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Wang ZJ, Deba F, Mohamed TS, Chiara DC, Ramos K, Hamouda AK. Unraveling amino acid residues critical for allosteric potentiation of (α4)3(β2)2-type nicotinic acetylcholine receptor responses. J Biol Chem 2017; 292:9988-10001. [PMID: 28446611 DOI: 10.1074/jbc.m116.771246] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/05/2016] [Revised: 04/20/2017] [Indexed: 01/29/2023] Open
Abstract
Neuronal nicotinic acetylcholine receptors (nAChRs) are promising drug targets to manage several neurological disorders and nicotine addiction. Growing evidence indicates that positive allosteric modulators of nAChRs improve pharmacological specificity by binding to unique sites present only in a subpopulation of nAChRs. Furthermore, nAChR positive allosteric modulators such as NS9283 and CMPI have been shown to potentiate responses of (α4)3(β2)2 but not (α4)2(β2)3 nAChR isoforms. This selective potentiation underlines that the α4:α4 interface, which is present only in the (α4)3(β2)2 nAChR, is an important and promising drug target. In this report we used site-directed mutagenesis to substitute specific amino acid residues and computational analyses to elucidate CMPI's binding mode at the α4:α4 subunit extracellular interface and identified a unique set of amino acid residues that determined its affinity. We found that amino acid residues α4Gly-41, α4Lys-64, and α4Thr-66 were critical for (α4)3(β2)2 nAChR potentiation by CMPI, but not by NS9283, whereas amino acid substitution at α4His-116, a known determinant of NS9283 and of agonist binding at the α4:α4 subunit interface, did not reduce CMPI potentiation. In contrast, substitutions at α4Gln-124 and α4Thr-126 reduced potentiation by CMPI and NS9283, indicating that their binding sites partially overlap. These results delineate the role of amino acid residues contributing to the α4:α4 subunit extracellular interface in nAChR potentiation. These findings also provide structural information that will facilitate the structure-based design of novel therapeutics that target selectively the (α4)3(β2)2 nAChR.
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Affiliation(s)
- Ze-Jun Wang
- From the Department of Pharmaceutical Sciences, Texas A&M Health Sciences Center, Kingsville, Texas 78363
| | - Farah Deba
- From the Department of Pharmaceutical Sciences, Texas A&M Health Sciences Center, Kingsville, Texas 78363
| | - Tasnim S Mohamed
- From the Department of Pharmaceutical Sciences, Texas A&M Health Sciences Center, Kingsville, Texas 78363
| | - David C Chiara
- Department of Neurobiology, Harvard Medical School, Boston, Massachusetts 02115
| | - Kara Ramos
- From the Department of Pharmaceutical Sciences, Texas A&M Health Sciences Center, Kingsville, Texas 78363
| | - Ayman K Hamouda
- From the Department of Pharmaceutical Sciences, Texas A&M Health Sciences Center, Kingsville, Texas 78363, .,Department of Neuroscience and Experimental Therapeutics, Texas A&M Health Sciences Center, Bryan, Texas 77807, and
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Jayakar SS, Ang G, Chiara DC, Hamouda AK. Photoaffinity Labeling of Pentameric Ligand-Gated Ion Channels: A Proteomic Approach to Identify Allosteric Modulator Binding Sites. Methods Mol Biol 2017; 1598:157-197. [PMID: 28508361 DOI: 10.1007/978-1-4939-6952-4_7] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/23/2023]
Abstract
Photoaffinity labeling techniques have been used for decades to identify drug binding sites and to study the structural biology of allosteric transitions in transmembrane proteins including pentameric ligand-gated ion channels (pLGIC). In a typical photoaffinity labeling experiment, to identify drug binding sites, UV light is used to introduce a covalent bond between a photoreactive ligand (which upon irradiation at the appropriate wavelength converts to a reactive intermediate) and amino acid residues that lie within its binding site. Then protein chemistry and peptide microsequencing techniques are used to identify these amino acids within the protein primary sequence. These amino acid residues are located within homology models of the receptor to identify the binding site of the photoreactive probe. Molecular modeling techniques are then used to model the binding of the photoreactive probe within the binding site using docking protocols. Photoaffinity labeling directly identifies amino acids that contribute to drug binding sites regardless of their location within the protein structure and distinguishes them from amino acids that are only involved in the transduction of the conformational changes mediated by the drug, but may not be part of its binding site (such as those identified by mutational studies). Major limitations of photoaffinity labeling include the availability of photoreactive ligands that faithfully mimic the properties of the parent molecule and protein preparations that supply large enough quantities suitable for photoaffinity labeling experiments. When the ligand of interest is not intrinsically photoreactive, chemical modifications to add a photoreactive group to the parent drug, and pharmacological evaluation of these chemical modifications become necessary. With few exceptions, expression and affinity-purification of proteins are required prior to photolabeling. Methods to isolate milligram quantities of highly enriched pLGIC suitable for photoaffinity labeling experiments have been developed. In this chapter, we discuss practical aspects of experimental strategies to identify allosteric modulator binding sites in pLGIC using photoaffinity labeling.
