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 alphaherpesvirus conserved pUS10 is important for natural infection and its expression is regulated by the conserved Herpesviridae protein kinase (CHPK)

Conserved Herpesviridae protein kinases (CHPK) are conserved among all members of the Herpesviridae. Herpesviruses lacking CHPK propagate in cell culture at varying degrees, depending on the virus and cell culture system. CHPK is dispensable for Marek's disease herpesvirus (MDV) replication in cell culture and experimental infection in chickens; however, CHPK-particularly its kinase activity-is essential for horizontal transmission in chickens, also known as natural infection. To address the importance of CHPK during natural infection in chickens, we used liquid chromatography-tandem mass spectrometry (LC-MS/MS) based proteomics of samples collected from live chickens. Comparing modification of viral proteins in feather follicle epithelial (FFE) cells infected with wildtype or a CHPK-null virus, we identified the US10 protein (pUS10) as a potential target for CHPK in vivo. When expression of pUS10 was evaluated in cell culture and in FFE skin cells during in vivo infection, pUS10 was severely reduced or abrogated in cells infected with CHPK mutant or CHPK-null viruses, respectively, indicating a potential role for pUS10 in transmission. To test this hypothesis, US10 was deleted from the MDV genome, and the reconstituted virus was tested for replication, horizontal transmission, and disease induction. Our results showed that removal of US10 had no effect on the ability of MDV to transmit in experimentally infected chickens, but disease induction in naturally infected chickens was significantly reduced. These results show CHPK is necessary for pUS10 expression both in cell culture and in the host, and pUS10 is important for disease induction during natural infection.

The Conserved Herpesviridae Protein Kinase (CHPK) of Gallid alphaherpesvirus 3 (GaHV3) Is Required for Horizontal Spread and Natural Infection in Chickens

We have formerly identified the conserved herpesvirus protein kinase (CHPK) as essential for horizontal transmission of Marek’s disease virus (MDV). Thus far, it has been confirmed that the mutation of the invariant lysine (K) of CHPKs abrogates kinase activity and that CHPK activity is required for MDV horizontal transmission. Since CHPK is conserved among all members of the Herpesviridae, we hypothesized that CHPK, and specifically its kinase activity, is important for the horizontal transmission of other herpesviruses. To test this hypothesis, we utilized our experimental and natural infection model in chickens with MD vaccine strain 301B/1 of Gallid alphaherpesvirus 3 (GaHV3). First, we mutated the invariant lysine (K) 157 of 301B/1 CHPK to alanine (A) and determined whether it was required for horizontal transmission. To confirm the requirement of 301B/1 CHPK activity for transmission, a rescued virus was generated in which the A157 was changed back to a K (A157K). Despite both the CHPK mutant (K157A) and rescuant (A157K) viruses having replication defects in vivo, only the CHPK mutant (K157A) was unable to spread to contact chickens, while both wild-type and rescuant (A157K) viruses transmitted efficiently, confirming the importance of CHPK activity for horizontal spread. The data confirm that CHPK is required for GaHV3 transmission and suggest that the requirement of avian CHPKs for natural infection is conserved.

Lamin post-translational modifications: emerging toggles of nuclear organization and function

Nuclear lamins are ancient type V intermediate filaments with diverse functions that include maintaining nuclear shape, mechanosignaling, tethering and stabilizing chromatin, regulating gene expression, and contributing to cell cycle progression. Despite these numerous roles, an outstanding question has been how lamins are regulated. Accumulating work indicates that a range of lamin post-translational modifications (PTMs) control their functions both in homeostatic cells and in disease states such as progeria, muscular dystrophy, and viral infection. Here, we review the current knowledge of the diverse types of PTMs that regulate lamins in a site-specific manner. We highlight methods that can be used to characterize lamin PTMs whose functions are currently unknown and provide a perspective on the future of the lamin PTM field.

