Department of Molecular and Integrative Physiology, Center for Biophysics and Computational Biology, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA.
Nature. 2011 May 22;474(7352):526-30. doi: 10.1038/nature10015.
Among ion channels, only the nicotinic-receptor superfamily has evolved to generate both cation- and anion-selective members. Although other, structurally unrelated, neurotransmitter-gated cation channels exist, no other type of neurotransmitter-gated anion channel, and thus no other source of fast synaptic inhibitory signals, has been described so far. In addition to the seemingly straightforward electrostatic effect of the presence (in the cation-selective members) or absence (in the anion-selective ones) of a ring of pore-facing carboxylates, mutational studies have identified other features of the amino-acid sequence near the intracellular end of the pore-lining transmembrane segments (M2) that are also required to achieve the high charge selectivity shown by native channels. However, the mechanism underlying this more subtle effect has remained elusive and a subject of speculation. Here we show, using single-channel electrophysiological recordings to estimate the protonation state of native ionizable side chains, that anion-selective-type sequences favour whereas cation-selective-type sequences prevent the protonation of the conserved, buried basic residues at the intracellular entrance of the pore (the M2 0' position). We conclude that the previously unrecognized tunable charge state of the 0' ring of buried basic side chains is an essential feature of these channels' versatile charge-selectivity filter.
在离子通道中，只有烟碱型受体超家族进化出了既能选择阳离子又能选择阴离子的成员。尽管存在其他结构上不相关的神经递质门控阳离子通道，但迄今为止还没有描述过其他类型的神经递质门控阴离子通道，也没有其他快速突触抑制信号的来源。除了存在（在阳离子选择性成员中）或不存在（在阴离子选择性成员中）面向孔的羧酸盐环的明显静电效应之外，突变研究还确定了位于孔衬跨膜片段（M2）的细胞内末端附近的氨基酸序列的其他特征，这些特征对于实现天然通道所示的高电荷选择性也是必需的。然而，这种更微妙效应的机制仍然难以捉摸，成为推测的主题。在这里，我们使用单通道电生理记录来估计天然可离解侧链的质子化状态，表明阴离子选择性序列有利于而阳离子选择性序列阻止孔内入口处保守的埋藏碱性残基（M2 0'位置）的质子化。我们得出结论，以前未被识别的埋藏碱性侧链 0'环的可调电荷状态是这些通道多功能电荷选择性过滤器的一个重要特征。