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Affiliation(s)
- Selwyn S Jayakar
- Department of Neurobiology, Harvard Medical School, Boston, MA, USA
| | - Gordon Ang
- Department of Pharmaceutical Sciences, College of Pharmacy, Texas A&M Health Sciences Center, Kingsville, TX, USA
| | - David C Chiara
- Department of Neurobiology, Harvard Medical School, Boston, MA, USA
| | - Ayman K Hamouda
- Department of Pharmaceutical Sciences, College of Pharmacy, Texas A&M Health Sciences Center, Kingsville, TX, USA. .,Department of Neuroscience and Experimental Therapeutics, College of Medicine, Texas A&M Health Science Center, Bryan, TX, USA. .,Department of Neuroscience and Experimental Therapeutics, College of Medicine, Texas A&M Health Science Center, Kingsville, TX, USA.
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3
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Hamouda AK, Deba F, Wang ZJ, Cohen JB. Photolabeling a Nicotinic Acetylcholine Receptor (nAChR) with an (α4)3(β2)2 nAChR-Selective Positive Allosteric Modulator. Mol Pharmacol 2016; 89:575-84. [PMID: 26976945 DOI: 10.1124/mol.116.103341] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/11/2016] [Accepted: 03/09/2016] [Indexed: 01/25/2023] Open
Abstract
Positive allosteric modulators (PAMs) of nicotinic acetylcholine (ACh) receptors (nAChRs) have potential clinical applications in the treatment of nicotine dependence and many neuropsychiatric conditions associated with decreased brain cholinergic activity, and 3-(2-chlorophenyl)-5-(5-methyl-1-(piperidin-4-yl)-1H-pyrrazol-4-yl)isoxazole (CMPI) has been identified as a PAM selective for neuronal nAChRs containing theα4 subunit. In this report, we compare CMPI interactions with low-sensitivity (α4)3(β2)2 and high-sensitivity (α4)2(β2)3 nAChRs, and with muscle-type nAChRs. In addition, we use the intrinsic reactivity of [(3)H]CMPI upon photolysis at 312 nm to identify its binding sites inTorpedonAChRs. Recording fromXenopusoocytes, we found that CMPI potentiated maximally the responses of (α4)3(β2)2nAChR to 10μM ACh (EC10) by 400% and with anEC50of ∼1µM. CMPI produced a left shift of the ACh concentration-response curve without altering ACh efficacy. In contrast, CMPI inhibited (∼35% at 10µM) ACh responses of (α4)2(β2)3nAChRs and fully inhibited human muscle andTorpedonAChRs with IC50values of ∼0.5µM. Upon irradiation at 312 nm, [(3)H]CMPI photoincorporated into eachTorpedo[(α1)2β1γδ] nAChR subunit. Sequencing of peptide fragments isolated from [(3)H]CMPI-photolabeled nAChR subunits established photolabeling of amino acids contributing to the ACh binding sites (αTyr(190),αTyr(198),γTrp(55),γTyr(111),γTyr(117),δTrp(57)) that was fully inhibitable by agonist and lower-efficiency, state-dependent [(3)H]CMPI photolabeling within the ion channel. Our results establish that CMPI is a potent potentiator of nAChRs containing anα4:α4 subunit interface, and that its intrinsic photoreactivy makes it of potential use to identify its binding sites in the (α4)3(β2)2nAChR.
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Affiliation(s)
- Ayman K Hamouda
- Department of Pharmaceutical Sciences, Rangel College of Pharmacy, Texas A&M Health Sciences Center, Kingsville, Texas (A.K.H., F.D., Z.-J.W.); and Department of Neurobiology, Harvard Medical School, Boston, Massachusetts (A.K.H., J.B.C.)
| | - Farah Deba
- Department of Pharmaceutical Sciences, Rangel College of Pharmacy, Texas A&M Health Sciences Center, Kingsville, Texas (A.K.H., F.D., Z.-J.W.); and Department of Neurobiology, Harvard Medical School, Boston, Massachusetts (A.K.H., J.B.C.)
| | - Ze-Jun Wang
- Department of Pharmaceutical Sciences, Rangel College of Pharmacy, Texas A&M Health Sciences Center, Kingsville, Texas (A.K.H., F.D., Z.-J.W.); and Department of Neurobiology, Harvard Medical School, Boston, Massachusetts (A.K.H., J.B.C.)
| | - Jonathan B Cohen
- Department of Pharmaceutical Sciences, Rangel College of Pharmacy, Texas A&M Health Sciences Center, Kingsville, Texas (A.K.H., F.D., Z.-J.W.); and Department of Neurobiology, Harvard Medical School, Boston, Massachusetts (A.K.H., J.B.C.)
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Niessen K, Seeger T, Tattersall J, Timperley C, Bird M, Green C, Thiermann H, Worek F. Affinities of bispyridinium non-oxime compounds to [3H]epibatidine binding sites of Torpedo californica nicotinic acetylcholine receptors depend on linker length. Chem Biol Interact 2013; 206:545-54. [DOI: 10.1016/j.cbi.2013.10.012] [Citation(s) in RCA: 20] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/06/2013] [Revised: 10/07/2013] [Accepted: 10/07/2013] [Indexed: 01/20/2023]
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Hamouda AK, Jayakar SS, Chiara DC, Cohen JB. Photoaffinity Labeling of Nicotinic Receptors: Diversity of Drug Binding Sites! J Mol Neurosci 2013; 53:480-6. [DOI: 10.1007/s12031-013-0150-1] [Citation(s) in RCA: 22] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/21/2013] [Accepted: 10/10/2013] [Indexed: 12/11/2022]
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Mazurov AA, Kombo DC, Hauser TA, Miao L, Dull G, Genus JF, Fedorov NB, