The latency-associated nuclear antigen of Kaposi's sarcoma-associated herpesvirus permits replication of terminal repeat-containing plasmids

Whole-genome transcription profiling of rhesus monkey rhadinovirus

Rhesus monkey rhadinovirus (RRV) and Kaposi's sarcoma-associated herpesvirus (KSHV; also called human herpesvirus 8) belong to the gamma-2 grouping of herpesviruses. RRV and KSHV share a high degree of sequence similarity, and their genomes are organized in a similar fashion. RRV serves as an excellent animal model system to study the gamma herpesvirus life cycle both in vitro and in vivo. We have developed a high-sensitivity, high-throughput, high-specificity real-time quantitative reverse transcriptase-based PCR assay for RRV and have used this assay to profile transcription from the whole RRV genome during de novo productive infection of rhesus fibroblasts. Using this assay, we demonstrate that the genome-wide transcription profile for RRV closely parallels the genome-wide transcription profile for KSHV.

The latency-associated nuclear antigen of rhesus monkey rhadinovirus inhibits viral replication through repression of Orf50/Rta transcriptional activation

Rhesus monkey rhadinovirus (RRV) is a gamma-2-herpesvirus that is closely related to Kaposi's sarcoma-associated herpesvirus/human herpesvirus-8. We have previously reported that the transcript for RRV latency-associated nuclear antigen (R-LANA) is expressed during lytic replication in rhesus fibroblasts. In this article, we report the development of a latent culture system for RRV and show that mRNA specific for R-LANA is expressed during latency as well. We have characterized the R-LANA protein and demonstrate that it exhibits a nuclear speckled localization and possesses the ability to homodimerize. When expressed in rhesus fibroblasts, R-LANA can inhibit RRV lytic replication in vitro. We have investigated the mechanism behind this inhibition and find that, while R-LANA itself has very little effect on lytic promoters, it can dramatically decrease the transactivation function of RRV Orf50 (Rta), which is the major viral transcription factor. We further show that the mechanism for this repression involves the recruitment of histone deacetylase complexes (HDACs), because R-LANA's ability to repress Orf50 transactivation is completely reversed by the addition of the HDAC inhibitor trichostatin A (TSA). We also report that TSA alone can significantly reactivate RRV from latently infected cells. We propose that the repressive effects of R-LANA on RRV Orf50 transactivation serve to downregulate the transcription of early genes at late times during the lytic cycle and also help to maintain viral latency by preventing viral reactivation.

Gamma-herpesvirus latency requires T cell evasion during episome maintenance

The gamma-herpesviruses persist as latent episomes in a dynamic lymphocyte pool. Their consequent need to express a viral episome maintenance protein presents a potential immune target. The glycine-alanine repeat of the Epstein-Barr virus episome maintenance protein, EBNA-1, limits EBNA-1 epitope presentation to CD8(+) T lymphocytes (CTLs). However, CTL recognition occurs in vitro, so the significance of such evasion for viral fitness is unclear. We used the murine gamma-herpesvirus-68 (MHV-68) to define the in vivo contribution of cis-acting CTL evasion to host colonisation. Although the ORF73 episome maintenance protein of MHV-68 lacks a glycine-alanine repeat, it was equivalent to EBNA-1 in conferring limited presentation on linked epitopes. This was associated with reduced protein synthesis and reduced protein degradation. We bypassed the cis-acting evasion of ORF73 by using an internal ribosome entry site to express in trans-a CTL target from the same mRNA. This led to a severe, MHC class I-restricted and CTL-dependent reduction in viral latency. Thus, despite MHV-68 encoding at least two trans-acting CTL evasion proteins, cis-acting evasion during episome maintenance was essential for normal host colonisation.

