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标题：Selectivity filter ion binding affinity determines inactivation in a potassium channel
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作者：Céline Boiteux ; David J. Posson ; Toby W. Allen 等
期刊名称：Proceedings of the National Academy of Sciences
印刷版ISSN：0027-8424
电子版ISSN：1091-6490
出版年度：2020
卷号：117
期号：47
页码：29968-29978
DOI：10.1073/pnas.2009624117
出版社：The National Academy of Sciences of the United States of America
摘要：Potassium channels can become nonconducting via inactivation at a gate inside the highly conserved selectivity filter (SF) region near the extracellular side of the membrane. In certain ligand-gated channels, such as BK channels and MthK, a Ca 2 -activated K channel from Methanobacterium thermoautotrophicum , the SF has been proposed to play a role in opening and closing rather than inactivation, although the underlying conformational changes are unknown. Using X-ray crystallography, identical conductive MthK structures were obtained in wide-ranging K concentrations (6 to 150 mM), unlike KcsA, whose SF collapses at low permeant ion concentrations. Surprisingly, three of the SF’s four binding sites remained almost fully occupied throughout this range, indicating high affinities (likely submillimolar), while only the central S2 site titrated, losing its ion at 6 mM, indicating low K affinity (∼50 mM). Molecular simulations showed that the MthK SF can also collapse in the absence of K , similar to KcsA, but that even a single K binding at any of the SF sites, except S4, can rescue the conductive state. The uneven titration across binding sites differs from KcsA, where SF sites display a uniform decrease in occupancy with K concentration, in the low millimolar range, leading to SF collapse. We found that ions were disfavored in MthK’s S2 site due to weaker coordination by carbonyl groups, arising from different interactions with the pore helix and water behind the SF. We conclude that these differences in interactions endow the seemingly identical SFs of KcsA and MthK with strikingly different inactivating phenotypes.
关键词：inactivation ; crystallography ; selectivity filter ; potassium channel ; molecular dynamics