Papillomavirus E2 proteins and the host BRD4 protein associate with transcriptionally active cellular chromatin.

The interaction of papillomavirus E2 proteins with cellular Brd4 protein is important for transcriptional regulation of viral genes and partitioning of viral genomes. Bovine papillomavirus type 1 (BPV-1) E2 binds cellular chromatin in complex with Brd4 in both mitotic and interphase cells. To identify specific sites of E2 interaction on cellular chromatin, a genome-wide chromatin immunoprecipitation-on-chip analysis was carried out using human promoter sequences. Both E2 and Brd4 were found bound to most transcriptionally active promoters in C33A cells. These promoters were also bound by RNA polymerase II and were modified by histone H3 acetylation and K4 trimethylation, all indicators of active transcription. E2 binding strongly correlated with Brd4 and RNA polymerase II occupancy and H3K4me3 modification at all human promoters, indicating that E2 bound to active promoters. E2 binding did not correlate with the presence of consensus E2 binding sites in the promoters. Furthermore, the mRNA levels of E2-bound cellular genes were not significantly changed by E2 expression. Thus, the papillomavirus E2 proteins bind to transcriptionally active cellular genes but do not change their activity. We propose that this may be a way for the virus to ensure that the viral genome is retained in transcriptionally active regions of the nucleus to escape silencing. Therefore, E2-mediated tethering of viral genomes to host chromatin has multiple roles: to partition the viral genome to daughter cells, to ensure that the genomes are retained in the nucleus, and to make certain that the genomes are retained in functionally active nuclear domains.