Bovine papillomavirus type 1 E2 transcriptional regulators directly bind two cellular transcription factors, TFIID and TFIIB

Protein-DNA Interactions Regulate Human Papillomavirus DNA Replication, Transcription, and Oncogenesis

Human papillomavirus (HPV) is a group of alpha papillomaviruses that cause various illnesses, including cancer. There are more than 160 types of HPV, with many being "high-risk" types that have been clinically linked to cervical and other types of cancer. "Low-risk" types of HPV cause less severe conditions, such as genital warts. Over the past few decades, numerous studies have shed light on how HPV induces carcinogenesis. The HPV genome is a circular double-stranded DNA molecule that is approximately 8 kilobases in size. Replication of this genome is strictly regulated and requires two virus-encoded proteins, E1 and E2. E1 is a DNA helicase that is necessary for replisome assembly and replication of the HPV genome. On the other hand, E2 is responsible for initiating DNA replication and regulating the transcription of HPV-encoded genes, most importantly the E6 and E7 oncogenes. This article explores the genetic characteristics of high-risk HPV types, the roles of HPV-encoded proteins in HPV DNA replication, the regulation of transcription of E6 and E7 oncogenes, and the development of oncogenesis.

Critical Involvement of TFIIB in Viral Pathogenesis

Viral infections and the harm they cause to their host are a perpetual threat to living organisms. Pathogenesis and subsequent spread of infection requires replication of the viral genome and expression of structural and non-structural proteins of the virus. Generally, viruses use transcription and translation machinery of the host cell to achieve this objective. The viral genome encodes transcriptional regulators that alter the expression of viral and host genes by manipulating initiation and termination steps of transcription. The regulation of the initiation step is often through interactions of viral factors with gene specific factors as well as general transcription factors (GTFs). Among the GTFs, TFIIB (Transcription Factor IIB) is a frequent target during viral pathogenesis. TFIIB is utilized by a plethora of viruses including human immunodeficiency virus, herpes simplex virus, vaccinia virus, Thogoto virus, hepatitis virus, Epstein-Barr virus and gammaherpesviruses to alter gene expression. A number of viral transcriptional regulators exhibit a direct interaction with host TFIIB in order to accomplish expression of their genes and to repress host transcription. Some viruses have evolved proteins with a three-dimensional structure very similar to TFIIB, demonstrating the importance of TFIIB for viral persistence. Upon viral infection, host transcription is selectively altered with viral transcription benefitting. The nature of viral utilization of TFIIB for expression of its own genes, along with selective repression of host antiviral genes and downregulation of general host transcription, makes TFIIB a potential candidate for antiviral therapies.

Design of peptides as inhibitors of human papillomavirus 16 transcriptional regulator E1-E2

Human papillomavirus 16 (HPV 16) is a DNA virus that is capable of infecting humans and causing cervical cancer. HPV16 E2 plays an important role in viral gene regulation. This work aims to predict the binding conformations and interactions between the dodecapeptides and HPV16 E2 as well as to design novel peptide inhibitors that are capable of binding to HPV16 E2 and disrupt the transcriptional regulator E1-E2 complex formation, using computational protein design techniques. Based on previously reported peptide4 (TWFWPYPYPHLP), novel peptide inhibitors were designed and five peptides that showed lower binding energy to HPV16 E2 than that of peptide4, were selected for in vitro experiments. Enzyme-linked immunosorbent (ELISA) assay showed that Y6R, W4H_Y6R, and W4H peptides bound to HPV16 E2 with higher affinity than peptide4 did. Moreover, Y6R, W4H_Y6R, and W4H peptides more effectively inhibited E1-E2 complex formation than peptide4. This work revealed important interactions between the peptides and E1-E2 complex, suggesting a strategy for development of more potent peptide inhibitors.

The papillomavirus E2 proteins

The papillomavirus E2 proteins are pivotal to the viral life cycle and have well characterized functions in transcriptional regulation, initiation of DNA replication and partitioning the viral genome. The E2 proteins also function in vegetative DNA replication, post-transcriptional processes and possibly packaging. This review describes structural and functional aspects of the E2 proteins and their binding sites on the viral genome. It is intended to be a reference guide to this viral protein.

Acetylation of conserved lysines in bovine papillomavirus E2 by p300

The p300, CBP, and pCAF lysine acetyltransferase (KAT) proteins have been reported to physically interact with bovine (BPV) and human (HPV) papillomavirus E2 proteins. While overexpression of these KAT proteins enhances E2-dependent transcription, the mechanism has not been determined. Using RNA interference (RNAi) to deplete these factors, we demonstrated that E2 transcriptional activity requires physiological levels of p300, CBP, and pCAF. Each protein appears to have a unique function in E2-dependent transcription, since overexpression of one KAT failed to compensate for RNAi knockdown of another KAT. Using an in vitro acetylation assay, we identified highly conserved lysines that are targeted by p300 for acetylation. The conservative changes of lysines at positions 111 and 112 to arginine were of particular interest. The K111R and the K111R/K112R mutants showed reduced transcriptional activity that was not responsive to p300 overexpression, while the K112R mutant retained activity. p300 and CBP were detected at the viral promoter; however, pCAF was not. We propose a model by which E2 transcriptional activity is controlled by p300-mediated acetylation of lysine 111. This model represents a novel mechanism regulating papillomavirus gene expression.

Small molecule inhibitors of human papillomavirus protein - protein interactions

Human papillomaviruses (HPV) have now been identified as a necessary cause of benign and malignant lesions of the differentiating epithelium, particularly cervical cancer, the second most prevalent cancer in women worldwide. While two prophylactic HPV vaccines and screening programs are available, there is currently no antiviral drug for the treatment of HPV infections and associated diseases. The recent progress toward the identification and characterization of specific molecular targets for small molecule-based approaches provides prospect for the development of effective HPV antiviral compounds. Traditionally, antiviral therapies target viral enzymes. HPV encode for few proteins, however, and rely extensively on the infected cell for completion of their life cycle. This article will review the functions of the viral E1 helicase, which encodes the only enzymatic function of the virus, of the E2 regulatory protein, and of the viral E6 and E7 oncogenes in viral replication and pathogenesis. Particular emphasis will be placed on the recent progress made towards the development of novel small molecule inhibitors that specifically target and inhibit the functions of these viral proteins, as well as their interactions with other viral and/or cellular proteins.

HPV16 E2 could act as down-regulator in cellular genes implicated in apoptosis, proliferation and cell differentiation

Background

Human Papillomavirus (HPV) E2 plays several important roles in the viral cycle, including the transcriptional regulation of the oncogenes E6 and E7, the regulation of the viral genome replication by its association with E1 helicase and participates in the viral genome segregation during mitosis by its association with the cellular protein Brd4. It has been shown that E2 protein can regulate negative or positively the activity of several cellular promoters, although the precise mechanism of this regulation is uncertain. In this work we constructed a recombinant adenoviral vector to overexpress HPV16 E2 and evaluated the global pattern of biological processes regulated by E2 using microarrays expression analysis.

Results

The gene expression profile was strongly modified in cells expressing HPV16 E2, finding 1048 down-regulated genes, and 581 up-regulated. The main cellular pathway modified was WNT since we found 28 genes down-regulated and 15 up-regulated. Interestingly, this pathway is a convergence point for regulating the expression of genes involved in several cellular processes, including apoptosis, proliferation and cell differentiation; MYCN, JAG1 and MAPK13 genes were selected to validate by RT-qPCR the microarray data as these genes in an altered level of expression, modify very important cellular processes. Additionally, we found that a large number of genes from pathways such as PDGF, angiogenesis and cytokines and chemokines mediated inflammation, were also modified in their expression.

