Reversible nucleolar translocation of Epstein-Barr virus-encoded EBNA-5 and hsp70 proteins after exposure to heat shock or cell density congestion

APOBEC3H Subcellular Localization Determinants Define Zipcode for Targeting HIV-1 for Restriction

APOBEC enzymes are DNA cytosine deaminases that normally serve as virus restriction factors, but several members, including APOBEC3H, also contribute to cancer mutagenesis. Despite their importance in multiple fields, little is known about cellular processes that regulate these DNA mutating enzymes. We show that APOBEC3H exists in two distinct subcellular compartments, cytoplasm and nucleolus, and that the structural determinants for each mechanism are genetically separable. First, native and fluorescently tagged APOBEC3Hs localize to these two compartments in multiple cell types. Second, a series of genetic, pharmacologic, and cell biological studies demonstrate active cytoplasmic and nucleolar retention mechanisms, whereas nuclear import and export occur through passive diffusion. Third, APOBEC3H cytoplasmic retention determinants relocalize APOBEC3A from a passive cell-wide state to the cytosol and, additionally, endow potent HIV-1 restriction activity. These results indicate that APOBEC3H has a structural zipcode for subcellular localization and selecting viral substrates for restriction.

Prognostic implication of HSPA (HSP70) in breast cancer patients treated with neoadjuvant anthracycline-based chemotherapy

Neoadjuvant chemotherapy is used in patients with locally advanced breast cancer to reduce tumor size before surgery. Unfortunately, resistance to chemotherapy may arise from a variety of mechanisms. Heat shock proteins (HSPs), which are highly expressed in mammary tumor cells, have been implicated in anticancer drug resistance. In spite of the widely described value of HSPs as molecular markers in cancer, their implications in breast tumors treated with anthracycline-based neoadjuvant chemotherapy has been poorly explored. In this study, we have evaluated, by immunohistochemistry, the expression of HSP27 (HSPB1) and HSP70 (HSPA) in serial biopsies from locally advanced breast cancer patients (n = 60) treated with doxorubicin (DOX)- or epirubicin (EPI)-based monochemotherapy. Serial biopsies were taken at days 1, 3, 7, and 21, and compared with prechemotherapy and surgical biopsies. After surgery, the patients received additional chemotherapy with cyclophosphamide, methotrexate, and 5-fluorouracil. High nuclear HSPB1 and HSPA expressions were found in invasive cells after DOX/EPI administration (P < 0.001), but the drug did not affect the cytoplasmic expression of the HSPs. Infiltrating lymphocytes showed high nuclear HSPA (P < 0.01) levels at postchemotherapy. No correlations were found between HSPs expression and the clinical and pathological response to neoadjuvant therapy. However, in postchemotherapy biopsies, high nuclear (>31 % of the cells) and cytoplasmic HSPA expressions (>11 % of the tumor cells) were associated with better DFS (P = 0.0348 and P = 0.0118, respectively). We conclude that HSPA expression may be a useful prognostic marker in breast cancer patients treated with neoadjuvant DOX/EPI chemotherapy indicating the need to change the administered drugs after surgery for overcoming drug resistance.

HspA1A facilitates DNA repair in human bronchial epithelial cells exposed to Benzo[a]pyrene and interacts with casein kinase 2

Benzo[a]pyrene (BaP) is a ubiquitously distributed environmental pollutant that induces deoxyribonucleic acid (DNA) damage. The inducible heat shock protein (HspA1A) can function as a molecular chaperone; however, its role in DNA repair remains largely unknown. In the present study, human bronchial epithelial cells (16HBE) stably transfected with plasmids carrying HspA1A gene or shRNAs against HspA1A were treated with BaP. DNA damage levels of the cells were evaluated by comet assay. Results suggest that HspA1A could protect cells against DNA damage and facilitate the decrease of DNA damage levels during the first 2 h of DNA repair. DNA repair capacity (DRC) of Benzo(a)pyrene diol epoxide (BPDE)-DNA adducts was evaluated by host cell reactivation assay in the stable 16HBE cells transfected with luciferase reporter vector PCMVluc pretreated with BPDE. Compared with control cells, cells overexpressing HspA1A showed higher DRC (p < 0.01 at 10 μM BPDE and p < 0.05 at 20 μM BPDE, respectively), while knockdown of HspA1A inhibited DNA repair (p < 0.05 at 10 μM BPDE). Moreover, casein kinase 2 (CK2) was shown to interact with HspA1A by mass spectrometry and co-immunoprecipitation assays. The two proteins were co-localized in the cell nucleus and perinuclear region during DNA repair, and were identified by confocal laser scanning microscope. In addition, cells overexpressing HspA1A showed an increased CK2 activity after BaP treatment compared with control cells (p < 0.01). Our results suggest that HspA1A facilitates DNA repair after BaP treatment. HspA1A also interacts with CK2 and enhances the kinase activities of CK2 during DNA repair.

Interaction of Epstein-Barr virus (EBV) with human B-lymphocytes

Epstein-Barr virus, EBV, and humans have a common history that reaches back to our primate ancestors. The virus co-evolved with man and has established a largely harmless and highly complex co-existence. It is carried as silent infection by almost all human adults. A serendipitous discovery established that it is the causative agent of infectious mononucleosis. Still, EBV became known first in 1964, in a rare, geographically prevalent malignant lymphoma of B-cell origin, Burkitt lymphoma BL. Its association with a malignancy prompted intensive studies and its capacity to immortalize B-lymphocytes in vitro was soon demonstrated. Consequently EBV was classified therefore as a potentially tumorigenic virus. Despite of this property however, the virus carrier state itself does not lead to malignancies because the transformed cells are recognized by the immune response. Consequently the EBV induced proliferation of EBV carrying B-lymphocytes is manifested only under immunosuppressive conditions. The expression of EBV encoded genes is regulated by the cell phenotype. The virus genome can be found in malignancies originating from cell types other than the B-lymphocyte. Even in the EBV infected B-cell, the direct transforming capacity is restricted to a defined window of differentiation. A complex interaction between virally encoded proteins and B-cell specific cellular proteins constitute the proliferation inducing program. In this short review we touch upon aspects which are the subject of our present work. We describe the mechanisms of some of the functional interactions between EBV encoded and cellular proteins that determine the phenotype of latently infected B-cells. The growth promoting EBV encoded genes are not expressed in the virus carrying BL cells. Still, EBV seems to contribute to the etiology of this tumor by modifying events that influence cell survival and proliferation. We describe a possible growth promoting mechanism in the genesis of Burkitt lymphoma that depends on the presence of EBV.

Involvement of the nucleolus in replication of human viruses

Viruses are intracellular pathogens that have to usurp some of the cellular machineries to provide an optimal environment for their own replication. An increasing number of reports reveal that many viruses induce modifications of nuclear substructures including nucleoli, whether they replicate or not in the nucleus of infected cells. Indeed, during infection of cells with various types of human viruses, nucleoli undergo important morphological modifications. A large number of viral components traffic to and from the nucleolus where they interact with different cellular and/or viral factors, numerous host nucleolar proteins are redistributed in other cell compartments or are modified and some cellular proteins are delocalised in the nucleolus of infected cells. Well‐documented studies have established that several of these nucleolar modifications play a role in some steps of the viral cycle, and also in fundamental cellular pathways. The nucleolus itself is the place where several essential steps of the viral cycle take place. In other cases, viruses divert host nucleolar proteins from their known functions in order to exert new unexpected role(s). Copyright © 2009 John Wiley & Sons, Ltd.

Correlations and co-localizations of Hsp70 with XPA, XPG in human bronchial epithelia cells exposed to benzo[a]pyrene

Benzo[a]pyrene (BaP) is a ubiquitously distributed environmental pollutant known to cause DNA damage, which may be repaired through nucleotide excision repair (NER). The significantly negative correlation between Hsp70 levels and the level of DNA damage in workers exposed to coke oven emission had been found. However, little is known about how Hsp70 modulate the DNA repair process. In a series of experiments using the human bronchial epithelia cells (16HBE) exposed to different concentrations of BaP for 24h, we measured expression of NER subunit xeroderma pigmentosum (XP) group A, C, F, G (XPA, XPC, XPF, XPG), excision repair cross-complementing 1 (ERCC1) and Hsp70, and analyzed their possible correlations. Co-localizations of Hsp70 with NER subunit were detected by confocal microscope. We found that in vitro exposure to BaP reduced cell viability in a dose-dependent manner ranging from 2 to 64 microM. Our results showed that levels of XPA, XPG and Hsp70 significantly increased at cells exposed to 1 or 2muM BaP. In addition, curve estimation showed there was a significant correlation between relative ratios of Hsp70 and XPA, XPG in cells exposed to different concentrations of BaP. Moreover, confocal microscopy demonstrated increased co-localization of Hsp70 with XPA, XPG in nuclei of cells exposed to BaP. These results suggested that Hsp70 might play a role in nucleotide excision repair. However, the mechanisms underlying this observation need further investigation.

