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

Abstract

The IkB kinases IKKα and IKKβ are critical in activating the NF-kB 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, IKKβ has a 16-20-fold higher level of kinase activity for IkBα than does IKKα. Furthemore, disruption of the murine IKKβ gene, but not the IKKα gene, results in severe defects in activating the NF-kB pathway, Mice lacking IKKβ succumb to severe hepatic apoptosis because of failure to activate the NF-kB pathway, whereas mice deficient in IKKα 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 IKKα and IKKβ 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 IKKα and IKKβ, which are involved in their unique regulatory roles.