1
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Bergh C, Rovšnik U, Howard R, Lindahl E. Discovery of lipid binding sites in a ligand-gated ion channel by integrating simulations and cryo-EM. eLife 2024; 12:RP86016. [PMID: 38289224 PMCID: PMC10945520 DOI: 10.7554/elife.86016] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/01/2024] Open
Abstract
Ligand-gated ion channels transduce electrochemical signals in neurons and other excitable cells. Aside from canonical ligands, phospholipids are thought to bind specifically to the transmembrane domain of several ion channels. However, structural details of such lipid contacts remain elusive, partly due to limited resolution of these regions in experimental structures. Here, we discovered multiple lipid interactions in the channel GLIC by integrating cryo-electron microscopy and large-scale molecular simulations. We identified 25 bound lipids in the GLIC closed state, a conformation where none, to our knowledge, were previously known. Three lipids were associated with each subunit in the inner leaflet, including a buried interaction disrupted in mutant simulations. In the outer leaflet, two intrasubunit sites were evident in both closed and open states, while a putative intersubunit site was preferred in open-state simulations. This work offers molecular details of GLIC-lipid contacts particularly in the ill-characterized closed state, testable hypotheses for state-dependent binding, and a multidisciplinary strategy for modeling protein-lipid interactions.
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Affiliation(s)
- Cathrine Bergh
- Science for Life Laboratory & Swedish e-Science Research Center, Department of Applied Physics, KTH Royal Institute of TechnologyStockholmSweden
| | - Urška Rovšnik
- Science for Life Laboratory, Department of Biochemistry and Biophysics, Stockholm UniversityStockholmSweden
| | - Rebecca Howard
- Science for Life Laboratory & Swedish e-Science Research Center, Department of Applied Physics, KTH Royal Institute of TechnologyStockholmSweden
- Science for Life Laboratory, Department of Biochemistry and Biophysics, Stockholm UniversityStockholmSweden
| | - Erik Lindahl
- Science for Life Laboratory & Swedish e-Science Research Center, Department of Applied Physics, KTH Royal Institute of TechnologyStockholmSweden
- Science for Life Laboratory, Department of Biochemistry and Biophysics, Stockholm UniversityStockholmSweden
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2
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Recent Insight into Lipid Binding and Lipid Modulation of Pentameric Ligand-Gated Ion Channels. Biomolecules 2022; 12:biom12060814. [PMID: 35740939 PMCID: PMC9221113 DOI: 10.3390/biom12060814] [Citation(s) in RCA: 3] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/05/2022] [Revised: 06/03/2022] [Accepted: 06/08/2022] [Indexed: 02/04/2023] Open
Abstract
Pentameric ligand-gated ion channels (pLGICs) play a leading role in synaptic communication, are implicated in a variety of neurological processes, and are important targets for the treatment of neurological and neuromuscular disorders. Endogenous lipids and lipophilic compounds are potent modulators of pLGIC function and may help shape synaptic communication. Increasing structural and biophysical data reveal sites for lipid binding to pLGICs. Here, we update our evolving understanding of pLGIC–lipid interactions highlighting newly identified modes of lipid binding along with the mechanistic understanding derived from the new structural data.
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3
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Elephants in the Dark: Insights and Incongruities in Pentameric Ligand-gated Ion Channel Models. J Mol Biol 2021; 433:167128. [PMID: 34224751 DOI: 10.1016/j.jmb.2021.167128] [Citation(s) in RCA: 14] [Impact Index Per Article: 4.7] [Reference Citation Analysis] [Abstract] [Key Words] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/14/2021] [Revised: 06/25/2021] [Accepted: 06/25/2021] [Indexed: 02/06/2023]
Abstract
The superfamily of pentameric ligand-gated ion channels (pLGICs) comprises key players in electrochemical signal transduction across evolution, including historic model systems for receptor allostery and targets for drug development. Accordingly, structural studies of these channels have steadily increased, and now approach 250 depositions in the protein data bank. This review contextualizes currently available structures in the pLGIC family, focusing on morphology, ligand binding, and gating in three model subfamilies: the prokaryotic channel GLIC, the cation-selective nicotinic acetylcholine receptor, and the anion-selective glycine receptor. Common themes include the challenging process of capturing and annotating channels in distinct functional states; partially conserved gating mechanisms, including remodeling at the extracellular/transmembrane-domain interface; and diversity beyond the protein level, arising from posttranslational modifications, ligands, lipids, and signaling partners. Interpreting pLGIC structures can be compared to describing an elephant in the dark, relying on touch alone to comprehend the many parts of a monumental beast: each structure represents a snapshot in time under specific experimental conditions, which must be integrated with further structure, function, and simulations data to build a comprehensive model, and understand how one channel may fundamentally differ from another.
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4
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Thompson MJ, Baenziger JE. Structural basis for the modulation of pentameric ligand-gated ion channel function by lipids. BIOCHIMICA ET BIOPHYSICA ACTA-BIOMEMBRANES 2020; 1862:183304. [DOI: 10.1016/j.bbamem.2020.183304] [Citation(s) in RCA: 15] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/31/2019] [Revised: 03/20/2020] [Accepted: 04/05/2020] [Indexed: 10/24/2022]
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5
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Hénault CM, Govaerts C, Spurny R, Brams M, Estrada-Mondragon A, Lynch J, Bertrand D, Pardon E, Evans GL, Woods K, Elberson BW, Cuello LG, Brannigan G, Nury H, Steyaert J, Baenziger JE, Ulens C. A lipid site shapes the agonist response of a pentameric ligand-gated ion channel. Nat Chem Biol 2019; 15:1156-1164. [PMID: 31591563 DOI: 10.1038/s41589-019-0369-4] [Citation(s) in RCA: 35] [Impact Index Per Article: 7.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/14/2018] [Accepted: 08/21/2019] [Indexed: 12/11/2022]
Abstract
Phospholipids are key components of cellular membranes and are emerging as important functional regulators of different membrane proteins, including pentameric ligand-gated ion channels (pLGICs). Here, we take advantage of the prokaryote channel ELIC (Erwinia ligand-gated ion channel) as a model to understand the determinants of phospholipid interactions in this family of receptors. A high-resolution structure of ELIC in a lipid-bound state reveals a phospholipid site at the lower half of pore-forming transmembrane helices M1 and M4 and at a nearby site for neurosteroids, cholesterol or general anesthetics. This site is shaped by an M4-helix kink and a Trp-Arg-Pro triad that is highly conserved in eukaryote GABAA/C and glycine receptors. A combined approach reveals that M4 is intrinsically flexible and that M4 deletions or disruptions of the lipid-binding site accelerate desensitization in ELIC, suggesting that lipid interactions shape the agonist response. Our data offer a structural context for understanding lipid modulation in pLGICs.
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Affiliation(s)
- Camille M Hénault
- Department of Biochemistry, Microbiology and Immunology, University of Ottawa, Ottawa, Ontario, Canada
| | - Cedric Govaerts
- Laboratory for the Structure and Function of Biological Membranes, Center for Structural Biology and Bioinformatics, Université libre de Bruxelles, Brussels, Belgium
| | - Radovan Spurny
- Laboratory of Structural Neurobiology, Department of Cellular and Molecular Medicine, KU Leuven, Leuven, Belgium
| | - Marijke Brams
- Laboratory of Structural Neurobiology, Department of Cellular and Molecular Medicine, KU Leuven, Leuven, Belgium
| | | | - Joseph Lynch
- Queensland Brain Institute, University of Queensland, Brisbane, Queensland, Australia
| | | | - Els Pardon
- Structural Biology Brussels, Vrije Universiteit Brussel, Brussels, Belgium.,VIB-VUB Center for Structural Biology, VIB, Brussels, Belgium
| | - Genevieve L Evans
- Laboratory of Structural Neurobiology, Department of Cellular and Molecular Medicine, KU Leuven, Leuven, Belgium
| | - Kristen Woods
- Center for Computational and Integrative Biology, Rutgers University-Camden, Camden, NJ, USA.,Department of Physics, Rutgers University-Camden, Camden, NJ, USA
| | - Benjamin W Elberson
- Department of Cell Physiology and Molecular Biophysics, Center for Membrane Protein Research, TTUHSC, Lubbock, TX, USA
| | - Luis G Cuello
- Department of Cell Physiology and Molecular Biophysics, Center for Membrane Protein Research, TTUHSC, Lubbock, TX, USA
| | - Grace Brannigan
- Center for Computational and Integrative Biology, Rutgers University-Camden, Camden, NJ, USA.,Department of Physics, Rutgers University-Camden, Camden, NJ, USA
| | - Hugues Nury
- University Grenoble Alpes, CNRS, IBS, Grenoble, France
| | - Jan Steyaert
- Structural Biology Brussels, Vrije Universiteit Brussel, Brussels, Belgium.,VIB-VUB Center for Structural Biology, VIB, Brussels, Belgium
| | - John E Baenziger
- Department of Biochemistry, Microbiology and Immunology, University of Ottawa, Ottawa, Ontario, Canada.
| | - Chris Ulens
- Laboratory of Structural Neurobiology, Department of Cellular and Molecular Medicine, KU Leuven, Leuven, Belgium.
