Constitutive phosphorylation of IkappaBalpha by casein kinase II occurs preferentially at serine 293: requirement for degradation of free IkappaBalpha

Ikappakappa mediates NF-kappaB activation in human immunodeficiency virus-infected cells

Human monocytes and macrophages are persistent reservoirs of human immunodeficiency virus (HIV) type-1. Persistent HIV infection of these cells results in increased levels of NF-kappaB in the nucleus secondary to increased IkappaBalpha, IkappaBbeta, and IkappaBepsilon degradation, a mechanism postulated to regulate viral persistence. To characterize the molecular mechanisms regulating HIV-mediated degradation of IkappaB, we have sought to identify the regulatory domains of IkappaBalpha targeted by HIV infection. Using monocytic cells stably expressing different transdominant molecules of IkappaBalpha, we determined that persistent HIV infection of these cells targets the NH2 but not the COOH terminus of IkappaBalpha. Further analysis demonstrated that phosphorylation at S32 and S36 is necessary for HIV-dependent IkappaBalpha degradation and NF-kappaB activation. Of the putative N-terminal IkappaBalpha kinases, we demonstrated that the Ikappakappa complex, but not p90(rsk), is activated by HIV infection and mediates HIV-dependent NF-kappaB activation. Analysis of viral replication in cells that constitutively express IkappaBalpha negative transdominant molecules demonstrated a lack of correlation between virus-induced NF-kappaB (p65/p50) nuclear translocation and degree of viral persistence in human monocytes.

NF-kappaB-mediated self defense of macrophages faced with bacteria

NF-kappaB is a ubiquitous transcription factor that is extensively exploited by immune cells involved in host defense mechanisms. Macrophages participate in the first line of defense against microorganisms, but little is known about whether and how NF-kappaB is involved in the handling of microbes by macrophages. To explore this issue, NF-kappaB-inactive macrophages, NIKMAC(NR), were created by overexpression of a super-repressor mutant of IkappaB alpha. When co-cultured with Escherichia coli, the NIKMAC(NR) macrophages exhibited impairment of bactercidal activity. Microscopic analysis revealed that NIKMAC(NR) cells faced with bacteria underwent rapid and fulminant apoptosis. Similary, NIKMAC(NR) macrophages cultured in the presence of a bacterial component, lipopolysaccharide, showed dramatic apoptosis. Inhibition of RNA synthesis or protein synthesis failed to block the apoptosis of NIKMAC(NR) cells, indicating that macrophages possess a pre-existing, apoptotic pathway that can be triggered by bacteria. Apoptosis was not observed in NIKMAC(NR) macrophages exposed to non-microbial stimuli including phorbol ester and opsonized zymosan. However, NIKMAC(NR) cells were more susceptible to apoptosis triggered by TNF-alpha and reactive oxygen intermediates, both of which are produced abundantly by macrophages when faced with bacteria. These data suggest a critical role for NF-kappaB in the survival of macrophages at the site of bacterial infection.

IkappaB-mediated inhibition of virus-induced beta interferon transcription

We have examined the consequences of overexpression of the IkappaBalpha and IkappaBbeta inhibitory proteins on the regulation of NF-kappaB-dependent beta interferon (IFN-beta) gene transcription in human cells after Sendai virus infection. In transient coexpression studies or in cell lines engineered to express different forms of IkappaB under tetracycline-inducible control, the IFN-beta promoter (-281 to +19) linked to the chloramphenicol acetyltransferase reporter gene was differentially inhibited in response to virus infection. IkappaBalpha exhibited a strong inhibitory effect on virus-induced IFN-beta expression, whereas IkappaBbeta exerted an inhibitory effect only at a high concentration. Despite activation of the IkappaB kinase complex by Sendai virus infection, overexpression of the double-point-mutated (S32A/S36A) dominant repressors of IkappaBalpha (TD-IkappaBalpha) completely blocked IFN-beta gene activation by Sendai virus. Endogenous IFN-beta RNA production was also inhibited in Tet-inducible TD-IkappaBalpha-expressing cells. Inhibition of IFN-beta expression directly correlated with a reduction in the binding of NF-kappaB (p50-RelA) complex to PRDII after Sendai virus infection in IkappaBalpha-expressing cells, whereas IFN-beta expression and NF-kappaB binding were only slightly reduced in IkappaBbeta-expressing cells. These experiments demonstrate a major role for IkappaBalpha in the regulation of NF-kappaB-induced IFN-beta gene activation and a minor role for IkappaBbeta in the activation process.

A conserved acidic motif in the N-terminal domain of nitrate reductase is necessary for the inactivation of the enzyme in the dark by phosphorylation and 14-3-3 binding

It has previously been shown that the N-terminal domain of tobacco (Nicotiana tabacum) nitrate reductase (NR) is involved in the inactivation of the enzyme by phosphorylation, which occurs in the dark (L. Nussaume, M. Vincentz, C. Meyer, J.P. Boutin, and M. Caboche [1995] Plant Cell 7: 611-621). The activity of a mutant NR protein lacking this N-terminal domain was no longer regulated by light-dark transitions. In this study smaller deletions were performed in the N-terminal domain of tobacco NR that removed protein motifs conserved among higher plant NRs. The resulting truncated NR-coding sequences were then fused to the cauliflower mosaic virus 35S RNA promoter and introduced in NR-deficient mutants of the closely related species Nicotiana plumbaginifolia. We found that the deletion of a conserved stretch of acidic residues led to an active NR protein that was more thermosensitive than the wild-type enzyme, but it was relatively insensitive to the inactivation by phosphorylation in the dark. Therefore, the removal of this acidic stretch seems to have the same effects on NR activation state as the deletion of the N-terminal domain. A hypothetical explanation for these observations is that a specific factor that impedes inactivation remains bound to the truncated enzyme. A synthetic peptide derived from this acidic protein motif was also found to be a good substrate for casein kinase II.