Expression of the Conserved Herpesvirus Protein Kinase (CHPK) of Marek's Disease Alphaherpesvirus in the Skin Reveals a Mechanistic Importance for CHPK during Interindividual Spread in Chickens

The Herpesviridae encode many conserved genes, including the conserved herpesvirus protein kinase (CHPK) that has multifunctional properties. In most cases, herpesviruses lacking CHPK can propagate in cell culture to various degrees, depending on the virus and cell culture system. However, in the natural animal model system of Marek's disease alphaherpesvirus (MDV) in chickens, CHPK is absolutely required for interindividual spread from chicken to chicken. The lack of biological reagents for chicken and MDV has limited our understanding of this important gene during interindividual spread. Here, we engineered epitope-tagged proteins in the context of virus infection in order to detect CHPK in the host. Using immunofluorescence assays and Western blotting during infection in cell culture and in chickens, we determined that the invariant lysine 170 (K170) of MDV CHPK is required for interindividual spread and autophosphorylation of CHPK and that mutation to methionine (M170) results in instability of the CHPK protein. Using these newly generated viruses allowed us to examine the expression of CHPK in infected chickens, and these results showed that mutant CHPK localization and late viral protein expression were severely affected in feather follicles wherein MDV is shed, providing important information on the requirement of CHPK for interindividual spread.IMPORTANCE Marek's disease in chickens is caused by Gallid alphaherpesvirus 2, better known as Marek's disease alphaherpesvirus (MDV). Current vaccines only reduce tumor formation but do not block interindividual spread from chicken to chicken. Understanding MDV interindividual spread provides important information for the development of potential therapies to protect against Marek's disease while also providing a reliable natural host in order to study herpesvirus replication and pathogenesis in animals. Here, we studied the conserved Herpesviridae protein kinase (CHPK) in cell culture and during infection in chickens. We determined that MDV CHPK is not required for cell-to-cell spread, for disease induction, and for oncogenicity. However, it is required for interindividual spread, and mutation of the invariant lysine (K170) results in stability issues and aberrant expression in chickens. This study is important because it addresses the critical role CHPK orthologs play in the natural host.

New biological research and understanding of Papanicolaou's test

The development of the Papanicolaou smear test by Dr. George Nicholas Papanicolaou (1883-1962) is one of the most significant achievements in screening for disease and cancer prevention in history. The Papanicolaou smear has been used for screening of cervical cancer since the 1950s. The test is technically straightforward and practical and based on a simple scientific observation: malignant cells have an aberrant nuclear morphology that can be distinguished from benign cells. Here, we review the scientific understanding that has been achieved and continues to be made on the causes and consequences of abnormal nuclear morphology, the basis of Dr. Papanicolaou's invention. The deformed nuclear shape is caused by the loss of lamina and nuclear envelope structural proteins. The consequences of a nuclear envelope defect include chromosomal numerical instability, altered chromatin organization and gene expression, and increased cell mobility because of a malleable nuclear envelope. HPV (Human Papilloma Virus) infection is recognized as the key etiology in the development of cervical cancer. Persistent HPV infection causes disruption of the nuclear lamina, which presents as a change in nuclear morphology detectable by a Papanicolaou smear. Thus, the causes and consequences of nuclear deformation are now linked to the mechanisms of viral carcinogenesis, and are still undergoing active investigation to reveal the details. Recently a statue was installed in front of the Papanicolaou's Cancer Research Building to honor the inventor. Remarkably, the invention nearly 60 years ago by Dr. Papanicolaou still exerts clinical impacts and inspires scientific inquiries.

Interindividual Spread of Herpesviruses

Interindividual spread of herpesviruses is essential for the virus life cycle and maintenance in host populations. For most herpesviruses, the virus-host relationship is close, having coevolved over millions of years resulting in comparatively high species specificity. The mechanisms governing interindividual spread or horizontal transmission are very complex, involving conserved herpesviral and cellular proteins during the attachment, entry, replication, and egress processes of infection. Also likely, specific herpesviruses have evolved unique viral and cellular interactions during cospeciation that are dependent on their relationship. Multiple steps are required for interindividual spread including virus assembly in infected cells; release into the environment, followed by virus attachment; and entry into new hosts. Should any of these steps be compromised, transmission is rendered impossible. This review will focus mainly on the natural virus-host model of Marek's disease virus (MDV) in chickens in order to delineate important steps during interindividual spread.

