The E2 protein of human papillomavirus type 16. Over-expression and purification of an active transcriptional regulator

trans activation by the full-length E2 proteins of human papillomavirus type 16 and bovine papillomavirus type 1 in vitro and in vivo: cooperation with activation domains of cellular transcription factors

Papillomaviral E2 genes encode proteins that regulate viral transcription. While the full-length bovine papillomavirus type 1 (BPV-1) E2 peptide is a strong trans activator, the homologous full-length E2 product of human papillomavirus type 16 (HPV-16) appeared to vary in function in previous studies. Here we show that when expressed from comparable constructs, the full-length E2 products of HPV-16 and BPV-1 trans activate a simple E2- and Sp1-dependent promoter up to approximately 100-fold in human keratinocytes and other epithelial cells as well as human and animal fibroblasts. Vaccinia virus-expressed, purified full-length HPV-16 and BPV-1 E2 proteins bound a consensus E2 site with high specific affinities (Kd = approximately 10(-9) M) and stimulated in vitro transcription up to six- to eightfold. In vivo and in vitro trans activation by either E2 protein required cooperation with another activator, such as Sp1, or other factors that interact with papillomavirus promoters, such as AP-1, Oct-1, nuclear factor 1/CTF, transcriptional enhancer factor 1, or USF. The glutamine-rich domain B of Sp1 or the mutually unrelated activation domains of other transcription factors were necessary and sufficient for cooperation with either E2 factor. We conclude that like BPV-1 E2, the HPV-16 E2 protein has the potential to function as a strong activator of viral gene expression in cooperation with cellular transcription factors.

During negative regulation of the human papillomavirus-16 E6 promoter, the viral E2 protein can displace Sp1 from a proximal promoter element

The principal early promoter of human papillomaviruses (HPVs), designated P97 in the case of HPV-16, contains four characteristically aligned cis-responsive elements, namely one binding site for Sp1, two for the viral E2 proteins, and the TATA box. The Sp1 binding site is needed to mediate activation of P97 by the remote epithelial-specific enhancer, and the two E2 binding sites contribute to a negative feedback-loop of viral gene expression. The Sp1 consensus motif and the TATA-box distal E2 binding site are spaced in all genital papillomaviruses by a single nucleotide. We show here that at physiological concentrations, the binding of E2 proteins and Sp1 are mutually exclusive events, since a bandshift analysis with nuclear extracts from ID13, a mouse cell line transformed by BPV-1, showed only the E2 or the Sp1 bandshift, but no complex indicative of the concomitant binding of both factors. Increasing concentrations of in vitro translated E2 protein compete efficiently with the Sp1 factor for binding to an oligonucleotide containing both binding sites. Interference between Sp1 and E2 protein binding is apparently relevant for P97 repression in vivo, since a mutational analysis revealed that both E2 binding sites are necessary for negative transcriptional regulation: Alone, neither the distal site, where E2 protein can induce Sp1 displacement, nor the proximal site, where E2 protein interferes with formation and function of the pre-initiation complex, have a significant effect, but two functional E2 binding sites lead to repression of P97.

Structural analysis of the human papillomavirus type 16-E2 transactivator with antipeptide antibodies reveals a high mobility region linking the transactivation and the DNA-binding domains

In order to probe the structure of the transcription factor encoded by the E2 Open Reading Frame of papillomaviruses, we raised polyclonal antibodies against a series of synthetic peptides that cover the HPV16-E2 protein. In gel shift experiments with the native form of the protein, we detected supershifts (caused by the binding of antibodies to the E2-DNA complex) with antibodies synthesized against peptides covering a central region 50 residues long in the E2 protein. On the contrary, antibodies raised against peptides from the NH2- and COOH-termini did not give any supershifted band. Western blot experiments showed that several of these non reacting antibodies did however interact with the denatured protein. These results suggest that the central region that connects the NH2-terminal domain responsible for transcriptional activation and the COOH-domain involved in DNA-binding is exposed and maintained in a conformation resembling the peptide, indicating a high mobility region. In contrast, the DNA-binding and transactivation domains were not recognized by the antipeptide antibodies, in line with secondary structure predictions and sequence comparisons indicating that the E2 protein consists of structured and conserved NH2 and COOH-terminal regions separated by a non-conserved and unstructured region. This flexible 'hinge' region may facilitate contacts between E2 dimers at distance in mechanisms of transcriptional activation steps that involve homosynergy or DNA-looping.

The E6/E7 promoter of human papillomavirus type 16 is activated in the absence of E2 proteins by a sequence-aberrant Sp1 distal element

The E6/E7 promoter of all genital human papillomaviruses is responsible for expression of the viral transforming genes. Centered 60 bp upstream of the transcription start, it contains a 20-bp segment with partially overlapping binding sites for the viral E2 proteins and for a cellular factor that was identified by footprint experiments. Bandshifts, bandshift competitions, and footprints revealed that protein complexes between nuclear extracts and these sequences have binding properties indistinguishable from those of the Sp1 factor that binds the simian virus 40 early promoter GC motif. Reactions of these complexes with anti-Sp1 antiserum were analyzed by superbandshifts and precipitation with protein A, and the results confirmed the identity of this transcription factor as Sp1. Sp1 binds in simian virus 40 and different human papillomavirus promoters the consensus sequence 5'-NGGNGN-3'. RNase protection analysis of in vitro or in vivo transcriptions with wild-type and mutant test vectors shows that the E6/E7 promoter of human papillomavirus type 16 is functionally dependent on the Sp1 distal promoter element. In all genital papillomaviruses, the Sp1 hexamer is invariably spaced by a single nucleotide from the distal E2 element, suggesting some precise interaction between Sp1 and E2 proteins. Published experimental evidence documents negative regulation of the E6/E7 promoter by E2 proteins through the proximal E2 element, whereas only minor quantitative differences in E6/E7 promoter function after cotransfection with E2 expression vectors were observed in this study. A detailed study of the interactions of Sp1 and E2 proteins with one another and with the corresponding three binding sites may reveal a complex modulation of this promoter.