Specific phosphorylation of exogenous protein and peptide substrates by the human cytomegalovirus UL97 protein kinase. Importance of the P+5 position

Mass Spectrometry-Based Characterization of the Virion Proteome, Phosphoproteome, and Associated Kinase Activity of Human Cytomegalovirus

The assembly of human cytomegalovirus (HCMV) virions is an orchestrated process that requires, as an essential prerequisite, the complex crosstalk between viral structural proteins. Currently, however, the mechanisms governing the successive steps in the constitution of virion protein complexes remain elusive. Protein phosphorylation is a key regulator determining the sequential changes in the conformation, binding, dynamics, and stability of proteins in the course of multiprotein assembly. In this review, we present a comprehensive map of the HCMV virion proteome, including a refined view on the virion phosphoproteome, based on previous publications supplemented by new results. Thus, a novel dataset of viral and cellular proteins contained in HCMV virions is generated, providing a basis for future analyses of individual phosphorylation steps and sites involved in the orchestrated assembly of HCMV virion-specific multiprotein complexes. Finally, we present the current knowledge on the activity of pUL97, the HCMV-encoded and virion-associated kinase, in phosphorylating viral and host proteins.

Human cytomegaloviral multifunctional protein kinase pUL97 impairs zebrafish embryonic development and increases mortality

Cytomegalovirus is a worldwide-distributed human pathogen, which is the leading cause of congenital virus infection, affecting 0.5 to 2% of live births. To date, it is largely unclear which molecular mechanisms underlie the symptomatic outcomes. This is mainly due to species specificity and limited homology among cytomegalovirus genomes. As it is not possible to infect model organisms with human cytomegalovirus, the aim of this study was to develop a heterologous system allowing in the future the elucidation of the pathological role of individual viral proteins. As a model organism the zebrafish has been chosen due to its ease of manipulation and characterization as well as its large offspring. As cytomegalovirus model protein, pUL97 was characterized because it is multiply involved in virus-host interaction. Here, we show in zebrafish embryos, that (i) pUL97 can be expressed in zebrafish, (ii) increasing pUL97 expression levels quantitatively correlate with both minor and major pathological defects, (iii) pUL97 expression impairs cell cycle progression and induces cell death, (iv) active pUL97, but not an inactive mutant, induces excess mortality, and (v) co-administration of a pUL97 inhibitor reduces embryonic pathology. Collectively, these data indicate the suitability of zebrafish to elucidate the pathological role of human cytomegaloviral proteins.

Impact of RNA polymerase I inhibitor CX-5461 on viral kinase-dependent and -independent cytomegalovirus replication

Human cytomegalovirus (HCMV) infections cause congenital birth defects and disease in immunosuppressed individuals. Antiviral compounds can control infection yet their use is restricted due to concerns of toxicity and the emergence of drug resistant strains. We have evaluated the impact of an RNA Polymerase I (Pol I) inhibitor, CX-5461 on HCMV replication. CX-5461 inhibits Pol I-mediated ribosomal DNA transcription by binding G-quadruplex DNA structures and also activates cellular stress response pathways. The addition of CX-5461 at both early and late stages of the HCMV infection inhibited viral DNA synthesis and virus production. Interestingly, adding CX-5461 after the onset of viral DNA synthesis resulted in a greater reduction compared to continuous treatment starting early during infection. We observed an accompanying increase in cyclin-dependent kinase inhibitor p21 in infected cells treated late but not early which likely explains the differences. Our previous studies demonstrated the importance of p21 in the antiviral activity of the HCMV kinase inhibitor, maribavir. Addition of CX-5461 increased the anti-HCMV activity of maribavir. Our data demonstrate that CX-5461 inhibits HCMV replication and synergizes with maribavir to disrupt infection.

Thymidine kinase and protein kinase in drug-resistant herpesviruses: Heads of a Lernaean Hydra

Herpesviruses thymidine kinase (TK) and protein kinase (PK) allow the activation of nucleoside analogues used in anti-herpesvirus treatments. Mutations emerging in these two genes often lead to emergence of drug-resistant strains responsible for life-threatening diseases in immunocompromised populations. In this review, we analyze the binding of different nucleoside analogues to the TK active site of the three α-herpesviruses [Herpes Simplex Virus 1 and 2 (HSV-1 and HSV-2) and Varicella-Zoster Virus (VZV)] and present the impact of known mutations on the structure of the viral TKs. Furthermore, models of β-herpesviruses [Human cytomegalovirus (HCMV) and human herpesvirus-6 (HHV-6)] PKs allow to link amino acid changes with resistance to ganciclovir and/or maribavir, an investigational chemotherapeutic used in patients with multidrug-resistant HCMV. Finally, we set the basis for the understanding of drug-resistance in γ-herpesviruses [Epstein-Barr virus (EBV) and Kaposi's sarcoma associated herpesvirus (KSHV)] TK and PK through the use of animal surrogate models.

Antagonistic Relationship between Human Cytomegalovirus pUL27 and pUL97 Activities during Infection

Human cytomegalovirus (HCMV) is a member of the betaherpesvirus family. During infection, an array of viral proteins manipulates the host cell cycle. We have previously shown that expression of HCMV pUL27 results in increased levels of the cyclin-dependent kinase (CDK) inhibitor p21(Cip1). In addition, pUL27 is necessary for the full antiviral activity of the pUL97 kinase inhibitor maribavir (MBV). The purpose of this study was to define the relationship between pUL27 and pUL97 and its role in MBV antiviral activity. We observed that expression of wild-type but not kinase-inactive pUL97 disrupted pUL27-dependent induction of p21(Cip1). Furthermore, pUL97 associated with and promoted the phosphorylation of pUL27. During infection, inhibition of the kinase resulted in elevated levels of p21(Cip1) in wild-type virus but not a pUL27-deficient virus. We manipulated the p21(Cip1) levels to evaluate the functional consequence to MBV. Overexpression of p21(Cip1) restored MBV activity against a pUL27-deficient virus, while disruption reduced activity against wild-type virus. We provide evidence that the functional target of p21(Cip1) in the context of MBV activity is CDK1. One CDK-like activity of pUL97 is to phosphorylate nuclear lamin A/C, resulting in altered nuclear morphology and increased viral egress. In the presence of MBV, we observed that infection using a pUL27-deficient virus still altered the nuclear morphology. This was prevented by the addition of a CDK inhibitor. Overall, our results demonstrate an antagonistic relationship between pUL27 and pUL97 activities centering on p21(Cip1) and support the idea that CDKs can complement some activities of pUL97.

