Department of Biochemistry, University of Zürich, Zürich, Switzerland.
Department Biozentrum, University of Basel, Basel, Switzerland.
Elife. 2020 Apr 8;9:e53683. doi: 10.7554/eLife.53683.
The TMEM175 family constitutes recently discovered Kchannels that are important for autophagosome turnover and lysosomal pH regulation and are associated with the early onset of Parkinson Disease. TMEM175 channels lack a P-loop selectivity filter, a hallmark of all known K channels, raising the question how selectivity is achieved. Here, we report the X-ray structure of a closed bacterial TMEM175 channel in complex with a nanobody fusion-protein disclosing bound K ions. Our analysis revealed that a highly conserved layer of threonine residues in the pore conveys a basal K selectivity. An additional layer comprising two serines in human TMEM175 increases selectivity further and renders this channel sensitive to 4-aminopyridine and Zn. Our findings suggest that large hydrophobic side chains occlude the pore, forming a physical gate, and that channel opening by iris-like motions simultaneously relocates the gate and exposes the otherwise concealed selectivity filter to the pore lumen.
TMEM175 家族构成了最近发现的 K 通道，这些通道对于自噬体周转和溶酶体 pH 调节很重要，并且与帕金森病的早期发病有关。TMEM175 通道缺乏 P 环选择性过滤器，这是所有已知 K 通道的标志，这引发了一个问题，即如何实现选择性。在这里，我们报告了一个封闭的细菌 TMEM175 通道与纳米体融合蛋白复合物的 X 射线结构，揭示了结合的 K 离子。我们的分析表明，在孔中的一层高度保守的苏氨酸残基传递基本的 K 选择性。在人类 TMEM175 中包含两个丝氨酸的额外层进一步增加了选择性，并使该通道对 4-氨基吡啶和 Zn 敏感。我们的发现表明，大的疏水性侧链阻塞孔，形成物理门，并且通过虹膜样运动打开通道同时重新定位门，并将原本隐藏的选择性过滤器暴露于孔腔中。