Definition of sequence requirements for latency-associated nuclear antigen 1 binding to Kaposi's sarcoma-associated herpesvirus DNA

In latent infection, Kaposi's sarcoma-associated herpesvirus (KSHV) latency-associated nuclear antigen 1 (LANA1)-specific binding to KSHV terminal repeat DNA mediates multicopy episome persistence. We now use electrophoretic mobility shift assays to investigate LANA1 binding to its 20-bp cognate sequence. Mutations at positions 6, 7, and 8 ((6)CCC(8)) severely reduced LANA1 binding, whereas mutations at other positions only modestly reduced binding. Since (6)CCC(8) is in the 5' half of an inverted repeat sequence, these results are consistent with an asymmetric role for the inverted repeat in LANA1 binding.

The latency-associated nuclear antigen of Kaposi's sarcoma-associated herpesvirus modulates cellular gene expression and protects lymphoid cells from p16 INK4A-induced cell cycle arrest

Latently infected Kaposi's sarcoma-associated herpes-virus (KSHV)-associated tumor cells have both endothelial and lymphoid origins and express a limited set of latent viral genes. One such gene, ORF73, encodes the latency-associated nuclear antigen (LANA), a multifunctional protein that plays roles in viral DNA replication, episome maintenance, and transcriptional regulation. LANA interacts with cellular proteins involved in transcriptional regulation such as the tumor suppressors, retinoblastoma (Rb) and p53, and RING3 family members. Although several reports about specific LANA-regulated promoters exist, only limited data are available that address how LANA expression in KSHV-infected cells globally affects cellular gene expression, thereby potentially contributing to KSHV pathogenicity. To investigate this question, we generated an Epstein-Barr virus-negative Burkitts lymphoma line that expresses LANA from a tetracycline-inducible promoter (BJAB/Tet-On/LANA), and we performed microarray-based gene expression profiling. Expression profiling at different time points post-induction revealed that 186 genes were activated or repressed over 2-fold in the presence of LANA. Of these genes, 41 are regulated in the Rb/E2F pathway, whereas 7 are related to p53 signaling. To determine whether these gene expression changes translate into LANA-dependent changes in cell cycle regulation, we overexpressed p16 INK4a, a CDK4/6 inhibitor that efficiently induces cell cycle arrest in Rb-positive cells. Under these conditions, LANA expression protects lymphoid cells from p16 INK4a-induced cell cycle arrest and induces S-phase entry.

Disruption of Kaposi's sarcoma-associated herpesvirus latent nuclear antigen leads to abortive episome persistence

Latent nuclear antigen (LNA) is implicated in Kaposi's sarcoma-associated herpesvirus (KSHV) episome persistence. LNA colocalizes with KSHV episomes on chromosomes in metaphase, and it maintains the stability and replication of KSHV terminal repeat-containing plasmids. In this study, we examined the function of LNA in episome persistence in the context of full-length KSHV genome by mutagenesis analysis. We generated a KSHV mutant, BAC36-DeltaLNA, with LNA disrupted by transposon-based mutagenesis with a KSHV BAC clone, BAC36, as a template. Immunofluorescence antibody staining revealed that the insertion of a transposon cassette into LNA disrupted its expression but had no effect on the expression of two adjacent genes, the vCyclin and vFLIP genes. Using a green fluorescent protein (GFP) cassette as a tracking marker for the KSHV episome, we found 8.7-fold-fewer GFP-positive cells in BAC36-DeltaLNA cultures than in wild-type BAC36 cultures at the early stage following episome delivery into 293 cells by transfection, which could be partially rescued by cotransfection with a LNA expression plasmid but not a control plasmid. Cells harboring BAC36-DeltaLNA with or without transient complementation rapidly lost episomes and became virus-free after 2 weeks of culture based on GFP expression and Gardella gel analysis and quantitative PCR assays for detecting KSHV genomes. In contrast, BAC36 episomes were stably maintained during the same period. Stable cultures with close to 100% of cells harboring KSHV episomes were readily established by hygromycin selection for BAC36 but not for BAC36-DeltaLNA. These results conclusively indicate that LNA is essential for the establishment and persistence of KSHV episomes in mammalian cells.

