Phosphorylation of p27Kip1 by Epstein-Barr virus protein kinase induces its degradation through SCFSkp2 ubiquitin ligase actions during viral lytic replication

Epstein-Barr Virus miR-BARTs 7 and 9 modulate viral cycle, cell proliferation, and proteomic profiles in Burkitt lymphoma

The Epstein-Barr Virus (EBV) encodes viral microRNAs (miRs) that have been implicated in the pathogenesis of nasopharyngeal and gastric carcinomas, yet their potential roles in lymphomas remain to be fully elucidated. This study evaluated the impact of CRISPR/Cas9-mediated knockdown of EBV miRs BART-7 and BART-9 in EBV-positive Burkitt lymphoma cells Akata. As anticipated, the Akata cells subjected to CRISPR/Cas9-mediated knockdown of either EBV BART-7 or BART-9 exhibited a significant reduction in the expression of these viral miRs compared to cells with wild-type (wt) EBV genomes. This outcome effectively validates the experimental model employed in this study. Knocking down either BART-7 or BART-9 resulted in a notable reduction in cell viability and proliferation rates, alongside an elevation in the expression of EBV lytic genes. Global proteomic analysis revealed that the knockdown of EBV BART-7 significantly decreased the expression of ubiquitin/proteasome proteins while concurrently increasing RNA binding proteins (RBPs). Conversely, BART-9 knockdown reduced proteins associated with oxidoreductase activity, particularly those involved in fatty acid metabolism. Our findings unveil previously undiscovered EBV miRs BARTs 7 and 9 roles in cellular pathways relevant to both viral biology and lymphomagenesis.

Tegument proteins of Epstein-Barr virus: Diverse functions, complex networks, and oncogenesis

The tegument is the structure between the envelope and nucleocapsid of herpesvirus particles. Viral (and cellular) proteins accumulate to create the layers of the tegument. Some Epstein-Barr virus (EBV) tegument proteins are conserved widely in Herpesviridae, but others are shared only by members of the gamma-herpesvirus subfamily. As the interface to envelope and nucleocapsid, the tegument functions in virion morphogenesis and budding of the nucleocapsid during progeny production. When a virus particle enters a cell, enzymes such as kinase and deubiquitinase, and transcriptional activators are released from the virion to promote virus infection. Moreover, some EBV tegument proteins are involved in oncogenesis. Here, we summarize the roles of EBV tegument proteins, in comparison to those of other herpesviruses.

Targeting the signaling in Epstein-Barr virus-associated diseases: mechanism, regulation, and clinical study

Epstein–Barr virus-associated diseases are important global health concerns. As a group I carcinogen, EBV accounts for 1.5% of human malignances, including both epithelial- and lymphatic-originated tumors. Moreover, EBV plays an etiological and pathogenic role in a number of non-neoplastic diseases, and is even involved in multiple autoimmune diseases (SADs). In this review, we summarize and discuss some recent exciting discoveries in EBV research area, which including DNA methylation alterations, metabolic reprogramming, the changes of mitochondria and ubiquitin-proteasome system (UPS), oxidative stress and EBV lytic reactivation, variations in non-coding RNA (ncRNA), radiochemotherapy and immunotherapy. Understanding and learning from this advancement will further confirm the far-reaching and future value of therapeutic strategies in EBV-associated diseases.

Conserved Herpesvirus Protein Kinases Target SAMHD1 to Facilitate Virus Replication

To ensure a successful infection, herpesviruses have developed elegant strategies to counterbalance the host anti-viral responses. Sterile alpha motif and HD domain 1 (SAMHD1) was recently identified as an intrinsic restriction factor for a variety of viruses. Aside from HIV-2 and the related simian immunodeficiency virus (SIV) Vpx proteins, the direct viral countermeasures against SAMHD1 restriction remain unknown. Using Epstein-Barr virus (EBV) as a primary model, we discover that SAMHD1-mediated anti-viral restriction is antagonized by EBV BGLF4, a member of the conserved viral protein kinases encoded by all herpesviruses. Mechanistically, we find that BGLF4 phosphorylates SAMHD1 and thereby inhibits its deoxynucleotide triphosphate triphosphohydrolase (dNTPase) activity. We further demonstrate that the targeting of SAMHD1 for phosphorylation is a common feature shared by beta- and gamma-herpesviruses. Together, our findings uncover an immune evasion mechanism whereby herpesviruses exploit the phosphorylation of SAMHD1 to thwart host defenses.

