1
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Tewes N, Kubitzki B, Bytyqi F, Metko N, Mach S, Thiel G, Rauh O. Mutation in pore-helix modulates interplay between filter gate and Ba2+ block in a Kcv channel pore. J Gen Physiol 2024; 156:e202313514. [PMID: 38652099 DOI: 10.1085/jgp.202313514] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/28/2023] [Revised: 03/05/2024] [Accepted: 04/09/2024] [Indexed: 04/25/2024] Open
Abstract
The selectivity filter of K+ channels catalyzes a rapid and highly selective transport of K+ while serving as a gate. To understand the control of this filter gate, we use the pore-only K+ channel KcvNTS in which gating is exclusively determined by the activity of the filter gate. It has been previously shown that a mutation at the C-terminus of the pore-helix (S42T) increases K+ permeability and introduces distinct voltage-dependent and K+-sensitive channel closures at depolarizing voltages. Here, we report that the latter are not generated by intrinsic conformational changes of the filter gate but by a voltage-dependent block caused by nanomolar trace contaminations of Ba2+ in the KCl solution. Channel closures can be alleviated by extreme positive voltages and they can be completely abolished by the high-affinity Ba2+ chelator 18C6TA. By contrast, the same channel closures can be augmented by adding Ba2+ at submicromolar concentrations to the cytosolic buffer. These data suggest that a conservative exchange of Ser for Thr in a crucial position of the filter gate increases the affinity of the filter for Ba2+ by >200-fold at positive voltages. While Ba2+ ions apparently remain only for a short time in the filter-binding sites of the WT channel before passing the pore, they remain much longer in the mutant channel. Our findings suggest that the dwell times of permeating and blocking ions in the filter-binding sites are tightly controlled by interactions between the pore-helix and the selectivity filter.
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Affiliation(s)
- Noel Tewes
- Membrane Biophysics, Technische Universität Darmstadt , Darmstadt, Germany
| | - Beatrice Kubitzki
- Membrane Biophysics, Technische Universität Darmstadt , Darmstadt, Germany
| | - Flandrit Bytyqi
- Membrane Biophysics, Technische Universität Darmstadt , Darmstadt, Germany
| | - Nikola Metko
- Membrane Biophysics, Technische Universität Darmstadt , Darmstadt, Germany
| | - Sebastian Mach
- Membrane Biophysics, Technische Universität Darmstadt , Darmstadt, Germany
| | - Gerhard Thiel
- Membrane Biophysics, Technische Universität Darmstadt , Darmstadt, Germany
- Centre for Synthetic Biology, Technische Universität Darmstadt , Darmstadt, Germany
| | - Oliver Rauh
- Membrane Biophysics, Technische Universität Darmstadt , Darmstadt, Germany
- Centre for Synthetic Biology, Technische Universität Darmstadt , Darmstadt, Germany
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2
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Rauh O, Opper J, Sturm M, Drexler N, Scheub DD, Hansen UP, Thiel G, Schroeder I. Role of ion distribution and energy barriers for concerted motion of subunits in selectivity filter gating of a K+ channel. J Mol Biol 2022; 434:167522. [DOI: 10.1016/j.jmb.2022.167522] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/20/2021] [Revised: 02/04/2022] [Accepted: 02/28/2022] [Indexed: 11/25/2022]
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3
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Nanazashvili M, Sánchez-Rodríguez JE, Fosque B, Bezanilla F, Sackin H. LRET Determination of Molecular Distances during pH Gating of the Mammalian Inward Rectifier Kir1.1b. Biophys J 2018; 114:88-97. [PMID: 29320699 PMCID: PMC5773755 DOI: 10.1016/j.bpj.2017.10.044] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/09/2017] [Revised: 09/23/2017] [Accepted: 10/19/2017] [Indexed: 01/31/2023] Open
Abstract
Gating of the mammalian inward rectifier Kir1.1 at the helix bundle crossing (HBC) by intracellular pH is believed to be mediated by conformational changes in the C-terminal domain (CTD). However, the exact motion of the CTD during Kir gating remains controversial. Crystal structures and single-molecule fluorescence resonance energy transfer of KirBac channels have implied a rigid body rotation and/or a contraction of the CTD as possible triggers for opening of the HBC gate. In our study, we used lanthanide-based resonance energy transfer on single-Cys dimeric constructs of the mammalian renal inward rectifier, Kir1.1b, incorporated into anionic liposomes plus PIP2, to determine unambiguous, state-dependent distances between paired Cys residues on diagonally opposite subunits. Functionality and pH dependence of our proteoliposome channels were verified in separate electrophysiological experiments. The lanthanide-based resonance energy transfer distances measured in closed (pH 6) and open (pH 8) conditions indicated neither expansion nor contraction of the CTD during gating, whereas the HBC gate widened by 8.8 ± 4 Å, from 6.3 ± 2 to 15.1 ± 6 Å, during opening. These results are consistent with a Kir gating model in which rigid body rotation of the large CTD around the permeation axis is correlated with opening of the HBC hydrophobic gate, allowing permeation of a 7 Å hydrated K ion.