Latency-associated nuclear antigen of Kaposi's sarcoma-associated herpesvirus up-regulates transcription of human telomerase reverse transcriptase promoter through interaction with transcription factor Sp1

Telomerase is required for the maintenance of telomere length and is an important determinant for cell immortalization. In human cells, telomerase activity is due to the expression of its enzymatic subunit, human telomerase reverse transcriptase (hTERT). The expression of hTERT is not typically detectable in healthy somatic human cells but is present in cancerous tissues and immortalized cells. We have previously shown that hTERT promoter activity is up-regulated by the Kaposi's sarcoma-associated herpesvirus (KSHV)-encoded latency-associated nuclear antigen (LANA). LANA is expressed in all forms of human malignancies associated with KSHV. The hTERT promoter sequence located at positions -130 to +5 contains several Sp1 binding motifs and was shown be important for up-regulation by LANA. In this report, we demonstrate that hTERT promoter activity is due to the direct interaction of LANA with Sp1. The interaction of LANA with Sp1 was demonstrated through in vitro binding experiments and coimmunoprecipitation and is supported by the colocalization of these two molecules in the nuclei of KSHV-infected cells. Moreover, LANA modulates Sp1-mediated transcription in transient GAL4 fusion reporter assays. Mapping of the regions involved in binding and transcriptional activation showed that the amino terminus of LANA is the major site for interaction and up-regulation but that it can cooperate with the carboxy terminus to enhance these functions. An analysis of Sp1 binding to its cognate sequence corroborated the binding data. Together, our results suggest that the interaction of LANA with Sp1 up-regulates the telomerase promoter, potentially contributing to the immortalization of KSHV-infected cells.

Inhibition of infectious human herpesvirus 8 production by gamma interferon and alpha interferon in BCBL-1 cells

Human herpesvirus-8 (HHV-8) is aetiologically linked to Kaposi's sarcoma and primary effusion lymphoma. Although interferon-alpha (IFN-alpha) and interferon-gamma (IFN-gamma) are both antiviral cytokines, IFN-alpha blocks entry of HHV-8 into the lytic phase, whereas IFN-gamma induces an increase in the percentage of cells undergoing lytic replication. Multiple events in the lytic cascade must be completed to produce infectious virus. The ability of both types of IFN to affect the production of infectious virus was explored. Both IFN-alpha and IFN-gamma induced expression of the antiviral proteins double-stranded RNA-activated protein kinase (PKR) and 2'5'-oligoadenylate synthetase (2'5'-OAS) in HHV-8-infected BCBL-1 cells. Higher levels resulted from incubation with IFN-alpha than with IFN-gamma, whereas IFN-gamma induced higher levels of IRF-1 than did IFN-alpha. IFN-gamma induced a minor increase in lytic viral gene expression, which was not accompanied by a detectable increase in infectious virus. When lytic replication of HHV-8 was induced using TPA, high levels of infectious virus appeared in the conditioned medium. When IFN-gamma was present during TPA stimulation, the production of infectious virus was reduced by at least a 60 %, and IFN-alpha fully blocked TPA-induced production of infectious virus. The greater reduction of viral production that occurred with IFN-alpha is consistent with the higher levels of the antiviral proteins PKR and 2'5'-OAS induced by IFN-alpha than by IFN-gamma. These studies indicate that the augmentation of cellular antiviral defences by IFN-gamma was sufficient to prevent production of infectious virus despite IFN-gamma-induced entry of some cells into the lytic phase of HHV-8 replication.

Poly(ADP-ribose) polymerase 1 binds to Kaposi's sarcoma-associated herpesvirus (KSHV) terminal repeat sequence and modulates KSHV replication in latency

During latency, Kaposi's sarcoma-associated herpesvirus (KSHV) is thought to replicate once and to be partitioned in synchrony with the cell cycle of the host. In this replication cycle, the KSHV terminal repeat (TR) sequence functions as a replication origin, assisted by the latency-associated nuclear antigen (LANA). Thus, TR seems to function as a cis element for the replication and partitioning of the KSHV genome. Viral replication and partitioning are also likely to require cellular factors that interact with TR in either a LANA-dependent or -independent manner. Here, we sought to identify factors that associate with TR by using a TR DNA column and found that poly(ADP-ribose) polymerase 1 (PARP1) and known replication factors, including ORC2, CDC6, and Mcm7, bound to TR. PARP1 bound directly to a specific region within TR independent of LANA, and LANA was poly(ADP-ribosyl)ated by PARP1. Drugs such as hydroxyurea and niacinamide, which raise or lower PARP activity, respectively, affected the virus copy number in infected cells. Thus, the poly(ADP-ribosyl)ation status of LANA appears to affect the replication and/or maintenance of the viral genome. Drugs that specifically up-regulate PARP activity may lead to the disappearance of latent KSHV.

