The Human Papillomavirus Type 8 E2 Tethering Protein Targets the Ribosomal DNA Loci of Host Mitotic Chromosomes

Abstract

For many papillomaviruses, the viral protein E2 tethers the viral genome to the host mitotic chromosomes to ensure persistent, long-term maintenance of the genome during cell division. Our previous studies of E2 proteins from different genera of papillomaviruses have shown that they bind to different regions of the host chromosomes during mitosis. For example, bovine papillomavirus type 1 (BPV-1) E2 binds to all chromosomes as small speckles in complex with the cellular protein Brd4. In contrast, the human papillomavirus type 8 (HPV-8) E2 protein binds as large speckles at the pericentromeric regions of chromosomes. Here we show that these speckles do not contain Brd4, and unlike that of BPV-1, the N-terminal Brd4-interacting domain of HPV-8 E2 is not required for chromosome binding. In contrast to BPV-1 E2, the HPV-8 E2 protein targets the short arms of acrocentric mitotic chromosomes. Furthermore, the E2 protein interacts with the repeated ribosomal DNA genes found in this location and colocalizes with UBF, the RNA polymerase I transcription factor. Therefore, HPV-8 E2 genome tethering occurs by a Brd4-independent mechanism through a novel interaction with specific regions of mitotic chromosomes. Thus, a wide range of viruses have adopted the strategy of linking their genomes to host chromosomes, but individual viruses use different chromosomal targets. Characterization of these targets will enable the development of antiviral therapies to eliminate the viral genomes from infected cells.Papillomaviruses infect the basal layer of stratified epithelia. The viral genomes are stably maintained in the nuclei of the dividing cells as low copy number, extrachromosomal elements for many years. Error-free replication and equal distribution of the replicated copies of the viral DNA to the dividing cells are key to the persistence of papillomavirus infection. The mechanism of viral genome partitioning is different from that of host chromosomes, as the viral DNA does not possess any sequence equivalent to the centromere to utilize the mitotic segregation machinery of the host. Instead, E2, a multifunctional viral protein, attaches the viral DNA to the host chromosomes (15,21,34). As the host cell chromosomes are divided equally into daughter cells during mitotic division, the viral DNA is passively segregated by being tethered to the host chromosomes. Chromosomal tethering mediated by a virus-encoded protein is a common tactic for maintaining the genomes of other persistent viruses. Gammaherpesviruses such as Epstein-Barr virus and Kaposi's sarcoma-associated herpesvirus use this strategy, which is mediated by the EBNA-1 and LANA proteins, respectively (reviewed in reference 23).Among the papillomaviruses, bovine papillomavirus type 1 (BPV-1) is best studied for chromosomal segregation. The multifunctional viral protein E2 has been shown to play an important role in this process. Both the BPV-1 E2 protein and viral genomes are localized on mitotic chromosomes as small speckles on the arms of all chromosomes, in complex wi...

Role of Kaposi's Sarcoma-Associated Herpesvirus C-Terminal LANA Chromosome Binding in Episome Persistence

“…For bovine papillomavirus type 1, E2 is targeted to mitotic chromosomes by binding to cellular Brd4 (8,54,84); however, this mechanism is not strictly conserved across the papillomavirus family (53,58). Intriguingly, the E2 protein of human papillomavirus type 8 associates with mitotic chromosomes in a paired dot pattern reminiscent of that of the C-terminal LANA domain (36,53,58,62), suggesting that these two proteins may share common features of chromosome association. EBNA1 has chromosome binding regions that are located between amino acids 8 to 67, 72 to 84, and 328 to 365 (34,52,81).…”

Brd4 Regulation of Papillomavirus Protein E2 Stability

“…Our initial studies indicated that Brd4 played a role in the tethering of BPV1 E2 and viral episomes to cellular chromosomes during mitosis (40) and that disruption of this interaction resulted in the loss of viral episomes (41). Factors in addition to Brd4 have now been implicated in E2's episome maintenance function (22), and it is clear that different PVs rely on distinct cellular proteins to ensure plasmid maintenance in dividing cells (28). Brd4 also mediates the transcriptional activation function of E2, a characteristic that has now been validated for all PVs examined to date (20,30,31).…”

Interaction of the Betapapillomavirus E2 Tethering Protein with Mitotic Chromosomes

“…UBF is a transcription factor required for rDNA transcription by RNA polymerase I (21); it remains bound to chromosomes during mitosis even when transcription is silenced (40,46). Studies in our laboratory have shown that the domains required for mitotic chromosomal association of HPV8 E2 are different from those required for BPV1 E2 binding; the hinge and C-terminal domain are sufficient and essential for chromosomal association (44). Unlike BPV1 E2, the HPV8 N-terminal transactivation domain is not required for binding.…”

“…In contrast, the HPV8 E2 protein showed a distinct pattern of large foci bound to the pericentromeric regions of chromosomes (39). Our laboratory has shown that the HPV8 E2 protein associates with the ribosomal DNA (rDNA) loci present on the short arms of human acrocentric chromosomes and colocalizes with upstream binding factor (UBF) (44). UBF is a transcription factor required for rDNA transcription by RNA polymerase I (21); it remains bound to chromosomes during mitosis even when transcription is silenced (40,46).…”