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Affiliation(s)
- Mikheil Nanazashvili
- Department of Physiology and Biophysics, The Chicago Medical School, Rosalind Franklin University, North Chicago, Illinois
| | - Jorge E Sánchez-Rodríguez
- Department of Biochemistry and Molecular Biology, University of Chicago, Chicago, Illinois; Departamento de Física, Universidad de Guadalajara, Guadalajara, Jalisco, Mexico
| | - Ben Fosque
- Department of Biochemistry and Molecular Biology, University of Chicago, Chicago, Illinois
| | - Francisco Bezanilla
- Department of Biochemistry and Molecular Biology, University of Chicago, Chicago, Illinois
| | - Henry Sackin
- Department of Physiology and Biophysics, The Chicago Medical School, Rosalind Franklin University, North Chicago, Illinois.
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4
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Zhao WS, Sun MY, Sun LF, Liu Y, Yang Y, Huang LD, Fan YZ, Cheng XY, Cao P, Hu YM, Li L, Tian Y, Wang R, Yu Y. A Highly Conserved Salt Bridge Stabilizes the Kinked Conformation of β2,3-Sheet Essential for Channel Function of P2X4 Receptors. J Biol Chem 2016; 291:7990-8003. [PMID: 26865631 DOI: 10.1074/jbc.m115.711127] [Citation(s) in RCA: 18] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Key Words] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/16/2015] [Indexed: 01/01/2023] Open
Abstract
Significant progress has been made in understanding the roles of crucial residues/motifs in the channel function of P2X receptors during the pre-structure era. The recent structural determination of P2X receptors allows us to reevaluate the role of those residues/motifs. Residues Arg-309 and Asp-85 (rat P2X4 numbering) are highly conserved throughout the P2X family and were involved in loss-of-function polymorphism in human P2X receptors. Previous studies proposed that they participated in direct ATP binding. However, the crystal structure of P2X demonstrated that those two residues form an intersubunit salt bridge located far away from the ATP-binding site. Therefore, it is necessary to reevaluate the role of this salt bridge in P2X receptors. Here, we suggest the crucial role of this structural element both in protein stability and in channel gating rather than direct ATP interaction and channel assembly. Combining mutagenesis, charge swap, and disulfide cross-linking, we revealed the stringent requirement of this salt bridge in normal P2X4 channel function. This salt bridge may contribute to stabilizing the bending conformation of the β2,3-sheet that is structurally coupled with this salt bridge and the α2-helix. Strongly kinked β2,3 is essential for domain-domain interactions between head domain, dorsal fin domain, right flipper domain, and loop β7,8 in P2X4 receptors. Disulfide cross-linking with directions opposing or along the bending angle of the β2,3-sheet toward the α2-helix led to loss-of-function and gain-of-function of P2X4 receptors, respectively. Further insertion of amino acids with bulky side chains into the linker between the β2,3-sheet or the conformational change of the α2-helix, interfering with the kinked conformation of β2,3, led to loss-of-function of P2X4 receptors. All these findings provided new insights in understanding the contribution of the salt bridge between Asp-85 and Arg-309 and its structurally coupled β2,3-sheet to the function of P2X receptors.
