Center for Biophysics and Quantitative Biology, Urbana, Illinois.
Center for Biophysics and Quantitative Biology, Urbana, Illinois; Department of Molecular and Integrative Physiology, Urbana, Illinois; Neuroscience Program, University of Illinois at Urbana-Champaign, Urbana, Illinois.
Biophys J. 2019 May 7;116(9):1667-1681. doi: 10.1016/j.bpj.2019.03.022. Epub 2019 Apr 2.
Whether synaptic transmission is excitatory or inhibitory depends, to a large extent, on whether the ion channels that open upon binding the released neurotransmitter conduct cations or anions. The mechanistic basis of the opposite charge selectivities of Cys-loop receptors has only recently begun to emerge. It is now clear that ionized side chains-whether pore-facing or buried-in the first α-helical turn of the second transmembrane segments underlie this phenomenon and that the electrostatics of backbone atoms are not critically involved. Moreover, on the basis of electrophysiological observations, it has recently been suggested that not only the sign of charged side chains but also their conformation are crucial determinants of cation-anion selectivity. To challenge these ideas with the chemical and structural rigor that electrophysiological observations naturally lack, we performed molecular dynamics, Brownian dynamics, and electrostatics calculations of ion permeation. To this end, we used structural models of the open-channel conformation of the α1 glutamate-gated Cl channel and the α1 glycine receptor. Our results provided full support to the notion that the conformation of charged sides chains matters for charge selectivity. Indeed, whereas some rotamers of the buried arginines at position 0' conferred high selectivity for anions, others supported the permeation of cations and anions at similar rates or even allowed the faster permeation of cations. Furthermore, we found that modeling glutamates at position -1' of the anion-selective α1 glycine receptor open-state structure-instead of the five native alanines-switches charge selectivity also in a conformation-dependent manner, with some glutamate rotamers being much more effective at conferring selectivity for cations than others. Regarding pore size, we found that the mere expansion of the pore has only a minimal impact on cation-anion selectivity. Overall, these results bring to light the previously unappreciated impact of side-chain conformation on charge selectivity in Cys-loop receptors.
无论突触传递是兴奋性还是抑制性的，在很大程度上取决于结合释放的神经递质后打开的离子通道是传导阳离子还是阴离子。Cys 环受体相反电荷选择性的机制基础最近才开始显现。现在很清楚，带电荷的侧链——无论是面向孔的还是埋藏在第二个跨膜片段的第一α螺旋转弯中的——是产生这种现象的基础，而骨架原子的静电作用并不起关键作用。此外，根据电生理观察，最近有人提出，不仅带电侧链的符号，而且它们的构象，也是阳离子-阴离子选择性的关键决定因素。为了用电生理观察自然缺乏的化学和结构严谨性来挑战这些观点，我们进行了离子渗透的分子动力学、布朗动力学和静电计算。为此，我们使用了α1 谷氨酸门控 Cl 通道和α1 甘氨酸受体的开放通道构象的结构模型。我们的结果完全支持这样一种观点，即带电侧链的构象对电荷选择性很重要。事实上，尽管位置 0'的埋藏精氨酸的一些旋转异构体赋予阴离子高选择性，而其他旋转异构体则支持阳离子和阴离子以相似的速率渗透，甚至允许阳离子更快地渗透。此外，我们发现，在阴离子选择性α1 甘氨酸受体开放态结构中模拟位置-1'的谷氨酸——而不是 5 个天然的丙氨酸——也以构象依赖的方式切换电荷选择性，一些谷氨酸旋转异构体比其他旋转异构体更有效地赋予对阳离子的选择性。关于孔径，我们发现仅仅扩大孔径对阳离子-阴离子选择性只有最小的影响。总的来说，这些结果揭示了侧链构象对 Cys 环受体中电荷选择性的以前未被认识到的影响。