Characterization of the nuclear and nucleolar localization signals of bovine herpesvirus-1 infected cell protein 27

Bovine herpesvirus-1 infected cell protein 27 (BICP27) was detected predominantly in the nucleolus. The open reading frame of BICP27 was fused with the enhanced yellow fluorescent protein (EYFP) gene to investigate its subcellular localization in live cells and BICP27 was able to direct monomeric, dimeric or trimeric EYFP exclusively to the nucleolus. By constructing a series of deletion mutants, the putative nuclear localization signal (NLS) and nucleolar localization signal (NoLS) were mapped to (81)RRAR(84) and (86)RPRRPRRRPRRR(97) respectively. Specific deletion of the putative NLS, NoLS or both abrogated nuclear localization, nucleolar localization or both respectively. Furthermore, NLS was able to direct trimeric EYFP predominantly to the nucleus but excluded from the nucleolus, whereas NoLS targeted trimeric EYFP primarily to the nucleus, and enriched in the nucleolus with faint staining in the cytoplasm. NLS+NoLS directed trimeric EYFP predominantly to the nucleolus with faint staining in the nucleus. Moreover, deletion of NLS+NoLS abolished the transactivating activity of BICP27 on gC promoter, whereas deletion of either NLS or NoLS did not. The study demonstrated that BICP27 is a nucleolar protein, adding BICP27 to the growing list of transactivators which localize to the nucleolus.

PRIMA-1MET induces nucleolar translocation of Epstein-Barr virus-encoded EBNA-5 protein

The low molecular weight compound, PRIMA-1^MET restores the transcriptional transactivation function of certain p53 mutants in tumor cells. We have previously shown that PRIMA-1^MET induces nucleolar translocation of p53, PML, CBP and Hsp70. The Epstein-Barr virus encoded, latency associated antigen EBNA-5 (also known as EBNA-LP) is required for the efficient transformation of human B lymphocytes by EBV. EBNA-5 associates with p53-hMDM2-p14ARF complexes. EBNA-5 is a nuclear protein that translocates to the nucleolus upon heat shock or inhibition of proteasomes along with p53, hMDM2, Hsp70, PML and proteasome subunits. Here we show that PRIMA-1^MET induces the nucleolar translocation of EBNA-5 in EBV transformed B lymphoblasts and in transfected tumor cells. The PRIMA-1^MET induced translocation of EBNA-5 is not dependent on the presence of mutant p53. It also occurs in p53 null cells or in cells that express wild type p53. Both the native and the EGFP or DSRed conjugated EBNA-5 respond to PRIMA-1^MET treatment in the same way. Image analysis of DSRed-EBNA-5 expressing cells, using confocal fluorescence time-lapse microscopy showed that the nucleolar translocation requires several hours to complete. FRAP (fluorescence recovery after photobleaching) and FLIP (fluorescence loss in photobleaching) measurements on live cells showed that the nucleolar translocation was accompanied by the formation of EBNA-5 aggregates. The process is reversible since the aggregates are dissolved upon removal of PRIMA-1^MET. Our results suggest that mutant p53 is not the sole target of PRIMA-1^MET. We propose that PRIMA-1^MET may reversibly inhibit cellular chaperons that prevent the aggregation of misfolded proteins, and that EBNA-5 may serve as a surrogate drug target for elucidating the precise molecular action of PRIMA-1^MET.

M148R and M149R are two virulence factors for myxoma virus pathogenesis in the European rabbit

Myxoma virus (MYXV), a member of the Poxviridae family, is the agent responsible for myxomatosis, a fatal disease in the European rabbit (Oryctolagus cuniculus). MYXV has a linear double-stranded DNA genome that encodes several factors important for evasion from the host immune system. Among them, four ankyrin (ANK) repeat proteins were identified: M148R, M149R, M150R and M-T5. To date, only M150R and M-T5 were studied and characterized as critical virulence factors. This article presents the first characterization of M148R and M149R. Green Fluorescent Protein (GFP) fusions allowed us to localize them in a viral context. Whereas M149R is only cytoplasmic, interestingly, M148R is in part located in the nucleolus, a unique feature for an ANK repeat poxviral protein. In order to evaluate their implication in viral pathogenicity, targeted M148R, M149R, or both deletions were constructed in the wild type T1 strain of myxoma virus. In vitro infection of rabbit and primate cultured cells as well as primary rabbit cells allowed us to conclude that M148R and M149R are not likely to be implicated in cell tropism or host range functions. However, in vivo experiments revealed that they are virulence factors since after infection of European rabbits with mutant viruses, a delay in the onset of clinical signs, an increase of survival time and a dramatic decrease in mortality rate were observed. Moreover, histological analysis suggests that M148R plays a role in the subversion of host inflammatory response by MYXV.

Identification of intracellular proteins associated with the EBV-encoded nuclear antigen 5 using an efficient TAP procedure and FT-ICR mass spectrometry

Epstein-Barr virus nuclear antigen 5 (EBNA5) is one of the first viral proteins detected after primary EBV infection and has been shown to be required for efficient transformation of B lymphocytes. EBNA5 is a protein that has many suggested functions but the underlying biology remains to be clarified. To gain further insight into the biological roles of the proposed multifunctional EBNA5, we isolated EBNA5 containing protein complexes using a modified tandem affinity purification (TAP) method and identified the protein components by LC-MS/MS analysis of tryptic digests on a LTQ-FT-ICR mass spectrometer. The modified TAP tag contained a Protein A domain and a StrepTagII sequence separated by two Tobacco Etch Virus protease cleavage sites and was fused to the C-terminus of EBNA5. Our results confirmed the wide applicability of this two-step affinity purification strategy for purification of protein complexes in mammalian cells. A total of 147 novel putative EBNA5 interaction partners were identified, 37 of which were validated with LC-MS/MS in split-tag experiments or in co-immuno precipitates from HEK293 cell extracts. This subgroup included the Bcl2-associated Athanogene 2 (BAG2) co-chaperone involved in protein folding and renaturation, the 26S proteasome subunit 2 involved in regulation of ubiquitin/proteasome protein degradation, and the heterogeneous ribonucleoprotein M (hnRNP M) involved in pre-mRNA processing. These EBNA5 interactors were further verified by co-immunoprecipitations from cell extracts of three EBV-positive lymphoblastoid lines. The combination of the Hsp70, Hsc70, BAG2 and 26S proteasome subunit 2 interactors suggests that EBNA5 might have a functional relationship with protein quality control systems that recognize proteins with abnormal structures and either refold them to normal conformation or target them for degradation. Our study also confirms previously identified interactors including HA95, Hsp70, Hsc70, Hsp27, HAX-1, Prolyl 4-hydroxylase, S3a, and alpha- and beta-tubulin.

Hsp72 up-regulates Epstein-Barr virus EBNALP coactivation with EBNA2

The Epstein-Barr virus (EBV) transcriptional coactivator EBNALP specifically associates and colocalizes with Hsp72 in lymphoblastoid cell lines. We now find that overexpression of Hsp72 more than doubled EBNALP coactivation with EBNA2 of a transfected EBV LMP1 promoter in B lymphoblasts, did not affect EBNA2 or EBNALP protein levels, and strongly up-regulated EBNA2 and EBNALP coactivation of LMP1 protein expression from the endogenous EBV genome in latency I infected Akata cells. The Hsp72 ATP, protein binding, and the C-terminal regulatory domains were required for full activity. An EBNALP deletion mutant, EBNALPd45, which does not associate with Hsp72, coactivated with EBNA2, but was not affected by Hsp72 overexpression, despite Hsp72 up-regulation of wild-type EBNALP coactivation with EBNA2 at all levels of EBNALP expression, indicating the importance of Hsp72 association with EBNALP for Hsp72 up-regulation of coactivation. Of importance, a 90% RNAi knockdown of Hsp72 reduced EBNALP coactivation with EBNA2 of transfected EBV LMP1 and Cp promoters by approximately 50%. Overexpression of the Hsp72 C-terminal interacting and regulatory protein, CHIP, strongly down-regulated EBNALP coactivation, independently of CHIP ubiquitin ligase activity. CHIP effects were Hsp72 dependent, indicating a background downmodulating role for CHIP in Hsp72 augmentation of EBNA2 and EBNALP coactivation. Based on these and other cited data, we favor a model in which Hsp72 chaperones EBNALP shuttling of repressors from EBNA2-enhanced promoters.

