Identification and characterization of an I kappa B kinase

Activation of the transcription factor NF-kappa B by tumor necrosis factor (TNF) and interleukin-1 (IL-1) requires the NF-kappa B-inducing kinase (NIK). In a yeast two-hybrid screen for NIK-interacting proteins, we have identified a protein kinase previously known as CHUK. Overexpression of CHUK activates a NF-kappa B-dependent reporter gene. A catalytically inactive mutant of CHUK is a dominant-negative inhibitor of TNF-, IL-1-, TRAF-, and NIK-induced NF-kappa B activation. CHUK associates with the NF-kappa B inhibitory protein, I kappa B-alpha, in mammalian cells. CHUK specifically phosphorylates I kappa B-alpha on both serine 32 and serine 36, modifications that are required for targeted degradation of I kappa B-alpha via the ubiquitin-proteasome pathway. This phosphorylation of I kappa B-alpha is greatly enhanced by NIK costimulation. Thus, CHUK is a NIK-activated I kappa B-alpha kinase that links TNF- and IL-1-induced kinase cascades to NF-kappa B activation.