The α and β Subunits of IκB Kinase (IKK) Mediate TRAF2-Dependent IKK Recruitment to Tumor Necrosis Factor (TNF) Receptor 1 in Response to TNF

The activation of IkappaB kinase (IKK) is a key step in the nuclear translocation of the transcription factor NF-kappaB. IKK is a complex composed of three subunits: IKKalpha, IKKbeta, and IKKgamma (also called NEMO). In response to the proinflammatory cytokine tumor necrosis factor (TNF), IKK is activated after being recruited to the TNF receptor 1 (TNF-R1) complex via TNF receptor-associated factor 2 (TRAF2). We found that the IKKalpha and IKKbeta catalytic subunits are required for IKK-TRAF2 interaction. This interaction occurs through the leucine zipper motif common to IKKalpha, IKKbeta, and the RING finger domain of TRAF2, and either IKKalpha or IKKbeta alone is sufficient for the recruitment of IKK to TNF-R1. Importantly, IKKgamma is not essential for TNF-induced IKK recruitment to TNF-R1, as this occurs efficiently in IKKgamma-deficient cells. Using TRAF2(-/-) cells, we demonstrated that the TNF-induced interaction between IKKgamma and the death domain kinase RIP is TRAF2 dependent and that one possible function of this interaction is to stabilize the IKK complex when it interacts with TRAF2.