in the same nucleus, even in the rare ICP4-expressing IFN-treated cells. IFN-dependent inhibition was nearly complete when the immediate-early promoter was in the viral genome but was minimal when the promoter was stably integrated into the cellular genome. These data reveal that IFN can completely block viral gene expression in infected cells and that enhancement of the ND10 structure, which is the site of initiation of HSV replication, correlates with the block in viral gene expression.

The host-cell architectural protein HMG I(Y) modulates binding of herpes simplex virus type 1 ICP4 to its cognate promoter

The productive infection cycle of herpes simplex virus is controlled in part by the action of ICP4, an immediate-early gene product that acts as both an activator and repressor of transcription. ICP4 is autoregulatory, and IE-3, the gene that encodes it, contains a high-affinity binding site for the protein at its cap site. Previously, we had demonstrated that this site could be occupied by proteins found in nuclear extracts from uninfected cells. A HeLa cell cDNA expression library was screened with a DNA probe containing the IE-3 gene cap site, and clones expressing the architectural chromatin proteins HMG I and HMG Y were identified by this technique. HMG I is shown to augment binding of ICP4 to its cognate site in in vitro assays and to enhance the activity of this protein in short-term transient expression assays.

Differential gene expression in neoplastic and human papillomavirus-immortalized oral keratinocytes

We have previously demonstrated that normal human oral keratinocytes immortalized by transfection with human papillomavirus type-16 Dna became tumorigenic after exposure to a chemical carcinogen. In an effort to detect differentially regulated genes associated with this transition from the immortal to the malignant phenotype, we employed representational differences analysis (a PCR-coupled subtractive hybridization technique). After analysing 50 colonies, 12 putative messages were identified. Northern analysis comparison using the identified cDNAs as probes was made between normal human oral keratinocyte, papillomavirus-immortalized human oral keratinocytes (HOK-16B), a neoplastic cell line derived from HOK-16B (HOK-16B-BaP-T) and the human oral cancer cell lines Hep-2, SCC-9 and Tu-177. We found that mRNAs encoding for cyclophilin A, c-myc binding protein 1, the heat shock protein 90alpha and one unknown transcript were up-regulated in the oral cancer cell lines analysed as well as in HOK-16B cells. We also detected a downregulation of the mRNAs encoding the skin-derived antileukoproteinase SKALP/elafin, the translationally regulated p23 protein and one unknown transcript. Whether these messages are associated to the neoplastic conversion of human keratinocytes remains to be determined.

How do animal DNA viruses get to the nucleus?

Genome and pre-genome replication in all animal DNA viruses except poxviruses occurs in the cell nucleus (Table 1). In order to reproduce, an infecting virion enters the cell and traverses through the cytoplasm toward the nucleus. Using the cell's own nuclear import machinery, the viral genome then enters the nucleus through the nuclear pore complex. Targeting of the infecting virion or viral genome to the multiplication site is therefore an essential process in productive viral infection as well as in latent infection and transformation. Yet little is known about how infecting genomes of animal DNA viruses reach the nucleus in order to reproduce. Moreover, this nuclear locus for viral multiplication is remarkable in that the sizes and composition of the infectious particles vary enormously. In this article, we discuss virion structure, life cycle to reproduce infectious particles, viral protein's nuclear import signal, and viral genome nuclear targeting.

HCF-dependent nuclear import of VP16

Transactivation by VP16 requires the formation of a multicomponent complex, the TAATGAAAT recognition factor complex (TRF.C), that contains in addition to VP16, two cellular proteins, Oct-1 and HCF. HCF binds directly to VP16 and this promotes subsequent interaction of the VP16-HCF complex with the POU DNA-binding domain of Oct-1 and selective assembly onto target sites. Here we demonstrate a novel role of HCF in the intracellular compartmentalization of VP16. We show that while VP16 does not contain a consensus nuclear localization signal (NLS) and is largely cytoplasmic, co-expression with HCF resulted in VP16 nuclear accumulation. A candidate NLS within the C-terminus of HCF was identified and insertion of this motif into green fluorescent protein (GFP) promoted nuclear accumulation. Conversely, removal of this signal from HCF (HCFDeltaNLS) resulted in its cytoplasmic accumulation. Co-expression of HCFDeltaNLS with wild-type (wt) VP16, or of wt HCF with VP16 mutants lacking HCF-binding activity failed to promote the nuclear enrichment of VP16. These results indicate that in addition to its role in stabilizing TRF.C, HCF acts as a nuclear import factor for VP16.

