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.
Lin
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YC
Laboratory of Immunoregulation, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Bethesda, MD 20892-1876.
Brown
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K
Siebenlist
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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