Herpesviruses have evolved elegant strategies to dampen the host anti-viral responses. Zhang et al. discover a mechanism by which herpesviruses evade SAMHD1-mediated host defenses through phosphorylation, expanding the functional repertoire of viral protein kinases in herpesvirus biology.

Cyclin-dependent kinases and CDK inhibitors in virus-associated cancers

The role of several risk factors, such as pollution, consumption of alcohol, age, sex and obesity in cancer progression is undeniable. Human malignancies are mainly characterized by deregulation of cyclin-dependent kinases (CDK) and cyclin inhibitor kinases (CIK) activities. Viruses express some onco-proteins which could interfere with CDK and CIKs function, and induce some signals to replicate their genome into host's cells. By reviewing some studies about the function of CDK and CIKs in cells infected with oncoviruses, such as HPV, HTLV, HERV, EBV, KSHV, HBV and HCV, we reviewed the mechanisms of different onco-proteins which could deregulate the cell cycle proteins.

[Dynamic changes of cellular environment during Epstein-Barr virus productive replication]

Productive (lytic) replication of DNA viruses elicits host cell DNA damage responses, which cause both beneficial and detrimental effects on viral replication. Viruses utilize them and selectively cancel the 'noisy' downstream signaling pathways, leading to maintain high S-phase CDK activities required for viral replication. To achieve this fine tuning of cellular environment, herpesviruses encode many (>70) genes in their genome, which are expressed in a strictly regulated temporal cascade (immediate-early, early, and late). Here, I introduce and discuss how Epstein-Barr virus, an oncogenic herpesvirus, hijacks the cellular environment and adapt it for the progeny production.

Oncogenesis of CAEBV revealed: Intragenic deletions in the viral genome and leaky expression of lytic genes

Epstein-Barr virus (EBV) is a causative agent of infectious mononucleosis and several malignancies involving lymphocytes and epithelial cells. We recently reported genomic analyses of chronic active EBV infection (CAEBV), a proliferative disorder of T and/or NK cells, as well as other lymphoid malignancies. We found that T and/or NK cells undergoing clonal expansion in CAEBV patients gain somatic driver mutations as the disorder progresses. Investigation of the viral genome revealed viral genomes harboring intragenic deletions in the BamHI-rightward transcripts (BART) region and in essential lytic genes. Interestingly, we observed that these deletions resulted in leaky expression of viral lytic genes. This increased expression of viral lytic genes is reminiscent of the "pre-latent abortive lytic" state, in which a substantial number of lytic genes are produced for weeks in the absence of progeny production, which contributes to cell survival upon de novo infection. It has been known that EBV can choose either latent or lytic state, but this dualistic concept may need to be reconsidered, as our data suggest the presence of the third, intermediate state; leaky expression of lytic genes that does not lead to completion of the full lytic amplification cycle. Leaky expression of lytic genes likely contributes to the formation and maintenance of several types of EBV-associated tumors. We also presented significant circumstantial evidence suggesting that EBV infects lymphoid progenitor cells in CAEBV before differentiation into T and NK cells. Taken together, our new data shed light on oncogenesis of CAEBV and other EBV-associated malignancies.

Direct Substrate Identification with an Analog Sensitive (AS) Viral Cyclin-Dependent Kinase (v-Cdk)

