Targeted mutagenesis of the human papillomavirus type 16 E2 transactivation domain reveals separable transcriptional activation and DNA replication functions

Experimental Support for Human Papillomavirus Genome Amplification Early after Infectious Delivery

Even though replication and transcription of human papillomavirus type 16 (HPV16) has been intensively studied, little is known about immediate-early events of the viral life cycle due to the lack of an efficient infection model allowing genetic dissection of viral factors. We employed the recently developed infection model (Bienkowska-Haba M, Luszczek W, Myers JE, Keiffer TR, et al. 2018. PLoS Pathog 14:e1006846) to investigate genome amplification and transcription immediately after infectious delivery of viral genome to nuclei of primary keratinocytes. Using 5-ethynyl-2'-deoxyuridine (EdU) pulse-labeling and highly sensitive fluorescence in situ hybridization, we observed that the HPV16 genome is replicated and amplified in an E1- and E2-dependent manner. Knockout of E1 resulted in failure of the viral genome to replicate and amplify. In contrast, knockout of the E8^E2 repressor led to increased viral genome copy number, confirming previous reports. Genome copy control by E8^E2 was confirmed for differentiation-induced genome amplification. Lack of functional E1 had no effect on transcription from the early promoter, suggesting that viral genome replication is not required for p97 promoter activity. However, infection with an HPV16 mutant virus defective for E2 transcriptional function revealed a requirement of E2 for efficient transcription from the early promoter. In the absence of the E8^E2 protein, early transcript levels are unaltered and even decreased when normalized to genome copy number. Surprisingly, a lack of functional E8^E2 repressor did not affect E8^E2 transcript levels when normalized to genome copy number. These data suggest that the main function of E8^E2 in the viral life cycle is to control genome copy number. IMPORTANCE It is being assumed that human papillomavirus (HPV) utilizes three different modes of replication during its life cycle: initial amplification during the establishment phase, genome maintenance, and differentiation-induced amplification. However, HPV16 initial amplification was never formally proven due to a lack of an infection model. Using our recently established infection model (Bienkowska-Haba M, Luszczek W, Myers JE, Keiffer TR, et al. 2018. PLoS Pathog 14:e1006846), we demonstrate herein that viral genome is indeed amplified in an E1- and E2-dependent manner. Furthermore, we find that the main function of the viral repressor E8^E2 is to control viral genome copy number. We did not find evidence that it regulates its own promoter in a negative feedback loop. Our data also suggest that the E2 transactivator function is required for stimulation of early promoter activity, which has been debated in the literature. Overall, this report confirms the usefulness of the infection model for studying early events of the HPV life cycle using mutational approaches.

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.

Multiple Roles of Brd4 in the Infectious Cycle of Human Papillomaviruses

Human Papillomaviruses (HPV) reproduce in stratified epithelia by establishing a reservoir of low- level infection in the dividing basal cells and restricting the production of viral particles to terminally differentiated cells. These small DNA viruses hijack pivotal cellular processes and pathways to support the persistent infectious cycle. One cellular factor that is key to multiple stages of viral replication and transcription is the BET (bromodomain and extra-terminal domain) protein, Brd4 (Bromodomain containing protein 4). Here we provide an overview of the multiple interactions of Brd4 that occur throughout the HPV infectious cycle.

Phosphorylation of the Human Papillomavirus E2 Protein at Tyrosine 138 Regulates Episomal Replication

The papillomavirus (PV) E2 protein is a critical regulator of viral transcription and genome replication. We previously reported that tyrosine (Y) 138 of HPV-31 E2 is phosphorylated by the fibroblast growth factor receptor 3 (FGFR3) kinase. In this study, we generated quasiviruses containing G418-selectable HPV-31 genomes with phosphodeficient phenylalanine mutant E2 Y138F and phosphomimetic glutamic acid mutant Y138E. We observed significantly fewer early viral transcripts immediately after infection with these Y138 mutant genomes even though E2 occupancy at the viral origin was equivalent to that of wild-type E2. Keratinocytes infected with Y138F quasiviruses formed stable colonies, and the genomes were maintained as episomes, while those infected with Y138E quasiviruses did not. We previously reported that the HPV-31 E2 Y138 mutation to glutamic acid did not bind to the Brd4 C-terminal motif (CTM). Here, we demonstrate that HPV-16 E2 Y138E bound to full-length Brd4 but not to the Brd4 CTM. We conclude that association of E2 with the Brd4 CTM is necessary for viral genome replication and suggest that this interaction can be regulated by phosphorylation of E2 Y138.IMPORTANCE Papillomavirus (PV) is a double-stranded DNA tumor virus infecting the cutaneous and mucosal epithelium. The PV E2 protein associates with a number of cellular factors to mediate replication of the HPV genome. Fibroblast growth factor receptor 3 (FGFR3) regulates HPV replication through phosphorylation of tyrosine 138 in the HPV E2 protein. Employing a quasivirus infection model and selection for G418 resistant genomes, we demonstrated that Y138 is a critical residue for Brd4 association and that inability to complex with Brd4 does not support episomal replication.

Activating the DNA Damage Response and Suppressing Innate Immunity: Human Papillomaviruses Walk the Line

Activation of the DNA damage response (DDR) by external agents can result in DNA fragments entering the cytoplasm and activating innate immune signaling pathways, including the stimulator of interferon genes (STING) pathway. The consequences of this activation can result in alterations in the cell cycle including the induction of cellular senescence, as well as boost the adaptive immune response following interferon production. Human papillomaviruses (HPV) are the causative agents in a host of human cancers including cervical and oropharyngeal; HPV are responsible for around 5% of all cancers. During infection, HPV replication activates the DDR in order to promote the viral life cycle. A striking feature of HPV-infected cells is their ability to continue to proliferate in the presence of an active DDR. Simultaneously, HPV suppress the innate immune response using a number of different mechanisms. The activation of the DDR and suppression of the innate immune response are essential for the progression of the viral life cycle. Here, we describe the mechanisms HPV use to turn on the DDR, while simultaneously suppressing the innate immune response. Pushing HPV from this fine line and tipping the balance towards activation of the innate immune response would be therapeutically beneficial.

Is viral E6 oncoprotein a viable target? A critical analysis in the context of cervical cancer

An understanding of the pathology of cervical cancer (CC) mediated by E6/E7 oncoproteins of high-risk human papillomavirus (HPV) was developed by late 80's. But if we look at the present scenario, not a single drug could be developed to inhibit these oncoproteins and in turn, be used specifically for the treatment of CC. The readers are advised not to presume the "viability of E6 protein" as mentioned in the title relates to just druggability of E6. The viability aspect will cover almost everything a researcher should know to develop E6 inhibitors until the preclinical stage. Herein, we have analysed the achievements and shortcomings of the scientific community in the last four decades in targeting HPV E6 against CC. Role of all HPV proteins has been briefly described for better perspective with a little detailed discussion of the role of E6. We have reviewed the articles from 1985 onward, reporting in vitro inhibition of E6. Recently, many computational studies have reported potent E6 inhibitors and these have also been reviewed. Subsequently, a critical analysis has been reported to cover the in vitro assay protocols and in vivo models to develop E6 inhibitors. A paragraph has been devoted to the role of public policy to fight CC employing vaccines and whether the vaccine against HPV has quenched the zeal to develop drugs against it. The review concludes with the challenges and the way forward.