The ability of herpes simplex virus type 1 immediate-early protein Vmw110 to bind to a ubiquitin-specific protease contributes to its roles in the activation of gene expression and stimulation of virus replication

Herpes simplex virus type 1 immediate-early protein Vmw110 stimulates the onset of virus infection and is required for efficient reactivation from latency. In transfection assays, Vmw110 is a potent activator of gene expression, but its mode of action has yet to be determined. Previous work has shown that Vmw110 localizes to specific intranuclear structures known as ND10, PML bodies, or PODs and causes the disruption of these domains. The ability of Vmw110 to disrupt ND10 correlates with its biological activities in infected and transfected cells. It has also been found that Vmw110 binds strongly and specifically to a ubiquitin-specific protease known as HAUSP, itself a component of a subset of ND10. In this study we have investigated the role of HAUSP in Vmw110 activity; single amino acid residues of Vmw110 required for the interaction were identified, and the effects of mutation of these residues in infected and transfected cells were then assayed. The results indicate that the ability to bind to HAUSP contributes to the functional activities of Vmw110.

Binding of the human cytomegalovirus 80-kDa immediate-early protein (IE2) to minor groove A/T-rich sequences bounded by CG dinucleotides is regulated by protein oligomerization and phosphorylation

The 80-kDa immediate-early regulatory protein IE2 of human cytomegalovirus (HCMV) functions as an essential positive transactivator of downstream viral promoters, but it also specifically down-regulates transcription from the major immediate-early promoter through a 14-bp DNA target motif known as the cis-repression signal (CRS) located at the transcription start site. The IE2 protein purified from bacteria as a fusion product of either staphylococcal Protein A/IE2(290-579) or glutathione-S-transferase (GST)/IE2(346-579) bound specifically to a [32P]-labeled CRS oligonucleotide probe in an in vitro electrophoretic mobility shift assay (EMSA). In contrast, no direct interaction with the CRS probes could be detected with IE2 wild-type protein in extracts from infected or transfected mammalian cells or when synthesized by in vitro translation. However, in vitro phosphorylation of GST/IE2(346-579) by incubation with either the catalytic subunit of protein kinase A (PKA) or a HeLa cell nuclear extract strongly inhibited its DNA-binding activity. This process required ATP hydrolysis and could be reversed by subsequent incubation with bacterial alkaline phosphatase. Importantly, dephosphorylation of the constitutively expressed native IE2 protein present in a nuclear extract from the U373(A45) cell line unmasked a specific CRS DNA-binding activity that could be supershifted with anti-IE2 monoclonal antibody (mAb). A series of high-molecular-weight hetero-oligomeric DNA-bound structures of intermediate mobility were formed in EMSA assays when a mixture of staphylococcal Protein A/IE2 and GST/IE2 was coincubated with the CRS probe. Coincubation with a DNA-binding negative but dimerization-competent GST/IE2 deletion mutant competitively inhibited DNA-binding by staphylococcal Protein A/IE2, whereas coincubation with a GST/IE2 deletion mutant that lacked the ability to both dimerize and bind to DNA failed to influence the mobility of the DNA-bound staphylococcal Protein A/IE2 protein. Therefore, IE2 appears to bind to DNA as a higher-order oligomer in which the presence of subunits with mutant DNA-binding domains interferes with the overall DNA-binding function. A series of point mutations introduced into each of nine conserved motifs throughout the DNA-binding and dimerization domain, all of which abolish the ability of the transfected intact IE2 protein to autoregulate the MIE promoter, also all lacked the ability to bind to CRS sequences as GST/IE2(346-379) fusion proteins. Detailed analysis of point mutations in the 14-bp CRS target DNA binding motif revealed that IE2 binds in a relatively sequence-independent manner to 10-bp-long A/T-rich DNA elements bounded on each side by CG dinucleotides. Moreover, the A/T-rich minor groove binding agent distamycin, but not the G/C-rich minor groove binding agent chromomycin-A3, actively competed with IE2 for binding to the CRS motif in a dose-dependent fashion. In conclusion, IE2 binds preferentially as multimerized dimers to A/T-rich sequences in the minor groove that are flanked on both sides by appropriately spaced CG dinucleotides, and inhibition of the DNA-binding or oligomerization activity by PKA phosphorylation probably accounts for the inactivity of the mammalian and in vitro translated forms of the protein.

