1
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Öster C, Lange S, Hendriks K, Lange A. Detecting Bound Ions in Ion Channels by Solid-State NMR Experiments on 15N-Labelled Ammonium Ions. Methods Mol Biol 2024; 2796:23-34. [PMID: 38856893 DOI: 10.1007/978-1-0716-3818-7_2] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 06/11/2024]
Abstract
Solid-state NMR allows for the study of membrane proteins under physiological conditions. Here we describe a method for detection of bound ions in the selectivity filter of ion channels using solid-state NMR. This method employs standard 1H-detected solid-state NMR setup and experiment types, which is enabled by using 15N-labelled ammonium ions to mimic potassium ions.
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Affiliation(s)
- Carl Öster
- Research Unit Molecular Biophysics, Leibniz-Forschungsinstitut für Molekulare Pharmakologie (FMP), Berlin, Germany.
| | - Sascha Lange
- Research Unit Molecular Biophysics, Leibniz-Forschungsinstitut für Molekulare Pharmakologie (FMP), Berlin, Germany
| | - Kitty Hendriks
- Research Unit Molecular Biophysics, Leibniz-Forschungsinstitut für Molekulare Pharmakologie (FMP), Berlin, Germany
| | - Adam Lange
- Research Unit Molecular Biophysics, Leibniz-Forschungsinstitut für Molekulare Pharmakologie (FMP), Berlin, Germany.
- Institut für Biologie, Humboldt-Universität zu Berlin, Berlin, Germany.
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2
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Minniberger S, Abdolvand S, Braunbeck S, Sun H, Plested AJR. Asymmetry and Ion Selectivity Properties of Bacterial Channel NaK Mutants Derived from Ionotropic Glutamate Receptors. J Mol Biol 2023; 435:167970. [PMID: 36682679 DOI: 10.1016/j.jmb.2023.167970] [Citation(s) in RCA: 3] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/28/2022] [Revised: 12/17/2022] [Accepted: 01/12/2023] [Indexed: 01/22/2023]
Abstract
Ionotropic glutamate receptors are ligand-gated cation channels that play essential roles in the excitatory synaptic transmission throughout the central nervous system. A number of open-pore structures of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic-acid (AMPA)-type glutamate receptors became available recently by cryo-electron microscopy (cryo-EM). These structures provide valuable insights into the conformation of the selectivity filter (SF), the part of the ion channel that determines the ion selectivity. Nonetheless, due to the moderate resolution of the cryo-EM structures, detailed information such as ion occupancy of monovalent and divalent cations as well as positioning of the side-chains in the SF is still missing. Here, in an attempt to obtain high-resolution information about glutamate receptor SFs, we incorporated partial SF sequences of the AMPA and kainate receptors into the bacterial tetrameric cation channel NaK, which served as a structural scaffold. We determined a series of X-ray structures of NaK-CDI, NaK-SDI and NaK-SELM mutants at 1.42-2.10 Å resolution, showing distinct ion occupation of different monovalent cations. Molecular dynamics (MD) simulations of NaK-CDI indicated the channel to be conductive to monovalent cations, which agrees well with our electrophysiology recordings. Moreover, previously unobserved structural asymmetry of the SF was revealed by the X-ray structures and MD simulations, implying its importance in ion non-selectivity of tetrameric cation channels.
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Affiliation(s)
- Sonja Minniberger
- Institute of Biology, Cellular Biophysics, Humboldt Universität zu Berlin, 10115 Berlin, Germany; NeuroCure, Charité Universitätsmedizin, 10117 Berlin, Germany; Leibniz Forschungsinstitut für Molekulare Pharmakologie (FMP), 13125 Berlin, Germany
| | - Saeid Abdolvand
- Institute of Biology, Cellular Biophysics, Humboldt Universität zu Berlin, 10115 Berlin, Germany; NeuroCure, Charité Universitätsmedizin, 10117 Berlin, Germany; Leibniz Forschungsinstitut für Molekulare Pharmakologie (FMP), 13125 Berlin, Germany
| | - Sebastian Braunbeck
- Institute of Biology, Cellular Biophysics, Humboldt Universität zu Berlin, 10115 Berlin, Germany; NeuroCure, Charité Universitätsmedizin, 10117 Berlin, Germany; Leibniz Forschungsinstitut für Molekulare Pharmakologie (FMP), 13125 Berlin, Germany
| | - Han Sun
- Leibniz Forschungsinstitut für Molekulare Pharmakologie (FMP), 13125 Berlin, Germany; Institute of Chemistry, Department of Chemistry, Technische Universität Berlin, 10623 Berlin, Germany.
