Tumor necrosis factor alpha-induced phosphorylation of I kappa B alpha is a signal for its degradation but not dissociation from NF-kappa B

Functional interactions between the pelle kinase, Toll receptor, and tube suggest a mechanism for activation of dorsal

A complex signal transduction pathway functions in the early Drosophila embryo to establish dorsal-ventral polarity. Activation of this pathway results in the nuclear transport of the protein dorsal (dl), a member of the rel/NF-kappaB family of transcription factors. Genetic studies have identified three intracellular components whose activity is required for activation of dl: Toll, a transmembrane receptor; pelle (pll), a serine/threonine protein kinase; and tube, a protein of unknown function. Here we examine the activities of these proteins when coexpressed in Drosophila Schneider cells. Coexpression of pll with dl enhanced dl nuclear localization and resulted in a modest increase in transcriptional activity. However, when pll was coexpressed with a specific mutant derivative of Toll (TlNaeI), although not with wild-type Toll, a striking synergistic activation of dl was detected. Unexpectedly, coexpression of pll plus TlNaeI, in the absence of dl, resulted in a similar synergistic activation of a GAL4-tube fusion protein. Based on these and other results, we propose a model in which pll receives a signal from activated Toll and phosphorylates tube, which then participates directly in dl activation.

The NF-kappa B and I kappa B proteins: new discoveries and insights

The transcription factor NF-kappa B has attracted widespread attention among researchers in many fields based on the following: its unusual and rapid regulation, the wide range of genes that it controls, its central role in immunological processes, the complexity of its subunits, and its apparent involvement in several diseases. A primary level of control for NF-kappa B is through interactions with an inhibitor protein called I kappa B. Recent evidence confirms the existence of multiple forms of I kappa B that appear to regulate NF-kappa B by distinct mechanisms. NF-kappa B can be activated by exposure of cells to LPS or inflammatory cytokines such as TNF or IL-1, viral infection or expression of certain viral gene products, UV irradiation, B or T cell activation, and by other physiological and nonphysiological stimuli. Activation of NF-kappa B to move into the nucleus is controlled by the targeted phosphorylation and subsequent degradation of I kappa B. Exciting new research has elaborated several important and unexpected findings that explain mechanisms involved in the activation of NF-kappa B. In the nucleus, NF-kappa B dimers bind to target DNA elements and activate transcription of genes encoding proteins involved with immune or inflammation responses and with cell growth control. Recent data provide evidence that NF-kappa B is constitutively active in several cell types, potentially playing unexpected roles in regulation of gene expression. In addition to advances in describing the mechanisms of NF-kappa B activation, excitement in NF-kappa B research has been generated by the first report of a crystal structure for one form of NF-kappa B, the first gene knockout studies for different forms of NF-kB and of I kappa B, and the implications for therapies of diseases thought to involve the inappropriate activation of NF-kappa B.

Constitutive expression of Bc1-3 in thymocytes increases the DNA binding of NF-kappaB1 (p50) homodimers in vivo

Previous studies have indicated that Bcl-3 interacts through its ankyrin repeats with the transcriptional factors NF-kappaB1 (p50) and NF-kappaB2 (p52), affecting their biological activities. To further investigate the role of Bcl-3 in vivo and its association with the NF-kappaB proteins, we have generated transgenic mice constitutively expressing Bcl-3 in thymocytes. The results indicate that Bcl-3 is associated with endogenous p50 and p52 in nuclear extracts from transgenic animals. Remarkably, constitutive expression of Bcl-3 in these cells augments the DNA binding activity of p52 homodimers. This effect could be reproduced in vitro and is blocked by anti-Bcl-3 antibodies. We have also shown that Bcl-3 is phosphorylated in thymocytes and that its dephosphorylation greatly decreases the effect on p50 homodimers.

