Subunit affinities and stoichiometries of the human papillomavirus type 11 E1:E2:DNA complex

The association between the papillomavirus E1 and E2 proteins is an important regulatory interaction, imparting coordinated control of:viral transcription and replication. Using fluorescence polarization, we have characterized the interactions between HPV-11 E1, HPV-11 E2, and DNA in solution at equilibrium. For these studies, two double-stranded fluorescein-labeled oligonucleotides were prepared. The first fluorescent oligonucleotide, designated F1-E2BS and containing a single E2 binding-site palindrome (ACCGN(6)CGGT), was used to determine the affinity of E2 for its DNA binding site. HPV-11 E2 bound F1-E2BS with an apparent K-d of 0.84 nM. Binding was saturable and consistent with a single class of noninteracting sites. The second oligonucleotide, designated F1-E1E2BS, contained both E1 and E2 sites in sequence derived directly from the HPV-11 origin of replication. Under titration conditions identical to those used for F1-E2BS, the E2 protein exhibited reduced affinity for F1-E1E2BS (K-d > 100 nM). F1 binding to F1-E1E2BS was of very low affinity. Addition of excess HPV-11, El to Fl-E1E2BS lowered the dissociation constant for the E2:F1-E1E2BS interaction to 2 nM. This effect was not dependent upon ATP or magnesium ion. Fluorescence polarization and other data suggest formation of a complex containing six E1 molecules and a single dimer of E2 bound to a single F1-E1E2BS oligonucleotide; E2 dissociation from the final complex did not occur. In summary, physical interaction between El and E2 increases the DNA binding affinity of each. The role of this energy coupling may be to promote origin-specific binding of both E1 and E2 to DNA.