Overexpressed heat shock protein 70 protects cells against DNA damage caused by ultraviolet C in a dose-dependent manner

Heat shock protein 70 (Hsp70) comprises proteins that have been reported to protect cells, tissues, and organisms against damage from a wide variety of stressful stimuli; however, little is known about whether Hsp70 protects against DNA damage. In this study, we investigated the relationship between Hsp70 expression and the levels of ultraviolet C (UVC)-induced DNA damage in A549 cells with normal, inhibited, and overexpressed Hsp70 levels. Hsp70 expression was inhibited by treatment with quercetin or overexpressed by transfection of plasmids harboring the hsp70 gene. The level of DNA damage was assessed by the comet assay. The results showed that the levels of DNA damage (shown as the percentage of comet cells) in A549 cells increased in all cells after exposure to an incident dose of 0, 10, 20, 40, and 80 J/m2 whether Hsp70 was inhibited or overexpressed. This response was dose dependent: a protection against UVC-induced DNA damage in cells with overexpressed Hsp70 was observed at UVC dose 20 J/m2 with a maximum at 40 J/m2 when compared with cells with normal Hsp70 levels and in quercetin-treated cells. This differential protection disappeared at 80 J/m2. These results suggest that overexpressed Hsp70 might play a role in protecting A549 cells from DNA damage caused by UVC irradiation, with a threshold of protection from at UVC irradiation-induced DNA damage by Hsp70. The detailed mechanism how Hsp70 is involved in DNA damage and possible DNA repair warrants further investigation.

CK2 phosphorylation weakens 90 kDa MFP1 association to the nuclear matrix in Allium cepa

MFP1 is a conserved plant coiled-coil protein located on the stroma side of the chloroplast thylakoids, as well as in the nuclear matrix. It displays species-specific variability in the number of genes, proteins, and expression. Allium cepa has two nuclear proteins antigenically related to MFP1 with different M(r), pI, distribution, and expression, but only the 90 kDa MFP1 protein is a nuclear matrix component that associates with both the nucleoskeletal filaments and a new category of nuclear bodies. The 90 kDa AcMFP1 migrates in two-dimensional blots as two sets of spots. The hypo-phosphorylated forms (pI approximately 9.5) are tightly bound to the nuclear matrix, while high ionic strength buffers release the more acidic hyper-phosphorylated ones (pI approximately 8.5), suggesting that the protein is post-translationally modified, and that these modifications control its attachment to the nuclear matrix. Dephosphorylation by exogenous alkaline phosphatase and phosphorylation by exogenous CK2, as well as specific inhibition and stimulation of endogenous CK2 with heparin and spermine and spermidine, respectively, revealed that the protein is an in vitro and in vivo substrate of this enzyme, and that CK2 phosphorylation weakens the strength of its binding to the nuclear matrix. In synchronized cells, the nuclear 90 kDa AcMFP1 phosphorylation levels vary during the cell cycle with a moderate peak in G2. These results provide the first evidence for AcMFP1 in vivo phosphorylation, and open up further research on its nuclear functions.

Development of a monoclonal antibody against Epstein-Barr virus nuclear antigen leader protein (EBNA-LP) that can detect EBNA-LP expressed in P3HR1 cells

A mouse monoclonal antibody, LP4D3, was raised against purified Epstein-Barr virus nuclear antigen leader protein (EBNA-LP) fused to glutathione-S-transferase. The antibody detected endogenous and exogenous EBNA-LP in immunoblotting, immunofluorescence and immunoprecipitation assays, and the epitope of the antibody was mapped in the W2 domain of EBNA-LP. While another monoclonal antibody to EBNA-LP, JF186, which is widely used for analyses of the viral protein, did not react with truncated forms of EBNA-LP expressed in P3HR1 cells, as reported earlier, the LP4D3 antibody did. The LP4D3 antibody will be a useful tool for further studies of EBNA-LP, especially investigations into the phenotypes of mutant EBNA-LP expressed in P3HR1 cells.

Mediation of Epstein-Barr virus EBNA-LP transcriptional coactivation by Sp100

The Epstein-Barr virus (EBV) EBNA-LP protein is important for EBV-mediated B-cell immortalization and is a potent gene-specific coactivator of the viral transcriptional activator, EBNA2. The mechanism(s) by which EBNA-LP functions as a coactivator remains an important question in the biology of EBV-induced B-cell immortalization. In this study, we found that EBNA-LP interacts with the promyelocytic leukemia nuclear body (PML NB)-associated protein Sp100 and displaces Sp100 and heterochromatin protein 1alpha (HP1alpha) from PML NBs. Interaction between EBNA-LP and Sp100 was mediated through conserved region 3 in EBNA-LP and the PML NB targeting domain in Sp100. Overexpression of Sp100 lacking the N-terminal PML NB targeting domain, but not a mutant form of Sp100 lacking the HP1alpha interaction domain, was sufficient to coactivate EBNA2 in a gene-specific manner independent of EBNA-LP. These findings suggest that Sp100 is a major mediator of EBNA-LP coactivation. These studies indicate that modulation of PML NB-associated proteins may be important for establishment of latent viral infections, and also identify a convenient model system to investigate the functions of Sp100.

The Epstein-Barr virus EBNA-LP protein preferentially coactivates EBNA2-mediated stimulation of latent membrane proteins expressed from the viral divergent promoter

The mechanistic contribution of the Epstein-Barr virus (EBV) EBNA-LP protein to B-cell immortalization remains an enigma. However, previous studies have indicated that EBNA-LP may contribute to immortalization by enhancing EBNA2-mediated transcriptional activation of the LMP-1 gene. To gain further insight into the potential role EBNA-LP has in EBV-mediated B-cell immortalization, we asked whether it is a global or gene-specific coactivator of EBNA2 and whether coactivation requires interaction between these proteins. In type I Burkitt's lymphoma cells, we found that EBNA-LP strongly coactivated EBNA2 stimulation of LMP-1 and LMP2B RNAs, which are expressed from the viral divergent promoter. Surprisingly, the viral LMP2A gene and cellular CD21 and Hes-1 genes were induced by EBNA2 but showed no further induction after EBNA-LP coexpression. We also found that EBNA-LP did not stably interact with EBNA2 in coimmunoprecipitation assays, even though the conditions were adequate to observe specific interactions between EBNA2 and its cellular cofactor, CBF1. Colocalization between EBNA2 and EBNA-LP was not detectable in EBV-transformed cell lines or transfected type I Burkitt's cells. Finally, no significant interactions between EBNA2 and EBNA-LP were found with mammalian two-hybrid assays. From this data, we conclude that EBNA-LP is not a global coactivator of EBNA2 targets, but it preferentially coactivates EBNA2 stimulation of the viral divergent promoter. While this may require specific transient interactions between these proteins that only occur in the context of the divergent promoter, our data strongly suggest that EBNA-LP also cooperates with EBNA2 through mechanisms that do not require direct or indirect complex formation between these proteins.

Epstein-Barr virus nuclear antigen leader protein induces expression of thymus- and activation-regulated chemokine in B cells

Epstein-Barr virus (EBV) nuclear antigen leader protein (EBNA-LP) plays a critical role in transformation of primary B lymphocytes to continuously proliferating lymphoblastoid cell lines (LCLs). To identify cellular genes in B cells whose expression is regulated by EBNA-LP, we performed microarray expression profiling on an EBV-negative human B-cell line, BJAB cells, that were transduced by a retroviral vector expressing the EBV EBNA-LP (BJAB-LP cells) and on BJAB cells that were transduced with a control vector (BJAB-vec cells). Microarray analysis led to the identification of a cellular gene encoding the CC chemokine TARC as a novel target gene that was induced by EBNA-LP. The levels of TARC mRNA expression and TARC secretion were significantly up-regulated in BJAB-LP compared with BJAB-vec cells. Induction of TARC was also observed when a subline of BJAB cells was converted by a recombinant EBV. Among the EBV-infected B-cell lines with the latency III phenotype that were tested, the LCLs especially secreted significantly high levels of TARC. The level of TARC secretion appeared to correlate with the level of full-length EBNA-LP expression. These results indicate that EBV infection induces TARC expression in B cells and that EBNA-LP is one of the viral gene products responsible for the induction.