Role of IkappaBalpha ubiquitination in signal-induced activation of NFkappaB in vivo

In unstimulated cells, the transcription factor NF-kappaB is held in the cytoplasm in an inactive state by the inhibitor protein IkappaBalpha. Stimulation of cells results in rapid phosphorylation and degradation of IkappaBalpha, thus releasing NF-kappaB, which translocates to the nucleus and activates transcription of responsive genes. Here we demonstrate that in cells where proteasomal degradation is inhibited, signal induction by tumor necrosis factor alpha results in the rapid accumulation of higher molecular weight forms of IkappaBalpha that dissociate from NF-kappaB and are consistent with ubiquitin conjugation. Removal of the high molecular weight forms of IkappaBalpha by a recombinant ubiquitin carboxyl-terminal hydrolase and reactivity of the immunopurified material with a monoclonal antibody specific for ubiquitin indicated that IkappaBalpha was conjugated to multiple copies of ubiquitin. Western blot analysis of immunopurified IkappaBalpha from cells expressing epitope-tagged versions of IkappaBalpha and ubiquitin revealed the presence of multiple copies of covalently bound tagged ubiquitin. An S32A/S36A mutant of IkappaBalpha that is neither phosphorylated nor degraded in response to signal induction fails to undergo inducible ubiquitination in vivo. Thus signal-induced activation of NF-kappaB involves phosphorylation-dependent ubiquitination of IkappaBalpha, which targets the protein for rapid degradation by the proteasome and releases NF-kappaB for translocation to the nucleus.

Site-specific phosphorylation of IkappaBalpha by a novel ubiquitination-dependent protein kinase activity

Signal-induced activation of the transcription factor NF-kappaB requires specific phosphorylation of the inhibitor IkappaBalpha and its subsequent proteolytic degradation. Phosphorylation of serine residues 32 and 36 targets IkappaBalpha to the ubiquitin (Ub)-proteasome pathway. Here we report the identification of a large, multisubunit kinase (molecular mass approximately 700 kDa) that phosphorylates IkappaBalpha at S32 and S36. Remarkably, the activity of this kinase requires the Ub-activating enzyme (E1), a specific Ub carrier protein (E2) of the Ubc4/Ubc5 family, and Ub. We also show that a ubiquitination event in the kinase complex is a prerequisite for specific phosphorylation of IkappaBalpha. Thus, ubiquitination serves a novel regulatory function that does not involve proteolysis.

Phosphorylation of p105 PEST sequence via a redox-insensitive pathway up-regulates processing of p50 NF-kappaB

The p105 Rel protein has dual functions; it is the precursor of the p5O subunit of NF-kappaB, and it acts as an IkappaB-like inhibitor to retain other Rel subunits in the cytoplasm. We have investigated the posttranslational regulation of p105 following activation of Jurkat T cells and find that a rapid and sustained phosphorylation of p105 is induced. The inducible phosphorylation occurs on multiple serines in the C-terminal-most 150 amino acids of the molecule, a region rich in Pro, Glu, Ser, and Thr residues. Phosphorylation of p105 in Jurkat cells treated with phorbol 12-myristate 13-acetate/ionomycin or with okadaic acid, another activator of NF-kappaB, is correlated with an increase in proteolytic processing to p5O. Intact PEST sequences are required for the phorbol 12-myristate 13-acetate/ionomycin-induced p105 processing, as a 68-amino acid C-terminal deletion abolishes the response to stimulation. When compounds that block Ikappa B alpha phosphorylation and degradation were tested, the serine protease inhibitors L-1-tosylamido-2-phenylethyl chloromethyl ketone and 1-chloro-3-tosyl-amido-7-amino-2-heptanone blocked inducible p105 phosphorylation, but the antioxidants pyrrolidine dithiocarbamate and butylated hydroxyanisol did not. Thus, while regulation of the p105 IkappaB resembles that of lkappaBa, involving inducible serine phosphorylation and proteolysis of the inhibitory ankyrin repeat domain, it depends on a different, redox-insensitive, signaling pathway.

Analysis of the signal transduction in the induction of nitric oxide synthase by lipoteichoic acid in macrophages

