Bridging the Gap: Composition, Regulation, and Physiological Function of the IκB Kinase Complex

Protein kinases that regulate the activity of specific transcription factors in response to extracellular stimuli not only are the subject of intense research but also are being chased as potential targets for development of new drugs for treatment of various human diseases. One such protein kinase is IKK, the IκB kinase that activates nuclear factor κB (NF-κB) through phosphorylation of IκB inhibitory proteins. In this review, we summarize the discovery of IKK and recent knowledge about its composition, regulation, and physiological functions. NF-κB transcription factors regulate the expression of a large number of genes that are necessary for proper functioning of the immune system and are key mediators of inflammatory responses to pathogens (2, 4, 5). NF-κB is also associated with cellular transformation and oncogenesis, and one of its most important, but lately discovered functions, is the activation of an antiapoptotic gene expression program (6, 29, 36, 38, 39, 42). As a transcription factor that orchestrates the inflammatory response, NF-κB is rapidly activated, independently of new protein synthesis, in response to signals produced during infection (e.g., bacterial endotoxins and viral double-stranded RNA) (for a review, see reference 3). NF-κB activation is also a transient response; this is of importance because many of the genes that are activated by NF-κB encode potentially toxic products such as tumor necrosis factor (TNF). The key to NF-κB regulation is the inhibitory κB (IκB) proteins which retain NF-κB in the cytoplasm (reviewed in reference 37). In response to diverse stimuli, IκBs are rapidly degraded and the freed NF-κB dimers translocate to the nucleus. Several years ago, it was established that the critical event which triggers the polyubiquitination and degradation of IκBs via the 26S proteasome is their stimulus-dependent phosphorylation at two serine residues (residues 32 and 36 in IκBα) that are located within their conserved N-terminal regulatory region (1, 10–12, 18, 20, 34, 40). The protein kinase that phosphorylates these regulatory sites remained elusive, and without detailed knowledge about its molecular identity, there was little progress towards a full understanding of the signaling pathways that control NF-κB activity. The initial hunt for such a protein kinase yielded many false candidates, such as protein kinase C, casein kinase II, and ribosomal S6 kinase (pp90rsk) (reviewed in reference 43). Although most of these kinases phosphorylate IκB proteins in the test tube on different serine, threonine, or tyrosine residues, none of them was found to phosphorylate the two regulatory sites that trigger the degradation of IκBs in a stimulus-dependent manner. A large-molecular-mass (700-kDa) protein kinase activity that phosphorylates IκBα on S32 and S36 in a ubiquitin-dependent manner was also detected in extracts of nonstimulated HeLa cells (13, 25). However, this activity was not reported to be stimulus dependent, and to date, its components and molecular identity are unknown. A careful consideration of IκB phosphorylation indicated that the physiological IκB kinase had to fulfill several criteria. Its activity should be stimulated by inducers of NF-κB with kinetics that are consistent with those of NF-κB activation, and it should phosphorylate both S32 and S36 in the N terminus of IκBα and both S19 and S23 in the N terminus of IκBβ. In addition, since substitution of threonines for these serines results in IκB mutants that are resistant to degradation, the physiological IκB kinase should be serine specific (18).