Detection of Epstein-Barr virus BGLF4 protein kinase in virus replication compartments and virus particles

Conserved herpesvirus protein kinases

Conserved herpesviral protein kinases (CHPKs) are a group of enzymes conserved throughout all subfamilies of Herpesviridae. Members of this group are serine/threonine protein kinases that are likely to play a conserved role in viral infection by interacting with common host cellular and viral factors; however, along with a conserved role, individual kinases may have unique functions in the context of viral infection in such a way that they are only partially replaceable even by close homologues. Recent studies demonstrated that CHPKs are crucial for viral infection and suggested their involvement in regulation of numerous processes at various infection steps (primary infection, nuclear egress, tegumentation), although the mechanisms of this regulation remain unknown. Notwithstanding, recent advances in discovery of new CHPK targets, and studies of CHPK knockout phenotypes have raised their attractiveness as targets for antiviral therapy. A number of compounds have been shown to inhibit the activity of human cytomegalovirus (HCMV)-encoded UL97 protein kinase and exhibit a pronounced antiviral effect, although the same compounds are inactive against Epstein-Barr virus (EBV)-encoded protein kinase BGLF4, illustrating the fact that low homology between the members of this group complicates development of compounds targeting the whole group, and suggesting that individualized, structure-based inhibitor design will be more effective. Determination of CHPK structures will greatly facilitate this task.

Epstein-Barr virus protein kinase BGLF4 is a virion tegument protein that dissociates from virions in a phosphorylation-dependent process and phosphorylates the viral immediate-early protein BZLF1

Epstein-Barr virus (EBV) BGLF4 is a viral protein kinase that is expressed in the lytic phase of infection and is packaged in virions. We report here that BGLF4 is a tegument protein that dissociates from the virion in a phosphorylation-dependent process. We also present evidence that BGLF4 interacts with and phosphorylates BZLF1, a key viral regulator of lytic infection. These conclusions are based on the following observations. (i) In in vitro tegument release assays, a significant fraction of BGLF4 was released from virions in the presence of physiological NaCl concentrations. (ii) Addition of physiological concentrations of ATP and MgCl(2) to virions enhanced BGLF4 release, but phosphatase treatment of virions significantly reduced BGLF4 release. (iii) A recombinant protein containing a domain of BZLF1 was specifically phosphorylated by purified recombinant BGLF4 in vitro, and BGLF4 altered BZLF1 posttranslational modification in vivo. (iv) BZLF1 was specifically coimmunoprecipitated with BGLF4 in 12-O-tetradecanoylphorbol-13-acetate-treated B95-8 cells and in COS-1 cells transiently expressing both of these viral proteins. (v) BGLF4 and BZLF1 were colocalized in intranuclear globular structures, resembling the viral replication compartment, in Akata cells treated with anti-human immunoglobulin G. Our results suggest that BGLF4 functions not only in lytically infected cells by phosphorylating viral and cellular targets but also immediately after viral penetration like other herpesvirus tegument proteins.

Epstein-Barr virus-encoded protein kinase (BGLF4) is involved in production of infectious virus

The Epstein-Barr virus (EBV) BGLF4 gene product is a protein kinase (PK). Although this kinase has been characterized and several of its targets have been identified, its biological role remains enigmatic. We have generated and assessed a BGLF4 knockdown phenotype by means of RNA interference and report the following: (i) BGLF4-targeting small interfering RNA effectively inhibited the expression of its product, the viral PK, during lytic reactivation, (ii) BGLF4 knockdown partially inhibited viral DNA replication and expression of selected late viral genes, (iii) the absence of EBV PK resulted in retention of the viral nucleocapsids in the nuclei, and (iv) as a result of the nuclear retention, release of infectious virions is significantly retarded. Our results provide evidence that EBV PK plays an important role in nuclear egress of the virus and ultimately is crucial for lytic virus replication.

Epstein-Barr virus BGLF4 kinase induces premature chromosome condensation through activation of condensin and topoisomerase II

Previous studies of Epstein-Barr virus (EBV) replication focused mainly on the viral and cellular factors involved in replication compartment assembly and controlling the cell cycle. However, little is known about how EBV reorganizes nuclear architecture and the chromatin territories. In EBV-positive nasopharyngeal carcinoma NA cells or Akata cells, we noticed that cellular chromatin becomes highly condensed upon EBV reactivation. In searching for the possible mechanisms involved, we found that transient expression of EBV BGLF4 kinase induces unscheduled chromosome condensation, nuclear lamina disassembly, and stress fiber rearrangements, independently of cellular DNA replication and Cdc2 activity. BGLF4 interacts with condensin complexes, the major components in mitotic chromosome assembly, and induces condensin phosphorylation at Cdc2 consensus motifs. BGLF4 also stimulates the decatenation activity of topoisomerase II, suggesting that it may induce chromosome condensation through condensin and topoisomerase II activation. The ability to induce chromosome condensation is conserved in another gammaherpesvirus kinase, murine herpesvirus 68 ORF36. Together, these findings suggest a novel mechanism by which gammaherpesvirus kinases may induce multiple premature mitotic events to provide more extrachromosomal space for viral DNA replication and successful egress of nucleocapsid from the nucleus.