Purification and characterization of a cellular protein that binds to the downstream activation sequence of the strict late UL38 promoter of herpes simplex virus type 1

Previous work on the strict late (gamma) UL38 promoter of herpes simplex virus type 1 identified three cis-acting elements required for wild-type levels of transcription: a TATA box at -31, a consensus mammalian initiator element at the transcription start site, and a downstream activation sequence (DAS) at +20 to +33. DAS is found in similar locations on several other late promoters, suggesting an important regulatory role in late gene expression. In this communication, we further characterize the interaction between DAS and a cellular protein which is found in both uninfected and infected nuclear extracts. This protein was purified from HeLa nuclear extracts and identified as the DNA binding component (Ku heterodimer) of DNA-dependent protein kinase (DNA-PK) by peptide mapping. Highly purified DNA-PK was able to stimulate UL38 transcription in vitro approximately 10-fold. DAS is similar in sequence to another element, nuclear regulatory element 1 (NRE1) of the glucocorticoid-responsive mouse mammary tumor virus long terminal repeat. NRE1 is known to specifically bind Ku in the absence of DNA ends. We demonstrated that NRE1 is able to substitute for DAS in the UL38 promoter to activate transcription as measured by in vitro transcription and in vivo during infection of tissue culture cells with recombinant virus. Also, we found that the binding of DNA-PK to DAS involves the bases demonstrated to be important in UL38 transcription and that the 70-kDa subunit of Ku binds to DAS.

Molecular cloning and characterization of a human brain-specific gene implicated in neuronal differentiation

A 2.5 kb human cDNA clone containing a CAG trinucleotide repeat, designated HB20, was isolated from a human fetal brain library. Northern analysis on multi-tissue blots and various cell lines confirmed that HB20 is specifically expressed in the brain. Its expression is low in two glioma cells, moderate in a neuron precursor cell, NT2, but absent in lymphoma, cervical carcinoma, or colonic carcinoma cells. Significant increase of HB20 mRNA was shown along with retinoic acid-induced terminal differentiation of NT2 cells into neuron cells, hNT. Homology comparison of the predicted HB20 amino acid sequence with the current database revealed that it belongs to a newly recognized protein family composed of nucleosome assembly proteins and SET proto-oncogene, which has been shown to interact specifically with B-type cyclins involved in the control of cell proliferation. Together with the detection of nuclear localization signals and apparent nuclear distribution of expressed protein, HB20 is likely to be a brain-specific nuclear protein, functioning in the process of neuronal differentiation.

Nuclear dots: actors on many stages

Nuclear dots (NDs), alternatively designated nuclear bodies (NBs), PML oncogenic domains (PODs), nuclear domain 10 (ND10) or Kr-bodies, became a major topic for researchers in many fields only recently. Originally described as an autoantigenic target in patients with primary biliary cirrhosis, they are now also known to play a role in development of acute promyelocytic leukemia (APL) and possibly other forms of neoplasia. Size, number and composition of NDs are regulated throughout the cell cycle. Infection with herpes simplex virus, adenovirus, cytomegalovirus, Epstein-Barr-virus, influenza virus and human T cell lymphotropic virus type I (HTLV I) strongly modifies ND structure through viral regulatory proteins. Due to this finding and because at least three of the cellular ND proteins are highly interferon-inducible, a function of NDs in early viral infection or in antiviral response has been postulated. Functional data are currently available only for two of the ND-associated proteins. The Sp100 protein seems to have transcriptional transactivating property, whereas the promyelocytic leukemia protein (PML) was reported to suppress growth and transformation. Here, we give a brief overview of the data currently available on NDs. Thus, we hope to link seemingly unrelated findings in the literature on oncology, virology, cell biology and immunology.

