Identification and Characterization of an IκB Kinase

tor–associated factor (TRAF) family of adaptor proteins Activation of the transcription factor NF-kB by tumor as signal transducers (Rothe et al., 1995a; Cao et al., necrosis factor (TNF) and interleukin-1 (IL-1) requires 1996a; Hsu et al., 1996a). TRAF proteins associate dithe NF-kB-inducing kinase (NIK). In a yeast two-hybrid rectly with the cytoplasmic domain of several members screen for NIK-interacting proteins, we have identified of the TNF receptor family including the 75 kDa TNF a protein kinase previously known as CHUK. Overexreceptor (TNFR2), CD40, and CD30 (Hu et al., 1994; pression of CHUK activates a NF-kB-dependent reRothe et al., 1994; Cheng et al., 1995; Mosialos et al., porter gene. A catalytically inactive mutant of CHUK 1995; Gedrich et al., 1996; Lee et al., 1996). In addition, is a dominant-negative inhibitor of TNF-, IL-1-, TRAF-, TRAF proteins are recruited indirectly to the 55 kDa and NIK-induced NF-kB activation. CHUK associates TNF receptor (TNFR1) and the related DR3/Wsl by the with the NF-kB inhibitory protein, IkB-a, in mammalian receptor-associated adaptor protein TRADD (Chinnaicells. CHUK specifically phosphorylates IkB-a on both yan et al, 1996a; Hsu et al., 1996a; Kitson et al., 1996; serine 32 and serine 36, modifications that are required Shu et al., 1996). TNFR1and presumably DR3/Wslfor targeted degradation of IkB-a via the ubiquitinmediated NF-kB activation also require the serine-threoproteasome pathway. This phosphorylation of IkB-a nine kinase RIP (Stanger et al., 1995; Ting et al., 1996), is greatly enhanced by NIK costimulation. Thus, CHUK which associates with the respective receptor comis a NIK-activated IkB-a kinase that links TNFand ILplexes via TRADD and also interacts with TRAF proteins 1-induced kinase cascades to NF-kB activation. (Chinnaiyan et al., 1996a; Hsu et al., 1996b). Three of