IMPORTANCE

HCMV infection results in severe disease upon immunosuppression and is a leading cause of congenital birth defects. Effective antiviral compounds exist, yet they exhibit high levels of toxicity, are not approved for use during pregnancy, and can result in antiviral resistance. Our studies have uncovered new information regarding the antiviral efficacy of the HCMV pUL97 kinase inhibitor MBV as it relates to the complex interplay between pUL97 and a second HCMV protein, pUL27. We demonstrate that pUL97 functions antagonistically against pUL27 by phosphorylation-dependent inactivation of pUL27-mediated induction of p21(Cip1). In contrast, we provide evidence that p21(Cip1) functions to antagonize overlapping activities between pUL97 and cellular CDKs. In addition, these studies further support the notion that CDK inhibitors or p21(Cip1) activators might be useful in combination with MBV to effectively inhibit HCMV infections.

Human cytomegalovirus pUL97 kinase induces global changes in the infected cell phosphoproteome

Replication of human cytomegalovirus (HCMV) is regulated in part by cellular kinases and the single viral Ser/Thr kinase, pUL97. The virus-coded kinase augments the replication of HCMV by enabling nuclear egress and altering cell cycle progression. These roles are accomplished through direct phosphorylation of nuclear lamins and the retinoblastoma protein, respectively. In an effort to identify additional pUL97 substrates, we analyzed the phosphoproteome of SILAC-labeled human fibroblasts during infection with either wild-type HCMV or a pUL97 kinase-dead mutant virus. Phosphopeptides were enriched over a titanium dioxide matrix and analyzed by high-resolution MS. We identified 157 unambiguous phosphosites from 106 cellular and 17 viral proteins whose phosphorylation required UL97. Analysis of peptides containing these sites allowed the identification of several candidate pUL97 phosphorylation motifs, including a completely novel phosphorylation motif, LxSP. Substrates harboring the LxSP motif were enriched in nucleocytoplasmic transport functions, including a number of components of the nuclear pore complex. These results extend the known functions of pUL97 and suggest that modulation of nuclear pore function may be important during HCMV replication.

Codon usage bias in human cytomegalovirus and its biological implication

Human cytomegalovirus (HCMV) infection, a worldwide contagion, causes a serious disorder in infected individuals. Analysis of codon usage can reveal much molecular information about this virus. The effective number of codon (ENC) values, relative synonymous codon usage (RSCU) values, codon adaptation index (CAI), and nucleotide contents was investigated in approximately 160 coding sequences (CDS) among 17 human cytomegalovirus genomes using the software CodonW. Linear regression analysis and logistic regression were performed to explore the preliminary data. The results showed that, overall, HCMV genomes had low codon usage bias (mean ENC=47.619). However, the ENC of individual CDS varied widely and was distributed unevenly between host-related genes and viral-self-function genes (P=0.002, odds ratio (OR)=3.194), as did the GC content (P=0.016, OR=2.178). The ENC values correlated with CAI, GC content, and the nucleotide composing at the 3rd codon position (GC3s) (P<0.001). There was a significant variation in the codon preference that depended on the RSCU data. The predicted ENC curve suggested that mutational pressure, rather than natural selection, was one of the main factors that determined the codon usage bias in HCMV. Among 123 genes with known function, the genes related to viral self-replication and viral-host interaction showed different ENC and CAI values, and GC and GC3s contents. In conclusion, the detailed codon usage bias theoretically revealed information concerning HCMV evolution and could be a valuable additional parameter for HCMV gene function research.

Host-virus interactions: from the perspectives of epigenetics

Chromatin structure and histone modifications play key roles in gene regulation. Some virus genomes are organized into chromatin-like structure, which undergoes different histone modifications facilitating complex functions in virus life cycles including replication. Here, we present a comprehensive summary of recent research in this field regarding the interaction between viruses and host epigenetic factors with emphasis on how chromatin modifications affect viral gene expression and virus infection. We also describe the strategies employed by viruses to manipulate the host epigenetic program to facilitate virus replication as well as the underlying mechanisms. Together, knowledge from this field not only generates novel insights into virus life cycles but may also have important therapeutic implications.

Human cytomegalovirus UL97 kinase alters the accumulation of CDK1

The UL97 protein kinase is a serine/threonine kinase expressed by human cytomegalovirus (CMV) that phosphorylates ganciclovir. An investigation of the subcellular localization of pUL97 in infected cells indicated that, early in infection, pUL97 localized to focal sites in the nucleus that transitioned to subnuclear compartments and eventually throughout the entire nucleus. When UL97 kinase activity was eliminated with a K355M mutation or pharmacologically inhibited with maribavir, the expansion and redistribution of pUL97 foci within the nucleus was delayed, nuclear reorganization did not occur and assembly complexes in the cytoplasm failed to form normally. As UL97 kinase and its homologues appear to be functionally related to CDK1, a known regulator of nuclear structural organization, the effects of the UL97 kinase on CDK1 were investigated. Expression of CDK1 in infected cells appeared to be induced by UL97 kinase activity at the level of transcription and was not tied to other virus life-cycle events, such as viral DNA replication or virion assembly. These results suggest that, in addition to phosphorylating CDK1 targets, the UL97 kinase modifies G₂/M cell-cycle checkpoint regulators, specifically CDK1, to promote virus 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.

Histone modifications in herpesvirus infections

In eukaryotic cells, gene expression is not only regulated by transcription factors but also by several epigenetic mechanisms including post-translational modifications of histone proteins. There are numerous histone modifications described to date and methylation, acetylation, ubiquitination and phosphorylation are amongst the best studied. In parallel, certain viruses interact with the very same regulatory mechanisms, hereby manipulating the normal epigenetic landscape of the host cell, to fit their own replication needs. This review concentrates on herpesviruses specifically and how they interfere with the histone-modifying enzymes to regulate their replication cycles. Herpesviruses vary greatly with respect to the cell types they infect and the clinical diseases they cause, yet they share various common features including their capacity to encode viral proteins which affect and interfere with the normal functions of histone-modifying enzymes. Studying the epigenetic manipulation/dysregulation of herpesvirus-host interactions not only generates novel insights into the pathogenesis of these viruses but may also have important therapeutic implications.

