Crystal structure of inhibitor of κB kinase β (IKKβ)

Inhibitor of κB (IκB) kinase (IKK) phosphorylates IκB proteins leading to their degradation and liberation of nuclear factor κB (NF-κB) for gene transcription. Here we report the crystal structure of IKKβ in complex with an inhibitor at 3.6 A resolution. The structure reveals a tri-modular architecture with the kinase domain (KD), a ubiquitin-like domain (ULD) and an elongated, α-helical scaffold/dimerization domain (SDD). Surprisingly, the predicted leucine zipper and helix-loop-helix motifs do not form these structures but are part of SDD. The ULD and SDD mediate a critical interaction with IκBα that restricts substrate specificity, and the ULD is also required for catalytic activity. The SDD mediates IKKβ dimerization, but dimerization per se is not important for maintaining IKKβ activity, and instead is required for IKKβ activation. Other IKK family members IKKα, TBK1 and IKKi may share the similar tri-modular architecture and function.