Hilf Ricarda J C, Dutzler Raimund
Department of Biochemistry, University of Zürich, Winterthurerstrasse 190, CH-8057 Zürich, Switzerland.
Nature. 2009 Jan 1;457(7225):115-8. doi: 10.1038/nature07461. Epub 2008 Nov 5.
The X-ray structure of a pentameric ligand-gated ion channel from Erwinia chrysanthemi (ELIC) has recently provided structural insight into this family of ion channels at high resolution. The structure shows a homo-pentameric protein with a barrel-stave architecture that defines an ion-conduction pore located on the fivefold axis of symmetry. In this structure, the wide aqueous vestibule that is encircled by the extracellular ligand-binding domains of the five subunits narrows to a discontinuous pore that spans the lipid bilayer. The pore is constricted by bulky hydrophobic residues towards the extracellular side, which probably serve as barriers that prevent the diffusion of ions. This interrupted pore architecture in ELIC thus depicts a non-conducting conformation of a pentameric ligand-gated ion channel, the thermodynamically stable state in the absence of bound ligand. As ligand binding promotes pore opening in these ion channels and the specific ligand for ELIC has not yet been identified, we have turned our attention towards a homologous protein from the cyanobacterium Gloebacter violaceus (GLIC). GLIC was shown to form proton-gated channels that are activated by a pH decrease on the extracellular side and that do not desensitize after activation. Both prokaryotic proteins, ELIC and GLIC form ion channels that are selective for cations over anions with poor discrimination among monovalent cations, characteristics that resemble the conduction properties of the cation-selective branch of the family that includes acetylcholine and serotonin receptors. Here we present the X-ray structure of GLIC at 3.1 A resolution. The structure reveals a conformation of the channel that is distinct from ELIC and that probably resembles the open state. In combination, both structures suggest a novel gating mechanism for pentameric ligand-gated ion channels where channel opening proceeds by a change in the tilt of the pore-forming helices.
来自菊欧文氏菌（ELIC）的五聚体配体门控离子通道的X射线结构最近在高分辨率下为该离子通道家族提供了结构上的见解。该结构显示了一个具有桶板结构的同五聚体蛋白，其定义了一个位于五重对称轴上的离子传导孔。在这个结构中，由五个亚基的细胞外配体结合域环绕的宽阔水相前庭变窄为一个跨越脂质双层的不连续孔。该孔在细胞外侧被庞大的疏水残基收缩，这些疏水残基可能作为阻止离子扩散的屏障。因此，ELIC中这种中断的孔结构描绘了五聚体配体门控离子通道的非传导构象，即无结合配体时的热力学稳定状态。由于配体结合促进这些离子通道的孔开放，且ELIC的特异性配体尚未确定，我们将注意力转向了来自蓝细菌紫球藻（GLIC）的同源蛋白。GLIC被证明能形成质子门控通道，该通道在细胞外侧pH降低时被激活，且激活后不会脱敏。这两种原核蛋白，ELIC和GLIC，都形成对阳离子具有选择性而对阴离子选择性较差的离子通道，在单价阳离子之间区分能力较差，这些特征类似于包括乙酰胆碱和5-羟色胺受体在内的该家族阳离子选择性分支的传导特性。在这里，我们展示了分辨率为3.1埃的GLIC的X射线结构。该结构揭示了一种与ELIC不同的通道构象，可能类似于开放状态。综合起来，这两种结构为五聚体配体门控离子通道提出了一种新的门控机制，即通道开放是通过成孔螺旋倾斜度的变化来实现的。
