DiDonato J A, Mercurio F, Karin M
Department of Pharmacology, University of California, San Diego School of Medicine, La Jolla 92093-0636.
Mol Cell Biol. 1995 Mar;15(3):1302-11. doi: 10.1128/MCB.15.3.1302.
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.
核因子κB是免疫和炎症反应基因的重要激活剂。在未受刺激的细胞中，核因子κB通过与IκB抑制因子相互作用而被隔离在细胞质中。这些无活性的复合物会响应多种细胞外信号而解离，从而使游离的核因子κB二聚体能够转运至细胞核并激活特定靶基因的转录。目前的主流观点认为，IκB的磷酸化导致无活性复合物的解离，而IκB的快速降解使这一过程不可逆。在此，我们表明，核因子κB活性诱导剂会刺激其中一种IκB即IκBα发生过度磷酸化。然而，与目前的主流观点相反，IκBα的过度磷酸化形式仍与核因子κB成分如RelA（p65）相关联。因此，IκBα的磷酸化不足以导致无活性的核因子κB：IκBα复合物解离。不过，该复合物会因响应细胞外信号而对磷酸化的IκBα进行选择性降解从而被破坏。使用多种蛋白酶抑制剂，其中一些对多催化蛋白酶复合物具有特异性，我们证明IκBα的降解是核因子κB激活所必需的。这些实验结果更符合一个新模型，即与核因子κB相关联的IκBα磷酸化标志其将被蛋白水解降解。IκBα在与核因子κB结合时被降解。IκBα的选择性降解释放出活性核因子κB二聚体，其可转运至细胞核以激活特定靶基因。