Conclusions

Our results demonstrate that HPV16 E2 has regulatory effects on cellular gene expression in HPV negative cells, independent of the other HPV proteins, and the gene profile observed indicates that these effects could be mediated by interactions with cellular proteins. The cellular processes affected suggest that E2 expression leads to the cells in to a convenient environment for a replicative cycle of the virus.

Tumor suppressor or oncogene? A critical role of the human papillomavirus (HPV) E2 protein in cervical cancer progression

The papillomavirus (PV) E2 proteins have been shown to exert many functions in the viral cycle including pivotal roles in transcriptional regulation and in viral DNA replication. Besides these historical roles, which rely on their aptitude to bind to specific DNA sequences, E2 has also been shown to modulate the host cells through direct protein interactions mainly through its amino terminal transactivation domain. We will describe here some of these new functions of E2 and their potential implication in the HPV-induced carcinogenesis. More particularly we will focus on E2-mediated modulation of the host cell cycle and consequences to cell transformation. In all, the HPV E2 proteins exhibit complex functions independent of transcription that can modulate the host cells in concert with the viral vegetative cycle and which could be involved in early carcinogenesis.

Persistent immune responses induced by a human immunodeficiency virus DNA vaccine delivered in association with electroporation in the skin of nonhuman primates

Strategies to improve vaccine efficacy are still required, especially in the case of chronic infections, including human immunodeficiency virus (HIV). DNA vaccines have potential advantages over conventional vaccines; however, low immunological efficacy has been demonstrated in many experiments involving large animals and in clinical trials. To improve the immunogenicity of DNA vaccines, we have designed a plasmid vector exploiting the binding capacity of the bovine papillomavirus E2 protein and we have used electroporation (EP) to increase DNA uptake after intradermal inoculation. We demonstrated, in nonhuman primates (NHPs), efficient induction of anti-HIV immunity with an improved DNA vaccine vector encoding an artificial fusion protein, consisting of several proteins and selected epitopes from HIV-1. We show that a DNA vaccine delivery method combining intradermal injection and noninvasive EP dramatically increased expression of the vaccine antigen selectively in the epidermis, and our observations strongly suggest the involvement of Langerhans cells in the strength and quality of the anti-HIV immune response. Although the humoral responses to the vaccine were transient, the cellular responses were exceptionally robust and persisted, at high levels, more than 2 years after the last vaccine boost. The immune responses were characterized by the induction of significant proportions of T cells producing both interferon-gamma and interleukin-2 cytokines, in both subpopulations, CD4(+) and CD8(+). This strategy is an attractive approach for vaccination in humans because of its high efficacy and the possible use of newly developed devices for EP.

Interaction of the betapapillomavirus E2 tethering protein with mitotic chromosomes

During persistent papillomavirus infection, the viral E2 protein tethers the viral genome to the host cell chromosomes, ensuring maintenance and segregation of the viral genome during cell division. However, E2 proteins from different papillomaviruses interact with distinct chromosomal regions and targets. The tethering mechanism has been best characterized for bovine papillomavirus type 1 (BPV1), where the E2 protein tethers the viral genome to mitotic chromosomes in complex with the cellular bromodomain protein, Brd4. In contrast, the betapapillomavirus human papillomavirus type 8 (HPV8) E2 protein binds to the repeated ribosomal DNA genes that are found on the short arm of human acrocentric chromosomes. In this study, we show that a short 16-amino-acid peptide from the hinge region and the C-terminal DNA binding domain of HPV8 E2 are necessary and sufficient for interaction with mitotic chromosomes. This 16-amino-acid region contains an RXXS motif that is highly conserved among betapapillomaviruses, and both arginine 250 and serine 253 residues within this motif are required for mitotic chromosome binding. The HPV8 E2 proteins are highly phosphorylated, and serine 253 is a site of phosphorylation. The HPV8 E2 chromosome binding sequence also has sequence similarity with chromosome binding regions in the gammaherpesvirus EBNA and LANA tethering proteins.

Human papillomavirus type 16 E2 protein transcriptionally activates the promoter of a key cellular splicing factor, SF2/ASF

Human papillomavirus (HPV) gene expression is regulated in concert with the epithelial differentiation program. In particular, expression of the virus capsid proteins L1 and L2 is tightly restricted to differentiated epithelial cells. For HPV16, the capsid proteins are encoded by 13 structurally different mRNAs that are produced by extensive alternative splicing. Previously, we demonstrated that upon epithelial differentiation, HPV16 infection upregulates hnRNP A1 and SF2/ASF, both key factors in alternative splicing regulation. Here we cloned a 1-kb region upstream of and including the transcriptional start site of the SF2ASF gene and used it in in vivo transcription assays to demonstrate that the HPV16 E2 transcription factor transactivates the SF2/ASF promoter. The transactivation domain but not the DNA binding domain of the protein is necessary for this. Active E2 association with the promoter was demonstrated using chromatin immunoprecipitation assays. Electrophoretic mobility shift assays indicated that E2 interacted with a region 482 to 684 bp upstream of the transcription initiation site in vitro. This is the first time that HPV16 E2 has been shown to regulate cellular gene expression and the first report of viral regulation of expression of an RNA processing factor. Such E2-mediated control during differentiation of infected epithelial cells may facilitate late capsid protein expression and completion of the virus life cycle.

Modification of papillomavirus E2 proteins by the small ubiquitin-like modifier family members (SUMOs)

Papillomavirus E2 proteins are critical regulatory proteins that function in replication, genome segregation, and viral transcription, including control of expression of the viral oncogenes, E6 and E7. Sumoylation is a post-translational modification that has been shown to target and modulate the function of many transcription factors, and we now demonstrate that E2 proteins are sumoylated. Both bovine and human papillomavirus E2 proteins bind to the SUMO conjugation enzyme, Ubc9, and using in vitro and E. coli sumoylation systems, these E2 proteins were readily modified by SUMO proteins. In vivo experiments further confirmed that E2 can be sumoylated by SUMO1, SUMO2, or SUMO3. Mapping studies identified lysine 292 as the principal residue for covalent conjugation of SUMO to HPV16 E2, and a lysine 292 to arginine mutant showed defects for both transcriptional activation and repression. The expression levels, intracellular localization, and the DNA-binding activity of HPV16 E2 were unchanged by this K292R mutation, suggesting that the transcriptional defect reflects a functional contribution by sumoylation at this residue. This study provides evidence that sumoylation has a role in the regulation of papillomavirus E2, and identifies a new mechanism for the modulation of E2 function at the post-translational level.

Search for cellular partners of human papillomavirus type 16 E2 protein

Human papillomaviruses (HPVs) are small, double-stranded DNA viruses that infect cutaneous and mucosal epithelia. Type 16 (HPV16) displays tropism to genital epithelia, giving rise to genital warts and cervical intraepithelial neoplasia (CIN), which is a precursor lesion to invasive carcinoma of the cervix. The great majority of human cervical cancers contain integrated HPV DNA where the E2 gene is usually disrupted, suggesting that the loss of the E2 protein is an important step in HPV-induced carcinogenesis. The HPV16 E2 protein is a regulatory protein that seems to be essential for creating favourable conditions for establishment of infection and proper completion of the viral life cycle. Recently, diverse activities of the E2 proteins have been described, but the molecular basis of these processes has not beenfully elucidated. Using a yeast two-hybrid system, we have identified epithelial cellular proteins that bind to the E2 protein of HPV16.

