Department of Molecular and Cellular Biochemistry, Indiana University, Bloomington, IN 47405, USA.
Biochem Biophys Res Commun. 2012 Jan 6;417(1):129-34. doi: 10.1016/j.bbrc.2011.11.070. Epub 2011 Nov 25.
Many Gram-negative bacteria utilize a type III secretion system (T3SS) to deliver protein effectors to target host cells. Transcriptional control of T3SS gene expression is generally coupled to secretion through the release of a regulatory protein. T3SS gene expression in Pseudomonas aeruginosa is regulated by extracellular secretion of ExsE. ExsE is a small 81 residue protein that appears to lack a stable structural core as indicated by previous studies. In this study, we employed various NMR methods to characterize the structure of ExsE alone and when bound to its secretion chaperone ExsC. We found that ExsE is largely unfolded throughout the polypeptide chain, belonging to a class of proteins that are intrinsically disordered. The unfolded, extended conformation of ExsE may expedite efficient secretion through the narrow path of the T3SS secretion channel to activate gene expression in a timely manner. We also found that the structurally flexible ExsE samples through conformations with localized structurally ordered regions. Importantly, these transiently ordered elements are related to the secondary structures involved in binding ExsC based on a prior crystal structure of the ExsC-ExsE complex. These findings support the notion that preexisting structured elements facilitate binding of intrinsically disordered proteins to their targets.
许多革兰氏阴性菌利用 III 型分泌系统（T3SS）将蛋白效应物递送至靶宿主细胞。T3SS 基因表达的转录控制通常与通过释放调节蛋白的分泌偶联。铜绿假单胞菌的 T3SS 基因表达受 ExsE 的细胞外分泌调控。ExsE 是一种小的 81 个残基的蛋白质，以前的研究表明，它似乎缺乏稳定的结构核心。在这项研究中，我们采用了各种 NMR 方法来单独表征 ExsE 的结构及其与分泌伴侣 ExsC 结合时的结构。我们发现 ExsE 在整个多肽链中主要是无规卷曲的，属于一类固有无序的蛋白质。ExsE 的无规卷曲、伸展构象可能会通过 T3SS 分泌通道的狭窄路径促进有效的分泌，从而及时激活基因表达。我们还发现结构灵活的 ExsE 可以通过构象变化来采样局部有序区域。重要的是，这些瞬态有序元素与根据 ExsC-ExsE 复合物的先前晶体结构涉及与 ExsC 结合的二级结构有关。这些发现支持了这样一种观点，即预先存在的结构元件有助于将固有无序的蛋白质与其靶标结合。