| | - Andrew J R Plested
- Institute of Biology, Cellular Biophysics, Humboldt Universität zu Berlin, 10115 Berlin, Germany; NeuroCure, Charité Universitätsmedizin, 10117 Berlin, Germany; Leibniz Forschungsinstitut für Molekulare Pharmakologie (FMP), 13125 Berlin, Germany.
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3
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Matamoros M, Ng XW, Brettmann JB, Piston DW, Nichols CG. Conformational plasticity of NaK2K and TREK2 potassium channel selectivity filters. Nat Commun 2023; 14:89. [PMID: 36609575 PMCID: PMC9822992 DOI: 10.1038/s41467-022-35756-7] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/18/2022] [Accepted: 12/23/2022] [Indexed: 01/07/2023] Open
Abstract
The K+ channel selectivity filter (SF) is defined by TxGYG amino acid sequences that generate four identical K+ binding sites (S1-S4). Only two sites (S3, S4) are present in the non-selective bacterial NaK channel, but a four-site K+-selective SF is obtained by mutating the wild-type TVGDGN SF sequence to a canonical K+ channel TVGYGD sequence (NaK2K mutant). Using single molecule FRET (smFRET), we show that the SF of NaK2K, but not of non-selective NaK, is ion-dependent, with the constricted SF configuration stabilized in high K+ conditions. Patch-clamp electrophysiology and non-canonical fluorescent amino acid incorporation show that NaK2K selectivity is reduced by crosslinking to limit SF conformational movement. Finally, the eukaryotic K+ channel TREK2 SF exhibits essentially identical smFRET-reported ion-dependent conformations as in prokaryotic K+ channels. Our results establish the generality of K+-induced SF conformational stability across the K+ channel superfamily, and introduce an approach to study manipulation of channel selectivity.
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Affiliation(s)
- Marcos Matamoros
- Center for Investigation of Membrane Excitability Diseases, Washington University School of Medicine, St. Louis, MO, USA
- Department of Cell Biology and Physiology, Washington University School of Medicine, St. Louis, MO, USA
| | - Xue Wen Ng
- Center for Investigation of Membrane Excitability Diseases, Washington University School of Medicine, St. Louis, MO, USA
- Department of Cell Biology and Physiology, Washington University School of Medicine, St. Louis, MO, USA
| | - Joshua B Brettmann
- Center for Investigation of Membrane Excitability Diseases, Washington University School of Medicine, St. Louis, MO, USA
- Millipore-Sigma Inc., St. Louis, MO, USA
| | - David W Piston
- Center for Investigation of Membrane Excitability Diseases, Washington University School of Medicine, St. Louis, MO, USA
- Department of Cell Biology and Physiology, Washington University School of Medicine, St. Louis, MO, USA
| | - Colin G Nichols
- Center for Investigation of Membrane Excitability Diseases, Washington University School of Medicine, St. Louis, MO, USA.
- Department of Cell Biology and Physiology, Washington University School of Medicine, St. Louis, MO, USA.