All three IkappaB isoforms and most Rel family members are stably associated with the IkappaB kinase 1/2 complex

Nuclear factor kappa B (NF-kappaB) is an important transcription factor for the genes of many pro-inflammatory proteins and is strongly activated by the cytokines interleukin-1 and tumor necrosis factor (TNF)alpha under various pathological conditions. In nonstimulated cells, NF-kappaB is present in the cytosol where it is complexed to its inhibitor IkappaB. Activation of NF-kappaB depends on the signal-induced phosphorylation of IkappaB by specific IkappaB kinases which initiates the inhibitor's conjugation to ubiquitin and subsequent degradation by the proteasome. We used both TNF-stimulated and okadaic-acid-stimulated HeLa cells to purify three biochemically distinct kinase activities targeting one or both of the two serines (S32 and S36) in IkappaBalpha which induce its rapid degradation upon cytokine stimulation. All three activities correspond to known IkappaB kinases: the mitogen-activated 90 kDa ribosomal S6 kinase (p90rsk1), the IkappaB kinase 1/2 complex (IKK1/2) and casein kinase II (CK II). However, we found that only one of the activities, namely the IKK1/2 complex, exists as a pre-assembled kinase-substrate complex in which the IKKs are directly or indirectly associated with several NF-kappaB-related and IkappaB-related proteins: RelA, RelB, cRel, p100, p105, Ikappa Balpha, Ikappa Bbeta and Ikappa Bepsilon. The existence of stable kinase-substrate complexes, the presence of all three known IkappaB isoforms in these complexes and our observation that the IKK complex is capable of phosphorylating Ikappa Balpha-, Ikappa Bbeta- and Ikappa Bepsilon-derived peptides at the respective degradation-relevant serines suggests that the IKK complex exerts a broad regulatory role for the activation of different NF-kappaB species. In contrast to previous studies, which locate CK II phosphorylation sites exclusively to the C-terminal PEST sequence of Ikappa Balpha, we observed efficient phosphorylation of serine 32 in Ikappa Balpha by the purified endogenous CK II complex. Therefore, both p90rsk1 and CK II have the same preference for phosphorylating only one of the two serines which are relevant for inducible degradation.

The crystal structure of the IkappaBalpha/NF-kappaB complex reveals mechanisms of NF-kappaB inactivation

IkappaBalpha regulates the transcription factor NF-kappaB through the formation of stable IkappaBalpha/NF-kappaB complexes. Prior to induction, IkappaBalpha retains NF-kappaB in the cytoplasm until the NF-kappaB activation signal is received. After activation, NF-kappaB is removed from gene promoters through association with nuclear IkappaBalpha, restoring the preinduction state. The 2.3 A crystal structure of IkappaBalpha in complex with the NF-kappaB p50/p65 heterodimer reveals mechanisms of these inhibitory activities. The presence of IkappaBalpha allows large en bloc movement of the NF-kappaB p65 subunit amino-terminal domain. This conformational change induces allosteric inhibition of NF-kappaB DNA binding. Amino acid residues immediately preceding the nuclear localization signals of both NF-kappaB p50 and p65 subunits are tethered to the IkappaBalpha amino-terminal ankyrin repeats, impeding NF-kappaB from nuclear import machinery recognition.

Ikappa Balpha functions through direct contacts with the nuclear localization signals and the DNA binding sequences of NF-kappaB

We have determined the binding energies of complexes formed between Ikappa Balpha and the wild type and mutational variants of three different Rel/NF-kappaB dimers, namely, the p50/p65 heterodimer and homodimers of p50 and p65. We show that although a common mode of interaction exists between the Rel/NF-kappaB dimers and Ikappa Balpha, IkappaB alpha binds the NF-kappaB p50/p65 heterodimer with 60- and 27-fold higher affinity than the p50 and p65 homodimers, respectively. Each of the three flexibly linked segments of the rel homology region of Rel/NF-kappaB proteins (the nuclear localization sequence, the dimerization domain, and the amino-terminal DNA binding domain) is directly engaged in forming the protein/protein interface with the ankyrin repeats and the carboxyl-terminal acidic tail/PEST sequence of Ikappa Balpha. In the cell, Ikappa Balpha functions to retain NF-kappaB in the cytoplasm and inhibit its DNA binding activity. These properties are a result of the direct involvement of the nuclear localization sequences and of the DNA binding region of NF-kappaB in complex with Ikappa Balpha. A model of the interactions in the complex is proposed based on our observations and the crystal structures of Rel/NF-kappaB dimers and the ankyrin domains of related proteins.

Regulation of erythroid Krüppel-like factor (EKLF) transcriptional activity by phosphorylation of a protein kinase casein kinase II site within its interaction domain

Erythroid Krüppel-like factor (EKLF) is a red cell-specific activator whose presence is crucial for establishing high levels of adult beta-globin expression in definitive cells during erythroid ontogeny. However, its simple presence within the erythroid lineage is not sufficient to activate the beta-globin promoter. One explanation that may account for this is that post-translational modification of EKLF differs within erythroid cell populations and regulates its activity. We have therefore addressed whether phosphorylation plays a role in modulating EKLF action. First, in vivo analyses implicate serine/threonine kinases as important players in the terminal differentiation of MEL cells, and demonstrate that EKLF is phosphorylated at serine and threonine residues within its transactivation region. Second, directed disruption of a protein kinase casein kinase (CK) II site, located within the EKLF interaction domain, abolishes EKLF transactivation and in vivo competition activity. Third, in vitro assays demonstrate that CKIIalpha interacts with EKLF, and that the EKLF interaction domain is phosphorylated by CKII only at Thr-41; however, the CKII-site mutant is not phosphorylated. Finally, the transactivation capability of EKLF is augmented by co-transfection of CKIIalpha. We conclude that EKLF is a phosphoprotein whose ability to transcriptionally activate an adjacent promoter is critically dependent on the phosphorylation status of a specific site located within the EKLF interaction domain, and that serine/threonine kinases play an important role in this process.

Sequential DNA damage-independent and -dependent activation of NF-kappaB by UV

NF-kappaB activation in response to UV irradiation of HeLa cells or of primary human skin fibroblasts occurs with two overlapping kinetics but totally different mechanisms. Although both mechanisms involve induced dissociation of NF-kappaB from IkappaBalpha and degradation of IkappaBalpha, targeting for degradation and signaling are different. Early IkappaBalpha degradation at 30 min to approximately 6 h is not initiated by UV-induced DNA damage. It does not require IkappaB kinase (IKK), as shown by introduction of a dominant-negative kinase subunit, and does not depend on the presence of the phosphorylatable substrate, IkappaBalpha, carrying serines at positions 32 and 36. Induced IkappaBalpha degradation requires, however, intact N- (positions 1-36) and C-terminal (positions 277-287) sequences. IkappaB degradation and NF-kappaB activation at late time points, 15-20 h after UV irradiation, is mediated through DNA damage-induced cleavage of IL-1alpha precursor, release of IL-1alpha and autocrine/paracrine action of IL-1alpha. Late-induced IkappaBalpha requires the presence of Ser32 and Ser36. The late mechanism indicates the existence of signal transfer from photoproducts in the nucleus to the cytoplasm. The release of the 'alarmone' IL-1alpha may account for some of the systemic effects of sunlight exposure.

