Analysis of domains in the IKKα and IKKβ proteins that regulate their kinase activity

The I kappa B kinases IKK alpha and IKK beta are critical in activating the NF-kappa B pathway. Although these proteins have a similar structure that includes kinase, leucine zipper, and helix-loop-helix domains, they exhibit marked differences in their kinase activity and functional properties. For example, IKK beta has a 10-20-fold higher level of kinase activity for I kappa B alpha than does IKK alpha. Furthermore, disruption of the murine IKK beta gene, but not the IKK alpha gene, results in severe defects in activating the NF-kappa B pathway. Mice lacking IKK beta succumb to severe hepatic apoptosis because of failure to activate the NF-kappa B pathway, whereas mice deficient in IKK alpha exhibit skin and skeletal abnormalities and an embryonic lethal phenotype. To better characterize differences in the functional properties of these kinases, hybrid IKK proteins were constructed by domain swapping, and their kinase activity was assayed. These studies demonstrated that differences in the IKK alpha and IKK beta helix-loop-helix domains are primarily responsible for differences in their kinase activity. In contrast, their kinase and leucine zipper domains exhibited relatively conserved function. These studies further define the properties of IKK alpha and IKK beta, which are involved in their unique regulatory roles.