[Show abstract][Hide abstract]ABSTRACT: Epstein-Barr virus (EBV) replication proteins are transported into the nucleus to synthesize viral genomes. We here report molecular mechanisms for nuclear transport of EBV DNA polymerase. The EBV DNA polymerase catalytic subunit, BALF5, was found to accumulate in the cytoplasm when expressed alone, while the EBV DNA polymerase processivity factor, BMRF1, moved into nucleus by itself. Co-expression of both proteins, however, resulted in efficient nuclear transport of BALF5. Deletion of nuclear localization signal of BMRF1 diminished their nuclear transport, although both proteins can still interact. These results suggest that BALF5 interacts with BMRF1 to effect transport into the nucleus. Interestingly, we found that Hsp90 inhibitors or knockdown of Hsp90β with short hairpin RNA prevented the BALF5 nuclear transport even in the presence of BMRF1, both in transfection assays and in the context of lytic replication. Immunoprecipitation analyses suggested that the molecular chaperone Hsp90 interacts with BALF5. Treatment with Hsp90 inhibitors blocked viral DNA replication almost completely during lytic infection, and knockdown of Hsp90β reduced viral genome synthesis. Collectively, we speculate that Hsp90 interacts with BALF5 in the cytoplasm to assist complex formation with BMRF1, leading to nuclear transport. Hsp90 inhibitors may be useful for the therapy for EBV associated diseases in future.
Full-text Article · Apr 2013 · Journal of Virology
[Show abstract][Hide abstract]ABSTRACT: Epstein-Barr virus (EBV), a human oncogenic herpesvirus that establishes a lifelong latent infection in the host, occasionally
enters lytic infection to produce progeny viruses. The EBV oncogene latent membrane protein 1 (LMP1), which is expressed in
both latent and lytic infection, constitutively activates the canonical NF-κB (p65) pathway. Such LMP1-mediated NF-κB activation
is necessary for proliferation of latently infected cells and inhibition of viral lytic cycle progression. Actually, canonical
NF-κB target gene expression was suppressed upon the onset of lytic infection. TRAF6, which is activated by conjugation of
polyubiquitin chains, associates with LMP1 to mediate NF-κB signal transduction. We have found that EBV-encoded BPLF1 interacts
with and deubiquitinates TRAF6 to inhibit NF-κB signaling during lytic infection. HEK293 cells with BPLF1-deficient recombinant
EBV exhibited poor viral DNA replication compared with the wild type. Furthermore, exogenous expression of BPLF1 or p65 knockdown
in cells restored DNA replication of BPLF1-deficient viruses, indicating that EBV BPLF1 deubiquitinates TRAF6 to inhibit NF-κB
signal transduction, leading to promotion of viral lytic DNA replication.
Full-text Article · Jan 2013 · Journal of Virology
[Show abstract][Hide abstract]ABSTRACT: The Epstein-Barr virus (EBV) predominantly establishes latent infection in B cells, and the reactivation of the virus from latency is dependent on the expression of the viral BZLF1 protein. The BZLF1 promoter (Zp) normally exhibits only low basal activity but is activated in response to chemical or biological inducers, such as 12-O-tetradecanoylphorbol-13-acetate (TPA), calcium ionophores, or histone deacetylase (HDAC) inhibitors. In some cell lines latently infected with EBV, an HDAC inhibitor alone can induce BZLF1 transcription, while the treatment does not enhance expression in other cell lines, such as B95-8 or Raji cells, suggesting unknown suppressive mechanisms besides histone deacetylation in those cells. Here, we found the epigenetic modification of the BZLF1 promoter in latent Raji cells by histone H3 lysine 27 trimethylation (H3K27me3), H3K9me2/me3, and H4K20me3. Levels of active markers such as histone acetylation and H3K4me3 were low in latent cells but increased upon reactivation. Treatment with 3-deazaneplanocin A (DZNep), an inhibitor of H3K27me3 and H4K20me3, significantly enhanced the BZLF1 transcription in Raji cells when in combination with an HDAC inhibitor, trichostatin A (TSA). The knockdown of Ezh2 or Suv420h1, histone methyltransferases for H3K27me3 or H4K20me3, respectively, further proved the suppression of Zp by the methylations. Taken together, the results indicate that H3K27 methylation and H4K20 methylation are involved, at least partly, in the maintenance of latency, and histone acetylation and H3K4 methylation correlate with the reactivation of the virus in Raji cells.
