The six conserved helicase motifs of the UL5 gene product, a component of the herpes simplex virus type 1 helicase-primase, are essential for its function

Channel catfish virus ORF25 and ORF63 genes are essential for viral replication in vitro

The channel catfish virus (CCV) is a lethal pathogen to aquatic animals that can provoke severe haemorrhagic disease in juvenile channel catfish. Although the CCV genome has been fully sequenced, the molecular mechanisms of CCV infection and pathogenesis are less well known. Genomic DNA replication is a necessary and key event for the CCV life cycle. In this study, the impacts of the putative helicase and primase encoded by viral ORF25 and ORF63 on CCV genome replication and infection were evaluated in channel catfish ovary (CCO) cells. The results showed that the number of CCV genome copies was decreased significantly in virus-infected CCO cells after knockdown of ORF25 and ORF63 using RNA interference. In contrast, the overexpression of ORF25 and ORF63 led to slight increase in the number of virus genome copies. Consistent with the above results, the present results also showed that the expressions of CCV true-late genes which strictly depend on viral DNA replication, were significantly increased or repressed by overexpression or RNA interference targeting viral ORF25 and ORF63 genes in virus-infected CCO cells. In addition, knockdown of ORF25 and ORF63 remarkably inhibited CCV-induced cytopathic effects and decreased progeny virus titres in CCO cells. Moreover, transmission electron microscopy observation of CCO cells infected with CCV accompanied by siRNA targeting the viral ORF25 and ORF63 genes showed that the number of virus particles was remarkably reduced. Taken together, these results indicated that ORF25 and ORF63 are essential for regulating CCV genome replication and CCV-induced infection. Our findings will provide an understanding of the replication mechanisms of CCV and contribute to the development of antiviral strategies for controlling CCV infection in channel catfish culture.

Characteristics of Helicase-primase Inhibitor Amenamevir-resistant Herpes Simplex Virus

The antiherpetic drug amenamevir (AMNV) inhibits the helicase-primase complex of herpes simplex virus type 1 (HSV-1), HSV-2 and varicella-zoster virus directly as well as inhibiting the replication of these viruses. Although several mutated HSV viruses resistant to helicase-primase inhibitors have been reported, the mutations contributing to the resistance remain unclear as recombinant viruses containing a single mutation have not been analyzed. We obtained AMNV-resistant viruses with amino acid substitutions by several passages under AMNV-treatment. Twenty HSV-1 and 19 HSV-2 mutants with mutation(s) in UL5 helicase and/or UL52 primase, but not in co-factor UL8, were isolated. The mutations in UL5 were located downstream of motif IV, with UL5 K356N in HSV-1 and K355N in HSV-2, in particular, identified as having the highest frequency: 9/20 and 9/19, respectively. We generated recombinant AMNV-resistant HSV-1 with a single amino acid substitution using BAC mutagenesis. As a result, G352C in UL5 helicase and F360C/V and N902T in UL52 primase were identified as novel mutations. The virus with K356N in UL5 showed 10-fold higher AMNV resistance than did other mutants, and showed equivalent viral growth in vitro and virulence in vivo as the parent HSV-1, although other mutants showed attenuated virulence. All recombinant viruses were susceptible to the other antiherpetic drugs, acyclovir and foscarnet. In conclusion, based on BAC mutagenesis, this study identified for the first time mutations in UL5 and UL52 that contributed to AMNV resistance, and found that a mutant with the most frequent K356N mutation in HSV-1 maintained viral growth and virulence equivalent to the parent virus.

The three-component helicase/primase complex of herpes simplex virus-1

Herpes simplex virus type 1 (HSV-1) is one of the nine herpesviruses that infect humans. HSV-1 encodes seven proteins to replicate its genome in the hijacked human cell. Among these are the herpes virus DNA helicase and primase that are essential components of its replication machinery. In the HSV-1 replisome, the helicase-primase complex is composed of three components including UL5 (helicase), UL52 (primase) and UL8 (non-catalytic subunit). UL5 and UL52 subunits are functionally interdependent, and the UL8 component is required for the coordination of UL5 and UL52 activities proceeding in opposite directions with respect to the viral replication fork. Anti-viral compounds currently under development target the functions of UL5 and UL52. Here, we review the structural and functional properties of the UL5/UL8/UL52 complex and highlight the gaps in knowledge to be filled to facilitate molecular characterization of the structure and function of the helicase-primase complex for development of alternative anti-viral treatments.

Identification of Amino Acids Essential for Viral Replication in the HCMV Helicase-Primase Complex

Promising new inhibitors that target the viral helicase-primase complex have been reported to block replication of herpes simplex and varicella-zoster viruses, but they have no activity against human cytomegalovirus (HCMV), another herpesvirus. The HCMV helicase-primase complex (pUL105-pUL102-pUL70) is essential for viral DNA replication and could thus be a relevant antiviral target. The roles of the individual subunits composing this complex remain to be defined. By using sequence alignment of herpesviruses homologs, we identified conserved amino acids in the putative pUL105 ATP binding site and in the putative pUL70 zinc finger pattern. Mutational analysis of several of these amino acids both in pUL105 and pUL70, proved that they are crucial for viral replication. We also constructed, by homology modeling, a theoretical structure of the pUL105 N-terminal domain which indicates that the mutated conserved amino acids in this domain could be involved in ATP hydrolysis.

Production and characterisation of Epstein-Barr virus helicase-primase complex and its accessory protein BBLF2/3

The helicase-primase complex is part of the lytic DNA replication machinery of herpesviruses, but up to now, almost nothing is known about its structure. For Epstein-Barr virus it consists in the helicase BBLF4, the primase BSLF1 and the accessory protein BBLF2/3. The accessory protein shows only weak sequence homology within the herpesvirus family but may be related to an inactive B-family polymerase. BSLF1 belongs to the archaeo-eukaryotic primase family, whereas the helicase BBLF4 has been related either to Dda helicases of caudovirales or to Pif1 helicases. We produced the helicase-primase complex in insect cells using a baculovirus coding for all three proteins simultaneously. The soluble monomeric helicase-primase complex containing the three proteins with 1:1:1 stoichiometry showed ATPase activity, which is strongly stimulated in the presence of ssDNA oligomers. Furthermore, we expressed BBLF2/3 as soluble monomeric protein and performed small-angle X-ray scattering experiments which yielded an envelope whose shape is compatible with B-family polymerases.

UL52 primase interactions in the herpes simplex virus 1 helicase-primase are affected by antiviral compounds and mutations causing drug resistance

Herpes simplex virus 1 (HSV-1) UL5/8/52 helicase-primase complex is required for DNA unwinding at the replication fork and synthesis of primers during virus replication, and it has become a promising novel target for antiviral therapy. Using molecular cloning, we have identified three separate domains of UL52. Co-immunoprecipitation experiments in extracts from cells transiently expressing HA-tagged UL5, FLAG-UL8, and enhanced GFP-tagged UL52 domains revealed that the N-terminal domain of UL52 primase binds UL5 helicase and the middle domain interacts with the UL8 accessory protein. In addition, an interaction between the single strand DNA-binding protein ICP8 and the UL52 middle domain was observed. The complex between UL5 and UL52 was stabilized by the antiviral compound BAY 54-6322, and mutations providing resistance to the drug obliterate this effect. Our results also suggest a mechanism for accommodating conformational strain resulting from movement of UL5 and UL52 in opposite directions on the lagging strand template, and they identify molecular complexes that can be further examined by structural biology techniques to resolve the mechanism of primer synthesis during herpesvirus replication. Finally, they help to explain the mechanism of action of a novel class of antiviral compounds currently being evaluated in clinical trials.

