The Beginning of the End: IκB Kinase (IKK) and NF-κB Activation

Rel homology domain nuclear localization sequence tumor necrosis factor interleukin IκB kinase leucine zipper helix-loop-helix IKK complex-associated protein IKK kinase NF-κB/Rel proteins are dimeric, sequence-specific transcription factors that control a variety of important biological decisions from formation of dorsal-ventral polarity in insects to activation of inflammatory and innate immune responses (reviewed in Ref. 1Baeuerle P.A. Baltimore D. Cell. 1996; 87: 13-20Abstract Full Text Full Text PDF PubMed Scopus (2954) Google Scholar). NF-κB proteins are related through the Rel homology domain (RHD),1 which subjects them to a particular type of regulation, centered around nuclear-cytoplasmic shuttling (reviewed in Ref. 2Baldwin A.S. Annu. Rev. Immunol. 1996; 14: 649-681Crossref PubMed Scopus (5649) Google Scholar). The RHD serves several functions. It is the dimerization and DNA binding domain, and we have learned in atomic detail how RHDs dimerize and interact with DNA (3Chen F.E. Huang D-B. Chen Y.-Q. Ghosh G. Nature. 1998; 391: 410-413Crossref PubMed Scopus (342) Google Scholar). In addition, the RHD contains a nuclear localization sequence (NLS), and most importantly it is the site for binding of inhibitors of NF-κB, the IκBs (reviewed in Ref. 2Baldwin A.S. Annu. Rev. Immunol. 1996; 14: 649-681Crossref PubMed Scopus (5649) Google Scholar). The IκBs also form a small family with a core composed of six or more ankyrin repeats, an N-terminal regulatory domain, and a C-terminal domain that contains a PEST motif (reviewed in Ref. 2Baldwin A.S. Annu. Rev. Immunol. 1996; 14: 649-681Crossref PubMed Scopus (5649) Google Scholar). By binding to NF-κB dimers, the IκBs mask their NLS and cause their cytoplasmic retention. Some IκBs, such as IκBα, contain a nuclear export sequence and when combining with NF-κB dimers in the nucleus (which the IκBs can presumably enter by diffusion) cause their exportin-mediated transport to the cytoplasm (4Arenzana-Seisdedos F. Turpin P. Rodriguez M. Thomas D. Hay R.T. Virelizier J.L. Dargemont C. J. Cell Sci. 1997; 110: 369-378Crossref PubMed Google Scholar). Recently the three-dimensional structures of NF-κB·IκB ternary complexes (composed of the RHDs of p50 and p65 and the ankyrin repeat core of IκBα) were solved (5Huxford T. Huang D.B. Malek S. Ghosh G. Cell. 1998; 95: 759-770Abstract Full Text Full Text PDF PubMed Scopus (508) Google Scholar, 6Jacobs M.D. Harrison S.C. Cell. 1998; 95: 749-758Abstract Full Text Full Text PDF PubMed Scopus (713) Google Scholar). These fascinating structures indicate that the ankyrin repeats of IκBα form a slightly bent cylinder through a stacked arrangement of α-helices that compose their ankyrin repeats. The peptide loops that connect these helices make specific contacts with the two RHDs, whose N-terminal Ig-like repeats flank the IκB core; the C-terminal Ig-like repeats (responsible for dimerization) contact each other with the IκB cylinder lying on top of them. Although the structures solved by two independent groups differ on the way by which IκB masks the NLS located next to the C-terminal Ig-like repeats of the RHDs (5Huxford T. Huang D.B. Malek S. Ghosh G. Cell. 1998; 95: 759-770Abstract Full Text Full Text PDF PubMed Scopus (508) Google Scholar, 6Jacobs M.D. Harrison S.C. Cell. 1998; 95: 749-758Abstract Full Text Full Text PDF PubMed Scopus (713) Google Scholar) it is likely that the first two ankyrin repeats sterically hinder the binding of importins to the NLS of NF-κB. Initially, NF-κB was thought to be a B cell-specific transcription factor (1Baeuerle P.A. Baltimore D. Cell. 1996; 87: 13-20Abstract Full Text Full Text PDF PubMed Scopus (2954) Google Scholar). However, it was quickly recognized that NF-κB activity can be induced in most cell types upon treatment with phorbol esters, the proinflammatory cytokines, tumor necrosis factor (TNF), and interleukin 1 (IL-1) and bacterial endotoxin. Subsequently, the list of NF-κB inducers has grown to contain double-stranded (ds) RNA, viruses, and the Tax protein of HTLV-1. It was also recognized that upon cell stimulation with these inducers, NF-κB dimers translocate from the cytoplasm to the nucleus where they bind target genes and regulate their transcription. Subsequently, the nuclear translocation of NF-κB was found to parallel and depend on induced degradation of IκBs (reviewed in Refs. 2Baldwin A.S. Annu. Rev. Immunol. 