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Kurauskas V, Tonelli M, Henzler-Wildman K. Full opening of helix bundle crossing does not lead to NaK channel activation. J Gen Physiol 2022; 154:213659. [PMID: 36326620 PMCID: PMC9640265 DOI: 10.1085/jgp.202213196] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/18/2022] [Revised: 08/11/2022] [Accepted: 10/10/2022] [Indexed: 11/05/2022] Open
Abstract
A critical part of ion channel function is the ability to open and close in response to stimuli and thus conduct ions in a regulated fashion. While x-ray diffraction studies of ion channels suggested a general steric gating mechanism located at the helix bundle crossing (HBC), recent functional studies on several channels indicate that the helix bundle crossing is wide-open even in functionally nonconductive channels. Two NaK channel variants were crystallized in very different open and closed conformations, which served as important models of the HBC gating hypothesis. However, neither of these NaK variants is conductive in liposomes unless phenylalanine 92 is mutated to alanine (F92A). Here, we use NMR to probe distances at near-atomic resolution of the two NaK variants in lipid bicelles. We demonstrate that in contrast to the crystal structures, both NaK variants are in a fully open conformation, akin to Ca2+-bound MthK channel structure where the HBC is widely open. While we were not able to determine what a conductive NaK structure is like, our further inquiry into the gating mechanism suggests that the selectivity filter and pore helix are coupled to the M2 helix below and undergo changes in the structure when F92 is mutated. Overall, our data show that NaK exhibits coupling between the selectivity filter and HBC, similar to K+ channels, and has a more complex gating mechanism than previously thought, where the full opening of HBC does not lead to channel activation.
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Affiliation(s)
- Vilius Kurauskas
- Department of Biochemistry, University of Wisconsin—Madison, Madison, WI
| | - Marco Tonelli
- National Magnetic Resonance Facility at Madison, University of Wisconsin—Madison, Madison, WI
| | - Katherine Henzler-Wildman
- Department of Biochemistry, University of Wisconsin—Madison, Madison, WI
- National Magnetic Resonance Facility at Madison, University of Wisconsin—Madison, Madison, WI
- Correspondence to Katherine Henzler-Wildman:
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5
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Molecular Events behind the Selectivity and Inactivation Properties of Model NaK-Derived Ion Channels. Int J Mol Sci 2022; 23:ijms23169246. [PMID: 36012519 PMCID: PMC9409022 DOI: 10.3390/ijms23169246] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/08/2022] [Revised: 08/10/2022] [Accepted: 08/14/2022] [Indexed: 11/17/2022] Open
Abstract
Y55W mutants of non-selective NaK and partly K+-selective NaK2K channels have been used to explore the conformational dynamics at the pore region of these channels as they interact with either Na+ or K+. A major conclusion is that these channels exhibit a remarkable pore conformational flexibility. Homo-FRET measurements reveal a large change in W55–W55 intersubunit distances, enabling the selectivity filter (SF) to admit different species, thus, favoring poor or no selectivity. Depending on the cation, these channels exhibit wide-open conformations of the SF in Na+, or tight induced-fit conformations in K+, most favored in the four binding sites containing NaK2K channels. Such conformational flexibility seems to arise from an altered pattern of restricting interactions between the SF and the protein scaffold behind it. Additionally, binding experiments provide clues to explain such poor selectivity. Compared to the K+-selective KcsA channel, these channels lack a high affinity K+ binding component and do not collapse in Na+. Thus, they cannot properly select K+ over competing cations, nor reject Na+ by collapsing, as K+-selective channels do. Finally, these channels do not show C-type inactivation, likely because their submillimolar K+ binding affinities prevent an efficient K+ loss from their SF, thus favoring permanently open channel states.