Full-text Article · Feb 2012 · Journal of Virology
[Show abstract][Hide abstract]ABSTRACT: Epstein-Barr virus LMP1, a major oncoprotein expressed in latent infection, is critical for primary B cell transformation,
functioning as a TNFR family member by aggregation in the plasma membrane resulting in constitutive activation of cellular
signals, such as NF-κB, MAPK, JAK/STAT, and AKT. Although transcription of LMP1 in latent type III cells is generally under the control of the viral coactivator EBNA2, little is known about EBNA2-independent
LMP1 expression in type II latency. We thus screened a cDNA library for factors that can activate the LMP1 promoter in an EBNA2-independent manner, using a reporter assay system. So far, we have screened >20,000 clones, and here
identified C/EBPϵ as a new transcriptional activator. Exogenous expression of C/EBPα, -β, or -ϵ efficiently augmented LMP1 mRNA and protein levels in EBV-positive cell lines, whereas other members of the C/EBP family exhibited modest or little
activity. It has been demonstrated that LMP1 gene transcription depends on two promoter regions: proximal (ED-L1) and distal (TR-L1). Interestingly, although we first used the proximal promoter for screening, we found that C/EBP increased transcription
from both promoters in latent EBV-positive cells. Mutagenesis in reporter assays and EMSA identified only one functional C/EBP
binding site, through which activation of both proximal and distal promoters is mediated. Introduction of point mutations
into the identified C/EBP site in EBV-BAC caused reduced LMP1 transcription from both LMP1 promoters in epithelial cells. In conclusion, C/EBP is a newly identified transcriptional activator of the LMP1 gene, independent of the EBNA2 coactivator.
Full-text Article · Dec 2011 · Journal of Biological Chemistry
[Show abstract][Hide abstract]ABSTRACT: Productive replication of Epstein-Barr virus occurs in discrete sites in nuclei, called replication compartments, where viral
DNA replication proteins and host homologous recombinational repair (HRR) and mismatch repair (MMR) factors are recruited.
Three-dimensional (3D) surface reconstruction imaging clarified the spatial arrangements of these factors within the replication
compartments. Subnuclear domains, designated BMRF1 cores, which were highly enriched in viral polymerase processivity factor
BMRF1 could be identified inside the replication compartments. Pulse-chase experiments revealed that newly synthesized viral
genomes organized around the BMRF1 cores were transferred inward. HRR factors could be demonstrated mainly outside BMRF1 cores,
where de novo synthesis of viral DNA was ongoing, whereas MMR factors were found predominantly inside. These results imply that de novo synthesis of viral DNA is coupled with HRR outside the cores, followed by MMR inside cores for quality control of replicated
viral genomes. Thus, our approach unveiled a viral genome manufacturing plant.
Full-text Article · Jul 2011 · Journal of Virology
[Show abstract][Hide abstract]ABSTRACT: Reactivation of the Epstein-Barr virus from latency is dependent on expression of the BZLF1 viral immediate-early protein.
The BZLF1 promoter (Zp) normally exhibits only low basal activity but is activated in response to chemical inducers such as 12-O-tetradecanoylphorbol-13-acetate and calcium ionophore. We found that Jun dimerization protein 2 (JDP2) plays a significant
role in suppressing Zp activity. Reporter, EMSA, and ChIP assays of a Zp mutant virus revealed JDP2 association with Zp at
the ZII cis-element, a binding site for CREB/ATF/AP-1. Suppression of Zp activity by JDP2 correlated with HDAC3 association and reduced
levels of histone acetylation. Although introduction of point mutations into the ZII element of the viral genome did not increase
the level of BZLF1 production, silencing of endogenous JDP2 gene expression by RNA interference increased the levels of viral early gene products and viral DNA replication. These results
indicate that JDP2 plays a role as a repressor of Zp and that its replacement by CREB/ATF/AP-1 at ZII is crucial to triggering
reactivation from latency to lytic replication.
Full-text Article · Jun 2011 · Journal of Biological Chemistry