Identification of a second transforming function in bovine papillomavirus type 1 E6 and the role of E6 interactions with paxillin, E6BP, and E6AP

Comparative structural studies on Bovine papillomavirus E6 oncoproteins: Novel insights into viral infection and cell transformation from homology modeling and molecular dynamics simulations

Bovine papillomavirus (BPV) infects cattle cells worldwide, leading to hyperproliferative lesions and the potential development of cancer, driven by E5, E6, and E7 oncoproteins along with other cofactors. E6 oncoprotein binds experimentally to various proteins, primarily paxillin and MAML1, as well as hMCM7 and CBP/p300. However, the molecular and structural mechanisms underlying BPV-induced malignant transformation remain unclear. Therefore, we have modeled the E6 oncoprotein structure from non-oncogenic BPV-5 and compared them with oncogenic BPV-1 to assess the relationship between structural features and oncogenic potential. Our analysis elucidated crucial structural aspects of E6, highlighting both conserved elements across genotypes and genotype-specific variations potentially implicated in the oncogenic process, particularly concerning primary target interactions. Additionally, we predicted the location of the hMCM7 binding site on the N-terminal of BPV-5 E6. This study enhances our understanding of the structural characteristics of BPV E6 oncoproteins and their interactions with host proteins, clarifying structural differences and similarities between high and low-risk BPVs. This is important to understand better the mechanisms involved in cell transformation in BPV infection, which could be used as a possible target for therapy.

Overexpression of Paxillin Correlates with Tumor Progression and Predicts Poor Survival in Glioblastoma

AIMS

To explore the prognostic and clinicopathological features of glioma with Paxillin (PXN) expression based on a large number of samples.

METHODS

RNA sequencing data of 325 glioma samples from Chinese Glioma Genome Atlas (CGGA) database were obtained as discovery set. Three additional datasets were further obtained as validation sets. The protein expression pattern of PXN in glioma was measured by IHC. Kaplan-Meier survival and multivariate Cox analysis were used to estimate the survival distributions. Moreover, the functional annotation of PXN was also analyzed.

RESULTS

In the discovery set, PXN overexpression was significantly associated with high-grade glioma as well as the higher mortality in survival analysis (log-rank test, P < 0.01). The results of the other validation datasets showed similar findings. PXN also served as an independent prognostic biomarker in glioblastoma patients. Functional assays showed that PXN contributed to glioma cell proliferation and invasion.

CONCLUSION

PXN plays as an oncogene in glioma progression and suggests a new potential biotarget for therapy.

Mutagenic Potential ofBos taurus Papillomavirus Type 1 E6 Recombinant Protein: First Description

Bovine papillomavirus (BPV) is considered a useful model to study HPV oncogenic process. BPV interacts with the host chromatin, resulting in DNA damage, which is attributed to E5, E6, and E7 viral oncoproteins activity. However, the oncogenic mechanisms of BPV E6 oncoprotein per se remain unknown. This study aimed to evaluate the mutagenic potential of Bos taurus papillomavirus type 1 (BPV-1) E6 recombinant oncoprotein by the cytokinesis-block micronucleus assay (CBMNA) and comet assay (CA). Peripheral blood samples of five calves were collected. Samples were subjected to molecular diagnosis, which did not reveal presence of BPV sequences. Samples were treated with 1 μg/mL of BPV-1 E6 oncoprotein and 50 μg/mL of cyclophosphamide (positive control). Negative controls were not submitted to any treatment. The samples were submitted to the CBMNA and CA. The results showed that BPV E6 oncoprotein induces clastogenesis per se, which is indicative of genomic instability. These results allowed better understanding the mechanism of cancer promotion associated with the BPV E6 oncoprotein and revealed that this oncoprotein can induce carcinogenesis per se. E6 recombinant oncoprotein has been suggested as a possible vaccine candidate. Results pointed out that BPV E6 recombinant oncoprotein modifications are required to use it as vaccine.