Viral cyclin-dependent kinases (v-Cdks) functionally emulate their cellular Cdk counterparts. Such viral mimicry is an established phenomenon that we extend here through chemical genetics. Kinases contain gatekeeper residues that limit the size of molecules that can be accommodated within the enzyme active site. Mutating gatekeeper residues to smaller amino acids allows larger molecules access to the active site. Such mutants can utilize bio-orthoganol ATPs for phosphate transfer and are inhibited by compounds ineffective against the wild type protein, and thus are referred to as analog-sensitive (AS) kinases. We identified the gatekeeper residues of the v-Cdks encoded by Epstein-Barr virus (EBV) and human cytomegalovirus (HCMV) and mutated them to generate AS kinases. The AS-v-Cdks are functional and utilize different ATP derivatives with a specificity closely matching their cellular ortholog, AS-Cdk2. The AS derivative of the EBV v-Cdk was used to transfer a thiolated phosphate group to targeted proteins which were then purified through covalent capture and identified by mass spectrometry. Pathway analysis of these newly identified direct substrates of the EBV v-Cdk extends the potential influence of this kinase into all stages of gene expression (transcription, splicing, mRNA export, and translation). Our work demonstrates the biochemical similarity of the cellular and viral Cdks, as well as the utility of AS v-Cdks for substrate identification to increase our understanding of both viral infections and Cdk biology.

Therapeutic Strategies against Epstein-Barr Virus-Associated Cancers Using Proteasome Inhibitors

Epstein-Barr virus (EBV) is closely associated with several lymphomas (endemic Burkitt lymphoma, Hodgkin lymphoma and nasal NK/T-cell lymphoma) and epithelial cancers (nasopharyngeal carcinoma and gastric carcinoma). To maintain its persistence in the host cells, the virus manipulates the ubiquitin-proteasome system to regulate viral lytic reactivation, modify cell cycle checkpoints, prevent apoptosis and evade immune surveillance. In this review, we aim to provide an overview of the mechanisms by which the virus manipulates the ubiquitin-proteasome system in EBV-associated lymphoid and epithelial malignancies, to evaluate the efficacy of proteasome inhibitors on the treatment of these cancers and discuss potential novel viral-targeted treatment strategies against the EBV-associated cancers.

Phosphoproteomic Profiling Reveals Epstein-Barr Virus Protein Kinase Integration of DNA Damage Response and Mitotic Signaling

Epstein-Barr virus (EBV) is etiologically linked to infectious mononucleosis and several human cancers. EBV encodes a conserved protein kinase BGLF4 that plays a key role in the viral life cycle. To provide new insight into the host proteins regulated by BGLF4, we utilized stable isotope labeling by amino acids in cell culture (SILAC)-based quantitative proteomics to compare site-specific phosphorylation in BGLF4-expressing Akata B cells. Our analysis revealed BGLF4-mediated hyperphosphorylation of 3,046 unique sites corresponding to 1,328 proteins. Frequency analysis of these phosphosites revealed a proline-rich motif signature downstream of BGLF4, indicating a broader substrate recognition for BGLF4 than its cellular ortholog cyclin-dependent kinase 1 (CDK1). Further, motif analysis of the hyperphosphorylated sites revealed enrichment in ATM, ATR and Aurora kinase substrates while functional analyses revealed significant enrichment of pathways related to the DNA damage response (DDR), mitosis and cell cycle. Phosphorylation of proteins associated with the mitotic spindle assembly checkpoint (SAC) indicated checkpoint activation, an event that inactivates the anaphase promoting complex/cyclosome, APC/C. Furthermore, we demonstrated that BGLF4 binds to and directly phosphorylates the key cellular proteins PP1, MPS1 and CDC20 that lie upstream of SAC activation and APC/C inhibition. Consistent with APC/C inactivation, we found that BGLF4 stabilizes the expression of many known APC/C substrates. We also noted hyperphosphorylation of 22 proteins associated the nuclear pore complex, which may contribute to nuclear pore disassembly and SAC activation. A drug that inhibits mitotic checkpoint activation also suppressed the accumulation of extracellular EBV virus. Taken together, our data reveal that, in addition to the DDR, manipulation of mitotic kinase signaling and SAC activation are mechanisms associated with lytic EBV replication. All MS data have been deposited in the ProteomeXchange with identifier PXD002411 (http://proteomecentral.proteomexchange.org/dataset/PXD002411).