Casein kinase II interacts with the bZIP domains of several transcription factors

Casein kinase II (CKII) is thought to regulate a broad range of transcription factors, but its mode of action is not well characterized. We previously showed that CKII is co-purified with the ATF family of transcription factors using DNA-affinity latex beads. Here we report a functional and physical interaction between CKII and transcription factors. We demonstrate that CKII binds through its catalytic alpha and alpha' subunits to the basic leucine zipper (bZIP) DNA-binding domains of many transcription factors, including ATF1. Kinetic analysis using a surface plasmon resonance sensor suggests that CKII loosely associates with ATF1 in vivo . Deletion of the bZIP domain of ATF1 markedly reduces its phosphorylation by CKII, suggesting that the bZIP recruits CKII to the vicinity of the target site. ATF1-CKII complex is also formed on DNA. Using CKIIalpha fusedto a heterologous DNA-binding domain, we also demonstrate that CKII, when bound to DNA, efficiently phosphorylates its substrate, which is bound to the same DNA molecule. Taken together, CKII may regulate transcription (and possibly other events) by phosphorylating proteins on DNA.

NF-kappaB-mediated inhibition of apoptosis is required for encephalomyocarditis virus virulence: a mechanism of resistance in p50 knockout mice

Apoptosis is a central host defense mechanism to eliminate virus-infected cells. Activation of NF-kappaB suppresses apoptosis following some types of stimulation in vitro. To test the physiological importance of this pathway in vivo, we studied murine encephalomyocarditis virus (EMCV) infection in mice and cell lines defective in NF-kappaB1 (p50) signaling. As previously reported, we find that all p50 knockout (p50 -/-) mice survive an EMCV infection that readily kills normal mice. By introducing the p50 mutation into interferon (IFN) type I receptor knockout (IFNRI -/-) mice, we find that this resistance is not mediated by IFN-beta as previously thought. While no IFNRI -/- mice survive, the double-knockout mice survive 60% of the time. The survival is tightly linked to the animals' ability to clear the virus from the heart in vivo. Using murine embryonic fibroblasts (MEF) derived from wild-type, p50 -/-, and p65 -/- embryos, we found that NF-kappaB is not required for the replication cycle of EMCV. However, during these experiments we observed that p50 -/- and p65 -/- MEF infected with EMCV undergo enhanced, premature cytotoxicity. Upon examination of this cell death, we found that EMCV infection induced both plasma membrane and nuclear changes typical of apoptosis in all cell lines. These apoptotic processes occurred in an accelerated and pronounced way in the NF-kappaB-defective cells, as soon as 6 h after infection, when virus is beginning to be released. Previously, only the RelA (p65) subunit of NF-kappaB has been shown to play a role in suppressing apoptosis. In our studies, we find that p50 is equally important in suppressing apoptosis during EMCV infection. Additionally, we show that suppression of apoptosis by NF-kappaB1 is required for EMCV virulence in vivo. The attenuation in p50 -/- mice can be explained by rapid apoptosis of infected cells which allows host phagocytes to clear infected cells before the viral burst leading to a reduction of the viral burden and survival of the mice.

NF-kappa B and Rel proteins: evolutionarily conserved mediators of immune responses

The transcription factor NF-kappa B, more than a decade after its discovery, remains an exciting and active area of study. The involvement of NF-kappa B in the expression of numerous cytokines and adhesion molecules has supported its role as an evolutionarily conserved coordinating element in the organism's response to situations of infection, stress, and injury. Recently, significant advances have been made in elucidating the details of the pathways through which signals are transmitted to the NF-kappa B:I kappa B complex in the cytosol. The field now awaits the discovery and characterization of the kinase responsible for the inducible phosphorylation of I kappa B proteins. Another exciting development has been the demonstration that in certain situations NF-kappa B acts as an anti-apoptotic protein; therefore, elucidation of the mechanism by which NF-kappa B protects against cell death is an important goal. Finally, the generation of knockouts of members of the NF-kappa B/I kappa B family has allowed the study of the roles of these proteins in normal development and physiology. In this review, we discuss some of these recent findings and their implications for the study of NF-kappa B.

The multisubunit IkappaB kinase complex shows random sequential kinetics and is activated by the C-terminal domain of IkappaB alpha

The multisubunit IkappaB kinase (IKK) catalyzes the signal-inducible phosphorylation of N-terminal serines of IkappaB. This phosphorylation is the key step in regulating the subsequent ubiquitination and proteolysis of IkappaB, which then releases NF-kappaB to promote gene transcription. As measured by 33P incorporation into a GST-IkappaB alpha fusion protein, varying both the concentration of GST-IkappaB alpha and [gamma-33P]ATP resulted in a kinetic pattern consistent with a random, sequential binding mechanism. Values of 55 nM and 7 microM were obtained for the dissociation constants of GST-IkappaB alpha and ATP, respectively. The value of alpha, a factor by which binding of one substrate changes the dissociation constant for the other substrate, was determined to be 0.11. This indicates that the two substrates bind in a cooperative fashion. Peptides corresponding to either amino acids 26-42 (N-terminal peptide) or amino acids 279-303 (C-terminal peptide) of IkappaB alpha inhibited the IKK-catalyzed phosphorylation of GST-IkappaB alpha; the C-terminal peptide, unexpectedly, was more potent. The inhibition by the C-terminal peptide was competitive with respect to GST-IkappaB alpha and mixed with respect to ATP, which verified the sequential binding mechanism. The C-terminal peptide was also a substrate for the enzyme, and a dissociation constant of 2.9-6.2 microM was obtained. Additionally, the N-terminal peptide was a substrate (Km = 140 microM). Competitive inhibition of the IKK-catalyzed phosphorylation of the C-terminal peptide by the N-terminal peptide indicated that the peptides are phosphorylated by the same active site. Surprisingly, the presence of the C-terminal peptide greatly accelerated the rate of phosphorylation of the N-terminal peptide as represented by a 160-fold increase in the apparent second-order rate constant (kcat/Km). These results are consistent with an allosteric site present within IKK that recognizes the C terminus of IkappaB alpha and activates the enzyme. This previously unobserved interaction with the C terminus may represent an important mechanism by which the enzyme recognizes and phosphorylates IkappaB.