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6
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Chen Q, Xu Y, Tang P. X-Ray Crystallographic Studies for Revealing Binding Sites of General Anesthetics in Pentameric Ligand-Gated Ion Channels. Methods Enzymol 2018; 603:21-47. [PMID: 29673527 DOI: 10.1016/bs.mie.2018.01.017] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/05/2023]
Abstract
X-ray crystallography is a powerful tool in structural biology and can offer insight into structured-based understanding of general anesthetic action on various relevant molecular targets, including pentameric ligand-gated ion channels (pLGICs). In this chapter, we outline the procedures for expression and purification of pLGICs. Optimization of crystallization conditions, especially to achieve high-resolution structures of pLGICs bound with general anesthetics, is also presented. Case studies of pLGICs bound with the volatile general anesthetic isoflurane, 2-bromoethanol, and the intravenous general anesthetic ketamine are revisited.
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Affiliation(s)
- Qiang Chen
- University of Pittsburgh School of Medicine, Pittsburgh, PA, United States
| | - Yan Xu
- University of Pittsburgh School of Medicine, Pittsburgh, PA, United States
| | - Pei Tang
- University of Pittsburgh School of Medicine, Pittsburgh, PA, United States.
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7
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An allosteric link connecting the lipid-protein interface to the gating of the nicotinic acetylcholine receptor. Sci Rep 2018; 8:3898. [PMID: 29497086 PMCID: PMC5832824 DOI: 10.1038/s41598-018-22150-x] [Citation(s) in RCA: 17] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/20/2017] [Accepted: 02/16/2018] [Indexed: 11/08/2022] Open
Abstract
The mechanisms underlying lipid-sensing by membrane proteins is of considerable biological importance. A unifying mechanistic question is how a change in structure at the lipid-protein interface is translated through the transmembrane domain to influence structures critical to protein function. Gating of the nicotinic acetylcholine receptor (nAChR) is sensitive to its lipid environment. To understand how changes at the lipid-protein interface influence gating, we examined how a mutation at position 418 on the lipid-facing surface of the outer most M4 transmembrane α-helix alters the energetic couplings between M4 and the remainder of the transmembrane domain. Human muscle nAChR is sensitive to mutations at position 418, with the Cys-to-Trp mutation resulting in a 16-fold potentiation in function that leads to a congenital myasthenic syndrome. Energetic coupling between M4 and the Cys-loop, a key structure implicated in gating, do not change with C418W. Instead, Trp418 and an adjacent residue couple energetically with residues on the M1 transmembrane α-helix, leading to a reorientation of M1 that stabilizes the open state. We thus identify an allosteric link connecting the lipid-protein interface of the nAChR to altered channel function.
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8
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Poveda JA, Marcela Giudici A, Lourdes Renart M, Morales A, González-Ros JM. Towards understanding the molecular basis of ion channel modulation by lipids: Mechanistic models and current paradigms. BIOCHIMICA ET BIOPHYSICA ACTA-BIOMEMBRANES 2017; 1859:1507-1516. [PMID: 28408206 DOI: 10.1016/j.bbamem.2017.04.003] [Citation(s) in RCA: 28] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/31/2016] [Revised: 03/27/2017] [Accepted: 04/09/2017] [Indexed: 10/19/2022]
Abstract
Research on ion channel modulation has become a hot topic because of the key roles these membrane proteins play in both prokaryotic and eukaryotic organisms. In this respect, lipid modulation adds to the overall modulatory mechanisms as a potential via to find new pharmacological targets for drug design based on interfering with lipid/channel interactions. However, our knowledge in this field is scarce and often circumscribed to the sites where lipids bind and/or its final functional consequences. To fully understand this process it is necessary to improve our knowledge on its molecular basis, from the binding sites to the signalling pathways that derive in structural and functional effects on the ion channel. In this review, we have compiled information about such mechanisms and established a classification into four different modes of action. Afterwards, we have revised in more detail the lipid modulation of Cys-loop receptors and of the potassium channel KcsA, which were chosen as model channels modulated by specific lipids. This article is part of a Special Issue entitled: Membrane Lipid Therapy: Drugs Targeting Biomembranes edited by Pablo V. Escribá.
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Affiliation(s)
- José A Poveda
- Instituto de Biología Molecular y Celular, Universidad Miguel Hernández, Elche, 03202 Alicante, Spain.
| | - A Marcela Giudici
- Instituto de Biología Molecular y Celular, Universidad Miguel Hernández, Elche, 03202 Alicante, Spain
| | - M Lourdes Renart
- Instituto de Biología Molecular y Celular, Universidad Miguel Hernández, Elche, 03202 Alicante, Spain
| | - Andrés Morales
- Departamento de Fisiología, Genética y Microbiología, Universidad de Alicante, 03080 Alicante, Spain
| | - José M González-Ros
- Instituto de Biología Molecular y Celular, Universidad Miguel Hernández, Elche, 03202 Alicante, Spain.
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9
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Therien JPD, Baenziger JE. Pentameric ligand-gated ion channels exhibit distinct transmembrane domain archetypes for folding/expression and function. Sci Rep 2017; 7:450. [PMID: 28348412 PMCID: PMC5428567 DOI: 10.1038/s41598-017-00573-2] [Citation(s) in RCA: 14] [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: 12/19/2016] [Accepted: 03/03/2017] [Indexed: 11/17/2022] Open
Abstract
Although transmembrane helix-helix interactions must be strong enough to drive folding, they must still permit the inter-helix movements associated with conformational change. Interactions between the outermost M4 and adjacent M1 and M3 α-helices of pentameric ligand-gated ion channels have been implicated in folding and function. Here, we evaluate the role of different physical interactions at this interface in the function of two prokaryotic homologs, GLIC and ELIC. Strikingly, disruption of most interactions in GLIC lead to either a reduction or a complete loss of expression and/or function, while analogous disruptions in ELIC often lead to gains in function. Structural comparisons suggest that GLIC and ELIC represent distinct transmembrane domain archetypes. One archetype, exemplified by GLIC, the glycine and GABA receptors and the glutamate activated chloride channel, has extensive aromatic contacts that govern M4-M1/M3 interactions and that are essential for expression and function. The other archetype, exemplified by ELIC and both the nicotinic acetylcholine and serotonin receptors, has relatively few aromatic contacts that are detrimental to function. These archetypes likely have evolved different mechanisms to balance the need for strong M4 "binding" to M1/M3 to promote folding/expression, and the need for weaker interactions that allow for greater conformational flexibility.
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Affiliation(s)
- J P Daniel Therien
- Department of Biochemistry, Microbiology, and Immunology University of Ottawa, Ottawa, ON, K1H 8M5, Canada
| | - John E Baenziger
- Department of Biochemistry, Microbiology, and Immunology University of Ottawa, Ottawa, ON, K1H 8M5, Canada.
