Department of Biochemistry, Dartmouth Medical School Hanover, NH, USA.
Front Mol Neurosci. 2012 Jan 2;4:56. doi: 10.3389/fnmol.2011.00056. eCollection 2012.
Ligand-gated ion channels couple the free energy of agonist binding to the gating of selective transmembrane ion pores, permitting cells to regulate ion flux in response to external chemical stimuli. However, the stereochemical mechanisms responsible for this coupling remain obscure. In the case of the ionotropic glutamate receptors (iGluRs), the modular nature of receptor subunits has facilitated structural analysis of the N-terminal domain (NTD), and of multiple conformations of the ligand-binding domain (LBD). Recently, the crystallographic structure of an antagonist-bound form of the receptor was determined. However, disulfide trapping of this conformation blocks channel opening, suggesting that channel activation involves additional quaternary packing arrangements. To explore the conformational space available to iGluR channels, we report here a second, clearly distinct domain architecture of homotetrameric, calcium-permeable AMPA receptors, determined by single-particle electron microscopy of untagged and fluorescently tagged constructs in a ligand-free state. It reveals a novel packing of NTD dimers, and a separation of LBD dimers across a central vestibule. In this arrangement, which reconciles diverse functional observations, agonist-induced cleft closure across LBD dimers can be converted into a twisting motion that provides a basis for receptor activation.
配体门控离子通道将激动剂结合的自由能与选择性跨膜离子孔的门控耦合起来，使细胞能够响应外部化学刺激来调节离子通量。然而，负责这种耦合的立体化学机制仍然不清楚。在离子型谷氨酸受体（iGluRs）的情况下，受体亚基的模块化性质促进了对 N 端结构域（NTD）和配体结合结构域（LBD）的多种构象的结构分析。最近，受体的拮抗剂结合形式的晶体结构被确定。然而，这种构象的二硫键捕获会阻止通道打开，这表明通道激活涉及额外的四级包装排列。为了探索 iGluR 通道可用的构象空间，我们在这里报告了另一种明显不同的同四聚体、钙通透型 AMPA 受体的结构域架构，这是通过未标记和荧光标记构建体在无配体状态下单粒子电子显微镜确定的。它揭示了 NTD 二聚体的一种新包装方式，以及 LBD 二聚体在中央前庭的分离。在这种排列中，不同的功能观察结果可以协调一致，配体诱导的 LBD 二聚体裂隙关闭可以转化为扭曲运动，为受体激活提供基础。