Proteasome-independent activation of nuclear factor kappaB in cytoplasmic extracts from human endothelial cells by Rickettsia rickettsii

Interaction of many infectious agents with eukaryotic host cells is known to cause activation of the ubiquitous transcription factor nuclear factor kappaB (NF-kappaB) (U. Siebenlist, G. Franzoso, and K. Brown, Annu. Rev. Cell Biol. 10:405-455, 1994). Recently, we reported a biphasic pattern of NF-kappaB activation in cultured human umbilical vein endothelial cells consequent to infection with Rickettsia rickettsii, an obligate intracellular gram-negative bacterium and the etiologic agent of Rocky Mountain spotted fever (L. A. Sporn, S. K. Sahni, N. B. Lerner, V. J. Marder, D. J. Silverman, L. C. Turpin, and A. L. Schwab, Infect. Immun. 65:2786-2791, 1997). In the present study, we describe activation of NF-kappaB in a cell-free system, accomplished by addition of partially purified R. rickettsii to endothelial cell cytoplasmic extracts. This activation was rapid, reaching maximal levels at 60 min, and was dependent on the number of R. rickettsii organisms added. Antibody supershift assays using monospecific antisera against NF-kappaB subunits (p50 and p65) confirmed the authenticity of the gel-shifted complexes and identified both p50-p50 homodimers and p50-p65 heterodimers as constituents of the activated NF-kappaB pool. Activation occurred independently of the presence of endothelial cell membranes and was not inhibited by removal of the endothelial cell proteasome. Lack of involvement of the proteasome was further confirmed in assays using the peptide-aldehyde proteasome inhibitor MG 132. Activation was not ATP dependent since no change in activation resulted from addition of an excess of the unhydrolyzable ATP analog ATPgammaS, supplementation with exogenous ATP, or hydrolysis of endogenous ATP with ATPase. Furthermore, Western blot analysis before and after in vitro activation failed to demonstrate phosphorylation of serine 32 or degradation of the cytoplasmic pool of IkappaB alpha. This lack of IkappaB alpha involvement was supported by the finding that R. rickettsii can induce NF-kappaB activation in cytoplasmic extracts prepared from T24 bladder carcinoma cells and human embryo fibroblasts stably transfected with a superrepressor phosphorylation mutant of IkappaB alpha, rendering NF-kappaB inactivatable by many known signals. Thus, evidence is provided for a potentially novel NF-kappaB activation pathway wherein R. rickettsii may interact with and activate host cell transcriptional machinery independently of the involvement of the proteasome or known signal transduction pathways.

Distinct domains of IkappaBalpha regulate c-Rel in the cytoplasm and in the nucleus

IkappaBalpha is a critical regulator of Rel/NF-KB-mediated gene activation. It controls the induction of NF-KB factors by retaining them in the cytoplasm and also functions in the nucleus to terminate the induction process. In this study, we show that IkappaBalpha regulates the transcriptional activity of c-Rel in the nuclear compartment. We also demonstrate that discrete functional domains of IkappaBalpha are responsible for the cytoplasmic and nuclear regulation of c-Rel. We show that the determinants for the cytoplasmic regulation of c-Rel reside in the N-terminal and central ankyrin regions of IkappaBalpha and that the N-terminal domain of IkappaBalpha is required to mask the c-Rel nuclear localization signal. Importantly, IkappaBalpha sequences necessary to regulate c-Rel in the nucleus map to its central ankyrin domain and to a few negatively charged amino acids that immediately follow in the C-terminal IkappaBalpha PEST domain. The mapping of the IkappaBalpha determinants that control the cytoplasmic and nuclear activities of c-Rel to specific regions of the molecule suggests that IkappaBalpha inhibitors could be designed to antagonize Rel/NF-kappaB activity in different subcellular compartments or at defined stages of activation.

Inhibition of human immunodeficiency virus type 1 replication by a bioavailable serine/threonine kinase inhibitor, fasudil hydrochloride

Replication of human immunodeficiency virus type 1 (HIV-1) is regulated by a host transcription factor, nuclear factor kappaB (NF-kappaB). NF-kappaB belongs to a group of inducible transcription factors and its activity is regulated by multiple cellular signal transduction pathways, including kinases. These kinases are known to be involved in signal-induced NF-kappaB activation and in the induction of HIV-1 gene expression from latently infected cells. In this study we have examined the effect of a newly developed serine/threonine kinase inhibitor, fasudil hydrochloride (FH), on the replication of HIV-1. Although FH was initially developed as a compound that inhibited a myosin light chain kinase (MLCK) and had been approved for clinical use in the treatment of vasospasm after subarachnoid hemorrhage, this study shows its efficacy in blocking HIV-1 replication in latently infected patients. When FH was added to monocytic cell lines latently infected with HIV-1, U1 and OM10.1, the induction of HIV-1 replication by TNF-alpha was blocked at noncytotoxic doses. The IC50 values of HIV-1 induction by FH were 9.3 and 24 microM for U1 and OM10.1, respectively. Because FH could block TNF-alpha-induced, NF-kappaB-dependent gene expression, as examined by the transient luciferase expression assay, the effect of FH was considered to be due to the blocking of the signal transduction pathway of NF-kappaB activation. Although the in vivo effect of FH in blocking HIV-1 induction is not yet known, these findings indicate the feasibility of clinical use of FH and its derivatives in decreasing viral load to prevent clinical development of acquired immunodeficiency syndrome (AIDS) among HIV-1-infected individuals.

Autoinhibition of casein kinase I epsilon (CKI epsilon) is relieved by protein phosphatases and limited proteolysis

Casein kinase I epsilon (CKI epsilon) is a member of the CKI gene family, members of which are involved in the control of SV40 DNA replication, DNA repair, and cell metabolism. The mechanisms that regulate CKI epsilon activity and substrate specificity are not well understood. We report that CKI epsilon, which contains a highly phosphorylated 123-amino acid carboxyl-terminal extension not present in CKI alpha, is substantially less active than CKI alpha in phosphorylating a number of substrates including SV40 large T antigen and is unable to inhibit the initiation of SV40 DNA replication. Two mechanisms for the activation of CKI epsilon have been identified. First, limited tryptic digestion of CKI epsilon produces a protease-resistant amino-terminal 39-kDa core kinase with several-fold enhanced activity. Second, phosphatase treatment of CKI epsilon activates CKI epsilon 5-20-fold toward T antigen. Similar treatment of a truncated form of CKI epsilon produced only a 2-fold activation. Notably, this activation was transient; reautophosphorylation led to a rapid down-regulation of the kinase within 5 min. Phosphatase treatment also activated CKI epsilon toward the novel substrates I kappa B alpha and Ets-1. These mechanisms may serve to regulate CKI epsilon and related forms of CKI in the cell, perhaps in response to DNA damage.

