IKAP is a scaffold protein of the IκB kinase complex

The transcription factor NF-κB coordinates the activation of numerous genes in response to pathogens and pro-inflammatory cytokines, and is, therefore, vital in the development of acute and chronic inflammatory diseases. NF-κB is activated by phsophorylation of its inhibitory subunit, IκB-α (ref. 7), on serine residues 32 and 36 by cytokine-activated IκB kinases (IKKs); this phosphorylation precedes rapid degradation of IκB. IKK-α and IKK-β isozymes are found in large complexes of relative molecular mass 700,000–900,000 (Mr 70K–90K), but little is known about other components that organize and regulate these complexes. IKK-α was independently discovered as a NF-κB-inducing kinase (NIK)-associated protein in a yeast two-hybrid screen, and IKK-β was also identified by homology screening. It is, however, unknown whether NIK is part of the IKK complex. Here we isolate large, interleukin-1-inducible IKK complexes that contain NIK, IKK-α, IKK-β, IκB-α, NF-κB/RelA and a protein of Mr 150K. This latter component is a new protein, termed IKK-complex-associated protein (IKAP), which can bind NIK and IKKs and assemble them into an active kinase complex. We show that IKAP is a scaffold protein and a regulator for threedifferent kinases involved in pro-inflammatory cytokine signalling.