Cooperative binding of the E2 protein of bovine papillomavirus to adjacent E2-responsive sequences

The DNA-binding properties of purified full-length E2 protein from bovine papillomavirus type 1 have been investigated by utilizing a quantitative gel shift analysis. By using a recombinant baculovirus which expresses the E2 open reading frame from the polyhedrin promoter, the full-length E2 protein was synthesized in insect cells and purified to homogeneity by using an E2 binding site (ACCGN₄CGGT)-specific oligonucleotide column. The K(d) of E2 binding to a 41-bp oligonucleotide containing a single binding site was found to be 2 x 10^-11 M. When two binding sites were included on an oligonucleotide, cooperative binding to these sites by the E2 protein was observed. A cooperativity parameter of 8.5 was determined for E2 binding to two sites. An 86-amino-acid peptide encompassing the C terminus of the protein retains the ability to bind E2 binding sites with a K(d) of 4 x 10^-10 M but exhibits slight cooperativity of binding to two adjacent sites. A major determinant for cooperative binding of the full-length E2 protein is thus encoded by the N-terminal amino acids outside the minimal DNA binding domains.