EBNA3C coactivation with EBNA2 requires a SUMO homology domain

Epstein-Barr virus (EBV) nuclear antigen 3C (EBNA3C) is critical for EBV immortalization of infected B lymphocytes and can coactivate the EBV LMP1 promoter with EBNA2. EBNA3C amino acids 365 to 545 are necessary and sufficient for coactivation and are required for SUMO-1 and SUMO-3 interaction. We found that EBNA3C but not EBNA3CDelta343-545 colocalized with SUMO-1 in nuclear bodies and was modified by SUMO-2, SUMO-3, and SUMO-1. EBNA3C amino acids 545 to 628 and amino acids 30 to 365 were also required for EBNA3C sumolation and nuclear body localization but were dispensable for coactivation, indicating that EBNA3C sumolation is not required for coactivation. Furthermore, EBNA3C amino acids 476 to 992 potently coactivated with EBNA2 but EBNA3C amino acids 516 to 922 lacked activity, indicating that amino acids 476 to 515 are critical for coactivation. EBNA3C amino acids 476 to 515 include DDDVIEV(507-513), which are similar to SUMO-1 EEDVIEV(84-90). EBNA3C m1 and m2 point mutations, DDD(507-509) mutated to AAA and DVIEVID(509-513) mutated to AVIAVIA, respectively, diminished SUMO-1 and SUMO-3 interaction in directed yeast two-hybrid and glutathione S-transferase pulldown assays. Furthermore, EBNA3C m1 and m2 did not coactivate the LMP1 promoter with EBNA2. Overexpression of wild-type SUMO-1, SUMO-3, and the SUMO-conjugating enzyme UBC9 coactivated the LMP1 promoter with EBNA2. Since EBNA2 activation is dependent on p300/CBP, the possible effect of EBNA3C on p300-mediated transcription was assayed. EBNA3C potentiated transcription of p300 fused to a heterologous DNA binding domain, whereas EBNA3C m1 and m2 did not. All of these data are consistent with a model in which EBNA3C upregulates EBNA2-mediated gene activation by binding to a sumolated repressor and inhibiting repressive effects on p300/CBP and other transcription factor(s) at EBNA2-regulated promoters.

Identification of protein kinases responsible for phosphorylation of Epstein-Barr virus nuclear antigen leader protein at serine-35, which regulates its coactivator function

Epstein-Barr virus (EBV) nuclear antigen leader protein (EBNA-LP) is a phosphoprotein suggested to play important roles in EBV-induced immortalization. Earlier studies have shown that the major site of phosphorylation of EBNA-LP by cellular kinase(s) is a serine residue at position 35 (Ser-35) and that the phosphorylation of Ser-35 is critical for regulation of the coactivator function of EBNA-LP (Yokoyama et al., J Virol 75, 5119-5128, 2001). In the present study, we have attempted to identify protein kinase(s) responsible for the phosphorylation of EBNA-LP at Ser-35. A purified chimeric protein consisting of glutathione S-transferase (GST) fused to a domain of EBNA-LP containing Ser-35 was found to be specifically phosphorylated by purified cdc2 in vitro, while GST fused to a mutated domain of EBNA-LP in which Ser-35 was replaced with alanine was not. In addition, overexpression of cdc2 in mammalian cells caused a significant increase in the phosphorylation of EBNA-LP, while this increased phosphorylation was eliminated if Ser-35 of EBNA-LP was replaced with alanine. These results indicate that the cellular protein kinase cdc2 mediates the phosphorylation of EBNA-LP at Ser-35. Recently, we reported that cdc2 and conserved protein kinases encoded by herpesviruses phosphorylate the same amino acid residue of target proteins (Kawaguchi et al., J Virol 77, 2359-2368, 2003). Consistent with this, the EBV-encoded conserved protein kinase BGLF4 specifically mediated the phosphorylation of EBNA-LP at Ser-35. These results indicate that the coactivator function of EBNA-LP can be regulated by the activity of these cellular and viral protein kinases.

EBV-encoded EBNA-5 associates with P14ARF in extranucleolar inclusions and prolongs the survival of P14ARF-expressing cells

Epstein-Barr virus (EBV) carrying lymphoblastoid cells of normal origin express the full program of all 9 virus-encoded, growth transformation associated proteins. They have an intact p53 pathway as a rule. This raises the question of whether any of the viral proteins impair the pathway functionally. Using a yeast 2-hybrid system, we have shown that EBNA-5 but not the other EBNAs interacts with the p14ARF protein, a regulator of the p53 pathway. The interaction was confirmed in vitro using a GST pull-down assay. Moreover, expression of EBNA-5 increased the survival of p14ARF-transfected cells. EBV infection of resting B cells induced the expression of p14ARF mRNA without increased level of the protein. A fraction of the p14ARF localized to the nucleoli but the bulk of the protein accumulated in nuclear but extranucleolar inclusions. Formation of the extranucleolar inclusions led to complete relocalization of EBNA-5 from nucleoplasm to these structures. The inclusions also contained p53 and HDM2, and were surrounded by PML bodies and proteasomes, which suggests that these inclusions could be targets for proteasome dependent protein degradation.

Epstein-Barr virus (EBV) nuclear antigen leader protein (EBNA-LP) forms complexes with a cellular anti-apoptosis protein Bcl-2 or its EBV counterpart BHRF1 through HS1-associated protein X-1

Epstein-Barr virus (EBV) nuclear antigen leader protein (EBNA-LP) plays a critical role in EBV-induced transformation. An earlier report (Y. Kawaguchi et al., J. Virol. 74: 10104-10111, 2000) showed that EBNA-LP interacts with a cellular protein HS1-associated protein X-1 (HAX-1). The predicted amino acid sequence of HAX-1 exhibits similarity to that of another cellular protein Nip3 which has been shown to interact with cellular and viral anti-apoptotic proteins such as Bcl-2 and BHRF1, an EBV homolog of Bcl-2. Here we investigated whether HAX-1, like Nip3, interacts with Bcl-2 proteins and report the following. (i) A purified chimeric protein consisting of gluthathione S-transferase (GST) fused to BHRF1 (GST-BHRF1) or Bcl-2 (GST-Bcl-2) specifically pulled down HAX-1 transiently expressed in COS-7 cells. (ii) GST-BHRF1 or GST-Bcl-2 was not able to pull down EBNA-LP transiently expressed in COS-7 cells, whereas each of the GST fusion proteins formed complexes with EBNA-LP in the presence of RAX-1. These results indicated that EBNA-LP interacts with the viral and cellular Bcl-2 proteins through HAX-1, suggesting that EBNA-LP possesses a potential function in the regulation of apoptosis in EBV-infected cells.

Physical interaction of Epstein-Barr virus (EBV) nuclear antigen leader protein (EBNA-LP) with human oestrogen-related receptor 1 (hERR1): hERR1 interacts with a conserved domain of EBNA-LP that is critical for EBV-induced B-cell immortalization

Epstein-Barr virus (EBV) nuclear antigen leader protein (EBNA-LP) consists of W1W2 repeats and a unique C-terminal Y1Y2 domain and plays a critical role in EBV-induced transformation. To identify the cellular proteins associating with EBNA-LP, we performed a yeast two-hybrid screen using EBNA-LP cDNA containing a single W1W2 domain as bait and an EBV-transformed human peripheral blood lymphocyte cDNA library as the source of cellular genes. Our results were as follows. (i) A cDNA in the positive yeast colony was found to encode a cellular protein, human oestrogen-related receptor 1 (hERR1), which is a constitutive transcriptional activator of the various types of oestrogen response elements. (ii) A purified chimeric protein consisting of glutathione S-transferase (GST) fused to hERR1 specifically formed complexes with EBNA-LPs containing one (EBNA-LPR1), two (EBNA-LPR2) or four W1W2 repeats (EBNA-LPR4) transiently expressed in COS-7 cells. Reciprocally, GST fused to EBNA-LPR1 or EBNA-LPR2 pulled down hERR1 transiently expressed in COS-7 cells. (iii) Mutational analyses of EBNA-LP revealed that the Y2 domain of EBNA-LP is responsible for the interaction with hERR1 and two leucines in the Y2 domain (Leu-78 and -82), which are conserved among a subset of primate gammaherpesviruses, are interactive sites for hERR1. So far, it has been reported that the only domain of EBNA-LP critical for EBV-induced transformation is the Y1Y2 domain. Potential roles of hERR1 in EBV-induced transformation are discussed.