Phosphorylation of a Conserved Tyrosine in the Papillomavirus E2 Protein Regulates Brd4 Binding and Viral Replication

The papillomavirus (PV) E2 protein coordinates viral transcription and genome replication. Following a strategy to identify amino acids in E2 that are posttranslationally modified, we reported that tyrosine kinase fibroblast growth factor receptor 3 (FGFR3) complexes with and phosphorylates E2, which inhibits viral DNA replication. Here, we present several lines of evidence indicating that tyrosine (Y) 138 of HPV-31 E2 is a substrate of FGFR3. The active form of FGFR3 bound to and phosphorylated the region of amino acids (aa) 107 to 175 in HPV-31 E2. The E2 phenylalanine (F) mutant Y138F displayed reduced FGFR3-induced phosphotyrosine. A constitutive kinase-active FGFR3 inhibited wild-type (WT) E2-induced E1-dependent DNA replication, while the 138F mutant retained activity. The tyrosine to glutamic acid (E) mutant Y138E, which can mimic phosphotyrosine, failed to induce transient DNA replication, although it maintained the ability to bind and localize the viral DNA helicase E1 to the viral origin. The bromodomain-containing protein 4 (Brd4) binds to E2 and is necessary for initiation of viral DNA synthesis. Interestingly, the Y138E protein coimmunoprecipitated with full-length Brd4 but was defective for association with its C-terminal domain (CTD). These results imply that the activity of the FGFR3 kinase in the infected epithelial cell restricts the HPV replication program through phosphorylation of E2 at Y138, which interferes with E2 binding to the Brd4 CTD, and that this interaction is required for initiation of viral DNA synthesis.IMPORTANCE Human papillomaviruses (HPVs) are highly infectious pathogens that commonly infect the oropharynx and uterine cervix. The idea that posttranslational modifications of viral proteins coordinates viral genome replication is less explored. We recently discovered that fibroblast growth factor receptor 3 (FGFR3) phosphorylates the viral E2 protein. The current study demonstrates that FGFR3 phosphorylates E2 at tyrosine 138, which inhibits association with the C-terminal peptide of Brd4. This study illustrates a novel regulatory mechanism of virus-host interaction and provides insight into the role of Brd4 in viral replication.

The SMC5/6 Complex Interacts with the Papillomavirus E2 Protein and Influences Maintenance of Viral Episomal DNA

The papillomavirus E2 protein executes numerous essential functions related to viral transcription, replication of viral DNA, and viral genome maintenance. Because E2 lacks enzymatic activity, many of these functions are mediated by interactions with host cellular proteins. Unbiased proteomics approaches have successfully identified a number of E2-host protein interactions. We have extended such studies and have identified and validated the cellular proteins structural maintenance of chromosome 5 (SMC5) and SMC6 as interactors of the viral E2 protein. These two proteins make up the core components of the SMC5/6 complex. The SMC5/6 complex is a member of the conserved structural maintenance of chromosomes (SMC) family of proteins, which are essential for genome maintenance. We have examined the role of SMC5/6 in various E2 functions. Our data suggest that SMC6 is not required for E2-mediated transcriptional activation, E1/E2-mediated transient replication, or differentiation-dependent amplification of viral DNA. Our data, however, suggest a role for SMC5/6 in viral genome maintenance.IMPORTANCE The high-risk human papillomaviruses (HPVs) are the etiological cause of cervical cancer and the most common sexually transmitted infection. While the majority of infections may be asymptomatic or cause only benign lesions, persistent infection with the oncogenic high-risk HPV types may lead to serious diseases, such as cervical cancer, anogenital carcinoma, or head and neck oropharyngeal squamous cell carcinoma. The identification of virus-host protein interactions provides insights into the mechanisms of viral DNA persistence, viral genome replication, and cellular transformation. Elucidating the mechanism of early events in the virus replication cycle as well as of integration of viral DNA into host chromatin may present novel antiviral strategies and targets for counteracting persistent infection. The E2 protein is an important viral regulatory protein whose functions are mediated through interactions with host cell proteins. Here we explore the interaction of E2 with SMC5/6 and the functional consequences.

Nuclear myosin 1 associates with papillomavirus E2 regulatory protein and influences viral replication

Nuclear myosin 1c (NM1) associates with RNA polymerases and is a partner in the chromatin remodeling complex B-WICH. This complex, which also contains WSTF and SNF2h proteins, is involved in transcriptional regulation. We report herein that papillomavirus protein E2 binds to NM1 and co-precipitates with the WSTF and SNF2h proteins. Our data suggest that E2 associates with the cellular B-WICH complex through binding to NM1. E2 and NM1 associate via their N-terminal domains and this interaction is ATP dependent. The cellular multifunctional protein Brd4 and beta-actin are also present in the NM1-E2 complex. NM1 downregulation by siRNA increases the replication of the BPV1 and HPV5 genomes but does not affect HPV18 genome replication. These results suggest that the B-WICH complex may play a role in the papillomavirus life cycle through NM1 and E2 protein interaction.

Design of peptide inhibitors of human papillomavirus 16 (HPV16) transcriptional regulator E1–E2 formation

Here, we have proposed a new scheme of the computational combinatorial design approach to identify potential inhibitor peptides. It consists of four steps: (i) using “multiple copy simultaneous search” (MCSS) procedure to locate specific functional groups on the protein surface; (ii) the peptide main chain is constructed based on the location of favored N-methylacetamide (NMA) groups; (iii) molecular dynamics simulations of the complex formed between the constructed peptides with the target protein in explicit water molecules are carried to select the peptides with strong binding to the protein and (iv) the sequences of the stable peptides selected from (iii) are aligned and the frequencies of the amino acids at each position of peptide are calculated. Sequence patterns of potential inhibitors are determined based on the frequency of amino acids at each position. It was applied to design peptide inhibitors that bind to the E2 protein of HPV16 so as to disrupt its transcriptional regulator of E1–E2 complex formation. The sequence pattern of these potential inhibitors is in agreement with known inhibitors obtained from phage display, and the MCSS calculations indicate that a hydrophobic pocket on HPV16 E2 plays a significant role in E1–E2 formation and inhibitor-E2 binding.

