Raf kinase inhibitor protein interacts with NF-kappa B-inducing kinase andTAK1 and inhibits NF-kappa 13 activation

The Raf kinase inhibitor protein (RKIP) acts as a negative regulator of the mitogen-activated protein (MAP) kinase (MAPK) cascade initiated by Raf-1. RKIP inhibits the phosphorylation of MAP/extracellular signal-regulated kin ase 1 (MEK1) by Raf-1 by disrupting the interaction between these two kinas es. We show here that RKIP also antagonizes the signal transduction pathway s that mediate the activation of the transcription factor nuclear factor ka ppa B (NF-kappaB) in response to stimulation with tumor necrosis factor alp ha (TNF-alpha) or interleukin 1 beta. Modulation of RKIP expression levels affected NF-kappaB signaling independent of the MAPK pathway. Genetic epist asis analysis involving the ectopic expression of kinases acting in the NF- kappaB pathway indicated that RKIP acts upstream of the kinase complex that mediates the phosphorylation and inactivation of the inhibitor of NF-kappa B (I kappaB). In vitro kinase assays showed that RKIP antagonizes the activ ation of the I kappaB kinase (IKK) activity elicited by TNF-alpha. RKIP phy sically interacted with four kinases of the NF-KB activation pathway, NF-ka ppaB-inducing kinase, transforming growth factor beta-activated kinase 1, I KK alpha, and IKK beta. This mode of action bears striking similarities to the interactions of RKIP with Raf-1 and MEK1 in the MAPK pathway. Emerging data from diverse organisms suggest that RKIP and RKIP-related proteins rep resent a new and evolutionarily highly conserved family of protein kinase r egulators. Since the MAPK and NF-kappaB pathways have physiologically disti nct roles, the function of RKIP may be, in part, to coordinate the regulati on of these pathways.