Maribavir and human cytomegalovirus-what happened in the clinical trials and why might the drug have failed?

We summarize the history of the clinical drug development of maribavir for its use as prophylaxis in stem-cell transplant recipients. We highlight key aspects in the design and interpretation of the results of the dose escalation phase II maribavir study that may have contributed to the negative findings on the phase III trials. We discuss key aspects of study design that should be considered in the study of new interventions needed to advance the prevention and treatment of CMV in transplant recipients.

How to control an infectious bead string: nucleosome-based regulation and targeting of herpesvirus chromatin

Herpesvirus infections of humans can cause a broad variety of symptoms ranging from mild afflictions to life-threatening disease. During infection, the large double-stranded DNA genomes of all herpesviruses are transcribed, replicated and encapsidated in the host cell nucleus, where DNA is typically structured and manoeuvred through nucleosomes. Nucleosomes individually assemble DNA around core histone octamers to form 'beads-on-a-string' chromatin fibres. Herpesviruses have responded to the advantages and challenges of chromatin formation in biologically unique ways. Although herpesvirus DNA is devoid of histones within nucleocapsids, nuclear viral genomes most likely form irregularly arranged or unstable nucleosomes during productive infection, and regular nucleosomal arrays resembling host cell chromatin in latently infected cells. Besides variations in nucleosome density, herpesvirus chromatin 'bead strings' undergo dynamic changes in histone composition and modification during the different stages of productive replication, latent infection and reactivation from latency, raising the likely possibility that epigenetic processes may dictate, at least in part, the outcome of infection and ensuing pathogenesis. Here, we summarise and discuss several new and important aspects regarding the nucleosome-based mechanisms that regulate herpesvirus chromatin structure and function in infected cells. Special emphasis is given to processes of histone deposition, histone variant exchange and covalent histone modification in relation to the transcription from the viral genome during productive and latent infections by human cytomegalovirus and herpes simplex virus type 1. We also present an overview on emerging histone-directed antiviral strategies that may be developed into 'epigenetic therapies' to improve current prevention and treatment options targeting herpesvirus infection and disease.

Sites and roles of phosphorylation of the human cytomegalovirus DNA polymerase subunit UL44

The human cytomegalovirus DNA polymerase subunit UL44 is a phosphoprotein, but its sites and roles of phosphorylation have not been investigated. We compared sites of phosphorylation of UL44 in vitro by the viral protein kinase UL97 and cyclin-dependent kinase 1 with those in infected cells. Transient treatment of infected cells with a UL97 inhibitor greatly reduced labeling of two minor UL44 phosphopeptides. Viruses containing alanine substitutions of most UL44 residues that are phosphorylated in infected cells exhibited at most modest effects on viral DNA synthesis and yield. However, substitution of highly phosphorylated sites adjacent to the nuclear localization signal abolished viral replication. The results taken together are consistent with UL44 being phosphorylated directly by UL97 during infection, and a crucial role for phosphorylation-mediated nuclear localization of UL44 for viral replication, but lend little support to the widely held hypothesis that UL97-mediated phosphorylation of UL44 is crucial for viral DNA synthesis.

Human cytomegalovirus UL97 kinase and nonkinase functions mediate viral cytoplasmic secondary envelopment

Previous studies have revealed critical roles for the human cytomegalovirus (HCMV) UL97 kinase in viral nuclear maturation events. We have shown recently that UL97 affects the morphology of the viral cytoplasmic assembly compartment (AC) (M. Azzeh, A. Honigman, A. Taraboulos, A. Rouvinski, and D. G. Wolf, Virology 354:69-79, 2006). Here, we employed a comprehensive ultrastructural analysis to dissect the impact of UL97 on cytoplasmic steps of HCMV assembly. Using UL97 deletion (ΔUL97) and kinase-null (K355M) mutants, as well as the UL97 kinase inhibitor NGIC-I, we demonstrated that the loss of UL97 kinase activity resulted in a unique combination of cytoplasmic features: (i) the formation of pp65-rich aberrant cytoplasmic tegument aggregates, (ii) distorted intracytoplasmic membranes, which replaced the normal architecture of the AC, and (iv) a paucity of cytoplasmic tegumented capsids and dense bodies (DBs). We further showed that these abnormal assembly intermediates did not result from impaired nuclear capsid maturation and egress per se by using 2-bromo-5,6-dichloro-1-(β-d-ribofuranosyl) benzimidizole (BDCRB) to induce the artificial inhibition of nuclear maturation and the nucleocytoplasmic translocation of capsids. The specific abrogation of UL97 kinase activity under low-multiplicity-of-infection conditions resulted in the improved release of extracellular virus compared to that of ΔUL97, despite similar rates of viral DNA accumulation and similar effects on nuclear capsid maturation and egress. The only ultrastructural correlate of the growth difference was a higher number of cytoplasmic DBs, tegumented capsids, and clustered viral particles observed upon the specific abrogation of UL97 kinase activity compared to that of ΔUL97. These combined findings reveal a novel role for UL97 in HCMV cytoplasmic secondary envelopment steps, with a further distinction of kinase-mediated function in the formation of the virus-induced AC and a nonkinase function enhancing the efficacy of viral tegumentation and release.

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.

Human cytomegalovirus UL97 kinase prevents the deposition of mutant protein aggregates in cellular models of Huntington's disease and ataxia