Structure based identification and characterization of flavonoids that disrupt human papillomavirus-16 E6 function

Expression and function of the human papillomavirus (HPV) early protein 6 (E6) is necessary for viral replication and oncogenesis in cervical cancers. HPV E6 targets the tumor suppressor protein p53 for degradation. To achieve this, "high-risk" HPV E6 proteins bind to and modify the target specificity of the ubiquitin ligase E6AP (E6 associated protein). This E6-dependent loss of p53 enables the virus to bypass host cell defenses and facilitates virally induced activation of the cell cycle progression during viral replication. Disruption of the interaction between E6 and E6AP and stabilization of p53 should decrease viability and proliferation of HPV positive cells. A new in vitro high-throughput binding assay was developed to assay binding between HPV-16 E6 and E6AP and to identify compounds that inhibit this interaction. The compound luteolin emerged from the screen and a library of novel flavones based on its structure was synthesized and characterized using this in vitro binding assay. The compounds identified in this study disrupt the E6/E6AP interaction, increase the levels of p53 and p21(Cip1/Waf1), and decrease proliferation of HPV positive cell lines. The new class of flavonoid E6 inhibitors displays a high degree of specificity for HPV positive cells. Docking analyses suggest that these compounds bind in a hydrophobic pocket at the interface between E6 and E6AP and mimic the leucines in the conserved α-helical motif of E6AP. The activity and specificity of these compounds represent a promising new lead for development as an antiviral therapy in the treatment of HPV infection and cervical cancer.

Papillomavirus E6 oncoproteins

Papillomaviruses induce benign and malignant epithelial tumors, and the viral E6 oncoprotein is essential for full transformation. E6 contributes to transformation by associating with cellular proteins, docking on specific acidic LXXLL peptide motifs found on these proteins. This review examines insights from recent studies of human and animal E6 proteins that determine the three-dimensional structure of E6 when bound to acidic LXXLL peptides. The structure of E6 is related to recent advances in the purification and identification of E6 associated protein complexes. These E6 protein-complexes, together with other proteins that bind to E6, alter a broad array of biological outcomes including modulation of cell survival, cellular transcription, host cell differentiation, growth factor dependence, DNA damage responses, and cell cycle progression.

Structural basis for hijacking of cellular LxxLL motifs by papillomavirus E6 oncoproteins

E6 viral oncoproteins are key players in epithelial tumors induced by papillomaviruses in vertebrates, including cervical cancer in humans. E6 proteins target many host proteins by specifically interacting with acidic LxxLL motifs. We solved the crystal structures of bovine (BPV1) and human (HPV16) papillomavirus E6 proteins bound to LxxLL peptides from the focal adhesion protein paxillin and the ubiquitin ligase E6AP, respectively. In both E6 proteins, two zinc domains and a linker helix form a basic-hydrophobic pocket, which captures helical LxxLL motifs in a way compatible with other interaction modes. Mutational inactivation of the LxxLL binding pocket disrupts the oncogenic activities of both E6 proteins. This work reveals the structural basis of both the multifunctionality and the oncogenicity of E6 proteins.

Bovine papillomavirus E5 and E7 oncoproteins in naturally occurring tumors: are two better than one?

Bovine papillomaviruses (BPVs) are oncogenic DNA viruses, which mainly induce benign lesions of cutaneous and/or mucosal epithelia in cattle. Thirteen (BPV 1-13) different viral genotypes have been characterized so far. BPVs are usually species-specific but BPV 1/2 may also infect equids as well as buffaloes and bison and cause tumors in these species. BPV-induced benign lesions usually regress, however occasionally they develop into cancer particularly in the presence of environmental carcinogenic co-factors. The major transforming protein of BPV is E5, a very short hydrophobic, transmembrane protein with many oncogenic activities. E5 contributes to cell transformation through the activation of the cellular β receptor for the platelet-derived growth factor (PDGFβ-r), it also decreases cell surface expression of major histocompatibility complex class I (MHCI) causing viral escape from immunosurveillance, and plays a role in the inhibition of the intracellular communication by means of aberrant connexin expression. E7 is considered as a weak transforming gene, it synergies with E5 in cell transformation during cancer development. E7 expression correlates in vivo with the over-expression of β1-integrin, which plays a role in the regulation of keratinocytes proliferation and differentiation. Additionally, E7 is involved in cell-mediated immune responses leading to tumour rejection, in anoikis process by direct binding to p600, and in invasion process by upregulation of Matrix metalloproteinase1 (MMP-1) expression. Studies on the role of BPV E5 and E7 oncoproteins in naturally occurring tumours are of scientific value, as they may shed new light on the biological role of these two oncogenes in cell transformation.