Novel IkappaB alpha proteolytic pathway in WEHI231 immature B cells

The Rel/NF-kappaB family of transcription factors is sequestered in the cytoplasm of most mammalian cells by inhibitor proteins belonging to the IkappaB family. Degradation of IkappaB by a phosphorylation-dependent ubiquitin-proteasome (inducible) pathway is believed to allow nuclear transport of active Rel/NF-kappaB dimers. Rel/NF-kappaB (a p50-c-Rel dimer) is constitutively nuclear in murine B cells, such as WEHI231 cells. In these cells, p50, c-Rel, and IkappaB alpha are synthesized at high levels but only IkappaB alpha is rapidly degraded. We have examined the mechanism of IkappaB alpha degradation and its relation to constitutive p50-c-Rel activation. We demonstrate that all IkappaB alpha is found complexed with c-Rel protein in the cytoplasm. Additionally, rapid IkappaB alpha proteolysis is independent of but coexistent with the inducible pathway and can be inhibited by calcium chelators and some calpain inhibitors. Conditions that prevent degradation of IkappaB alpha also inhibit nuclear p50-c-Rel activity. Furthermore, the half-life of nuclear c-Rel is much shorter than that of the cytoplasmic form, underscoring the necessity for its continuous nuclear transport to maintain constitutive p50-c-Rel activity. We observed that IkappaB beta, another NF-kappaB inhibitor, is also complexed with c-Rel but slowly degraded by a proteasome-dependent process in WEHI231 cells. In addition, IkappaB beta is basally phosphorylated and cytoplasmic. We thus suggest that calcium-dependent IkappaB alpha proteolysis maintains nuclear transport of a p50-c-Rel heterodimer which in turn activates the synthesis of IkappaB alpha, p50, and c-Rel to sustain this dynamic process in WEHI231 B cells.

A PEST-like sequence mediates phosphorylation and efficient ubiquitination of yeast uracil permease

Uptake of uracil by the yeast Saccharomyces cerevisiae is mediated by a specific permease encoded by the FUR4 gene. Uracil permease located at the cell surface is subject to two covalent modifications: phosphorylation and ubiquitination. The ubiquitination step is necessary prior to permease endocytosis and subsequent vacuolar degradation. Here, we demonstrate that a PEST-like sequence located within the cytoplasmic N terminus of the protein is essential for uracil permease turnover. Internalization of the transporter was reduced when some of the serines within the region were converted to alanines and severely impaired when all five serines within the region were mutated or when this region was absent. The phosphorylation and degree of ubiquitination of variant permeases were inversely correlated with the number of serines replaced by alanines. A serine-free version of this sequence was very poorly phosphorylated, and elimination of this sequence prevented ubiquitination. Thus, it appears that the serine residues in the PEST-like sequence are required for phosphorylation and ubiquitination of uracil permease. A PEST-like sequence in which the serines were replaced by glutamic acids allowed efficient permease turnover, suggesting that the PEST serines are phosphoacceptors.

A role for CKII phosphorylation of the cactus PEST domain in dorsoventral patterning of the Drosophila embryo

Regulated proteolysis of Cactus, the cytoplasmic inhibitor of the Rel-related transcription factor Dorsal, is an essential step in patterning of the Drosophila embryo. Signal-induced Cactus degradation frees Dorsal for nuclear translocation on the ventral and lateral sides of the embryo, establishing zones of gene expression along the dorsoventral axis. Cactus stability is regulated by amino-terminal serine residues necessary for signal responsiveness, as well as by a carboxy-terminal PEST domain. We have identified Drosophila casein kinase II (CKII) as a Cactus kinase and shown that CKII specifically phosphorylates a set of serine residues within the Cactus PEST domain. These serines are phosphorylated in vivo and are required for wild-type Cactus activity. Conversion of these serines to alanine or glutamic acid residues differentially affects the levels and activity of Cactus in embryos, but does not inhibit the binding of Cactus to Dorsal. Taken together, these data indicate that wild-type axis formation requires CKII-catalyzed phosphorylation of the Cactus PEST domain.

Phosphorylation of A170 stress protein by casein kinase II-like activity in macrophages

A170 is an oxidative stress-inducible protein having a Zinc finger domain, two PEST sequences, and many potential phosphorylation sites for serine/threonine kinases. These structural features suggest that the phosphorylation of A170 affects its function and degradation. We have found that A170 is phosphorylated in cultured murine peritoneal macrophages. In addition, using recombinant A170 proteins, we found two proteins of 40 and 44 kDa with kinase activity in cell extracts using an in-gel kinase assay. We compared the properties of the intrinsic A170 kinases with those of mitogen-activated protein kinase (ERK 2), protein kinase A (PKA), casein kinase II (CK II), and protein kinase C, since their catalytic subunits have molecular masses similar to A170 kinases. ERK 2, CK II, and PKA phosphorylated recombinant A170 as a substrate. The 40 and 44 kDa kinases present in the macrophage extract were similar to alpha and alpha' subunits of CK II in respect to substrate specificity, pharmacological properties, immuno-reactivities, and ubiquitous expression in tissues.

Two putative protein kinase CK2 phosphorylation sites are important for Myf-5 activity

Myf-5, a member of a family of muscle-specific transcription factors, is important for myogenic cell determination and differentiation. Here, we report that Myf-5 protein constitutes a substrate for phosphorylation in vitro by protein kinase CK2. We identified two potential phosphorylation sites at serine49 and serine133, both of which seem to be necessary for Myf-5 activity. Mutants which can no longer be phosphorylated fail to transactivate E-box-dependent reporter genes and act as trans-dominant repressors of wild-type Myf-5. Normal activity can be restored by replacing the serine residues with glutamate suggesting that a negative charge at these sites is obligatory for Myf-5 activity. Although serine133 is part of helix 2 which mediates dimerization, we find no evidence for impaired DNA-binding or heterodimerization of the Ser-Ala133 mutant. Some serine49 mutations exhibit reduced nuclear localization and/or protein stability. Our data suggest that CK2-mediated phosphorylation of Myf-5 is required for Myf-5 activity.

Regulation of NF-kappa B activity by I kappa B alpha and I kappa B beta stability

Transcription factor NF-kappa B must be released from cytoplasmic inhibitory molecules (I kappa Bs) in order to move to the nucleus and to activate its target genes. Little is known about the mechanisms regulating the maintenance of constitutive nuclear NF-kappa B in some cell-types and of sustained nuclear NF-kappa B activity after stimulation. Increased turnover has been implicated in the regulation of constitutive NF-kappa B activity in mature B cells. We therefore compared the turnover of I kappa B alpha and I kappa B beta in mature B cells and HeLa cells. Both proteins display a high turnover in B cells although I kappa B beta is considerably more stable than I kappa B alpha. The half-life of both inhibitors is increased in HeLa cells. In contrast, all other NF-kappa B/I kappa B molecules tested are relatively stable in both cell-types. The elevated turnover of endogenous I kappa B alpha in Namalwa cells is inhibited by a proteasome inhibitor and thus seems to be driven by the same degradation machinery as the slower turnover in non-B cells. Furthermore, we investigated the processes involved in persistent activation of NF-kappa B. TNF-alpha signaling leads to a rapid depletion of cellular I kappa B beta pools. I kappa B alpha is efficiently resynthesized whereas I kappa B beta levels stay low for a prolonged time. NF-kappa B binding activity can be detected for several hours after stimulation. We found that removal of the TNF-alpha containing medium causes a rapid decrease in nuclear NF-kappa B. A phosphoform of newly synthesized I kappa B alpha is visible when degradation by the proteasome is inhibited and new I kappa B alpha displays the same properties regarding phosphorylation and degradation in response to a second inducer. There is no significant difference in the turnover of pre- and post-inductive I kappa B alpha. These observations suggest that resynthesis of I kappa B alpha and removal of the stimulus are obligatory steps for the inactivation of nuclear NF kappa B.