The presence of aggregates of abnormally expanded polyglutamine (polyQ)-containing proteins are a pathological hallmark of a number of neurodegenerative diseases including Huntington's disease (HD) and spinocerebellar ataxia-3 (SCA3). Previous studies in cellular, Drosophila, and mouse models of HD and SCA have shown that neurodegeneration can be prevented by manipulations that inhibit polyQ aggregation. We have shown that the UL97 kinase of the human cytomegalovirus (HCMV) prevents aggregation of the pp71 and pp65 viral tegument proteins. To explore whether UL97 may act as a general antiaggregation factor, we examined whether UL97 prevents aggregation of cellular non-polyQ and polyQ proteins. We report that UL97 prevents the deposition of aggregates of two non-polyQ proteins: a protein chimera (GFP170*) composed of the green fluorescent protein and a fragment of the Golgi Complex protein (GCP-170) and a chimera composed of the red fluorescent protein (RFP) fused to the Werner syndrome protein (WRN), a RecQ helicase and exonuclease involved in DNA repair. Furthermore, we show that UL97 inhibits aggregate deposition in cellular models of HD and SCA3. UL97 prevents the deposition of aggregates of the mutant huntingtin exon 1 containing 82 glutamine repeats (HttExon1-Q82) or full length ataxin-3 containing a 72 polyQ track (AT3-72Q). The kinase activity of UL97 appears critical, as the kinase-dead UL97 mutant (K335M) fails to prevent aggregate formation. We further show that UL97 disrupts nuclear PML bodies and decreases p53-mediated transcription. The universality of the antiaggregation effect of UL97 suggests that UL97 targets a key cellular factor that regulates cellular aggregation mechanisms. Our results identify UL97 as a novel means to modulate polyQ aggregation and suggest that UL97 can serve as a novel tool to probe the cellular mechanisms that contribute to the formation of aggregates in polyglutamine disorders.

Inactivation and disassembly of the anaphase-promoting complex during human cytomegalovirus infection is associated with degradation of the APC5 and APC4 subunits and does not require UL97-mediated phosphorylation of Cdh1

Infection of quiescent cells by human cytomegalovirus (HCMV) elicits severe cell cycle deregulation, resulting in a G(1)/S arrest, which can be partly attributed to the inactivation of the anaphase-promoting complex (APC). As we previously reported, the premature phosphorylation of its coactivator Cdh1 and/or the dissociation of the core complex can account for the inactivation. We have expanded on these results and further delineated the key components required for disabling the APC during HCMV infection. The viral protein kinase UL97 was hypothesized to phosphorylate Cdh1, and consistent with this, phosphatase assays utilizing a virus with a UL97 deletion mutation (ΔUL97 virus) indicated that Cdh1 is hypophosphorylated at early times in the infection. Mass spectrometry analysis demonstrated that UL97 can phosphorylate Cdh1 in vitro, and the majority of the sites identified correlated with previously characterized cyclin-dependent kinase (Cdk) consensus sites. Analysis of the APC core complex during ΔUL97 virus infection showed APC dissociation occurring at the same time as during infection with wild-type virus, suggesting that the UL97-mediated phosphorylation of Cdh1 is not required for this to occur. Further investigation of the APC subunits showed a proteasome-dependent loss of the APC5 and APC4 subunits that was temporally associated with the disassembly of the APC. Immediate early viral gene expression was not sufficient for the degradation of APC4 and APC5, indicating that a viral early gene product(s), possibly in association with a de novo-synthesized cellular protein(s), is involved.

Function of human cytomegalovirus UL97 kinase in viral infection and its inhibition by maribavir

The serine/threonine kinase expressed by human cytomegalovirus from gene UL97 phosphorylates the antiviral drug ganciclovir, but its biological function is the phosphorylation of its natural viral and cellular protein substrates which affect viral replication at many levels. The UL97 kinase null phenotype is therefore complex, as is the mechanism of action of maribavir, a highly specific inhibitor of its enzymatic activity. Studies that utilise the drug corroborate results from genetic approaches and together have elucidated many functions of the UL97 kinase that are critical for viral replication. The kinase phosphorylates eukaryotic elongation factor 1delta, the carboxyl terminal domain of the large subunit of RNA polymerase II, the retinoblastoma tumour suppressor and lamins A and C. Each of these is also phosphorylated and regulated by cdc2/cyclin-dependent kinase 1, suggesting that the viral kinase may perform a similar function. These and other activities of the UL97 kinase appear to stimulate the cell cycle to support viral DNA synthesis, enhance the expression of viral genes, promote virion morphogenesis and facilitate the egress of mature capsids from the nucleus. In the absence of UL97 kinase activity, viral DNA synthesis is inefficient and structural proteins are sequestered in nuclear aggresomes, reducing the efficiency of virion morphogenesis. Mature capsids that do form fail to egress the nucleus as the nuclear lamina are not dispersed by the kinase. The critical functions performed by the UL97 kinase illustrate its importance in viral replication and confirm that the kinase is a target for the development of antiviral therapies.

Viral-encoded enzymes that target host chromatin functions

Ever since their existence, there has been an everlasting arms race between viruses and their host cells. Host cells have developed numerous strategies to silence viral gene expression whereas viruses always find their ways to overcome these obstacles. Recent studies show that viruses have also evolved to take full advantage of existing cellular chromatin components to activate or repress its own genes when needed. While in most cases viruses encode certain proteins to recruit or inhibit cellular factors through physical interactions, growing examples show that viral-encoded enzymes affect host chromatin structure through post-translationally modifying histones or other cellular proteins important for chromatin function. The most well-studied example is vSET encoded by paramecium bursaria chlorella virus 1. vSET specifically methylates histone H3 at lysine 27, causing genome-wide silencing of Polycomb target genes upon infection, thus mimicking the function of Polycomb repressive complex 2 (PRC2) in eukaryotes. Other examples include BGLF4 from Epstein-Barr virus that affects both condensin and topoisomerase II activity and Us3 from Herpes Simplex virus 1 that inhibits HDAC1 function through phosphorylation.

Conserved domains and structure prediction of human cytomegalovirus UL27 protein

Background

The human cytomegalovirus (HCMV) nuclear UL27 protein (pUL27) could be involved at the stage of nuclear egress. Maribavir is a new anti-HCMV drug that targets nuclear egress through direct inhibition of the HCMV serine–threonine kinase, UL97 protein (pUL97). Because maribavir- resistance-related mutations are observed in both proteins, pUL27 is thought to interfere with pUL97 activity; however, its mechanism of action remains unclear.

Methods

As there is no available crystal structure for pUL27 or any known structures of its homologous proteins, we attempted to identify pUL27 functional domains by sequence analysis, identification of conserved domains, structure prediction and matching with previously known maribavir resistance mutations.

Results

The UL27 sequence analysis of 20 HCMV wild-type strains and 8 ganciclovir-resistant HCMV strains allowed us to describe four conserved domains, to localize the putative phosphorylation sites and to identify protein–protein interface domains, suggesting that pUL27 could interact with either pUL97 or itself.