Functional Domains within the Nucleus of a Cell Infected with HSV-1

HSV-1 is a nuclear replicating DNA virus capable of establishing both lytic and latent infections in mammalian cells. Expression of the more than 80 HSV genes (the majority of which do not contain introns) requires complex coordination of viral and cellular factors both temporally, at appropriate points during the infectious cycle, and spatially as the virus transcription, replication and DNA packaging factories develop in the cell nucleus. Whilst the HSV genome encodes sufficient proteins to sustain viral DNA replication, it is reliant upon its host cell for RNA polymerase II and RNA processing machinery, in addition to an increasing number of cellular cofactors, for gene expression. As HSV establishes a lytic infection, cellular gene expression and splicing are inhibited as cellular chromatin is displaced and a dramatic reorganisation of the host cell nucleus occurs. The formation of large protein-rich factories synthesising viral RNA and replicating and packaging the viral genomes is the most striking alteration. In addition to the synthetic factories, large clumps of cellular and viral intron-containing RNAs accumulate in the nucleus as a result of the inhibition of splicing, at locations which colocalise with splicing factors, but are separate from transcription sites. An essential HSV protein IE63, discussed here, has been identified with a role in the organisation of the nucleus at many levels including replication and transcription site formation, splicing factor organisation and the transport of RNA. This review is a summary of our present understanding of the organisation of the HSV infected cell nucleus, relating viral genomes, RNA, DNA and proteins in the context of the nucleus. However this is a rapidly evolving field and new factors (both viral and cellular) involved in the regulation of these functional domains are constantly being identified. Copyright 1997 by John Wiley & Sons, Ltd.

Experimental investigation of herpes simplex virus latency

The clinical manifestations of herpes simplex virus infection generally involve a mild and localized primary infection followed by asymptomatic (latent) infection interrupted sporadically by periods of recrudescence (reactivation) where virus replication and associated cytopathologic findings are manifest at the site of initial infection. During the latent phase of infection, viral genomes, but not infectious virus itself, can be detected in sensory and autonomic neurons. The process of latent infection and reactivation has been subject to continuing investigation in animal models and, more recently, in cultured cells. The initiation and maintenance of latent infection in neurons are apparently passive phenomena in that no virus gene products need be expressed or are required. Despite this, a single latency-associated transcript (LAT) encoded by DNA encompassing about 6% of the viral genome is expressed during latent infection in a minority of neurons containing viral DNA. This transcript is spliced, and the intron derived from this splicing is stably maintained in the nucleus of neurons expressing it. Reactivation, which can be induced by stress and assayed in several animal models, is facilitated by the expression of LAT. Although the mechanism of action of LAT-mediated facilitation of reactivation is not clear, all available evidence argues against its involving the expression of a protein. Rather, the most consistent models of action involve LAT expression playing a cis-acting role in a very early stage of the reactivation process.

Assembly of complete, functionally active herpes simplex virus DNA replication compartments and recruitment of associated viral and cellular proteins in transient cotransfection assays

Early during the herpes simplex virus (HSV) lytic cycle or in the presence of DNA synthesis inhibitors, core viral replication machinery proteins accumulate in intranuclear speckled punctate prereplicative foci, some of which colocalize with numerous sites of host cellular DNA synthesis initiation known as replisomes. At later times, in the absence of inhibitors, several globular or large irregularly shaped replication compartments are formed; these compartments also contain progeny viral DNA and incorporate the IE175(ICP4) transcription factor together with several cellular proteins involved in DNA replication and repair. In this study, we demonstrate that several forms of both prereplication foci and active viral replication compartments that display an appearance similar to that of the compartments in HSV-infected cells can be successfully assembled in transient assays in DNA-transfected cells receiving genes encoding all seven essential HSV replication fork proteins together with oriS target plasmid DNA. Furthermore, bromodeoxyuridine (BrdU)-pulse-labeled DNA synthesis initiation sites colocalized with the HSV single-stranded DNA-binding protein (SSB) in these replication compartments, implying that active viral DNA replication may be occurring. The assembly of complete HSV replication compartments and incorporation of BrdU were both abolished by treatment with phosphonoacetic acid (PAA) and by omission of any one of the seven viral replication proteins, UL5, UL8, UL9, UL42, UL52, SSB, and Pol, that are essential for viral DNA replication. Consistent with the fact that both HSV IE175 and IE63(ICP27) localize within replication compartments in HSV-infected cells, the assembled HSV replication compartments were also able to recruit both of these essential regulatory proteins. Blocking viral DNA synthesis with PAA, but not omission of oriS, prevented the association of IE175 with prereplication structures. The assembled HSV replication compartments also redistributed cotransfected cellular p53 into the viral replication compartments. However, the other two HSV immediate-early nuclear proteins IE110(ICP0) and IE68(ICP22) did not enter the replication compartments in either infected or transfected cells.