A new member of the I kappaB protein family, I kappaB epsilon, inhibits RelA (p65)-mediated NF-kappaB transcription

A novel member of the I kappaB family has been identified as a protein that associated with the p50 subunit of NF-kappaB in a yeast two-hybrid screen. Similar to previously known I kappaB proteins, this member, I kappaB epsilon, has six consecutive ankyrin repeats. I kappaB epsilon mRNA is widely expressed in different human tissues, with highest levels in spleen, testis, and lung. I kappaB epsilon interacts with different NF-kappaB proteins, including p65 (RelA), c-Rel, p50, and p52, in vitro and in vivo and inhibits the DNA-binding activity of both p50-p65 and p50-c-Rel complexes effectively. Endogenous and transfected NF-kappaB (RelA-dependent) transcriptional activation is inhibited by I kappaB epsilon. I kappaB epsilon mRNA is expressed at different levels in specific cell types and is synthesized constitutively in transformed B-cell lines. It also displays differential induction in response to tumor necrosis factor alpha, interleukin-1, or phorbol ester stimulation compared to I kappaB alpha in non-B-cell lines. Therefore, I kappaB epsilon represents a novel I kappaB family member which provides an alternative mechanism for regulation of NF-kappaB-dependent transcription.

Distinct functional properties of IkappaB alpha and IkappaB beta

The biological activity of the transcription factor NF-kappaB is controlled mainly by the IkappaB alpha and IkappaB beta proteins, which restrict NF-kappaB to the cytoplasm and inhibit its DNA binding activity. Here, we carried out experiments to determine and compare the mechanisms by which IkappaB alpha and IkappaB beta inhibit NF-kappaB-dependent transcriptional activation. First, we found that in vivo IkappaB alpha is a stronger inhibitor of NF-kappaB than is IkappaB beta. This difference is directly correlated with their abilities to inhibit NF-kappaB binding to DNA in vitro and in vivo. Moreover, IkappaB alpha, but not IkappaB beta, can remove NF-kappaB from functional preinitiation complexes in in vitro transcription experiments. Second, we showed that both IkappaBs function in vivo not only in the cytoplasm but also in the nucleus, where they inhibit NF-kappaB binding to DNA. Third, the inhibitory activity of IkappaB beta, but not that of IkappaB alpha, is facilitated by phosphorylation of the C-terminal PEST sequence by casein kinase II and/or by the interaction of NF-kappaB with high-mobility group protein I (HMG I) on selected promoters. The unphosphorylated form of IkappaB beta forms stable ternary complexes with NF-kappaB on the DNA either in vitro or in vivo. These experiments suggest that IkappaB alpha works as a postinduction repressor of NF-kappaB independently of HMG I, whereas IkappaB beta functions preferentially in promoters regulated by the NF-kappaB/HMG I complexes.

Novel inhibitors of cytokine-induced IkappaBalpha phosphorylation and endothelial cell adhesion molecule expression show anti-inflammatory effects in vivo

We have identified two compounds that inhibit the expression of endothelial-leukocyte adhesion molecules intercellular adhesion molecule-1, vascular cell adhesion molecule-1, and E-selectin. These compounds act by inhibiting tumor necrosis factor-alpha-induced phosphorylation of IkappaB-alpha, resulting in decreased nuclear factor-kappaB and decreased expression of adhesion molecules. The effects on both IkappaB-alpha phosphorylation and surface expression of E-selectin were irreversible and occurred at an IC50 of approximately 10 microM. These agents selectively and irreversibly inhibited the tumor necrosis factor-alpha-inducible phosphorylation of IkappaB-alpha without affecting the constitutive IkappaB-alpha phosphorylation. Although these compounds exhibited other activities, including stimulation of the stress-activated protein kinases, p38 and JNK-1, and activation of tyrosine phosphorylation of a 130-140-kDa protein, these effects are probably distinct from the effects on adhesion molecule expression since they were reversible. One compound was evaluated in vivo and shown to be a potent anti-inflammatory drug in two animal models of inflammation. The compound reduced edema formation in a dose-dependent manner in the rat carrageenan paw edema assay and reduced paw swelling in a rat adjuvant arthritis model. These studies suggest that inhibitors of cytokine-inducible IkappaBalpha phosphorylation exert anti-inflammatory activity in vivo.

Mothers against dpp participates in a DDP/TGF-beta responsive serine-threonine kinase signal transduction cascade

Mothers against dpp (Mad) is the prototype of a family of genes required for signaling by TGF-beta related ligands. In Drosophila, Mad is specifically required in cells responding to Decapentaplegic (DPP) signals. We further specify the role of Mad in DPP-mediated signaling by utilizing tkvQ199D, an activated form of the DPP type I receptor serine-threonine kinase thick veins (tkv). In the embryonic midgut, tkvQ199D mimics DPP-mediated inductive interactions. Homozygous Mad mutations block signaling by tkvQ199D. Appropriate responses to signaling by tkvQ199D are restored by expression of MAD protein in DPP-target cells. Endogenous MAD is phosphorylated in a ligand-dependent manner in Drosophila cell culture. DPP overexpression in the embryonic midgut induces MAD nuclear accumulation; after withdrawal of the overexpressed DPP signal, MAD is detected only in the cytoplasm. However, in three different tissues and developmental stages actively responding to endogenous DPP, MAD protein is detected in the cytoplasm but not in the nucleus. From these observations, we discuss possible roles for MAD in a DPP-dependent serine-threonine kinase signal transduction cascade integral to the proper interpretation of DPP signals.