1996; 14: 649-681Crossref PubMed Scopus (5649) Google Scholar and 7Verma I.M. Stevenson J.K. Schwarz E.M. Van Antwerp D. Miyamoto S. Genes Dev. 1995; 9: 2723-2735Crossref PubMed Scopus (1669) Google Scholar). Potent NF-κB activators can induce almost complete degradation of IκBs (especially IκBα) within minutes. This process, which is mediated by the 26 S proteasome (8Alkalay I. Yaron A. Hatzubai A. Jung S. Avraham A. Gerlitz O. Pashut-Lavon I. Ben-Neriah Y. Mol. Cell. Biol. 1995; 15: 1294-1301Crossref PubMed Google Scholar, 9DiDonato J.A. Mercurio F. Karin M. Mol. Cell. Biol. 1995; 15: 1302-1311Crossref PubMed Google Scholar), depends on phosphorylation of two conserved serines (Ser-32 and Ser-36 in IκBα) in the N-terminal regulatory domain of IκB (10Brown K. Gerstberger S. Carlson L. Franzoso G. Siebenlist U. Science. 1995; 267: 1485-1491Crossref PubMed Scopus (1332) Google Scholar, 11Chen Z. Hagler J. Palombella V.J. Melandri F. Scherer D. Ballard D. Maniatis T. Genes Dev. 1995; 9: 1586-1597Crossref PubMed Scopus (1178) Google Scholar, 12DiDonato J.A. Mercurio F. Rosette C. Wu-li J. Suyang H. Ghosh S. Karin M. Mol. Cell. Biol. 1996; 16: 1295-1304Crossref PubMed Google Scholar). Homologous serines are also required for degradation of the Drosophila IκB homolog, Cactus (13Reach M. Galindo R.L. Towb P. Allen J.L. Karin M. Wasserman S.A. Dev. Biol. 1996; 180: 353-364Crossref PubMed Scopus (101) Google Scholar). Even the substitution of a single serine can considerably inhibit IκB degradation. Furthermore, these serines cannot be replaced by threonine, indicating that the kinase that phosphorylates them is serine-specific (12DiDonato J.A. Mercurio F. Rosette C. Wu-li J. Suyang H. Ghosh S. Karin M. Mol. Cell. Biol. 1996; 16: 1295-1304Crossref PubMed Google Scholar). In the presence of proteasome inhibitors, N-terminally phosphorylated IκBα accumulates very rapidly, indicating that its phosphorylation precedes its degradation and does not result in dissociation from NF-κB (8Alkalay I. Yaron A. Hatzubai A. Jung S. Avraham A. Gerlitz O. Pashut-Lavon I. Ben-Neriah Y. Mol. Cell. Biol. 1995; 15: 1294-1301Crossref PubMed Google Scholar,9DiDonato J.A. Mercurio F. Karin M. Mol. Cell. Biol. 1995; 15: 1302-1311Crossref PubMed Google Scholar). Phosphorylated IκBs undergo without delay a second post-translational modification, polyubiquitination. The major acceptor sites for ubiquitin in IκBα are arginines 21 and 22, whose substitution with lysines considerably retards its degradation (12DiDonato J.A. Mercurio F. Rosette C. Wu-li J. Suyang H. Ghosh S. Karin M. Mol. Cell. Biol. 1996; 16: 1295-1304Crossref PubMed Google Scholar,14Scherer D.C. Brockman J. Chen Z. Maniatis T. Ballard D. Proc. Natl. Acad. Sci. U. S. A. 1995; 92: 11259-11263Crossref PubMed Scopus (503) Google Scholar). Polyubiquitination involves a cascade of enzymatic reactions, the first of which is ATP-dependent and catalyzed by E1 ubiquitin-activating enzyme to form an E1-ubiquitin thioester. The second reaction is catalyzed by the E2 ubiquitin-conjugating enzymes, which receive activated ubiquitin from E1. The last step in the cascade, the transfer of activated ubiquitin from the E2-ubiquitin intermediate to the substrate, is catalyzed by a third group of enzymes, the E3 ubiquitin-protein ligases (15Hershko A. Ciechanover A. Annu. Rev. Biochem. 