Helicase-primase as a target of new therapies for herpes simplex virus infections

The seminal discovery of acyclovir 40 years ago heralded the modern era of truly selective antiviral therapies and this drug remains the therapy of choice for herpes simplex virus infections. Yet by modern standards, its antiviral activity is modest and new drugs against novel molecular targets such as the helicase-primase have the potential to improve clinical outcome, particularly in high-risk patients. A brief synopsis of current therapies for these infections and clinical need is provided to help provide an initial perspective. The function of the helicase-primase complex is then summarized and the development of new inhibitors of the helicase-primase complex, such as pritelivir and amenamevir, is discussed. We review their mechanism of action, propensity for drug resistance, and pharmacokinetic characteristics and discuss their potential to advance current therapeutic options. Strategies that include combinations of these inhibitors with acyclovir are also considered, as they will likely maximize clinical efficacy.

Helicase–primase inhibitors for herpes simplex virus: looking to the future of non-nucleoside inhibitors for treating herpes virus infections

Helicase–primase inhibitors (HPIs) are the first new family of potent herpes virus (herpes simplex and varicella-zoster virus) inhibitors to go beyond the preliminary stages of investigation since the emergence of the original nucleoside analog inhibitors. To consider the clinical future of HPIs, this review puts the exciting new findings with two HPIs, amenamevir and pritelivir, into the historical context of antiviral development for the prevention and treatment of herpes simplex virus over the last century and, on this basis, the authors speculate on the potential evolution of these and other non-nucleoside inhibitors in the future.

Deep sequencing of viral genomes provides insight into the evolution and pathogenesis of varicella zoster virus and its vaccine in humans

Immunization with the vOka vaccine prevents varicella (chickenpox) in children and susceptible adults. The vOka vaccine strain comprises a mixture of genotypes and, despite attenuation, causes rashes in small numbers of recipients. Like wild-type virus, the vaccine establishes latency in neuronal tissue and can later reactivate to cause Herpes zoster (shingles). Using hybridization-based methodologies, we have purified and sequenced vOka directly from skin lesions. We show that alleles present in the vaccine can be recovered from the lesions and demonstrate the presence of a severe bottleneck between inoculation and lesion formation. Genotypes in any one lesion appear to be descended from one to three vaccine-genotypes with a low frequency of novel mutations. No single vOka haplotype and no novel mutations are consistently present in rashes, indicating that neither new mutations nor recombination with wild type are critical to the evolution of vOka rashes. Instead, alleles arising from attenuation (i.e., not derived from free-living virus) are present at lower frequencies in rash genotypes. We identify 11 loci at which the ancestral allele is selected for in vOka rash formation and show genotypes in rashes that have reactivated from latency cannot be distinguished from rashes occurring immediately after inoculation. We conclude that the vOka vaccine, although heterogeneous, has not evolved to form rashes through positive selection in the mode of a quasispecies, but rather alleles that were essentially neutral during the vaccine production have been selected against in the human subjects, allowing us to identify key loci for rash formation.

The DNA helicase-primase complex as a target for herpes viral infection

INTRODUCTION

The Herpesviridae are responsible for debilitating acute and chronic infections, and some members of this family are associated with human cancers. Conventional anti-herpesviral therapy targets the viral DNA polymerase and has been extremely successful; however, the emergence of drug-resistant virus strains, especially in neonates and immunocompromised patients, underscores the need for continued development of anti-herpes drugs. In this article, we explore an alternative target for antiviral therapy, the HSV helicase/primase complex.

AREAS COVERED

This review addresses the current state of knowledge of HSV DNA replication and the important roles played by the herpesvirus helicase- primase complex. In the last 10 years several helicase/primase inhibitors (HPIs) have been described, and in this article, we discuss and contrast these new agents with established inhibitors.

EXPERT OPINION

The outstanding safety profile of existing nucleoside analogues for α-herpesvirus infection make the development of new therapeutic agents a challenge. Currently used nucleoside analogues exhibit few side effects and have low occurrence of clinically relevant resistance. For HCMV, however, existing drugs have significant toxicity issues and the frequency of drug resistance is high, and no antiviral therapies are available for EBV and KSHV. The development of new anti-herpesvirus drugs is thus well worth pursuing especially for immunocompromised patients and those who develop drug-resistant infections. Although the HPIs are promising, limitations to their development into a successful drug strategy remain.

Antiviral agents for herpes simplex virus

This review starts with a brief description of herpes simplex virus types 1 and 2 (HSV-1 and HSV-2), the clinical diseases they cause, and the continuing clinical need for antiviral chemotherapy. A historical overview describes the progress from the early, rather toxic antivirals to acyclovir (ACV) which led the way for its prodrug, valacyclovir, to penciclovir and its prodrug, famciclovir (FCV). These compounds have been the mainstay of HSV therapy for two decades and have established a remarkable safety record. This review focuses on these compounds, the preclinical studies which reveal potentially important differences, the clinical trials, and the clinical experience through two decades. Some possible areas for further investigation are suggested. The focus shifts to new approaches and novel compounds, in particular, the combination of ACV with hydrocortisone, known as ME609 or zovirax duo, an HSV helicase-primase inhibitor, pritelivir (AIC316), and CMX001, the cidofovir prodrug for treating resistant HSV infection in immunocompromised patients. Letermovir has established that the human cytomegalovirus terminase enzyme is a valid target and that similar compounds could be sought for HSV. We discuss the difficulties facing the progression of new compounds. In our concluding remarks, we summarize the present situation including a discussion on the reclassification of FCV from prescription-only to pharmacist-controlled for herpes labialis in New Zealand in 2010; should this be repeated more widely? We conclude that HSV research is emerging from a quiescent phase.

The helicase-primase complex as a target for effective herpesvirus antivirals

Herpes simplex virus and varicella-zoster virus have been treated for more that half a century using nucleoside analogues. However, there is still an unmet clinical need for improved herpes antivirals. The successful compounds, acyclovir; penciclovir and their orally bioavailable prodrugs valaciclovir and famciclovir, ultimately block virus replication by inhibiting virus-specific DNA-polymerase. The helicase-primase (HP) complex offers a distinctly different target for specific inhibition of virus DNA synthesis. This review describes the synthetic programmes that have already led to two HP-inhibitors (HPI) that have commenced clinical trials in man. One of these (known as AIC 316) continues in clinical development to date. The specificity of HPI is reflected by the ability to select drug-resistant mutants. The role of HP-antiviral resistance will be considered and how the study of cross--resistance among mutants already shows subtle differences between compounds in this respect. The impact of resistance on the drug development in the clinic will also be considered. Finally, herpesvirus latency remains as the most important barrier to a therapeutic cure. Whether or not helicase primase inhibitors alone or in combination with nucleoside analogues can impact on this elusive goal remains to be seen.