Cutaneous papillomavirus E6 oncoproteins associate with MAML1 to repress transactivation and NOTCH signaling

Papillomavirus E6 oncoproteins associate with LXXLL motifs on target cellular proteins to alter their function. Using a proteomic approach, we found the E6 oncoproteins of cutaneous papillomaviruses Bovine Papillomavirus Type 1 (BE6) and HPV types 1 and 8 (1E6 and 8E6) associated with the MAML1 transcriptional co-activator. All three E6 proteins bind to an acidic LXXLL motif at the carboxy-terminus of MAML1 and repress transactivation by MAML1. MAML1 is best known as the co-activator and effector of NOTCH induced transcription, and BPV-1 E6 represses synthetic NOTCH responsive promoters, endogenous NOTCH responsive promoters, and is found in a complex with MAML1 in stably transformed cells. BPV-1 induced papillomas show characteristics of repressed NOTCH signal transduction, including suprabasal expression of integrins, talin, and basal type keratins, and delayed expression of the NOTCH dependent HES1 transcription factor. These observations give rise to a model whereby papillomavirus oncoproteins including BPV-1 E6 and the cancer associated HPV-8 E6 repress Notch induced transcription, thereby delaying keratinocyte differentiation.

Strategies for bacterial expression of protein-peptide complexes: application to solubilization of papillomavirus E6

E6 is a small oncoprotein involved in tumorigenesis induced by papillomaviruses (PVs). E6 often recognizes its cellular targets by binding to short motifs presenting the consensus LXXLL. E6 proteins have long resisted structural analysis. We found that bovine papillomavirus type 1 (BPV1) E6 binds the N-terminal LXXLL motif of the cellular protein paxillin with significantly higher affinity as compared to other E6/peptide interactions. Although recombinant BPV1 E6 was poorly soluble in the free state, provision of the paxillin LXXLL peptide during BPV1 E6 biosynthesis greatly enhanced the protein's solubility. Expression of BPV1 E6/LXXLL peptide complexes was carried out in bacteria in the form of triple fusion constructs comprising, from N- to C-terminus, the soluble carrier protein maltose binding protein (MBP), the LXXLL motif and the E6 protein. A TEV protease cleavage site was placed either between MBP and LXXLL motif or between LXXLL motif and E6. These constructs allowed us to produce highly concentrated samples of BPV1 E6, either covalently fused to the C-terminus of the LXXLL motif (intra-molecular complex) or non-covalently bound to it (inter-molecular complex). Heteronuclear NMR measurements were performed and showed that the E6 protein was folded with similar conformations in both covalent and non-covalent complexes. These data open the way to novel structural and functional studies of the BPV1 E6 in complex with its preferential target motif.

Transformation by bovine papillomavirus type 1 E6 requires paxillin

Papillomavirus E6 proteins are adapters that change the function of cellular regulatory proteins. The bovine papillomavirus type 1 E6 (BE6) binds to LXXLL peptide sequences termed LD motifs (consensus sequence LDXLLXXL) on the cellular protein paxillin that is a substrate of Src and focal adhesion kinases. Anchorage-independent transformation induced by BE6 required both paxillin and BE6-binding LD motifs on paxillin but was independent of the major tyrosine phosphorylation sites of paxillin. The essential role of paxillin in transformation by BE6 highlights the role of paxillin in the transduction of cellular signals that result in anchorage-independent cell proliferation.