Epstein-Barr virus nuclear antigen 5 inhibits pre-mRNA cleavage and polyadenylation

The long-standing suspicion that Epstein-Barr virus nuclear antigen 5 (EBNA5) is involved in transcription regulation was recently confirmed by the observation by several groups that EBNA5 cooperates with EBNA2 in activation of the LMP1 promoter. In attempts to elucidate the molecular basis for the EBNA5-mediated enhancement of EBNA2 transactivation, we obtained evidence of an additional function of EBNA5: at high but still biologically relevant levels, EBNA5 acted as a repressor of gene expression by interfering with the processing of pre-mRNA. Transient transfections with reporter plasmids revealed that EBNA5 repressed reporter mRNA and protein expression in the cytoplasm, but did not lower the steady-state level of reporter RNA in the total cellular RNA fraction. We have excluded that repression occurred as a consequence of cell death induced by EBNA5. Using the RNase protection assay with a probe comprising the pre-mRNA cleavage and polyadenylation site, EBNA5 was found to inhibit 3'-end cleavage and polyadenylation of pre-mRNAs from the reporter plasmids investigated. The effect of inhibitory levels of EBNA5 on chromosomal genes was examined in transient transfections by expression profiling using a cDNA microarray panel containing 588 genes. The results showed that EBNA5 could also inhibit the expression of chromosomal genes and did it in a discriminatory manner. This is consistent with the notion that a regulatory mechanism exists in the cell that confers specificity to the selection by EBNA5 of target genes for repression.

Conserved region CR2 of Epstein-Barr virus nuclear antigen leader protein is a multifunctional domain that mediates self-association as well as nuclear localization and nuclear matrix association

Self-association of viral proteins is important for many of their functions, including enzymatic, transcriptional, and transformational activities. Epstein-Barr virus (EBV) nuclear antigen leader protein (EBNA-LP) contains various numbers of W1W2 repeats and a unique carboxyl-terminal Y1Y2 domain. It was reported that EBNA-LP associates with a variety of cellular proteins and plays a critical role in EBV-induced transformation. We report here that EBNA-LP self-associates in vivo and the domain responsible for the homotypic association is a multifunctional domain mediating nuclear localization, nuclear matrix association, and EBNA-2-dependent coactivator function of the protein. Our conclusions are based on the following observations. (i) EBNA-LP interacts with itself or its derivatives in the yeast two-hybrid system. (ii) A purified chimeric protein consisting of glutathione S-transferase fused to EBNA-LP specifically formed complexes with EBNA-LP transiently expressed in COS-7 cells. (iii) When Flag epitope-tagged EBNA-LP with either one or two W1W2 repeats and EBNA-LP containing four W1W2 repeats were coexpressed in COS-7 cells, the latter was specifically coimmunoprecipitated with the former. (iv) Mutational analyses of EBNA-LP with deletion mutants revealed that the region between codons 19 and 39 (relative to the first amino acid residue of the W2 domain) is essential for self-association of the protein. The mapped region almost completely overlaps with CR2 and CR3, regions conserved among a subset of primate gamma-herpesviruses and critical for EBNA-2-dependent coactivator function. Amino acid substitutions in CR2 alone abolished the ability of the protein to self-interact. This laboratory previously reported that CR2 is also responsible for nuclear localization and nuclear matrix association (A. Yokoyama, Y. Kawaguchi, I. Kitabayashi, M. Ohki, and K. Hirai, Virology 279:401-413, 2001). (v) Sucrose gradient sedimentation showed that amino acid substitutions in CR2 reduced the ability of the protein to form protein complexes in B cells. These results suggest that self-association of EBNA-LP may be important for its various functions and interactions of the protein with multiple cellular proteins.

Genetic analysis of the Epstein-Barr virus-coded leader protein EBNA-LP as a co-activator of EBNA2 function

Co-operation between the Epstein-Barr virus (EBV)-coded leader protein EBNA-LP and the nuclear antigen EBNA2 appears to be critical for efficient virus-induced B cell transformation. Here we report the genetic analysis of EBNA-LP function using two transient co-transfection assays of co-operativity, activation of latent membrane protein 1 (LMP1) expression from a resident EBV genome in Akata-BL cells and activation of an EBNA2-responsive reporter construct. Small deletions were introduced into each of five conserved regions (CRs) of EBNA-LP sequence present in type 1 and type 2 EBV strains and in several primate lymphocryptovirus EBNA-LP homologues. Deletions within all three CRs in the EBNA-LP W1W2 repeat domain completely abrogated function, through inhibition of nuclear localization in the cases of CR1 and CR2 but not of CR3; deletions within CR4 and CR5 in the Y1Y2 unique domain had relatively little effect, yet loss of the whole Y2 sequence blocked activity. Alanine substitution of serine residues within potential phosphorylation sites identified two mutants of particular interest. Substitution of three such residues (S34,36,63) within W1W2 not only abrogated EBNA-LP activity but was associated with a complete loss of EBNA2 detectability in co-transfected cells, implying possible destabilization of the co-expressed EBNA2 protein. More importantly the individual substitution of S36 completely blocked EBNA-LP/EBNA2 co-operativity while retaining EBNA2 expression. We infer critical roles for the CR3 domain and for the S36 residue in EBNA-LP's co-operative function.

Intranuclear localization of the transcription coadaptor CBP/p300 and the transcription factor RBP-Jk in relation to EBNA-2 and -5 in B lymphocytes

We have studied the expression and the localization of the cellular proteins CBP/p300 and RBP-Jk in in vitro EBV-infected human B lymphocytes in relation to the EBNA-2 and EBNA-5 proteins. We found that the level of CBP/p300 was elevated drastically by EBV infection and also after activation by CD40 ligation. Thus the increase in CBP/p300 expression in the EBV-infected cells is related to the virus-induced activation and proliferation of the cells. EBNA-2 and RBP-Jk colocalized in the nucleoplasm, which is in accordance with their functional interaction. We confirmed earlier reports about the presence and colocalization of EBNA-5 and CBP in the nuclear POD bodies. On the other hand, neither EBNA-2 nor p300 was detected in the PODs. The expression of these two proteins overlapped in some distinct dots of the nucleoplasm. Taken together, the different patterns of CBP and p300 expression and their different localization in relation to the PML bodies and two EBV-encoded proteins in the B cells may provide some clue to their distinct functional roles.

Epstein-Barr virus nuclear antigen 5 interacts with HAX-1, a possible component of the B-cell receptor signalling pathway

Using a yeast two-hybrid screen of a B-cell cDNA library with an Epstein-Barr nuclear antigen 5 (EBNA5) molecule containing seven repeats of the W(1)W(2) domain as bait, we have isolated the EBNA5-interacting protein HAX-1. HAX-1 has previously been shown to associate with HS1, a protein specifically expressed in cells of the haematopoietic lineage, and is thought to be involved in signal transduction in B-cells. Immunofluorescence experiments showed that HAX-1 co-localized with the hsp60 protein that is associated with the mitochondria in the cell cytoplasm. Pull down experiments with a fusion protein between glutathione S-transferase and the seven copy repeat EBNA5 synthesized in bacteria and in yeast cells confirmed that HAX-1 can interact with EBNA5 in vitro. Conventionally, EBNA5 is regarded as a nuclear protein. However, we show here that the smallest EBNA5 species, composed of the unique Y domain and only one copy of the W(1)W(2) repeat domain, like HAX-1, co-localizes with the mitochondrial hsp60 protein in the B-cell cytoplasm. Furthermore, immunoprecipitation experiments demonstrate that the single repeat EBNA5 associates with HAX-1 in transfected B-lymphoblastoid cells.

Identification of major phosphorylation sites of Epstein-Barr virus nuclear antigen leader protein (EBNA-LP): ability of EBNA-LP to induce latent membrane protein 1 cooperatively with EBNA-2 is regulated by phosphorylation

Epstein-Barr virus (EBV) nuclear antigen leader protein (EBNA-LP) is a phosphoprotein suggested to play important roles in EBV-induced immortalization of B cells. One of the potential functions of EBNA-LP is a cooperative induction with EBNA-2 of viral and cellular gene expression, including that of the genes for viral latent membrane protein 1 (LMP-1) and cellular cyclin D2. We report here that the phosphorylation of EBNA-LP by cellular kinase(s) is critical to its ability to cooperate with EBNA-2 in up-regulating the expression of LMP-1 in a B-lymphoma cell line. Our conclusion is based on the following observations. (i) Mass-spectrometric analysis of purified EBNA-LP and mutational analyses of EBNA-LP revealed that the serine residue at position 35 in the W2 repeat domain is the major phosphorylation site of EBNA-LP in vivo. (ii) Substitutions of this site in each W2 repeat domain with alanine markedly reduced the ability of the protein to induce LMP-1 expression in combination with EBNA-2 in Akata cells. (iii) Replacement at the major phosphorylation sites with glutamic acids restored the wild-type phenotype. It is well established that this substitution mimics constitutive phosphorylation. These results indicated that the coactivator function of EBNA-LP is regulated by phosphorylation.

