The transcriptional activator nuclear factor kappa B (NF-B) is required for the upregulation of a large number of genes in response to inflammation, viral and bacterial infection, and other stress stimuli. Genes that respond to NF-B encode a variety of cytokines, cell adhesion molecules, and acute-phase response proteins as well as apoptotic suppressor and effector proteins. It is believed that this reprogramming of gene expression is essential for cell survival during situations of physiological crisis (61). The activation of NF-B in response to stimulation by the proinflammatory cytokines tumor necrosis factor alpha (TNF-âŁ) and interleukin 1 beta (IL-1â€) has been extensively studied (17, 30); however, the mechanisms that modulate and eventually limit these responses are still poorly understood (61).We report here that the recently discovered protein kinase inhibitor protein RKIP (Raf kinase inhibitor protein) acts to inhibit NF-B activation. RKIP was first identified as an interacting partner of Raf-1 and shown to function as a negative regulator of the mitogen-activated protein (MAP) kinase (MAPK) cascade initiated by 76). The Raf-1-initiated pathway is comprised of three sequentially acting protein kinases: a MAP kinase kinase kinase (MAPKKK), a MAP kinase kinase (MAPKK), and a MAPK. This basic relationship has now been found to be conserved in several protein kinase pathways. In the Raf-1 pathway the MAPK is ERK1/2 (extracellular signal-regulated kinase 1 and 2), the MAPKK is MEK1 (MAP/ERK kinase 1), and the MAPKKK is Raf-1 itself. Functional studies using both gain-of-function and loss-of-function approaches demonstrated that RKIP disrupts the interaction between 76). Depletion of endogenous RKIP upregulated Raf-1 kinase activity and MAPK signaling, whereas ectopic expression of RKIP suppressed Raf-1 kinase activity and MAPK signaling as well as v-Raf-mediated transformation. Biochemical studies showed that RKIP efficiently dissociated preformed Raf/MEK complexes and behaved kinetically as a competitive inhibitor of MEK phosphorylation. In vivo, the association of endogenous RKIP with Raf-1 correlated inversely with Raf-1 kinase activity during serum stimulation of quiescent cells.Active NF-B is a dimer that can be assembled from several members of the Rel family of transcription factors, and some form of NF-B is expressed in most cell types (61). In unstimulated cells, NF-B is retained in the cytoplasm in an inactive form bound to a family of inhibitory proteins known as IB (inhibitors of B). Activation of NF-B requires the phosphorylation and degradation of IB, which allows the NF-B dimer to translocate into the nucleus. Virtually all of the many stimuli that can activate NF-B cause the phosphorylation of IB on