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10
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Menny A, Lefebvre SN, Schmidpeter PA, Drège E, Fourati Z, Delarue M, Edelstein SJ, Nimigean CM, Joseph D, Corringer PJ. Identification of a pre-active conformation of a pentameric channel receptor. eLife 2017; 6. [PMID: 28294942 PMCID: PMC5398890 DOI: 10.7554/elife.23955] [Citation(s) in RCA: 29] [Impact Index Per Article: 4.1] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/07/2016] [Accepted: 03/14/2017] [Indexed: 11/26/2022] Open
Abstract
Pentameric ligand-gated ion channels (pLGICs) mediate fast chemical signaling through global allosteric transitions. Despite the existence of several high-resolution structures of pLGICs, their dynamical properties remain elusive. Using the proton-gated channel GLIC, we engineered multiple fluorescent reporters, each incorporating a bimane and a tryptophan/tyrosine, whose close distance causes fluorescence quenching. We show that proton application causes a global compaction of the extracellular subunit interface, coupled to an outward motion of the M2-M3 loop near the channel gate. These movements are highly similar in lipid vesicles and detergent micelles. These reorganizations are essentially completed within 2 ms and occur without channel opening at low proton concentration, indicating that they report a pre-active intermediate state in the transition pathway toward activation. This provides a template to investigate the gating of eukaryotic neurotransmitter receptors, for which intermediate states also participate in activation. DOI:http://dx.doi.org/10.7554/eLife.23955.001 In the nervous system, proteins of the pLGIC family are found in the membrane that surrounds each neuron. These proteins have channels that can allow ions to pass through the membrane and are responsible for transmitting electrical signals from one neuron to the next. Small molecules called neurotransmitters interact with the pLGICs to open or close the ion channel. If the ability of the pLGIC channels to open is altered, it can lead to behavioral changes like addiction, or diseases such as schizophrenia or epilepsy. For a pLGIC channel to switch between the “open” and “closed” states, specific parts of the protein need to move in relation to each other. However, to study these transitions researchers have previously relied on comparing the three-dimensional structures of open and closed pLGICs extracted out of the cell membrane. Different techniques are needed to directly follow these movements within membranes. Bacteria also have proteins belonging to the pLGIC family, and Menny et al. have now investigated one such bacterial protein to understand how pLGICs open. First, a small fluorescent molecule that glows differently if the environment around it changes was attached to various parts of the bacterial channel. These fluorescent markers revealed how several parts of the protein move and they also made it possible to measure how quickly these movements take place. Some of these movements happen before the channel opens, suggesting that the activation of this pLGIC protein happens in stages and involves the protein adopting a temporary intermediate state. The next step will be to better understand the structure of the intermediate state, which could help us to understand how pLGICs work in the nervous systems of animals. In future this may aid the design of new drugs that can modify the activity of these channels in patients with neurological conditions or addictions. DOI:http://dx.doi.org/10.7554/eLife.23955.002
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Affiliation(s)
- Anaïs Menny
- Channel Receptors Unit, Institut Pasteur, Paris, France.,Unité Mixte de Recherche 3571, Centre National de la Recherche Scientifique, Paris, France.,Université Pierre et Marie Curie, Cellule Pasteur, Paris, France
| | - Solène N Lefebvre
- Channel Receptors Unit, Institut Pasteur, Paris, France.,Unité Mixte de Recherche 3571, Centre National de la Recherche Scientifique, Paris, France.,Université Pierre et Marie Curie, Cellule Pasteur, Paris, France
| | - Philipp Am Schmidpeter
- Departments of Anesthesiology, Physiology and Biophysics, Biochemistry, Weill Cornell Medicine, New York, United States
| | - Emmanuelle Drège
- BioCIS, Université Paris-Sud, CNRS, Université Paris-Saclay, Châtenay-Malabry, France
| | - Zaineb Fourati
- Unité de Dynamique Structurale des Macromolécules, Institut Pasteur, Paris, France.,Unité Mixte de Recherche 3528, Centre National de la Recherche Scientifique, Paris, France
| | - Marc Delarue
- Unité de Dynamique Structurale des Macromolécules, Institut Pasteur, Paris, France.,Unité Mixte de Recherche 3528, Centre National de la Recherche Scientifique, Paris, France
| | - Stuart J Edelstein
- Biologie Cellulaire de la Synapse, Institute of Biology, Ecole Normale Supérieure, Paris, France
| | - Crina M Nimigean
- Departments of Anesthesiology, Physiology and Biophysics, Biochemistry, Weill Cornell Medicine, New York, United States
| | - Delphine Joseph
- BioCIS, Université Paris-Sud, CNRS, Université Paris-Saclay, Châtenay-Malabry, France
| | - Pierre-Jean Corringer
- Channel Receptors Unit, Institut Pasteur, Paris, France.,Unité Mixte de Recherche 3571, Centre National de la Recherche Scientifique, Paris, France
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11
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Basak S, Schmandt N, Gicheru Y, Chakrapani S. Crystal structure and dynamics of a lipid-induced potential desensitized-state of a pentameric ligand-gated channel. eLife 2017; 6:23886. [PMID: 28262093 PMCID: PMC5378477 DOI: 10.7554/elife.23886] [Citation(s) in RCA: 52] [Impact Index Per Article: 7.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/05/2016] [Accepted: 03/04/2017] [Indexed: 12/14/2022] Open
Abstract
Desensitization in pentameric ligand-gated ion channels plays an important role in regulating neuronal excitability. Here, we show that docosahexaenoic acid (DHA), a key ω−3 polyunsaturated fatty acid in synaptic membranes, enhances the agonist-induced transition to the desensitized state in the prokaryotic channel GLIC. We determined a 3.25 Å crystal structure of the GLIC-DHA complex in a potentially desensitized conformation. The DHA molecule is bound at the channel-periphery near the M4 helix and exerts a long-range allosteric effect on the pore across domain-interfaces. In this previously unobserved conformation, the extracellular-half of the pore-lining M2 is splayed open, reminiscent of the open conformation, while the intracellular-half is constricted, leading to a loss of both water and permeant ions. These findings, in combination with spin-labeling/EPR spectroscopic measurements in reconstituted-membranes, provide novel mechanistic details of desensitization in pentameric channels. DOI:http://dx.doi.org/10.7554/eLife.23886.001 The nerve cells (or neurons) in the brain communicate with each other by releasing chemicals called neurotransmitters that bind to ion channels on neighboring neurons. This ultimately causes ions to flow in or out of the receiving neuron through these ion channels; this ion flow determines how the neuron responds. One family of ion channels that is found at the junction between neurons, and between neurons and muscle fibers, is known as the pentameric ligand-gated ion channels (or pLGICs). These channels act as ‘gates’ that open to allow ions through them when a neurotransmitter binds to the channel. In addition to the open ‘active’ state, the channels can take on two different ‘inactive’ states that do not allow ions to pass through the channel: a closed (resting) state and a desensitized state (that is still bound to the neurotransmitter). Understanding how channels switch between these states is important for designing drugs that correct problems that cause the channels to work incorrectly. Problems that affect the desensitized state have been linked to neurological disorders such as epilepsy. Medically important molecules such as anesthetics and alcohols are thought to affect desensitization, and drugs that target desensitized ion channels may present ways of treating neurological disorders with fewer side effects. Docosahexaenoic acid (DHA) is an abundant lipid molecule that is present in the membranes of neurons. It is one of the key ingredients in fish oil supplements and is thought to enhance learning and memory. DHA affects the desensitization of pLGICs but it is not clear exactly how it does so. Basak et al. now show that DHA affects a bacterial pLGIC in the same way as it affects human channels – by enhancing desensitization. Using a technique called X-ray crystallography to analyze the channel while bound to DHA revealed a previously unobserved channel structure. The DHA molecule binds to a site at the edge of the channel and causes a change in its structure that leaves the upper part of the channel open while the lower part is constricted. Basak et al. predict that molecules such as anesthetics target this desensitized state. The next step will be to obtain the structures of bacterial and human pLGIC channels in a natural membrane environment. This will allow us to better understand the changes in structure that the channels go through as they transmit signals between neurons, and so help in the development of new treatments for neurological disorders. DOI:http://dx.doi.org/10.7554/eLife.23886.002
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Affiliation(s)
- Sandip Basak