Simian varicella virus gene expression during acute and latent infection of rhesus macaques

Varicella zoster virus (VZV) is a neurotropic α-herpesvirus that causes chickenpox during primary infection and establishes latency in sensory ganglia. Reactivation of VZV results in herpes zoster and other neurological complications. Our understanding of the VZV transcriptome during acute and latent infection in immune competent individuals remains incomplete. Infection of rhesus macaques with the homologous simian varicella virus (SVV) recapitulates the hallmarks of VZV infection. We therefore characterized the SVV transcriptome by quantitative real-time reverse transcriptase PCR during acute infection in bronchial alveolar lavage (BAL) cells and peripheral blood mononuclear cells, and during latency in sensory ganglia obtained from the same rhesus macaques. During acute infection, all known SVV open reading frames (ORFs) were detected, and the most abundantly expressed ORFs are involved in virus replication and assembly such as the transcriptional activator ORF 63 and the structural proteins ORF 41 and ORF 49. In contrast, latent SVV gene expression is highly restricted. ORF 61, a viral transactivator and latency-associated transcript, is the most prevalent transcript detected in sensory ganglia. We also detected ORFs A, B, 4, 10, 63, 64, 65, 66, and 68 though significantly less frequently than ORF 61. This comprehensive analysis has revealed genes that potentially play a role in the establishment and/or maintenance of SVV latency.

Helicase–primase inhibitors as the potential next generation of highly active drugs against herpes simplex viruses

Since the introduction of the nucleoside analogs decades ago, treatment of herpes simplex virus (HSV) infections has not seen much innovation, except for the development of their respective prodrugs. The inhibitors of the helicase–primase complex of HSV represent a very innovative approach to the treatment of herpesvirus disease, and this article considers the development of some representatives of this class of therapeutics. The molecular and biochemical features of the helicase–primase complex are considered and the development of three inhibitors of helicase–primase, BILS 179 BS, AIC316 and ASP2151, is described. The clinical development of AIC316 is at an advanced stage and displays general safety as well as favorable, long-lasting exposures in healthy volunteers. The first efficacy data from a Phase II trial with more than 150 HSV-2-positive subjects demonstrated dose-dependent antiviral activity.

Antiviral drug resistance and helicase-primase inhibitors of herpes simplex virus

A new class of chemical inhibitors has been discovered that interferes with the process of herpesvirus DNA replication. To date, the majority of useful herpesvirus antivirals are nucleoside analogues that block herpesvirus DNA replication by targeting the DNA polymerase. The new helicase-primase inhibitors (HPI) target a different enzyme complex that is also essential for herpesvirus DNA replication. This review will place the HPI in the context of previous work on the nucleoside analogues. Several promising highly potent HPI will be described with a particular focus on the identification of drug-resistance mutations. Several HPI have good pharmacological profiles and are now at the outset of phase II clinical trials. Provided there are no safety issues to stop their progress, this new class of compound will be a major advance in the herpesvirus antiviral field. Furthermore, HPI are likely to have a major impact on the therapy and prevention of herpes simplex virus and varicella zoster in both immunocompetent and immunocompromised patients alone or in combination with current nucleoside analogues. The possibility of acquired drug-resistance to HPI will then become an issue of great practical importance.

Herpes simplex virus type 1 helicase-primase: DNA binding and consequent protein oligomerization and primase activation

The heterotrimeric helicase-primase complex of herpes simplex virus type I (HSV-1), consisting of UL5, UL8, and UL52, possesses 5' to 3' helicase, single-stranded DNA (ssDNA)-dependent ATPase, primase, and DNA binding activities. In this study we confirm that the UL5-UL8-UL52 complex has higher affinity for forked DNA than for ssDNA and fails to bind to fully annealed double-stranded DNA substrates. In addition, we show that a single-stranded overhang of greater than 6 nucleotides is required for efficient enzyme loading and unwinding. Electrophoretic mobility shift assays and surface plasmon resonance analysis provide additional quantitative information about how the UL5-UL8-UL52 complex associates with the replication fork. Although it has previously been reported that in the absence of DNA and nucleoside triphosphates the UL5-UL8-UL52 complex exists as a monomer in solution, we now present evidence that in the presence of forked DNA and AMP-PNP, higher-order complexes can form. Electrophoretic mobility shift assays reveal two discrete complexes with different mobilities only when helicase-primase is bound to DNA containing a single-stranded region, and surface plasmon resonance analysis confirms larger amounts of the complex bound to forked substrates than to single-overhang substrates. Furthermore, we show that primase activity exhibits a cooperative dependence on protein concentration while ATPase and helicase activities do not. Taken together, these data suggest that the primase activity of the helicase-primase requires formation of a dimer or higher-order structure while ATPase activity does not. Importantly, this provides a simple mechanism for generating a two-polymerase replisome at the replication fork.

Mismatch primer-based PCR reveals that helicase-primase inhibitor resistance mutations pre-exist in herpes simplex virus type 1 clinical isolates and are not induced during incubation with the inhibitor

OBJECTIVES

Previous studies suggested that helicase-primase inhibitor (HPI) resistance mutations can be selected at relatively high frequency from some isolates of herpes simplex virus type 1 (HSV-1). An intentional mismatch primer (IMP) PCR was developed to detect three known HPI resistance mutations well above the expected background frequency. The objective of this study was to provide proof that HPI resistance mutations pre-exist at relatively high frequency in some clinical isolates obtained from individuals naive to HPIs.

METHODS

Three different IMP PCRs were standardized to detect critical HPI resistance mutations (K356N or K356T in UL5, or A899T in UL52) at 10-100 times the expected background frequency (<10(-6)). Thirty HSV-1 clinical isolates were then screened for the resistance mutations in the absence of the inhibitor using IMP PCR.

RESULTS

Among 30 clinical isolates that were all susceptible to the HPI, BAY 57-1293, 5 were shown to contain UL5 mutations at 10-100 times higher than the expected frequency. No UL52 resistance mutations were encountered in this study.

CONCLUSIONS

The detection of HPI-resistant mutations in some clinical isolates by means of IMP PCR proved that the mutations pre-exist and showed that they are not induced during incubation with the inhibitor.

Herpes simplex virus-1 helicase-primase: roles of each subunit in DNA binding and phosphodiester bond formation

The helicase-primase complex from herpes simplex virus-1 contains three subunits, UL5, UL52, and UL8. We generated each of the potential two-subunit complexes, UL5-UL52, UL5-UL8, and UL52-UL8, and used a series of kinetic and photo-cross-linking studies to provide further insights into the roles of each subunit in DNA binding and primer synthesis. UL8 increases the rate of primer synthesis by UL5-UL52 by increasing the rate of primer initiation (two NTPs --> pppNpN), the rate-limiting step in primer synthesis. The UL5-UL8 complex lacked any detectable catalytic activity (DNA-dependent ATPase, primase, or RNA polymerase using a RNA primer-template and NTPs as substrates) but could still bind DNA, indicating that UL52 must provide some key amino acids needed for helicase function. The UL52-UL8 complex lacked detectable DNA-dependent ATPase activity and could not synthesize primers on single-stranded DNA. However, it exhibited robust RNA polymerase activity using a RNA primer-template and NTPs as substrates. Thus, UL52 must contain the entire primase active site needed for phosphodiester bond formation, while UL5 minimally contributes amino acids needed for the initiation of primer synthesis. Photo-cross-linking experiments using single-stranded templates containing 5-iodouracil either before, in, or after the canonical 3'-GPyPy (Py is T or C) initiation site for primer synthesis showed that only UL5 cross-linked to the DNA. This occurred for the UL5-UL52, UL5-UL52-UL8, and UL5-UL8 complexes and whether the reaction mixtures contained NTPs. Photo-cross-linking of a RNA primer-template, the product of primer synthesis, containing 5-iodouracil in the template generated the same apparent cross-linked species.