Brd4-independent transcriptional repression function of the papillomavirus e2 proteins

The papillomavirus E2 protein is a critical viral regulatory protein with transcription, DNA replication, and genome maintenance functions. We have previously identified the cellular bromodomain protein Brd4 as a major E2-interacting protein and established that it participates in tethering bovine papillomavirus type 1 E2 and viral genomes to host cell mitotic chromosomes. We have also shown that Brd4 mediates E2-dependent transcriptional activation, which is strongly inhibited by the disruption of E2/Brd4 binding as well as by short hairpin RNA (shRNA) knockdown of Brd4 expression levels. Since several mutants harboring single amino acid substitutions within the E2 transactivation domain that are defective for both transcriptional transactivation and Brd4 binding are also defective for transcriptional repression, we examined the role of Brd4 in E2 repression of the human papillomavirus E6/E7 promoter. Surprisingly, in a variety of in vivo assays, including transcription reporter assays, HeLa cell proliferation and colony reduction assays, and Northern blot analyses, neither blocking of the binding of E2 to Brd4 nor shRNA knockdown of Brd4 affected the E2 repression function. Our study provides evidence for a Brd4-independent mechanism of E2-mediated repression and suggests that different cellular factors must be involved in E2-mediated transcriptional activation and repression functions.

Interaction of papillomavirus E2 protein with the Brm chromatin remodeling complex leads to enhanced transcriptional activation

Papillomavirus E2 is a sequence-specific DNA binding protein that regulates transcription and replication of the viral genome. The transcriptional activities of E2 are typically evaluated by transient transfection of nonreplicating E2-dependent reporters. We sought to address whether E2 activates transcription in an episomal context and its potential interaction with the chromatin remodeling proteins. Using an Epstein-Barr virus-based episomal reporter, we demonstrate that E2 stimulates transcription from an E2-dependent promoter in a chromatin context. This activation is enhanced by the presence of proteins associated with SWI/SNF complexes, which are ATP-dependent chromatin remodeling enzymes. We show that exogenous expression of the Brm ATPase enhances E2 activity in SWI/SNF-deficient cell lines and that the amino-terminal transactivation domain of E2 mediates association with the Brm complex in vivo. Using chromatin immunoprecipitation assays, we demonstrate that Brm enhances promoter occupancy by E2 in an episomal context. Our results demonstrate that E2 activates transcription from an episomal reporter system and reveal a novel property of E2 in collaborating with the Brm chromatin remodeling complex in enhancing transcriptional activation.

Bromodomain protein 4 mediates the papillomavirus E2 transcriptional activation function

The papillomavirus E2 regulatory protein has essential roles in viral transcription and the initiation of viral DNA replication as well as for viral genome maintenance. Brd4 has recently been identified as a major E2-interacting protein and, in the case of the bovine papillomavirus type 1, serves to tether E2 and the viral genomes to mitotic chromosomes in dividing cells, thus ensuring viral genome maintenance. We have explored the possibility that Brd4 is involved in other E2 functions. By analyzing the binding of Brd4 to a series of alanine-scanning substitution mutants of the human papillomavirus type 16 E2 N-terminal transactivation domain, we found that amino acids required for Brd4 binding were also required for transcriptional activation but not for viral DNA replication. Functional studies of cells expressing either the C-terminal domain of Brd4 that can bind E2 and compete its binding to Brd4 or short interfering RNA to knock down Brd4 protein levels revealed a role for Brd4 in the transcriptional activation function of E2 but not for its viral DNA replication function. Therefore, these studies establish a broader role for Brd4 in the papillomavirus life cycle than as the chromosome tether for E2 during mitosis.

Association of bovine papillomavirus E2 protein with nuclear structures in vivo

Papillomaviruses are small DNA viruses which have the capacity to establish a persistent infection in mammalian epithelial cells. The papillomavirus E2 protein is a central coordinator of viral gene expression, genome replication, and maintenance. We have investigated the distribution of bovine papillomavirus E2 protein in nuclei of proliferating cells and found that E2 is associated with cellular chromatin. This distribution does not change during the entire cell cycle. The N-terminal transactivation domain, but not the C-terminal DNA-binding domain, of the E2 protein is responsible for this association. The majority of the full-length E2 protein can only be detected in chromatin-enriched fractions but not as a free protein in the nucleus. Limited micrococcal nuclease digestion revealed that the E2 protein partitioned to different chromatin regions. A fraction of the E2 protein was located at nuclear sites that are resistant against nuclease attack, whereas the remaining E2 resided on compact chromatin accessible to micrococcal nuclease. These data suggest that there are two pools of E2 in the cell nucleus: one that localizes on transcriptionally inactive compact chromatin and the other, which compartmentalizes to transcriptionally active nuclear structures of the cell. Our data also suggest that E2 associates with chromatin through cellular protein(s), which in turn is released from chromatin at 0.4 M salt.

Interaction of bovine papillomavirus E2 protein with Brd4 stabilizes its association with chromatin

The bovine papillomavirus E2 protein maintains and segregates the viral extrachromosomal genomes by tethering them to cellular mitotic chromosomes. E2 interacts with a cellular bromodomain protein, Brd4, to mediate the segregation of viral genomes into daughter cells. Brd4 binds acetylated histones and has been observed to diffusely coat mitotic chromosomes in several cell types. In this study, we show that in mitotic C127 cells, Brd4 diffusely coated the condensed chromosomes. However, in the presence of the E2 protein, E2 and Brd4 colocalized in punctate dots that were randomly distributed over the chromosomes. A similar pattern of E2 and Brd4 colocalization on mitotic chromosomes was observed in CV-1 cells, whereas only a faint chromosomal coating of Brd4 was detected in the absence of the E2 protein. Therefore, the viral E2 protein relocalizes and/or stabilizes the association of Brd4 with chromosomes in mitotic cells. The colocalization of E2 and Brd4 was also observed in interphase cells, indicating that this protein-protein interaction persists throughout the cell cycle. The interaction of E2 with Brd4 greatly stabilized the association of Brd4 with interphase chromatin. In both mitotic and interphase cells, this stabilization required a transcriptionally competent transactivation domain, but not the DNA binding function of the E2 protein. Thus, the E2 protein modulates the chromatin association of Brd4 during both interphase and mitosis. This study demonstrates that the segregation of papillomavirus genomes is not simply due to the passive hitchhiking of the E2/genome complex with a convenient cellular chromosomal protein.

Specific in vitro interaction between papillomavirus E2 proteins and TBP-associated factors

The bovine and human papillomavirus (BPV/HPV) E2 proteins bind specifically to palindromic sequences ACCGN4CGGT that are concentrated within the viral long control region, where they regulate viral oncogene transcription. E2 can activate viral promoters over relatively large distances within the viral genome and was shown to cooperate with a number of cellular transcription factors. Transcriptional activator proteins, such as E2, are thought to act, at least in part, by influencing the assembly and/or stability of preinitiation complexes and it has been suggested that the transcription factor IID, composed by the TATA-binding protein (TBP) and numerous TBP-associated factors (TAFs), is a possible target of this important viral protein. In this paper, we demonstrate that E2 proteins associate in vitro with several TAFs, in particular with TAFII250 and TAFII80. In addition, we observed that the association of TAFII250 with BPV1 E2 is stronger than with HPV18 E2 and that the carboxy terminal domain of both viral proteins is involved in this interaction. On the other hand, TAFII80 binds with similar strength to both E2 proteins through their amino terminal region. These observations may help to explain the different behavior of bovine and human E2 proteins, since BPV E2 is a stronger transcriptional activator than HPV18 E2.

