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Abstract
Potassium channels are present in every living cell and essential to setting up a stable, non-zero transmembrane electrostatic potential which manifests the off-equilibrium livelihood of the cell. They are involved in other cellular activities and regulation, such as the controlled release of hormones, the activation of T-cells for immune response, the firing of action potential in muscle cells and neurons, etc. Pharmacological reagents targeting potassium channels are important for treating various human diseases linked to dysfunction of the channels. High-resolution structures of these channels are very useful tools for delineating the detailed chemical basis underlying channel functions and for structure-based design and optimization of their pharmacological and pharmaceutical agents. Structural studies of potassium channels have revolutionized biophysical understandings of key concepts in the field - ion selectivity, conduction, channel gating, and modulation, making them multi-modality targets of pharmacological regulation. In this chapter, I will select a few high-resolution structures to illustrate key structural insights, proposed allostery behind channel functions, disagreements still open to debate, and channel-lipid interactions and co-evolution. The known structural consensus allows the inference of conserved molecular mechanisms shared among subfamilies of K+ channels and makes it possible to develop channel-specific pharmaceutical agents.
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Affiliation(s)
- Qiu-Xing Jiang
- Laboratory of Molecular Physiology and Biophysics and the Cryo-EM Center, Hauptmann-Woodward Medical Research Institute, Buffalo, NY, USA.
- Department of Medicinal Chemistry, University of Florida, Gainesville, FL, USA.
- Departments of Materials Design and Invention and Physiology and Biophysics, University of Buffalo (SUNY), Buffalo, NY, USA.
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Cholesterol-Dependent Gating Effects on Ion Channels. ADVANCES IN EXPERIMENTAL MEDICINE AND BIOLOGY 2019; 1115:167-190. [PMID: 30649760 DOI: 10.1007/978-3-030-04278-3_8] [Citation(s) in RCA: 17] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/24/2022]
Abstract
Biomembranes separate a live cell from its environment and keep it in an off-equilibrium, steady state. They contain both phospholipids and nonphospholipids, depending on whether there are phosphate groups in the headgroup regions. Cholesterol (CHOL) is one type of nonphospholipids, and one of the most abundant lipid molecules in humans. Its content in plasma membranes and intracellular membranes varies and is tightly regulated. Voltage-gated ion channels are universally present in every cell and are fairly diversified in the eukaryotic domain of life. Our lipid-dependent gating hypothesis postulates that the controlled switch of the voltage-sensor domains (VSDs) in a voltage-gated potassium (Kv) channel between the "down" and the "up" state (gating) is sensitive to the ratio of phospholipids:nonphospholipids in the annular layer around the channel. High CHOL content is found to exert strong inhibitory effects on Kv channels. Such effects have been observed in in vitro membranes, cultured cells, and animal models for cholesterol metabolic defects. Thermodynamic analysis of the CHOL-dependent gating suggests that the inhibitory effects of CHOL result from collective interactions between annular CHOL molecules and the channel, which appear to be a more generic principle behind the CHOL effects on other ion channels and transporters. We will review the recent progress in the CHOL-dependent gating of voltage-gated ion channels, discuss the current technical limitations, and then expand briefly the learned principles to other ion channels that are known to be sensitive to the CHOL-channel interactions.
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Rangl M, Miyagi A, Kowal J, Stahlberg H, Nimigean CM, Scheuring S. Real-time visualization of conformational changes within single MloK1 cyclic nucleotide-modulated channels. Nat Commun 2016; 7:12789. [PMID: 27647260 PMCID: PMC5034309 DOI: 10.1038/ncomms12789] [Citation(s) in RCA: 24] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/11/2016] [Accepted: 08/01/2016] [Indexed: 01/11/2023] Open
Abstract
Eukaryotic cyclic nucleotide-modulated (CNM) ion channels perform various physiological roles by opening in response to cyclic nucleotides binding to a specialized cyclic nucleotide-binding domain. Despite progress in structure-function analysis, the conformational rearrangements underlying the gating of these channels are still unknown. Here, we image ligand-induced conformational changes in single CNM channels from Mesorhizobium loti (MloK1) in real-time, using high-speed atomic force microscopy. In the presence of cAMP, most channels are in a stable conformation, but a few molecules dynamically switch back and forth (blink) between at least two conformations with different heights. Upon cAMP depletion, more channels start blinking, with blinking heights increasing over time, suggestive of slow, progressive loss of ligands from the tetramer. We propose that during gating, MloK1 transitions from a set of mobile conformations in the absence to a stable conformation in the presence of ligand and that these conformations are central for gating the pore.