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Affiliation(s)
- Wen-Shan Zhao
- From the School of Life Sciences and Key Laboratory of Preclinical Study for New Drugs of Gansu Province School of Basic Medical Sciences, Lanzhou University, Lanzhou 730000, China, the Institute of Medical Sciences and Department of Pharmacology and Chemical Biology, Shanghai Jiao Tong University School of Medicine, Shanghai 200025, China
| | - Meng-Yang Sun
- From the School of Life Sciences and Key Laboratory of Preclinical Study for New Drugs of Gansu Province School of Basic Medical Sciences, Lanzhou University, Lanzhou 730000, China, the Institute of Medical Sciences and Department of Pharmacology and Chemical Biology, Shanghai Jiao Tong University School of Medicine, Shanghai 200025, China
| | - Liang-Fei Sun
- the Institute of Medical Sciences and Department of Pharmacology and Chemical Biology, Shanghai Jiao Tong University School of Medicine, Shanghai 200025, China
| | - Yan Liu
- the Institute of Medical Sciences and Department of Pharmacology and Chemical Biology, Shanghai Jiao Tong University School of Medicine, Shanghai 200025, China
| | - Yang Yang
- the Institute of Medical Sciences and Department of Pharmacology and Chemical Biology, Shanghai Jiao Tong University School of Medicine, Shanghai 200025, China
| | - Li-Dong Huang
- the Institute of Medical Sciences and Department of Pharmacology and Chemical Biology, Shanghai Jiao Tong University School of Medicine, Shanghai 200025, China
| | - Ying-Zhe Fan
- the Putuo District Center Hospital, Shanghai University of Chinese Traditional Medicine, Shanghai 200062, China
| | - Xiao-Yang Cheng
- the Institute of Medical Sciences and Department of Pharmacology and Chemical Biology, Shanghai Jiao Tong University School of Medicine, Shanghai 200025, China
| | - Peng Cao
- the Hospital of Integrated Traditional Chinese and Western Medicine, Nanjing University of Chinese Medicine, Nanjing 210023, China, and the Laboratory of Cellular and Molecular Biology, Jiangsu Province Academy of Traditional Chinese Medicine, Nanjing 210028, China
| | - You-Min Hu
- the Institute of Medical Sciences and Department of Pharmacology and Chemical Biology, Shanghai Jiao Tong University School of Medicine, Shanghai 200025, China
| | - Lingyong Li
- the Department of Anesthesiology and Perioperative Medicine, University of Texas M. D. Anderson Cancer Center, Houston, Texas 77030
| | - Yun Tian
- the College of Bioscience and Biotechnology, Hunan Agricultural University, Changsha 410128, China
| | - Rui Wang
- From the School of Life Sciences and Key Laboratory of Preclinical Study for New Drugs of Gansu Province School of Basic Medical Sciences, Lanzhou University, Lanzhou 730000, China,
| | - Ye Yu
- the Institute of Medical Sciences and Department of Pharmacology and Chemical Biology, Shanghai Jiao Tong University School of Medicine, Shanghai 200025, China, the College of Bioscience and Biotechnology, Hunan Agricultural University, Changsha 410128, China,
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5
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Kan HI, Chen IY, Zulfajri M, Wang CC. Subunit disassembly pathway of human hemoglobin revealing the site-specific role of its cysteine residues. J Phys Chem B 2013; 117:9831-9. [PMID: 23902424 DOI: 10.1021/jp402292b] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
Cysteine residues play a unique role in human hemoglobin (Hb) by affecting its cooperative oxygen binding behavior and the stability of its tetrameric structure. However, how these cysteine residues fulfill their biophysical functions from the molecular level is yet unclear. Here we study the subunit disassembly pathway of human hemoglobin using the sulfhydryl reagent, p-hydroxymercuribenzoate (PMB) and investigate the functional roles of cysteine residues in human hemoglobin. We show evidence from the matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectrometry that all three types of cysteine residues, including the surface-exposed βCys93 and the shielded αCys104 and βCys112 are reactive to PMB, resolving an issue long under debate. It is demonstrated that all three types of cysteine residues must be blocked by PMB to accomplish the subunit disassembly, and the PMB-cysteine reactions proceed in a stepwise manner with an order of βCys93, αCys104, and βCys112. The PMB reactions with the three different cysteine residues demonstrate strong site-specificity. The possible influence of PMB-cysteine reactions to the stability of various intersubunit salt bridges has been discussed based on the crystallographic structure of hemoglobin, providing insights in understanding the hemoglobin subunit disassembly pathway and the site-specific functional role of each cysteine residue in hemoglobin.