- Department of Physiology and Biophysics, School of Medicine, Case Western Reserve University, Cleveland, United States
| | - Nicolaus Schmandt
- Department of Neuroscience, School of Medicine, Case Western Reserve University, Cleveland, United States
| | - Yvonne Gicheru
- Department of Physiology and Biophysics, School of Medicine, Case Western Reserve University, Cleveland, United States
| | - Sudha Chakrapani
- Department of Physiology and Biophysics, School of Medicine, Case Western Reserve University, Cleveland, United States
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12
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Baenziger JE, Domville JA, Therien JD. The Role of Cholesterol in the Activation of Nicotinic Acetylcholine Receptors. CURRENT TOPICS IN MEMBRANES 2017; 80:95-137. [DOI: 10.1016/bs.ctm.2017.05.002] [Citation(s) in RCA: 23] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/21/2022]
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13
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Hénault CM, Baenziger JE. Functional characterization of two prokaryotic pentameric ligand-gated ion channel chimeras - role of the GLIC transmembrane domain in proton sensing. BIOCHIMICA ET BIOPHYSICA ACTA-BIOMEMBRANES 2016; 1859:218-227. [PMID: 27845033 DOI: 10.1016/j.bbamem.2016.11.006] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/05/2016] [Revised: 10/21/2016] [Accepted: 11/10/2016] [Indexed: 10/20/2022]
Abstract
With the long-term goal of using a chimeric approach to dissect the distinct lipid sensitivities and thermal stabilities of the pentameric ligand-gated ion channels (pLGIC), GLIC and ELIC, we constructed chimeras by cross-combining their extracellular (ECD) and transmembrane (TMD) domains. As expected, the chimera formed between GLIC-ECD and ELIC-TMD (GE) responded to protons, the agonist for GLIC, but not cysteamine, the agonist for ELIC, although GE exhibited a 25-fold decrease in proton-sensitivity relative to wild type. The chimera formed between ELIC-ECD and the GLIC-TMD (EG) was usually toxic, unless it contained a pore-lining Ile9'Ala gain-of-function mutation. No significant improvements in expression/toxicity were observed with extensive loop substitutions at the ECD/TMD interface. Surprisingly, oocytes expressing EG-I9'A responded to both the ELIC agonist, cysteamine and the GLIC agonist, protons - the latter at pH values ≤4.0. The cysteamine- and proton-induced currents in EG-I9'A were inhibited by the GLIC TMD pore blocker, amantadine. The cysteamine-induced response of EG-I9'A was also inhibited by protons at pH values down to 4.5, but potentiated at lower pH values. Proton-induced gating at low pH was not abolished by mutation of an intramembrane histidine residue previously implicated in GLIC TMD function. We show that the TMD plays a major role governing the thermal stability of a pLGIC, and identify three distinct mechanisms by which agonists and protons influence the gating of the EG chimera. A structural basis for the impaired function of GE is suggested.
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Affiliation(s)
- Camille M Hénault
- Department of Biochemistry, Microbiology, and Immunology, University of Ottawa, 451 Smyth Rd, Ottawa, ON K1H 8M5, Canada
| | - John E Baenziger
- Department of Biochemistry, Microbiology, and Immunology, University of Ottawa, 451 Smyth Rd, Ottawa, ON K1H 8M5, Canada.
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14
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Morales-Perez CL, Noviello CM, Hibbs RE. X-ray structure of the human α4β2 nicotinic receptor. Nature 2016; 538:411-415. [PMID: 27698419 PMCID: PMC5161573 DOI: 10.1038/nature19785] [Citation(s) in RCA: 305] [Impact Index Per Article: 38.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/13/2016] [Accepted: 08/22/2016] [Indexed: 02/07/2023]
Abstract
Nicotinic acetylcholine receptors are ligand-gated ion channels that mediate fast chemical neurotransmission at the neuromuscular junction and have diverse signalling roles in the central nervous system. The nicotinic receptor has been a model system for cell-surface receptors, and specifically for ligand-gated ion channels, for well over a century. In addition to the receptors' prominent roles in the development of the fields of pharmacology and neurobiology, nicotinic receptors are important therapeutic targets for neuromuscular disease, addiction, epilepsy and for neuromuscular blocking agents used during surgery. The overall architecture of the receptor was described in landmark studies of the nicotinic receptor isolated from the electric organ of Torpedo marmorata. Structures of a soluble ligand-binding domain have provided atomic-scale insights into receptor-ligand interactions, while high-resolution structures of other members of the pentameric receptor superfamily provide touchstones for an emerging allosteric gating mechanism. All available high-resolution structures are of homopentameric receptors. However, the vast majority of pentameric receptors (called Cys-loop receptors in eukaryotes) present physiologically are heteromeric. Here we present the X-ray crystallographic structure of the human α4β2 nicotinic receptor, the most abundant nicotinic subtype in the brain. This structure provides insights into the architectural principles governing ligand recognition, heteromer assembly, ion permeation and desensitization in this prototypical receptor class.
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Affiliation(s)
- Claudio L. Morales-Perez
- Departments of Neuroscience and Biophysics, University of Texas
Southwestern Medical Center, Dallas, TX 75390, USA
| | - Colleen M. Noviello
- Departments of Neuroscience and Biophysics, University of Texas
Southwestern Medical Center, Dallas, TX 75390, USA
| | - Ryan E. Hibbs
- Departments of Neuroscience and Biophysics, University of Texas
Southwestern Medical Center, Dallas, TX 75390, USA
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15
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Uncovering the lipidic basis for the preparation of functional nicotinic acetylcholine receptor detergent complexes for structural studies. Sci Rep 2016; 6:32766. [PMID: 27641515 PMCID: PMC5027579 DOI: 10.1038/srep32766] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/11/2016] [Accepted: 08/15/2016] [Indexed: 01/10/2023] Open
Abstract
This study compares the lipid composition, including individual phospholipid molecular species of solubilized nAChR detergent complexes (nAChR-DCs) with those of the bulk lipids from their source, Torpedo californica (Tc) electric tissue. This lipidomic analysis revealed seventy-seven (77) phospholipid species in the Tc tissue. Analysis of affinity-purified nAChR-DCs prepared with C-12 to C-16 phospholipid analog detergents alkylphosphocholine (FC) and lysofoscholine (LFC) demonstrated that nAChR-DCs prepared with FC12, LFC14, and LFC16 contained >60 phospholipids/nAChR, which was more than twice of those prepared with FC14, FC16, and LFC12. Significantly, all the nAChR-DCs lacked ethanolamine and anionic phospholipids, contained only four cholesterol molecules, and a limited number of phospholipid molecular species per nAChR. Upon incorporation into oocytes, FC12 produce significant functionality, whereas LFC14 and LFC16 nAChR-DCs displayed an increased functionality as compared to the crude Tc membrane. All three nAChR-DCs displayed different degrees of alterations in macroscopic activation and desensitization kinetics.
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16
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Tillman TS, Alvarez FJD, Reinert NJ, Liu C, Wang D, Xu Y, Xiao K, Zhang P, Tang P. Functional Human α7 Nicotinic Acetylcholine Receptor (nAChR) Generated from Escherichia coli. J Biol Chem 2016; 291:18276-82. [PMID: 27385587 DOI: 10.1074/jbc.m116.729970] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/29/2016] [Indexed: 11/06/2022] Open
Abstract
Human Cys-loop receptors are important therapeutic targets. High-resolution structures are essential for rational drug design, but only a few are available due to difficulties in obtaining sufficient quantities of protein suitable for structural studies. Although expression of proteins in E. coli offers advantages of high yield, low cost, and fast turnover, this approach has not been thoroughly explored for full-length human Cys-loop receptors because of the conventional wisdom that E. coli lacks the specific chaperones and post-translational modifications potentially required for expression of human Cys-loop receptors. Here we report the successful production of full-length wild type human α7nAChR from E. coli Chemically induced chaperones promote high expression levels of well-folded proteins. The choice of detergents, lipids, and ligands during purification determines the final protein quality. The purified α7nAChR not only forms pentamers as imaged by negative-stain electron microscopy, but also retains pharmacological characteristics of native α7nAChR, including binding to bungarotoxin and positive allosteric modulators specific to α7nAChR. Moreover, the purified α7nAChR injected into Xenopus oocytes can be activated by acetylcholine, choline, and nicotine, inhibited by the channel blockers QX-222 and phencyclidine, and potentiated by the α7nAChR specific modulators PNU-120596 and TQS. The successful generation of functional human α7nAChR from E. coli opens a new avenue for producing mammalian Cys-loop receptors to facilitate structure-based rational drug design.