Physical and functional interactions between herpes simplex virus immediate-early proteins ICP4 and ICP27

The ordered expression of herpes simplex virus type 1 (HSV-1) genes, during the course of a productive infection, requires the action of the virus immediate-early regulatory proteins. Using a protein interaction assay, we demonstrate specific in vitro protein-protein interactions between ICP4 and ICP27, two immediate-early proteins of HSV-1 that are essential for virus replication. We map multiple points of contact between these proteins. Furthermore, by coimmunoprecipitation experiments, we demonstrate the following. (i) ICP4-ICP27 complexes are present in extracts from HSV-1 infected cells. (ii) ICP27 binds preferentially to less modified forms of ICP4, a protein that is extensively modified posttranslationally. We also demonstrate, by performing electrophoretic mobility shift assays and supershifts with monoclonal antibodies to ICP4 or ICP27, that both proteins are present in a DNA-protein complex with a noncanonical ICP4 binding site present in the HSV thymidine kinase (TK) gene. ICP4, in extracts from cells infected with ICP27-deficient viruses, is impaired in its ability to form complexes with the TK site but not with the canonical site from the alpha4 gene. However, ICP4 is able to form complexes with the TK probe, in the absence of ICP27, when overproduced in mammalian cells or expressed in bacteria. These data suggest that the inability of ICP4 from infected cell extracts to bind the TK probe in the absence of ICP27 does not reflect a requirement for the physical presence of ICP27 in the complex. Rather, they imply that ICP27 is likely to modulate the DNA binding activity of ICP4 by affecting its posttranslational modification status. Therefore, we propose that ICP27, in addition to its established role as a posttranscriptional regulator of virus gene expression, may also modulate transcription either through direct or indirect interactions with HSV regulatory regions, or through its ability to modulate the DNA binding activity of ICP4.

Attenuation of DNA-dependent protein kinase activity and its catalytic subunit by the herpes simplex virus type 1 transactivator ICP0

The DNA-dependent protein kinase (DNA-PK) is involved in several fundamental nuclear processes, including DNA double-strand break repair, V(D)J recombination, and transcription by RNA polymerases I and II. In this study, we show that infection of mammalian cells with herpes simplex virus type 1 attenuates DNA-PK activity by specifically depleting the p350/DNA-PKcs catalytic subunit. The half-life of the p350/DNA-PKcs protein decreases from greater than 24 h to less than 4 h following infection. The depletion of DNA-PK activity and p350/DNA-PKcs abundance is dependent on expression of the viral immediate-early protein ICP0. As ICP0 acts as a promoter-independent transactivator of gene expression, these data suggest that ICP0 may function by directly or indirectly targeting the p350/DNA-PKcs subunit of DNA-PK, thereby altering the inhibitory effects of DNA-PK on RNA polymerase II transcription.

Mutational analysis of ICP0R, a transrepressor protein created by alternative splicing of the ICP0 gene of herpes simplex virus type 1

The immediate-early protein ICP0 (infected-cell polypeptide 0) of herpes simplex virus type 1 (HSV-1) is a promiscuous transactivator of both viral and nonviral promoters in transient expression assays. Failure to splice the second of two introns in the ICP0 gene results in the utilization of an alternate stop codon that generates a truncated form of ICP0 called ICP0R. This protein exists in low levels in HSV-1-infected cells and functions as a dominant negative repressor of ICP0-mediated transactivation in transient expression assays. To conduct a detailed structure-function analysis of ICP0R, a series of insertion and deletion mutants of this protein were generated and analyzed in transfection assays. These studies indicated that segments of ICP0R that were rich in acidic amino acid residues (amino acids 9 to 76 and 233 to 241) or glycine residues (amino acids 242 to 262) were dispensable for the dominant negative phenotype. In contrast, the RING finger domain (amino acids 116 to 156) and surprisingly the sequences carboxy terminal to it (amino acids 157 to 232) were absolutely essential for transdominant repression. Consistent with these findings, the amino acid sequences of these two regions were conserved among other alphaherpesvirus ICP0 homologs. A construct containing only amino acids 76 to 232 inhibited ICP0-mediated transactivation almost as efficiently as wild-type ICP0R and represented the minimal sequences necessary for the dominant negative phenotype. These results demonstrated that the critical functional domain shared by both ICP0R and ICP0 is much more complex than a simple RING finger motif. Western blot (immunoblot) analyses of transfected cell lysates revealed that nearly all of the mutant constructs directed the expression of stable ICP0R proteins of the predicted molecular weight. However, there was a striking inverse correlation between the ability of a mutant construct to mediate transrepression and the amount of protein that it synthesized, indicating that dominant negative inhibition is achieved through the action of very little ICP0R protein.