Differentially expressed genes between high-risk human papillomavirus types in human cervical cancer cells

Cervical carcinoma (CC) is one of the most common cancers among women worldwide and the first cause of death among the Mexican female population. Human papillomavirus (HPV) infection is the most important etiologic factor for CC. Of the oncogenic types, HPV16 and HPV18 are found in 60-70% of invasive CCs worldwide. HPV18 appears to be associated with a more aggressive form of cervical neoplasia than HPV16 infection. At present, there are no studies on differentially expressed cellular genes between transformed cells harboring HPV16 and HPV18 sequences. Based on previous complementary DNA microarray data from our group, 13 genes were found to be differentially overexpressed between HPV16- and HPV18-transformed cells. These genes were as follows: E6BP, UBE4A, C20orf14, ATF7, ABCC8, SLC6A12, WASF3, SUV39H1, SPAG8, CCNC, E2FFE, BIRC5, and DEDD. Differential expression of six selected genes was confirmed by real-time reverse transcription-polymerase chain reaction (RT-PCR). All real-time RT-PCRs confirmed differential expression between HPV18 and HPV(-) samples. The present work identifies genes from signaling pathways triggered by HPV transformation that could be differentially deregulated between HPV16(+) and HPV18(+) samples.

Structures of a human papillomavirus (HPV) E6 polypeptide bound to MAGUK proteins: mechanisms of targeting tumor suppressors by a high-risk HPV oncoprotein

Human papillomavirus (HPV) E6 oncoprotein targets certain tumor suppressors such as MAGI-1 and SAP97/hDlg for degradation. A short peptide at the C terminus of E6 interacts specifically with the PDZ domains of these tumor suppressors, which is a property unique to high-risk HPVs that are associated with cervical cancer. The detailed recognition mechanisms between HPV E6 and PDZ proteins are unclear. To understand the specific binding of cellular PDZ substrates by HPV E6, we have solved the crystal structures of the complexes containing a peptide from HPV18 E6 bound to three PDZ domains from MAGI-1 and SAP97/Dlg. The complex crystal structures reveal novel features of PDZ peptide recognition that explain why high-risk HPV E6 can specifically target these cellular tumor suppressors for destruction. Moreover, a new peptide-binding loop on these PDZs is identified as interacting with the E6 peptide. Furthermore, we have identified an arginine residue, unique to high-risk HPV E6 but outside the canonical core PDZ recognition motif, that plays an important role in the binding of the PDZs of both MAGI-I and SAP97/Dlg, the mutation of which abolishes E6's ability to degrade the two proteins. Finally, we have identified a dimer form of MAGI-1 PDZ domain 1 in the cocrystal structure with E6 peptide, which may have functional relevance for MAGI-1 activity. In addition to its novel insights into the biochemistry of PDZ interactions, this study is important for understanding HPV-induced oncogenesis; this could provide a basis for developing antiviral and anticancer compounds.

Suprabasal overexpression of beta-1 integrin is induced by bovine papillomavirus type 1

In a normal stratified squamous epithelium, beta1-integrin is expressed in basal epithelial cells. In BPV-induced fibropapillomas beta1-integrin is overexpressed and aberrantly localized, with uniform expression in the lower spinous layer, and sporadic expression within the mid-spinous region that co-localizes with expression of the viral E5 and E7 oncoproteins. In situ hybridization of fibropapillomas for beta1-integrin RNA revealed sporadic hybridization in the spinous layer, indicating transcriptional induction. Beta1-integrin expression in cultured keratinocytes requires exogenous EGF in the media, but this requirement is lost if E7 is expressed, and E7 was able to abrogate the EGF-requirement of normal keratinocytes for the activation of ERK and DNA synthesis. Within fibropapillomas, suprabasal expression of E5 and E7 correlated with suprabasal expression of beta1-integrin and PCNA, indicating that vegetative viral replication in the spinous layer correlated with the expression of E7 and beta1 integrin. The ability of BPV-1 E7 to support beta1-integrin expression and EGF independent DNA synthesis and the activation of ERK are the first biochemical correlates of its expression in keratinocytes.