Conclusions

Although the function of pUL27 is still unknown in the HCMV replication cycle, our approach identified target domains that appeared to be essential to the function of pUL27. This work provides a better understanding on the relative importance of each pUL27 mutation and could form the basis of later comparison analyses, when a three-dimensional structure of a pUL27 homologue will be available.

Viral mimicry of Cdc2/cyclin-dependent kinase 1 mediates disruption of nuclear lamina during human cytomegalovirus nuclear egress

The nuclear lamina is a major obstacle encountered by herpesvirus nucleocapsids in their passage from the nucleus to the cytoplasm (nuclear egress). We found that the human cytomegalovirus (HCMV)-encoded protein kinase UL97, which is required for efficient nuclear egress, phosphorylates the nuclear lamina component lamin A/C in vitro on sites targeted by Cdc2/cyclin-dependent kinase 1, the enzyme that is responsible for breaking down the nuclear lamina during mitosis. Quantitative mass spectrometry analyses, comparing lamin A/C isolated from cells infected with viruses either expressing or lacking UL97 activity, revealed UL97-dependent phosphorylation of lamin A/C on the serine at residue 22 (Ser(22)). Transient treatment of HCMV-infected cells with maribavir, an inhibitor of UL97 kinase activity, reduced lamin A/C phosphorylation by approximately 50%, consistent with UL97 directly phosphorylating lamin A/C during HCMV replication. Phosphorylation of lamin A/C during viral replication was accompanied by changes in the shape of the nucleus, as well as thinning, invaginations, and discrete breaks in the nuclear lamina, all of which required UL97 activity. As Ser(22) is a phosphorylation site of particularly strong relevance for lamin A/C disassembly, our data support a model wherein viral mimicry of a mitotic host cell kinase activity promotes nuclear egress while accommodating viral arrest of the cell cycle.

Conserved retinoblastoma protein-binding motif in human cytomegalovirus UL97 kinase minimally impacts viral replication but affects susceptibility to maribavir

The UL97 kinase has been shown to phosphorylate and inactivate the retinoblastoma protein (Rb) and has three consensus Rb-binding motifs that might contribute to this activity. Recombinant viruses containing mutations in the Rb-binding motifs generally replicated well in human foreskin fibroblasts with only a slight delay in replication kinetics. Their susceptibility to the specific UL97 kinase inhibitor, maribavir, was also examined. Mutation of the amino terminal motif, which is involved in the inactivation of Rb, also renders the virus hypersensitive to the drug and suggests that the motif may play a role in its mechanism of action.

Cytomegalovirus UL97 mutations in the era of ganciclovir and maribavir

Mutations in the human CMV UL97 kinase gene are a major mechanism of viral resistance to two anti-CMV drugs, ganciclovir (GCV) and maribavir (MBV). GCV, the most widely used and established therapy for CMV, is a substrate for the UL97 kinase. Well-characterised GCV-resistance mutations at UL97 codons 460, 520 and 590-607 impair the phosphorylation of GCV that is necessary for its antiviral activity, presumably by altering substrate recognition. In contrast, MBV is an inhibitor of the UL97 kinase and is the first new CMV therapy to reach later stage clinical trials in many years. No MBV-resistant CMV isolates have yet been detected in clinical trials, but after culture propagation under drug, UL97 mutations that confer moderate to high-level MBV resistance have been identified at codons 353, 397, 409 and 411. These mutations are located upstream of the GCV-resistance mutations and are close to the ATP-binding and catalytic domains common to all kinases, consistent with MBV acting as a small molecule ATP-competitive kinase inhibitor. So far, no UL97 mutations are known to confer resistance to both GCV and MBV.

Cellular serine/threonine phosphatase activity during human cytomegalovirus infection

While the importance of cellular and viral kinases in HCMV replication has been demonstrated, relatively little is known about the activity of cellular phosphatases. We conducted a series of experiments designed to investigate the effect of HCMV infection on cellular serine/threonine phosphatase activity. We found that the abundance of two major cellular serine/threonine phosphatases, PP1 and PP2A, increases during HCMV infection. This was associated with an increase in threonine phosphatase activity in HCMV-infected cells. HCMV infection conferred resistance to the effects of the phosphatase inhibitors calyculin A (CA) and okadaic acid with regards to global protein hyperphosphorylation and the shutoff of protein synthesis. The protective effect of HCMV infection could be overcome at a high concentration of CA, suggesting that cellular phosphatase activity is required for critical cellular processes during HCMV infection. Specifically, phosphatase activity was required to limit the accumulation of phospho-eIF2alpha, but not phospho-PKR, during HCMV infection.

Substrate specificity of the herpes simplex virus type 2 UL13 protein kinase

The UL13 protein kinase is conserved among many herpesviruses but HSV-2 UL13 specificity is not known. Here, we found that HSV-2 UL13 is a phosphoprotein that autophosphorylates, and that serines within ERK and Cdc2 motifs were important for autophosphorylation but not for UL13 phosphorylation of exogenous substrates. HSV-2 UL13 phosphorylated a peptide also recognized by ERK and Cdc2. However, mutation of substrate residues critical for Cdc2 or Erk phosphorylation did not alter HSV-2 UL13 phosphorylation of the peptide, and HSV-2 UL13 did not phosphorylate standard Cdc2 or Erk peptide substrates. Mutation of prolines surrounding the peptide phosphoacceptor site reduced phosphorylation by HSV-2 UL13, and a peptide containing serine-proline amid alanines and glycines was phosphorylated. Thus, HSV-2 UL13 does not mimic ERK or Cdc2 substrate recognition and its minimal recognition motif can be serine-proline. This motif's simplicity indicates that distal sequence or protein structure contributes to HSV-2 UL13 substrate specificity.

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.