Characterization of the uracil-DNA glycosylase activity of Epstein-Barr virus BKRF3 and its role in lytic viral DNA replication

Uracil-DNA glycosylases (UDGs) of the uracil-N-glycosylase (UNG) family are the primary DNA repair enzymes responsible for removal of inappropriate uracil from DNA. Recent studies further suggest that the nuclear human UNG2 and the UDGs of large DNA viruses may coordinate with their DNA polymerase accessory factors to enhance DNA replication. Based on its amino acid sequence, the putative UDG of Epstein-Barr virus (EBV), BKRF3, belongs to the UNG family of proteins, and it was demonstrated previously to enhance oriLyt-dependent DNA replication in a cotransfection replication assay. However, the expression and enzyme activity of EBV BKRF3 have not yet been characterized. In this study, His-BKRF3 was expressed in bacteria and purified for biochemical analysis. Similar to the case for the Escherichia coli and human UNG enzymes, His-BKRF3 excised uracil from single-stranded DNA more efficiently than from double-stranded DNA and was inhibited by the purified bacteriophage PBS1 inhibitor Ugi. In addition, BKRF3 was able to complement an E. coli ung mutant in rifampin and nalidixic acid resistance mutator assays. The expression kinetics and subcellular localization of BKRF3 products were detected in EBV-positive lymphoid and epithelial cells by using BKRF3-specific mouse antibodies. Expression of BKRF3 is regulated mainly by the immediate-early transcription activator Rta. The efficiency of EBV lytic DNA replication was slightly affected by BKRF3 small interfering RNA (siRNA), whereas cellular UNG2 siRNA or inhibition of cellular and viral UNG activities by expressing Ugi repressed EBV lytic DNA replication. Taking these results together, we demonstrate the UNG activity of BKRF3 in vitro and in vivo and suggest that UNGs may participate in DNA replication or repair and thereby promote efficient production of viral DNA.

Kaposi's sarcoma-associated herpesvirus-encoded protein kinase and its interaction with K-bZIP

The oncogenic herpesvirus, Kaposi's sarcoma-associated herpesvirus, also identified as human herpesvirus 8, contains genes producing proteins that control transcription and influence cell signaling. Open reading frame 36 (ORF36) of this virus encodes a serine/threonine protein kinase, which is designated the viral protein kinase (vPK). Our recent efforts to elucidate the role of vPK in the viral life cycle have focused on identifying viral protein substrates and determining the effects of vPK-mediated phosphorylation on specific steps in viral replication. The vPK gene was transcribed into 4.2-kb and 3.6-kb mRNAs during the early and late phases of viral reactivation. vPK is colocalized with viral DNA replication/transcription compartments as marked by a polymerase processivity factor, and K-bZIP, a protein known to bind the viral DNA replication origin (Ori-Lyt) and to regulate viral transcription. The vPK physically associated with and strongly phosphorylated K-bZIP at threonine 111, a site also recognized by the cyclin-dependent kinase Cdk2. Both K-bZIP and vPK were corecruited to viral promoters targeted by K-bZIP as well as to the Ori-Lyt region. Phosphorylation of K-bZIP by vPK had a negative impact on K-bZIP transcription repression activity. The extent of posttranslational modification of K-bZIP by sumoylation, a process that influences its repression function, was decreased by vPK phosphorylation at threonine 111. Our data thus identify a new role of vPK as a modulator of viral transcription.

Lytic replication of Epstein–Barr virus

Epstein–Barr virus (EBV) is a gammaherpesvirus with a 172kb genome and many genes encoding enzymes for lytic viral DNA replication. Recent observations indicate that an S-phase-like environment and the activated DNA repair system are required for viral lytic DNA replication. The virally encoded DNA replication-associated enzymes are then expressed in two clusters, suggesting their participation at different stages of replication. Simultaneously, EBV-encoded regulatory proteins may modulate cell-cycle control to enhance virus replication efficiency. The interactions among proteins in the viral replication complex and cellular proteins may either generate structural specificities for replication proteins or stabilize the protein complexes. During infection, EBV has evolved several strategies to overcome the host defense mechanism, such as interfering with innate immunity and withdrawing into a latent state. This review discusses the latest progress in viral control of lytic replication and the interactions among viral lytic replication compartment and cellular machineries. The possible contribution of EBV lytic gene products to human malignancy is also discussed.