Direct Phosphorylation of IκB by IKKα and IKKβ: Discrimination Between Free and NF-κB-Bound Substrate

Abstract

A large protein complex mediates the phosphorylation of the inhibitor of κB (IκB), which results in the activation of nuclear factor κB (NF-κB). Two subunits of this complex, IκB kinase α (IKKα) and IκB kinase β (IKKβ), are required for NF-κB activation. Purified recombinant IKKα and IKKβ expressed in insect cells were used to demonstrate that each protein can directly phosphorylate IκB proteins. IKKα and IKKβ were found to form both homodimers and heterodimers. Both IKKα and IKKβ phosphorylated IκB bound to NF-κB more efficiently than they phosphorylated free IκB. This result explains how free IκB can accumulate in cells in which IKK is still active and thus can contribute to the termination of NF-κB activation.