We have determined the crystal structure, at 3.2 A, of a ternary complex 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 octamer site, and its polypeptide backbone forms a pair of hydrogen bonds with the adenine base at position 5 of the octamer DNA. Numerous protein-protein 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 hydrophobic pocket on the POU-specific domain. The structure of this ternary complex is consistent with previous biochemical studies and shows how peptide-DNA and peptide-protein contacts from OCA-B provide structural and functional specificity in the regulation of immunoglobulin transcription.
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