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Affiliation(s)
- Heng-I Kan
- Department of Chemistry, National Sun Yat-sen University, Kaohsiung, Taiwan, R.O.C. 80424
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6
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Weingarth M, Prokofyev A, van der Cruijsen EAW, Nand D, Bonvin AMJJ, Pongs O, Baldus M. Structural determinants of specific lipid binding to potassium channels. J Am Chem Soc 2013; 135:3983-8. [PMID: 23425320 DOI: 10.1021/ja3119114] [Citation(s) in RCA: 64] [Impact Index Per Article: 5.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
We have investigated specific lipid binding to the pore domain of potassium channels KcsA and chimeric KcsA-Kv1.3 on the structural and functional level using extensive coarse-grained and atomistic molecular dynamics simulations, solid-state NMR, and single channel measurements. We show that, while KcsA activity is critically modulated by the specific and cooperative binding of anionic nonannular lipids close to the channel's selectivity filter, the influence of nonannular lipid binding on KcsA-Kv1.3 is much reduced. The diminished impact of specific lipid binding on KcsA-Kv1.3 results from a point-mutation at the corresponding nonannular lipid binding site leading to a salt-bridge between adjacent KcsA-Kv1.3 subunits, which is conserved in many voltage-gated potassium channels and prevents strong nonannular lipid binding to the pore domain. Our findings elucidate how protein-lipid and protein-protein interactions modulate K(+) channel activity. The combination of MD, NMR, and functional studies as shown here may help to dissect the structural and dynamical processes that are critical for the functioning of larger membrane proteins, including Kv channels in a membrane setting.
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Affiliation(s)
- Markus Weingarth
- Bijvoet Center for Biomolecular Research, Faculty of Science, Utrecht University, Padualaan 8, 3584 CH Utrecht, The Netherlands
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7
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Köpfer DA, Hahn U, Ohmert I, Vriend G, Pongs O, de Groot BL, Zachariae U. A molecular switch driving inactivation in the cardiac K+ channel HERG. PLoS One 2012; 7:e41023. [PMID: 22848423 PMCID: PMC3404103 DOI: 10.1371/journal.pone.0041023] [Citation(s) in RCA: 16] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/10/2012] [Accepted: 06/15/2012] [Indexed: 12/30/2022] Open
Abstract
K+ channels control transmembrane action potentials by gating open or closed in response to external stimuli. Inactivation gating, involving a conformational change at the K+ selectivity filter, has recently been recognized as a major K+ channel regulatory mechanism. In the K+ channel hERG, inactivation controls the length of the human cardiac action potential. Mutations impairing hERG inactivation cause life-threatening cardiac arrhythmia, which also occur as undesired side effects of drugs. In this paper, we report atomistic molecular dynamics simulations, complemented by mutational and electrophysiological studies, which suggest that the selectivity filter adopts a collapsed conformation in the inactivated state of hERG. The selectivity filter is gated by an intricate hydrogen bond network around residues S620 and N629. Mutations of this hydrogen bond network are shown to cause inactivation deficiency in electrophysiological measurements. In addition, drug-related conformational changes around the central cavity and pore helix provide a functional mechanism for newly discovered hERG activators.
