Effects of mutations within two hydrophilic regions of the bovine papillomavirus type 1 E1 DNA-binding domain on E1-E2 interaction

The interaction between papillomavirus E1 and E2 proteins is essential for viral genome replication. Using both in vivo and in vitro assays to evaluat e the regions of the two proteins necessary for the E1-E2 interaction, thre e independent interactions were identified for bovine papillomavirus E1: th e N terminus of E1 (E1N, residues 1-311) interacts with the E2 transactivat ion domain (E2TAD) and the E2 DNA-binding domain (E2DBD) and the C terminus of E1 (E1C, residues 315-605) interacts with E2. Nine mutations within E1N were evaluated for their effects on E2 interaction. Five mutations elimina ted interaction with the E2TAD; four of these were located within two previ ously identified conserved, hydrophilic regions, HR1 and HR3. Since HR1 and HR3 residues appear to comprise the origin of replication recognition elem ent for El, simultaneous interaction with the E2TAD during initiation compl ex formation would seem unlikely. Consistent with this inference is the fac t that three of the five mutants defective for E2TAD binding exhibited wild -type levels of replication. The replication-positive phenotype of these mu tants suggests that the E1N-E2TAD interaction is not essential for replicat ion function and is probably involved in some other E1-E2 function, such as regulating transcription. Only one of the five mutations defective for E2T AD binding also prevented E2DBD interaction, indicating that the regions of E1N that interact with the E2TAD and the E2DBD are not identical. The abil ity of E1N to cooperatively interact with E2 bound to E2-binding site (E2BS ) 11 versus EBS12 was also examined, and cooperative binding was only obser ved when E2 was bound to E2BS12.