Activation of the IκB Kinases by RIP via IKKγ/NEMO-mediated Oligomerization

To understand the mechanism of activation of the IκB kinase (IKK) complex in the tumor necrosis factor (TNF) receptor 1 pathway, we examined the possibility that oligomerization of the IKK complex triggered by ligand-induced trimerization of the TNF receptor 1 complex is responsible for activation of the IKKs. Gel filtration analysis of the IKK complex revealed that TNFα stimulation induces a large increase in the size of this complex, suggesting oligomerization. Substitution of the C-terminal region of IKKγ, which interacts with RIP, with a truncated DR4 lacking its cytoplasmic death domain, produced a molecule that could induce IKK and NF-κB activation in cells in response to TRAIL. Enforced oligomerization of the N terminus of IKKγ or truncated IKKα or IKKβ lacking their serine-cluster domains can also induce IKK and NF-κB activation. These data suggest that IKKγ functions as a signaling adaptor between the upstream regulators such as RIP and the IKKs and that oligomerization of the IKK complex by upstream regulators is a critical step in activation of this complex.