HPV16 and 18 genome amplification show different E4-dependence, with 16E4 enhancing E1 nuclear accumulation and replicative efficiency via its cell cycle arrest and kinase activation functions

To clarify E1^E4’s role during high-risk HPV infection, the E4 proteins of HPV16 and 18 were compared side by side using an isogenic keratinocyte differentiation model. While no effect on cell proliferation or viral genome copy number was observed during the early phase of either virus life cycle, time-course experiments showed that viral genome amplification and L1 expression were differently affected upon differentiation, with HPV16 showing a much clearer E4 dependency. Although E4 loss never completely abolished genome amplification, its more obvious contribution in HPV16 focused our efforts on 16E4. As previously suggested, in the context of the virus life cycle, 16E4s G2-arrest capability was found to contribute to both genome amplification success and L1 accumulation. Loss of 16E4 also lead to a reduced maintenance of ERK, JNK and p38MAPK activity throughout the genome amplifying cell layers, with 16E4 (but not 18E4) co-localizing precisely with activated cytoplasmic JNK in both wild type raft tissue, and HPV16-induced patient biopsy tissue. When 16E1 was co-expressed with E4, as occurs during genome amplification in vivo, the E1 replication helicase accumulated preferentially in the nucleus, and in transient replication assays, E4 stimulated viral genome amplification. Interestingly, a 16E1 mutant deficient in its regulatory phosphorylation sites no longer accumulated in the nucleus following E4 co-expression. E4-mediated stabilisation of 16E2 was also apparent, with E2 levels declining in organotypic raft culture when 16E4 was absent. These results suggest that 16E4-mediated enhancement of genome amplification involves its cell cycle inhibition and cellular kinase activation functions, with E4 modifying the activity and function of viral replication proteins including E1. These activities of 16E4, and the different kinase patterns seen here with HPV18, 31 and 45, may reflect natural differences in the biology and tropisms of these viruses, as well as differences in E4 function.

In HPV induced lesions, the most abundant protein expressed in the productive stage of viral life cycle is E1^E4 (E4), with its expression being coincident with viral genome amplification. To clarify the role of E4 in the high-risk HPV life cycle, we carried out a comparative analysis of E4 function in HPV16 and 18 using an isogenic keratinocyte cell-line background. Our results show that E1^E4 contributes to virus genome replication efficiency and life cycle completion rather than being essential. These effects were seen more dramatically with HPV16. The difference between HPV16 and HPV18 in our system suggests important tropism differences between these viruses. HPV16 E4’s contribution to the virus life cycle is mediated by several activities, including its G2 arrest function, as well as its role in activating members of the MAPK pathway, including ERK, p38, and most notably pJNK. These 16 E4 functions facilitated the nuclear localization of the E1 virus helicase and enhanced E1/E2 dependent viral genome amplification as well as stabilising E2. We suspect that the massive accumulation of E4 in the upper epithelial layers may however underlie a more critical role for E4 post-genome amplification.

Brd4 Activates Early Viral Transcription upon Human Papillomavirus 18 Infection of Primary Keratinocytes

Human papillomaviruses (HPVs) replicate in the cutaneous and mucosal epithelia, and the infectious cycle is synchronous with the differentiation program of the host keratinocytes. The virus initially infects dividing cells in the lower layers of the epithelium, where it establishes a persistent infection. The viral genome is maintained as a low-copy-number, extrachromosomal element in these proliferating cells but switches to the late stage of the life cycle in differentiated cells. The cellular chromatin adaptor protein Brd4 is involved in several stages and processes of the viral life cycle. In concert with the viral transcriptional regulator E2, Brd4 can repress transcription from the early viral promoter. Brd4 and E2 form a complex with the viral genome that associates with host chromosomes to partition the viral genome in dividing cells; Brd4 also localizes to active sites of productive HPV DNA replication. However, because of the difficulties in producing HPV viral particles, the role of Brd4 in modulating viral transcription and replication at the initial stage of infection is unclear. In this study, we have used an HPV18 quasivirus-based genome delivery system to assess the role of Brd4 in the initial infectivity of primary human keratinocytes. We show that, upon infection of primary human keratinocytes with HPV18 quasivirus, Brd4 activates viral transcription and replication. Furthermore, this activation is independent of the functional interaction between Brd4 and the HPV18 E2 protein.

HPVs lack encapsidated proteins and so rely exquisitely on host cellular factors to initiate their gene expression programs in newly infected cells. Brd4 is an important cellular chromatin adaptor molecule that normally activates host transcription initiation and elongation. In this study, we further optimize and utilize a quasivirus infection system to show that Brd4 activates HPV18 transcription at early stages of infection. HPVs are important human pathogens causing a wide range of cutaneous and tumorigenic morbidities. Therefore, specifically targeting this protein could provide a new target of therapeutic prevention of establishment of HPV infections.

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.

Failure to interact with Brd4 alters the ability of HPV16 E2 to regulate host genome expression and cellular movement

The E2 protein of the carcinogen human papillomavirus 16 (HPV16) regulates replication and transcription of the viral genome in association with viral and cellular proteins. Our previous work demonstrated that E2 can regulate transcription from the host genome. E2 can activate transcription from adjacent promoters when located upstream using E2 DNA binding sequences and this activation is dependent upon the cellular protein Brd4; this report demonstrates that a Brd4 binding E2 mutant alters host genome expression differently from wild type E2. Of particular note is that highly down regulated genes are mostly not affected by failure to interact with Brd4 suggesting that the E2-Brd4 interaction is more responsible for the transcriptional activation of host genes rather than repression. Therefore failure to interact efficiently with Brd4, or altered levels of Brd4, would alter the ability of E2 to regulate the host genome and could contribute to determining the outcome of infection.

Characterization of the Human Papillomavirus 16 E8 Promoter

Persistent infections with certain human papillomaviruses (HPV) such as HPV16 are a necessary risk factor for the development of anogenital and oropharyngeal cancers. HPV16 genomes replicate as low-copy-number plasmids in the nucleus of undifferentiated keratinocytes, which requires the viral E1 and E2 replication proteins. The HPV16 E8^E2C (or E8^E2) protein limits genome replication by repressing both viral transcription and the E1/E2-dependent DNA replication. How E8^E2C expression is regulated is not understood. Previous transcript analyses indicated that the spliced E8^E2C RNA is initiated at a promoter located in the E1 region upstream of the E8 gene. Deletion and mutational analyses of the E8 promoter region identify two conserved elements that are required for basal promoter activity in HPV-negative keratinocytes. In contrast, the transcriptional enhancer in the upstream regulatory region of HPV16 does not modulate basal E8 promoter activity. Cotransfection studies indicate that E8^E2C inhibits, whereas E2 weakly activates, the E8 promoter. Interestingly, the cotransfection of E1 and E2 induces the E8 promoter much more strongly than the major early promoter, and this is partially dependent upon binding of E2 to Brd4. Mutation of E8 promoter elements in the context of HPV16 genomes results in an increased genome copy number and elevated levels of viral early and late transcripts. In summary, the promoter responsible for the expression of E8^E2C is both positively and negatively regulated by viral and cellular factors, and this regulatory circuit may be crucial to maintain a low but constant copy number of HPV16 genomes in undifferentiated cells.