Human cytomegalovirus protein kinase UL97 forms a complex with the tegument phosphoprotein pp65

UL97 is a protein kinase encoded by human cytomegalovirus (HCMV) and is an important target for antiviral drugs against this ubiquitous herpesvirus, which is a major cause of life-threatening opportunistic infections in the immunocompromised host. In an effort to better understand the function(s) of UL97 during HCMV replication, a recombinant HCMV, NTAP97, which expresses a tandem affinity purification (TAP) tag at the amino terminus of UL97, was used to obtain UL97 protein complexes from infected cells. pp65 (also known as UL83), the 65-kDa virion tegument phosphoprotein, specifically copurified with UL97 during TAP, as shown by mass spectrometry and Western blot analyses. Reciprocal coimmunoprecipitation experiments using lysates of infected cells also indicated an interaction between UL97 and pp65. Moreover, in a glutathione S-transferase (GST) pull-down experiment, purified GST-pp65 fusion protein specifically bound in vitro-translated UL97, suggesting that UL97 and pp65 do not require other viral proteins to form a complex and may directly interact. Notably, pp65 has been previously reported to form unusual aggregates during viral replication when UL97 is pharmacologically inhibited or genetically ablated, and a pp65 deletion mutant was observed to exhibit modest resistance to a UL97 inhibitor (M. N. Prichard, W. J. Britt, S. L. Daily, C. B. Hartline, and E. R. Kern, J. Virol. 79:15494-15502, 2005). A stable protein-protein interaction between pp65 and UL97 may be relevant to incorporation of these proteins into HCMV particles during virion morphogenesis, with potential implications for immunomodulation by HCMV, and may also be a mechanism by which UL97 is negatively regulated during HCMV replication.

Ligand profiling and identification technology for searching bioactive ligands

We introduce a new methodology named ligand profiling and identification for effective discovery of bioactive ligands such as peptide hormones. This technology was developed from a new concept of parallel column chromatography and active fraction profiling by nano-LC MS. Traditional methods use sequential column chromatography, and thus are inevitably limited by the low abundance of the peptide of interest and by a low yield due to the many column steps. Using this new technology, insulin was successfully identified and diarginylinsulin, a minor intermediate form of insulin, was unexpectedly also identified simultaneously from 100 mg of porcine pancreatic tissue. This integrative technology could be used to search for various low-abundance peptides (or bioactive molecules) rapidly and simultaneously, by applying this to the later stages of traditional sequential purification.

Human cytomegalovirus UL97 Kinase is required for the normal intranuclear distribution of pp65 and virion morphogenesis

Recombinant human cytomegaloviruses that do not express UL97 kinase activity exhibit a distinctive plaque morphology characterized by the formation of highly refractile bodies late in infection. These structures were also observed in infected cells treated with the UL97 kinase inhibitor maribavir. Nuclear inclusions were purified to near homogeneity, and the constituent proteins were identified by matrix-assisted laser desorption ionization-time-of-flight mass spectrometry. This analysis demonstrated that the aggregates were formed principally of the tegument proteins pp65 and ppUL25 but also contained additional virion structural proteins including the major capsid protein. Immunoblotting experiments confirmed these results and identified a number of additional viral proteins present in the purified tegument aggregates. Interestingly, the formation of these structures appeared to be dependent on pp65, since it was not induced in cells infected with a recombinant virus with this open reading frame deleted. Morphologically similar aggregates could be reproduced in nuclei of uninfected cells by overexpressing pp65, and their formation was prevented by coexpressing the UL97 kinase. Inhibition of UL97 kinase activity with maribavir or mutation of an essential amino acid in the kinase abolished its ability to prevent aggregate formation. These data taken together suggest that the UL97 kinase impacts the aggregation of pp65 in the nuclei of infected cells. We propose that the kinase plays an important role in the acquisition of tegument during virion morphogenesis in the nucleus and that this activity represents an important step in the production of mature virus particles.

Update on human herpesvirus 6 biology, clinical features, and therapy

Human herpesvirus 6 (HHV-6) is a betaherpesvirus that is closely related to human cytomegalovirus. It was discovered in 1986, and HHV-6 literature has expanded considerably in the past 10 years. We here present an up-to-date and complete overview of the recent developments concerning HHV-6 biological features, clinical associations, and therapeutic approaches. HHV-6 gene expression regulation and gene products have been systematically characterized, and the multiple interactions between HHV-6 and the host immune system have been explored. Moreover, the discovery of the cellular receptor for HHV-6, CD46, has shed a new light on HHV-6 cell tropism. Furthermore, the in vitro interactions between HHV-6 and other viruses, particularly human immunodeficiency virus, and their relevance for the in vivo situation are discussed, as well as the transactivating capacities of several HHV-6 proteins. The insight into the clinical spectrum of HHV-6 is still evolving and, apart from being recognized as a major pathogen in transplant recipients (as exemplified by the rising number of prospective clinical studies), its role in central nervous system disease has become increasingly apparent. Finally, we present an overview of therapeutic options for HHV-6 therapy (including modes of action and resistance mechanisms).

Expression and localization of the Epstein-Barr virus-encoded protein kinase

The protein kinase (PK) encoded by the Epstein-Barr Virus (EBV) BGLF4 gene is the only EBV protein kinase. The expression pattern of EBV PK during the reactivation of the viral lytic cycle and the subcellular localization of the protein were analyzed with a polyclonal antiserum raised against a peptide corresponding to the N terminus of EBV PK. Based on previously published data (E. Gershburg and J. S. Pagano, J. Virol. 76:998-1003, 2002) and the expression pattern described here, we conclude that EBV PK is an early protein that requires viral-DNA replication for maximum expression. By biochemical fractionation, the protein could be detected mainly in the nuclear fraction 4 h after viral reactivation in Akata cells. Nuclear localization could be visualized by indirect immunofluorescence in HeLa cells transiently expressing EBV BGLF4 in the absence of other viral products. Transient expression of 3'-terminal deletion mutants of EBV BGLF4 resulted in cytoplasmic localization, confirming the presence of a nuclear localization site in the C-terminal region of the protein. In contrast to the wild-type EBV PK, all of the mutants were unable to hyperphosphorylate EA-D during coexpression or to phosphorylate ganciclovir, as measured by an in-cell activity assay. Thus, the results demonstrate that the nuclear localization, as well as the kinase activity, of BGFL4 is dependent on an intact C-terminal region.

Phosphorylation of the RNA polymerase II carboxyl-terminal domain in human cytomegalovirus-infected cells and in vitro by the viral UL97 protein kinase

The carboxyl-terminal domain (CTD) of the largest subunit of RNA polymerase II (RNAP II) ordinarily exists in electrophoretically distinct hypophosphorylated and hyperphosphorylated forms. Human cytomegalovirus infection induced forms of this subunit whose electrophoretic mobilities were intermediate without decreases in abundance of the original forms. Phosphatase treatment nearly eliminated the intermediate migrating forms. In vitro, the viral protein kinase, UL97, phosphorylated this subunit, a recombinant protein containing the CTD, and peptides containing the CTD consensus sequence, YSPTSPS. Phosphorylation occurred predominantly on serine 5 and was substantially reduced when either serine 2 or 5 was already phosphorylated. The abundance of the intermediate and hypophosphorylated forms was reduced at most twofold during infections in which UL97 was genetically or pharmacologically inhibited. These results identify a new pattern of RNA polymerase II modification induced by virus infection and a viral enzyme that phosphorylates the CTD in vitro, but only possibly in vivo.

