Adjacent residues in the E1 initiator beta-hairpin define different roles of the beta-hairpin in Ori melting, helicase loading, and helicase activity.

We have analyzed two residues in the helicase domain of the E1 initiator protein. These residues are part of a highly conserved structural motif, the beta-hairpin, which is present in the helicase domain of all papovavirus initiator proteins. These proteins are unique in their ability to transition from local template melting activity to unwinding. We demonstrate that the beta-hairpin has two functions. First, it is the tool used by the E1 double trimer (DT) to pry open and melt double-stranded DNA. Second, it is required for the unwinding activity of the hexameric E1 helicase. The fact that the same structural element, but not the same residues, contacts both dsDNA in the DT for melting and ssDNA in the double hexamer (DH) for helicase activity provides a link between local origin melting and DNA helicase activity and suggests how the transition between these two states comes about.