Crystal structure of an OCA-B peptide bound to an Oct-1 POU domain/octamerDNA complex: specific recognition of a protein-DNA interface

We have determined the crystal structure, at 3.2 Angstrom, of a ternary com plex containing an OCA-B peptide, the Oct-1 POU domain, and an octamer DNA site. The OCA-B peptide binds in the major groove near the center of the oc tamer site, and its polypeptide backbone forms a pair of hydrogen bonds wit h the adenine base at position 5 of the octamer DNA. Numerous protein-prote in contacts between the OCA-B peptide and the POU domain are also involved in the ternary complex. In particular, the hydrophobic surface from a short alpha-helix of OCA-B helps to stabilize the complex by binding to a hydrop hobic pocket on the POU-specific domain. The structure of this tertiary com plex is consistent with previous biochemical studies and shows how peptide- DNA and peptide-protein contacts from OCA-B provide structural and function al specificity in the regulation of immunoglobulin transcription.