1. This study investigates the signal transduction mechanisms leading to the enhanced formation of nitric oxide (NO) due to the induction of NO synthase (iNOS) in murine J774.2 macrophages in culture activated with lipoteichoic acid (LTA), a cell wall component of the gram-positive bacterium Staphylococcus aureus. 2. LTA (10 microgram ml-1) caused within 24 h an enhanced accumulation of nitrite (an indicator of NO biosynthesis) in the supernatant of J774.2 macrophages which was prevented by the non-selective NOS inhibitor NG-monomethyl-L-arginine (L-NMMA; IC50: 35 microM) or by the iNOS-selective NOS inhibitor, aminoethyl-isothiourea (AE-ITU; IC50: 6 microM). The inhibition of nitrite formation afforded by these agents was prevented by excess L-arginine (3-30 mM), but not by D-arginine (3-30 mM). Furthermore, the degree of iNOS inhibition was similar when these NOS inhibitors were added to the macrophages 10 h after LTA. 3. Pretreatment of J774.2 macrophages with cyclohexamide or dexamethasone prevented the enhanced formation of nitrite caused by LTA. This inhibition did not occur when dexamethasone or cyclohexamide were added to the cells 10 h after LTA. The increase in nitrite formation stimulated by LTA (10 micrograms ml-1) was not affected by polymyxin B (0.05-0.5 microgram ml-1), an agent which binds and inactivates endotoxin. 4. A specific inhibitor of phosphatidylcholine-phospholipase C (PC-PLC), D609, prevented the increase in nitrite formation (IC50 = 20 micrograms ml-1) caused by LTA. The inhibition afforded by D609 was significantly smaller when this agent was added to the cells 10 h after LTA. 5. The structurally distinct tyrosine kinase inhibitors, erbstatin, genistein, and tyrphostin AG126 prevented the formation of nitrite caused by LTA. The inhibition afforded by these compounds was significantly attenuated when they were added to the cells 10 h after LTA. In contrast, daidzein or tyrphostin A-1, which are inactive analogues of genistein and tyrphostin (up to a concentration of 10 microM) did not affect the nitrite formation caused by LTA. 6. Inhibitors of the activation of the nuclear transcription factor NF-kappa B such as pyrrolidine dithiocarbamate (PDTC; an antioxidant and a metal chelator), butylated hydroxyanisole (BHA; an antioxidant), L-1-tosylamido-2-phenylethyl chloromethyl ketone (TPCK), calpain inhibitor I (both I kappa B-protease inhibitors), or rotenone (an antioxidant which inhibits electron transport) prevented the nitrite formation stimulated by LTA. The inhibition afforded by these agents was significantly smaller when they were added to the macrophages 10 h after LTA. 7. Incubation of J774.2 cells with LTA over 24 h resulted in the expression of iNOS protein (130 kDa) as identified by Western blot analysis. The expression of iNOS protein by LTA was significantly attenuated by cyclohexamide, D609, tyrphostin AG126, PDTC or by TPCK. 8. Thus, the signal transduction leading to the expression of iNOS protein and activity caused by LTA in murine J774.2 macrophages involves (i) the activation of PC-PLC, (ii) phosphorylation of tyrosine kinase, and (iii) the activation of the transcription factor NF-kappa B.

The Rel family of eukaryotic transcription factors

Members of the Rel transcription factor family mediate the response of eukaryotic cells to a broad range of environmental threats, in addition to serving an essential role in the development of certain vertebrate and insect cells. It is now apparent that there are two classes of Rel proteins, which differ in whether they bind DNA as monomers or dimers and which use markedly different mechanisms to transduce intracellular signals. Recent progress has been made towards understanding the structural basis for the fascinating biology of these proteins.

Critical role for lysines 21 and 22 in signal-induced, ubiquitin-mediated proteolysis of I kappa B-alpha

The NF-kappa B transcription factor induces rapid transcription of many genes in response to a variety of extracellular signals. NF-kappa B is readily activated from normally inhibited cytoplasmic stores by induced proteolytic degradation of I kappa B-alpha, a principal inhibitor of this transcription factor. Following the inhibitor's degradation, NF-kappa B is free to translocate to the nucleus and induce gene transcription. The I kappa B-alpha inhibitor is targeted for degradation by signal-induced phosphorylation of two closely spaced serines in its NH2 terminus (Ser32 and Ser36). Proteolytic degradation appears to be carried out by proteasomes which can recognize ubiquitinated intermediates of the I kappa B-alpha inhibitor. We provide evidence which supports a ubiquitin-mediated mechanism. Amino acid substitutions of two adjacent potential ubiquitination sites in the NH2 terminus of I kappa B-alpha (Lys21 and Lys22) almost completely block the rapid, signal-induced degradation of the mutant protein, while they do not interfere with induced phosphorylation. The mutant I kappa B-alpha also does not permit signal-induced activation of NF-kappa B bound to it. The data suggest that ubiquitination at either of the two adjacent lysines (21 and 22) is required for degradation following induced phosphorylation at nearby serines 32 and 36. Such dependence on ubiquitination of specific sites for protein degradation is unusual. This mechanism of degradation may also apply to I kappa B-beta, an inhibitor related to and functionally overlapping with I kappa B-alpha, as well as to cactus, an I kappa B homolog of Drosophila.