Mutations in the human cytomegalovirus UL27 gene that confer resistance to maribavir

Previous drug selection experiments resulted in the isolation of a human cytomegalovirus (CMV) UL97 phosphotransferase mutant resistant to the benzimidazole compound maribavir (1263W94), reflecting the anti-UL97 effect of this drug. Three other CMV strains were plaque purified during these experiments. These strains showed lower-grade resistance to maribavir than the UL97 mutant. Extensive DNA sequence analyses showed no changes from the baseline strain AD169 in UL97, the genes involved in DNA replication, and most structural proteins. However, changes were identified in UL27 where each strain contained a different mutation (R233S, W362R, or a combination of A406V and a stop at codon 415). The mutation at codon 415 is predicted to truncate the expressed UL27 protein by 193 codons (32% of UL27) with a loss of nuclear localization. The expression of full-length UL27 as a green fluorescent fusion protein in uninfected fibroblasts resulted in nuclear and nucleolar fluorescence, whereas cytoplasmic localization was observed when codons 1 to 415 were similarly expressed. Viable UL27 deletion mutants were created by recombination and showed slight growth attenuation and maribavir resistance in cell culture. Marker transfer experiments confirmed that UL27 mutations conferred maribavir resistance. The UL27 sequence was well conserved in a sample of 16 diverse clinical isolates. Mutation in UL27, a betaherpesvirus-specific early gene of unknown biological function, may adapt the virus for growth in the absence of UL97 activity.

Effects of maribavir and selected indolocarbazoles on Epstein-Barr virus protein kinase BGLF4 and on viral lytic replication

The human cytomegalovirus (HCMV) homolog of the Epstein-Barr virus (EBV) protein kinase (PK), UL97, is inhibited by maribavir (1263W94) and selected indolocarbazoles. Here we show that only one of these indolocarbazoles (K252a), but not maribavir, inhibits autophosphorylation of the EBV PK, BGLF4. However, maribavir and another indolocarbazole, NGIC-I, do inhibit EBV DNA synthesis, suggesting that although these last compounds inhibit both HCMV and EBV, they seem to operate through differ-ent pathways.

The UL97 protein kinase of human cytomegalovirus and homologues in other herpesviruses: impact on virus and host

The human herpesviruses, herpes simplex virus 1 (HSV-1), HSV-2, varicella zoster virus (VZV), Epstein-Barr virus (EBV), human cytomegalovirus (HCMV), human herpesvirus 6A (HHV-6A), HHV-6B, HHV-7 and HHV-8, establish persistent infections with possible recurrence during immunosuppression. HCMV replication is inhibited by the nucleoside analogue ganciclovir (GCV), the compound of choice for the treatment of HCMV diseases and preemptive treatment of infections. The viral UL97 protein (pUL97) which shares homologies with protein kinases and bacterial phosphotransferases is able to monophosphorylate GCV. Homologues of pUL97 are found in HSV (UL13), VZV (ORF47), EBV (BGLF4), HHV-6 (U69), HHV-8 (ORF36) as well as in murine CMV (M97) or rat CMV (R97). Several indolocarbazoles have been reported to be specific inhibitors of pUL97. The protein is important for efficient replication of the virus. Autophosphorylation of pUL97 was observed using different experimental systems. Most recently, it has been shown that pUL97 interacts with the DNA polymerase processivity factor pUL44. Indolocarbazole protein kinase inhibitors are promising lead compounds for the development of more specific inhibitors of HCMV.

Resistance of human cytomegalovirus to the benzimidazole L-ribonucleoside maribavir maps to UL27

1-(beta-D-Ribofuranosyl)-2,5,6-trichlorobenzimidazole (TCRB) and its 2-bromo analog, BDCRB, are potent and selective inhibitors of human cytomegalovirus (HCMV) DNA processing and packaging. Since they are readily metabolized in vivo, analogs were synthesized to improve biostability. One of these, 1-(beta-L-ribofuranosyl)-2-isopropylamino-5,6-dichlorobenzimidazole (1263W94; maribavir), inhibits viral DNA synthesis and nuclear egress. Resistance to maribavir was mapped to UL97, and this viral kinase was shown to be a direct target of maribavir. In the present study, an HCMV strain resistant to TCRB and BDCRB was passaged in increasing concentrations of maribavir, and resistant virus was isolated. This strain (G2) grew at the same rate as the wild-type virus and was resistant to both BDCRB and maribavir. Resistance to BDCRB was expected, because the parent strain from which G2 was isolated was resistant due to known mutations in UL56 and UL89. However, no mutations were found in UL97 or other relevant open reading frames that could explain resistance to maribavir. Because sequencing of selected HCMV genes did not identify the resistance mutation, a cosmid library was made from G2, and a series of recombinant G2 wild-type viruses were constructed. Testing the recombinants for sensitivity to maribavir narrowed the locus of resistance to genes UL26 to UL32. Sequencing identified a single coding mutation in ORF UL27 (Leu335Pro) as the one responsible for resistance to maribavir. These results establish that UL27 is either directly or indirectly involved in the mechanism of action of maribavir. They also suggest that UL27 could play a role in HCMV DNA synthesis or egress of HCMV particles from the nucleus.

Protein kinases conserved in herpesviruses potentially share a function mimicking the cellular protein kinase cdc2

Herpesviruses encode protein kinases. A subset of these proteins, represented by HSV-1 UL13, are conserved throughout all members of the Herpesviridae, and here, are designated CHPKs (conserved herpesvirus protein kinases). In addition to conserved gene products like CHPKs, herpesviruses encode genes specific to respective herpesviruses. When acting upon conserved viral gene products or cellular factors, CHPKs may play conserved roles in the life cycles of herpesviruses. CHPKs may also express unique functions within the infectious process of individual herpesviruses when specific viral gene products are targeted. CHPKs demonstrate specific activity in multiple herpesvirus infections, functioning in the regulation of viral gene expression in HSV-1, tissue tropism in VZV, and viral DNA synthesis, encapsidation and egress from the nucleus in HCMV. The HCMV CHPK, however, can partially substitute for the HSV-1 CHPK. Representative CHPKs from all Herpesviridae subfamilies can also facilitate the hyperphosphorylation of the cellular translation factor, EF-1delta. This indicates that CHPKs have conserved functions. Recent data have shown that both CHPKs and a cellular protein kinase, cdc2, phosphorylate the same amino acid residues of target proteins. Thus, CHPKs may mimic cdc2 function in infected cells.