Proteasome inhibitor induces nucleolar translocation of Epstein-Barr virus-encoded EBNA-5

We have previously shown that Epstein-Barr virus (EBV)-encoded EBNA-5 is localized to PML bodies (PODs) in EBV-immortalized lymphoblastoid cell lines (LCLs). Here we have extended our study of the subnuclear localization of EBNA-5 and found a strict co-localization with PML in LCLs and in BL lines with an immunoblastic, LCL-like phenotype. Moreover, GFP-EBNA-5 accumulated in PML bodies upon transfection into LCLs. In contrast, transfection of cell lines of non-immunoblastic origin with an EBNA-5 expression construct showed preferential localization of the protein to the nucleoplasm. Since PML is involved in proteasome-dependent protein degradation, we investigated the total levels and sub-cellular localization of EBNA-5 upon inhibition of proteasome activity. We found that a proteasome inhibitor, MG132, induced the translocation of both endogenous and transfected EBNA-5 to the nucleoli in every cell line tested. The total EBNA-5 protein levels were not affected by the proteasomal block. EBNA-5 forms complexes with heat shock protein Hsp70. The proteasome inhibitor induced a rise in total levels of Hsp70 and dramatically changed its homogeneous nuclear and cytoplasmic distribution into nucleolar and cytoplasmic. This effect was EBNA-5-independent. The nucleolar localization of Hsp70 was enhanced by the presence of EBNA-5, however. EBNA-5 also enhanced the nucleolar translocation of a mutant p53 in a colon cancer line, SW480, treated with MG132. The coordinated changes in EBNA-5 and Hsp70 localization and the effect of EBNA-5 on mutant p53 distribution upon MG132 treatment might reflect the involvement of EBNA-5 in the regulation of intracellular protein trafficking associated with the proteasome-mediated degradation.

Proteins associated with the promyelocytic leukemia gene product (PML)-containing nuclear body move to the nucleolus upon inhibition of proteasome-dependent protein degradation

Several recent findings have indicated that the promyelocytic leukemia gene product (PML) oncogenic domains (PODs) are involved in proteasome-mediated degradation of ubiquitinated proteins. We wanted to examine the intracellular distribution of PML protein in the presence of a proteasome inhibitor. We used high-resolution microscopy to study the distribution of PML protein and other POD-associated proteins along with the proteasomes themselves under normal conditions and in cells treated with the proteasome inhibitor, MG132. Inhibition of the proteasomes in MCF-7, HeLa, and IB-4 cell lines resulted in a radical redistribution of the POD-associated proteins PML, Sp100, and SUMO-1. After 6-10 h of MG132 treatment, PML, Sp100, and SUMO-1 were no longer detectable in the PODs and accumulated mainly in the nucleolus. Moreover, MG132 treatment changed the cellular distribution of the proteasomes. Interestingly, this included the accumulation in euchromatin areas of the nucleus and within the nucleoli. Several non-POD-associated proteins did not change their cellular distribution under the same conditions. The accumulation of POD-associated proteins and proteasomes in the nucleoli of MG132-treated cells indicates that these proteins may target the nucleoli under normal conditions and that the nucleolus may have a function in the regulation of proteasomal protein degradation.

Proteins associated with the promyelocytic leukemia gene product (PML)-containing nuclear body move to the nucleolus upon inhibition of proteasome-dependent protein degradation

Several recent findings have indicated that the promyelocytic leukemia gene product (PML) oncogenic domains (PODs) are involved in proteasome-mediated degradation of ubiquitinated proteins. We wanted to examine the intracellular distribution of PML protein in the presence of a proteasome inhibitor. We used high-resolution microscopy to study the distribution of PML protein and other POD-associated proteins along with the proteasomes themselves under normal conditions and in cells treated with the proteasome inhibitor, MG132. Inhibition of the proteasomes in MCF-7, HeLa, and IB-4 cell lines resulted in a radical redistribution of the POD-associated proteins PML, Sp100, and SUMO-1. After 6-10 h of MG132 treatment, PML, Sp100, and SUMO-1 were no longer detectable in the PODs and accumulated mainly in the nucleolus. Moreover, MG132 treatment changed the cellular distribution of the proteasomes. Interestingly, this included the accumulation in euchromatin areas of the nucleus and within the nucleoli. Several non-POD-associated proteins did not change their cellular distribution under the same conditions. The accumulation of POD-associated proteins and proteasomes in the nucleoli of MG132-treated cells indicates that these proteins may target the nucleoli under normal conditions and that the nucleolus may have a function in the regulation of proteasomal protein degradation.

The conserved domain CR2 of Epstein-Barr virus nuclear antigen leader protein is responsible not only for nuclear matrix association but also for nuclear localization

There is a growing body of evidence for the importance of the nuclear matrix in various nuclear events including gene expression and DNA replication. Epstein-Barr virus (EBV) nuclear antigen leader protein (EBNA-LP) is a nuclear matrix-associated protein that has been suggested to play an important role in EBV-induced transformation. To define the biological significance of the association of EBNA-LP with the nuclear matrix, we mapped the domain of EBNA-LP responsible for nuclear matrix association and investigated the functions of the EBNA-LP mutant mutagenized by substitution of alanines for the cluster of arginine residues in the mapped region. The results of the present study were as follows. (i) Transiently expressed EBNA-LP in COS-7 or BOSC23 cells was associated with the nuclear matrix, similarly to that in EBV-infected B cells. (ii) Mutational analysis of EBNA-LP revealed that a 10-amino acid segment of EBNA-LP is critical for nuclear matrix association of the protein. Interestingly, the identified region overlapped with the region CR2 of EBNA-LP conserved among a subset of primate gammaherpesviruses. The identified segment is referred to as EBNA-LP NMTS (nuclear matrix targeting signal). (iii) The EBNA-LP mutant with the arginine to alanine substitutions in NMTS was no longer localized not only to the nuclear matrix but also to the nucleus. (iv) The EBNA-LP mutant lacked its ability to coactivate EBNA-2-dependent transactivation. These results indicated that EBNA-LP needs to be localized in the nucleus and/or associated with the nuclear matrix through CR2 to elicit its function such as the coactivation of the EBNA-2-dependent transcriptional activation.

Interaction of Epstein-Barr virus nuclear antigen leader protein (EBNA-LP) with HS1-associated protein X-1: implication of cytoplasmic function of EBNA-LP

Epstein-Barr virus (EBV) nuclear antigen leader protein (EBNA-LP) consists of W1W2 repeats and a unique C-terminal Y1Y2 domain and has been suggested to play an important role in EBV-induced transformation. To identify the cellular factors interacting with EBNA-LP, we performed a yeast two-hybrid screen, using EBNA-LP cDNA containing four W1W2 repeats as bait and an EBV-transformed human peripheral blood lymphocyte cDNA library as the source of cellular genes. Our results were as follows. (i) All three cDNAs in positive yeast colonies were found to encode the same cellular protein, HS1-associated protein X-1 (HAX-1), which is localized mainly in the cytoplasm and has been suggested to be involved in the regulation of B-cell signal transduction and apoptosis. (ii) Mutational analysis of EBNA-LP revealed that the association with HAX-1 is mediated by the W1W2 repeat domain. (iii) A purified chimeric protein consisting of glutathione S-transferase fused to EBNA-LP specifically formed complexes with HAX-1 transiently expressed in COS-7 cells. (iv) When EBNA-LP and HAX-1 were coexpressed in COS-7 cells, EBNA-LP was specifically coimmunoprecipitated with HAX-1. (v) Careful cell fractionation experiments of an EBV-infected lymphoblastoid cell line revealed that EBNA-LP is localized in the cytoplasm as well as in the nucleus. (vi) When EBNA-LP containing four W1W2 repeats was expressed in COS-7 cells, EBNA-LP was detected mainly in the nucleus by immunofluorescence assay. Interestingly, when EBNA-LP containing a single W1W2 repeat was expressed in COS-7 cells, EBNA-LP was localized predominantly in the cytoplasm and was colocalized with HAX-1. These results indicate that EBNA-LP is in fact present and may have a significant function in the cytoplasm, possibly by interacting with and affecting the function of HAX-1.