The interferon-inducible p202 protein as a modulator of transcription: inhibition of NF-kappa B, c-Fos, and c-Jun activities

The antimicrobial, immunomodulatory, and cell growth-regulatory activities of the interferons are mediated by interferon-inducible proteins. One of these is p202, a nuclear protein that is encoded by the Ifi 202 gene from the interferon-activatable gene 200 cluster. Overexpression of p202 in transfected cells slows down cell proliferation. As shown earlier, p202 binds to the hypophosphorylated form of the retinoblastoma susceptibility protein. Here we report that p202 inhibits the activities of the NF-kappa B and the AP-1 enhancers both in transiently transfected cells and in transfected stable cell lines overexpressing p202. Furthermore, p202 binds the NF-kappa B p50 and p65 and the AP-1 c-Fos and c-Jun transcription factors in vitro and in vivo. NF-kappa B, c-Fos, and c-Jun participate in the transcription of various cellular and viral genes, and thus p202 can modulate the expression of these genes in response to interferons.

Activin increases phosphorylation and decreases stability of the transcription factor Pit-1 in MtTW15 somatotrope cells

Activin is a polypeptide growth factor which exerts endocrine, paracrine, and autocrine effects in a variety of tissues. In the pituitary somatotrope, activin represses proliferation and growth hormone (GH) biosynthesis and secretion. We previously demonstrated that decreases in GH biosynthesis in MtTW15 somatotrope cells are due at least in part to decreased binding of the tissue-specific transcription factor, Pit-1, to the GH promoter, resulting in decreased transcription of the GH gene. The objective of the current study was to determine the extent to which activin-mediated decreases in GH transcription were the result of decreased Pit-1 activity and/or decreased Pit-1 protein content in MtTW15 cells. Activin caused rapid increases in Pit-1 phosphorylation, which were temporally correlated with decreases in GH DNA binding. Pit-1 phosphorylation preceded marked decreases in steady-state levels of Pit-1 protein. The rate of Pit-1 synthesis was only moderately decreased by activin, with a time-course similar to that observed for decreases in GH biosynthesis. However, Pit-1 stability was markedly decreased after more than 4 h of activin treatment. These data demonstrate that activin decreases GH expression in MtTW15 cells through multilevel regulation of Pit-1, which may represent a more general mechanism whereby activin and other transforming growth factor beta family members modulate gene expression through regulation of transcription factor activity as well as content.

Signal-induced degradation of I kappa B alpha requires site-specific ubiquitination

The inhibitor protein I kappa B alpha controls the nuclear import of the transcription factor NF-kappa B. The inhibitory activity of I kappa B alpha is regulated from the cytoplasmic compartment by signal-induced proteolysis. Previous studies have shown that signal-dependent phosphorylation of serine residues 32 and 36 targets I kappa B alpha to the ubiquitin-proteasome pathway. Here we provide evidence that lysine residues 21 and 22 serve as the primary sites for signal-induced ubiquitination of I kappa B alpha. Conservative Lys-->Arg substitutions at both Lys-21 and Lys-22 produce dominant-negative mutants of I kappa B alpha in vivo. These constitutive inhibitors are appropriately phosphorylated but fail to release NF-kappa B in response to multiple inducers, including viral proteins, cytokines, and agents that mimic antigenic stimulation through the T-cell receptor. Moreover, these Lys-->Arg mutations prevent signal-dependent degradation of I kappa B alpha in vivo and ubiquitin conjugation in vitro. We conclude that site-specific ubiquitination of phosphorylated I kappa B alpha at Lys-21 and/or Lys-22 is an obligatory step in the activation of NF-kappa B.