Selective inhibition of l kappaB alpha phosphorylation and HIV-1 LTR-directed gene expression by novel antioxidant compounds

Oxidative stress activates the NF-kappaB/Rel transcription factors which are involved in the activation of numerous immunoregulatory genes and the human immunodeficiency virus type 1 (HIV-1) long terminal repeat (LTR). In the present study, we examined the effects of established and novel compounds including antioxidants, ribonucleotide reductase inhibitors, and iron chelators on NF-kappaB activation and HIV LTR-mediated gene expression induced by TNF-alpha. N-Acetylcysteine (NAC), pyrrolidinedithiocarbamate (PDTC), and Trimidox (TD) at various concentrations inhibited TNF-alpha-induced NF-kappaB binding in Jurkat cells. Pretreatment of cells with these compounds prior to stimulation prevented I kappaB alpha degradation. Phosphorylation of I kappaB alpha, a prerequisite for its signal-induced degradation, was abrogated in these cells, indicating that oxidative stress is an essential step in the NF-kappaB activation pathway. On the other hand, iron chelators desferrioxamine, pyridoxal isonicotinoyl hydrazone (PIH), and salicylaldehyde isonicotinoyl hydrazone (SIH) showed no inhibition of TNF-alpha-induced NF-kappaB DNA-binding activity. Synergistic induction of HIV-1 LTR-mediated gene expression by TNF-alpha and the HIV-1 transactivator Tat in Jurkat cells was significantly suppressed in the presence of NAC and TD, but not PDTC. The inhibition of NAC and TD on LTR-directed gene expression was diminished when NF-kappaB-binding sites in the LTR were deleted, indicating that these compounds affected the NF-kappaB component of the synergism. Iron chelators PIH and SIH also showed some inhibitory effect on LTR-mediated gene activation, presumably through an NF-kappaB-independent mechanism. These experiments demonstrate that TD, at concentration 50 times lower than the effective concentration of NAC, potently inhibits NF-kappaB activity and suppresses HIV LTR expression.

Direct involvement of the ubiquitin-conjugating enzyme Ubc9/Hus5 in the degradation of IkappaBalpha

The NF-kappaB/Rel proteins are sequestered in the cytoplasm in association with IkappaBalpha. In response to external signals, IkappaBalpha is phosphorylated, multi-ubiquitinated, and degraded by proteasomes, thereby releasing NF-kappaB/Rel proteins to migrate to the nucleus. We have cloned a mouse ubiquitin-conjugating enzyme (mE2), which associates with IkappaBalpha. mE2 is homologous to the yeast Ubc9/Hus5 ubiquitin-conjugating enzyme. A transdominant-negative mutant of mE2 had no effect on phosphorylation of IkappaBalpha, but delayed its degradation. Correspondingly, tumor necrosis factor-alpha-inducible NF-kappaB activity was diminished. We propose that mE2 is directly involved in the ubiquitin conjugation of IkappaBalpha, a pivotal step in its degradation pathway.

Casein kinase II is a selective target of HIV-1 transcriptional inhibitors

The identification of cellular factors that are required to complete various steps of the HIV-1 life cycle may lead to the development of new therapeutics. One key step, transcription from the integrated provirus, is inhibited by members of two distinct classes of compounds, the flavonoids and the benzothiophenes, via an unknown mechanism, possibly involving a cellular factor. A marked specificity toward inhibiting HIV-1 transcription is evidenced by the ability of drug-treated cells to retain their proliferative and differentiation capabilities. In addition, the compounds do not impede the activation and function of the transcriptional factor NF-kappaB. Here we report on the identification of several cellular proteins that mediate the HIV-1 transcriptional inhibitory property of the flavonoid chrysin. Chemical and immunologic analyses identified these cellular proteins as the individual subunits of casein kinase II (CKII). Though structurally unrelated to chrysin, an HIV-1 inhibitory benzothiophene also bound selectively to CKII. Both chrysin and the benzothiophenes inhibited human recombinant CKII enzymatic activity and showed competitive kinetics with respect to ATP, analogous to the classic CKII inhibitor 5, 6-dichloro-1-beta-D-ribofuranosylbenzimidazole (DRB). Moreover, DRB potently inhibited HIV-1 expression in chronically infected cells. CKII may regulate HIV-1 transcription by phosphorylating cellular proteins involved in HIV-1 transactivation that contain multiple CKII phosphorylation consensus sequences.

The BASP1 family of myristoylated proteins abundant in axonal termini. Primary structure analysis and physico-chemical properties

Proteins BASP1 and GAP-43/B-50, which are abundant in nerve endings, show a number of similar physico-chemical properties. Nevertheless, they belong to different protein families. In this work, complete amino acid sequences of bovine BASP1 and human BASP1 were established. They proved to be very similar to the sequences of rat brain protein NAP-22 and chicken brain protein CAP-23. Relatively to human BASP1 its bovine, rat and chicken analogues show 80%, 70% and 45% sequence identity respectively, confirming their membership of a definite protein family (BASP1 family). All members of BASP1 family contain several 'good' PEST sequences characteristic for short-living proteins. Conservation of PEST sequences in BASP1 of different species points to their significance for BASP1 functions. In contrast to GAP-43/B-50 showing high immunological cross-reactivity between the proteins belonging to different species of mammals, immunological properties of BASP1 are species specific. BASP1 shows both high hydrophilicity and some properties characteristic for hydrophobic proteins. These properties are caused by N-terminal myristoylation of BASP1 molecules. Unlike GAP-43/B-50, BASP1 is present in high amounts also in some non-nervous tissues: testis, kidney and lymphoid organs (spleen, thymus). So far examined characteristics, including myristoylation, peptide maps and detected by isoelectrofocusing microheterogeneity, proved to be the same for BASP1 samples isolated from both brain and non-nervous tissues. Therefore, in spite of different physiological consequences, biochemical functions of BASP1 must also be similar in different tissues.

The signal response of IkappaB alpha is regulated by transferable N- and C-terminal domains

IkappaB alpha retains the transcription factor NF-kappaB in the cytoplasm, thus inhibiting its function. Various stimuli inactivate IkappaB alpha by triggering phosphorylation of the N-terminal residues Ser32 and Ser36. Phosphorylation of both serines is demonstrated directly by phosphopeptide mapping utilizing calpain protease, which cuts approximately 60 residues from the N terminus, and by analysis of mutants lacking one or both serine residues. Phosphorylation is followed by rapid proteolysis, and the liberated NF-kappaB translocates to the nucleus, where it activates transcription of its target genes. Transfer of the N-terminal domain of IkappaB alpha to the ankyrin domain of the related oncoprotein Bcl-3 or to the unrelated protein glutathione S-transferase confers signal-induced phosphorylation on the resulting chimeric proteins. If the C-terminal domain of IkappaB alpha is transferred as well, the resulting chimeras exhibit both signal-induced phosphorylation and rapid proteolysis. Thus, the signal response of IkappaB alpha is controlled by transferable N-terminal and C-terminal domains.