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6
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Medeiros-Silva J, Somberg NH, Wang HK, McKay MJ, Mandala VS, Dregni AJ, Hong M. pH- and Calcium-Dependent Aromatic Network in the SARS-CoV-2 Envelope Protein. J Am Chem Soc 2022; 144:6839-6850. [PMID: 35380805 DOI: 10.1021/jacs.2c00973] [Citation(s) in RCA: 16] [Impact Index Per Article: 8.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/24/2022]
Abstract
The envelope (E) protein of the SARS-CoV-2 virus is a membrane-bound viroporin that conducts cations across the endoplasmic reticulum Golgi intermediate compartment (ERGIC) membrane of the host cell to cause virus pathogenicity. The structure of the closed state of the E transmembrane (TM) domain, ETM, was recently determined using solid-state NMR spectroscopy. However, how the channel pore opens to mediate cation transport is unclear. Here, we use 13C and 19F solid-state NMR spectroscopy to investigate the conformation and dynamics of ETM at acidic pH and in the presence of calcium ions, which mimic the ERGIC and lysosomal environment experienced by the E protein in the cell. Acidic pH and calcium ions increased the conformational disorder of the N- and C-terminal residues and also increased the water accessibility of the protein, indicating that the pore lumen has become more spacious. ETM contains three regularly spaced phenylalanine (Phe) residues in the center of the peptide. 19F NMR spectra of para-fluorinated Phe20 and Phe26 indicate that both residues exhibit two sidechain conformations, which coexist within each channel. These two Phe conformations differ in their water accessibility, lipid contact, and dynamics. Channel opening by acidic pH and Ca2+ increases the population of the dynamic lipid-facing conformation. These results suggest an intricate aromatic network that regulates the opening of the ETM channel pore. This aromatic network may be a target for E inhibitors against SARS-CoV-2 and related coronaviruses.
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Affiliation(s)
- João Medeiros-Silva
- Department of Chemistry, Massachusetts Institute of Technology, 170 Albany Street, Cambridge, Massachusetts 02139, United States
| | - Noah H Somberg
- Department of Chemistry, Massachusetts Institute of Technology, 170 Albany Street, Cambridge, Massachusetts 02139, United States
| | - Harrison K Wang
- Department of Chemistry, Massachusetts Institute of Technology, 170 Albany Street, Cambridge, Massachusetts 02139, United States
| | - Matthew J McKay
- Department of Chemistry, Massachusetts Institute of Technology, 170 Albany Street, Cambridge, Massachusetts 02139, United States
| | - Venkata S Mandala
- Department of Chemistry, Massachusetts Institute of Technology, 170 Albany Street, Cambridge, Massachusetts 02139, United States
| | - Aurelio J Dregni
- Department of Chemistry, Massachusetts Institute of Technology, 170 Albany Street, Cambridge, Massachusetts 02139, United States
| | - Mei Hong
- Department of Chemistry, Massachusetts Institute of Technology, 170 Albany Street, Cambridge, Massachusetts 02139, United States
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7
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A distinct mechanism of C-type inactivation in the Kv-like KcsA mutant E71V. Nat Commun 2022; 13:1574. [PMID: 35322021 PMCID: PMC8943062 DOI: 10.1038/s41467-022-28866-9] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/13/2021] [Accepted: 02/01/2022] [Indexed: 11/08/2022] Open
Abstract
C-type inactivation is of great physiological importance in voltage-activated K+ channels (Kv), but its structural basis remains unresolved. Knowledge about C-type inactivation has been largely deduced from the bacterial K+ channel KcsA, whose selectivity filter constricts under inactivating conditions. However, the filter is highly sensitive to its molecular environment, which is different in Kv channels than in KcsA. In particular, a glutamic acid residue at position 71 along the pore helix in KcsA is substituted by a valine conserved in most Kv channels, suggesting that this side chain is a molecular determinant of function. Here, a combination of X-ray crystallography, solid-state NMR and MD simulations of the E71V KcsA mutant is undertaken to explore inactivation in this Kv-like construct. X-ray and ssNMR data show that the filter of the Kv-like mutant does not constrict under inactivating conditions. Rather, the filter adopts a conformation that is slightly narrowed and rigidified. On the other hand, MD simulations indicate that the constricted conformation can nonetheless be stably established in the mutant channel. Together, these findings suggest that the Kv-like KcsA mutant may be associated with different modes of C-type inactivation, showing that distinct filter environments entail distinct C-type inactivation mechanisms.