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Affiliation(s)
| | | | | | | | | | - Yan Xu
- From the Departments of Anesthesiology, Structural Biology, Pharmacology and Chemical Biology, and
| | | | | | - Pei Tang
- From the Departments of Anesthesiology, Pharmacology and Chemical Biology, and Computational & Systems Biology, University of Pittsburgh School of Medicine, Pittsburgh, Pennsylvania 15260
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17
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Basak S, Chatterjee S, Chakrapani S. Site Directed Spin Labeling and EPR Spectroscopic Studies of Pentameric Ligand-Gated Ion Channels. J Vis Exp 2016. [PMID: 27403967 DOI: 10.3791/54127] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/19/2023] Open
Abstract
Ion channel gating is a stimulus-driven orchestration of protein motions that leads to transitions between closed, open, and desensitized states. Fundamental to these transitions is the intrinsic flexibility of the protein, which is critically modulated by membrane lipid-composition. To better understand the structural basis of channel function, it is necessary to study protein dynamics in a physiological membrane environment. Electron Paramagnetic Resonance (EPR) spectroscopy is an important tool to characterize conformational transitions between functional states. In comparison to NMR and X-ray crystallography, the information obtained from EPR is intrinsically of lower resolution. However, unlike in other techniques, in EPR there is no upper-limit to the molecular weight of the protein, the sample requirements are significantly lower, and more importantly the protein is not constrained by the crystal lattice forces. Therefore, EPR is uniquely suited for studying large protein complexes and proteins in reconstituted systems. In this article, we will discuss general protocols for site-directed spin labeling and membrane reconstitution using a prokaryotic proton-gated pentameric Ligand-Gated Ion Channel (pLGIC) from Gloeobacter violaceus (GLIC) as an example. A combination of steady-state Continuous Wave (CW) and Pulsed (Double Electron Electron Resonance-DEER) EPR approaches will be described that will enable a complete quantitative characterization of channel dynamics.
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Affiliation(s)
- Sandip Basak
- Department of Physiology and Biophysics, Case Western Reserve University
| | - Soumili Chatterjee
- Department of Physiology and Biophysics, Case Western Reserve University
| | - Sudha Chakrapani
- Department of Physiology and Biophysics, Case Western Reserve University; School of Medicine, Case Western Reserve University;
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18
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Carswell CL, Hénault CM, Murlidaran S, Therien JPD, Juranka PF, Surujballi JA, Brannigan G, Baenziger JE. Role of the Fourth Transmembrane α Helix in the Allosteric Modulation of Pentameric Ligand-Gated Ion Channels. Structure 2015; 23:1655-1664. [PMID: 26235032 PMCID: PMC4824752 DOI: 10.1016/j.str.2015.06.020] [Citation(s) in RCA: 29] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/18/2014] [Revised: 06/01/2015] [Accepted: 06/27/2015] [Indexed: 01/22/2023]
Abstract
The gating of pentameric ligand-gated ion channels is sensitive to a variety of allosteric modulators that act on structures peripheral to those involved in the allosteric pathway leading from the agonist site to the channel gate. One such structure, the lipid-exposed transmembrane α helix, M4, is the target of lipids, neurosteroids, and disease-causing mutations. Here we show that M4 interactions with the adjacent transmembrane α helices, M1 and M3, modulate pLGIC function. Enhanced M4 interactions promote channel function while ineffective interactions reduce channel function. The interface chemistry governs the intrinsic strength of M4-M1/M3 inter-helical interactions, both influencing channel gating and imparting distinct susceptibilities to the potentiating effects of a lipid-facing M4 congenital myasthenic syndrome mutation. Through aromatic substitutions, functional studies, and molecular dynamics simulations, we elucidate a mechanism by which M4 modulates channel function.
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Affiliation(s)
- Casey L Carswell
- Department of Biochemistry, Microbiology, and Immunology, 451 Smyth Road, Ottawa, ON K1H 8M5, Canada
| | - Camille M Hénault
- Department of Biochemistry, Microbiology, and Immunology, 451 Smyth Road, Ottawa, ON K1H 8M5, Canada
| | - Sruthi Murlidaran
- Center for Computational and Integrative Biology, Rutgers University-Camden, Camden, NJ 08102, USA
| | - J P Daniel Therien
- Department of Biochemistry, Microbiology, and Immunology, 451 Smyth Road, Ottawa, ON K1H 8M5, Canada
| | - Peter F Juranka
- Department of Biochemistry, Microbiology, and Immunology, 451 Smyth Road, Ottawa, ON K1H 8M5, Canada
| | - Julian A Surujballi
- Department of Biochemistry, Microbiology, and Immunology, 451 Smyth Road, Ottawa, ON K1H 8M5, Canada
| | - Grace Brannigan
- Center for Computational and Integrative Biology, Rutgers University-Camden, Camden, NJ 08102, USA; Department of Physics, Rutgers University-Camden, Camden, NJ 08103, USA
| | - John E Baenziger
- Department of Biochemistry, Microbiology, and Immunology, 451 Smyth Road, Ottawa, ON K1H 8M5, Canada.
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19
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Hénault CM, Juranka PF, Baenziger JE. The M4 Transmembrane α-Helix Contributes Differently to Both the Maturation and Function of Two Prokaryotic Pentameric Ligand-gated Ion Channels. J Biol Chem 2015; 290:25118-28. [PMID: 26318456 DOI: 10.1074/jbc.m115.676833] [Citation(s) in RCA: 24] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/13/2015] [Indexed: 01/22/2023] Open
Abstract
The role of the outermost transmembrane α-helix in both the maturation and function of the prokaryotic pentameric ligand-gated ion channels, GLIC and ELIC, was examined by Ala scanning mutagenesis, deletion mutations, and mutant cycle analyses. Ala mutations at the M4-M1/M3 interface lead to loss-of-function phenotypes in GLIC, with the largest negative effects occurring near the M4 C terminus. In particular, two aromatic residues at the M4 C terminus form a network of π-π and/or cation-π interactions with residues on M3 and the β6-β7 loop that is essential for both maturation and function. M4-M1/M3 interactions appear to be optimized in GLIC with even subtle structural changes at this interface leading to detrimental effects. In contrast, mutations along the M4-M1/M3 interface of ELIC typically lead to gain-of-function phenotypes, suggesting that these interactions in ELIC are not optimized for channel function. In addition, no cluster of interacting residues involving the M4 C terminus, M3, and the β6-β7 loop was found, suggesting that the M4 C terminus plays little role in ELIC maturation or function. This study shows that M4 makes distinct contributions to the maturation and gating of these two closely related homologs, suggesting that GLIC and ELIC exhibit divergent features of channel function.
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Affiliation(s)
- Camille M Hénault
- From the Department of Biochemistry, Microbiology, and Immunology, University of Ottawa, Ottawa, Ontario K1H 8M5, Canada
| | - Peter F Juranka
- From the Department of Biochemistry, Microbiology, and Immunology, University of Ottawa, Ottawa, Ontario K1H 8M5, Canada
| | - John E Baenziger
- From the Department of Biochemistry, Microbiology, and Immunology, University of Ottawa, Ottawa, Ontario K1H 8M5, Canada
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20
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Mnatsakanyan N, Nishtala SN, Pandhare A, Fiori MC, Goyal R, Pauwels JE, Navetta AF, Ahrorov A, Jansen M. Functional Chimeras of GLIC Obtained by Adding the Intracellular Domain of Anion- and Cation-Conducting Cys-Loop Receptors. Biochemistry 2015; 54:2670-2682. [PMID: 25861708 DOI: 10.1021/acs.biochem.5b00203] [Citation(s) in RCA: 16] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
Pentameric ligand-gated ion channels (pLGICs), also called Cys-loop receptors in eukaryotic superfamily members, play diverse roles in neurotransmission and serve as primary targets for many therapeutic drugs. Structural studies of full-length eukaryotic pLGICs have been challenging because of glycosylation, large size, pentameric assembly, and hydrophobicity. X-ray structures of prokaryotic pLGICs, including the Gloeobacter violaceus LGIC (GLIC) and the Erwinia chrysanthemi LGIC (ELIC), and truncated eukaryotic pLGICs have significantly improved and complemented the understanding of structural details previously obtained with acetylcholine-binding protein and Torpedo nicotinic acetylcholine receptors. Prokaryotic pLGICs share their overall structural features with eukaryotic pLGICs for the ligand-binding extracellular and channel-lining transmembrane domains. The large intracellular domain (ICD) is present only in eukaryotic members and is characterized by a low level of sequence conservation and significant variability in length (50-250 amino acids), making the ICD a potential target for the modulation of specific pLGIC subunits. None of the structures includes a complete ICD. Here, we created chimeras by adding the ICD of cation-conducting (nAChR-α7) and anion-conducting (GABAρ1, Glyα1) eukaryotic homopentamer-forming pLGICs to GLIC. GLIC-ICD chimeras assemble into pentamers to form proton-gated channels, as does the parent GLIC. Additionally, the sensitivity of the chimeras toward modulation of functional maturation by chaperone protein RIC-3 is preserved as in those of the parent eukaryotic channels. For a previously described GLIC-5HT3A-ICD chimera, we now provide evidence of its successful large-scale expression and purification to homogeneity. Overall, the chimeras provide valuable tools for functional and structural studies of eukaryotic pLGIC ICDs.