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Affiliation(s)
- David A. Köpfer
- Computational Biomolecular Dynamics Group, Max Planck Institute for Biophysical Chemistry, Göttingen, Germany
| | - Ulrike Hahn
- Center for Molecular Neurobiology, Institute for Neural Signal Transduction, University of Hamburg, Hamburg, Germany
| | - Iris Ohmert
- Center for Molecular Neurobiology, Institute for Neural Signal Transduction, University of Hamburg, Hamburg, Germany
| | - Gert Vriend
- Center for Molecular and Biomolecular Informatics, Radboud University Nijmegen Medical Center, Nijmegen, The Netherlands
| | - Olaf Pongs
- Center for Molecular Neurobiology, Institute for Neural Signal Transduction, University of Hamburg, Hamburg, Germany
| | - Bert L. de Groot
- Computational Biomolecular Dynamics Group, Max Planck Institute for Biophysical Chemistry, Göttingen, Germany
| | - Ulrich Zachariae
- Computational Biomolecular Dynamics Group, Max Planck Institute for Biophysical Chemistry, Göttingen, Germany
- Center for Molecular and Biomolecular Informatics, Radboud University Nijmegen Medical Center, Nijmegen, The Netherlands
- Scottish Universities Physics Alliance, School of Physics and Astronomy, The University of Edinburgh, Edinburgh, United Kingdom
- * E-mail:
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8
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Sackin H, Nanazashvili M, Li H, Palmer LG, Yang L. Residues at the outer mouth of Kir1.1 determine K-dependent gating. Biophys J 2012; 102:2742-50. [PMID: 22735524 DOI: 10.1016/j.bpj.2012.05.018] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/17/2012] [Revised: 05/09/2012] [Accepted: 05/10/2012] [Indexed: 10/28/2022] Open
Abstract
Three residues (E132, F127, and R128) at the outer mouth of Kir1.1b directly affected inward rectifier gating by external K, independent of pH gating. Each of the individual mutations E132Q, F127V, F127D, and R128Y changed the normal K dependence of macroscopic conductance from hyperbolic (Km = 6 ± 2 mM) to linear, up to 500 mM, without changing the hyperbolic K dependence of single-channel conductance. This suggests that E132, F127, and R128 are responsible for maximal Kir1.1b activation by external K. In addition, these same residues were also essential for recovery of Kir1.1b activity after complete removal of external K by 18-Crown-6 polyether. In contrast, charge-altering mutations at neighboring residues (E92A, E104A, D97V, or Q133E) near the outer mouth of the channel did not affect Kir1.1b recovery after chelation of external K. The collective role of E132, R128, and F127 in preventing Kir1.1b inactivation by either cytoplasmic acidification or external K removal implies that pH inactivation and the external K sensor share a common mechanism, whereby E132, R128, and F127 stabilize the Kir1.1b selectivity filter gate in an open conformation, allowing rapid recovery of channel activity after a period of external K depletion.
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Affiliation(s)
- Henry Sackin
- Department of Physiology, The Chicago Medical School, Rosalind Franklin University, North Chicago, Illinois, USA.
