Activation of the I kappa B kinases by RIP via IKK gamma/NEMO-mediated oligomerization

To understand the mechanism of activation of the I kappaB kinase (IKK) comp lex in the tumor necrosis factor (TNF) receptor 1 pathway, we examined the possibility that oligomerization of the IKK complex triggered by ligand-ind uced trimerization of the TNF receptor 1 complex is responsible for activat ion of the IKKs. Gel filtration analysis of the IKK complex revealed that T NF alpha stimulation induces a large increase in the size of this complex, suggesting oligomerization. Substitution of the C-terminal region of IKK ga mma, which interacts with RIP, with a truncated DR4 lacking its cytoplasmic death domain, produced a molecule that could induce IKK and NF-kappaB acti vation in cells in response to TRAIL. Enforced oligomerization of the N ter minus of IKK gamma or truncated IKK alpha or IKK beta lacking their serine- cluster domains can also induce IKK and NF-kappaB activation. These data su ggest that IKK gamma functions as a signaling adaptor between the upstream regulators such as RIP and the IKKs and that oligomerization of the IKK com plex by upstream regulators is a critical step in activation of this comple x.