The cellular DNA helicase ChlR1 regulates chromatin and nuclear matrix attachment of the human papillomavirus type 16 E2 protein and high copy viral genome establishment: ChlR1 regulates the chromatin association of HPV16 E2

In papillomavirus infections, the viral genome is established as a double-stranded DNA episome. To segregate the episomes to daughter cells during mitosis, they are tethered to cellular chromatin by the viral E2 protein. We previously demonstrated that the E2 proteins of diverse papillomavirus types, including bovine papillomavirus (BPV) and human papillomavirus type 16 (HPV16), associate with the cellular DNA helicase ChlR1. This virus-host interaction is important for the tethering of BPV E2 to mitotic chromatin and the stable maintenance of BPV episomes. The role of the association between E2 and ChlR1 in the HPV16 life cycle is unresolved. Here, we show that the HPV16 E2 mutation Y131A has significantly reduced binding to ChlR1, but retains transcriptional activation and viral origin-dependent replication functions. Subcellular fractionation of keratinocytes expressing E2Y131A shows a marked change in the localization of the protein. In comparison to wild type E2, the chromatin-bound pool of E2Y131A is decreased, concomitant with an increase in nuclear matrix-associated protein. Cell cycle synchronization indicates that the shift in subcellular localization of E2Y131A occurs in mid-S phase. A similar alteration between the subcellular pools of E2WT protein occurred upon ChlR1 silencing. Notably, in an HPV16 life cycle model in primary human keratinocytes, mutant E2Y131A genomes were established as episomes, but at a markedly lower copy number than wild type HPV16 genomes, and were not maintained upon cell passage. Our studies indicate that ChlR1 is an important regulator of the chromatin association of E2 and of the establishment and maintenance of HPV16 episomes.