Interaction of bovine papillomavirus E2 protein with Brd4 stabilizes its association with chromatin

The bovine papillomavirus E2 protein maintains and segregates the viral extrachromosomal genomes by tethering them to cellular mitotic chromosomes. E2 interacts with a cellular bromodomain protein, Brd4, to mediate the segregation of viral genomes into daughter cells. Brd4 binds acetylated histones and has been observed to diffusely coat mitotic chromosomes in several cell types. In this study, we show that in mitotic C127 cells, Brd4 diffusely coated the condensed chromosomes. However, in the presence of the E2 protein, E2 and Brd4 colocalized in punctate dots that were randomly distributed over the chromosomes. A similar pattern of E2 and Brd4 colocalization on mitotic chromosomes was observed in CV-1 cells, whereas only a faint chromosomal coating of Brd4 was detected in the absence of the E2 protein. Therefore, the viral E2 protein relocalizes and/or stabilizes the association of Brd4 with chromosomes in mitotic cells. The colocalization of E2 and Brd4 was also observed in interphase cells, indicating that this protein-protein interaction persists throughout the cell cycle. The interaction of E2 with Brd4 greatly stabilized the association of Brd4 with interphase chromatin. In both mitotic and interphase cells, this stabilization required a transcriptionally competent transactivation domain, but not the DNA binding function of the E2 protein. Thus, the E2 protein modulates the chromatin association of Brd4 during both interphase and mitosis. This study demonstrates that the segregation of papillomavirus genomes is not simply due to the passive hitchhiking of the E2/genome complex with a convenient cellular chromosomal protein.