Opposing effects of bovine papillomavirus type 1 E6 and E7 genes on Fas-mediated apoptosis

Programmed cell death (PCD), best exemplified by apoptosis, is a genetically programmed process of cellular destruction that is indispensable for normal development and homeostasis of multicellular organisms. Tumor necrosis factor alpha (TNF) and related cytokines are employed by host defenses to eliminate virally infected cells through induction of apoptosis. Many viruses have evolved specific gene products to modulate this process. We have recently shown that the bovine papillomavirus type 1 (BPV-1) E6 and E7 genes independently sensitize mouse cells to TNF-induced apoptosis. In this report, we investigated the effect of E6 and E7 expression on Fas-mediated apoptosis. In contrast to TNF-mediated apoptosis, E6 and E7 demonstrated opposite effects: while E7 potentiated apoptosis triggered by an agonistic Fas antibody, E6 attenuated the effect. The mitochondrial pathway leading to the activation of caspases appears to be involved in Fas-mediated apoptosis in C127 cells. To further explore the mechanisms by which E6 and E7 modulate Fas-mediated apoptosis, we examined the surface expression of Fas in cells expressing E6 and E7. Significantly, levels of surface Fas expression correlated with the opposing effects of E6 and E7 on Fas-mediated apoptosis. Specifically, while E7 increased the surface expression of Fas, E6 reduced surface Fas expression. Mutational analysis demonstrated a correlation of E6's ability to downregulate surface Fas expression and apoptosis. Since the tumor suppressor p53 can be targeted for degradation by human papillomavirus and has been shown to induce apoptosis by upregulating surface Fas expression, we investigated the role of p53 in BPV-1 E6 and E7 modulation of Fas-mediated apoptosis. Our results demonstrated that the modulatory effects by E6 and E7 could occur in the absence of p53. Interestingly, the reduced Fas protein level on the cell surface is not accompanied by a decrease in total Fas levels in E6-expressing cells. Instead, considerably more Fas protein is found in the cytoplasm of cells expressing E6. These results highlight a novel activity of E6 and E7 that may be involved in viral pathogenesis.

Role of the PDZ domain-binding motif of the oncoprotein E6 in the pathogenesis of human papillomavirus type 31

A number of PDZ domain-containing proteins have been identified as binding partners for the oncoprotein E6 of the high-risk type human papillomaviruses (HPVs). These include hDlg, hScrib, MAGI-1, MAGI-2, MAGI-3, and MUPP1. The PDZ domain-binding motif (-X-T-X-V) at the carboxy terminus of E6 is essential for targeting PDZ proteins for proteasomal degradation. The presence of this motif only in the high-risk HPVs suggests its possible role in HPV-induced oncogenesis. To investigate the role of the PDZ domain-binding motif of E6 in the HPV life cycle, two mutant HPV31 genomes were constructed: E6ValDelta, with a deletion of the last amino acid residue of E6 (valine), and E6ETQVDelta, with a deletion of the entire PDZ domain-binding motif of E6 (ETQV). Three human foreskin keratinocyte (HFK) cell lines were established which maintained transfected wild-type HPV31 or either of two mutant genomes. Cells containing either of two mutant genomes were significantly retarded in their growth rates and reduced in their viral copy numbers compared to those transfected with wild-type genomes. Western analysis did not reveal any significant changes in the levels of PDZ proteins following stable transfection of any HPV31 genomes into HFKs. Although the E6ETQVDelta-transfected HFKs exhibited a pattern of morphological differentiation that appeared different from the HPV31 wild-type-transfected HFKs in organotypic raft cultures, immunohistochemical analysis failed to identify substantial changes in the differentiation-dependent membrane localization of hDlg proteins. These results suggest that binding of E6 to PDZ proteins modulates the early viral functions such as proliferation and maintenance of the viral copy number in undifferentiated cells.