Stimulation-dependent I kappa B alpha phosphorylation marks the NF-kappa B inhibitor for degradation via the ubiquitin-proteasome pathway

The nuclear translocation of NF-kappa B follows the degradation of its inhibitor, I kappa B alpha, an event coupled with stimulation-dependent inhibitor phosphorylation. Prevention of the stimulation-dependent phosphorylation of I kappa B alpha, either by treating cells with various reagents or by mutagenesis of certain putative I kappa B alpha phosphorylation sites, abolishes the inducible degradation of I kappa B alpha. Yet, the mechanism coupling the stimulation-induced phosphorylation with the degradation has not been resolved. Recent reports suggest a role for the proteasome in I kappa B alpha degradation, but the mode of substrate recognition and the involvement of ubiquitin conjugation as a targeting signal have not been addressed. We show that of the two forms of I kappa B alpha recovered from stimulated cells in a complex with RelA and p50, only the newly phosphorylated form, pI kappa B alpha, is a substrate for an in vitro reconstituted ubiquitin-proteasome system. Proteolysis requires ATP, ubiquitin, a specific ubiquitin-conjugating enzyme, and other ubiquitin-proteasome components. In vivo, inducible I kappa B alpha degradation requires a functional ubiquitin-activating enzyme and is associated with the appearance of high molecular weight adducts of I kappa B alpha. Ubiquitin-mediated protein degradation may, therefore, constitute an integral step of a signal transduction process.

Interleukin-4 inhibits kappa light chain expression and NF kappa B activation but not I kappa B alpha degradation in 70Z/3 murine pre-B cells

The murine pre-B cell line 70Z/3 responds to lipopolysaccharide by up-regulating the surface expression of kappa (kappa) light chain through activation of the transcription factor NF kappa B. Interleukin-4 (IL-4), a T cell cytokine, is a known inhibitor of some LPS-mediated events. We investigated whether IL-4 could inhibit the up-regulation of kappa light chain and activation of NF kappa B by LPS in 70Z/3. IL-4 partially inhibited both the LPS-induced expression of kappa light chain and also the activation of NF kappa B as judged by an NF kappa B reporter gene assay. Additionally, electrophoretic mobility shift assays confirmed this effect on LPS-induced NF kappa B DNA binding activity in the nucleus. Surprisingly, proteolytic degradation of I kappa B alpha (MAD3), a prerequisite for NF kappa B activation, was unaffected by IL-4, implying that this cytokine inhibits some subsequent undefined event in the activation of NF kappa B. IL-4 was also found partially to inhibit NF kappa B activity induced by tumor necrosis factor-alpha (TNF-alpha) and interleukin-1-beta (IL-1 beta). These results indicate that there may be a common mechanism for the well-documented anti-inflammatory effects of IL-4 and that this mechanism involves the transcription factor NF kappa B.

N- and C-terminal sequences control degradation of MAD3/I kappa B alpha in response to inducers of NF-kappa B activity

The proteolytic degradation of the inhibitory protein MAD3/I kappa B alpha in response to extracellular stimulation is a prerequisite step in the activation of the transcription factor NF-kappa B. Analysis of the expression of human I kappa B alpha protein in stable transfectants of mouse 70Z/3 cells shows that, as for the endogenous murine protein, exogenous I kappa B alpha is degraded in response to inducers of NF-kappa B activity, such as phorbol myristate acetate or lipopolysaccharide. In addition, pretreatment of the cells with the proteasome inhibitor N-Ac-Leu-Leu-norleucinal inhibits this ligand-induced degradation and, in agreement with previous studies, stabilizes a hyperphosphorylated form of the human I kappa B alpha protein. By expressing mutant forms of the human protein in this cell line, we have been able to delineate the sequences responsible for both the ligand-induced phosphorylation and the degradation of I kappa B alpha. Our results show that deletion of the C terminus of the I kappa B alpha molecule up to amino acid 279 abolishes constitutive but not ligand-inducible phosphorylation and inhibits ligand-inducible degradation. Further analysis reveals that the inducible phosphorylation of I kappa B alpha maps to two serines in the N terminus of the protein (residues 32 and 36) and that the mutation of either residue is sufficient to abolish ligand-induced degradation, whereas both residues must be mutated to abolish inducible phosphorylation of the protein. We propose that treatment of 70Z/3 cells with either phorbol myristate acetate or lipopolysaccharide induces a kinase activity which phosphorylates serines 32 and that these phosphorylations target the protein for rapid proteolytic degradation, possibly by the ubiquitin-26S proteasome pathway, thus allowing NF-kappa B to translocate to the nucleus and to activate gene expression.

