Conformational alteration of Oct-1 upon DNA binding dictates selectivity in differential interactions with related transcriptional coactivators.

VP16 (termed VP16-H here) of herpes simplex virus (HSV) belongs to a family of related regulatory proteins which includes VP16-B of bovine herpesvirus (BHV). We show that VP16-B, while also being a powerful transactivator of transcription dependent on Oct-1 binding sites in its target promoters, has virtually no activity on a defined VP16-H-responsive, octamer-containing target promoter. While Oct-1 binds equally well to the VP16-B-responsive and -nonresponsive sites, VP16-B interacts with Oct-1 only when Oct-1 is bound to the BHV octamer site and not when it is bound to the HSV site. We show from the analysis of chimeric proteins that the ability of VP16-B to discriminate between the Oct-1 forms depends on features of its N-terminal region. We also show from an analysis of chimeric DNA motifs that sequences that lie 3' to the POU domain-contacting region of the HSV octamer site play a role in making it unresponsive to VP16-B. Finally, we show by high-resolution hydroxyl radical footprint analysis that the conformation of Oct-l is different on the two sites. These results augment our previous report on an allosteric effect of DNA signals on the conformation of bound proteins and indicate that different conformations of the same DNA binding protein can be recognized selectively by related members of interacting regulatory proteins. The possible implications of our observations for selective gene regulation by Oct-1, a ubiquitous transcription factor, and other multimember transcription families are discussed.