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Affiliation(s)
- Nelli Mnatsakanyan
- Department of Cell Physiology and Molecular Biophysics, School of Medicine, Texas Tech University Health Sciences Center, Lubbock, Texas 79430, United States.,Center for Membrane Protein Research, School of Medicine, Texas Tech University Health Sciences Center, Lubbock, Texas 79430, United States
| | - Sita Nirupama Nishtala
- Department of Cell Physiology and Molecular Biophysics, School of Medicine, Texas Tech University Health Sciences Center, Lubbock, Texas 79430, United States.,Center for Membrane Protein Research, School of Medicine, Texas Tech University Health Sciences Center, Lubbock, Texas 79430, United States
| | - Akash Pandhare
- Department of Cell Physiology and Molecular Biophysics, School of Medicine, Texas Tech University Health Sciences Center, Lubbock, Texas 79430, United States.,Center for Membrane Protein Research, School of Medicine, Texas Tech University Health Sciences Center, Lubbock, Texas 79430, United States
| | - Mariana C Fiori
- Department of Cell Physiology and Molecular Biophysics, School of Medicine, Texas Tech University Health Sciences Center, Lubbock, Texas 79430, United States.,Center for Membrane Protein Research, School of Medicine, Texas Tech University Health Sciences Center, Lubbock, Texas 79430, United States
| | - Raman Goyal
- Department of Cell Physiology and Molecular Biophysics, School of Medicine, Texas Tech University Health Sciences Center, Lubbock, Texas 79430, United States.,Center for Membrane Protein Research, School of Medicine, Texas Tech University Health Sciences Center, Lubbock, Texas 79430, United States
| | - Jonathan E Pauwels
- Department of Cell Physiology and Molecular Biophysics, School of Medicine, Texas Tech University Health Sciences Center, Lubbock, Texas 79430, United States.,Center for Membrane Protein Research, School of Medicine, Texas Tech University Health Sciences Center, Lubbock, Texas 79430, United States.,Center for Biotechnology and Genomics, Texas Tech University, Lubbock, Texas 79430, United States
| | - Andrew F Navetta
- Department of Cell Physiology and Molecular Biophysics, School of Medicine, Texas Tech University Health Sciences Center, Lubbock, Texas 79430, United States.,Medical Student Summer Research Program, School of Medicine, Texas Tech University Health Sciences Center, Lubbock, Texas 79430, United States
| | - Afzal Ahrorov
- Department of Cell Physiology and Molecular Biophysics, School of Medicine, Texas Tech University Health Sciences Center, Lubbock, Texas 79430, United States.,Undergraduate Science Education Program of the Howard Hughes Medical Institute, Texas Tech University, Lubbock, Texas 79430, United States
| | - Michaela Jansen
- Department of Cell Physiology and Molecular Biophysics, School of Medicine, Texas Tech University Health Sciences Center, Lubbock, Texas 79430, United States.,Center for Membrane Protein Research, School of Medicine, Texas Tech University Health Sciences Center, Lubbock, Texas 79430, United States
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21
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Abstract
Ion channels open and close in response to diverse stimuli, and the molecular events underlying these processes are extensively modulated by ligands of both endogenous and exogenous origin. In the past decade, high-resolution structures of several channel types have been solved, providing unprecedented details of the molecular architecture of these membrane proteins. Intrinsic conformational flexibility of ion channels critically governs their functions. However, the dynamics underlying gating mechanisms and modulations are obscured in the information from crystal structures. While nuclear magnetic resonance spectroscopic methods allow direct measurements of protein dynamics, they are limited by the large size of these membrane protein assemblies in detergent micelles or lipid membranes. Electron paramagnetic resonance (EPR) spectroscopy has emerged as a key biophysical tool to characterize structural dynamics of ion channels and to determine stimulus-driven conformational transition between functional states in a physiological environment. This review will provide an overview of the recent advances in the field of voltage- and ligand-gated channels and highlight some of the challenges and controversies surrounding the structural information available. It will discuss general methods used in site-directed spin labeling and EPR spectroscopy and illustrate how findings from these studies have narrowed the gap between high-resolution structures and gating mechanisms in membranes, and have thereby helped reconcile seemingly disparate models of ion channel function.
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22
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Nicotinic acetylcholine receptor-lipid interactions: Mechanistic insight and biological function. BIOCHIMICA ET BIOPHYSICA ACTA-BIOMEMBRANES 2015; 1848:1806-17. [PMID: 25791350 DOI: 10.1016/j.bbamem.2015.03.010] [Citation(s) in RCA: 56] [Impact Index Per Article: 6.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/07/2015] [Revised: 02/15/2015] [Accepted: 03/09/2015] [Indexed: 01/14/2023]
Abstract
Membrane lipids are potent modulators of the nicotinic acetylcholine receptor (nAChR) from Torpedo. Lipids influence nAChR function by both conformational selection and kinetic mechanisms, stabilizing varying proportions of activatable versus non-activatable conformations, as well as influencing the transitions between these conformational states. Of note, some membranes stabilize an electrically silent uncoupled conformation that binds agonist but does not undergo agonist-induced conformational transitions. The uncoupled nAChR, however, does transition to activatable conformations in relatively thick lipid bilayers, such as those found in lipid rafts. In this review, we discuss current understanding of lipid-nAChR interactions in the context of increasingly available high resolution structural and functional data. These data highlight different sites of lipid action, including the lipid-exposed M4 transmembrane α-helix. Current evidence suggests that lipids alter nAChR function by modulating interactions between M4 and the adjacent transmembrane α-helices, M1 and M3. These interactions have also been implicated in both the folding and trafficking of nAChRs to the cell surface. We review current mechanistic understanding of lipid-nAChR interactions, and highlight potential biological roles for lipid-nAChR interactions in modulating the synaptic response. This article is part of a Special Issue entitled: Lipid-protein interactions.
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23
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Carswell CL, Sun J, Baenziger JE. Intramembrane aromatic interactions influence the lipid sensitivities of pentameric ligand-gated ion channels. J Biol Chem 2014; 290:2496-507. [PMID: 25519904 DOI: 10.1074/jbc.m114.624395] [Citation(s) in RCA: 35] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Although the Torpedo nicotinic acetylcholine receptor (nAChR) reconstituted into phosphatidylcholine (PC) membranes lacking cholesterol and anionic lipids adopts a conformation where agonist binding is uncoupled from channel gating, the underlying mechanism remains to be defined. Here, we examine the mechanism behind lipid-dependent uncoupling by comparing the propensities of two prokaryotic homologs, Gloebacter and Erwinia ligand-gated ion channel (GLIC and ELIC, respectively), to adopt a similar uncoupled conformation. Membrane-reconstituted GLIC and ELIC both exhibit folded structures in the minimal PC membranes that stabilize an uncoupled nAChR. GLIC, with a large number of aromatic interactions at the interface between the outermost transmembrane α-helix, M4, and the adjacent transmembrane α-helices, M1 and M3, retains the ability to flux cations in this uncoupling PC membrane environment. In contrast, ELIC, with a level of aromatic interactions intermediate between that of the nAChR and GLIC, does not undergo agonist-induced channel gating, although it does not exhibit the expected biophysical characteristics of the uncoupled state. Engineering new aromatic interactions at the M4-M1/M3 interface to promote effective M4 interactions with M1/M3, however, increases the stability of the transmembrane domain to restore channel function. Our data provide direct evidence that M4 interactions with M1/M3 are modulated during lipid sensing. Aromatic residues strengthen M4 interactions with M1/M3 to reduce the sensitivities of pentameric ligand-gated ion channels to their surrounding membrane environment.
