Phosphorylation of I kappa B alpha precedes but is not sufficient for its dissociation from NF-kappa B.
NF-kappa B is an important activator of immune and inflammatory response genes. NF-kappa B is sequestered in the cytoplasm of nonstimulated cells through interaction with the I kappa B inhibitors. These inactive complexes are dissociated in response to a variety of extracellular signals, thereby allowing free NF-kappa B dimers to translocate to the nucleus and active transcription of specific target genes. The current dogma is that phosphorylation of the I kappa Bs is responsible for dissociation of the inactive complexes, an event that is rendered irreversible by rapid I kappa B degradation. Here, we show that inducers of NF-kappa B activity stimulate the hyperphosphorylation of one of the I kappa Bs, I kappa B alpha. However, contrary to the present dogma the hyperphosphorylated form of I kappa B alpha remains associated with NF-kappa B components such as RelA (p65). Thus, phosphorylation of I kappa B alpha is not sufficient to cause dissociation of the inactive NF-kappa B:I kappa B alpha complex. However, that complex is disrupted through the selective degradation of phosphorylated I kappa B alpha in response to extracellular signals. Using a variety of protease inhibitors, some of which have specificity towards the multicatalytic proteinase complex, we demonstrate that degradation of I kappa B alpha is required for NF-kappa B activation. The results of these experiments are more consistent with a new model according to which phosphorylation of I kappa B alpha associated with NF-kappa B marks it for proteolytic degradation. I kappa B alpha is degraded while bound to NF-kappa B. The selective degradation of I kappa B alpha releases active NF-kappa B dimers which can translocate to the nucleus to activate specific target genes.
DiDonato
J A
JA
Department of Pharmacology, University of California, San Diego School of Medicine, La Jolla 92093-0636.
Mercurio
F
F
Karin
M
M
D000595:Amino Acid Sequence; D006528:Carcinoma, Hepatocellular; D003546:Cysteine Endopeptidases / Q000378:metabolism; D004268:DNA-Binding Proteins / Q000302:isolation & purification / Q000378:metabolism*; D004591:Electrophoresis, Polyacrylamide Gel; D006367:HeLa Cells; D006801:Humans; D020672:I-kappa B Proteins; D008113:Liver Neoplasms; D046911:Macromolecular Substances; D008954:Models, Biological; D008969:Molecular Sequence Data; D009097:Multienzyme Complexes / Q000378:metabolism; D000072000:NF-KappaB Inhibitor alpha; D016328:NF-kappa B / Q000037:antagonists & inhibitors / Q000302:isolation & purification / Q000378:metabolism*; D009842:Oligopeptides / Q000494:pharmacology; D010766:Phosphorylation; D011480:Protease Inhibitors / Q000494:pharmacology; D046988:Proteasome Endopeptidase Complex; D015398:Signal Transduction; D051996:Transcription Factor RelA; D014162:Transfection; D014407:Tumor Cells, Cultured
D004268:DNA-Binding Proteins; D020672:I-kappa B Proteins; D046911:Macromolecular Substances; D009097:Multienzyme Complexes; D016328:NF-kappa B; C000606204:NFKBIA protein, human; D009842:Oligopeptides; D011480:Protease Inhibitors; D051996:Transcription Factor RelA; D000072000:NF-KappaB Inhibitor alpha; D003546:Cysteine Endopeptidases; D046988:Proteasome Endopeptidase Complex