The human cytomegalovirus UL44 protein is a substrate for the UL97 protein kinase

The human cytomegalovirus UL97 protein is an unusual protein kinase that is able to autophosphorylate and to phosphorylate certain exogenous substrates, including nucleoside analogs such as ganciclovir. However, no natural substrate of UL97 in infected cells has been identified. We report here that recombinant UL44 protein became radiolabeled when incubated with recombinant UL97 and [(32)P]ATP and that both proteins could be coimmunoprecipitated by an antibody that recognizes either protein. Subsequent studies showed that highly purified, recombinant UL97 phosphorylated purified, recombinant UL44. This phosphorylation occurred on serine and threonine residues and was sensitive to inhibition by maribavir and to a mutation that inactivates UL97 catalytic activity. Two-dimensional gel electrophoresis revealed the absence of specific phosphorylated forms of UL44 in immunoprecipitates from lysates of cells infected with a UL97 null mutant virus or with wild-type virus in the presence of maribavir. The results indicate that UL97 is sufficient to phosphorylate UL44 in vitro and is necessary for the normal phosphorylation of UL44 in infected cells. This strongly suggests that UL44 is a natural substrate of UL97.

The protein kinase pUL97 of human cytomegalovirus interacts with and phosphorylates the DNA polymerase processivity factor pUL44

The protein kinase pUL97 of human cytomegalovirus plays important but incompletely defined roles in viral replication. Concerning the early phase of infection, it is postulated that pUL97 possesses regulatory functions in gene expression and/or DNA synthesis. Here we report that pUL97 interacts with an essential component of the replication complex, the DNA polymerase processivity factor pUL44. Interaction was determined by yeast two-hybrid and coimmunoprecipitation analyses and was mapped to the pUL97 region 366-459. In vitro kinase assays demonstrated that pUL44, coimmunoprecipitated either from transfected or from infected cells, is phosphorylated by pUL97 (but not by a catalytically inactive pUL97-mutant). In infected fibroblasts, immunofluorescence analysis revealed that pUL97 and pUL44 accumulate in the nucleus and are both incorporated into viral replication centers. The treatment with inhibitors of DNA synthesis or pUL97 kinase activity largely prevented colocalization. Thus, pUL97 may be indirectly involved in viral genome replication by modifying the replication cofactor pUL44.

Antiherpesvirus drugs: a promising spectrum of new drugs and drug targets

In the absence of effective vaccines to control herpesvirus infections, nucleosidic antiviral drugs have been the mainstay of clinical treatment since their development in the late 1970s. However, given the drawbacks of these drugs, including the increasing emergence of drug-resistant clinical isolates, new strategies for treating herpesvirus infections are warranted. A range of promising new drugs with novel molecular targets has been developed, but will they cure latent infections?

The human cytomegalovirus UL97 protein kinase, an antiviral drug target, is required at the stage of nuclear egress

Human cytomegalovirus encodes an unusual protein kinase, UL97, that activates the established antiviral drug ganciclovir and is specifically inhibited by a new antiviral drug, maribavir. We used maribavir and a UL97 null mutant, which is severely deficient in viral replication, to determine what stage of virus infection critically requires UL97. Compared with wild-type virus, there was little or no decrease in immediate-early gene expression, viral DNA synthesis, late gene expression, or packaging of viral DNA into nuclease-resistant structures in mutant-infected or maribavir-treated cells under conditions where the virus yield was severely impaired. Electron microscopy studies revealed similar proportions of various capsid forms, including DNA-containing capsids, in the nuclei of wild-type- and mutant-infected cells. However, capsids were rare in the cytoplasm of mutant-infected or maribavir-treated cells; the magnitudes of these decreases in cytoplasmic capsids were similar to those for virus yield. Thus, genetic and pharmacological evidence indicates that UL97 is required at the stage of infection when nucleocapsids exit from the nucleus (nuclear egress), and this poorly understood stage of virus infection can be targeted by antiviral drugs. Understanding UL97 function and maribavir action should help elucidate this interesting biological process and help identify new antiviral drug targets for an important pathogen in immunocompromised patients.

Relationship between autophosphorylation and phosphorylation of exogenous substrates by the human cytomegalovirus UL97 protein kinase

Human cytomegalovirus encodes an unusual protein kinase, UL97, which is a member of the HvU(L) family of protein kinases encoded by diverse herpesviruses. UL97 is able to autophosphorylate and to phosphorylate certain exogenous substrates, including nucleoside analogs such as ganciclovir. It has previously been concluded that phosphorylation of UL97 is essential for its phosphorylation of ganciclovir. We examined the relationship between autophosphorylation of UL97 and its activity on exogenous substrates. Glutathione S-transferase-UL97 fusion protein purified from insect cells was found to be already partially phosphorylated, but neither extensive autophosphorylation nor phosphatase treatment meaningfully altered the time course of its phosphorylation of the exogenous substrate, histone H2B. Sequencing and mass spectrometric analyses of (32)P-labeled tryptic peptides of the UL97 fusion protein identified nine sites of autophosphorylation, all within the first 200 residues of the protein, outside of conserved protein kinase subdomains. A peptide corresponding to the N-terminal UL97 segment that was most extensively autophosphorylated was readily phosphorylated by UL97, confirming that fusion protein sequences are not required for phosphorylation at this site. Deletion mutants lacking at least the first 239 residues exhibited drastically reduced autophosphorylation (<5%) but retained near-wild-type H2B phosphorylation activity. Baculoviruses expressing these mutants efficiently directed the phosphorylation of ganciclovir in insect cells. Taken together, these results identify the autophosphorylation sites of a herpesvirus protein kinase and show that autophosphorylation of UL97 is not required for phosphorylation of exogenous substrates.