Adenovirus early region 4 34-kilodalton protein directs the nuclear localization of the early region 1B 55-kilodalton protein in primate cells

The localization of the adenovirus type 5 34-kDa E4 and 55-kDa E1B proteins was determined in the absence of other adenovirus proteins. When expressed by transfection in human, monkey, hamster, rat, and mouse cell lines, the E1B protein was predominantly cytoplasmic and typically was excluded from the nucleus. When expressed by transfection, the E4 protein accumulated in the nucleus. Strikingly, when coexpressed by transfection in human, monkey, or baby hamster kidney cells, the E1B protein colocalized in the nucleus with the E4 protein. A complex of the E4 and E1B proteins was identified by coimmunoprecipitation in transfected HeLa cells. By contrast to the interaction observed in primate and baby hamster kidney cells, the E4 protein failed to direct the E1B protein to the nucleus in rat and mouse cell lines as well as CHO and V79 hamster cell lines. This failure of the E4 protein to direct the nuclear localization of the E1B protein in REF-52 rat cells was overcome by fusion with HeLa cells. Within 4 h of heterokaryon formation and with protein synthesis inhibited, a portion of the E4 protein present in the REF-52 nuclei migrated to the HeLa nuclei. Simultaneously, the previously cytoplasmic E1B protein colocalized with the E4 protein in both human and rat cell nuclei. These results suggest that a primate cell-specific factor mediates the functional interaction of the E1B and E4 proteins of adenovirus.

Overexpression of the herpes simplex virus type 1 immediate-early regulatory protein, ICP27, is responsible for the aberrant localization of ICP0 and mutant forms of ICP4 in ICP4 mutant virus-infected cells

ICP0 and ICP4 are immediate-early regulatory proteins of herpes simplex virus type 1. Previous studies by Knipe and Smith demonstrated that these two proteins are characteristically observed in the nuclei of wild-type virus-infected cells but predominantly in the cytoplasms of cells infected with several ICP4 temperature-sensitive (ts) mutant viruses at the nonpermissive temperature (NPT) (D. M. Knipe and J. L. Smith, Mol. Cell. Biol. 6:2371-2381, 1986). Consistent with this observation, it has been shown previously that ICP0 is present predominantly in the cytoplasms of cells infected with an ICP4 null mutant virus (n12) at high multiplicities of infection and that the level of ICP27, a third viral regulatory protein, plays an important role in determining the intracellular localization of ICP0 (Z. Zhu, W. Cai, and P. A. Schaffer, J. Virol. 68:3027-3040, 1994). To address whether the cytoplasmic localization of ICP0 is a common feature of cells infected with all ICP4 mutant viruses or whether mutant ICP4 polypeptides, together with ICP27, determine the intracellular localization of ICP0, we used double-staining immunofluorescence tests to examine the intracellular staining patterns of ICP0 and ICP4 in cells infected with an extensive series of ICP4 mutant viruses. In these tests, compared with the localization pattern of ICP0 in wild-type virus-infected cells, more ICP0 was detected in the cytoplasms of cells infected with all ICP4 mutants tested at high multiplicities of infection. Each of the mutant forms of ICP4 exhibiting predominantly cytoplasmic staining contains both the nuclear localization signal and the previously mapped ICP27-responsive region (Z. Zhu and P. A. Schaffer, J. Virol. 69:49-59, 1995). No correlation between the intracellular staining patterns of ICP0 and mutant forms of ICP4 was demonstrated, suggesting that mutant ICP4 polypeptides per se are not responsible for retention of ICP0 in the cytoplasm. This observation was confirmed in studies of cells cotransfected with plasmids expressing ICP0 and mutant forms of ICP4, in which the staining pattern of ICP0 was not changed in the presence of mutant ICP4 proteins. Studies of cells infected at low multiplicities with a variety of ICP4 ts mutant viruses at the NPT showed that both ICP0 and ts forms of ICP4 were localized predominantly within the nucleus. These observations are a further indication that the aberrant localization of the ts forms of ICP4 at the NPT is not a direct result of specific mutations in the ICP4 gene. In the final series of tests, the localization of ICP0 in cells infected with a double-mutant virus unable to express either ICP4 or ICP27 was examined. In these tests, ICP0 was detected exclusively in the nuclei of Vero cells but in both the nuclei and the cytoplasms of ICP27-expressing cells infected with the double mutant. These results demonstrate that ICP27, rather than the absence of functional ICP4, is responsible for the cytoplasmic localization of ICP0 in ICP4 mutant virus-infected cells. Taken together, these findings demonstrate that the aberrant localization of ICP0 and certain mutant forms of ICP4 in cells infected with ICP4 mutant viruses is mediated by high levels of ICP27 resulting from the inability of mutant forms of ICP4 to repress the expression of ICP27.