Ubiquitin-mediated processing of NF-kappa B transcriptional activator precursor p105. Reconstitution of a cell-free system and identification of the ubiquitin-carrier protein, E2, and a novel ubiquitin-protein ligase, E3, involved in conjugation

In most cases, the transcriptional factor NF-kappa B is a heterodimer consisting of two subunits, p50 and p65, which are encoded by two distinct genes of the Rel family. p50 is translated as a precursor of 105 kDa. The C-terminal domain of the precursor is rapidly degraded, forming the mature p50 subunit consisted of the N-terminal region of the molecule. The mechanism of generation of p50 is not known. It has been suggested that the ubiquitin-proteasome system is involved in the process; however, the specific enzymes involved and the mechanism of limited proteolysis, in which half of the molecule is spared, have been obscure. Palombella and colleagues (Palombella, V. J., Rando, O. J., Goldberg, A. L., and Maniatis, T. (1994) Cell 78, 773-785) have shown that ubiquitin is required for the processing in a cell-free system of a truncated, artificially constructed, 60-kDa precursor. They have also shown that proteasome inhibitors block the processing both in vitro and in vivo. In this study, we demonstrate reconstitution of a cell-free processing system and demonstrate directly that: (a) the ubiquitin-proteasome system is involved in processing of the intact p105 precursor, (b) conjugation of ubiquitin to the precursor is an essential intermediate step in the processing, (c) the recently discovered novel species of the ubiquitin-carrier protein, E2-F1, that is involved in the conjugation and degradation of p53, is also required for the limited processing of the p105 precursor, and (d) a novel, approximately 320-kDa species of ubiquitin-protein ligase, is involved in the process. This novel enzyme is distinct from E6-AP, the p53-conjugating ligase, and from E3 alpha, the "N-end rule" ligase.

Activation of NF-kappa B by phosphatase inhibitors involves the phosphorylation of I kappa B alpha at phosphatase 2A-sensitive sites

Activation of NF-kappa B by various cellular stimuli involves the phosphorylation and subsequent degradation of its inhibitor, I kappa B alpha, although the underlying mechanism remains unclear. In the present study, the role of serine/threonine phosphatases in the regulation of I kappa B alpha phosphorylation was investigated. Our studies demonstrate that incubation of human T cells with low concentrations (approximately 1-5 nM) of calyculin A or okadaic acid, potent inhibitors of protein phosphatase type 1 (PP-1) and type 2A (PP-2A), induces the phosphorylation of I kappa B alpha even in the absence of any cellular stimulus. This action of the phosphatase inhibitors, which is associated with the activation of the RelA.p50 NF-kappa B heterodimer, is not affected by agents that block the induction of I kappa B alpha phosphorylation by tumor necrosis factor alpha (TNF-alpha). Furthermore, the phosphorylated I kappa B alpha from calyculin A-treated cells, but not that from TNF-alpha-stimulated cells, is sensitive to PP-2A in vitro, suggesting the existence of fundamental differences in the phosphorylation of I kappa B alpha induced by the two different NF-kappa B inducers. However, induction of I kappa B alpha phosphorylation by both TNF-alpha and the phosphatase inhibitors is associated with the subsequent degradation of I kappa B alpha. We further demonstrate that TNF-alpha- and calyculin A-induced I kappa B alpha degradation exhibits similar but not identical sensitivities to a proteasome inhibitor. Together, these results suggest that phosphorylation of I kappa B alpha, mediated through both the TNF-alpha-inducible and the PP-2A-opposing kinases, may serve to target I kappa B alpha for proteasome-mediated degradation.

Transcriptional control by protein phosphorylation: signal transmission from the cell surface to the nucleus

Two general mechanisms have evolved for the rapid and accurate transmission of signals from cell-surface receptors to the nucleus, both involving protein phosphorylation. One mechanism depends on the regulated translocation of activated protein kinases from the cytoplasm to the nucleus, where they phosphorylate target transcription factors. In the second mechanism, transcription factors are kept in a latent state in the cytoplasm and are translocated into the nucleus upon activation.