IMPORTANCE

HPV16 replicates in differentiating epithelia and can cause cancer. How HPV16 maintains its genome in undifferentiated cells at a low but constant level is not well understood but may be relevant for the immunological escape of HPV16 in the basal layers of the infected epithelium. This study demonstrates that the expression of the viral E8^E2C protein, which is a potent inhibitor of viral replication in undifferentiated cells, is driven by a separate promoter. The E8 promoter is both positively and negatively regulated by viral proteins and thus most likely acts as a sensor and modulator of viral copy number.

Evidence supporting a role for TopBP1 and Brd4 in the initiation but not continuation of human papillomavirus 16 E1/E2-mediated DNA replication

To replicate the double-stranded human papillomavirus 16 (HPV16) DNA genome, viral proteins E1 and E2 associate with the viral origin of replication, and E2 can also regulate transcription from adjacent promoters. E2 interacts with host proteins in order to regulate both transcription and replication; TopBP1 and Brd4 are cellular proteins that interact with HPV16 E2. Previous work with E2 mutants demonstrated the Brd4 requirement for the transactivation properties of E2, while TopBP1 is required for DNA replication induced by E2 from the viral origin of replication in association with E1. More-recent studies have also implicated Brd4 in the regulation of DNA replication by E2 and E1. Here, we demonstrate that both TopBP1 and Brd4 are present at the viral origin of replication and that interaction with E2 is required for optimal initiation of DNA replication. Both cellular proteins are present in E1-E2-containing nuclear foci, and the viral origin of replication is required for the efficient formation of these foci. Short hairpin RNA (shRNA) against either TopBP1 or Brd4 destroys the E1-E2 nuclear bodies but has no effect on E1-E2-mediated levels of DNA replication. An E2 mutation in the context of the complete HPV16 genome that compromises Brd4 interaction fails to efficiently establish episomes in primary human keratinocytes. Overall, the results suggest that interactions between TopBP1 and E2 and between Brd4 and E2 are required to correctly initiate DNA replication but are not required for continuing DNA replication, which may be mediated by alternative processes such as rolling circle amplification and/or homologous recombination.

IMPORTANCE

Human papillomavirus 16 (HPV16) is causative in many human cancers, including cervical and head and neck cancers, and is responsible for the annual deaths of hundreds of thousands of people worldwide. The current vaccine will save lives in future generations, but antivirals targeting HPV16 are required for the alleviation of disease burden on the current, and future, generations. Targeting viral DNA replication that is mediated by two viral proteins, E1 and E2, in association with cellular proteins such as TopBP1 and Brd4 would have therapeutic benefits. This report suggests a role for these cellular proteins in the initiation of viral DNA replication by HPV16 E1-E2 but not for continuing replication. This is important if viral replication is to be effectively targeted; we need to understand the viral and cellular proteins required at each phase of viral DNA replication so that it can be effectively disrupted.

Regulation of human genome expression and RNA splicing by human papillomavirus 16 E2 protein

Human papillomavirus 16 (HPV16) is causative in human cancer. The E2 protein regulates transcription from and replication of the viral genome; the role of E2 in regulating the host genome has been less well studied. We have expressed HPV16 E2 (E2) stably in U2OS cells; these cells tolerate E2 expression well and gene expression analysis identified 74 genes showing differential expression specific to E2. Analysis of published gene expression data sets during cervical cancer progression identified 20 of the genes as being altered in a similar direction as the E2 specific genes. In addition, E2 altered the splicing of many genes implicated in cancer and cell motility. The E2 expressing cells showed no alteration in cell growth but were altered in cell motility, consistent with the E2 induced altered splicing predicted to affect this cellular function. The results present a model system for investigating E2 regulation of the host genome.

Brd4 is displaced from HPV replication factories as they expand and amplify viral DNA

Replication foci are generated by many viruses to concentrate and localize viral DNA synthesis to specific regions of the cell. Expression of the HPV16 E1 and E2 replication proteins in keratinocytes results in nuclear foci that recruit proteins associated with the host DNA damage response. We show that the Brd4 protein localizes to these foci and is essential for their formation. However, when E1 and E2 begin amplifying viral DNA, Brd4 is displaced from the foci and cellular factors associated with DNA synthesis and homologous recombination are recruited. Differentiated HPV-infected keratinocytes form similar nuclear foci that contain amplifying viral DNA. We compare the different foci and show that, while they have many characteristics in common, there is a switch between early Brd4-dependent foci and mature Brd4-independent replication foci. However, HPV genomes encoding mutated E2 proteins that are unable to bind Brd4 can replicate and amplify the viral genome. We propose that, while E1, E2 and Brd4 might bind host chromatin at early stages of infection, there is a temporal and functional switch at later stages and increased E1 and E2 levels promote viral DNA amplification, displacement of Brd4 and growth of a replication factory. The concomitant DNA damage response recruits proteins required for DNA synthesis and repair, which could then be utilized for viral DNA replication. Hence, while Brd4 can enhance replication by concentrating viral processes in specific regions of the host nucleus, this interaction is not absolutely essential for HPV replication.

Papillomaviruses have a remarkable infection cycle that depends on the development of a stratified epithelium. The virus infects the lower, dividing layers of the epithelium and viral genomes replicate at low copy number, and are maintained in these cells, for long periods of time. As infected cells differentiate and move to the surface of the epithelium, they switch on high level viral DNA replication, synthesize capsid proteins and form new viral particles. Viral replication takes place in nuclear foci and is dependent on the E1 and E2 replication proteins. Brd4 is a cellular chromatin binding protein that interacts with E2 and is important for transcriptional regulation of papillomaviruses. In this study we examine the role of Brd4 at different stages in the formation of viral replication foci. In the absence of viral DNA replication, Brd4 links the viral proteins to host chromatin. However, when viral genomes begin to amplify to high levels, Brd4 is displaced from nuclear foci and is not required for replication.

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.

Current understanding of the role of the Brd4 protein in the papillomavirus lifecycle

The Brd4 protein is an epigenetic reader that is central to regulation of cellular transcription and mitotic bookmarking. The transcription and replication proteins of many viruses interact with Brd4. We describe the multiple roles of Brd4 in the papillomavirus lifecycle.

Targeting human papillomavirus genome replication for antiviral drug discovery

Human papillomavirus (HPV) infections are a major human health problem; they are the cause of recurrent benign warts and of several cancers of the anogenital tract and head and neck region. Although there are two prophylactic HPV vaccines that could, if used universally, prevent as many as two-thirds of HPV-induced cancers, as well as several cytotoxic and immunomodulatory agents for localized treatment of infections, there are currently no HPV antiviral drugs in our arsenal of therapeutic agents. This review examines the status of past and ongoing research into the development of HPV antivirals, focused primarily upon approaches targeting the replication of the viral genome. The only HPV enzyme, E1, is a DNA helicase that interfaces with the cellular DNA replication machinery to replicate the HPV genome. To date, searches for small molecule inhibitors of E1 for use as antivirals have met with limited success. The lack of other viral enzymes has meant that the search for antivirals has shifted to a large degree to the modulation of protein-protein interactions. There has been some success in identifying small molecule inhibitors targeting interactions between HPV proteins but with activity against a small subset of viral types only. As noted in this review, it is thought that targeting E1 interactions with cellular replication proteins may provide inhibitors with broader activity against multiple HPV types. Herein, we outline the steps in HPV DNA replication and discuss those that appear to provide the most advantageous targets for the development of anti-HPV therapeutics.