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Affiliation(s)
- Martina Rangl
- INSERM U1006, Aix-Marseille Université, Parc Scientifique et Technologique de Luminy, 163 Avenue de Luminy, Marseille 13009, France
| | - Atsushi Miyagi
- INSERM U1006, Aix-Marseille Université, Parc Scientifique et Technologique de Luminy, 163 Avenue de Luminy, Marseille 13009, France
| | - Julia Kowal
- Center for Cellular Imaging and NanoAnalytics, Biozentrum, University of Basel, Mattenstrasse 26, Basel CH-4058, Switzerland
| | - Henning Stahlberg
- Center for Cellular Imaging and NanoAnalytics, Biozentrum, University of Basel, Mattenstrasse 26, Basel CH-4058, Switzerland
| | - Crina M Nimigean
- INSERM U1006, Aix-Marseille Université, Parc Scientifique et Technologique de Luminy, 163 Avenue de Luminy, Marseille 13009, France.,Departments of Anesthesiology, Physiology and Biophysics, and Biochemistry, Weill Cornell Medical College, 1300 York Avenue, New York, New York 10065, USA
| | - Simon Scheuring
- INSERM U1006, Aix-Marseille Université, Parc Scientifique et Technologique de Luminy, 163 Avenue de Luminy, Marseille 13009, France
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Kowal J, Chami M, Baumgartner P, Arheit M, Chiu PL, Rangl M, Scheuring S, Schröder GF, Nimigean CM, Stahlberg H. Ligand-induced structural changes in the cyclic nucleotide-modulated potassium channel MloK1. Nat Commun 2015; 5:3106. [PMID: 24469021 PMCID: PMC4086158 DOI: 10.1038/ncomms4106] [Citation(s) in RCA: 53] [Impact Index Per Article: 5.9] [Reference Citation Analysis] [Abstract] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/10/2013] [Accepted: 12/13/2013] [Indexed: 12/25/2022] Open
Abstract
Cyclic nucleotide-modulated ion channels are important for signal transduction and pacemaking in eukaryotes. The molecular determinants of ligand gating in these channels are still unknown, mainly because of a lack of direct structural information. Here we report ligand-induced conformational changes in full-length MloK1, a cyclic nucleotide-modulated potassium channel from the bacterium Mesorhizobium loti, analysed by electron crystallography and atomic force microscopy. Upon cAMP binding, the cyclic nucleotide-binding domains move vertically towards the membrane, and directly contact the S1–S4 voltage sensor domains. This is accompanied by a significant shift and tilt of the voltage sensor domain helices. In both states, the inner pore-lining helices are in an ‘open’ conformation. We propose a mechanism in which ligand binding can favour pore opening via a direct interaction between the cyclic nucleotide-binding domains and voltage sensors. This offers a simple mechanistic hypothesis for the coupling between ligand gating and voltage sensing in eukaryotic HCN channels. The molecular determinants underlying ligand gating of cyclic nucleotide-modulated ion channels remain unclear. Kowal et al. determine the conformational changes underlying cAMP binding to the bacterial channel MloK1, and propose a mechanism for coupling of ligand gating and voltage sensing in eukaryotic HCN channels.
