Center for Integrated Protein Science Munich and Department of Biology I, Mikrobiologie, Biozentrum, Ludwig-Maximilians-Universität München, Martinsried, Germany.
J Mol Biol. 2012 Nov 23;424(1-2):15-27. doi: 10.1016/j.jmb.2012.08.023. Epub 2012 Sep 18.
At acidic pH and in the presence of lysine, the pH sensor CadC activates transcription of the cadBA operon encoding the lysine/cadaverine antiporter CadB and the lysine decarboxylase CadA. In effect, these proteins contribute to acid stress adaptation in Escherichia coli. cadBA expression is feedback inhibited by cadaverine, and a cadaverine binding site is predicted within the central cavity of the periplasmic domain of CadC on the basis of its crystallographic analysis. Our present study demonstrates that this site only partially accounts for the cadaverine response in vivo. Instead, evidence for a second, pivotal binding site was collected, which overlaps with the pH-responsive patch of amino acids located at the dimer interface of the periplasmic domain. The temporal response of the E. coli Cad module upon acid shock was measured and modeled for two CadC variants with mutated cadaverine binding sites. These studies supported a cascade-like binding and deactivation model for the CadC dimer: binding of cadaverine within the pair of central cavities triggers a conformational transition that exposes two further binding sites at the dimer interface, and the occupation of those stabilizes the inactive conformation. Altogether, these data represent a striking example for the deactivation of a pH sensor.
在酸性 pH 值条件下并存在赖氨酸的情况下，pH 值传感器 CadC 会激活编码赖氨酸/尸胺反向转运蛋白 CadB 和赖氨酸脱羧酶 CadA 的 cadBA 操纵子的转录。实际上，这些蛋白有助于大肠杆菌适应酸应激。CadBA 的表达受到尸胺的反馈抑制，并且根据其晶体学分析，预测 CadC 的周质域中央腔中存在尸胺结合位点。我们目前的研究表明，该位点仅部分解释了体内的尸胺反应。相反，收集了第二个关键结合位点的证据，该位点与位于周质域二聚体界面的 pH 响应氨基酸补丁重叠。在酸冲击下，测量并模拟了具有突变尸胺结合位点的两种 CadC 变体的大肠杆菌 Cad 模块的时间响应。这些研究支持 CadC 二聚体的级联式结合和失活模型：在一对中央腔中结合尸胺会触发构象转变，从而在二聚体界面上暴露另外两个结合位点，并且这些位点的占据稳定了非活性构象。总之，这些数据代表了 pH 值传感器失活的一个显著例子。
