Institute of Biology, Cellular Biophysics, Humboldt Universität zu Berlin, 10115 Berlin, Germany.
NeuroCure Cluster of Excellence, Charité-Universitätsmedizin Berlin, corporate member of Freie Universität Berlin and Humboldt-Universität zu Berlin, and Berlin Institute of Health, 10117 Berlin, Germany.
Proc Natl Acad Sci U S A. 2021 Feb 23;118(8). doi: 10.1073/pnas.2012843118.
Fast excitatory synaptic transmission in the central nervous system relies on the AMPA-type glutamate receptor (AMPAR). This receptor incorporates a nonselective cation channel, which is opened by the binding of glutamate. Although the open pore structure has recently became available from cryo-electron microscopy (Cryo-EM), the molecular mechanisms governing cation permeability in AMPA receptors are not understood. Here, we combined microsecond molecular dynamic (MD) simulations on a putative open-state structure of GluA2 with electrophysiology on cloned channels to elucidate ion permeation mechanisms. Na, K, and Cs permeated at physiological rates, consistent with a structure that represents a true open state. A single major ion binding site for Na and K in the pore represents the simplest selectivity filter (SF) structure for any tetrameric cation channel of known structure. The minimal SF comprised only Q586 and Q587, and other residues on the cytoplasmic side formed a water-filled cavity with a cone shape that lacked major interactions with ions. We observed that Cl readily enters the upper pore, explaining anion permeation in the RNA-edited (Q586R) form of GluA2. A permissive architecture of the SF accommodated different alkali metals in distinct solvation states to allow rapid, nonselective cation permeation and copermeation by water. Simulations suggested Cs uses two equally populated ion binding sites in the filter, and we confirmed with electrophysiology of GluA2 that Cs is slightly more permeant than Na, consistent with serial binding sites preferentially driving selectivity.
中枢神经系统中的快速兴奋性突触传递依赖于 AMPA 型谷氨酸受体 (AMPAR)。该受体包含一个非选择性阳离子通道，其由谷氨酸结合而打开。尽管最近已经从冷冻电子显微镜 (Cryo-EM) 获得了开放孔结构，但 AMPA 受体中阳离子通透性的分子机制尚不清楚。在这里，我们结合了对假定的 GluA2 开放状态结构的微秒分子动力学 (MD) 模拟和对克隆通道的电生理学研究，以阐明离子渗透机制。Na+、K+和 Cs+以生理速率渗透，这与代表真实开放状态的结构一致。孔中的单个主要 Na+和 K+离子结合位点代表了已知结构的任何四聚体阳离子通道的最简单选择性过滤器 (SF) 结构。最小 SF 仅由 Q586 和 Q587 组成，细胞质侧的其他残基形成一个充满水的锥形空腔，与离子没有主要相互作用。我们观察到 Cl- 很容易进入上孔，这解释了 GluA2 的 RNA 编辑 (Q586R) 形式中的阴离子渗透。SF 的许可结构容纳了不同的碱金属处于不同的溶剂化状态，从而允许快速、非选择性的阳离子渗透和水的共渗透。模拟表明 Cs+在过滤器中使用两个同样占据的离子结合位点，我们通过 GluA2 的电生理学证实了 Cs+比 Na+稍微更容易渗透，这与优先驱动选择性的串联结合位点一致。