Epstein-Barr virus (EBV) is a herpesvirus that is associated with B cell and epithelial human cancers. Herpesviruses encode a protein kinase which is an important regulator of lytic virus replication and is consequently a target for anti-viral drug development. The EBV genome encodes for a serine/threonine protein kinase called BGLF4. Previous work on BGLF4 has largely focused on its cyclin-dependent kinase 1 (CDK1)-like activity. The range of BGLF4 cellular substrates and the full impact of BGLF4 on the intracellular microenvironment still remain to be elucidated. Here, we utilized unbiased quantitative phosphoproteomic approach to dissect the changes in the cellular phosphoproteome that are mediated by BGLF4. Our MS analyses revealed extensive hyperphosphorylation of substrates that are normally targeted by CDK1, Ataxia telangiectasia mutated (ATM), Ataxia telangiectasia and Rad3-related (ATR) proteins and Aurora kinases. The up-regulated phosphoproteins were functionally linked to the DNA damage response, mitosis and cell cycle pathways. Our data demonstrate widespread changes in the cellular phosphoproteome that occur upon BGLF4 expression and suggest that manipulation of the DNA damage and mitotic kinase signaling pathways are central to efficient EBV lytic replication.

Chromatin Structure of Epstein-Barr Virus Latent Episomes

EBV latent infection is characterized by a highly restricted pattern of viral gene expression. EBV can establish latent infections in multiple different tissue types with remarkable variation and plasticity in viral transcription and replication. During latency, the viral genome persists as a multi-copy episome, a non-integrated-closed circular DNA with nucleosome structure similar to cellular chromosomes. Chromatin assembly and histone modifications contribute to the regulation of viral gene expression, DNA replication, and episome persistence during latency. This review focuses on how EBV latency is regulated by chromatin and its associated processes.

A locus encompassing the Epstein-Barr virus bglf4 kinase regulates expression of genes encoding viral structural proteins

The mechanism regulating expression of late genes, encoding viral structural components, is an unresolved problem in the biology of DNA tumor viruses. Here we show that BGLF4, the only protein kinase encoded by Epstein-Barr virus (EBV), controls expression of late genes independent of its effect on viral DNA replication. Ectopic expression of BGLF4 in cells lacking the kinase gene stimulated the transcript levels of six late genes by 8- to 10-fold. Introduction of a BGLF4 mutant that eliminated its kinase activity did not stimulate late gene expression. In cells infected with wild-type EBV, siRNA to BGLF4 (siG4) markedly reduced late gene expression without compromising viral DNA replication. Synthesis of late products was restored upon expression of a form of BGLF4 resistant to the siRNA. Studying the EBV transcriptome using mRNA-seq during the late phase of the lytic cycle in the absence and presence of siG4 showed that BGLF4 controlled expression of 31 late genes. Analysis of the EBV transcriptome identified BGLF3 as a gene whose expression was reduced as a result of silencing BGLF4. Knockdown of BGLF3 markedly reduced late gene expression but had no effect on viral DNA replication or expression of BGLF4. Our findings reveal the presence of a late control locus encompassing BGLF3 and BGLF4 in the EBV genome, and provide evidence for the importance of both proteins in post-replication events that are necessary for expression of late genes.

Hsp90 inhibitor 17-DMAG decreases expression of conserved herpesvirus protein kinases and reduces virus production in Epstein-Barr virus-infected cells

All eight human herpesviruses have a conserved herpesvirus protein kinase (CHPK) that is important for the lytic phase of the viral life cycle. In this study, we show that heat shock protein 90 (Hsp90) interacts directly with each of the eight CHPKs, and we demonstrate that an Hsp90 inhibitor drug, 17-dimethylaminoethylamino-17-demethoxygeldanamycin (17-DMAG), decreases expression of all eight CHPKs in transfected HeLa cells. 17-DMAG also decreases expression the of the endogenous Epstein-Barr virus protein kinase (EBV PK, encoded by the BGLF4 gene) in lytically infected EBV-positive cells and inhibits phosphorylation of several different known EBV PK target proteins. Furthermore, 17-DMAG treatment abrogates expression of the human cytomegalovirus (HCMV) kinase UL97 in HCMV-infected human fibroblasts. Importantly, 17-DMAG treatment decreased the EBV titer approximately 100-fold in lytically infected AGS-Akata cells without causing significant cellular toxicity during the same time frame. Increased EBV PK expression in 17-DMAG-treated AGS-Akata cells did not restore EBV titers, suggesting that 17-DMAG simultaneously targets multiple viral and/or cellular proteins required for efficient viral replication. These results suggest that Hsp90 inhibitors, including 17-DMAG, may be a promising group of drugs that could have profound antiviral effects on herpesviruses.