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8
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Öster C, Tekwani Movellan K, Goold B, Hendriks K, Lange S, Becker S, de Groot BL, Kopec W, Andreas LB, Lange A. Direct Detection of Bound Ammonium Ions in the Selectivity Filter of Ion Channels by Solid-State NMR. J Am Chem Soc 2022; 144:4147-4157. [PMID: 35200002 PMCID: PMC8915258 DOI: 10.1021/jacs.1c13247] [Citation(s) in RCA: 4] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/20/2021] [Indexed: 01/16/2023]
Abstract
The flow of ions across cell membranes facilitated by ion channels is an important function for all living cells. Despite the huge amount of structural data provided by crystallography, elucidating the exact interactions between the selectivity filter atoms and bound ions is challenging. Here, we detect bound 15N-labeled ammonium ions as a mimic for potassium ions in ion channels using solid-state NMR under near-native conditions. The non-selective ion channel NaK showed two ammonium peaks corresponding to its two ion binding sites, while its potassium-selective mutant NaK2K that has a signature potassium-selective selectivity filter with four ion binding sites gave rise to four ammonium peaks. Ions bound in specific ion binding sites were identified based on magnetization transfer between the ions and carbon atoms in the selectivity filters. Magnetization transfer between bound ions and water molecules revealed that only one out of four ions in the selectivity filter of NaK2K is in close contact with water, which is in agreement with the direct knock-on ion conduction mechanism where ions are conducted through the channel by means of direct interactions without water molecules in between. Interestingly, the potassium-selective ion channels investigated here (NaK2K and, additionally, KcsA-Kv1.3) showed remarkably different chemical shifts for their bound ions, despite having identical amino acid sequences and crystal structures of their selectivity filters. Molecular dynamics simulations show similar ion binding and conduction behavior between ammonium and potassium ions and identify the origin of the differences between the investigated potassium channels.
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Affiliation(s)
- Carl Öster
- Department
of Molecular Biophysics, Leibniz-Forschungsinstitut
für Molekulare Pharmakologie, Robert-Rössle-Str. 10, 13125 Berlin, Germany
| | - Kumar Tekwani Movellan
- Department
of NMR-Based Structural Biology, Max Planck
Institute for Multidisciplinary Sciences, Am Fassberg 11, 37077 Göttingen, Germany
| | - Benjamin Goold
- Faculty
of Engineering and Physical Sciences, University
of Southampton, University Road, SO17 1BJ Southampton, U.K.
- Computational
Biomolecular Dynamics Group, Max Planck
Institute for Multidisciplinary Sciences, Am Fassberg 11, 37077 Göttingen, Germany
| | - Kitty Hendriks
- Department
of Molecular Biophysics, Leibniz-Forschungsinstitut
für Molekulare Pharmakologie, Robert-Rössle-Str. 10, 13125 Berlin, Germany
| | - Sascha Lange
- Department
of Molecular Biophysics, Leibniz-Forschungsinstitut
für Molekulare Pharmakologie, Robert-Rössle-Str. 10, 13125 Berlin, Germany
| | - Stefan Becker
- Department
of NMR-Based Structural Biology, Max Planck
Institute for Multidisciplinary Sciences, Am Fassberg 11, 37077 Göttingen, Germany
| | - Bert L. de Groot
- Computational
Biomolecular Dynamics Group, Max Planck
Institute for Multidisciplinary Sciences, Am Fassberg 11, 37077 Göttingen, Germany
| | - Wojciech Kopec
- Computational
Biomolecular Dynamics Group, Max Planck
Institute for Multidisciplinary Sciences, Am Fassberg 11, 37077 Göttingen, Germany
| | - Loren B. Andreas
- Department
of NMR-Based Structural Biology, Max Planck
Institute for Multidisciplinary Sciences, Am Fassberg 11, 37077 Göttingen, Germany
| | - Adam Lange
- Department
of Molecular Biophysics, Leibniz-Forschungsinstitut
für Molekulare Pharmakologie, Robert-Rössle-Str. 10, 13125 Berlin, Germany
- Institut
für Biologie, Humboldt-Universität
zu Berlin, Invalidenstr.