Human cytomegalovirus exploits ESCRT machinery in the process of virion maturation

The endosomal sorting complex required for transport (ESCRT) machinery controls the incorporation of cargo into intraluminal vesicles of multivesicular bodies. This machinery is used during envelopment of many RNA viruses and some DNA viruses, including herpes simplex virus type 1. Other viruses mature independent of ESCRT components, instead relying on the intrinsic behavior of viral matrix and envelope proteins to drive envelopment. Human cytomegalovirus (HCMV) maturation has been reported to proceed independent of ESCRT components (A. Fraile-Ramos et al. Cell. Microbiol. 9:2955-2967, 2007). A virus complementation assay was used to evaluate the role of dominant-negative (DN) form of a key ESCRT ATPase, vacuolar protein sorting-4 (Vps4DN) in HCMV replication. Vps4DN specifically inhibited viral replication, whereas wild-type-Vps4 had no effect. In addition, a DN form of charged multivesicular body protein 1 (CHMP1DN) was found to inhibit HCMV. In contrast, DN tumor susceptibility gene-101 (Tsg101DN) did not impact viral replication despite the presence of a PTAP motif within pp150/ppUL32, an essential tegument protein involved in the last steps of viral maturation and release. Either Vps4DN or CHMP1DN blocked viral replication at a step after the accumulation of late viral proteins, suggesting that both are involved in maturation. Both Vps4A and CHMP1A localized in the vicinity of viral cytoplasmic assembly compartments, sites of viral maturation that develop in CMV-infected cells. Thus, ESCRT machinery is involved in the final steps of HCMV replication.

A mutation in helicase motif IV of herpes simplex virus type 1 UL5 that results in reduced growth in vitro and lower virulence in a murine infection model is related to the predicted helicase structure

A variant was selected from a clinical isolate of herpes simplex virus type 1 (HSV-1) during a single passage in the presence of a helicase-primase inhibitor (HPI) at eight times the IC(50). The variant was approximately 40-fold resistant to the HPI BAY 57-1293 and it showed significantly reduced growth in tissue culture with a concomitant reduction in virulence in a murine infection model. The variant contained a single mutation (Asn342Lys) in the UL5 predicted functional helicase motif IV. The Asn342Lys mutation was transferred to a laboratory strain, PDK cl-1, and the recombinant acquired the expected resistance and reduced growth characteristics. Comparative modelling and docking studies predicted the Asn342 position to be physically distant from the HPI interaction pocket formed by UL5 and UL52 (primase). We suggest that this mutation results in steric/allosteric modification of the HPI-binding pocket, conferring an indirect resistance to the HPI. Slower growth and moderately reduced virulence suggest that this mutation might also interfere with the helicase-primase activity.

Comparative analysis of the genes UL1 through UL7 of the duck enteritis virus and other herpesviruses of the subfamily Alphaherpesvirinae

The nucleotide sequences of eight open reading frames (ORFs) located at the 5' end of the unique long region of the duck enteritis virus (DEV) Clone-03 strain were determined. The genes identified were designated UL1, UL2, UL3, UL4, UL5, UL6 and UL7 homologues of the herpes simplex virus 1 (HSV-1). The DEV UL3.5 located between UL3 and UL4 had no homologue in the HSV-1. The arrangement and transcription orientation of the eight genes were collinear with their homologues in the HSV-1. Phylogenetic trees were constructed based on the alignments of the deduced amino acids of eight proteins with their homologues in 12 alpha-herpesviruses. In the UL1, UL3, UL3.5, UL5 and UL7 proteins trees, the branches were more closely related to the genus Mardivirus. However, the UL2, UL4, and UL6 proteins phylogenetic trees indicated a large distance from Mardivirus, indicating that the DEV evolved differently from other viruses in the subfamily Alphaherpesvirinae and formed a single branch within this subfamily.

Herpes simplex virus helicase-primase inhibitors: recent findings from the study of drug resistance mutations

After several decades during which nucleoside analogues (especially acyclovir and penciclovir and their prodrugs) have benefited many patients suffering from herpes simplex virus (HSV) infections, the discovery of the helicase-primase inhibitors (HPIs) represents an interesting new approach. Although antiviral resistance has not been a major problem for nucleoside analogues in immunocompetent patients, the problem of acyclovir resistance in immunocompromised patients is well documented. Several HPIs are extremely potent antiviral compounds and may, therefore, offer an important alternative therapy in these patients. The potential for synergy, not just for the inhibition of virus replication but also to delay the appearance of drug-resistant virus, needs to be thoroughly investigated. The study of resistance to HPIs has been important towards understanding the mechanism of action of these compounds and confirming the target function. However, during the course of our studies on HPI resistance, we have made a number of interesting observations that may be relevant to their clinical use. This article draws attention to the major observations on HPI resistance reported by others and to our own recently published observations that have extended this expanding area of antiviral research.

Identification of putative functional motifs in viral proteins essential for human cytomegalovirus DNA replication

Six of the eleven genes essential for Human cytomegalovirus (HCMV) DNA synthesis have been analyzed for putative structural motifs that may have a significant functional role in DNA replication. The genes studied encode for the DNA polymerase accessory protein (UL44), single-stranded DNA binding protein (UL57), primase-helicase complex (UL70, UL102, and UL105), and the putative initiator protein (UL84). The full-length open reading frames of these genes were highly conserved between ten isolates with amino acid sequence identity of >97% for all genes. Using ScanProsite software from the Expert Protein Analysis System (ExPASy) proteomics server, we have mapped putative motifs throughout these HCMV replication genes. Interesting motifs identified include casein kinase-2 (CKII) phosphorylation sites, a microbodies signal motif in UL57, and an ATP binding site in the putative UL105 helicase. Our investigations have also elucidated motif-rich regions of the UL44 DNA polymerase accessory protein and identified cysteine motifs that have potential implications for UL57 and UL70 primase. Taken together, these findings provide insights to regions of these HCMV replication proteins that are important for post-translation modification, activation, and overall function, and this information can be utilized to target further research into these proteins and advance the development of novel antiviral agents that target these processes.

Helicases as prospective targets for anti-cancer therapy

It has been proposed that selective inactivation of a DNA repair pathway may enhance anti-cancer therapies that eliminate cancerous cells through the cytotoxic effects of DNA damaging agents or radiation. Given the unique and critically important roles of DNA helicases in the DNA damage response, DNA repair, and maintenance of genomic stability, a number of strategies currently being explored or in use to combat cancer may be either mediated or enhanced through the modulation of helicase function. The focus of this review will be to examine the roles of helicases in DNA repair that might be suitably targeted by cancer therapeutic approaches. Treatment of cancers with anti-cancer drugs such as small molecule compounds that modulate helicase expression or function is a viable approach to selectively kill cancer cells through the inactivation of helicase-dependent DNA repair pathways, particularly those associated with DNA recombination, replication restart, and cell cycle checkpoint.