SF2/ASF binds the human papillomavirus type 16 late RNA control element and is regulated during differentiation of virus-infected epithelial cells

Pre-mRNA splicing occurs in the spliceosome, which is composed of small ribonucleoprotein particles (snRNPs) and many non-snRNP components. SR proteins, so called because of their C-terminal arginine- and serine-rich domains (RS domains), are essential members of this class. Recruitment of snRNPs to 5' and 3' splice sites is mediated and promoted by SR proteins. SR proteins also bridge splicing factors across exons to help to define these units and have a central role in alternative and enhancer-dependent splicing. Here, we show that the SR protein SF2/ASF is part of a complex that forms upon the 79-nucleotide negative regulatory element (NRE) that is thought to be pivotal in posttranscriptional regulation of late gene expression in human papillomavirus type 16 (HPV-16). However, the NRE does not contain any active splice sites, is located in the viral late 3' untranslated region, and regulates RNA-processing events other than splicing. The level of expression and extent of phosphorylation of SF2/ASF are upregulated with epithelial differentiation, as is subcellular distribution, specifically in HPV-16-infected epithelial cells, and expression levels are controlled, at least in part, by the virus transcription regulator E2.

Antigen-binding properties of monoclonal antibodies reactive with human TATA-binding protein and use in immunoaffinity chromatography

The TATA-binding protein (TBP) plays a central role in the assembly of most eukaryotic transcription initiation complexes. We have characterized 3 monoclonal antibodies (mAbs) that react in the far amino-terminal (N-terminal) domain of the human TBP molecule (residues 1-99). One of these mAbs (designated 1TBP22) is a polyol-responsive monoclonal antibody (PR-mAb) and was adapted to an immunoaffinity chromatography procedure for purifying bacterially expressed, recombinant human TBP. The epitope for mAb 1TBP22 maps to residues 55-99, which includes the polyglutamine region. However, mAb 1TBP22 does not react with poly-l-glutamine. Human TBP, contained on the pET11a plasmid, was expressed in Escherichia coli Rosetta (DE3)pLysS. The cell lysate from 330 ml of induced culture was treated with polyethyleneimine (PEI) at 0.5 M NaCl to precipitate the nucleic acids. After centrifugation, the supernatant fluid was applied to an immunoadsorbent containing mAb 1TBP22. After extensive washing, the TBP was eluted with buffer containing 0.75 M ammonium sulfate and 40% propylene glycol. Human TPB purified by the immunoaffinity chromatography method was found to be active in gel-shift assays and transcription assays. Preliminary data indicate that this mAb might be useful for purifying protein complexes containing TBP from HeLa cell extracts.

Direct interaction between nucleosome assembly protein 1 and the papillomavirus E2 proteins involved in activation of transcription

Using a yeast two-hybrid screen, we identified human nucleosome assembly protein 1 (hNAP-1) as a protein interacting with the activation domain of the transcriptional activator encoded by papillomaviruses (PVs), the E2 protein. We show that the interaction between E2 and hNAP-1 is direct and not merely mediated by the transcriptional coactivator p300, which is bound by both proteins. Coexpression of hNAP-1 strongly enhances activation by E2, indicating a functional interaction as well. E2 binds to at least two separate domains within hNAP-1, one within the C terminus and an internal domain. The binding of E2 to hNAP-1 is necessary for cooperativity between the factors. Moreover, the N-terminal 91 amino acids are crucial for the transcriptional activity of hNAP-1, since deletion mutants lacking this N-terminal portion fail to cooperate with E2. We provide evidence that hNAP-1, E2, and p300 can form a ternary complex efficient in the activation of transcription. We also show that p53 directly interacts with hNAP-1, indicating that transcriptional activators in addition to PV E2 interact with hNAP-1. These results suggest that the binding of sequence-specific DNA binding proteins to hNAP-1 may be an important step contributing to the activation of transcription.

Analysis of chromatin attachment and partitioning functions of bovine papillomavirus type 1 E2 protein

Recent studies have suggested that the tethering of viral genomes to host cell chromosomes could provide one of the ways to achieve their nuclear retention and partitioning during extrachromosomal maintenance in dividing cells. The data we present here provide firm evidence that the partitioning of the bovine papillomavirus type 1 (BPV1) genome is dependent on the chromatin attachment process mediated by viral E2 protein and its multiple binding sites. On the other hand, the attachment of E2 and the E2-mediated tethering of reporter plasmids to host chromosomes are not necessarily sufficient for efficient partitioning, suggesting that additional E2-dependent activities might be involved in the latter process. The activity of E2 protein in chromatin attachment and partitioning is more sensitive to the point mutations in the N-terminal domain than its transactivation and replication initiation functions. Therefore, at least part of the interactions of the E2 N-terminal domain with its targets during the chromatin attachment and partitioning processes are likely to involve specific receptors not involved in transactivation and replication activities of the protein. The mutational analysis also indicates that the binding of E2 to chromatin is not achieved through interaction of linear N-terminal subsequences of the E2 protein with putative receptors. Instead, the composite surface elements of the N-terminal domain build up the receptor-binding surface of E2. In this regard, the interaction of BPV1 E2 with its chromosomal targets clearly differs from the interactions of LANA1 protein from Kaposi's sarcoma-associated human herpesvirus and EBNA1 from Epstein-Barr virus with their specific receptors.

Direct interaction of TFIIB and the IE protein of equine herpesvirus 1 is required for maximal trans-activation function

Recently, we reported that the immediate-early (IE) protein of equine herpesvirus 1 (EHV-1) associates with transcription factor TFIIB [J. Virol. 75 (2001), 10219]. In the current study, the IE protein purified as a glutathione-S-transferase (GST) fusion protein was shown to interact directly with purified TFIIB in GST-pulldown assays. A panel of TFIIB mutants employed in protein-binding assays revealed that residues 125 to 174 within the first direct repeat of TFIIB mediate its interaction with the IE protein. This interaction is physiologically relevant as transient transfection assays demonstrated that (1). exogenous native TFIIB did not perturb IE protein function, and (2). ectopic expression of a TFIIB mutant that lacked the IE protein interactive domain significantly diminished the ability of the IE protein to trans-activate EHV-1 promoters. These results suggest that an interaction of the IE protein with TFIIB is an important aspect of the regulatory role of the IE protein in the trans-activation of EHV-1 promoters.

Crystal structure of the E2 transactivation domain of human papillomavirus type 11 bound to a protein interaction inhibitor

Interaction between the E2 protein and E1 helicase of human papillomaviruses (HPVs) is essential for the initiation of viral DNA replication. We recently described a series of small molecules that bind to the N-terminal transactivation domain (TAD) of HPV type 11 E2 and inhibits its interaction with E1 in vitro and in cellular assays. Here we report the crystal structures of both the HPV11 TAD and of a complex between this domain and an inhibitor, at 2.5- and 2.4-A resolution, respectively. The HPV11 TAD structure is very similar to that of the analogous domain of HPV16. Inhibitor binding caused no significant alteration of the protein backbone, but movements of several amino acid side chains at the binding site, in particular those of Tyr-19, His-32, Leu-94, and Glu-100, resulted in the formation of a deep hydrophobic pocket that accommodates the indandione moiety of the inhibitor. Mutational analysis provides functional evidence for specific interactions between Tyr-19 and E1 and between His-32 and the inhibitor. A second inhibitor molecule is also present at the binding pocket. Although evidence is presented that this second molecule makes only weak interactions with the protein and is likely an artifact of crystallization, its presence defines additional regions of the binding pocket that could be exploited to design more potent inhibitors.