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9
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Yang Y, Yu Y, Cheng J, Liu Y, Liu DS, Wang J, Zhu MX, Wang R, Xu TL. Highly conserved salt bridge stabilizes rigid signal patch at extracellular loop critical for surface expression of acid-sensing ion channels. J Biol Chem 2012; 287:14443-55. [PMID: 22399291 DOI: 10.1074/jbc.m111.334250] [Citation(s) in RCA: 17] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Acid-sensing ion channels (ASICs) are non-selective cation channels activated by extracellular acidosis associated with many physiological and pathological conditions. A detailed understanding of the mechanisms that govern cell surface expression of ASICs, therefore, is critical for better understanding of the cell signaling under acidosis conditions. In this study, we examined the role of a highly conserved salt bridge residing at the extracellular loop of rat ASIC3 (Asp(107)-Arg(153)) and human ASIC1a (Asp(107)-Arg(160)) channels. Comprehensive mutagenesis and electrophysiological recordings revealed that the salt bridge is essential for functional expression of ASICs in a pH sensing-independent manner. Surface biotinylation and immunolabeling of an extracellular epitope indicated that mutations, including even minor alterations, at the salt bridge impaired cell surface expression of ASICs. Molecular dynamics simulations, normal mode analysis, and further mutagenesis studies suggested a high stability and structural constrain of the salt bridge, which serves to separate an adjacent structurally rigid signal patch, important for surface expression, from a flexible gating domain. Thus, we provide the first evidence of structural requirement that involves a stabilizing salt bridge and an exposed rigid signal patch at the destined extracellular loop for normal surface expression of ASICs. These findings will allow evaluation of new strategies aimed at preventing excessive excitability and neuronal injury associated with tissue acidosis and ASIC activation.
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Affiliation(s)
- Yang Yang
- Key Laboratory of Preclinical Study for New Drugs of Gansu Province, School of Basic Medical Sciences, Lanzhou University, Lanzhou 730000, China
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10
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Edvinsson JM, Shah AJ, Palmer LG. Potassium-dependent activation of Kir4.2 K⁺ channels. J Physiol 2011; 589:5949-63. [PMID: 22025665 DOI: 10.1113/jphysiol.2011.220731] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/24/2023] Open
Abstract
The inwardly rectifying potassium channel Kir4.2 is sensitive to changes in the extracellular potassium concentration ([K(+)](o)). This form of regulation is manifest as a slow (tens of minutes) increase in the whole-cell currents when [K(+)](o) is increased. Here we have investigated the mechanism of K(o)(+) sensitivity of Kir4.2 expressed in Xenopus oocytes. Using two-electrode voltage clamp we found that the sensitivity is specific for the homomeric form of the channel and is completely abolished when coexpressed with Kir5.1. Furthermore, unlike Kir1.1, there is no coupling between the intracellular pH sensitivity and K(o)(+) sensitivity, as is evident by introducing a mutation (K66M), which greatly decreases the pH(i) sensitivity while the K(o)(+) sensitivity remains unchanged. K(o)(+)-dependent activation of Kir4.2 does not involve an increase in the surface expression of the channel, nor is there a difference in the open probability between high and low [K(+)] as determined through patch-clamp measurements. We also found that there is an inverse relationship between the rates of both activation and deactivation and [K(+)](o). Using a kinetic model we argue that Kir4.2 exists in at least three states at the plasma membrane: a deactivated state, an intermediate unstable state and an active state, and that [K(+)](o) affects the rate of transition from the intermediate state to the active state.