Molecular cloning and sequence analysis of the duck enteritis virus UL5 gene

Duck enteritis virus (DEV) is a herpesvirus that causes an acute, contagious, and fatal disease. In the present article, the DEV UL5 gene was cloned and sequenced from a vaccine virus. According to the consensus sequence of herpesvirus UL5 and UL3 gene degenerate oligonucleotide primers were designed and were used in the polymerase chain reaction (PCR) to amplify DNA products with 4577 bp in size. DNA sequence analysis revealed a 2568 bp open reading frame (ORF) encoding a 855 amino acid polypeptide homologous to herpesvirus UL5 proteins. The DEV UL5 gene has a base composition of 769 adenine (29.95%), 556 cytosine (21.65%), 533 guanine (20.76%) and 710 thymine (27.65%). Sequence comparison revealed that the nucleotide sequence of the DEV UL5 gene was highly similar to other alphaherpesviruses. Phylogenetic tree analysis showed that the fifteen herpesviruses viruses analyzed fell into four large groups, and the duck enteritis virus itself branched and was most closely related to meleagrid herpesvirus 1, gallid herpesvirus 2 and gallid herpesvirus 3 subtrees.

Mutations close to functional motif IV in HSV-1 UL5 helicase that confer resistance to HSV helicase-primase inhibitors, variously affect virus growth rate and pathogenicity

Herpes simplex virus (HSV) helicase-primase (HP) is the target for a novel class of antiviral compounds, the helicase-primase inhibitors (HPIs), e.g. BAY 57-1293. Although mutations in herpesviruses conferring resistance to nucleoside analogues are commonly associated with attenuation in vivo, to date, this is not necessarily true for HPIs. HPI-resistant HSV mutants selected in tissue culture are reported to be equally pathogenic compared to parental virus in animal models. Here we demonstrate that a slow-growing HSV-1 mutant, with the BAY 57-1293-resistance mutation Gly352Arg in UL5 helicase, is clearly less virulent than its wild-type parent in a murine zosteriform infection model. This contrasts with published results obtained for a mutant containing a different HPI-resistance substitution (Gly352Val) at the same location, since this mutant was reported to be fully pathogenic. We believe our report to be the first to describe an HPI-resistant HSV-1 mutant, that is markedly less virulent in vivo and slowly growing in tissue culture compared to the parental strain. Another BAY 57-1293-resistant UL5 mutant (Lys356Gln), which showed faster growth characteristics in cell culture, however, was at least equally virulent compared to the parent strain.

Role of the herpes simplex virus helicase-primase complex during adeno-associated virus DNA replication

A subset of DNA replication proteins of herpes simplex virus (HSV) comprising the single-strand DNA-binding protein, ICP8 (UL29), and the helicase-primase complex (UL5, UL8, and UL52 proteins) has previously been shown to be sufficient for the replication of adeno-associated virus (AAV). We recently demonstrated complex formation between ICP8, AAV Rep78, and the single-stranded DNA AAV genome, both in vitro and in the nuclear HSV replication domains of coinfected cells. In this study the functional role(s) of HSV helicase and primase during AAV DNA replication were analyzed. To differentiate between their necessity as structural components of the HSV replication complex or as active enzymes, point mutations within the helicase and primase catalytic domains were analyzed. In two complementary approaches the remaining HSV helper functions were either provided by infection with HSV mutants or by plasmid transfection. We show here that upon cotransfection of the minimal four HSV proteins (i.e., the four proteins constituting the minimal requirements for basal AAV replication), UL52 primase catalytic activity was not required for AAV DNA replication. In contrast, UL5 helicase activity was necessary for fully efficient replication. Confocal microscopy confirmed that all mutants retained the ability to support formation of ICP8-positive nuclear replication foci, to which AAV Rep78 colocalized in a manner strictly dependent on the presence of AAV single-stranded DNA (ssDNA). The data indicate that recruitment of AAV Rep78 and ssDNA to nuclear replication sites by the four HSV helper proteins is maintained in the absence of catalytic primase or helicase activities and suggest an involvement of the HSV UL5 helicase activity during AAV DNA replication.

A mutation in the human herpes simplex virus type 1 UL52 zinc finger motif results in defective primase activity but can recruit viral polymerase and support viral replication efficiently

Herpes simplex virus type 1 (HSV-1) encodes a heterotrimeric helicase/primase complex consisting of UL5, UL8, and UL52. UL5 contains conserved helicase motifs, while UL52 contains conserved primase motifs, including a zinc finger motif. Although HSV-1 and HSV-2 UL52s contain a leucine residue at position 986, most other herpesvirus primase homologues contain a phenylalanine at this position. We constructed an HSV-1 UL52 L986F mutation and found that it can complement a UL52 null virus more efficiently than the wild type (WT). We thus predicted that the UL5/8/52 complex containing the L986F mutation might possess increased primase activity; however, it exhibited only 25% of the WT level of primase activity. Interestingly, the mutant complex displayed elevated levels of DNA binding and single-stranded DNA-dependent ATPase and helicase activities. This result confirms a complex interdependence between the helicase and primase subunits. We previously showed that primase-defective mutants failed to recruit the polymerase catalytic subunit UL30 to prereplicative sites, suggesting that an active primase, or primer synthesis, is required for polymerase recruitment. Although L986F exhibits decreased primase activity, it can support efficient replication and recruit UL30 efficiently to replication compartments, indicating that a partially active primase is capable of recruiting polymerase. Extraction with detergents prior to fixation can extract nucleosolic proteins but not proteins bound to chromatin or the nuclear matrix. We showed that UL30 was extracted from replication compartments while UL42 remained bound, suggesting that UL30 may be tethered to the replication fork by protein-protein interactions.

Detection of HSV-1 variants highly resistant to the helicase-primase inhibitor BAY 57-1293 at high frequency in 2 of 10 recent clinical isolates of HSV-1

OBJECTIVES

BAY 57-1293 is a helicase-primase inhibitor (HPI) from a new class of antivirals that are highly efficacious in herpes simplex virus (HSV)-1 animal infection models. Resistant mutants with point mutations in the helicase (UL5) were reported to be present in laboratory isolates at a low frequency of approximately 10(-6). In contrast, we have shown elsewhere that some laboratory isolates contain resistant variants at higher frequency (10(-4)). Therefore, we screened 10 recent clinical isolates of HSV-1 for BAY 57-1293-resistant virions.

METHODS

Clinical isolates were screened by a plaque reduction assay in Vero cells to determine the frequency of occurrence of BAY 57-1293-resistant variants. The helicase gene for the resistant variants was sequenced.

RESULTS

One isolate contained highly resistant variants at 10(-4) and another at 10(-5). Both variants contained a previously reported BAY 57-1293 resistance mutation (K356N) in UL5 and were >5000-fold resistant.

CONCLUSIONS

Occurrence of HPI-resistant viruses at high frequency in a clinical isolate is intriguing. Two alternative hypotheses are proposed to explain this phenomenon. It is also surprising that two unrelated clinical isolates contain an identical HPI resistance mutation. These results have important implications for HPI drug-resistance monitoring during subsequent clinical trials.

