A DNA-binding activity in BPV initiator protein E1 required for melting duplex ori DNA but not processive helicase activity initiated on partially single-stranded DNA

The papillomavirus replication protein E1 assembles on the viral origin of replication (ori) as a series of complexes. It has been proposed that the ori DNA is first melted by a head-to-tail double trimer of E1 that evolves into two hexamers that encircle and unwind DNA bi-directionally. Here the role of a conserved lysine residue in the smaller tier or collar of the E1 helicase domain in ori processing is described. Unlike the residues of the AAA+ domain DNA-binding segments (β-hairpin and hydrophobic loop; larger tier), this residue functions in the initial melting of duplex ori DNA but not in the processive DNA unwinding of partially single-stranded test substrates. These data therefore define a new DNA-binding related activity in the E1 protein and demonstrate that separate functional elements for DNA melting and helicase activity can be distinguished. New insights into the mechanism of ori melting are elaborated, suggesting the coordinated involvement of rigid and flexible DNA-binding components in E1.