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Affiliation(s)
- Johan M Edvinsson
- Graduate Program in Physiology, Biophysics, and Systems Biology, Weill Cornell Medical College, New York, NY 10065, USA
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11
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Nimigean CM, Allen TW. Origins of ion selectivity in potassium channels from the perspective of channel block. ACTA ACUST UNITED AC 2011; 137:405-13. [PMID: 21518829 PMCID: PMC3082928 DOI: 10.1085/jgp.201010551] [Citation(s) in RCA: 60] [Impact Index Per Article: 4.6] [Reference Citation Analysis] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Affiliation(s)
- Crina M Nimigean
- Department of Anesthesiology, Weill Medical College, Cornell University, New York, NY 10065, USA. crn2002@med.cornell.edu
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12
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Sackin H, Nanazashvili M, Li H, Palmer LG, Walters DE. A conserved arginine near the filter of Kir1.1 controls Rb/K selectivity. Channels (Austin) 2011; 4:203-14. [PMID: 20458182 DOI: 10.4161/chan.4.3.11982] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/19/2022] Open
Abstract
ROMK (Kir1.1) channels are important for K secretion and recycling in the collecting duct, connecting tubule and thick ascending limb of the mammalian nephron. We have identified a highly conserved Arg in the P loop of the channel near the selectivity filter that controls Rb/K selectivity. Mutation of this Arg to a Tyr (R128Y-Kir1.1b, R147Y-Kir1.1a) increased the macroscopic conductance ratio, G(Rb)/G(K) by 17 ± 3 fold and altered the selectivity sequence from NH(4) > K > Tl > Rb >> Cs in wt-Kir1.1 to: Rb > Cs > Tl > NH(4) >> K in R128Y, without significant change in the high K/Na permeability ratio of Kir1.1. R128M produced similar, but smaller, increases in Rb, Tl, NH(4) and Cs conductance relative to K. R128Y remained susceptible to block by both external Ba and the honeybee toxin, TPNQ, although R128Y had a reduced affinity for TPNQ, relative to wild-type. The effect of R128Y-Kir1.1b on the G(Rb)/G(K) ratio can be partly explained by a larger single-channel Rb conductance (12.4 ± 0.5 pS) than K conductance (<1.5 pS) in this mutant. The kinetics of R128Y gating at -120 mV with Rb as the permeant ion were similar to those of wt-Kir1.1 conducting Rb, but with a longer open time (129 ms vs. 6 ms for wt) and two closed states (13 ms, 905 ms), resulting in an open probability (Po) of 0.5, compared to a Po of 0.9 for wt-Kir1.1, which had a single closed state of 1 ms at -120 mV. Single-channel R128Y rectification was eliminated in excised, insideout patches with symmetrical Rb solutions. The large increase in the Rb/K conductance ratio, with no change in K/Na permeability or rectification, is consistent with R128Y-Kir1.1b causing a subtle change in the selectivity filter, perhaps by disruption of an intra-subunit salt bridge (R128-E118) near the filter.
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Affiliation(s)
- Henry Sackin
- Dept. of Physiology, The Chicago Medical School, RFU, IL, USA.
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13
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Sackin H, Nanazashvili M, Li H, Palmer LG, Yang L. Modulation of Kir1.1 inactivation by extracellular Ca and Mg. Biophys J 2011; 100:1207-15. [PMID: 21354393 DOI: 10.1016/j.bpj.2011.01.032] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/13/2010] [Accepted: 01/12/2011] [Indexed: 10/18/2022] Open
Abstract
Kir1.1 inactivation, associated with transient internal acidification, is strongly dependent on external K, Ca, and Mg. Here, we show that in 1 mM K, a 15 min internal acidification (pH 6.3) followed by a 30 min recovery (pH 8.0) produced 84 ± 3% inactivation in 2 mM Ca but only 18 ± 4% inactivation in the absence of external Ca and Mg. In 100 mM external K, the same acidification protocol produced 29 ± 4% inactivation in 10 mM external Ca but no inactivation when extracellular Ca was reduced below 2 mM (with 0 Mg). However, chelation of external K with 15 mM of 18-Crown-6 (a crown ether) restored inactivation even in the absence of external divalents. External Ca was more effective than external Mg at producing inactivation, but Mg caused a greater degree of open channel block than Ca, making it unlikely that Kir1.1 inactivation arises from divalent block per se. Because the Ca sensitivity of inactivation persisted in 100 mM external K, it is also unlikely that Ca enhanced Kir1.1 inactivation by reducing the local K concentration at the outer mouth of the channel. The removal of four surface, negative side chains at E92, D97, E104, and E132 (Kir1.1b) increased the sensitivity of inactivation to external Ca (and Mg), whereas addition of a negative surface charge (N105E-Kir1.1b) decreased the sensitivity of inactivation to Ca and Mg. This result is consistent with the notion that negative surface charges stabilize external K in the selectivity filter or at the S(0)-K binding site just outside the filter. Extracellular Ca and Mg probably potentiate the slow, K-dependent inactivation of Kir1.1 by decreasing the affinity of the channel for external K independently of divalent block. The removal of external Ca and Mg largely eliminated both Kir1.1 inactivation and the K-dependence of pH gating, thereby uncoupling the selectivity filter gate from the cytoplasmic-side bundle-crossing gate.
