Tetrameric Oligomerization of IκB Kinase γ (IKKγ) Is Obligatory for IKK Complex Activity and NF-κB Activation.

The IκB kinase (IKK) complex mediates activation of transcription factor NF-κB by phosphorylation of IκB proteins. Its catalytic subunits, IKKα and IKKβ, require association with the regulatory IKK-γ (NEMO) component to gain full basal and inducible kinase activity. However, the oligomeric composition of the IKK complex and its regulation by IKK-γ are poorly understood. We show here that IKKγ predominantly forms tetramers and interacts with IKKα or IKKβ in this state. We propose that tetramerization is accomplished by a prerequisite dimerization through a C-terminal coiled-coil minimal oligomerization domain (MOD). This is followed by dimerization of the dimers with their N-terminal sequences. Tetrameric IKKγ sequesters four kinase molecules, yielding a γ[sub 4](α/β)[sub 4] stoichiometry. Deletion of the MOD leads to loss of tetramerization and of phosphorylation of IKKβ and IKKgamma;, although the kinase can still interact with the resultant IKKγ, monomers and dimers. Likewise, MOD-mediated IKKγ tetramerization is required to enhance IKKβ kinase activity when overexpressed in 293 cells and to reconstitute a lipopolysaccharide-responsive IKK complex in pre-B cells. These data thus suggest that IKKγ tetramerization enforces a spatial positioning of two kinase dimers to facilitate transautophosphorylation and activation. [ABSTRACT FROM AUTHOR]