1998; 67: 425-479Crossref PubMed Scopus (7091) Google Scholar). The E3 group is very heterogeneous, and most of its members are poorly characterized. Recently, a cell-free system that catalyzes the ubiquitination of N-terminally phosphorylated IκBα was established and used to show that the only regulated step in the IκB degradation pathway is the phosphorylation reaction (16Yaron A. Gonen H. Alkalay I. Hatzubai A. Jung S. Beyth S. Mercurio F. Manning A.M. Ciechanover A. BenNeriah Y. EMBO J. 1997; 16: 6486-6494Crossref PubMed Scopus (204) Google Scholar). By contrast, the ubiquitinating activity that specifically recognizes phosphorylated IκB is constitutive. Most importantly, Yaron et al. (17Yaron A. Hatzubai A. Davis M. Lavon I. Amit S. Manning A.M. Andersen J.S. Mann M. Mercurio F. Ben-Neriah Y. Nature. 1998; 396: 590-594Crossref PubMed Scopus (579) Google Scholar) have elegantly employed this cell-free system and cutting edge protein purification and sequence determination technology to molecularly identify the recognition component of the phospho-IκB-specific E3 activity. This protein, named E3RSIκB, is a member of the F-box/WD-repeat family (reviewed in Ref. 18Patton E.E. Willems A.R. Tyers M. Trends Genet. 1998; 14: 236-243Abstract Full Text Full Text PDF PubMed Scopus (446) Google Scholar). Interestingly, other members of this family, which contain an F-box and one or two WD or leucine-rich repeats, are essential components of E3 activities involved in regulated protein degradation (19Bai C. Sen P. Hofmann K. Ma L. Goebl M. Harper J.W. Elledge S.J. Cell. 1996; 86: 263-274Abstract Full Text Full Text PDF PubMed Scopus (1007) Google Scholar, 20Feldman R.M.R. Correll C.C. Kaplan K.B. Deshaies R.J. Cell. 1997; 91: 221-230Abstract Full Text Full Text PDF PubMed Scopus (723) Google Scholar, 21Skowyra D. Craig K.L. Tyers M. Elledge S.J. Harper J.W. Cell. 1997; 91: 209-219Abstract Full Text Full Text PDF PubMed Scopus (1040) Google Scholar). In the case of E3RSIκB, Cdc4, and Grr1, recognition of the phosphoamino acid embedded within a specific sequence is believed to be mediated by the WD repeats (17Yaron A. Hatzubai A. Davis M. Lavon I. Amit S. Manning A.M. Andersen J.S. Mann M. Mercurio F. Ben-Neriah Y. Nature. 1998; 396: 590-594Crossref PubMed Scopus (579) Google Scholar, 18Patton E.E. Willems A.R. Tyers M. Trends Genet. 1998; 14: 236-243Abstract Full Text Full Text PDF PubMed Scopus (446) Google Scholar). The F box, on the other hand, is responsible for binding to Skp1, which in turn binds to members of the Cullin family, such as Cdc53 (20Feldman R.M.R. Correll C.C. Kaplan K.B. Deshaies R.J. Cell. 1997; 91: 221-230Abstract Full Text Full Text PDF PubMed Scopus (723) Google Scholar, 21Skowyra D. Craig K.L. Tyers M. Elledge S.J. Harper J.W. Cell. 1997; 91: 209-219Abstract Full Text Full Text PDF PubMed Scopus (1040) Google Scholar). The Cullin subunit of the E3 complex appears to be responsible for recruitment of E2-ubiquitin onto the phosphorylated substrate (18Patton E.E. Willems A.R. Tyers M. Trends Genet. 1998; 14: 236-243Abstract Full Text Full Text PDF PubMed Scopus (446) Google Scholar). E3RSIκB is identical to β-TrCP, which was previously isolated via a two-hybrid screen as a protein that binds to the phosphorylated version of the HIV protein Vpu (22Margottin F. Bour S.P. Durand H. Selig L. Benichou S. Richard V. Thomas D. Strebel K. Benarous R. Mol. Cell. 1998; 1: 565-574Abstract Full Text Full Text PDF PubMed Scopus (581) Google Scholar). Phospho-Vpu binds CD4, a T cell membrane protein, to induce its ubiquitination and degradation. Curiously, Vpu contains a sequence very similar to the one surrounding the phosphoacceptor sites of IκBs (Fig. 1). The same sequence is also present in β-catenin, another protein whose abundance is regulated via a ubiquitin-dependent degradation pathway (23Aberle H. Bauer A. Stappert J. Kispert A. Kemler R. EMBO J. 1997; 16: 3797-3804Crossref PubMed Scopus (2204) Google Scholar). Furthermore, genetic analysis has shown that degradation of theDrosophila β-catenin homolog Armadillo depends on a homolog of E3RSIκB called Slimb (24Jiang J. Struhl G. Nature. 