Conserved interaction of the papillomavirus E2 transcriptional activator proteins with human and yeast TFIIB proteins

The HPV E2-Host Protein-Protein Interactions: A Complex Hijacking of the Cellular Network

Over 100 genotypes of human papillomaviruses (HPVs) have been identified as being responsible for unapparent infections or for lesions ranging from benign skin or genital warts to cancer. The pathogenesis of HPV results from complex relationships between viral and host factors, driven in particular by the interplay between the host proteome and the early viral proteins. The E2 protein regulates the transcription, the replication as well as the mitotic segregation of the viral genome through the recruitment of host cell factors to the HPV regulatory region. It is thereby a pivotal factor for the productive viral life cycle and for viral persistence, a major risk factor for cancer development. In addition, the E2 proteins have been shown to engage numerous interactions through which they play important roles in modulating the host cell. Such E2 activities are probably contributing to create cell conditions appropriate for the successive stages of the viral life cycle, and some of these activities have been demonstrated only for the oncogenic high-risk HPV. The recent mapping of E2-host protein-protein interactions with 12 genotypes representative of HPV diversity has shed some light on the large complexity of the host cell hijacking and on its diversity according to viral genotypes. This article reviews the functions of E2 as they emerge from the E2/host proteome interplay, taking into account the large-scale comparative interactomic study.

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.

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.

TFIIB and the regulation of transcription by RNA polymerase II

Accurate transcription of a gene by RNA polymerase II requires the assembly of a group of general transcription factors at the promoter. The general transcription factor TFIIB plays a central role in preinitiation complex assembly, providing a bridge between promoter-bound TFIID and RNA polymerase II. TFIIB makes extensive contact with the core promoter via two independent DNA-recognition modules. In addition to interacting with other general transcription factors, TFIIB directly modulates the catalytic center of RNA polymerase II in the transcription complex. Moreover, TFIIB has been proposed as a target of transcriptional activator proteins that act to stimulate preinitiation complex assembly. In this review, we will discuss our current understanding of these activities of TFIIB.

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.

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.

Model of the TBP–TFIIB Complex from Plasmodium falciparum: Interface Analysis and Perspectives as a New Target for Antimalarial Design

BACKGROUND

Malaria affects 200-300 million individuals per year worldwide. Plasmodium falciparum is the causative agent of the most severe and mortal type of malaria. The need for new antimalarials comes from the widespread resistance to those in current use. New antimalarial targets are required to increase chemical diversity and effectiveness of the drugs. The research for such new targets and drug chemotypes is aided by structure-based drug design. We present a model of the TBP-TFIIB complex from P. falciparum (pfTBP-pfTFIIB) and a detailed study of the interactions at the TBP-TFIIB interface.

METHODS

The model was built using standard methodology, optimized energetically and evaluated structurally. We carried out an analysis of the interface considering its evolution, available experimental data on TBP and TFIIB mutants, and the main conserved and non-conserved interactions. To support the perspective of using this complex as a new target for rational antimalarial design, we present the comparison of the pfTBP-pfTFIIB interface with its human homolog.

RESULTS

Despite the high residue conservation at the interface, we identified a potential region, composed of species-specific residues that can be used for rational antimalarial design.

CONCLUSIONS

Currently there are no antimalarial drugs targeted to stop the nuclear transcription process, a vital event for all replication stages of P. falciparum. Due to its absolute requirement in transcription initiation, we consider the pfTBP-pfTFIIB interface as a new potential target for novel antimalarial chemotypes.

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.

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.

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.

COUP-TF (chicken ovalbumin upstream promoter transcription factor)-interacting protein 1 (CTIP1) is a sequence-specific DNA binding protein

Chicken ovalbumin upstream promoter transcription factor (COUP-TF)-interacting proteins 1 and 2 [CTIP1/Evi9/B cell leukaemia (Bcl) l1a and CTIP2/Bcl11b respectively] are highly related C(2)H(2) zinc finger proteins that are abundantly expressed in brain and the immune system, and are associated with immune system malignancies. A selection procedure was employed to isolate high-affinity DNA binding sites for CTIP1. The core binding site on DNA identified in these studies, 5'-GGCCGG-3' (upper strand), is highly related to the canonical GC box and was bound by a CTIP1 oligomeric complex(es) in vitro. Furthermore, both CTIP1 and CTIP2 repressed transcription of a reporter gene harbouring a multimerized CTIP binding site, and this repression was neither reversed by trichostatin A (an inhibitor of known class I and II histone deacetylases) nor stimulated by co-transfection of a COUP-TF family member. These results demonstrate that CTIP1 is a sequence-specific DNA binding protein and a bona fide transcriptional repressor that is capable of functioning independently of COUP-TF family members. These findings may be relevant to the physiological and/or pathological action(s) of CTIPs in cells that do not express COUP-TF family members, such as cells of the haematopoietic and immune systems.