Conserved regions in the Epstein-Barr virus leader protein define distinct domains required for nuclear localization and transcriptional cooperation with EBNA2

Epstein-Barr virus (EBV) EBNA-LP is a latent protein whose function is not fully understood. Recent studies have shown that EBNA-LP may be an important EBNA2 cofactor by enhancing EBNA2 stimulation of the latency C and LMP-1 promoters. To further our understanding of EBNA-LP function, we have introduced a series of mutations into evolutionarily conserved regions and tested the mutant proteins for the ability to enhance EBNA2 stimulation of the latency C and LMP-1 promoters. Three conserved regions (CR1 to CR3) are located in the repeat domains that are essential for the EBNA2 cooperativity function. In addition, three serine residues are also well conserved in the repeat domains. Clustered alanine mutations were introduced into CR1 to CR3, and the conserved serines were also changed to alanine residues in an EBNA-LP with two repeats, which is the minimal protein able to cooperate with EBNA2. Mutations introduced into CR1a had no effect on EBNA-LP function, while mutations introduced into CR1b resulted in EBNA-LP with slightly decreased activity. Mutations in CR1c and CR2 resulted in proteins that no longer localized exclusively to the nucleus and also had no EBNA2 cooperation activity. Mutations introduced into conserved serines S5/71 resulted in proteins with slightly higher activity, while mutations introduced into conserved serines S35/101 or in CR3 (which contains S60/126) resulted in EBNA-LP proteins with substantially reduced activity. The potential karyophilic signals within EBNA-LP CR1c and CR2 were also examined by introducing oligonucleotides encoding these positively charged amino acid groupings into a cytoplasmic test protein, herpes simplex virus DeltaIE175, and by examining the intracellular localization of the resulting proteins. This assay identified a strong nuclear localization signal between EBNA-LP amino acids 43 and 50 (109 to 117 in the second W repeat) comprising CR2, while EBNA-LP amino acids 29 to 36 (91 to 98 in the second W repeat) were unable to function independently as a nuclear localization signal. However, a combination of amino acids 29 to 50 resulted in more efficient nuclear localization than with amino acids 43 to 50 alone. These results indicate that EBNA-LP has a bipartite nuclear localization signal and that efficient nuclear localization is essential for EBNA2 cooperativity function. Interestingly, EBNA-LP with only a single repeat localized exclusively to the cytoplasm, providing an explanation for why this isoform has no activity. In addition, two conserved serine residues which are distinct from nuclear import functions are important for EBNA2 cooperativity function.

Sequence and functional analysis of EBNA-LP and EBNA2 proteins from nonhuman primate lymphocryptoviruses

The Epstein-Barr virus (EBV) EBNA-LP and EBNA2 proteins are the first to be synthesized during establishment of latent infection in B lymphocytes. EBNA2 is a key transcriptional regulator of both viral and cellular gene expression and is essential for EBV-induced immortalization of B lymphocytes. EBNA-LP is also important for EBV-induced immortalization of B lymphocytes, but far less is known about the functional domains and cellular cofactors that mediate EBNA-LP function. While recent studies suggest that serine phosphorylation of EBNA-LP and coactivation of EBNA2-mediated transactivation are important, more detailed mutational and genetic studies are complicated by the repeat regions that comprise the majority of the EBNA-LP sequence. Therefore, we have used a comparative approach by studying the EBNA-LP homologues from baboon and rhesus macaque lymphocryptoviruses (LCVs) (baboon LCV and rhesus LCV). The predicted baboon and rhesus LCV EBNA-LP amino acid sequences are 61 and 64% identical to the EBV EBNA-LP W1 and W2 exons and 51% identical to the EBV EBNA-LP Y1 and Y2 exons. Five evolutionarily conserved regions can be defined, and four of eight potential serine residues are conserved among all three EBNA-LPs. The major internal repeat sequence also revealed a highly conserved Wp EBNA promoter with strong conservation of upstream activating sequences important for Wp transcriptional regulation. To test whether transcriptional coactivating properties were common to the rhesus LCV EBNA-LP, a rhesus LCV EBNA2 homologue was cloned and expressed. The rhesus LCV EBNA2 transcriptionally transactivates EBNA2-responsive promoters through a CBF1-dependent mechanism. The rhesus LCV EBNA-LP was able to further enhance rhesus LCV or EBV EBNA2 transactivation 5- to 12-fold. Thus, there is strong structural and functional conservation among the simian EBNA-LP homologues. Identification of evolutionarily conserved serine residues and regions in EBNA-LP homologues provides important clues for identifying the cellular cofactors and molecular mechanisms mediating these conserved viral functions.

Human herpesvirus-8-encoded LNA-1 accumulates in heterochromatin- associated nuclear bodies

Subnuclear distribution of the human herpesvirus-8 (HHV-8)- encoded nuclear protein LNA-1 was analysed at high resolution in body cavity (BC) lymphoma-derived cell lines, in cell hybrids between BC cells and various human and mouse cells and in freshly infected K562 and ECV cell lines. Three-dimensional reconstruction of nuclei from optical sections and quantitative analysis of the distribution of LNA-1 fluorescence in relation to chromatin showed that LNA-1 associates preferentially with the border of heterochromatin in the interphase nuclei. This was further confirmed in the following systems: in endo- and exonuclease-digested nuclei, in human-mouse (BC-1-Sp2- 0) hybrids and on chromatin spreads. LNA-1 was found to bind to mitotic chromosomes at random. Epstein-Barr virus (EBV), but not HHV-8, was rapidly lost from mouse-human hybrid cells in parallel with the loss of human chromosomes. HHV-8 could persist on the residual mouse background for more than 8 months. In early human-mouse hybrids that contain a single fused nucleus, LNA-1 preferentially associates with human chromatin. After the gradual loss of the human chromosomes, LNA-1 becomes associated with the murine pericentromeric heterochromatin. In human-human hybrids derived from the fusion of the HHV-8-carrying BCBL-1 cells and the EBV-immortalized lymphoblastoid cell line IB4, LNA-1 did not co-localize with EBNA-1, EBNA-2, EBNA-5 or EBNA-6. LNA-1 was not associated with PML containing ND10 bodies either. DNase but not RNase or detergent treatment of isolated nuclei destroys LNA-1 bodies. In advanced apoptotic cells LNA- 1 bodies remain intact but are not included in the apoptotic bodies themselves.

Nucleolar localization of the UL3 protein of herpes simplex virus type 2

A rabbit polyclonal antiserum was raised against a recombinant 6 x His-UL3 fusion protein expressed in Escherichia coli and used to examine the intracellular localization of the UL3 protein of herpes simplex virus type 2 (HSV-2). The antiserum reacted specifically with 31 and 34 kDa proteins in HSV-2 186-infected Vero cells and with 31 and 35 kDa proteins in UL3-expressing COS-7 cells. The UL3 protein localized both in the cytoplasm and in five to ten bright fluorescent granules in the nucleus close to the nuclear membrane at 4 h post-infection (p.i.). These structures became bigger at 5 h p.i. and showed doughnut-like forms at 6 h p.i. In transfected Vero cells, the UL3 protein localized exclusively in the nucleoplasm and specifically in the nucleolus. Five deletion mutants of the UL3 protein were constructed for transfection assays and the results showed that the region containing amino acids 100-164 was important for nucleolar localization. Moreover, green fluorescent protein (GFP)-targetting experiments showed that the region containing amino acids 100-164 was able to transport non-nucleolar GFP to the nucleolus as a fusion protein.

Simultaneous detection of two independent antigens by double staining with two mouse monoclonal antibodies

Simultaneous detection of two antigens by immunostaining usually requires primary antibodies from two different species or a hapten modification of one of the antibodies if they are from the same species. A novel double staining method is described for immunodetection of two independent antigens using two mouse monoclonal antibodies. The principle of the method is the following: The first antigen is detected by a monoclonal antibody that is diluted so extensively that it cannot be recognized with conventional detection systems. A highly sensitive biotin-tyramide amplification system is used to identify this antibody. The second antigen is stained with a monoclonal antibody by dilution and detected by conventional immunostaining. The method was tested for both alkaline-phosphatase staining on paraffin sections and immunofluorescence staining on cultured cells in cytospin preparation. The absence of cross-reaction in the former system was demonstrated by the mutually exclusive detection of T- and B-cells in human lymph nodes or T-cells and carcinoma cells in nasopharyngeal carcinoma biopsies. Similarly, the EBV encoded EBNA2 and ZEBRA proteins showed a mutually exclusive staining by immunofluorescence on B95-8 cells. The method could be used to demonstrate co-expression of two independent antigens in the same cells, such as PCNA and keratin in carcinoma cells in paraffin sections and for EBNA2 and LMP1 EBV proteins in immunofluorescence preparations of B95-8 cells.

Common and emerging infectious causes of hematological malignancies in the young

Comparative epidemiological studies have for a long time suggested a link (or links) between infectious agents and hematological malignancies in the young. Identification of Epstein-Barr virus (EBV) as the major cause of specific subtypes of Burkitt's lymphoma and Hodgkin's disease 20 and 10 years ago, respectively, and the recent involvement of human T-cell leukemia virus in non-Hodgkin's lymphomas of the T-cell lineage in young adults in Jamaica have given further credit to early presumptions that these diseases have an infectious etiology. The spectrum of possibly involved viruses: old, EBV, and new, herpesviruses 6, 7 and 8, and unknown retroviruses - as well as the list of partially or totally unresolved disease entities: Hodgkin's disease in adolescents, non-Hodgkin's lymphomas in the immunocompromised, and acute lymphocytic leukemia - is rapidly expanding. Both direct and indirect transforming effects of the above-mentioned viruses are being rapidly disclosed. However, the complex interaction between the different viruses and other causes of hematological malignancies in the young guarantees that many things remain to be discovered also in the future.