Signal-induced site-specific phosphorylation targets I kappa B alpha to the ubiquitin-proteasome pathway

The transcription factor NF-kappa B is sequestered in the cytoplasm by the inhibitor protein I kappa B alpha. Extracellular inducers of NF-kappa B activate signal transduction pathways that result in the phosphorylation and subsequent degradation of I kappa B alpha. At present, the link between phosphorylation of I kappa B alpha and its degradation is not understood. In this report we provide evidence that phosphorylation of serine residues 32 and 36 of I kappa B alpha targets the protein to the ubiquitin-proteasome pathway. I kappa B alpha is ubiquitinated in vivo and in vitro following phosphorylation, and mutations that abolish phosphorylation and degradation of I kappa B alpha in vivo prevent ubiquitination in vitro. Ubiquitinated I kappa B alpha remains associated with NF-kappa B, and the bound I kappa B alpha is degraded by the 26S proteasome. Thus, ubiquitination provides a mechanistic link between phosphorylation and degradation of I kappa B alpha.

Correction of radiation sensitivity in ataxia telangiectasia cells by a truncated I kappa B-alpha

Cells from patients with ataxia telangiectasia (AT) are hypersensitive to ionizing radiation and are defective in the regulation of DNA synthesis. A complementary DNA that corrects the radiation sensitivity and DNA synthesis defects in fibroblasts from an AT group D patient was isolated by expression cloning and shown to encode a truncated form of I kappa B-alpha, an inhibitor of the nuclear factor kappa B (NF-kappa B) transcriptional activator. The parental AT fibroblasts expressed large amounts of the I kappa B-alpha transcript and showed constitutive activation of NF-kappa B. The AT fibroblasts transfected with the truncated I kappa B-alpha expressed normal amounts of the I kappa B-alpha transcript and showed regulated activation of NF-kappa B. These results suggest that aberrant regulation of NF-kappa B and I kappa B-alpha contribute to the cellular defect in AT.

Phosphorylation of human I kappa B-alpha on serines 32 and 36 controls I kappa B-alpha proteolysis and NF-kappa B activation in response to diverse stimuli

Post-translational activation of the higher eukaryotic transcription factor NF-kappa B requires both phosphorylation and proteolytic degradation of the inhibitory subunit I kappa B-alpha. Inhibition of proteasome activity can stabilize an inducibly phosphorylated form of I kappa B-alpha in intact cells, suggesting that phosphorylation targets the protein for degradation. In this study, we have identified serines 32 and 36 in human I kappa B-alpha as essential for the control of I kappa B-alpha stability and the activation of NF-kappa B in HeLa cells. A point mutant substituting serines 32 and 36 by alanine residues was no longer phosphorylated in response to okadaic acid (OA) stimulation. This and various other Ser32 and Ser36 mutants behaved as potent dominant negative I kappa B proteins attenuating kappa B-dependent transactivation in response to OA, phorbol 12-myristate 13-acetate (PMA) and tumor necrosis factor-alpha (TNF). While both endogenous and transiently expressed wild-type I kappa B-alpha were proteolytically degraded in response to PMA and TNF stimulation of cells, the S32/36A mutant of I kappa B-alpha remained largely intact under these conditions. Our data suggest that such diverse stimuli as OA, TNF and PMA use the same kinase system to phosphorylate and thereby destabilize I kappa B-alpha, leading to NF-kappa B activation.

Cactus protein degradation mediates Drosophila dorsal-ventral signaling

Dorsal-ventral patterning in the Drosophila embryo relies on a signal transduction pathway that is similar to a signaling pathway leading to the activation of the mammalian transcription factor NF-kappa B. Stimulation of this Drosophila pathway on the ventral side of the embryo causes the nuclear translocation of Dorsal, the Drosophila NF-kappa B homolog. Cactus, like its mammalian homolog I kappa B, inhibits nuclear translocation by binding Dorsal and retaining it in the cytoplasm. We show that Cactus, like I kappa B, is rapidly degraded in response to signaling. More importantly, signal-dependent degradation of Cactus does not require the presence of Dorsal, indicating that Cactus degradation is a direct response to signaling, and that disruption of the Dorsal/Cactus complex is a secondary result of Cactus degradation. Mutant alleles of cactus that encode more stable forms of the protein block signaling, showing that efficient degradation is necessary for signaling. We find that Cactus protein stability is regulated by two independent processes that rely on different regions within the protein: signal-dependent degradation requires sequences in the amino terminus or ankyrin repeats, whereas signal-independent degradation of free Cactus requires the carboxy-terminal region of the protein that includes a PEST sequence.