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.