Epstein-Barr virus deubiquitinase downregulates TRAF6-mediated NF-κB signaling during productive replication

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.

Epstein-Barr virus BGLF4 kinase retards cellular S-phase progression and induces chromosomal abnormality

Epstein-Barr virus (EBV) induces an uncoordinated S-phase-like cellular environment coupled with multiple prophase-like events in cells replicating the virus. The EBV encoded Ser/Thr kinase BGLF4 has been shown to induce premature chromosome condensation through activation of condensin and topoisomerase II and reorganization of the nuclear lamina to facilitate the nuclear egress of nucleocapsids in a pathway mimicking Cdk1. However, the observation that RB is hyperphosphorylated in the presence of BGLF4 raised the possibility that BGLF4 may have a Cdk2-like activity to promote S-phase progression. Here, we investigated the regulatory effects of BGLF4 on cell cycle progression and found that S-phase progression and DNA synthesis were interrupted by BGLF4 in mammalian cells. Expression of BGLF4 did not compensate Cdk1 defects for DNA replication in S. cerevisiae. Using time-lapse microscopy, we found the fate of individual HeLa cells was determined by the expression level of BGLF4. In addition to slight cell growth retardation, BGLF4 elicits abnormal chromosomal structure and micronucleus formation in 293 and NCP-TW01 cells. In Saos-2 cells, BGLF4 induced the hyperphosphorylation of co-transfected RB, while E2F1 was not released from RB-E2F1 complexes. The E2F1 regulated activities of the cyclin D1 and ZBRK1 promoters were suppressed by BGLF4 in a dose dependent manner. Detection with phosphoamino acid specific antibodies revealed that, in addition to Ser780, phosphorylation of the DNA damage-responsive Ser612 on RB was enhanced by BGLF4. Taken together, our study indicates that BGLF4 may directly or indirectly induce a DNA damage signal that eventually interferes with host DNA synthesis and delays S-phase progression.

SUMO binding by the Epstein-Barr virus protein kinase BGLF4 is crucial for BGLF4 function

An Epstein-Barr virus (EBV) protein microarray was used to screen for proteins binding noncovalently to the small ubiquitin-like modifier SUMO2. Among the 11 SUMO binding proteins identified was the conserved protein kinase BGLF4. The mutation of potential SUMO interaction motifs (SIMs) in BGLF4 identified N- and C-terminal SIMs. The mutation of both SIMs changed the intracellular localization of BGLF4 from nuclear to cytoplasmic, while BGLF4 mutated in the N-terminal SIM remained predominantly nuclear. The mutation of the C-terminal SIM yielded an intermediate phenotype with nuclear and cytoplasmic staining. The transfer of BGLF4 amino acids 342 to 359 to a nuclear green fluorescent protein (GFP)-tagged reporter protein led to the relocalization of the reporter to the cytoplasm. Thus, the C-terminal SIM lies adjacent to a nuclear export signal, and coordinated SUMO binding by the N- and C-terminal SIMs blocks export and allows the nuclear accumulation of BGLF4. The mutation of either SIM prevented SUMO binding in vitro. The ability of BGLF4 to abolish the SUMOylation of the EBV lytic cycle transactivator ZTA was dependent on both BGLF4 SUMO binding and BGLF4 kinase activity. The global profile of SUMOylated cell proteins was also suppressed by BGLF4 but not by the SIM or kinase-dead BGLF4 mutant. The effective BGLF4-mediated dispersion of promyelocytic leukemia (PML) bodies was dependent on SUMO binding. The SUMO binding function of BGLF4 was also required to induce the cellular DNA damage response and to enhance the production of extracellular virus during EBV lytic replication. Thus, SUMO binding by BGLF4 modulates BGLF4 function and affects the efficiency of lytic EBV replication.