Mitotic chromosome-binding activity of latency-associated nuclear antigen 1 is required for DNA replication from terminal repeat sequence of Kaposi's sarcoma-associated herpesvirus

Latency-associated nuclear antigen 1 (LANA1) of Kaposi's sarcoma-associated herpesvirus (KSHV) is implicated in the persistence of the viral genome during latent infection. It has been suggested that LANA1 tethers the viral genome to the host chromosome and also participates actively in DNA replication from the terminal repeat of KSHV. Here we show by mutational analysis that the mitotic chromosome-binding activity of LANA1 is tightly coupled to its replication activity. Thus, KSHV appears to have evolved a unique tactic for its stable maintenance.

Kaposi's sarcoma-associated herpesvirus-encoded latency-associated nuclear antigen inhibits lytic replication by targeting Rta: a potential mechanism for virus-mediated control of latency

Like other herpesviruses, Kaposi's sarcoma-associated herpesvirus (KSHV, also designated human herpesvirus 8) can establish a latent infection in the infected host. During latency a small number of genes are expressed. One of those genes encodes latency-associated nuclear antigen (LANA), which is constitutively expressed in cells during latent as well as lytic infection. LANA has previously been shown to be important for the establishment of latent episome maintenance through tethering of the viral genome to the host chromosomes. Under specific conditions, KSHV can undergo lytic replication, with the production of viral progeny. The immediate-early Rta, encoded by open reading frame 50 of KSHV, has been shown to play a critical role in switching from viral latent replication to lytic replication. Overexpression of Rta from a heterologous promoter is sufficient for driving KSHV lytic replication and the production of viral progeny. In the present study, we show that LANA down-modulates Rta's promoter activity in transient reporter assays, thus repressing Rta-mediated transactivation. This results in a decrease in the production of KSHV progeny virions. We also found that LANA interacts physically with Rta both in vivo and in vitro. Taken together, our results demonstrate that LANA can inhibit viral lytic replication by inhibiting expression as well as antagonizing the function of Rta. This suggests that LANA may play a critical role in maintaining latency by controlling the switch between viral latency and lytic replication.

KSHV LANA1 binds DNA as an oligomer and residues N-terminal to the oligomerization domain are essential for DNA binding, replication, and episome persistence

Latency-associated nuclear antigen 1 (LANA1) binds to Kaposi's sarcoma-associated herpesvirus (KSHV) terminal repeat (TR) DNA to mediate episome replication and persistence. LANA1 concentrates at sites of TR DNA along mitotic chromosomes, consistent with tethering KSHV DNA to chromosomes for efficient segregation of episomes to progeny nuclei. We now investigate LANA1 C-terminus self-association and DNA binding. The TR DNA binding domain was localized to LANA1 residues 996-1139. Scanning deletions within this region ablated both LANA1 oligomerization and DNA binding, consistent with a requirement for oligomerization to bind DNA. Furthermore, LANA1 bound TR DNA as an oligomer. Deletion of amino acids 1007-1021, N-terminal to the LANA1 oligomerization domain, ablated DNA binding, DNA replication, and episome persistence, implicating these residues in contacting DNA. Indeed, LANA1 residues 1007-1021 correspond to EBNA1 residues that contact the cognate sequence. Like EBNA1, the LANA1 DNA-binding domain has oligomerization activity and critical residues essential for recognizing DNA.

