Roles for homotypic interactions and transautophosphorylation in IκB kinase (IKKβ) activation

Abstract

The nuclear factor κB (NFκB)/Rel family of transcription factors participates in a wide range of biological activities including inflammation, immunity, and apoptosis. NF-κB is kept inactive in the cytoplasm in unstimulated cells by virtue of the masking of its nuclear localization sequence by bound IκB protein. Cellular stimuli trigger the destruction of IκB proteins and the liberation of NF-κB to enter the nucleus and activate gene expression. A multisubunit IκB kinase complex (IKK) phosphorylates IκB proteins and mediates the activation of NF-κB by proinflammatory stimuli such as tumor necrosis factor α. Phosphorylation of IκB proteins triggers their polyubiquitination and their subsequent recognition and degradation by the proteasome. The IKK complex contains two catalytic subunits, IKKα and IKKβ, and a noncatalytic subunit, NF-κB essential modifier/IKKγ. IKK activation depends upon the phosphorylation of residues in the activation loop of IKKβ and the subsequent activation of IKKβ kinase activity. However, the events contributing to IKKβ phosphorylation are not well understood. Here, we present evidence that the activation of IKKβ depends on its ability to form homotypic interactions and to transautophosphorylate. We find that an intact leucine zipper in IKKβ is necessary for homotypic interactions, kinase activation, and phosphorylation on its activation loop. Enforced oligomerization of an IKKβ mutant defective in forming homotypic interactions restores kinase activation. Homotypic interactions allow IKKβ molecules to transautophosphorylate one another on their activation loops. Finally, the oligomerization of IKKβ is stimulated by tumor necrosis factor α in cultured cells. Our findings support a model whereby ligand-induced homotypic interactions between IKKβ molecules result in IKKβ phosphorylation and consequently IKK activation.