1998; 391: 493-496Crossref PubMed Scopus (542) Google Scholar). Thus, rather than serving as a recognition sequence for the IκB kinase, the conserved sequence surrounding the IκB N-terminal phosphoacceptor sites is a recognition site for E3RSIκB, whose binding to IκB is strictly dependent on phosphorylation of these sites (17Yaron A. Hatzubai A. Davis M. Lavon I. Amit S. Manning A.M. Andersen J.S. Mann M. Mercurio F. Ben-Neriah Y. Nature. 1998; 396: 590-594Crossref PubMed Scopus (579) Google Scholar). Indeed, the sequence similarity between the IκB and β-catenin phosphorylation sites led other investigators to examine and confirm the involvement of β-TrCP in IκB ubiquitination and degradation (25Winston J.T. Strack P. Beer-Romero P. Chu C.Y. Elledge S.J. Harper J.W. Genes Dev. 1999; 13: 270-283Crossref PubMed Scopus (827) Google Scholar). Coimmunoprecipitation experiments show that like other F box proteins, E3RSIκB also associates with Skp1 and Cul1 (25Winston J.T. Strack P. Beer-Romero P. Chu C.Y. Elledge S.J. Harper J.W. Genes Dev. 1999; 13: 270-283Crossref PubMed Scopus (827) Google Scholar). However, it remains to be established whether Cul1 rather than other Cullins is a physiological component of the E3IκB complex. In addition, the particular E2 that works in conjunction with E3IκB in vivo needs to be identified. The enzymes that catalyze the ubiquitination of phospho-IκB are constitutively active. Therefore the regulated step that dictates the fate of IκB is in most cases phosphorylation of the two N-terminal serines. As the E3IκB complex may also be involved in degradation of CD4 and β-catenin, the phosphorylation step is also the one responsible for specificity in this pathway. There are only two exceptions to this universal pathway for NF-κB activation. The first is activation of NF-κB in response to UV radiation, which although dependent on IκB degradation does not involve IκB phosphorylation at the N-terminal sites (26Bender K. Gottlicher M. Whiteside S. Rahmsdorf H.J. Herrlich P. EMBO J. 1998; 17: 5170-5181Crossref PubMed Scopus (202) Google Scholar, 27Li N. Karin M. Proc. Natl. Acad. Sci. U. S. A. 1998; 95: 13012-13017Crossref PubMed Scopus (406) Google Scholar). The second exception is anoxia, which stimulates phosphorylation of IκBα at tyrosine 42 (28Imbert V. Rupec R.A. Livolsi A. Pahl H.L. Traenckner E.B. Mueller-Dieckmann C. Farahifar D. Rossi B. Auberger P. Baeuerle P.A. Peyron J.F. Cell. 1996; 86: 787-798Abstract Full Text Full Text PDF PubMed Scopus (631) Google Scholar). The tyrosine-phosphorylated IκBα was suggested to bind to the SH2 domain of phosphatidylinositol 3-kinase, which yanks it away from NF-κB (29Beraud C. Henzel W.J. Baeuerle P.A. Proc. Natl. Acad. Sci. U. S. A. 1999; 96: 429-434Crossref PubMed Scopus (265) Google Scholar). Tyrosine 42, however, is not conserved in other IκBs, and therefore the universality of this pathway is questionable. The control of IκB phosphorylation in response to all other NF-κB activating stimuli rests on the shoulders of the IκB kinase (IKK) complex. Once it became clear that the key step in NF-κB activation was IκB phosphorylation, a search for a stimulus-responsive protein kinase catalyzing this event was initiated. This effort bore fruit when a protein kinase activity that is specific for the N-terminal regulatory serines of IκBs was identified (30DiDonato J.A. Hayakawa M. Rothwarf D.M. 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In other the subunit and not serves as the target for activators involved in proinflammatory that are to the complex via is also involved in of IKK activity. In to the activation is phosphorylated at its C-terminal which contains serines M. Hayakawa M. Chen Y. Karin M. Science. 