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Affiliation(s)
- Casey L Carswell
- From the Department of Biochemistry, Microbiology, and Immunology, University of Ottawa, Ottawa Ontario, K1H 8M5, Canada
| | - Jiayin Sun
- From the Department of Biochemistry, Microbiology, and Immunology, University of Ottawa, Ottawa Ontario, K1H 8M5, Canada
| | - John E Baenziger
- From the Department of Biochemistry, Microbiology, and Immunology, University of Ottawa, Ottawa Ontario, K1H 8M5, Canada
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24
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The role of the M4 lipid-sensor in the folding, trafficking, and allosteric modulation of nicotinic acetylcholine receptors. Neuropharmacology 2014; 96:157-68. [PMID: 25433148 DOI: 10.1016/j.neuropharm.2014.11.011] [Citation(s) in RCA: 34] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/21/2014] [Revised: 10/31/2014] [Accepted: 11/18/2014] [Indexed: 11/24/2022]
Abstract
With the availability of high resolution structural data, increasing attention has focused on the mechanisms by which drugs and endogenous compounds allosterically modulate nicotinic acetylcholine receptor (nAChR) function. Lipids are potent modulators of the nAChR from Torpedo. Membrane lipids influence nAChR function by both conformational selection and kinetic mechanisms, stabilizing varying proportions of pre-existing resting, open, desensitized, and uncoupled conformations, as well as influencing the transitions between these conformational states. Structural and functional data highlight a role for the lipid-exposed M4 transmembrane α-helix of each subunit in lipid sensing, and suggest that lipids influence gating by altering the binding of M4 to the adjacent transmembrane α-helices, M1 and M3. M4 has also been implicated in both the folding and trafficking of nAChRs to the cell surface, as well as in the potentiation of nAChR gating by neurosteroids. Here, we discuss the roles of M4 in the folding, trafficking, and allosteric modulation of nAChRs. We also consider the hypothesis that variable chemistry at the M4-M1/M3 transmembrane α-helical interface in different nAChR subunits governs the capacity for potentiation by activating lipids. This article is part of the Special Issue entitled 'The Nicotinic Acetylcholine Receptor: From Molecular Biology to Cognition'.
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25
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Forman SA, Chiara DC, Miller KW. Anesthetics target interfacial transmembrane sites in nicotinic acetylcholine receptors. Neuropharmacology 2014; 96:169-77. [PMID: 25316107 DOI: 10.1016/j.neuropharm.2014.10.002] [Citation(s) in RCA: 33] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/22/2014] [Revised: 09/22/2014] [Accepted: 10/02/2014] [Indexed: 11/25/2022]
Abstract
General anesthetics are a heterogeneous group of small amphiphilic ligands that interact weakly at multiple allosteric sites on many pentameric ligand gated ion channels (pLGICs), resulting in either inhibition, potentiation of channel activity, or both. Allosteric principles imply that modulator sites must change configuration and ligand affinity during receptor state transitions. Thus, general anesthetics and related compounds are useful both as state-dependent probes of receptor structure and as potentially selective modulators of pLGIC functions. This review focuses on general anesthetic sites in nicotinic acetylcholine receptors, which were among the first anesthetic-sensitive pLGIC experimental models studied, with particular focus on sites formed by transmembrane domain elements. Structural models place many of these sites at interfaces between two or more pLGIC transmembrane helices both within subunits and between adjacent subunits, and between transmembrane helices and either lipids (the lipid-protein interface) or water (i.e. the ion channel). A single general anesthetic may bind at multiple allosteric sites in pLGICs, producing a net effect of either inhibition (e.g. blocking the ion channel) or enhanced channel gating (e.g. inter-subunit sites). Other general anesthetic sites identified by photolabeling or crystallography are tentatively linked to functional effects, including intra-subunit helix bundle sites and the lipid-protein interface. This article is part of the Special Issue entitled 'The Nicotinic Acetylcholine Receptor: From Molecular Biology to Cognition'.
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Affiliation(s)
- Stuart A Forman
- Dept. of Anesthesia Critical Care & Pain Medicine, Massachusetts General Hospital, Boston, 55 Fruit Street, MA 02114, USA; Dept. of Anaesthesia, Harvard Medical School, 25 Shattuck Street, Boston, MA 02115, USA.
| | - David C Chiara
- Dept. of Neurobiology, Harvard Medical School, 25 Shattuck Street, Boston, MA 02115, USA.
| | - Keith W Miller
- Dept. of Anesthesia Critical Care & Pain Medicine, Massachusetts General Hospital, Boston, 55 Fruit Street, MA 02114, USA; Dept. of Anaesthesia, Harvard Medical School, 25 Shattuck Street, Boston, MA 02115, USA.
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26
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Kabbani N, Nordman JC, Corgiat BA, Veltri DP, Shehu A, Seymour VA, Adams DJ. Are nicotinic acetylcholine receptors coupled to G proteins? Bioessays 2014; 35:1025-34. [PMID: 24185813 DOI: 10.1002/bies.201300082] [Citation(s) in RCA: 59] [Impact Index Per Article: 5.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022]
Abstract
It was, until recently, accepted that the two classes of acetylcholine (ACh) receptors are distinct in an important sense: muscarinic ACh receptors signal via heterotrimeric GTP binding proteins (G proteins), whereas nicotinic ACh receptors (nAChRs) open to allow flux of Na+, Ca2+, and K+ ions into the cell after activation. Here we present evidence of direct coupling between G proteins and nAChRs in neurons. Based on proteomic, biophysical, and functional evidence, we hypothesize that binding to G proteins modulates the activity and signaling of nAChRs in cells. It is important to note that while this hypothesis is new for the nAChR, it is consistent with known interactions between G proteins and structurally related ligand-gated ion channels. Therefore, it underscores an evolutionarily conserved metabotropic mechanism of G protein signaling via nAChR channels.
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27
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Crystallographic studies of pharmacological sites in pentameric ligand-gated ion channels. Biochim Biophys Acta Gen Subj 2014; 1850:511-23. [PMID: 24836522 DOI: 10.1016/j.bbagen.2014.05.007] [Citation(s) in RCA: 43] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/16/2014] [Revised: 05/05/2014] [Accepted: 05/06/2014] [Indexed: 12/18/2022]
Abstract
BACKGROUND Pentameric ligand-gated ion channels (pLGICs) mediate fast chemical transmission of nerve signals in the central and peripheral nervous system. On the functional side, these molecules respond to the binding of a neurotransmitter (glycine, GABA, acetylcholine or 5HT3) in the extracellular domain (ECD) by opening their ionotropic pore in the transmembrane domain (TMD). The response to the neurotransmitter binding can be modulated by several chemical compounds acting at topographically distinct sites, as documented by a large body of literature. Notably, these receptors are the target of several classes of world-wide prescribed drugs, including general anesthetics, smoking cessation aids, anxiolytics, anticonvulsants, muscle relaxants, hypnotics and anti-emetics. On the structural side recent progress has been made on the crystallization of pLGICs in its different allosteric states, especially pLGICs of bacterial origin. Therefore, structure-function relationships can now be discussed at the atomic level for pLGICs. SCOPE OF REVIEW This review focuses on the crystallographic structure of complexes of pLGICs with a number of ligands of pharmacological interest. First, we review structural data on two key functional aspects of these receptors: the agonist-induced activation and ion transport itself. The molecular understanding of both these functional aspects is important, as they are those that most pharmacological compounds target. Next, we describe modulation sites that have recently been documented by X-ray crystallography. Finally, we propose a simple geometric classification of all these pharmacological sites in pLGICs, based on icosahedrons. MAJOR CONCLUSIONS This review illustrates the wealth of structural insight gained by comparing all available structures of members of the pLGIC family to rationalize the pharmacology of structurally diverse drugs acting at topographically distinct sites. It will be highlighted how sites that had been described earlier using biochemical techniques can be rationalized using structural data. Surprisingly, the use of icosahedral symmetry allows to link together several modulation sites, in a way that was totally unanticipated. GENERAL SIGNIFICANCE Overall, understanding the interplay between the different modulation sites at the structural level should help the design of future drugs targeting pLGICs. This article is part of a Special Issue entitled structural biochemistry and biophysics of membrane proteins.