I kappa B epsilon, a novel member of the I kappa B family, controls RelA and cRel NF-kappa B activity

We have isolated a human cDNA which encodes a novel I kappa B family member using a yeast two-hybrid screen for proteins able to interact with the p52 subunit of the transcription factor NF-kappa B. The protein is found in many cell types and its expression is up-regulated following NF-kappa B activation and during myelopoiesis. Consistent with its proposed role as an I kappa B molecule, I kappa B-epsilon is able to inhibit NF-kappa B-directed transactivation via cytoplasmic retention of rel proteins. I kappa B-epsilon translation initiates from an internal ATG codon to give rise to a protein of 45 kDa, which exists as multiple phosphorylated isoforms in resting cells. Unlike the other inhibitors, it is found almost exclusively in complexes containing RelA and/or cRel. Upon activation, I kappa B-epsilon protein is degraded with slow kinetics by a proteasome-dependent mechanism. Similarly to I kappa B-alpha and I kappa B, I kappa B-epsilon contains multiple ankyrin repeats and two conserved serines which are necessary for signal-induced degradation of the molecule. A unique lysine residue located N-terminal of the serines appears to be not strictly required for degradation. Unlike I kappa B- alpha and I kappa B-beta, I kappa B-epsilon does not contain a C-terminal PEST-like sequence. I kappa B-epsilon would, therefore, appear to regulate a late, transient activation of a subset of genes, regulated by RelA/cRel NF-kappa B complexes, distinct from those regulated by other I kappa B proteins.

Immunological defects in mice with a targeted disruption in Bcl-3

The proto-oncogene bcl-3 is a member of the IkappaB family. The Bcl-3 protein is known to interact specifically with the p50 and p52 subunits of NFkappaB. However, the function of this interaction is not well understood. To determine the in vivo role of Bcl-3, mice were generated that lack the bcl-3 gene, Bcl 3(-/-). Here we report that Bcl 3(-/-) mice appear developmentally normal, but exhibit severe defects in humoral immune responses and protection from in vivo pathogenic challenges. Relative to wild-type mice, Bcl 3(-/-) mice are unable to clear L. monocytogenes and are more susceptible to infection with S. pneumoniae. This phenotype is similar to that observed in the p50(-/-) mice and the cross between the Bcl-3(-/-) and p50(-/-) mice generates animals with an enhanced phenotype. In accordance with the observed defects in their immune response, the Bcl 3(-/-) mice have normal immunoglobulin levels before and after immunization, but fail to produce antigen-specific antibodies. Additionally, spleens from Bcl-3(-/-) mice are abnormal and void of germinal centers. In contrast, the p50(-/-) mice have normal germinal centers. We propose that in in vivo, Bcl-3 can function independently of p50.

Different mechanisms control signal-induced degradation and basal turnover of the NF-kappaB inhibitor IkappaB alpha in vivo

The transcription factor NF-kappaB is sequestered in the cytoplasm by a family of IkappaB molecules. Upon cellular stimulation with diverse agents, one of these molecules, IkappaB alpha, is rapidly phosphorylated and subsequently degraded. This process triggers nuclear translocation of NF-kappaB and the successive activation of target genes. Independent of its rapid stimulation-induced breakdown, IkappaB alpha is inherently unstable and undergoes a continuous turnover. To compare the mechanisms and protein domains involved in inducible and basal degradation of IkappaB alpha in intact cells we employed a transfection strategy using tagged IkappaB alpha and ubiquitin molecules. We show that tumor necrosis factor alpha (TNFalpha) induced breakdown of IkappaB alpha but not its basal turnover coincides with ubiquitination in the amino-terminal signal response domain (SRD) of IkappaB alpha. Neither the SRD nor the carboxy-terminal PEST sequence is needed for basal turnover, which instead depends only on the core ankyrin repeat domain. Despite the differences in the requirements of protein domains and ubiquitin-conjugation for both degradation pathways, each one is mediated by the proteasome. This finding is important for understanding alternative modes of controlling NF-kappaB activity.

Basal phosphorylation of the PEST domain in the I(kappa)B(beta) regulates its functional interaction with the c-rel proto-oncogene product

The product of the c-rel proto-oncogene (c-Rel) belongs to the NF-kappaB/Rel family of polypeptides and has been implicated in the transcriptional control of cell proliferation and immune function. In human T lymphocytes, c-Rel is sequestered in the cytoplasmic compartment by constitutively phosphorylated inhibitors, including I(kappa)B(alpha) and I(kappa)B(beta). Studies with bacterially expressed forms of these inhibitory proteins revealed that unphosphorylated I(kappa)B(alpha) but not I(kappa)B(beta) assembles with c-Rel and inhibits its DNA binding activity. Furthermore, latent I(kappa)B(beta)-c-Rel complexes derived from mammalian cells were sensitive to phosphatase treatment, whereas I(kappa)B(alpha)-c-Rel complexes were resistant. We have identified a constitutive protein kinase in unstimulated T cells that associates with and phosphorylates I(kappa)B(beta) in vitro. The substrate specificity, electrophoretic mobility, and antigenic properties of this I(kappa)B(beta)-associated kinase (BAK) suggest identity with casein kinase II (CKII), an enzyme known to mediate basal phosphorylation of I(kappa)B(alpha). Phosphorylation of recombinant I(kappa)B(beta) by either BAK or CKII restored the capacity of this inhibitor to antagonize the DNA binding activity of c-Rel. Peptide mapping and mutational analyses localized the bulk of the basal phosphorylation sites in I(kappa)B(beta) to the C-terminal PEST domain, which contains two potential acceptors for CKII-mediated phosphoryl group transfer (Ser-313 and Ser-315). Point mutations introduced into the full-length inhibitor at Ser-313 and Ser-315 led to a significant reduction in the phosphorylation of I(kappa)B(beta) and severely impaired its c-Rel inhibitory function in vivo. Taken together, these findings strongly suggest that basal phosphorylation of the PEST domain of I(kappa)B(beta) at consensus CKII sites is required for the efficient formation of latent I(kappa)B(beta)-c-Rel complexes.

Signal-induced degradation of I(kappa)B(alpha): association with NF-kappaB and the PEST sequence in I(kappa)B(alpha) are not required

Signal-induced degradation of I(kappa)B(alpha) via the ubiquitin-proteasome pathway requires phosphorylation on residues serine 32 and serine 36 followed by ubiquitination on lysines 21 and 22. We investigated the role of other regions of I(kappa)B(alpha) which may be involved in its degradation. Here we report that the carboxy-terminal PEST sequence is not required for I(kappa)B(alpha) signal-induced degradation. However, removal of the PEST sequence stabilizes free I(kappa)B(alpha) in unstimulated cells. We further report that a PEST deletion mutant does not associate well with NF-(kappa)B proteins but is degraded in response to signal. Therefore, we conclude that both association with NF-(kappa)B and a PEST sequence are not required for signal-induced I(kappa)B(alpha) degradation. Additionally, the PEST sequence may be required for constitutive turnover of free I(kappa)B(alpha).