Paxillin: adapting to change

Molecular scaffold or adaptor proteins facilitate precise spatiotemporal regulation and integration of multiple signaling pathways to effect the optimal cellular response to changes in the immediate environment. Paxillin is a multidomain adaptor that recruits both structural and signaling molecules to focal adhesions, sites of integrin engagement with the extracellular matrix, where it performs a critical role in transducing adhesion and growth factor signals to elicit changes in cell migration and gene expression.

The human papillomavirus type 11 and 16 E6 proteins modulate the cell-cycle regulator and transcription cofactor TRIP-Br1

The genital human papillomaviruses (HPVs) are a taxonomic group including HPV types that preferentially cause genital and laryngeal warts ("low-risk types"), such as HPV-6 and HPV-11, or cancer of the cervix and its precursor lesions ("high-risk types"), such as HPV-16. The transforming processes induced by these viruses depend on the proteins E5, E6, and E7. Among these oncoproteins, the E6 protein stands out because it supports a particularly large number of functions and interactions with cellular proteins, some of which are specific for the carcinogenic HPVs, while others are shared among low- and high-risk HPVs. Here we report yeast two-hybrid screens with HPV-6 and -11 E6 proteins that identified TRIP-Br1 as a novel cellular target. TRIP-Br1 was recently detected by two research groups, which described two separate functions, namely that of a transcriptional integrator of the E2F1/DP1/RB cell-cycle regulatory pathway (and then named TRIP-Br1), and that of an antagonist of the cyclin-dependent kinase suppression of p16INK4a (and then named p34SEI-1). We observed that TRIP-Br1 interacts with low- and high-risk HPV E6 proteins in yeast, in vitro and in mammalian cell cultures. Transcription activation of a complex consisting of E2F1, DP1, and TRIP-Br1 was efficiently stimulated by both E6 proteins. TRIP-Br1 has an LLG E6 interaction motif, which contributed to the binding of E6 proteins. Apparently, E6 does not promote degradation of TRIP-Br1. Our observations imply that the cell-cycle promoting transcription factor E2F1/DP1 is dually targeted by HPV oncoproteins, namely (i) by interference of the E7 protein with repression by RB, and (ii) by the transcriptional cofactor function of the E6 protein. Our data reveal the natural context of the transcription activator function of E6, which has been predicted without knowledge of the E2F1/DP1/TRIP-Br/E6 complex by studying chimeric constructs, and add a function to the limited number of transforming properties shared by low- and high-risk HPVs.

Requirement of E6AP and the features of human papillomavirus E6 necessary to support degradation of p53

E6 oncoproteins from human papillomavirus type 16 (16E6) and Bovine Papillomavirus type 1 (BE6) bind to leucine rich peptides (called charged leucine, LXXLL, or signature peptides) found on target cellular proteins. BE6 and 16E6 both bind the product of the UBE3A gene called E6AP on a charged leucine peptide, LQELL. E6AP is an E3 ubiquitin ligase that together with 16E6 interacts with p53 to target p53 degradation. Although both BE6 and 16E6 bind the LQELL peptide of E6AP, only 16E6 acts as an adapter to then bring p53 to E6AP. In order to determine how E6 proteins function as adapters, 16E6, p53, and E6AP were expressed in yeast, and were shown to form a tri-molecular complex. 16E6 mutants were selected that retained interactions with E6AP yet were defective for interaction with p53. Such 16E6 mutations were typically within the amino-terminus of 16E6. Through the use of E6AP null cells, transfected E6AP was shown to be necessary and sufficient for the degradation of p53 in the presence of 16E6. However, the interaction of 16E6 with E6AP was complex. While BE6 interacts only with the LQELL motif of E6AP, an intact LQELL motif is not necessary either for interaction of 16E6 with E6AP or for p53 degradation. In addition, 16E6 mutants that fail to bind the LQELL motif of E6AP can support p53 degradation. These results indicate that 16E6 may have multiple modes of interaction with E6AP and that assembly of p53 containing complexes for targeted degradation by E6AP may occur in more than one way. These results have implications for potential targeting of the interaction of 16E6 and E6AP in the therapy of HPV-induced cancer.