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daCosta CJB, Baenziger JE. Gating of pentameric ligand-gated ion channels: structural insights and ambiguities. Structure 2014; 21:1271-83. [PMID: 23931140 DOI: 10.1016/j.str.2013.06.019] [Citation(s) in RCA: 79] [Impact Index Per Article: 7.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/29/2013] [Revised: 05/31/2013] [Accepted: 06/26/2013] [Indexed: 01/09/2023]
Abstract
Pentameric ligand-gated ion channels (pLGICs) mediate fast synaptic communication by converting chemical signals into an electrical response. Recently solved agonist-bound and agonist-free structures greatly extend our understanding of these complex molecular machines. A key challenge to a full description of function, however, is the ability to unequivocally relate determined structures to the canonical resting, open, and desensitized states. Here, we review current understanding of pLGIC structure, with a focus on the conformational changes underlying channel gating. We compare available structural information and review the evidence supporting the assignment of each structure to a particular conformational state. We discuss multiple factors that may complicate the interpretation of crystal structures, highlighting the potential influence of lipids and detergents. We contend that further advances in the structural biology of pLGICs will require deeper insight into the nature of pLGIC-lipid interactions.
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Affiliation(s)
- Corrie J B daCosta
- Receptor Biology Laboratory, Departments of Physiology and Biomedical Engineering, Mayo Clinic College of Medicine, Rochester, MN 55905, USA
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Velisetty P, Chalamalasetti SV, Chakrapani S. Structural basis for allosteric coupling at the membrane-protein interface in Gloeobacter violaceus ligand-gated ion channel (GLIC). J Biol Chem 2013; 289:3013-25. [PMID: 24338475 DOI: 10.1074/jbc.m113.523050] [Citation(s) in RCA: 35] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Ligand binding at the extracellular domain of pentameric ligand-gated ion channels initiates a relay of conformational changes that culminates at the gate within the transmembrane domain. The interface between the two domains is a key structural entity that governs gating. Molecular events in signal transduction at the interface are poorly defined because of its intrinsically dynamic nature combined with functional modulation by membrane lipid and water vestibules. Here we used electron paramagnetic resonance spectroscopy to delineate protein motions underlying Gloeobacter violaceus ligand-gated ion channel gating in a membrane environment and report the interface conformation in the closed and the desensitized states. Extensive intrasubunit interactions were observed in the closed state that are weakened upon desensitization and replaced by newer intersubunit contacts. Gating involves major rearrangements of the interfacial loops, accompanied by reorganization of the protein-lipid-water interface. These structural changes may serve as targets for modulation of gating by lipids, alcohols, and amphipathic drug molecules.
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Affiliation(s)
- Phanindra Velisetty
- From the Department of Physiology and Biophysics, School of Medicine, Case Western Reserve University, Cleveland, Ohio 44106-4970
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Dellisanti CD, Ghosh B, Hanson SM, Raspanti JM, Grant VA, Diarra GM, Schuh AM, Satyshur K, Klug CS, Czajkowski C. Site-directed spin labeling reveals pentameric ligand-gated ion channel gating motions. PLoS Biol 2013; 11:e1001714. [PMID: 24260024 PMCID: PMC3833874 DOI: 10.1371/journal.pbio.1001714] [Citation(s) in RCA: 40] [Impact Index Per Article: 3.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/03/2013] [Accepted: 10/08/2013] [Indexed: 11/21/2022] Open
Abstract
Pentameric ligand-gated ion channels (pLGICs) are neurotransmitter-activated receptors that mediate fast synaptic transmission. In pLGICs, binding of agonist to the extracellular domain triggers a structural rearrangement that leads to the opening of an ion-conducting pore in the transmembrane domain and, in the continued presence of neurotransmitter, the channels desensitize (close). The flexible loops in each subunit that connect the extracellular binding domain (loops 2, 7, and 9) to the transmembrane channel domain (M2–M3 loop) are essential for coupling ligand binding to channel gating. Comparing the crystal structures of two bacterial pLGIC homologues, ELIC and the proton-activated GLIC, suggests channel gating is associated with rearrangements in these loops, but whether these motions accurately predict the motions in functional lipid-embedded pLGICs is unknown. Here, using site-directed spin labeling (SDSL) electron paramagnetic resonance (EPR) spectroscopy and functional GLIC channels reconstituted into liposomes, we examined if, and how far, the loops at the ECD/TMD gating interface move during proton-dependent gating transitions from the resting to desensitized state. Loop 9 moves ∼9 Å inward toward the channel lumen in response to proton-induced desensitization. Loop 9 motions were not observed when GLIC was in detergent micelles, suggesting detergent solubilization traps the protein in a nonactivatable state and lipids are required for functional gating transitions. Proton-induced desensitization immobilizes loop 2 with little change in position. Proton-induced motion of the M2–M3 loop was not observed, suggesting its conformation is nearly identical in closed and desensitized states. Our experimentally derived distance measurements of spin-labeled GLIC suggest ELIC is not a good model for the functional resting state of GLIC, and that the crystal structure of GLIC does not correspond to a desensitized state. These findings advance our understanding of the molecular mechanisms underlying pLGIC gating. Ligand-gated ion channels reside in the membranes of nerve and muscle cells. These proteins form channels that span the membrane, where they transduce chemical signals into changes in electrical excitability. Neurotransmitters bind to the extracellular surface of these proteins to trigger global structural rearrangements that open the channel, allowing ions to flow across the cell membrane. In the continued presence of neurotransmitters, the channels desensitize and close. Channel opening and closing regulate muscle contraction and signaling in the brain, and defects in these channels lead to a variety of diseases. While crystal structures have provided frozen snapshots of these proteins in presumed closed and open channel states, little is known about how the channels desensitize and move during actual signaling events. Here, we applied a technique to investigate the structure and local dynamics of proteins known as site-directed spin labeling to a prototypical ligand-gated channel, GLIC. We directly quantified ligand-induced motions in regions at the boundary between the binding domain (loops 2 and 9) and the channel domain (M2–M3 loop). We show that a large movement of loop 9 and an immobilization of loop 2, which rearranges the interface between the binding and channel domains, accompanies GLIC channel gating transitions into a desensitized state. These data provide new insights into the protein movements that underlie electrochemical transmission of signals between cells.
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Affiliation(s)
- Cosma D. Dellisanti
- Department of Neuroscience, University of Wisconsin, Madison, Wisconsin, United States of America
| | - Borna Ghosh
- Department of Neuroscience, University of Wisconsin, Madison, Wisconsin, United States of America
| | - Susan M. Hanson
- Department of Neuroscience, University of Wisconsin, Madison, Wisconsin, United States of America
| | - James M. Raspanti
- Department of Neuroscience, University of Wisconsin, Madison, Wisconsin, United States of America
| | - Valerie A. Grant
- Department of Neuroscience, University of Wisconsin, Madison, Wisconsin, United States of America
| | - Gaoussou M. Diarra
- Department of Neuroscience, University of Wisconsin, Madison, Wisconsin, United States of America
| | - Abby M. Schuh
- Department of Neuroscience, University of Wisconsin, Madison, Wisconsin, United States of America
| | - Kenneth Satyshur
- Department of Neuroscience, University of Wisconsin, Madison, Wisconsin, United States of America
| | - Candice S. Klug
- Department of Biophysics, Medical College of Wisconsin, Milwaukee, Wisconsin, United States of America
| | - Cynthia Czajkowski
- Department of Neuroscience, University of Wisconsin, Madison, Wisconsin, United States of America
- * E-mail:
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