Roles of Epstein-Barr virus BGLF3.5 gene and two upstream open reading frames in lytic viral replication in HEK293 cells

The Epstein-Barr virus (EBV) predominantly establishes a latent infection in B lymphocytes, but a small percentage of infected cells switch from the latent state to the lytic cycle, leading to potent viral DNA replication and progeny viruses production. We here focused on a lytic gene BGLF3.5, and first established BGLF3.5 mutants by marker cassette insertion. Unexpectedly, this insertion mutant failed to produce BGLF4 protein and thus progeny production was severely inhibited. Then we carefully made two point mutant viruses (stop codon insertion or frame-shift mutation) and found that BGLF3.5 is not essential for EBV lytic replication processes, such as viral gene expression, DNA replication, or progeny production in the HEK293 cells although its homolog in murine gammaherpesvirus 68 (MHV-68) was reported to be essential. In addition, we examined the roles of two short, upstream open reading frames within the 5'UTR of BGLF3.5 gene in translation of BGLF4.

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.

A conserved gammaherpesvirus protein kinase targets histone deacetylases 1 and 2 to facilitate viral replication in primary macrophages

Gammaherpesviruses are ubiquitious pathogens that establish lifelong infection and are associated with several malignancies. All gammaherpesviruses encode a conserved protein kinase that facilitates viral replication and chronic infection and thus represents an attractive therapeutic target. In this study, we identify a novel function of gammaherpesvirus protein kinase as a regulator of class I histone deacetylases (HDAC). Mouse gammaherpesvirus 68 (MHV68)-encoded protein kinase orf36 interacted with HDAC1 and 2 and prevented association of these HDACs with the viral promoter driving expression of RTA, a critical immediate early transcriptional activator. Furthermore, the ability to interact with HDAC1 and 2 was not limited to the MHV68 orf36, as BGLF4, a related viral protein kinase encoded by Epstein-Barr virus, interacted with HDAC1 in vitro. Importantly, targeting of HDAC1 and 2 by orf36 was independent of the kinase's enzymatic activity. Additionally, orf36 expression, but not its enzymatic activity, induced changes in the global deacetylase activity observed in infected primary macrophages. Combined deficiency of HDAC1 and 2 rescued attenuated replication and viral DNA synthesis of the orf36 null MHV68 mutant, indicating that the regulation of HDAC1 and 2 by orf36 was relevant for viral replication. Understanding the mechanism by which orf36 facilitates viral replication, including through HDAC targeting, will facilitate the development of improved therapeutics against gammaherpesvirus kinases.

The cellular ataxia telangiectasia-mutated kinase promotes epstein-barr virus lytic reactivation in response to multiple different types of lytic reactivation-inducing stimuli

The Epstein-Barr virus (EBV) latent-to-lytic switch is mediated by the viral proteins BZLF1 (Z), BRLF1 (R), and BRRF1 (Na). Since we previously showed that DNA-damaging agents (including chemotherapy and irradiation) can induce EBV lytic reactivation and recently demonstrated that wild-type p53 contributes to lytic reactivation, we investigated the role of the ATM kinase during EBV reactivation. ATM phosphorylates and activates p53, as well as numerous other substrates involved in the cellular DNA damage response. Using an ATM inhibitor (KU55933), we found that ATM activity is required for efficient induction of EBV lytic gene expression by a variety of different stimuli, including a histone deacetylase (HDAC) inhibitor, the transforming growth factor β (TGF-β) cytokine, a demethylating agent (5-azacytidine), B cell receptor engagement with anti-IgG antibody, hydrogen peroxide, and the proteosome inhibitor bortezomib. In EBV-infected AGS (gastric) cells, knockdown of ATM, or p53, expression inhibits EBV reactivation. Conversely, treatment of these cells with nutlin-3 (which activates p53 and ATM) robustly induces lytic reactivation in a p53- and ATM-dependent manner. The ability of the EBV R and Na proteins to induce lytic reactivation in EBV-infected AGS cells is ATM dependent. However, overexpression of Z induces lytic gene expression in the presence or absence of ATM activity. Our results suggest that ATM enhances Z promoter activity in the context of the intact EBV genome and that p53 contributes to the ATM effect. Nevertheless, since we found that ATM inhibitors also reduce lytic reactivation in Burkitt lymphoma cells that have no p53, additional ATM substrates must also contribute to the ATM effect.