Conserved herpesvirus kinases target the DNA damage response pathway and TIP60 histone acetyltransferase to promote virus replication

Herpesviruses, which are major human pathogens, establish life-long persistent infections. Although the α, β, and γ herpesviruses infect different tissues and cause distinct diseases, they each encode a conserved serine/threonine kinase that is critical for virus replication and spread. The extent of substrate conservation and the key common cell-signaling pathways targeted by these kinases are unknown. Using a human protein microarray high-throughput approach, we identify shared substrates of the conserved kinases from herpes simplex virus, human cytomegalovirus, Epstein-Barr virus (EBV), and Kaposi's sarcoma-associated herpesvirus. DNA damage response (DDR) proteins were statistically enriched, and the histone acetyltransferase TIP60, an upstream regulator of the DDR pathway, was required for efficient herpesvirus replication. During EBV replication, TIP60 activation by the BGLF4 kinase triggers EBV-induced DDR and also mediates induction of viral lytic gene expression. Identification of key cellular targets of the conserved herpesvirus kinases will facilitate the development of broadly effective antiviral strategies.

The Ying-Yang of the virus-host interaction: control of the DNA damage response

Evaluation of: Nikitin PA, Yan CM, Forte E et al.: An ATM/Chk2-mediated DNA damage-responsive signaling pathway suppresses Epstein-Barr virus transformation of primary human B cells. Cell. Host Microbe 8(6), 510-522 (2010). Viruses have evolved elegant strategies to manipulate the host while the host counters with defense systems including the interferon response, apoptosis and the DNA damage response (DDR). Viruses have multiple strategies for manipulating the DDR and the same virus can even activate or inhibit the DDR at different stages of infection. Epstein-Barr virus (EBV) is implicated in several human cancers, including Burkitt's lymphoma, nasopharyngeal carcinoma, post-transplant lymphoproliferative disease and HIV-associated lymphomas. Although multiple viral proteins have been implicated in EBV-associated malignancies, the cellular pathways that control EBV-induced transformation and tumorigenesis remain incompletely understood. In this study, Nikitin et al. demonstrate that early EBV infection induces a cellular DDR that restricts virus-mediated transformation. The EBV-encoded EBNA3C protein subsequently attenuates this response to favor transformation and immortalization of host cells.

Noise cancellation: viral fine tuning of the cellular environment for its own genome replication

Productive replication of DNA viruses elicits host cell DNA damage responses, which cause both beneficial and detrimental effects on viral replication. In response to the viral productive replication, host cells attempt to attenuate the S-phase cyclin-dependent kinase (CDK) activities to inhibit viral replication. However, accumulating evidence regarding interactions between viral factors and cellular signaling molecules indicate that viruses utilize them and selectively block the downstream signaling pathways that lead to attenuation of the high S-phase CDK activities required for viral replication. In this review, we describe the sophisticated strategy of Epstein-Barr virus to cancel such “noisy” host defense signals in order to hijack the cellular environment.

Key motifs in EBV (Epstein-Barr virus)-encoded protein kinase for phosphorylation activity and nuclear localization

A sole EBV (Epstein-Barr virus)-encoded protein kinase (EBV-PK) (the BGLF4 gene product) plays important roles in viral infection. Although a number of targets of this protein have been identified, the kinase itself remains largely unstudied with regard to its enzymology and structure. In the present study, site-directed mutagenesis has been employed to generate mutations targeting residues involved in nuclear localization of the EBV-PK, core residues in subdomain III of the protein kinase domain conserved in most protein kinases or residues in subdomain VIa conserved only within the HPK (herpesvirus-encoded protein kinase) group. Deletion of amino acids 389-391 resulted in exclusive cytoplasmic localization of the protein, indicating the involvement of this region in nuclear translocation of the EBV-PK. Mutations at the amino acids Glu113 (core component), Phe175, Leu178, Phe184, Leu185 and Asn186 (conserved in HPKs) resulted in loss of EBV-PK autophosphorylation, protein substrate [EBV EA-D (early antigen diffused)] phosphorylation, and ability to facilitate ganciclovir phosphorylation. These results reiterate the unique features of this group of kinases and present an opportunity for designing more specific antiviral compounds.