UVB irradiation reduces the half-life and transactivation potential of the human papillomavirus 16 E2 protein

Human papillomaviruses (HPV) are causative agents of human cancers including those of the cervix and also of the head and neck; HPV16 is the most commonly found type in these diseases. The viral E2 protein regulates transcription from the viral genome by interacting with DNA-binding sequences in the HPV transcriptional control region; it also regulates replication by interacting with and recruiting the HPV replication factor E1 to the viral origin. Therefore, E2 is essential for the viral life cycle. The E2 protein interacts with several proteins involved in the cellular response to DNA damage including p53, TopBP1, and PARP. We therefore set out to establish whether DNA-damaging agents can regulate E2 activity. Here we show that UVB irradiation downregulates transcriptional activity of both HPV16 and HPV8 E2, while hydroxyurea and etoposide do not. This downregulation of E2 activity is independent of p53 function as it occurs in p53 wild type and null cell types as well as in the presence of functional HPV16 E6 that degrades p53. Using stable cell lines expressing E2 we show that this downregulation of E2 function by UVB is due to a reduction of the E2 protein half-life. The identification of the pathway(s) through which UVB downregulates E2 transcriptional activity and protein levels will present a novel target for the treatment of HPV-related diseases.

The Papillomavirus E2 protein binds to and synergizes with C/EBP factors involved in keratinocyte differentiation

The papillomavirus life cycle is closely linked to the differentiation program of the host keratinocyte. Thus, late gene expression and viral maturation are restricted to terminally differentiated keratinocytes. A variety of cellular transcription factors including those of the C/EBP family are involved in the regulation of keratinocyte differentiation. In this study we show that the papillomavirus transcription factor E2 cooperates with C/EBPalpha and -beta in transcriptional activation. This synergism was independent of an E2 binding site. E2 and C/EBP factors synergistically transactivated a synthetic promoter construct containing classical C/EBPbeta sites and the C/EBPalpha-responsive proximal promoter of the involucrin gene, which is naturally expressed in differentiating keratinocytes. C/EBPalpha or -beta coprecipitated with E2 proteins derived from human papillomavirus type 8 (HPV8), HPV16, HPV18, and bovine papillomavirus type 1 in vitro and in vivo, indicating complex formation by the cellular and viral factors. The interaction domains could be mapped to the C terminus of E2 and amino acids 261 to 302 located within the bZIP motif of C/EBPbeta. Our data suggest that E2, via its interaction with C/EBP factors, may contribute to enhancing keratinocyte differentiation, which is suppressed by the viral oncoproteins E6 and E7 in HPV-induced lesions.

Novel cellular interacting partners of the human papillomavirus 16 transcription/replication factor E2

Human papillomaviruses (HPVs) are causative agents in a number of human diseases. HPV can be divided into two groups: low risk that cause diseases such as genital warts, and high risk that cause ano-genital cancers. Of the high-risk group, HPV16 is the most commonly found in cervical cancer. All HPV encode an E2 protein and this protein regulates transcription from, and replication of, the viral genome making it essential for the viral life cycle. In order to function E2 must interact with cellular proteins; identification of these cellular partners will provide targets for disruption of the viral life cycle and will also provide insights into the processes of transcription and replication. To identify the cellular interacting partners for HPV16 E2, we carried out a yeast two-hybrid screen with the amino-terminus of E2 that is essential for mediating transcription and replication. Here we describe how this screen was carried out and detail the interacting partners that were identified; these include the proteins TopBP1, RACK1, POMP, p27(BBP), ODC antizyme, and Delta-adaptin. Several of these partners have characteristics that make them ideal candidates for mediating E2 function.

Transcriptional activity among high and low risk human papillomavirus E2 proteins correlates with E2 DNA binding

The full-length E2 protein, encoded by human papillomaviruses (HPVs), is a sequence-specific transcription factor found in all HPVs, including cancer-causing high risk HPV types 16 and 18 and wart-inducing low risk HPV types 6 and 11. To investigate whether E2 proteins encoded by high risk HPVs may function differentially from E2 proteins encoded by low risk HPVs and animal papillomaviruses, we conducted comparative DNA-binding and transcription studies using electrophoretic mobility shift assays and cell-free transcription systems reconstituted with purified general transcription factors, cofactor, RNA polymerase II, and with E2 proteins encoded by HPV-16, HPV-18, HPV-11, and bovine papillomavirus type 1 (BPV-1). We found that although different types of E2 proteins all exhibited transactivation and repression activities, depending on the sequence context of the E2-binding sites, HPV-16 E2 shows stronger transcription activity and greater DNA-binding affinity than those displayed by the other E2 proteins. Surprisingly, HPV-18 E2 behaves more similarly to BPV-1 E2 than HPV-16 E2 in its functional properties. Our studies thus categorize HPV-18 E2 and BPV-1 E2 in the same protein family, a finding consistent with the available E2 structural data that separate the closely related HPV-16 and HPV-18 E2 proteins but classify together the more divergent BPV-1 and HPV-18 E2 proteins.

Cooperative activation of human papillomavirus type 8 gene expression by the E2 protein and the cellular coactivator p300

The E2 proteins of papillomaviruses (PV) bind to the coactivator CBP/p300 as do many other transcription factors, but the precise role of CBP/p300 in E2-specific functions is not yet understood. We show that the E2 protein of human PV type 8 (HPV8) directly binds to p300. Activation of HPV8 gene expression by low amounts of HPV8 E2 was stimulated up to sevenfold by coexpression of p300. The interaction between E2 and p300 may play a role in differentiation-dependent activation of PV gene expression, since we can show that the expression level of p300 increases during keratinocyte differentiation. Surprisingly, sequence-specific binding of E2 to its recognition sites within the regulatory region of HPV8 is not necessary for this cooperation, indicating that E2 can be recruited to the promoter via protein-protein interaction. HPV8 E2 binds via its N-terminal activation domain (AD), its C-terminal DNA binding domain (DBD), and its internal hinge region to p300 in vitro. Transient-transfection assays revealed that the AD is necessary and sufficient for cooperative activation with p300. However, we provide evidence that the interaction of the hinge and the DBD of HPV8 E2 with p300 may contribute. Our data suggest an important role of p300 in regulation of HPV8 gene expression and reveal a new mechanism by which E2 may be recruited to a promoter to activate transcription without sequence specific DNA binding.

Functional interaction between human papillomavirus type 18 E2 and poly(ADP-ribose) polymerase 1

Human papillomavirus E2 protein is a transcription factor of viral gene expression and DNA replication. Here we show that PARP is a positive regulator of the E2 protein of human papillomavirus type 18 (HPV-18). PARP interacted with the COOH terminal region of HPV-18 E2 in vitro. The E2 interaction domain within PARP is located in the NH(2)-terminal zinc finger motif and the BRCT motif included in the automodification domain. Overexpression of either wild type or the NH(2)-terminal region of PARP containing zinc finger and BRCT stimulated E2-dependent transcription. Gel retardation assay indicates that PARP augments DNA binding activity of E2 in vitro. We also show that PARP-1 is recruited to E2-dependent promoter in vivo using ChIP assay. These results suggest that PARP serves a transcriptional co-activator in E2-dependent transcription by interacting directly with the HPV E2 protein.