High frequency of spontaneous helicase-primase inhibitor (BAY 57-1293) drug-resistant variants in certain laboratory isolates of HSV-1

Herpes simplex virus (HSV) helicase-primase is the target for a new group of potent antivirals that show great promise in vivo. A claimed advantage of this class of compounds is the low rate of drug resistance, which is reported to occur at a lesser rate than acyclovir (ACV)-resistance in cell culture. We confirmed that BAY 57-1293 is highly active against HSV-1 and superior to ACV when tested in Vero cells. Notably, drug resistance was detected in laboratory working stocks in two different strains of HSV at 10(-4) to 10(-5) and there was evidence that the resistant variants were present in the virus population before the selection was applied. Plaque-purified clones obtained from the parental viruses showed a lower level of resistance selection in the presence of drug (10-6) and this value is similar to published reports. In the case of HSV-1 SC16, no difference was observed between a working stock and a plaque-pure clone in the rate of resistance to the nucleoside analogue ACV. The working stocks were found to contain variants with resistance to BAY 57-1293 ranging from approximately 15-fold to 4,000-fold suggesting that these viruses have the potential to subvert effective therapy. Sequence analysis of HSV-1 helicase protein showed that most of the amino acid substitutions in the variants described in this study tallied with published results, with some interesting exceptions in the case of HSV-1 strain PDK. Resistant variants did not readily revert to a sensitive phenotype in the absence of the inhibitor and representative BAY 57-1293-resistant variants were cross-resistant to an alternative helicase-primase inhibitor, BILS 22 BS. Variants resistant to BAY 57-1293 retained sensitivity to the nucleoside analogue, ACV.

Single amino acid substitutions in the HSV-1 helicase protein that confer resistance to the helicase-primase inhibitor BAY 57-1293 are associated with increased or decreased virus growth characteristics in tissue culture

Two mutants (BAYr1 and BAYr2) that are 100-fold and >3000-fold resistant, respectively, to the helicase-primase inhibitor (HPI) BAY 57-1293 were derived from a plaque-pure parental strain, HSV-1 SC16 cl-2. BAYr1 has two substitutions in the HSV-1 helicase (UL5) protein (A4 to V; K356 to Q) and BAYr2 has one (G352 to R). It was shown reproducibly that BAYr1 grows to higher titres in tissue culture while BAYr2 grows more slowly than wild-type. Marker transfer experiments confirmed that K356Q and G352R are the drug-resistance mutations and that they are directly associated with differences in virus growth in tissue culture. When BAYr1 was tested in a murine infection model, this virus was shown to be fully pathogenic. We present evidence that single mutations close to a predicted functional domain of an essential HSV-1 replication enzyme (helicase) are associated with drug resistance and virus growth characteristics.

High-molecular-weight protein (pUL48) of human cytomegalovirus is a competent deubiquitinating protease: mutant viruses altered in its active-site cysteine or histidine are viable

We show here that the high-molecular-weight protein (HMWP or pUL48; 253 kDa) of human cytomegalovirus (HCMV) is a functionally competent deubiquitinating protease (DUB). By using a suicide substrate probe specific for ubiquitin-binding cysteine proteases (DUB probe) to screen lysates of HCMV-infected cells, we found just one infected-cell-specific DUB. Characteristics of this protein, including its large size, expression at late times of infection, presence in extracellular virus particles, and reactivity with an antiserum to the HMWP, identified it as the HMWP. This was confirmed by constructing mutant viruses with substitutions in two of the putative active-site residues, Cys24Ile and His162Ala. HMWP with these mutations either failed to bind the DUB probe (C24I) or had significantly reduced reactivity with it (H162A). More compellingly, the deubiquitinating activity detected in wild-type virus particles was completely abolished in both the C24I and H162A mutants, thereby directly linking HMWP with deubiquitinating enzyme activity. Mutations in these active-site residues were not lethal to virus replication but slowed production of infectious virus relative to wild type and mutations of other conserved residues. Initial studies, by electron microscopy, of cells infected with the mutants revealed no obvious differences at late times of replication in the general appearance of the cells or in the distribution, relative numbers, or appearance of virus particles in the cytoplasm or nucleus.

Understanding helicases as a means of virus control

Helicases are promising antiviral drug targets because their enzymatic activities are essential for viral genome replication, transcription, and translation. Numerous potent inhibitors of helicases encoded by herpes simplex virus, severe acute respiratory syndrome coronavirus, hepatitis C virus, Japanese encephalitis virus, West Nile virus, and human papillomavirus have been recently reported in the scientific literature. Some inhibitors have also been shown to decrease viral replication in cell culture and animal models. This review discusses recent progress in understanding the structure and function of viral helicases to help clarify how these potential antiviral compounds function and to facilitate the design of better inhibitors. The above helicases and all related viral proteins are classified here based on their evolutionary and functional similarities, and the key mechanistic features of each group are noted. All helicases share a common motor function fueled by ATP hydrolysis, but differ in exactly how the motor moves the protein and its cargo on a nucleic acid chain. The helicase inhibitors discussed here influence rates of helicase-catalyzed DNA (or RNA) unwinding by preventing ATP hydrolysis, nucleic acid binding, nucleic acid release, or by disrupting the interaction of a helicase with a required cofactor.

Mutations in the putative zinc-binding motif of UL52 demonstrate a complex interdependence between the UL5 and UL52 subunits of the human herpes simplex virus type 1 helicase/primase complex

Herpes simplex virus type 1 (HSV-1) encodes a heterotrimeric helicase-primase (UL5/8/52) complex. UL5 contains seven motifs found in helicase superfamily 1, and UL52 contains conserved motifs found in primases. The contributions of each subunit to the biochemical activities of the complex, however, remain unclear. We have previously demonstrated that a mutation in the putative zinc finger at UL52 C terminus abrogates not only primase but also ATPase, helicase, and DNA-binding activities of a UL5/UL52 subcomplex, indicating a complex interdependence between the two subunits. To test this hypothesis and to further investigate the role of the zinc finger in the enzymatic activities of the helicase-primase, a series of mutations were constructed in this motif. They differed in their ability to complement a UL52 null virus: totally defective, partial complementation, and potentiating. In this study, four of these mutants were studied biochemically after expression and purification from insect cells infected with recombinant baculoviruses. All mutants show greatly reduced primase activity. Complementation-defective mutants exhibited severe defects in ATPase, helicase, and DNA-binding activities. Partially complementing mutants displayed intermediate levels of these activities, except that one showed a wild-type level of helicase activity. These data suggest that the UL52 zinc finger motif plays an important role in the activities of the helicase-primase complex. The observation that mutations in UL52 affected helicase, ATPase, and DNA-binding activities indicates that UL52 binding to DNA via the zinc finger may be necessary for loading UL5. Alternatively, UL5 and UL52 may share a DNA-binding interface.