Genetic analysis of the E2 transactivation domain dimerization interface from bovine papillomavirus type 1

The bovine papillomavirus type 1 (BPV1) E2 protein binds as a dimer to the viral genome to promote its transcription, replication and maintenance in keratinocytes. Although BPV1 E2 dimerizes primarily through its DNA-binding domain, it was shown previously that its transactivation domain (TAD) can also dimerize in vitro through formation of a disulfide bond between cysteine 57 (C57) of adjacent monomers and of an ion pair between arginine 172 (R172) and aspartic acid 175 (D175). The function of this TAD dimerization interface in vivo remains unknown. Here, we report the effects of substituting C57, R172 and D175 by alanine on the transactivation activity of BPV E2 as well as on its ability to support viral DNA replication using a novel luciferase-based assay. Results for this mutational analysis suggest that the TAD dimerization interface is not essential for either process but may contribute to the DNA replication activity of BPV1 E2.

Recruitment of Brd4 to the human papillomavirus type 16 DNA replication complex is essential for replication of viral DNA

Replication of the human papillomavirus (HPV) DNA genome relies on viral factors E1 and E2 and the cellular replication machinery. Bromodomain-containing protein 4 (Brd4) interacts with viral E2 protein to mediate papillomavirus (PV) genome maintenance and viral transcription. However, the functional role of Brd4 in the HPV life cycle remains to be clearly defined. In this study, we provide the first look into the E2-Brd4 interaction in the presence of other important viral factors, such as the HPV16 E1 protein and the viral genome. We show that Brd4 is recruited to actively replicating HPV16 origin foci together with HPV16 E1, E2, and a number of the cellular replication factors: replication protein A70 (RPA70), replication factor C1 (RFC1), and DNA polymerase δ. Mutagenesis disrupting the E2-Brd4 interaction abolishes the formation of the HPV16 replication complex and impairs HPV16 DNA replication in cells. Brd4 was further demonstrated to be necessary for HPV16 viral DNA replication using a cell-free replication system in which depletion of Brd4 by small interfering RNA (siRNA) silencing leads to impaired HPV16 viral DNA replication and recombinant Brd4 protein is able to rescue viral DNA replication. In addition, releasing endogenous Brd4 from cellular chromatin by using the bromodomain inhibitor JQ1(+) enhances HPV16 DNA replication, demonstrating that the role of Brd4 in HPV DNA replication could be uncoupled from its function in chromatin-associated transcriptional regulation and cell cycle control. Our study reveals a new role for Brd4 in HPV genome replication, providing novel insights into understanding the life cycle of this oncogenic DNA virus.

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.

An interaction between human papillomavirus 16 E2 and TopBP1 is required for optimum viral DNA replication and episomal genome establishment

In human papillomavirus DNA replication, the viral protein E2 forms homodimers and binds to 12-bp palindromic DNA sequences surrounding the origin of DNA replication. Via a protein-protein interaction, it then recruits the viral helicase E1 to an A/T-rich origin of replication, whereupon a dihexamer forms, resulting in DNA replication initiation. In order to carry out DNA replication, the viral proteins must interact with host factors that are currently not all known. An attractive cellular candidate for regulating viral replication is TopBP1, a known interactor of the E2 protein. In mammalian DNA replication, TopBP1 loads DNA polymerases onto the replicative helicase after the G(1)-to-S transition, and this process is tightly cell cycle controlled. The direct interaction between E2 and TopBP1 would allow E2 to bypass this cell cycle control, resulting in DNA replication more than once per cell cycle, which is a requirement for the viral life cycle. We report here the generation of an HPV16 E2 mutant compromised in TopBP1 interaction in vivo and demonstrate that this mutant retains transcriptional activation and repression functions but has suboptimal DNA replication potential. Introduction of this mutant into a viral life cycle model results in the failure to establish viral episomes. The results present a potential new antiviral target, the E2-TopBP1 interaction, and increase our understanding of the viral life cycle, suggesting that the E2-TopBP1 interaction is essential.

A conserved N-terminal domain mediates required DNA replication activities and phosphorylation of the transcriptional activator IE1 of Autographa californica multicapsid nucleopolyhedrovirus

IE1 is the principal transcriptional regulator of the baculoviruses. Like multifunctional transcription factors of other large DNA viruses, IE1 is an essential, site-specific DNA-binding phosphoprotein that activates virus gene expression and promotes genome replication. To define the poorly understood mechanisms by which IE1 achieves its diverse functions, we identified IE1 domains that contribute to productive infection of Autographa californica multicapsid nucleopolyhedrovirus (AcMNPV), the baculovirus prototype. Site-directed mutagenesis revealed that the N-terminal 23 residues of IE1 are required for origin-specific DNA replication and AcMNPV propagation, but not for DNA-binding-dependent transcriptional activation. Within this defined replication domain, we identified an invariant TPXR/H motif that resembles a consensus cyclin-dependent kinase phosphorylation site. Amino acid substitutions of potential phosphorylation sites within or near this motif caused loss of IE1-mediated DNA replication activity. Remarkably, substitution of the single threonine (residue 15) within the TPXR/H motif caused complete loss of AcMNPV multiplication. The replication domain was required for IE1 phosphorylation. It was also sufficient for conferring phosphorylation of a heterologous protein. Importantly, IE1 hyperphosphorylation coincided exclusively with AcMNPV DNA replication. The temporal regulation of IE1 phosphorylation and the essential nature of the TPXR/H motif suggest that phosphorylation critically alters and possibly activates DNA replication activity of IE1 during infection. The striking conservation of the TPXR/H motif among IE1 proteins further suggests that this molecular switch may be a common mechanism by which the alphabaculoviruses coordinate DNA replication and gene expression by using a single regulator.

Evolutionary variation of papillomavirus E2 protein and E2 binding sites

Background

In an effort to identify the evolutionary changes relevant to E2 function, within and between papillomavirus genera, we evaluated the E2 binding sites (E2BS)s inside the long-control-region (LCR), and throughout the genomes. We identified E2BSs in the six largest genera of papillomaviruses: Alpha, Beta, Gamma, Delta, Lambda, and Xi-papillomaviruses (128 genomes), by comparing the sequences with a model consensus we created from known functional E2BSs (HPV16, HPV18, BPV1). We analyzed the sequence conservation and nucleotide content of the 4-nucleotide spacer within E2BSs. We determined that there is a statistically significant difference in GC content of the four-nucleotide E2BS spacer, between Alpha and Delta-papillomaviruses, as compared to each of the other groups. Additionally, we performed multiple alignments of E2 protein sequences using members of each genus in order to identify evolutionary changes within the E2 protein.

