Activation of NF-kappa B requires proteolysis of the inhibitor I kappa B-alpha: signal-induced phosphorylation of I kappa B-alpha alone does not release active NF-kappa B.
The transcription factor NF-kappa B is retained in the cytoplasm by its inhibitor I kappa B-alpha. Upon cellular stimulation with a variety of pathogen- or stress-related agents, I kappa B-alpha is functionally inactivated and NF-kappa B translocates to the nucleus to trigger transcription of a large array of genes, many of which encode proteins critical for immune or stress responses. Here, we demonstrate that signal-induced proteolysis of I kappa B-alpha is an obligatory step for activation of NF-kappa B: calpain inhibitors I and II, which inhibit cysteine proteases, block activation of NF-kappa B by blocking degradation of I kappa B-alpha without affecting signal-induced phosphorylation of this inhibitor. This contrasts with previous models in which phosphorylation of I kappa B-alpha was postulated to be sufficient for activation. We demonstrate further that signal-induced phosphorylation of I kappa B-alpha does not by itself lead to dissociation of the inhibitor from NF-kappa B, providing a rationale for and confirmation of the need to proteolyze I kappa B-alpha in order to activate NF-kappa B. Signal-controlled, target-specific proteolysis is an unexpected, yet likely more general, mechanism for regulating transcription factors.
Laboratory of Immunoregulation, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Bethesda, MD 20892-1876.
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Siebenlist
D002154:Calpain / Q000037:antagonists & inhibitors; D002478:Cells, Cultured; D003546:Cysteine Endopeptidases / Q000187:drug effects / Q000378:metabolism*; D015853:Cysteine Proteinase Inhibitors / Q000494:pharmacology; D004268:DNA-Binding Proteins / Q000187:drug effects / Q000378:metabolism*; D006023:Glycoproteins / Q000494:pharmacology; D020672:I-kappa B Proteins; D000072000:NF-KappaB Inhibitor alpha; D016328:NF-kappa B / Q000037:antagonists & inhibitors / Q000378:metabolism*; D010766:Phosphorylation; D011485:Protein Binding; D015398:Signal Transduction
D015853:Cysteine Proteinase Inhibitors; D004268:DNA-Binding Proteins; D006023:Glycoproteins; D020672:I-kappa B Proteins; D016328:NF-kappa B; C039304:calpain inhibitors; D000072000:NF-KappaB Inhibitor alpha; D002154:Calpain; D003546:Cysteine Endopeptidases