Point mutations in exon I of the herpes simplex virus putative terminase subunit, UL15, indicate that the most conserved residues are essential for cleavage and packaging

The herpes simplex virus UL15 and UL28 genes are believed to encode two subunits of the terminase involved in cleavage and packaging of viral genomes. Analysis of the UL15 protein sequence and its herpesvirus homologues revealed the presence of 20 conserved regions. Twelve of the twenty regions conserved among herpesviruses are also conserved in terminases from DNA bacteriophage. Point mutations in UL15 were designed in four conserved regions: L120N (CR1), Q205E (CR2), Q251E (CR3), G263A (CR3), and Y285S (CR4). Transfection experiments indicated that each mutant gene could produce stable UL15 protein at wild-type levels; however, only one mutant (Q251E) was able to complement the UL15-null virus. Each mutation was introduced into the viral genome by marker transfer, and all mutants except Q251E were unable to form plaques on Vero cells. Furthermore, failure to form plaques on Vero cells correlated with a defect in cleavage and packaging. Immunofluorescence experiments indicated that in cells infected with all mutant viruses the UL15 protein could be detected and was found to localize to replication compartments. Although wild-type and mutant Q251E were able to produce A, B, and C capsids, the rest of the mutants were only able to produce B capsids, a finding consistent with their defects in cleavage and packaging. In addition, all mutant UL15 proteins retained their ability to interact with B capsids. Therefore, amino acid residues 120, 205, 263, and 285 are essential for the cleavage and packaging process rather than for association with capsids or localization to replication compartments.

Current and potential therapies for the treatment of herpes-virus infections

Human herpesviruses are found worldwide and are among the most frequent causes of viral infections in immunocompetent as well as in immunocompromised patients. During the past decade and a half a better understanding of the replication and disease-causing state of herpes simplex virus types 1 and 2 (HSV-1 and HSV-2), varicella zoster virus (VZV), and human cytomegalovirus (HCMV) has been achieved due in part to the development of potent antiviral compounds that target these viruses. While some of these antiviral therapies are considered safe and efficacious (acyclovir, penciclovir), some have toxicities associated with them (ganciclovir and foscarnet). In addition, the increased and prolonged use of these compounds in the clinical setting, especially for the treatment of immunocompromised patients, has led to the emergence of viral resistance against most of these drugs. While resistance is not a serious issue for immunocompetent individuals, it is a real concern for immunocompromised patients, especially those with AIDS and the ones that have undergone organ transplantation. All the currently approved treatments target the viral DNA polymerase. It is clear that new drugs that are more efficacious than the present ones, are not toxic, and target a different viral function would be of great use especially for immunocompromised patients. Here, an overview is provided of the diseases caused by the herpesviruses as well as the replication strategy of the better studied members of this family for which treatments are available. We also discuss the various drugs that have been approved for the treatment of some herpesviruses in terms of structure, mechanism of action, and development of resistance. Finally, we present a discussion of viral targets other than the DNA polymerase, for which new antiviral compounds are being considered.

Complete sequence and genomic analysis of rhesus cytomegalovirus

The complete DNA sequence of rhesus cytomegalovirus (RhCMV) strain 68-1 was determined with the whole-genome shotgun approach on virion DNA. The RhCMV genome is 221,459 bp in length and possesses a 49% G+C base composition. The genome contains 230 potential open reading frames (ORFs) of 100 or more codons that are arranged colinearly with counterparts of previously sequenced betaherpesviruses such as human cytomegalovirus (HCMV). Of the 230 RhCMV ORFs, 138 (60%) are homologous to known HCMV proteins. The conserved ORFs include the structural, replicative, and transcriptional regulatory proteins, immune evasion elements, G protein-coupled receptors, and immunoglobulin homologues. Interestingly, the RhCMV genome also contains sequences with homology to cyclooxygenase-2, an enzyme associated with inflammatory processes. Closer examination identified a series of candidate exons with the capacity to encode a full-length cyclooxygenase-2 protein. Counterparts of cyclooxygenase-2 have not been found in other sequenced herpesviruses. The availability of the complete RhCMV sequence along with the ability to grow RhCMV in vitro will facilitate the construction of recombinant viral strains for identifying viral determinants of CMV pathogenicity in the experimentally infected rhesus macaque and to the development of CMV as a vaccine vector.

Interactions between human cytomegalovirus helicase-primase proteins

The human cytomegalovirus (HCMV) UL70, UL102, and UL105 genes are predicted to encode essential proteins that assemble the replicative helicase-primase complex based on sequence and genome position similarities to putative herpes simplex virus type 1 (HSV-1) counterparts. Consistent with this prediction, they are required for transient complementation of DNA synthesis. However, little is known about their physical interactions and biochemical activities, primarily because of their restricted expression in HCMV-infected cells. To look for assembly of the predicted complexes, we prepared rabbit polyclonal antisera and used Semliki Forest Virus (SFV) vectors to express untagged and glutathione-S-transferase (GST)-tagged UL70, UL102 and UL105 proteins. The UL70 and UL105 proteins co-purified with the GST-tagged UL102 protein from triply-infected baby hamster kidney cells (BHK-21), and pUL70, but not pUL105, co-purified with pGST-UL102 from dually infected BHK-21 cells. In immunoprecipitation experiments with untagged SFV-expressed proteins, pUL70 or pUL105 coprecipitated with pUL102, pUL102 or pUL70 co-precipitated with pUL105; and pUL102 or pUL105 coprecipitated with pUL70. Comparison of the GST-pull down and immunoprecipitation experiments suggested that the amino-terminal GST-tag interfered with certain pairwise interactions. These results support the prediction that the HCMV helicase-primase proteins assemble a three-protein heteromeric complex, and show that each protein contacts both partners.

DNA primases

DNA primases are enzymes whose continual activity is required at the DNA replication fork. They catalyze the synthesis of short RNA molecules used as primers for DNA polymerases. Primers are synthesized from ribonucleoside triphosphates and are four to fifteen nucleotides long. Most DNA primases can be divided into two classes. The first class contains bacterial and bacteriophage enzymes found associated with replicative DNA helicases. These prokaryotic primases contain three distinct domains: an amino terminal domain with a zinc ribbon motif involved in binding template DNA, a middle RNA polymerase domain, and a carboxyl-terminal region that either is itself a DNA helicase or interacts with a DNA helicase. The second major primase class comprises heterodimeric eukaryotic primases that form a complex with DNA polymerase alpha and its accessory B subunit. The small eukaryotic primase subunit contains the active site for RNA synthesis, and its activity correlates with DNA replication during the cell cycle.

A tale of two HSV-1 helicases: roles of phage and animal virus helicases in DNA replication and recombination

Helicases play essential roles in many important biological processes such as DNA replication, repair, recombination, transcription, splicing, and translation. Many bacteriophages and plant and animal viruses encode one or more helicases, and these enzymes have been shown to play many roles in their respective viral life cycles. In this review we concentrate primarily on the roles of helicases in DNA replication and recombination with special emphasis on the bacteriophages T4, T7, and A as model systems. We explore comparisons between these model systems and the herpesviruses--primarily herpes simplex virus. Bacteriophage utilize various pathways of recombination-dependent DNA replication during the replication of their genomes. In fact the study of recombination in the phage systems has greatly enhanced our understanding of the importance of recombination in the replication strategies of bacteria, yeast, and higher eukaryotes. The ability to "restart" the replication process after a replication fork has stalled or has become disrupted for other reasons is a critical feature in the replication of all organisms studied. Phage helicases and other recombination proteins play critical roles in the "restart" process. Parallels between DNA replication and recombination in phage and in the herpesviruses is explored. We and others have proposed that recombination plays an important role in the life cycle of the herpesviruses, and in this review, we discuss models for herpes simplex virus type 1 (HSV-1) DNA replication. HSV-1 encodes two helicases. UL9 binds specifically to the origins of replication and is believed to initiate HSV DNA replication by unwinding at the origin; the heterotrimeric helicase-primase complex, encoded by UL5, UL8, and UL52 genes, is believed to unwind duplex viral DNA at replication forks. Structure-function analyses of UL9 and the helicase-primase are discussed with attention to the roles these proteins might play during HSV replication.