Results

When a phylogenetic tree was generated from E2 amino acid sequences, it was discovered that the alpha-papillomavirus genera segregates into two distinct subgroups (α1 and α2). When these subgroups were individually analyzed, it was determined that the subgroup α1 consensus E2BS favored a spacer of AAAA, whereas subgroup α2 favored the opposite orientation of the same spacer; TTTT. This observation suggests that these conserved inverted linkers could have functional importance.

Small molecule inhibitors of the human papillomavirus E1-E2 interaction

Human papillomaviruses are responsible for multiple human diseases, including cervical cancer caused by multiple high-risk types and genital warts caused by the low-risk types 6 and 11. Based on the research indicating that low-risk HPV could be successfully targeted by inhibitors of viral DNA replication, we carried out several high-throughput screens for inhibitors of DNA replication activities. Two series were identified in screens for inhibitors of the interaction between the viral proteins E1 and E2. The two series were demonstrated to bind to overlapping sites on the transactivation domain of E2, at the E1-binding interface, by a series of biochemical and biophysical experiments. A member of the first series was also cocrystallized with the E2 transactivation domain. For both series, structure-activity investigations are described, which resulted in several hundred fold improvements in activity. The best compounds in each series had low nanomolar activity against the HPV11 E1-E2 interaction, and EC(50) values in cellular DNA replication assays of approximately 1 μM. Binding modes for the two series are compared, and some general conclusions about the discovery of protein-protein interaction inhibitors are drawn from the work described.

Human papillomavirus 16 E2 stability and transcriptional activation is enhanced by E1 via a direct protein-protein interaction

Human papillomavirus 16 E1 and E2 interact with cellular factors to replicate the viral genome. E2 forms homodimers and binds to 12 bp palindromic sequences adjacent to the viral origin and recruits E1 to the origin. E1 forms a di-hexameric helicase complex that replicates the viral genome. This manuscript demonstrates that E1 stabilises the E2 protein, increasing the half life in both C33a and 293 T cells respectively. This stabilisation requires a direct protein--protein interaction. In addition, the E1 protein enhances E2 transcription function in a manner that suggests the E1 protein itself can contribute to transcriptional regulation not simply by E2 stabilisation but by direct stimulation of transcription. This activation of E2 transcription is again dependent upon an interaction with E1. Overall the results suggest that in the viral life cycle, co-expression of E1 with E2 can increase E2 stability and enhance E2 function.

Regulation of human papillomavirus type 16 early gene expression in trophoblastic and cervical cells

We compared the outcome of different cellular and viral factors on the regulation of the HPV-16 early viral gene expression in trophoblastic and cervical cancer cells. A high variability of the long control (LCR) activity was observed, prompting us to evaluate the role of secreted factors in the control of the early gene expression in trophoblastic cell lines. Endogenous progesterone and exogenous dexamethasone were found to activate LCR driven transcriptional activity. Since host cells express HPV early proteins to regulate LCR activity, we investigated the effect of the combined HPV-16 early proteins on the LCR driven transcription and the possible involvement of E2. A physiological level of HPV-16 early proteins expression strongly induced the LCR driven reporter activity. According to mutational analysis, E1 and E2 proteins, indispensable for viral replication, were not involved in LCR extrachromosomal transcriptional regulation. This suggests that E5 and/or E6 and/or E7, consequently, activated viral transcription.

Nuclear export of human papillomavirus type 31 E1 is regulated by Cdk2 phosphorylation and required for viral genome maintenance

The initiator protein E1 from human papillomavirus (HPV) is a helicase essential for replication of the viral genome. E1 contains three functional domains: a C-terminal enzymatic domain that has ATPase/helicase activity, a central DNA-binding domain that recognizes specific sequences in the origin of replication, and a N-terminal region necessary for viral DNA replication in vivo but dispensable in vitro. This N-terminal portion of E1 contains a conserved nuclear export signal (NES) whose function in the viral life cycle remains unclear. In this study, we provide evidence that nuclear export of HPV31 E1 is inhibited by cyclin E/A-Cdk2 phosphorylation of two serines residues, S92 and S106, located near and within the E1 NES, respectively. Using E1 mutant proteins that are confined to the nucleus, we determined that nuclear export of E1 is not essential for transient viral DNA replication but is important for the long-term maintenance of the HPV episome in undifferentiated keratinocytes. The findings that E1 nuclear export is not required for viral DNA replication but needed for genome maintenance over multiple cell divisions raised the possibility that continuous nuclear accumulation of E1 is detrimental to cellular growth. In support of this possibility, we observed that nuclear accumulation of E1 dramatically reduces cellular proliferation by delaying cell cycle progression in S phase. On the basis of these results, we propose that nuclear export of E1 is required, at least in part, to limit accumulation of this viral helicase in the nucleus in order to prevent its detrimental effect on cellular proliferation.

NCoR1 mediates papillomavirus E8;E2C transcriptional repression

The papillomavirus E2 open reading frame encodes the full-length E2 protein as well as an alternatively spliced product called E8;E2C. E8;E2C has been best studied for the high-risk human papillomaviruses, where it has been shown to regulate viral genome levels and, like the full-length E2 protein, to repress transcription from the viral promoter that directs the expression of the viral E6 and E7 oncogenes. The repression function of E8;E2C is dependent on the 12-amino-acid N-terminal sequence from the E8 open reading frame (ORF). In order to understand the mechanism by which E8;E2C mediates transcriptional repression, we performed an unbiased proteomic analysis from which we identified six high-confidence candidate interacting proteins (HCIPs) for E8;E2C; the top two are NCoR1 and TBLR1. We established an interaction of E8;E2C with an NCoR1/HDAC3 complex and demonstrated that this interaction requires the wild-type E8 open reading frame. Small interfering RNA (siRNA) knockdown studies demonstrated the involvement of NCoR1/HDAC3 in the E8;E2C-dependent repression of the viral long control region (LCR) promoter. Additional genetic work confirmed that the papillomavirus E2 and E8;E2C proteins repress transcription through distinct mechanisms.

Development of quantitative and high-throughput assays of polyomavirus and papillomavirus DNA replication

Polyoma- and papillomaviruses genome replication is initiated by the binding of large T antigen (LT) and of E1 and E2, respectively, at the viral origin (ori). Replication of an ori-containing plasmid occurs in cells transiently expressing these viral proteins and is typically quantified by Southern blotting or PCR. To facilitate the study of SV40 and HPV31 DNA replication, we developed cellular assays in which transient replication of the ori-plasmid is quantified using a firefly luciferase gene located in cis to the ori. Under optimized conditions, replication of the SV40 and HPV31 ori-plasmids resulted in a 50- and 150-fold increase in firefly luciferase levels, respectively. These results were validated using replication-defective mutants of LT, E1 and E2 and with inhibitors of DNA replication and cell-cycle progression. These quantitative and high-throughput assays should greatly facilitate the study of SV40 and HPV31 DNA replication and the identification of small-molecule inhibitors of this process.