42, 10115 Berlin, Germany
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9
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Hendriks K, Öster C, Lange A. Structural Plasticity of the Selectivity Filter in Cation Channels. Front Physiol 2021; 12:792958. [PMID: 34950061 PMCID: PMC8689586 DOI: 10.3389/fphys.2021.792958] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/11/2021] [Accepted: 11/17/2021] [Indexed: 11/13/2022] Open
Abstract
Ion channels allow for the passage of ions across biological membranes, which is essential for the functioning of a cell. In pore loop channels the selectivity filter (SF) is a conserved sequence that forms a constriction with multiple ion binding sites. It is becoming increasingly clear that there are several conformations and dynamic states of the SF in cation channels. Here we outline specific modes of structural plasticity observed in the SFs of various pore loop channels: disorder, asymmetry, and collapse. We summarize the multiple atomic structures with varying SF conformations as well as asymmetric and more dynamic states that were discovered recently using structural biology, spectroscopic, and computational methods. Overall, we discuss here that structural plasticity within the SF is a key molecular determinant of ion channel gating behavior.
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Affiliation(s)
- Kitty Hendriks
- Department of Molecular Biophysics, Leibniz-Forschungsinstitut für Molekulare Pharmakologie, Berlin, Germany
| | - Carl Öster
- Department of Molecular Biophysics, Leibniz-Forschungsinstitut für Molekulare Pharmakologie, Berlin, Germany
| | - Adam Lange
- Department of Molecular Biophysics, Leibniz-Forschungsinstitut für Molekulare Pharmakologie, Berlin, Germany.,Institut für Biologie, Humboldt-Universität zu Berlin, Berlin, Germany
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10
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Lewis A, Kurauskas V, Tonelli M, Henzler-Wildman K. Ion-dependent structure, dynamics, and allosteric coupling in a non-selective cation channel. Nat Commun 2021; 12:6225. [PMID: 34711838 PMCID: PMC8553846 DOI: 10.1038/s41467-021-26538-8] [Citation(s) in RCA: 9] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/16/2021] [Accepted: 10/05/2021] [Indexed: 11/09/2022] Open
Abstract
The selectivity filter (SF) determines which ions are efficiently conducted through ion channel pores. NaK is a non-selective cation channel that conducts Na+ and K+ with equal efficiency. Crystal structures of NaK suggested a rigid SF structure, but later solid-state NMR and MD simulations questioned this interpretation. Here, we use solution NMR to characterize how bound Na+ vs. K+ affects NaK SF structure and dynamics. We find that the extracellular end of the SF is flexible on the ps-ns timescale regardless of bound ion. On a slower timescale, we observe a structural change between the Na+ and K+-bound states, accompanied by increased structural heterogeneity in Na+. We also show direct evidence that the SF structure is communicated to the pore via I88 on the M2 helix. These results support a dynamic SF with multiple conformations involved in non-selective conduction. Our data also demonstrate allosteric coupling between the SF and pore-lining helices in a non-selective cation channel that is analogous to the allosteric coupling previously demonstrated for K+-selective channels, supporting the generality of this model. NaK is a bacterial non-selective cation channel. Here, the authors use solution NMR to show that selectivity filter (SF) in NaK is dynamic, with structural differences between the Na+ and K + -bound states. The conformation of the SF is communicated to the pore-lining helices similarly as in the K + -selective channels.
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Affiliation(s)
- Adam Lewis
- Department of Biochemistry, University of Wisconsin-Madison, Madison, WI, 53706, USA
| | - Vilius Kurauskas
- Department of Biochemistry, University of Wisconsin-Madison, Madison, WI, 53706, USA
| | - Marco Tonelli
- National Magnetic Resonance Facility at Madison (NMRFAM), University of Wisconsin-Madison, Madison, WI, 53706, USA
| | - Katherine Henzler-Wildman
- Department of Biochemistry, University of Wisconsin-Madison, Madison, WI, 53706, USA. .,National Magnetic Resonance Facility at Madison (NMRFAM), University of Wisconsin-Madison, Madison, WI, 53706, USA.
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