Human papillomavirus E2 down-regulates the human telomerase reverse transcriptase promoter

The transcriptional regulation of the human telomerase reverse transcriptase (hTERT) gene is a critical step in transformation and differentiation. Human papillomavirus E2 protein inhibits cell growth in HPV-infected cells and triggers apoptosis in HeLa cells. Because E2 induces cell growth suppression and senescence, we hypothesize that the protein may modulate cellular gene expression related to these processes. In this report, we demonstrate that E2 inhibits the hTERT promoter. The mapping of the E2-responsive region of hTERT reveals that Sp1 is important for E2-mediated repression of this promoter in 293T cells. Site-directed mutagenesis data on the hTERT promoter show that E2 does not abolish E-Box-mediated transcription and represses promoter activity via the Sp1 binding site. Furthermore, chromatin immunoprecipitation assays indicate that E2 is actively recruited to the hTERT promoter region. Our findings provide novel insights into the biological function of human papillomavirus E2.

A Functional interaction between the human papillomavirus 16 transcription/replication factor E2 and the DNA damage response protein TopBP1

The human papillomavirus (HPV) transcription/replication factor E2 is essential for the life cycle of HPVs. E2 protein binds to DNA target sequences in the viral long control regions to regulate transcription of the viral genome. It also enhances viral DNA replication by interacting with the viral replication factor E1 and recruiting it to the origin of replication and may also play a more direct role in replication. The cellular proteins with which E2 interacts to carry out these functions are largely unknown. To identify these proteins a yeast two-hybrid screen was carried out with the transcription/replication domain of HPV16 E2. This screen identified several candidate interacting partners for E2 including TopBP1 (topoisomerase II beta-binding protein 1). TopBP1 has eight BRCA1 carboxyl-terminal domains that are found in proteins regulating the DNA damage response, transcription, and replication. Here we demonstrate that HPV16 E2 and TopBP1 interact in vitro and in vivo and that TopBP1 can enhance the ability of E2 to activate transcription and replication. This is the first time that TopBP1 has been shown to function as a transcriptional coactivator and that E2 interacts with TopBP1. Removal of the amino-terminal domain of TopBP1 abolishes coactivation of transcription and replication. This interaction may have functional consequences upon the viral life cycle.

Induction of the bovine papillomavirus origin "onion skin"-type DNA replication at high E1 protein concentrations in vivo

We have studied the replication of plasmids composed of bovine papillomavirus type 1 (BPV1) origin of replication and expression cartridges for viral proteins E1 and E2 in hamster and mouse cells. We found that the replication mode changed dramatically at different expression levels of the E1 protein. At high levels of the E1 protein, overreplication of the origin region of the plasmid was observed. Analysis of the replication products by one-dimensional and two-dimensional gel electrophoresis suggested that initially "onion skin"-type replication intermediates were generated, presumably resulting from initiation of the new replication forks before the leading fork completed the synthesis of the DNA on the episomal plasmid. These replication intermediates served as templates for generation of a heterogeneous set of origin region-containing linear fragments by displacement synthesis at the partially replicated plasmid. Additionally, the linear fragments may have been generated by DNA break-up of the onion skin-type intermediates. Analysis of replication products indicated that generated linear fragments recombined and formed concatemers or circular molecules, which presumably were able to replicate in an E1- and E2-dependent fashion. At moderate and low levels of E1, generated by transcription of the E1 open reading frame using weaker promoters, DNA replication was initiated at much lower levels, which allowed elongation of the replication fork starting from the origin to be more balanced and resulted in the generation of full-sized replication products.

Human TATA binding protein inhibits human papillomavirus type 11 DNA replication by antagonizing E1-E2 protein complex formation on the viral origin of replication

The human papillomavirus (HPV) protein E2 possesses dual roles in the viral life cycle. By interacting directly with host transcription factors in basal keratinocytes, E2 promotes viral transcription. As keratinocyte differentiation progresses, E2 associates with the viral helicase, E1, to activate vegetative viral DNA replication. How E2's major role switches from transcription to replication during keratinocyte differentiation is not understood, but the presence of a TATA site near the viral origin of replication led us to hypothesize that TATA-binding protein (TBP) could affect HPV replication. Here we show that the C-terminal domain of TBP (TBPc) is a potent inhibitor of E2-stimulated HPV DNA replication in vitro (50% inhibitory concentration = 0.56 nM). Increasing the E1 concentration could not overcome TBPc inhibition in replication assays, indicating that TBPc is a noncompetitive inhibitor of E1 binding. While direct E2-TBPc association could be demonstrated, this interaction could not fully account for the mechanism of TBPc-mediated inhibition of viral replication. Because E2 supports sequence-specific binding of E1 to the viral ori, we proposed that TBPc antagonizes E1-ori association indirectly through inhibition of E2-DNA binding. Indeed, TBPc potently antagonized E2 binding to DNA in the absence (K(i) = 0.5 +/- 0.1 nM) and presence (K(i) = 0.6 +/- 0.3 nM) of E1. Since E2 and TBPc cannot be coadjacent on viral sequences, direct DNA-binding competition between TBPc and E2 was responsible for replication inhibition. Given the ability of TBPc to inhibit HPV DNA replication in vitro and data indicating that TBPc antagonized E2-ori association, we propose that transcription factors regulate HPV DNA replication as well as viral transcription.

Minute virus of mice NS1 interacts with the SMN protein, and they colocalize in novel nuclear bodies induced by parvovirus infection

The human survival motor neuron (SMN) gene is the spinal muscular atrophy-determining gene, and a knockout of the murine Smn gene results in preembryonic lethality. Here we show that SMN can directly interact in vitro and in vivo with the large nonstructural protein NS1 of the autonomous parvovirus minute virus of mice (MVM), a protein essential for viral replication and a potent transcriptional activator. Typically, SMN localizes within nuclear Cajal bodies and diffusely in the cytoplasm. Following transient NS1expression, SMN and NS1 colocalize within Cajal bodies. At early time points following parvovirus infection, NS1 fails to colocalize with SMN within Cajal bodies; however, during the course of MVM infection, dramatic nuclear alterations occur. Formerly distinct nuclear bodies such as Cajal bodies, promyelocytic leukemia gene product (PML) oncogenic domains (PODs), speckles, and autonomous parvovirus-associated replication (APAR) bodies are seen aggregating at later points in infection. These newly formed large nuclear bodies (termed SMN-associated APAR bodies) are active sites of viral replication and viral capsid assembly. These results highlight the transient nature of nuclear bodies and their contents and identify a novel nuclear body formed during infection. Furthermore, simple transient expression of the viral nonstructural proteins is insufficient to induce this nuclear reorganization, suggesting that this event is induced specifically by a step in the viral infection process.