Gammaherpesvirus gene expression and DNA synthesis are facilitated by viral protein kinase and histone variant H2AX

Gammaherpesvirus protein kinases are an attractive therapeutic target as they support lytic replication and latency. Via an unknown mechanism these kinases enhance expression of select viral genes and DNA synthesis. Importantly, the kinase phenotypes have not been examined in primary cell types. Mouse gammaherpesvirus-68 (MHV68) protein kinase orf36 activates the DNA damage response (DDR) and facilitates lytic replication in primary macrophages. Significantly, H2AX, a DDR component and putative orf36 substrate, enhances MHV68 replication. Here we report that orf36 facilitated expression of RTA, an immediate early MHV68 gene, and DNA synthesis during de novo infection of primary macrophages. H2AX expression supported efficient RTA transcription and phosphorylated H2AX associated with RTA promoter. Furthermore, viral DNA synthesis was attenuated in H2AX-deficient macrophages, suggesting that the DDR system was exploited throughout the replication cycle. The interactions between a cancer-associated gammaherpesvirus and host tumor suppressor system have important implications for the pathogenesis of gammaherpesvirus infection.

Cellular transcription factor Oct-1 interacts with the Epstein-Barr virus BRLF1 protein to promote disruption of viral latency

The Epstein-Barr virus (EBV) latent-to-lytic switch is an essential part of the viral life cycle, but the cellular factors that promote viral reactivation are not well defined. In this report, we demonstrate that the cellular transcription factor Oct-1 cooperates with the EBV immediate-early protein BRLF1 (R, Rta) to induce lytic viral reactivation. We show that cotransfected Oct-1 enhances the ability of BRLF1 to activate lytic gene expression in 293 cells stably infected with a BRLF1-defective EBV mutant (BRLF1-stop) and that Oct-1 increases BRLF1-mediated activation of lytic EBV promoters in reporter gene assays. We find that Oct-1 interacts directly with BRLF1 in vitro and that a mutant BRLF1 protein (the M140A mutant) attenuated for the ability to interact with Oct-1 in vitro is also resistant to Oct-1-mediated transcriptional enhancement in 293 BRLF1-stop cells. Furthermore, we show that cotransfected Oct-1 augments BRLF1 binding to a variety of lytic EBV promoters in chromatin immunoprecipitation (ChIP) assays (including the BZLF1, BMRF1, and SM promoters) and that BRLF1 tethers Oct-1 to lytic EBV promoters. In addition, we demonstrate that an Oct-1 mutant defective in DNA binding (the S335D mutant) still retains the ability to enhance BRLF1 transcriptional effects. Finally, we show that knockdown of endogenous Oct-1 expression reduces the level of constitutive lytic EBV gene expression in both EBV-positive B-cell and EBV-positive epithelial cell lines. These results suggest that Oct-1 acts as a positive regulator of EBV lytic gene expression and that this effect is at least partially mediated through its interaction with the viral protein BRLF1.

