Transcriptional activation of interferon β (IFN-β), an antiviral cytokine, requires the assembly of IRF-3 and CBP/p300 at the promoter region of the IFN-β gene. The crystal structure of IRF-3 in complex with CBP reveals that CBP interacts with a hydrophobic surface on IRF-3, which in latent IRF-3 is covered by its autoinhibitory elements. This structural organization suggests that virus-induced phosphoactivation of IRF-3 triggers unfolding of the autoinhibitory elements and exposes the same hydrophobic surface for CBP interaction. The structure also reveals that the interacting CBP segment can exist in drastically different conformations, depending on the identity of the associating transcription cofactor. The finding suggests a possible regulatory mechanism in CBP/p300, by which the interacting transcription factor can specify the coactivator's conformation and influence the transcriptional outcome.