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Affiliation(s)
- Julia Kowal
- Center for Cellular Imaging and NanoAnalytics, Biozentrum, University of Basel, Mattenstrasse 26, CH-4058 Basel, Switzerland
| | - Mohamed Chami
- Center for Cellular Imaging and NanoAnalytics, Biozentrum, University of Basel, Mattenstrasse 26, CH-4058 Basel, Switzerland
| | - Paul Baumgartner
- Center for Cellular Imaging and NanoAnalytics, Biozentrum, University of Basel, Mattenstrasse 26, CH-4058 Basel, Switzerland
| | - Marcel Arheit
- Center for Cellular Imaging and NanoAnalytics, Biozentrum, University of Basel, Mattenstrasse 26, CH-4058 Basel, Switzerland
| | | | - Martina Rangl
- U1006 INSERM, Aix-Marseille Université, Parc Scientifique et Technologique de Luminy, 163 Avenue de Luminy, 13009 Marseille, France
| | - Simon Scheuring
- U1006 INSERM, Aix-Marseille Université, Parc Scientifique et Technologique de Luminy, 163 Avenue de Luminy, 13009 Marseille, France
| | - Gunnar F Schröder
- 1] Forschungszentrum Jülich, Institute of Complex Systems, ICS-6: Structural Biochemistry, 52425 Jülich, Germany [2] Department of Physics, Heinrich-Heine University Düsseldorf, 40225 Düsseldorf, Germany
| | - Crina M Nimigean
- Departments of Anesthesiology, Physiology and Biophysics, and Biochemistry, Weill Cornell Medical College, 1300 York Ave, New York, New York 10065, USA
| | - Henning Stahlberg
- Center for Cellular Imaging and NanoAnalytics, Biozentrum, University of Basel, Mattenstrasse 26, CH-4058 Basel, Switzerland
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Kuang Q, Purhonen P, Jegerschöld C, Koeck PJB, Hebert H. Free RCK arrangement in Kch, a putative escherichia coli potassium channel, as suggested by electron crystallography. Structure 2014; 23:199-205. [PMID: 25497729 DOI: 10.1016/j.str.2014.10.018] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/15/2014] [Revised: 10/08/2014] [Accepted: 10/21/2014] [Indexed: 01/24/2023]
Abstract
The ligand-gated potassium channels are stimulated by various kinds of messengers. Previous studies showed that ligand-gated potassium channels containing RCK domains (the regulator of the conductance of potassium ion) form a dimer of tetramer structure through the RCK octameric gating ring in the presence of detergent. Here, we have analyzed the structure of Kch, a channel of this type from Escherichia coli, in a lipid environment using electron crystallography. By combining information from the 3D map of the transmembrane part of the protein and docking of an atomic model of a potassium channel, we conclude that the RCK domains face the solution and that an RCK octameric gating ring arrangement does not form under our crystallization condition. Our findings may be applied to other potassium channels that have an RCK gating ring arrangement.
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Affiliation(s)
- Qie Kuang
- Department of Biosciences and Nutrition, Karolinska Institutet, 14183 Huddinge, Sweden; School of Technology and Health, KTH Royal Institute of Technology, 14183 Huddinge, Sweden.
| | - Pasi Purhonen
- Department of Biosciences and Nutrition, Karolinska Institutet, 14183 Huddinge, Sweden
| | - Caroline Jegerschöld
- Department of Biosciences and Nutrition, Karolinska Institutet, 14183 Huddinge, Sweden
| | - Philip J B Koeck
- Department of Biosciences and Nutrition, Karolinska Institutet, 14183 Huddinge, Sweden; School of Technology and Health, KTH Royal Institute of Technology, 14183 Huddinge, Sweden
| | - Hans Hebert
- Department of Biosciences and Nutrition, Karolinska Institutet, 14183 Huddinge, Sweden; School of Technology and Health, KTH Royal Institute of Technology, 14183 Huddinge, Sweden.
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Kuang Q, Purhonen P, Jegerschöld C, Hebert H. The projection structure of Kch, a putative potassium channel in Escherichia coli, by electron crystallography. BIOCHIMICA ET BIOPHYSICA ACTA-BIOMEMBRANES 2013; 1838:237-43. [PMID: 24055821 DOI: 10.1016/j.bbamem.2013.09.006] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/30/2013] [Revised: 09/07/2013] [Accepted: 09/10/2013] [Indexed: 11/30/2022]
Abstract
The kch gene, the only potassium channel gene in Escherichia coli, has the property to express both full-length Kch and its cytosolic domain (RCK) due to a methionine at position 240. The RCK domains are believed to form an octameric ring structure and regulate the gating of the potassium channels after having bound certain ligands. Several different gating ring structures have been reported for the soluble RCK domains, however, these were studied isolated from their transmembrane parts. We previously reported an octameric structure of Kch in solution by electron microscopy and single particle reconstruction, composed of two tetrameric full-length proteins through RCK interaction. To exclude the effect of the detergent, we have now performed an electron crystallographic study of the full-length Kch in membrane bound form. Well-ordered two-dimensional crystals were grown in a natural phospholipid environment. A projection map merged from the fifteen best images extended to 6Å resolution. The c12 two-sided plane group of the two-dimensional crystals showed that Kch crystallized as two symmetrically related overlapping layers. The arrangement suggests that the two layers of RCK domains are shifted with respect to each other and the RCK octameric gating ring of Kch does not form under the crystallization condition.
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Affiliation(s)
- Qie Kuang
- Karolinska Institutet, Dept of Biosciences and Nutrition and KTH Royal Institute of Technology, School of Technology and Health, Novum, S-14183 Huddinge, Sweden.