The hinge region of the human papillomavirus type 8 E2 protein activates the human p21(WAF1/CIP1) promoter via interaction with Sp1

The E2 proteins regulate papillomavirus (PV) gene expression by sequence-specific DNA binding. However, E2 is also able to activate in the absence of E2 binding sites. We show here that the E2 protein of human PV type 8 (HPV8) can activate the expression of p21(WAF1/CIP1) via promoter-proximal 200 nucleotides, which contain several Sp1 binding sites and no E2 binding sites. HPV8 E2 lacking the activation domain, which is rather conserved among E2 proteins, cooperated with co-expressed Sp1 in stimulation of the p21(WAF1/CIP1) promoter, in contrast to HPV18 E2 lacking the activation domain. We can demonstrate that the internal non-conserved hinge region of HPV8 E2 is sufficient for this functional cooperativity with Sp1. In correlation, the hinge of HPV8 E2 directly binds to Sp1. These results suggest that HPV8 E2 might be able to 'super'-activate Sp1-mediated transcription by a direct interaction via the non-conserved hinge region.

Functional interaction between p/CAF and human papillomavirus E2 protein

p300/CREB-binding protein-associated factor (p/CAF), a transcriptional co-activator, interacts with co-activator p300/CBP and acidic transcription factors. p/CAF mediates transcriptional activation by acetylating nucleosomal histones and cellular factors. Previously we reported that CBP binds to human papillomavirus E2 and activates E2-dependent transcription (Lee, D., Lee, B., Kim, J., Kim, D. W., and Choe, J. (2000) J. Biol. Chem. 275, 7045-7051). Here we show that p/CAF binds to the human papillomavirus E2 protein in vivo and in vitro and activates E2-dependent transcription. CBP along with p/CAF synergistically activates E2-dependent transcription. In addition, the histone acetylase activity of p/CAF is required for efficient activation of E2 transcriptional activity. These results suggest that p/CAF is a transcriptional co-activator of the human papillomavirus E2 protein.

A new cellular factor recognizes E2 binding sites of papillomaviruses which mediate transcriptional repression by E2

Repression of transcription by the full-length E2 protein of papillomaviruses (PV) seems to occur when the E2 binding sites and those of positively acting cellular factors overlap. Previously, we showed that RUNX1 (formerly called CBF) binds to the repression-mediating E2 binding site P2 of human PV type 8 (HPV8). By a yeast one-hybrid system we could identify an unknown protein binding also to P2, tentatively called PBF (papillomavirus binding factor). PBF recognizes the sequence CCGG, which represents the 3' half of the E2 binding site just adjacent to the RUNX1 motif. PBF also binds to the repression-mediating E2 BS-1 in BPV1, which is conserved to P2 of HPV8. Point mutations destroying PBF binding to HPV8 P2 and BPV-1 E2 BS-1 in vitro reduce promoter activity in corresponding reporter constructs. Our results suggest that PBF might play a role in transcription of PV genes and in E2-mediated repression.

Mapping the sequences that mediate interaction of the equine herpesvirus 1 immediate-early protein and human TFIIB

The sole immediate-early (IE) gene of equine herpesvirus 1 encodes a 1,487-amino-acid (aa) regulatory phosphoprotein that independently activates expression of early viral genes. Coimmunoprecipitation assays demonstrated that the IE protein physically interacts with the general transcription factor TFIIB. Using a variety of protein-binding assays that employed a panel of IE truncation and deletion mutants expressed as in vitro-synthesized or glutathione S-transferase fusion proteins, we mapped a TFIIB-binding domain to aa 407 to 757 of the IE protein. IE mutants carrying internal deletions of aa 426 to 578 and 621 to 757 were partially defective for TFIIB binding, indicating that aa 407 to 757 may harbor more than one TFIIB-binding domain. The interaction between the IE protein and TFIIB is of physiological importance, as evidenced by transient-cotransfection assays. Partial deletion of the TFIIB-binding domain within the IE protein inhibited its ability to activate expression of the viral thymidine kinase gene, a representative early promoter, and of the IR5 gene, a representative late promoter, by greater than 20 and 50%, respectively. These results indicate that the interaction of the IE protein with TFIIB is necessary for its full transactivation function and that the IE-TFIIB interaction may be part of the mechanism by which the IE protein activates transcription.

TATA-binding protein-associated factors enhance the recruitment of RNA polymerase II by transcriptional activators

Transcription factor (TF) IID, comprised of the TATA-binding protein (TBP) and TBP-associated factors (TAFs), is a general transcription factor required for RNA polymerase II (pol II) transcription on most eukaryotic genes. Recent findings that TAFs may not be globally required for activator-dependent transcription in vivo and in vitro and that both TAF-dependent and TAF-independent promoters are found in yeast suggest that transcriptional activation can occur through at least two different pathways, depending on the presence or absence of TAFs. Using order-of-addition and template challenge assays performed in a human cell-free transcription system reconstituted with recombinant general transcription factors (TFIIB, TBP, TFIIE, TFIIF), a recombinant general cofactor (PC4), and highly purified epitope-tagged multiprotein complexes (TFIID, TFIIH, pol II), we demonstrate that when TBP is used as the TATA-binding factor transcriptional activators such as Gal4-VP16 and human papillomavirus E2 mainly function by facilitating pol II entry to the promoter region. In contrast, when TFIID is used as the TATA-binding factor, promoter recognition by TFIID appears to be the rate-limiting step facilitated by transcriptional activators during preinitiation complex assembly. Using protein-protein pull-down and far-Western analyses, we further show that the presence of TAFs in TFIID facilitates the recruitment of pol II by transcriptional activators, thereby switching the rate-limiting step from pol II entry to promoter recognition. Our findings thus provide distinct molecular mechanisms for TAF-independent and TAF-dependent activation.

Orientation of a novel DNA binding site affects human papillomavirus-mediated transcription and replication

A consensus binding site for the human papillomavirus (HPV) E2 protein was determined from an unbiased set of degenerate oligonucleotides using cyclic amplification and selection of targets (CASTing). Detectable DNA-protein complexes were formed after six to nine cycles of CASTing. A population of selected binding sites was cloned, and a consensus was determined by statistical analysis of the DNA sequences of individual isolates. Starting from a pool with 20 random bases, a consensus binding site of ACAC-N(5)-GGT was derived. CASTing and electrophoretic mobility shift analyses demonstrate that human but not bovine papillomavirus E2 proteins recognize this sequence. The presence of this sequence in papillomavirus genomes suggests a role for its function. We demonstrate that this site functionally substitutes for the canonical E2 binding site (ACCG-N(4)-CGGT) in both transient-transcription and DNA replication assays. This sequence, in most instances, is interchangeable with the resident E2 binding sites in the context of the HPV type 16 long control region. Where the novel sequence does not support E2-mediated effects on gene expression or DNA replication, we demonstrate that changing the orientation of the novel sequence restores this effect.

High and low levels of cottontail rabbit papillomavirus E2 protein generate opposite effects on gene expression

The papillomavirus E2 protein plays an important role in viral transcriptional regulation and replication. We chose to study the cottontail rabbit papillomavirus (CRPV) E2 protein as a transcriptional regulator because of the availability of an animal model for papilloma formation, which may be relevant for human papillomavirus (HPV) infection and replication. We studied the effect of expression levels of E2 on the long control region, which contains transcriptional promoter and enhancer elements, and synthetic E2-dependent artificial promoters in which the E2 was the dominant factor in the transcriptional activation. These experiments indicated that high levels of E2 were inhibitory and low levels were stimulatory for transactivation. In addition, we showed that the complex formed between CRPV E2 and the cognate binding site was less stable than the complex formed between HPV E2 and the same cognate binding site. Furthermore, we showed that CRPV E2 binding to its transcriptional regulatory sequence was stabilized by other proteins such as E1, which produced increments in transcriptional activation of E2-dependent genes. The data may be used to define conditions in which the rabbit model can be used for the screening of drugs which are inhibitory to the HPV and CRPV replication and gene expression.