Investigation of the susceptibility of human cell lines to bovine herpesvirus 4 infection: demonstration that human cells can support a nonpermissive persistent infection which protects them against tumor necrosis factor alpha-induced apoptosis

Bovine herpesvirus 4 (BoHV-4) is a gammaherpesvirus that has a worldwide distribution in the population of cattle. Many factors make human contamination by BoHV-4 likely to occur. In this study, we performed in vitro experiments to assess the risk and the consequences of human infection by BoHV-4. First, by using a recombinant BoHV-4 strain expressing enhanced green fluorescent protein under the control of the human cytomegalovirus immediate-early gene promoter, we tested 21 human cell lines for their sensitivity and their permissiveness to BoHV-4 infection. These experiments revealed that human cell lines from lymphoid and myeloid origins were resistant to infection, whereas epithelial cells, carcinoma cells, or adenocarcinoma cells isolated from various organs were sensitive but poorly permissive to BoHV-4 infection. Second, by using the HeLa cell line as a model of human cells sensitive but not permissive to BoHV-4 infection, we investigated the resistance of infected cells to apoptosis and the persistence of the infection through cellular divisions. The results obtained can be summarized as follows. (i) BoHV-4 nonpermissive infection of HeLa cells protects them against tumor necrosis factor alpha-induced apoptosis. (ii) BoHV-4 infection of HeLa cells persists in cell culture; however, the percentage of infected cells decreases with time due to erratic transmission of the viral genome through cell division. (iii) BoHV-4 infection has no effect on the rate of HeLa cell division. Altogether, these data suggest that BoHV-4 could infect humans. This study also stresses the importance of considering the insidious effects of nonpermissive infection when the biosafety of animal gammaherpesviruses for humans is being considered.

Inefficient establishment of KSHV latency suggests an additional role for continued lytic replication in Kaposi sarcoma pathogenesis

Kaposi sarcoma-associated (KS-associated) herpesvirus (KSHV) infection is linked to the development of both KS and several lymphoproliferative diseases. In all cases, the resulting tumor cells predominantly display latent viral infection. KS tumorigenesis requires ongoing lytic viral replication as well, however, for reasons that are unclear but have been suggested to involve the production of angiogenic or mitogenic factors by lytically infected cells. Here we demonstrate that proliferating cells infected with KSHV in vitro display a marked propensity to segregate latent viral genomes, with only a variable but small subpopulation being capable of stable episome maintenance. Stable maintenance is not due to the enhanced production of viral or host trans-acting factors, but is associated with cis-acting, epigenetic changes in the viral chromosome. These results indicate that acquisition of stable KSHV latency is a multistep process that proceeds with varying degrees of efficiency in different cell types. They also suggest an additional role for lytic replication in sustaining KS tumorigenesis: namely, the recruitment of new cells to latency to replace those that have segregated the viral episome.

Inefficient establishment of KSHV latency suggests an additional role for continued lytic replication in Kaposi sarcoma pathogenesis

Kaposi sarcoma-associated (KS-associated) herpesvirus (KSHV) infection is linked to the development of both KS and several lymphoproliferative diseases. In all cases, the resulting tumor cells predominantly display latent viral infection. KS tumorigenesis requires ongoing lytic viral replication as well, however, for reasons that are unclear but have been suggested to involve the production of angiogenic or mitogenic factors by lytically infected cells. Here we demonstrate that proliferating cells infected with KSHV in vitro display a marked propensity to segregate latent viral genomes, with only a variable but small subpopulation being capable of stable episome maintenance. Stable maintenance is not due to the enhanced production of viral or host trans-acting factors, but is associated with cis-acting, epigenetic changes in the viral chromosome. These results indicate that acquisition of stable KSHV latency is a multistep process that proceeds with varying degrees of efficiency in different cell types. They also suggest an additional role for lytic replication in sustaining KS tumorigenesis: namely, the recruitment of new cells to latency to replace those that have segregated the viral episome.

