Department of Chemistry and Biochemistry, University of California, San Diego, 9500 Gilman Drive, La Jolla, CA 92092-0378, USA.
Proc Natl Acad Sci U S A. 2011 Jun 21;108(25):10178-83. doi: 10.1073/pnas.1102226108. Epub 2011 May 31.
IκBα is a crucial regulator of NFκB transcription. NFκB-mediated gene activation is robust because levels of free IκBα are kept extremely low by rapid, ubiquitin-independent degradation of newly synthesized IκBα. IκBα has a weakly folded ankyrin repeat 5-6 (AR5-6) region that is critical in establishing its short intracellular half-life. The AR5-6 region of IκBα folds upon binding to NFκB. The NFκB-bound IκBα has a long half-life and requires ubiquitin-targeted degradation. We present single molecule FRET evidence that the native state of IκBα transiently populates an intrinsically disordered state characterized by a more extended structure and fluctuations on the millisecond time scale. Binding to NFκB or introduction of stabilizing mutations in AR 6 suppressed the fluctuations, whereas higher temperature or small amounts of urea increased them. The results reveal that intrinsically disordered protein regions transition between collapsed and extended conformations under native conditions.
IκBα 是 NFκB 转录的关键调节因子。NFκB 介导的基因激活非常强劲，因为新合成的 IκBα 会通过快速、非泛素依赖性降解，使游离 IκBα 的水平保持极低。IκBα 具有一个折叠较弱的锚蛋白重复序列 5-6（AR5-6）区域，该区域对于其短的细胞内半衰期至关重要。IκBα 的 AR5-6 区域在与 NFκB 结合时会折叠。与 NFκB 结合的 IκBα 半衰期较长，需要泛素靶向降解。我们提供了单分子 FRET 证据，表明 IκBα 的天然状态会短暂地进入一种固有无序状态，其特征是结构更伸展，在毫秒时间尺度上存在波动。与 NFκB 结合或在 AR6 中引入稳定突变会抑制波动，而较高的温度或少量的脲则会增加波动。这些结果表明，在天然条件下，固有无序的蛋白质区域会在折叠和伸展构象之间发生转变。