Loss of the EP2 prostaglandin E2 receptor in immortalized human keratinocytes results in increased invasiveness and decreased paxillin expression

Prostaglandin E(2) (PGE(2)) receptor subtype EP(2), which is coupled to cAMP metabolism, is known to mediate proliferation of primary human keratinocytes in vitro. The effect of gain or loss of EP(2) receptors in immortalized human keratinocytes (HaCat cells) was examined. HaCat keratinocytes were transfected with sense or anti-sense constructs of the EP(2) receptor. Loss or gain of EP(2) expression was documented by immunoblot and associated changes in agonist-stimulated cAMP production. Loss or gain of EP(2) receptor expression correlated with alterations in plating efficiencies but with modest affects on growth. When cell lines were studied in an organ culture model, anti-sense clones were highly invasive compared with vector controls and sense transfectants. A marked increase in prostaglandin production is commonly seen in malignant lesions. Because prostaglandin receptors are known to undergo ligand-induced receptor down-regulation, we sought to determine whether EP(2) receptor down-regulation results in increased invasiveness. In vector controls, invasiveness was reproduced by ligand-dependent EP(2) receptor down-regulation as assessed by immunohistochemistry. In addition, loss of EP(2) receptor expression was associated with decreased paxillin expression, a critical component of focal adhesion assembly. Thus, down-regulation of EP(2) receptors represents a potential mechanism for neoplastic progression to an invasive phenotype.

Chimaeric HPV E6 proteins allow dissection of the proteolytic pathways regulating different E6 cellular target proteins

The ability of HPV E6 oncoproteins to induce the degradation of PDZ domain-containing MAGUK proteins correlates with their malignant potential. We previously showed that the HPV-6 E6 protein, when provided with the PDZ-binding domain from HPV-18 E6, acquires the ability to bind the Discs Large (Dlg) tumour suppressor and target it for degradation. Based on this finding we have extended this analysis to E6 proteins from a variety of different papillomavirus types. Cloning a PDZ-binding sequence onto the C-terminus of E6 proteins derived from low-risk mucosal, and low and high-risk cutaneous papillomavirus types, enables them to bind Dlg and a second MAGUK family member, MAGI-1. This renders the mucosally-derived low-risk chimaeric HPV E6 proteins capable of targeting Dlg for degradation, but they are unable to induce significant levels of degradation of MAGI-1. In contrast, none of the E6 proteins derived from cutaneous papillomavirus types induce significant degradation of either MAGI-1 or Dlg when provided with a PDZ-binding domain. These results demonstrate significant differences, both between mucosal and cutaneous HPV E6 proteins and in the pathways required for Dlg and MAGI-1 degradation.

Bovine papillomavirus type 1 E6-induced sensitization to apoptosis is distinct from its transforming activity

The bovine papillomavirus type 1 (BPV-1) E6 oncoprotein induces tumorigenic transformation of murine C127 cells and stimulates transcription when targeted to a promoter. We have previously shown that C127 cells expressing BPV-1 E6 exhibited increased tumor necrosis factor alpha (TNF)-mediated apoptosis. To understand the mechanisms by which BPV-1 E6 sensitizes cells to apoptosis and to investigate the relevance of E6-enhanced apoptosis to its other biological activities, we analyzed a BPV-1 E6 mutant (491, with four amino acids deleted at the C-terminus) for its ability to sensitize C127 cells to apoptosis. The result was then compared with the E6 mutant's ability to transform cells, to activate transcription, and to associate with known cellular binding proteins. Our data indicated that the transcriptional activation function of BPV-1 E6 correlated with sensitization of cells to TNF-mediated apoptosis. Moreover, functions required for BPV-1 E6-mediated sensitization of cells to apoptosis are distinct from those required for transformation. A potential role of paxillin in E6 sensitization of cells to apoptosis is implicated. These results thus indicate that sensitization of cells to TNF-induced apoptosis represents a novel function of BPV-1 E6.