The Epstein-Barr virus-encoded nuclear antigen EBNA-5 accumulates in PML-containing bodies

EBNA-5 is one of the Epstein-Barr virus (EBV)-encoded nuclear proteins required for immortalization of human B lymphocytes. In the nuclei of EBV-transformed lymphoblastoid cell lines EBNA-5 is preferentially targetted to distinct nuclear foci. Previously we have shown (W.Q. Jiang, L. Szekely, V. Wendel-Hansen, N. Ringertz, G. Klein, and A. Rosen, Exp. Cell Res. 197:314-318, 1991) that the same foci also contained the retinoblastoma (Rb) protein. Using a similar double immunofluorescence technique, we now show that these foci colocalize with nuclear bodies positive for PML, the promyelocytic leukemia-associated protein. Artificial spreading of the chromatin by exposure to the forces of fluid surface tension disrupts this colocalization gradually, suggesting that the bodies consist of at least two subcomponents. Heat shock or metabolic stress induced by high cell density leads to the release of EBNA-5 from the PML-positive nuclear bodies and induces it to translocate to the nucleoli. In addition to their presence in nuclear bodies, both proteins are occasionally present in nuclear aggregates and doughnut-like structures in which PML is concentrated in an outer shell. Nuclear bodies with prominent PML staining are seen in resting B lymphocytes. This staining pattern does not change upon EBV infection. In freshly infected cells EBNA-5 antigens are first distributed throughout the nucleoplasm. After a few days intensely staining foci develop. These foci coincide with PML-positive nuclear bodies. At a later stage and in established lymphoblastoid cell lines EBNA-5 is almost exclusively present in the PML-positive nuclear foci. The colocalization is restricted to EBV-infected human lymphoblasts. The data presented indicate that the distinct EBNA-5 foci are not newly formed structures but the result of translocation of the viral protein to a specialized domain present already in the nuclei of uninfected cells.

Herpes simplex virus trans-regulatory protein ICP27 stabilizes and binds to 3' ends of labile mRNA

Previous work demonstrated that a herpes simplex virus type 1 (HSV-1) immediate-early function up-regulates beta interferon but not chloramphenicol acetyltransferase reporter genes driven by the strong simian virus 40 (SV40) or cytomegalovirus promoter-enhancer regions in both transient assays and stable cell lines. The different 3' mRNA stabilization and RNA-processing signals from these two reporter genes appeared to be primarily responsible for this phenomenon. We now report that the HSV-1 ICP27 itself is sufficient to stimulate both steady-state accumulation and increased half-life of beta interferon reporter gene mRNA. Furthermore, the ability to respond directly to cotransfected ICP27 can be transferred to chloramphenicol acetyltransferase reporter genes by replacement of their SV40-derived splicing and poly(A) signals with the 3' AU-rich and poly(A) RNA-processing signals from the normally highly labile beta interferon and c-myc mRNA species. ICP27 expressed in bacteria bound specifically to in vitro-generated RNA from both the beta interferon and c-myc intronless AU-rich 3' RNA-processing regions, but not to the SV40-derived early-region splice signal and poly(A) sequences. By site-specific mutagenesis, we also show that individual ICP27 C-terminal amino acid residues that are positionally conserved in ICP27 homologs in other herpesviruses (D-357, E-358, H-479, C-400, C-483, and C-488) are critical for trans-regulatory activity. Importantly, several of these positions match mutations that are known to be essential for the role of ICP27 in the early-to-late switch during the virus lytic cycle. Therefore, our findings support the notion that HSV ICP27 modulates gene expression posttranscriptionally in part by targeting RNA.