Escape of herpesviruses from the nucleus

The nuclear envelope of eukaryotic cells is composed of double lipid-bilayer membranes, the membrane-connected nuclear pore complexes and an underlying nuclear lamina network. The nuclear pore complexes serve as gates for regulating the transport of macromolecules between cytoplasm and nucleus. The nuclear lamina not only provides an intact meshwork for maintaining the nuclear stiffness but also presents a natural barrier against most DNA viruses. Herpesviruses are large DNA viruses associated with multiple human and animal diseases. The complex herpesviral virion contains more than 30 viral proteins. After viral DNA replication, the newly synthesised genome is packaged into the pre-assembled intranuclear capsid. The nucleocapsid must then transverse through the nuclear envelope to the cytoplasm for the subsequent maturation process. Information regarding how nucleocapsid breaches the rigid nuclear lamina barrier and accesses the inner nuclear membrane for primary envelopment has emerged recently. From the point of view of both viral components and nuclear structure, this review summarises recent advances in the complicated protein-protein interactions and the phosphorylation regulations involved in the nuclear egress of herpesviral nucleocapsids.

Cyclin-dependent kinase-like function is shared by the beta- and gamma- subset of the conserved herpesvirus protein kinases

The UL97 protein of human cytomegalovirus (HCMV, or HHV-5 (human herpesvirus 5)), is a kinase that phosphorylates the cellular retinoblastoma (Rb) tumor suppressor and lamin A/C proteins that are also substrates of cellular cyclin-dependent kinases (Cdks). A functional complementation assay has further shown that UL97 has authentic Cdk-like activity. The other seven human herpesviruses each encode a kinase with sequence and positional homology to UL97. These UL97-homologous proteins have been termed the conserved herpesvirus protein kinases (CHPKs) to distinguish them from other human herpesvirus-encoded kinases. To determine if the Cdk-like activities of UL97 were shared by all of the CHPKs, we individually expressed epitope-tagged alleles of each protein in human Saos-2 cells to test for Rb phosphorylation, human U-2 OS cells to monitor nuclear lamina disruption and lamin A phosphorylation, or S. cerevisiae cdc28-13 mutant cells to directly assay for Cdk function. We found that the ability to phosphorylate Rb and lamin A, and to disrupt the nuclear lamina, was shared by all CHPKs from the beta- and gamma-herpesvirus families, but not by their alpha-herpesvirus homologs. Similarly, all but one of the beta and gamma CHPKs displayed bona fide Cdk activity in S. cerevisiae, while the alpha proteins did not. Thus, we have identified novel virally-encoded Cdk-like kinases, a nomenclature we abbreviate as v-Cdks. Interestingly, we found that other, non-Cdk-related activities reported for UL97 (dispersion of promyelocytic leukemia protein nuclear bodies (PML-NBs) and disruption of cytoplasmic or nuclear aggresomes) showed weak conservation among the CHPKs that, in general, did not segregate to specific viral families. Therefore, the genomic and evolutionary conservation of these kinases has not been fully maintained at the functional level. Our data indicate that these related kinases, some of which are targets of approved or developmental antiviral drugs, are likely to serve both overlapping and non-overlapping functions during viral infections.

Simian virus 40 T/t antigens and lamin A/C small interfering RNA rescue the phenotype of an Epstein-Barr virus protein kinase (BGLF4) mutant

The Epstein-Barr virus (EBV)-encoded viral protein kinase, EBV-PK (the BGLF4 gene product), is required for efficient nuclear viral egress in 293 cells. However, since EBV-PK phosphorylates a number of different viral and cellular proteins (including lamin A/C), the relative importance of each target during lytic viral replication remains unclear. We show here that an EBV PK mutant (PKmut; containing stop codons at residues 1 and 5 in EBV-PK) is highly defective for release of infectious virus from 293 cells but not 293T cells. Furthermore, the phenotype of the PKmut in 293 cells is substantially reversed by expression of the simian virus 40 (SV40) large (T) and small (t) T antigens. Efficient rescue requires the presence of both SV40 T/t proteins. We show that 293T cells have a much higher level of constitutive lamin A/C phosphorylation than do 293 cells over residues (S22 and S392) that promote phosphorylation-dependent nuclear disassembly and that both large T and small t contribute to enhanced lamin A/C phosphorylation. Finally, we demonstrate that knockdown of lamin A/C expression using small interfering RNA also rescues the PKmut phenotype in 293 cells. These results suggest that essential roles of EBV-PK during lytic viral replication include the phosphorylation and dispersion of lamin A/C.