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Affiliation(s)
- Henry Sackin
- Department of Physiology and Biophysics, Chicago Medical School, Rosalind Franklin University, North Chicago, Illinois, USA.
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14
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Edvinsson JM, Shah AJ, Palmer LG. Kir4.1 K+ channels are regulated by external cations. Channels (Austin) 2011; 5:269-79. [PMID: 21532341 DOI: 10.4161/chan.5.3.15827] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/27/2022] Open
Abstract
The inwardly rectifying potassium channel (Kir), Kir4.1 mediates spatial K(+)-buffering in the CNS. In this process the channel is potentially exposed to a large range of extracellular K(+) concentrations ([K(+)]o). We found that Kir4.1 is regulated by K(+)o. Increased [K(+)]o leads to a slow (mins) increase in the whole-cell currents of Xenopus oocytes expressing Kir4.1. Conversely, removing K(+) from the bath solution results in a slow decrease of the currents. This regulation is not coupled to the pHi-sensitive gate of the channel, nor does it require the presence of K67, a residue necessary for K(+)o-dependent regulation of Kir1.1. The voltage-dependent blockers Cs(+) and Ba(2+) substitute for K(+) and prevent deactivation of the channel in the absence of K(+)o. Cs(+) blocks and regulates the channel with similar affinity, consistent with the regulatory sites being in the selectivity-filter of the channel. Although both Rb(+) and NH4(+) permeate Kir4.1, only Rb(+) is able to regulate the channel. We conclude that Kir4.1 is regulated by ions interacting with specific sites in the selectivity filter. Using a kinetic model of the permeation process we show the plausibility of the channel's sensing the extracellular ionic environment through changes in the selectivity occupancy pattern, and that it is feasible for an ion with the selectivity properties of NH4(+) to permeate the channel without inducing these changes.
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Affiliation(s)
- Johan M Edvinsson
- Graduate Program in Physiology, Biophysics and Systems Biology, Department of Physiology and Biophysics, Weill Cornell Medical College, New York, NY, USA
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Mechanism for selectivity-inactivation coupling in KcsA potassium channels. Proc Natl Acad Sci U S A 2011; 108:5272-7. [PMID: 21402935 DOI: 10.1073/pnas.1014186108] [Citation(s) in RCA: 71] [Impact Index Per Article: 5.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Structures of the prokaryotic K(+) channel, KcsA, highlight the role of the selectivity filter carbonyls from the GYG signature sequence in determining a highly selective pore, but channels displaying this sequence vary widely in their cation selectivity. Furthermore, variable selectivity can be found within the same channel during a process called C-type inactivation. We investigated the mechanism for changes in selectivity associated with inactivation in a model K(+) channel, KcsA. We found that E71A, a noninactivating KcsA mutant in which a hydrogen-bond behind the selectivity filter is disrupted, also displays decreased K(+) selectivity. In E71A channels, Na(+) permeates at higher rates as seen with and flux measurements and analysis of intracellular Na(+) block. Crystal structures of E71A reveal that the selectivity filter no longer assumes the "collapsed," presumed inactivated, conformation in low K(+), but a "flipped" conformation, that is also observed in high K(+), high Na(+), and even Na(+) only conditions. The data reveal the importance of the E71-D80 interaction in both favoring inactivation and maintaining high K(+) selectivity. We propose a molecular mechanism by which inactivation and K(+) selectivity are linked, a mechanism that may also be at work in other channels containing the canonical GYG signature sequence.
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