Multiple functions within the Epstein-Barr virus EBNA-3A protein

Two regions of the EBNA-3A protein of Epstein-Barr virus were shown to be capable of binding to the cell protein RBP-Jk (also known as CBF-1), a component of the Notch signaling pathway. Consistent with this binding, EBNA-3A inhibited reporter gene expression from plasmids containing RBP-Jk DNA binding sites within their promoters, including the Cp promoter. When EBNA-3A was linked to a GAL4 DNA binding domain, it repressed the activity of a promoter containing GAL4 binding sites at all plasmid concentrations tested. However, a deletion mutant of EBNA-3A lacking amino acids 100 to 364 showed a biphasic response in the GAL4 assay: it inhibited transcription at low DNA concentrations but activated it at high DNA concentrations. There appears to be a gene activation function within EBNA-3A that is masked in the full-length protein in this assay. Current models for EBNA-3 function have stressed transcription repression through binding to RBP-Jk, but we consider an alternative scheme in which the role of the binding of EBNA-3A, -3B, and -3C to RBP-Jk is to buffer the levels of active EBNA-3 protein. We have also found that the behavior of EBNA-3A in a cell fractionation procedure that distinguishes insoluble matrix from soluble cell fractions is modified by EBNA-LP, indicating a further novel level of interplay between the EBNA proteins.

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.

Specific intranucleolar distribution of Hsp70 during heat shock in polytene cells

Hsp70, the most abundant and conserved heat shock protein, has been described as strongly concentrating in the nucleolus during heat shock. The important metabolic processes that take place in the nucleolus, rDNA transcription, processing, and assembling with ribosomal proteins, and the nucleolar architecture itself are very sensitive to temperature changes. In this work, we have analyzed in detail the nucleolar changes, in structure and activity, induced by temperature in Chironomus thummi salivary gland cells and the fine subnucleolar localization of Hsp70 during heat shock. The optimum temperature chosen to induce the heat shock response was 35 degrees C. Under these conditions transcription of heat shock genes, inactivation of previously active genes and maximum synthesis of Hsps take place, while survival of larvae and recovery were ensured. After 1 h at 35 degrees C, nucleoli change from a uniform control pattern to a segregated pattern of nucleolar components that can be observed even at the light microscopic level. The dense fibrillar component (DFC) and the granular component appeared perfectly differentiated and spatially separated, the former occupying mainly the central inner region surrounded by a rim of granular component. Hsp70 was specifically localized within the DFC upon heat shock as shown by immunolocalization by both light and electron microscopy. Pulse labeling with [3H]uridine proves that rRNA transcription continues during heat shock. The pattern of Hsp70 distribution within the nucleolus correlates with that of newly produced rRNA transcripts. Hsp70 also colocalizes with RNA polymerase I, both being restricted to the DFC. These data show that the DFC seems to be the intranucleolar target for Hsp70 in heat-shocked cells. We discuss these results in relation to the possible function of Hsp70 in the first steps of preribosome synthesis.

Epstein-Barr virus leader protein enhances EBNA-2-mediated transactivation of latent membrane protein 1 expression: a role for the W1W2 repeat domain

The Epstein-Barr virus (EBV)-encoded leader protein EBNA-LP is made up of several 66-amino-acid repeats (the W1W2 domains) linked to a unique 45-amino-acid C-terminal sequence (the Y1Y2 domain). This protein is highly expressed along with a second nuclear antigen, EBNA-2, during the initial stages of virus-induced B-cell transformation. While EBNA-2's essential role in transformation as a transcriptional activatory is well documented, very little is known about EBNA-LP function except that recombinant viruses lacking the EBNA-LP Y1Y2 exons show reduced, but still detectable, transforming ability. This was taken as evidence that EBNA-LP plays an auxiliary role but is not essential for transformation. A recent study showed that EBNA-LP could cooperate with EBNA-2 in activating cyclin D2 transcription in resting B cells (A.J. Sinclair, L Palmero, G. Peters, and P.J. Farrell, EMBO J. 13:3321-3328, 1994). Here we report that EBNA-LP can also cooperate with EBNA-2 in up-regulating expression of the major EBV effector protein of B-cell transformation, latent membrane protein 1 (LMP1). In transient-transfection assays, EBNA-LP enhanced the level of EBNA-2-induced LMP1 expression by 5- to 10-fold in one Latency I Burkitt's lymphoma cell line, Eli-BL, and was absolutely required, along with EBNA-2, to induce LMP1 in a second line, Akata-BL. These changes in LMP1 protein expression appeared to be reflected at the transcriptional level. A study of EBNA-LP mutants showed that this cooperative function mapped to the W1W2 repeat domain rather than to Y1Y2. Because a Y1Y2-deleted form of EBNA-LP may therefore retain some aspects of wild-type function, the original data from virus recombinants leave open the possibility that EBNA-LP is actually an essential transforming gene.

Nucleolar and nuclear localization properties of a herpesvirus bZIP oncoprotein, MEQ

Marek's disease virus (MDV) is one of the most oncogenic herpesviruses and induces T lymphomas in chickens within weeks after infection. Only a limited number of viral transcripts are detected in MDV tumor samples and cell lines. One of the major transcripts encodes MEQ, a 339-amino-acid bZIP protein which is homologous to the Jun/Fos family of transcription factors. The C-terminal half of MEQ contains proline-rich repeats and, when fused to the DNA-binding domain of a yeast transcription factor, Gal4 (residues 1 to 147), exhibits transactivation function. MEQ can dimerize with itself and with c-Jun. The MEQ-c-Jun heterodimers bind to an AP-1-like enhancer within the MEQ promoter region with greater affinity than do homodimers of either protein, and they transactivate MEQ expression. Here we show that MEQ is expressed in the nucleus but, interestingly, with a predominant fraction in the nucleoli and coiled bodies. This makes MEQ the first bZIP protein to be identified in the nucleoli. MEQ contains two stretches of basic residues, designated basic region 1 (BR1) and basic region 2 (BR2). Using a series of deletion mutants, we have mapped the primary nuclear localization signal (NLS) and the sole nucleolar localization signal (NoLS) to the BR2 region. BR1 was shown to provide an auxiliary signal in nuclear translocation. To demonstrate that BR2 is an authentic NoLS, BR2 was fused to cytoplasmic v-Raf (delta gag) kinase. The BR2-Raf fusion protein was observed to migrate into the nucleoplasm and the nucleolus. The BR2 region can be further divided into two long arginine-lysine stretches, BR2N and BR2C, which are separated by the five amino acids Asn-Arg-Asp-Ala-Ala (NRDAA). We provide evidence that the requirement for nuclear translocation is less stringent than that for nucleolar translocation, as either BR2N or BR2C alone is sufficient to translocate the cytoplasmic v-Raf (delta gag) into the nucleus, but only in combination can they translocate v-Raf (delta gag) into the nucleolus. Our studies demonstrate that MEQ is both a nuclear and nucleolar protein, adding MEQ to the growing list of transactivators which localize to the nucleolus.

Cell cycle stage-specific phosphorylation of the Epstein-Barr virus immortalization protein EBNA-LP

EBNA-LP is a viral nuclear oncoprotein implicated in the immortalization of B lymphocytes by Epstein-Barr virus. An analysis of EBNA-LP migration on polyacrylamide gels was performed with protein derived from the X50-7 lymphoblastoid cell line blocked by hydroxyurea or aphidicolin at the G1/S phase of the cell cycle or by nocodazole at the G2/M phase. More slowly migrating species of EBNA-LP were detected in G2/M phase-arrested cell extracts. Release from nocodazole G2/M block or treatment with phosphatase caused the more slowly migrating species of EBNA-LP to disappear. Analyses of 32PO(4)(3-)-labeled EBNA-LP protein immunoprecipitated from the drug-synchronized cells showed that phosphorylated EBNA-LP was present throughout the cell cycle but that phosphorylation increased in G2 and was maximal at G2/M. Phosphoamino acid analysis revealed that all phosphorylation was on serine residues only. The ability of EBNA-LP to be phosphorylated by p34(cdc2) kinase and casein kinase II exclusively on serines implicates these enzymes as being potentially involved in EBNA-LP phosphorylation.

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.