The C-terminal repressor region of herpes simplex virus type 1 ICP27 is required for the redistribution of small nuclear ribonucleoprotein particles and splicing factor SC35; however, these alterations are not sufficient to inhibit host cell splicing

Herpes simplex virus type 1 infection results in a reorganization of antigens associated with the small nuclear ribonucleoprotein particles (snRNPs), resulting in the formation of prominent clusters near the nuclear periphery. In this study, we show that the immediate-early protein ICP27, which is involved in the impairment of host cell splicing and in the changes in the distribution of snRNPs, is also required for reassorting the SR domain splicing factor SC35. Other viral processes, such as adsorption and penetration, shutoff of host protein synthesis, early and late gene expression, and DNA replication, do not appear to play a role in changing the staining pattern of splicing antigens. Furthermore, the C-terminal repressor region of ICP27, which is required for the inhibitory effects on splicing, also is involved in redistributing the snRNPs and SC35. During infection or transfection with five different repressor mutants, the speckled staining pattern characteristic of uninfected cells was seen and the level of a spliced target mRNA was not reduced. Infections in the presence of activator mutants showed a redistributed snRNP pattern and a decreased accumulation of spliced target mRNA. Moreover, two arginine-rich regions in the N-terminal half of ICP27 were not required for the redistribution of snRNPs or SC35. Substitution of these regions with a lysine-rich sequence from simian virus 40 large-T antigen resulted in a redistribution of splicing antigens. Unexpectedly, a repressor mutant with a ts phenotype showed a redistributed staining pattern like that seen with wild-type infected cells. During infections with this ts mutant, splicing was not inhibited, as shown in this and previous studies, confirming its repressor phenotype. Furthermore, both the mutant and the wild-type protein colocalized with snRNPs. Therefore, the redistribution of snRNPs and SC35 correlates with ICP27-mediated impairment of host cell splicing, but these alterations are not sufficient to fully inhibit splicing. This indicates that active splicing complexes are still present even after dramatic changes in the organization of the snRNPs.

Arginine-rich regions succeeding the nuclear localization region of the herpes simplex virus type 1 regulatory protein ICP27 are required for efficient nuclear localization and late gene expression

The herpes simplex virus type 1 (HSV-1) immediate-early protein ICP27 is an essential regulatory protein that localizes to the nuclei of infected cells. The strong nuclear localization signal (NLS) of ICP27 was identified recently and shown to reside in the amino-terminal portion of the polypeptide from residues 110 to 137 (W.E. Mears, V. Lam, and S.A. Rice, J. Virol. 69:935-947, 1995). There are also two arginine-rich regions directly succeeding the NLS. The first of these arginine-rich sequences (residues 141 to 151), together with the NLS, has been shown by Mears et al. to form the nucleolar localization signal. Arginine-rich motifs are common in domains involved in nuclear localization and RNA binding. To analyze the role of the arginine-rich regions in ICP27, we constructed stably transformed cell lines containing ICP27 mutants with deletions of all or parts of the NLS and arginine-rich regions. We also constructed mutants in which these regions were replaced with heterologous NLSs or RNA-binding domains. Characterization of these mutants indicated that the arginine-rich regions were required but not sufficient for wild-type localization of ICP27. More importantly, the NLS and arginine-rich regions were also essential to the function of ICP27. Mutants lacking these sequences were defective in late gene expression during infection even when ICP27 was properly localized to the nucleus by substitution of the NLS from simian virus 40 large T antigen. Further, the defect in late gene expression could not be overcome by replacement with the highly basic RNA-binding domain of human immunodeficiency virus type 1 Tat. The deficiency in late gene expression was independent of ICP27's role in stimulating viral DNA replication. In addition, localization of the HSV-1 proteins ICP4, ICP0, and ICP8 was unaffected by ICP27 mutants in this region. These results suggest that the arginine-rich regions are required for efficient nuclear localization and for the regulatory activity of ICP27 involved in viral late gene expression.