Herpesviruses remodel host membranes for virus egress

Herpesviruses replicate their DNA and package this DNA into capsids in the nucleus. These capsids then face substantial obstacles to their release from cells. Unlike other DNA viruses, herpesviruses do not depend on disruption of nuclear and cytoplasmic membranes for their release. Enveloped particles are formed by budding through inner nuclear membranes, and then these perinuclear enveloped particles fuse with outer nuclear membranes. Unenveloped capsids in the cytoplasm are decorated with tegument proteins and then undergo secondary envelopment by budding into trans-Golgi network membranes, producing infectious particles that are released. In this Review, we describe the remodelling of host membranes that facilitates herpesvirus egress.

Epstein-Barr virus genetics: talking about the BAC generation

Genetic mutant organisms pervade all areas of Biology. Early on, herpesviruses (HV) were found to be amenable to genetic analysis using homologous recombination techniques in eukaryotic cells. More recently, HV genomes cloned onto a bacterial artificial chromosome (BAC) have become available. HV BACs can be easily modified in E.coli and reintroduced in eukaryotic cells to produce infectious viruses. Mutants derived from HV BACs have been used both to understand the functions of all types of genetic elements present on the virus genome, but also to generate mutants with potentially medically relevant properties such as preventative vaccines. Here we retrace the development of the BAC technology applied to the Epstein-Barr virus (EBV) and review the strategies available for the construction of mutants. We expand on the appropriate controls required for proper use of the EBV BACs, and on the technical hurdles researchers face in working with these recombinants. We then discuss how further technological developments might successfully overcome these difficulties. Finally, we catalog the EBV BAC mutants that are currently available and illustrate their contributions to the field using a few representative examples.

Viral serine/threonine protein kinases

Phosphorylation represents one the most abundant and important posttranslational modifications of proteins, including viral proteins. Virus-encoded serine/threonine protein kinases appear to be a feature that is unique to large DNA viruses. Although the importance of these kinases for virus replication in cell culture is variable, they invariably play important roles in virus virulence. The current review provides an overview of the different viral serine/threonine protein kinases of several large DNA viruses and discusses their function, importance, and potential as antiviral drug targets.

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.

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.

The Epstein-Barr virus (EBV)-encoded protein kinase, EBV-PK, but not the thymidine kinase (EBV-TK), is required for ganciclovir and acyclovir inhibition of lytic viral production

Ganciclovir (GCV) and acyclovir (ACV) are guanine nucleoside analogues that inhibit lytic herpesvirus replication. GCV and ACV must be monophosphorylated by virally encoded enzymes to be converted into nucleotides and incorporated into viral DNA. However, whether GCV and/or ACV phosphorylation in Epstein-Barr virus (EBV)-infected cells is mediated primarily by the EBV-encoded protein kinase (EBV-PK), the EBV-encoded thymidine kinase (EBV-TK), or both is controversial. To examine this question, we constructed EBV mutants containing stop codons in either the EBV-PK or EBV-TK open reading frame and selected for stable 293T clones latently infected with wild-type EBV or each of the mutant viruses. Cells were induced to the lytic form of viral replication with a BZLF1 expression vector in the presence and absence of various doses of GCV and ACV, and infectious viral titers were determined by a green Raji cell assay. As expected, virus production in wild-type EBV-infected 293T cells was inhibited by both GCV (50% inhibitory concentration [IC(50)] = 1.5 microM) and ACV (IC(50) = 4.1 microM). However, the EBV-PK mutant (which replicates as well as the wild-type (WT) virus in 293T cells) was resistant to both GCV (IC(50) = 19.6 microM) and ACV (IC(50) = 36.4 microM). Expression of the EBV-PK protein in trans restored GCV and ACV sensitivity in cells infected with the PK mutant virus. In contrast, in 293T cells infected with the TK mutant virus, viral replication remained sensitive to both GCV (IC(50) = 1.2 microM) and ACV (IC(50) = 2.8 microM), although susceptibility to the thymine nucleoside analogue, bromodeoxyuridine, was reduced. Thus, EBV-PK but not EBV-TK mediates ACV and GCV susceptibilities.