Current and potential therapies for the treatment of herpesvirus infections

Human herpesviruses are found worldwide and are among the most frequent causes of viral infections in immunocompetent as well as in immunocompromised patients. During the past decade and a half a better understanding of the replication and disease causing state of herpes simplex virus types 1 and 2 (HSV-1 and HSV-2), varicella-zoster virus (VZV), and human cytomegalovirus (HCMV) has been achieved due in part to the development of potent antiviral compounds that target these viruses. While some of these antiviral therapies are considered safe and efficacious (acyclovir, penciclovir), some have toxicities associated with them (ganciclovir and foscarnet). In addition, the increased and prolonged use of these compounds in the clinical setting, especially for the treatment of immunocompromised patients, has led to the emergence of viral resistance against most of these drugs. While resistance is not a serious issue for immunocompetent individuals, it is a real concern for immunocompromised patients, especially those with AIDS and the ones that have undergone organ transplantation. All the currently approved treatments target the viral DNA polymerase. It is clear that new drugs that are more efficacious than the present ones, are not toxic, and target a different viral function would be of great use especially for immunocompromised patients. Here, we provide an overview of the diseases caused by the herpesviruses as well as the replication strategy of the better studiedmembers of this family for which treatments are available. We also discuss the various drugs that have been approved for the treatment of some herpesviruses in terms of structure, mechanism of action, and development of resistance. Finally, we present a discussion of viral targets other than the DNA polymerase, for which new antiviral compounds are being considered.

Current and potential therapies for the treatment of herpesvirus infections

Human herpesviruses are found worldwide and are among the most frequent causes of viral infections in immunocompetent as well as in immunocompromised patients. During the past decade and a half a better understanding of the replication and disease causing state of herpes simplex virus types 1 and 2 (HSV-1 and HSV-2), varicella-zoster virus (VZV), and human cytomegalovirus (HCMV) has been achieved due in part to the development of potent antiviral compounds that target these viruses. While some of these antiviral therapies are considered safe and efficacious (acyclovir, penciclovir), some have toxicities associated with them (ganciclovir and foscarnet). In addition, the increased and prolonged use of these compounds in the clinical setting, especially for the treatment of immunocompromised patients, has led to the emergence of viral resistance against most of these drugs. While resistance is not a serious issue for immunocompetent individuals, it is a real concern for immunocompromised patients, especially those with AIDS and the ones that have undergone organ transplantation. All the currently approved treatments target the viral DNA polymerase. It is clear that new drugs that are more efficacious than the present ones, are not toxic, and target a different viral function would be of great use especially for immunocompromised patients. Here, we provide an overview of the diseases caused by the herpesviruses as well as the replication strategy of the better studied members of this family for which treatments are available. We also discuss the various drugs that have been approved for the treatment of some herpesviruses in terms of structure, mechanism of action, and development of resistance. Finally, we present a discussion of viral targets other than the DNA polymerase, for which new antiviral compounds are being considered.

The Autographa californica nuclear polyhedrosis virus p143 gene encodes a DNA helicase

The P143 protein of Autographa californica nuclear polyhedrosis virus is essential for replication of viral DNA. To determine the function of P143, the protein was purified to near homogeneity from recombinant baculovirus-infected cells that overexpress P143. ATPase activity copurified with P143 protein during purification and also during gel filtration at a high salt concentration. The ATPase activity did not require the presence of single-stranded DNA, but was stimulated fourfold by the addition of single-stranded DNA. The ATPase activity of P143 had a K(m) of 60 microM and a turnover of 4.5 molecules of ATP hydrolyzed/s/molecule of enzyme, indicating moderate affinity for ATP and high catalytic efficiency. P143 unwound a 40-nucleotide primer in an ATP-dependent manner, indicating that the enzyme possesses in vitro DNA helicase activity. Based on this result, it seems likely that P143 functions as a helicase in viral DNA replication.

Helicase motifs: the engine that powers DNA unwinding

Helicases play essential roles in nearly all DNA metabolic transactions and have been implicated in a variety of human genetic disorders. A hallmark of these enzymes is the existence of a set of highly conserved amino acid sequences termed the 'helicase motifs' that were hypothesized to be critical for helicase function. These motifs are shared by another group of enzymes involved in chromatin remodelling. Numerous structure-function studies, targeting highly conserved residues within the helicase motifs, have been instrumental in uncovering the functional significance of these regions. Recently, the results of these mutational studies were augmented by the solution of the three-dimensional crystal structure of three different helicases. The structural model for each helicase revealed that the conserved motifs are clustered together, forming a nucleotide-binding pocket and a portion of the nucleic acid binding site. This result is gratifying, as it is consistent with structure-function studies suggesting that all the conserved motifs are involved in the nucleotide hydrolysis reaction. Here, we review helicase structure-function studies in the light of the recent crystal structure reports. The current data support a model for helicase action in which the conserved motifs define an engine that powers the unwinding of duplex nucleic acids, using energy derived from nucleotide hydrolysis and conformational changes that allow the transduction of energy between the nucleotide and nucleic acid binding sites. In addition, this ATP-hydrolysing engine is apparently also associated with proteins involved in chromatin remodelling and provides the energy required to alter protein-DNA structure, rather than duplex DNA or RNA structure.

Assembly of the epstein-barr virus BBLF4, BSLF1 and BBLF2/3 proteins and their interactive properties

Epstein-Barr virus (EBV) encodes putative helicase-primase proteins BBLF4, BSLF1 and BBLF2/3, which are essential for the lytic phase of viral DNA replication. The BSLF1, BBLF4 and BBLF2/3 proteins were expressed in B95-8 cells after induction of a virus productive cycle, possessing apparent molecular masses of 89 kDa, 90 kDa and 80 kDa, respectively. The anti-BSLF1 or anti-BBLF2/3 protein-specific antibody, which recognizes its target protein in both Western blotting and immunoprecipitation analyses, immunoprecipitated all of the BSLF1, BBLF4 and BBLF2/3 proteins from the extract of the cells with a virus productive cycle, indicating that these viral proteins are assembled together in vivo. To characterize their protein-protein interactions in detail, recombinant baculoviruses capable of expressing each of these viral gene products in insect cells were constructed. The assembly of the three virus replication proteins was reproduced in insect cells co- infected with the three recombinant baculoviruses, indicating that complex formation does not require other EBV replication proteins. Furthermore, experiments performed by using the extracts from insect cells co-infected with each pair of the recombinant viruses demonstrated that the BSLF1 protein could interact separately with both the BBLF4 and BBLF2/3 proteins and that the BBLF2/3 protein also interacted with the BBLF4 protein. These observations strongly suggest that within the BBLF4-BSLF1-BBLF2/3 complex each component interacts directly with the other two, resulting in helicase-primase enzyme activity.