The paxillin LD motifs

Adapter/scaffold proteins, through their multidomain structure, perform a fundamental role in facilitating signal transduction within cells. Paxillin is a focal adhesion adapter protein implicated in growth factor- as well as integrin-mediated signaling pathways. The amino-terminus of paxillin contains five leucine-rich sequences termed LD motifs. These paxillin LD motifs are highly conserved between species as well as within the paxillin superfamily. They mediate interactions with several structural and regulatory proteins important for coordinating changes in the actin cytoskeleton associated with cell motility and cell adhesion as well as in the regulation of gene expression.

Paxillin: a focal adhesion-associated adaptor protein

Paxillin is a focal adhesion-associated, phosphotyrosine-containing protein that may play a role in several signaling pathways. Paxillin contains a number of motifs that mediate protein-protein interactions, including LD motifs, LIM domains, an SH3 domain-binding site and SH2 domain-binding sites. These motifs serve as docking sites for cytoskeletal proteins, tyrosine kinases, serine/threonine kinases, GTPase activating proteins and other adaptor proteins that recruit additional enzymes into complex with paxillin. Thus paxillin itself serves as a docking protein to recruit signaling molecules to a specific cellular compartment, the focal adhesions, and/or to recruit specific combinations of signaling molecules into a complex to coordinate downstream signaling. The biological function of paxillin coordinated signaling is likely to regulate cell spreading and motility.

Interaction with CBP/p300 enables the bovine papillomavirus type 1 E6 oncoprotein to downregulate CBP/p300-mediated transactivation by p53

The E6 oncoprotein of bovine papillomavirus type 1 (BPV-1) can transform cells independently of p53 degradation. The precise mechanisms underlying this transformation are not yet completely understood. Here it is shown that BPV-1 E6 interacts with CBP/p300 in the same way as described for the E6 proteins of oncogenic human papillomaviruses. This interaction results in an inhibition of the transcriptional coactivator function of CBP/p300 required by p53 and probably by other transcription factors. The comparison of the CBP/p300-binding properties of BPV-1 E6 mutants previously characterized in transcription and transformation studies suggests (i) that the E6-CBP/p300 interaction may be necessary, but not sufficient, for cell transformation, and (ii) that the transcriptional activator function, inherent to the E6 protein, is not derived from forming a complex with CBP/p300.

Competitive binding to a charged leucine motif represses transformation by a papillomavirus E6 oncoprotein

E6 oncoproteins from HPV-16 and bovine papillomavirus type 1 (BPV-1) bind to similar leucine-rich peptides termed charged leucine motifs found on the cellular focal adhesion protein paxillin and the E3 ubiquitin ligase E6AP. BPV-1 E6 (BE6) mutants that do not bind to paxillin are defective at inducing cellular transformation. It is possible, however, that BE6 mutants that do not bind paxillin are defective for transformation for an unrelated reason than the ability to bind to charged leucine motifs. To address the role of BE6 interaction with charged leucine motifs, we fused a BE6-binding charged leucine motif to the amino terminus of BE6, thereby creating an autoinhibitory binding domain. We found that the fusion protein failed to bind to paxillin or transform murine C127 cells. Mutation of the amino terminal binding motif in the fusion protein restored both interaction with paxillin and transformation. This demonstrates that BE6 transformation requires binding to charged leucine motifs on particular cellular proteins and that transformation by papillomavirus oncoproteins can be repressed by competitive interactions with charged leucine motifs.