Proteasomal degradation of the papillomavirus E2 protein is inhibited by overexpression of bromodomain-containing protein 4

The E2 protein of human papillomavirus (HPV) binds to specific sites in the viral genome to regulate its transcription, replication, and maintenance in infected cells. Like most regulatory proteins, E2 is rapidly turned over. A high-throughput assay was developed to quantify the expression and stability of E2 in vivo, based on its fusion to Renilla luciferase (RLuc). The steady-state levels of Rluc-E2 were quantified by measuring the amounts of associated luciferase activity, and its degradation was measured by monitoring the decrease in enzymatic activity occurring after a block of translation with cycloheximide. Using this assay, the E2 proteins from a low-risk (HPV11) and a high-risk (HPV31) human papillomavirus (HPV) type were found to have short half-lives of 60 min in C33A cervical carcinoma cells and to be ubiquitinated and degraded by the proteasome. Analysis of mutant proteins showed that the instability of E2 is independent of its DNA-binding and transcriptional activities but is encoded within its transactivation domain, the region that binds to the cellular chromatin factor bromodomain-containing protein 4 (Brd4) to regulate viral gene transcription. Overexpression of Brd4, or of its C-terminal E2-interaction domain, was found to increase the steady-state levels and stability of wild-type E2 but not of E2 mutants defective for binding Brd4. These results indicate that the stability of E2 is increased upon complex formation with Brd4 and highlight the value of the luciferase assay for the study of E2 degradation.

Human papillomavirus type 16 E2 and E6 are RNA-binding proteins and inhibit in vitro splicing of pre-mRNAs with suboptimal splice sites

Human papillomavirus type 16 (HPV16) genome expresses six regulatory proteins (E1, E2, E4, E5, E6, and E7) which regulate viral DNA replication, gene expression, and cell function. We expressed HPV16 E2, E4, E6, and E7 from bacteria as GST fusion proteins and examined their possible functions in RNA splicing. Both HPV16 E2, a viral transactivator protein, and E6, a viral oncoprotein, inhibited splicing of pre-mRNAs containing an intron with suboptimal splice sites, whereas HPV5 E2 did not. The N-terminal half and the hinge region of HPV16 E2 as well as the N-terminal and central portions of HPV16 E6 are responsible for the suppression. HPV16 E2 interacts with pre-mRNAs through its C-terminal DNA-binding domain. HPV16 E6 binds pre-mRNAs via nuclear localization signal (NLS3) in its C-terminal half. Low-risk HPV6 E6, a cytoplasmic protein, does not bind RNA. Notably, both HPV16 E2 and E6 selectively bind to the intron region of pre-mRNAs and interact with a subset of cellular SR proteins. Together, these findings suggest that HPV16 E2 and E6 are RNA binding proteins and might play roles in posttranscriptional regulation during virus infection.

Replication and partitioning of papillomavirus genomes

Papillomaviruses establish persistent infection in the dividing, basal epithelial cells of the host. The viral genome is maintained as a circular, double-stranded DNA, extrachromosomal element within these cells. Viral genome amplification occurs only when the epithelial cells differentiate and viral particles are shed in squames that are sloughed from the surface of the epithelium. There are three modes of replication in the papillomavirus life cycle. Upon entry, in the establishment phase, the viral genome is amplified to a low copy number. In the second maintenance phase, the genome replicates in dividing cells at a constant copy number, in synchrony with the cellular DNA. And finally, in the vegetative or productive phase, the viral DNA is amplified to a high copy number in differentiated cells and is destined to be packaged in viral capsids. This review discusses the cis elements and protein factors required for each stage of papillomavirus replication.

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.

The E8--E2 gene product of human papillomavirus type 16 represses early transcription and replication but is dispensable for viral plasmid persistence in keratinocytes

A conserved E8(wedge)E2 spliced mRNA is detected in keratinocytes transfected with human papillomavirus type 16 (HPV-16) plasmid DNA. Expression of HPV-16 E8--E2 (16-E8--E2) is independent of the major early promoter, P97, and is modulated by both specific splicing events and conserved cis elements in the upstream regulatory region in a manner that differs from transcriptional regulation of other early viral genes. Mutations that disrupt the predicted 16-E8--E2 message also increase initial HPV-16 plasmid amplification 8- to 15-fold and major early gene (P97) transcription 4- to 5-fold over those of the wild type (wt). Expressing the 16-E8--E2 gene product from the cytomegalovirus (CMV) promoter represses HPV-16 early gene transcription from P97 in a dose-dependent manner, as detected by RNase protection assays. When expressed from the CMV promoter, 16-E8--E2 also inhibits the amplification of an HPV-16 plasmid and a heterologous simian virus 40 (SV40) ori plasmid that contains E2 binding sites in cis. In contrast, cotransfections with HPV-16 wt genomes that express physiologic levels of 16-E8--E2 are sufficient to repress HPV-16 plasmid amplification but are limiting and insufficient for the repression of SV40 amplification. 16-E8--E2-dependent repression of HPV-16 E1 expression is sufficient to account for this observed inhibition of initial HPV-16 plasmid amplification. Unlike with other papillomaviruses, primary human keratinocytes immortalized by the HPV-16 E8 mutant genome contain more than eightfold-higher levels of unintegrated plasmid than the wt, demonstrating that 16-E8(wedge)E2 limits the viral copy number but is not required for plasmid persistence and maintenance.

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.

Dimerization of the human papillomavirus type 16 E2 N terminus results in DNA looping within the upstream regulatory region

Papillomavirus E2 proteins play a central role in regulating viral gene expression and replication. DNA-binding activity is associated with the C-terminal domain of E2, which forms a stable dimer, while the N-terminal domain is responsible for E2's replication and transactivation functions. The crystal structure of the latter domain revealed a second dimerization interface on E2 which may be responsible for DNA loop formation in the regulatory region of the human papillomavirus (HPV) genome. We investigated the biological significance of the N-terminal dimerization by introducing single amino acid substitutions into the dimerization interface. As expected, these substitutions did not influence the C-terminal dimerization and DNA-binding functions of E2. However, the mutations led to reduced transactivation of a synthetic E2-responsive reporter gene, while HPV DNA replication was unaffected. The effect of the mutations on DNA looping was visualized by atomic force microscopy. While wild-type E2 was able to generate DNA loops, all three mutant E2 proteins were defective in this ability. Our results suggest that N-terminal dimerization plays a role in E2-mediated transactivation, probably via DNA looping, a common mechanism for remote regulation of gene transcription.

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.