The Kaposi's sarcoma-associated herpesvirus latency-associated nuclear antigen 1 N terminus is essential for chromosome association, DNA replication, and episome persistence

To persist in latently infected, proliferating cells, Kaposi's sarcoma-associated herpesvirus (KSHV) episomes must replicate and efficiently segregate to progeny nuclei. Episome persistence in uninfected cells requires latency-associated nuclear antigen 1 (LANA1) in trans and cis-acting KSHV terminal repeat (TR) DNA. The LANA1 C terminus binds TR DNA, and LANA1 mediates TR-associated DNA replication in transient assays. LANA1 also concentrates at sites of KSHV TR DNA episomes along mitotic chromosomes, consistent with a tethering role to efficiently segregate episomes to progeny nuclei. LANA1 amino acids 5 to 22 constitute a chromosome association region (Piolot et al., J. Virol. 75:3948-3959, 2001). We now investigate LANA1 residues 5 to 22 with scanning alanine substitutions. Mutations targeting LANA1 5GMR7, 8LRS10, and 11GRS13 eliminated chromosome association, DNA replication, and episome persistence. LANA1 mutated at 14TG15 retained the ability to associate with chromosomes but was partially deficient in DNA replication and episome persistence. These results provide genetic support for a key role of the LANA1 N terminus in chromosome association, LANA1-mediated DNA replication, and episome persistence.

ORF73 of murine herpesvirus-68 is critical for the establishment and maintenance of latency

In vitro studies have established that the latency-associated nuclear antigen encoded by human Kaposi's sarcoma-associated herpesvirus and the related ORF73 gene product of herpesvirus saimiri interact with virus origins of replication to facilitate maintenance of episomal DNA. Such a function implies a critical role for ORF73 in the establishment and maintenance of latency in vivo. To determine the role of ORF73 in virus pathogenesis, the ORF73 gene product encoded by murine herpesvirus-68 (MHV-68) was disrupted by making an ORF73 deletion mutant, Delta73, and an independent ORF73 frameshift mutant, FS73. The effect of the mutations introduced in ORF73 on MHV-68 pathogenesis was analysed in vivo using a well-characterized murine model system. These studies have revealed that ORF73 is not required for efficient lytic replication either in vitro or in vivo. In contrast, a severe latency deficit is observed in splenocytes of animals infected with an ORF73 mutant, as assessed by infectious centre reactivation assay or by in situ hybridization detection of latent virus. Assessment of viral genome-positive cells in sorted splenocyte populations confirmed the absence of ORF73 mutant virus from splenic latency reservoirs, including germinal centre B cells. These data indicate a crucial role for ORF73 in the establishment of latency and for virus persistence in the host.

Manipulation of glycogen-synthase kinase-3 activity in KSHV-associated cancers

The Kaposi's sarcoma-associated herpesvirus, KSHV, is associated with cancers that have increased incidence in patients who are also HIV positive or who have undergone organ transplantation. It has recently been observed that beta-catenin is overexpressed in two KSHV-associated cancers, Kaposi's sarcoma and primary effusion lymphoma. Investigation of the underlying defect in beta-catenin regulation revealed that the KSHV-encoded LANA protein stabilizes beta-catenin by binding to the negative regulator GSK-3, causing a cell-cycle-dependent nuclear accumulation of GSK-3. Thus, redistribution of GSK-3 has been identified as yet another mechanism through which beta-catenin can be dysregulated and contribute to human cancer.

ORF73 of herpesvirus Saimiri strain C488 tethers the viral genome to metaphase chromosomes and binds to cis-acting DNA sequences in the terminal repeats