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.

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.

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.

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.

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.

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.

Interaction of the papillomavirus E2 protein with mitotic chromosomes

The bovine papillomavirus E2 transactivator protein is a multifunctional protein that activates viral transcription, cooperates in initiation of viral DNA replication, and is required for long-term episomal maintenance of viral genomes. We have shown previously that the E2 transactivator protein and bovine papillomavirus type 1 genomes are associated with mitotic chromosomes and have proposed that E2 links the genomes to cellular chromosomes to ensure segregation to daughter nuclei. In this study, we show that E2 is associated with cellular chromosomes at all stages of mitosis. We also further map the regions of E2 that are required for this association. The transactivation domain of E2 is necessary and sufficient to mediate the interaction with mitotic chromosomes; the DNA binding domain, and the flexible hinge region that separates the two domains, is not required. Furthermore, mutation of previously identified phosphorylation sites (serine residues 235, 298, and 301) has no effect on the ability of the E2 protein to bind mitotic chromosomes.

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.

Papillomavirus E2 induces p53-independent apoptosis in HeLa cells

We have previously shown that expression of the papillomavirus E2 protein in HeLa cells induces p53 accumulation and causes both cell cycle arrest and apoptosis. In contrast to growth arrest, onset of apoptosis was not correlated with an increase of p53 transcriptional activity. In the present study, we conducted biochemical and genetic experiments in order to determine whether E2-induced apoptosis was independent of p53 induction. We showed that E2 did not alter the transcription of Bax, a known p53-activated cell death inducer. The time course of apoptotic cell death preceded p53 induction by several hours. Overexpression of the HPV18 E6 oncogene prevented E2-mediated p53 accumulation, but did not alter the rate of cell death. Finally, point mutants of the HPV18 E2 transactivation domain induced apoptosis, although they were unable to induce high p53 accumulation or cell cycle arrest. In addition, the results obtained with these mutants indicated that both transcriptional activation and replication functions of E2 were dispensable for the induction of cell death. These observations show that E2-induced apoptosis is an early event, independent of p53 accumulation and unrelated to downstream p53-dependent transcriptional events.

The linking regions of EBNA1 are essential for its support of replication and transcription

The ability of distant cis-acting DNA elements to interact functionally has been proposed to be mediated by the interaction of proteins associated site specifically with those cis-acting elements. We have found that the DNA-linking regions of EBNA1 are essential for its contribution to both replication and transcription. The synthesis of plasmids containing the Epstein-Barr virus (EBV) origin of plasmid replication (oriP) can be mediated entirely by the cellular machinery; however, the replicated molecules are lost rapidly from proliferating cells. When EBNA1 is provided in trans, plasmids containing oriP (oriP plasmids) are synthesized during repeated S phases, and the newly formed daughter molecules are precisely segregated to the daughter cells. The contribution(s) of EBNA1 to the stable replication of oriP plasmids is therefore likely to be postsynthetic. In latently infected cells, EBNA1 also regulates the expression of multiple EBV promoters located as many as 10 kbp away. EBNA1 supports replication and transcription through binding to oriP; both the ability of EBNA1 to bind to DNA and the integrity of its binding sites in oriP are required. However, DNA binding by EBNA1 is not sufficient to support replication or transcription, indicating that an additional activity (or activities) is required. EBNA1 links DNAs to which it binds and can form a loop between the two subelements of oriP, the family of repeats and the region of dyad symmetry, each of which contains multiple binding sites for EBNA1. We have constructed a set of derivatives of EBNA1 which contain both, one, or neither of its linking regions in various contexts. Analyses of these derivatives demonstrate that the linking regions of EBNA1 are essential for its support of replication and transcription and that the ability of derivatives of EBNA1 to link DNAs correlates strongly with their support of these activities in cells. These findings indicate that protein-protein associations of the linking regions of EBNA1 underlie its long-range contributions to replication and transcription.