Interaction of the papillomavirus transcription/replication factor, E2, and the viral capsid protein, L2

The minor capsid protein L2 of papillomaviruses (PVs) likely plays a role in the selective encapsidation of PV DNA in viral capsids and in the infectivity of PV virions. The L2 protein also can cause the relocalization of the PV early protein, E2TA, to nuclear subdomains known as promyelocytic leukemia oncogenic domains (PODs) in which it is localized. E2TA is a transcriptional transactivator that also plays a critical role in viral DNA replication. In this study, we investigated whether L2, in causing the relocalization of E2TA, alters the activities of E2TA. We provide evidence that L2 inhibits the transcriptional transactivation function of E2, but it does not specifically inhibit the capacity of E2 to support viral DNA replication. We also investigated whether the colocalization of E2 and L2 to PODs and the ability of L2 to inhibit the transcriptional transactivation activity of E2TA might be mediated through a direct interaction between these two proteins. Using an in vitro protein-protein association assay, we found that L2 binds to E2TA. Two regions in E2TA were found to mediate this interaction. One of those domains is present in an alternative E2 gene product, E2TR, which is an antagonist to E2TA. Here we show that the L2 protein also relocalizes the E2 transcriptional repressor, E2TR, to the nuclear subdomains. These data suggest that the ability of L2 to relocalize E2 proteins to PODs is mediated through a direct interaction with L2.

BPV1 E2 protein enhances packaging of full-length plasmid DNA in BPV1 pseudovirions

We studied determinants of efficient encapsidation of circular DNA, incorporating a PV early region DNA sequence (nt 584-1978) previously shown to enhance packaging of DNA within papillomavirus (PV)-like particles (VLPs). Insect coelomic cells (Sf-9) and cultured monkey kidney cells (Cos-1) were transfected with an 8-kb reporter plasmid incorporating the putative BPV packaging sequence and infected with BPV1 L1 and L2 recombinant baculovirus or vaccinia virus. Heavy (1.34 g/ml) and light (1.30 g/ml) VLPs were produced, and each packaged some of the input plasmid. In light VLPs, truncated plasmids, which nevertheless incorporated the PV-derived DNA packaging sequence, were more common than full-length plasmids. Packaging efficiency of the plasmid was estimated at 1 plasmid per 10(4) VLPs in both Cos-1 and Sf-9 cells. In each cell type, expression of the BPV1 early region protein E2 in trans doubled the quantity of heavy but not light VLPs and also increased the packaging efficiency of full-length circular plasmids by threefold in heavy VLPs. The resultant pseudovirions incorporated significant amounts of E2 protein. Pseudovirions, comprising plasmids packaged within heavy VLPs, mediated the delivery of packaged plasmid into Cos-1 cells, whereby "infectivity" was blocked by antisera to BPV1 L1, but not antisera to BPV1 E4. We conclude that (a) packaging of DNA within PV L1+L2 pseudovirions is enhanced by BPV1 E2 acting in trans, (b) E2 may be packaged with the pseudovirion, and (c) E2-mediated enhancement of packaging favors 8-kb plasmid incorporation over incorporation of shorter DNA sequences.

The bovine papillomavirus E2 transactivator is stimulated by the E1 initiator through the E2 activation domain

Bovine papillomavirus type 1 (BPV-1) encodes two regulatory proteins, E1 and E2, that are essential for viral replication and transcription. E1, an ATP-dependent helicase, binds to the viral ori and is essential for viral replication, while the viral transcriptional activator, E2, plays cis-dominant roles in both viral replication and transcription. At low reporter concentrations, E1 stimulates E2 enhancer function, while at high reporter concentrations, repression results. An analysis of cis requirements revealed that neither replication nor specific E1-binding sites are required for the initiators' effect on E2 transactivator function. Though no dependence on E1-binding sites was found, analysis of E1 DNA binding and ATPase mutants revealed that both domains are required for E1 modulation of E2. Through the use of E2 fusion-gene constructs we showed that a heterologous DNA-binding domain could be substituted for the E2 DNA-binding domain and this recombinant protein remained responsive to E1. Furthermore, E1 could rescue activation domain mutants of E2 defective for transactivation. These data suggest that E1 stimulation of E2 involves interactions between E1 and the E2 activation domain on DNA. We speculate that E1 may allosterically interact with the E2 activation domain, perhaps stabilizing a particular structure, which increases the enhancer function of E2.

Searching for Antiviral Drugs for Human Papillomaviruses

The human papillomaviruses (HPVs) are ubiquitous human pathogens that cause a wide variety of benign and pre-malignant epithelial tumours. Of the almost 100 different types of HPV that have been characterized to date, approximately two dozen specifically infect genital and oral mucosa. Mucosal HPVs are most frequently sexually transmitted and, with an incidence roughly twice that of herpes simplex virus infection, are considered one of the most common sexually transmitted diseases throughout the world. A subset of genital HPVs, termed ‘high-risk’ HPVs, is highly associated with the development of genital cancers including cervical carcinoma. The absence of a simple monolayer cell culture system for analysis and propagation of the virus has substantially retarded progress in the development of diagnostic and therapeutic strategies for HPV infection. In spite of these difficulties, great progress has been made in the elucidation of the molecular controls of virus gene expression, replication and pathogenesis. With this knowledge and some important new tools, there is great potential for the development of improved diagnostic and prognostic tests, prophylactic and therapeutic vaccines, and traditional antiviral medicines.

Mechanisms of human papillomavirus E2-mediated repression of viral oncogene expression and cervical cancer cell growth inhibition

The papillomavirus E2 gene product plays a pivotal role in viral replication. E2 has multiple functions, including (i) transcriptional activation and repression of viral promoters and (ii) the enhancement of viral DNA replication. It was previously reported that E2 suppressed the growth of papillomavirus-positive cervical carcinoma cell lines. In the present study, we investigated the mechanisms of E2 growth inhibition. We found that the transcriptional activation function of E2 is required for inhibition of the growth of HeLa cells as well as for transcriptional repression of the viral E6/E7 promoter. It had been previously postulated that transcriptional repression of the E6/E7 promoter results from E2 binding its cognate sites proximal to the E6/E7 promoter and displacing other cellular transcriptional factors. In this study, we report a requirement for the transcription activation function for the binding of E2 to transcriptionally active templates.

Repression of the integrated papillomavirus E6/E7 promoter is required for growth suppression of cervical cancer cells

The human papillomavirus (HPV) E2 protein is an important regulator of viral E6 and E7 gene expression. E2 can repress the viral promoter for E6 and E7 expression as well as block progression of the cell cycle in cancer cells harboring the DNA of "high-risk" HPV types. Although the phenomenon of E2-mediated growth arrest of HeLa cells and other HPV-positive cancer cells has been well documented, the specific mechanism by which E2 affects cellular proliferation has not yet been elucidated. Here, we show that bovine papillomavirus (BPV) E2-induced growth arrest of HeLa cells requires the repression of the E6 and E7 promoter. This repression is specific for E2TA and not E2TR, a BPV E2 variant that lacks the N-terminal transactivation domain. We demonstrate that expression of HPV16 E6 and E7 from a heterologous promoter that is not regulated by E2 rescues HeLa cells from E2-mediated growth arrest. Our data indicate that the pathway of E2-mediated growth arrest of HeLa cells requires repression of E6 and E7 expression through an activity specified by the transactivation domain of E2TA.