Kaposi's sarcoma (KS)-associated herpesvirus (KSHV), a human oncogenic gamma-2-herpesvirus, transforms human endothelial cells and establishes latent infection at a low efficiency in vitro. During latent infection, only a limited number of genes are expressed, and the circularized viral genome is maintained as a multicopy episome. Latency-associated nuclear antigen (LANA), exclusively expressed during latency, has been shown to have a multifunctional role in KS pathogenesis. LANA tethers the viral episome to the host chromosome, thus ensuring efficient persistence of the viral genome during successive rounds of cell division. Besides episome maintenance, LANA modulates the expression of genes of various cellular and viral pathways, including those of retinoblastoma protein and p53. Herpesvirus saimiri (HVS), another gamma-2-herpesvirus, primarily infects New World primates. Orf73, encoding the nuclear antigen of HVS, is the positional homolog of the LANA gene, and the ORF73 protein has some sequence homology to KSHV LANA. However, the function of ORF73 of HVS has not been thoroughly investigated. In this report, we show that HVS ORF73 may be important for episome persistence and colocalizes with the HVS genomic DNA on metaphase chromosomes. Furthermore, HVS terminal repeats (TRs) contain a cis-acting sequence similar to that in KSHV TRs, suggesting that the LANA binding sequence is conserved between these two viruses. This cis-acting element is sufficient to bind HVS ORF73 from strains C488 and A11, and plasmids containing the HVS C488 TR element are maintained and replicate in HVS C488 ORF73-expressing cells.

Generation and precise modification of a herpesvirus saimiri bacterial artificial chromosome demonstrates that the terminal repeats are required for both virus production and episomal persistence

Herpesvirus saimiri (HVS) is the prototype gamma-2 herpesvirus, and shares considerable homology with the human gammaherpesviruses Kaposi's sarcoma-associated herpesvirus and Epstein–Barr virus. The generation of herpesvirus mutants is a key facet in the study of virus biology. The use of F-factor-based bacterial artificial chromosomes (BACs) to clone and modify the genomes of herpesviruses has enhanced the variety, precision and simplicity of mutant production. Here we describe the cloning of the genome of HVS non-transforming strain A11-S4 into a BAC. The cloning of the BAC elements disrupts open reading frame (ORF) 15 but the HVS-BAC can still replicate at levels similar to wild-type virus, and can persistently infect fibroblasts. The HVS-BAC was modified by RecA-mediated recombination initially to substitute reporter genes and also to delete the terminal repeats (TR). After deletion of the TR, the HVS-BAC fails to enter a productive virus lytic cycle, and cannot establish a persistent episomal infection when transfected into fibroblast cell lines. This shows that while ORF 15 is dispensable for virus function in vitro, the TR is required for both virus latency and lytic virus production. In addition, the HVS-BAC promises to be a valuable tool that can be used for the routine and precise production and analysis of viral mutants to further explore gammaherpesvirus biology.

Identification of a novel cellular transcriptional repressor interacting with the latent nuclear antigen of Kaposi's sarcoma-associated herpesvirus

The latent nuclear antigen (LNA) of Kaposi's sarcoma-associated herpesvirus (KSHV) has an essential role in viral latent infection. LNA maintains the stability of KSHV episomes and modulates the expression of cellular genes. A novel cellular protein KLIP1 was identified to interact with LNA through yeast two-hybrid screening, and confirmed by a glutathione S-transferase pull down assay. Domain mapping showed that KLIP1 interacted with the N-terminal domain of LNA. Northern blot hybridization with a KLIP1 probe identified a major transcript of 1.8 kb and a minor transcript of 2.8 kb. cDNA library screening and 5'-RACE revealed that the major transcript encoded an open-reading-frame of 1,257 bp and had a 5'-untranslated region of 73 nucleotides. The major KLIP1 transcript was ubiquitously present in different cell types examined. A KLIP1 synthetic peptide antibody detected a doublet of 58-kDa and 63-kDa proteins in a Western blot assay. KLIP1 had two putative nuclear localization signals and showed punctate nuclear localization when expressed as a GFP-fusion protein. KLIP1 interacted with LNA in vivo, as demonstrated by coimmunoprecipitation using KSHV-infected cells and colocalization when they were expressed as GFP- and DsRed-fusion proteins, respectively. Consistent with its interaction with LNA, nuclear localization, and possession of two leucine zipper motifs, KLIP1 behaved like a transcriptional factor and repressed herpes simplex virus thymidine kinase (TK) promoter activity in a mammalian one-hybrid assay. In addition, cotransfection with LNA alleviated the transcriptional repression effect of KLIP1 on TK promoter activity. These results suggest that KLIP1 is a new member of cellular transcriptional repressors, and that LNA is involved in deregulating cellular transcription process.