P53 represses human papillomavirus type 16 DNA replication via the viral E2 protein

BACKGROUND

Human papillomavirus (HPV) DNA replication can be inhibited by the cellular tumour suppressor protein p53. However, the mechanism through which p53 inhibits viral replication and the role that this might play in the HPV life cycle are not known. The papillomavirus E2 protein is required for efficient HPV DNA replication and also regulates viral gene expression. E2 represses transcription of the HPV E6 and E7 oncogenes and can thereby modulate indirectly host cell proliferation and survival. In addition, the E2 protein from HPV 16 has been shown to bind p53 and to be capable of inducing apoptosis independently of E6 and E7.

RESULTS

Here we use a panel of E2 mutants to confirm that mutations which block the induction of apoptosis via this E6/E7-independent pathway, have little or no effect on the induction of apoptosis by the E6/E7-dependent pathway. Although these mutations in E2 do not affect the ability of the protein to mediate HPV DNA replication, they do abrogate the repressive effects of p53 on the transcriptional activity of E2 and prevent the inhibition of E2-dependent HPV DNA replication by p53.

CONCLUSION

These data suggest that p53 down-regulates HPV 16 DNA replication via the E2 protein.

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.

Structure of the papillomavirus DNA-tethering complex E2:Brd4 and a peptide that ablates HPV chromosomal association

Many DNA viruses that are latent in dividing cells are noncovalent passengers on mitotic chromosomes and require specific viral-encoded and cellular factors for this activity. The chromosomal protein Brd4 is implicated in the hitchhiking of bovine papillomavirus-1 (BPV-1), and the viral protein E2 binds to both plasmids and Brd4. Here, we present the X-ray crystal structure of the carboxy-terminal domain of Brd4 in complex with HPV-16 E2, and with this information have developed a Brd4-Tat fusion protein that is efficiently taken up by different transformed cells harboring HPV plasmids. In cells treated with these fusion proteins for only 2 hr and arrested in metaphase, the HPV DNA, either HPV-16 or -31, is displaced from mitotic chromosomes. Mutant Brd4 peptides are deficient in ablating this association. We suggest that such peptides may lead to the development of inhibitors of latency for many, if not all, papillomaviruses.

Brd4 is required for e2-mediated transcriptional activation but not genome partitioning of all papillomaviruses

Bromodomain protein 4 (Brd4) has been identified as the cellular binding target through which the E2 protein of bovine papillomavirus type 1 links the viral genome to mitotic chromosomes. This tethering ensures retention and efficient partitioning of genomes to daughter cells following cell division. E2 is also a regulator of viral gene expression and a replication factor, in association with the viral E1 protein. In this study, we show that E2 proteins from a wide range of papillomaviruses interact with Brd4, albeit with variations in efficiency. Moreover, disruption of the E2-Brd4 interaction abrogates the transactivation function of E2, indicating that Brd4 is required for E2-mediated transactivation of all papillomaviruses. However, the interaction of E2 and Brd4 is not required for genome partitioning of all papillomaviruses since a number of papillomavirus E2 proteins associate with mitotic chromosomes independently of Brd4 binding. Furthermore, mutations in E2 that disrupt the interaction with Brd4 do not affect the ability of these E2s to associate with chromosomes. Thus, while all papillomaviruses attach their genomes to cellular chromosomes to facilitate genome segregation, they target different cellular binding partners. In summary, the E2 proteins from many papillomaviruses, including the clinically important alpha genus human papillomaviruses, interact with Brd4 to mediate transcriptional activation function but not all depend on this interaction to efficiently associate with mitotic chromosomes.

Amino acid substitutions that specifically impair the transcriptional activity of papillomavirus E2 affect binding to the long isoform of Brd4

The E2 protein of papillomaviruses binds to specific sites in the viral genome to regulate its transcription, replication and segregation in mitosis. Amino acid substitutions in the transactivation domain (TAD) of E2, of Arg37 and Ile73, have been shown previously to impair the transcriptional activity of the protein but not its ability to support viral DNA replication. To understand the biochemical basis of this defect, we have used the TADs of a low-risk (HPV11) and a high-risk (HPV31) human papillomavirus (HPV) as affinity ligands to capture proteins from whole cell extracts that can associate with these domains. The major TAD-binding protein was identified by mass spectrometry and western blotting as the long isoform of Brd4. Binding to Brd4 was also demonstrated for the E2 TADs of other papillomaviruses including cutaneous and animal types. For HPV11, HPV31 and CRPV E2, we found that binding to Brd4 is significantly reduced by substitutions of Arg37 and Ile73. Since these amino acids are located near each other in the 3-dimensional structure of the TAD, we suggest that they define a conserved surface involved in binding Brd4 to regulate viral gene transcription.

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.

Repression of HPV16 early region transcription by the E2 protein

HPV16 DNA is often integrated in cancers, disrupting the E1 or E2 genes. E2 can repress the E6/E7 promoter, but other models have been proposed to explain why integration promotes malignant progression. E1 and E2 are required for viral replication, and so genetic analysis of their role in transcriptional regulation is complex. Therefore, we developed an extrachromosomal vector containing HPV16 to undertake a genetic analysis of the E1 and E2 genes. We demonstrate that the E2 protein is primarily a transcriptional repressor when expressed from the virus. Furthermore, repression requires both the transactivation function of E2 and specific binding of E2 to the LCR. We find no evidence that the E1 protein directly modulates HPV16 gene expression. However, certain E1 mutations modulated transcription indirectly by altering splicing of E2 mRNA species. These data provide important insight into which E1 and E2 functions are optimal targets for anti-viral therapies.

Human papillomavirus type 16 E1circumflexE4 contributes to multiple facets of the papillomavirus life cycle

The life cycle of human papillomaviruses (HPVs) is tightly linked to the differentiation program of the host's stratified epithelia that it infects. E1(circumflex)E4 is a viral protein that has been ascribed multiple biochemical properties of potential biological relevance to the viral life cycle. To identify the role(s) of the viral E1(circumflex)E4 protein in the HPV life cycle, we characterized the properties of HPV type 16 (HPV16) genomes harboring mutations in the E4 gene in NIKS cells, a spontaneously immortalized keratinocyte cell line that when grown in organotypic raft cultures supports the HPV life cycle. We learned that E1(circumflex)E4 contributes to the replication of the viral plasmid genome as a nuclear plasmid in basal cells, in which we also found E1(circumflex)E4 protein to be expressed at low levels. In the suprabasal compartment of organotypic raft cultures harboring E1(circumflex)E4 mutant HPV16 genomes there were alterations in the frequency of suprabasal cells supporting DNA synthesis, the levels of viral DNA amplification, and the degree to which the virus perturbs differentiation. Interestingly, the comparison of the phenotypes of various mutations in E4 indicated that the E1(circumflex)E4 protein-encoding requirements for these various processes differed. These data support the hypothesis that E1(circumflex)E4 is a multifunctional protein and that the different properties of E1(circumflex)E4 contribute to different processes in both the early and late stages of the virus life cycle.