1999; PubMed Scopus Google Scholar). of these sites depends on activity. experiments indicate that the C-terminal sites are involved in of kinase activity M. Hayakawa M. Chen Y. Karin M. Science. 1999; PubMed Scopus Google Scholar). of or of the C-terminal serines with in a whose activation than that of the substitution of the same sites with acid in a enzyme that can be on these a was to the of IKK activity Initially, the IKK complex is not phosphorylated on its In response to are activated and to the complex via This in phosphorylation of and activation of that only a small of IKK is activated through phosphorylation by However, through the activated subunit can the which can be or the case of a as as other IKK complexes through an Indeed, the of in is for its which depends on at the activation The activated IKK complexes the IκB of NF-κB·IκB their ubiquitin-dependent degradation and activation of NF-κB. the activated presumably the as undergo C-terminal This which is to be as a such that when at of the C-terminal serines are phosphorylated the enzyme a activity This of IKK by the has As the C-terminal sites are to the motif they may their on kinase activity by the of this domain and its with the kinase This of IKK is activated in a of the of to its via at the activation it is important to have an way to kinase activity and it to by a this IKK activation result in NF-κB activation by of and inflammatory As these can to NF-κB activation Karin M. N. J. 1997; PubMed Scopus Google Scholar), is a that in the of an way to IKK and NF-κB activities a proinflammatory result in a major such as Interestingly, IKK activation was in D. F. U. E. B. C. 1999; PubMed Scopus Google Scholar). This in NF-κB which these from of by and F. D. K. E. A. C. B. J. 1997; PubMed Scopus Google Scholar). In NF-κB and IKK activity may types of from J. 1997; PubMed Scopus Google Scholar). Thus, IKK a target for of The presence of two within the IKK complex is a that a and and and have identical each subunit may be responsible for phosphorylation of and may be to Initially, between and were and it was that the two have identical more suggested that and not is involved in IKK activation by proinflammatory stimuli M. Hayakawa M. Chen Y. Karin M. Science. 1999; PubMed Scopus Google Scholar). It therefore became important to a genetic to examine the of and This was through the of technology to in The first result by these experiments was the of The complete of in are within Y. V. M. P. R. Karin M. Science. 1999; PubMed Scopus Google Scholar). and a and most is their which is and of that the contain of almost they are their Y. V. M. P. R. Karin M. Science. 1999; PubMed Scopus Google Scholar). The major with the is and of more a and of the the most is in the of the the is to than the appears of the is to of at the (which is one cell In addition, appears to be a to such that of a the are by a of The the of in and that and are to the Y. V. M. P. R. Karin M. Science. 1999; PubMed Scopus Google Scholar). As such as the are an important for that control of the be to a in The second was that is not required for IKK activation by proinflammatory stimulation of or with or IKK activation and IκBα degradation were Y. V. M. P. R. Karin M. 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Thus, is required for activation of IKK and phosphorylation of Although these experiments rather for the and of and they a its as a component of the IKK complex composed of and or does it also as a kinase or a component of a complex to this analysis of IKK complexes in the cell type in which its Once the form of involved in is it be to how its activity is regulated and which of its a in these the it is to the in IKK and regulation, the rather of this important protein B. for and M. Y. and D. Rothwarf for