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Cukkemane A, Baldus M. Characterization of a cyclic nucleotide-activated K(+) channel and its lipid environment by using solid-state NMR spectroscopy. Chembiochem 2013; 14:1789-98. [PMID: 23956185 DOI: 10.1002/cbic.201300182] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/26/2013] [Indexed: 01/31/2023]
Abstract
Voltage-gated ion channels are large tetrameric multidomain membrane proteins that play crucial roles in various cellular transduction pathways. Because of their large size and domain-related mobility, structural characterization has proved challenging. We analyzed high-resolution solid-state NMR data on different isotope-labeled protein constructs of a bacterial cyclic nucleotide-activated K(+) channel (MlCNG) in lipid bilayers. We could identify the different subdomains of the 4×355 residue protein, such as the voltage-sensing domain and the cyclic nucleotide binding domain. Comparison to ssNMR data obtained on isotope-labeled cell membranes suggests a tight association of negatively charged lipids to the channel. We detected spectroscopic polymorphism that extends beyond the ligand binding site, and the corresponding protein segments have been associated with mutant channel types in eukaryotic systems. These findings illustrate the potential of ssNMR for structural investigations on large membrane-embedded proteins, even in the presence of local disorder.
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Affiliation(s)
- Abhishek Cukkemane
- NMR Spectroscopy, Bijvoet Center for Biomolecular Research, Department of Chemistry, Faculty of Science, Utrecht University, Padualaan 8, 3584 Utrecht (The Netherlands)
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Whited AM, Park PSH. Atomic force microscopy: a multifaceted tool to study membrane proteins and their interactions with ligands. BIOCHIMICA ET BIOPHYSICA ACTA-BIOMEMBRANES 2013; 1838:56-68. [PMID: 23603221 DOI: 10.1016/j.bbamem.2013.04.011] [Citation(s) in RCA: 83] [Impact Index Per Article: 7.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 02/04/2013] [Revised: 03/22/2013] [Accepted: 04/09/2013] [Indexed: 01/31/2023]
Abstract
Membrane proteins are embedded in lipid bilayers and facilitate the communication between the external environment and the interior of the cell. This communication is often mediated by the binding of ligands to the membrane protein. Understanding the nature of the interaction between a ligand and a membrane protein is required to both understand the mechanism of action of these proteins and for the development of novel pharmacological drugs. The highly hydrophobic nature of membrane proteins and the requirement of a lipid bilayer for native function have hampered the structural and molecular characterizations of these proteins under physiologically relevant conditions. Atomic force microscopy offers a solution to studying membrane proteins and their interactions with ligands under physiologically relevant conditions and can provide novel insights about the nature of these critical molecular interactions that facilitate cellular communication. In this review, we provide an overview of the atomic force microscopy technique and discuss its application in the study of a variety of questions related to the interaction between a membrane protein and a ligand. This article is part of a Special Issue entitled: Structural and biophysical characterization of membrane protein-ligand binding.
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Affiliation(s)
- Allison M Whited
- Department of Ophthalmology and Visual Sciences, Case Western Reserve University, Cleveland, OH 44106, USA
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Gating of the MlotiK1 potassium channel involves large rearrangements of the cyclic nucleotide-binding domains. Proc Natl Acad Sci U S A 2011; 108:20802-7. [PMID: 22135457 DOI: 10.1073/pnas.1111149108] [Citation(s) in RCA: 42] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Cyclic nucleotide-regulated ion channels are present in bacteria, plants, vertebrates, and humans. In higher organisms, they are closely involved in signaling networks of vision and olfaction. Binding of cAMP or cGMP favors the activation of these ion channels. Despite a wealth of structural and studies, there is a lack of structural data describing the gating process in a full-length cyclic nucleotide-regulated channel. We used high-resolution atomic force microscopy (AFM) to directly observe the conformational change of the membrane embedded bacterial cyclic nucleotide-regulated channel MlotiK1. In the nucleotide-bound conformation, the cytoplasmic cyclic nucleotide-binding (CNB) domains of MlotiK1 are disposed in a fourfold symmetric arrangement forming a pore-like vestibule. Upon nucleotide-unbinding, the four CNB domains undergo a large rearrangement, stand up by ∼1.7 nm, and adopt a structurally variable grouped conformation that closes the cytoplasmic vestibule. This fully reversible conformational change provides insight into how CNB domains rearrange when regulating the potassium channel.
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