Characterization of the minimal DNA binding domain of the human papillomavirus E1 helicase: Fluorescence anisotropy studies and characterization of a dimerization-defective mutant protein

The El helicase of papillomaviruses is required for replication of the viral double-stranded DNA genome, in conjunction with cellular factors. DNA replication is initiated at the viral origin by the assembly of El monomers into oligomeric complexes that have unwinding activity. In vivo, this process is catalyzed by the viral E2 protein, which recruits El specifically at the origin. For bovine papillomavirus (BPV) El a minimal DNA-binding domain (DBD) has been identified N-terminal to the enzymatic domain. In this study, we characterized the DBD of human papillomavirus 11 (HPV11), HPV18, and BPV El using a quantitative DNA binding assay based on fluorescence anisotropy. We found that the HPV11 DBD binds DNA with an affinity and sequence requirement comparable to those of the analogous domain of BPV but that the HPV18 DBD has a higher affinity for nonspecific DNA. By comparing the DNA-binding properties of a dimerization-defective protein to those of the wild type, we provide evidence that dimerization of the HPV11 DBD occurs only on two appropriately positioned El binding-sites and contributes approximately a 10-fold increase in binding affinity. In contrast, the HPV11 El helicase purified as preformed hexamers binds DNA with little sequence specificity, similarly to a dimerization-defective DBD. Finally, we show that the amino acid substitution that prevents dimerization reduces the ability of a longer El protein to bind to the origin in vitro and to support transient HPV DNA replication in vivo, but has little effect on its ATPase activity or ability to oligomerize into hexamers. These results are discussed in light of a model of the assembly of replication-competent double hexameric El complexes at the origin.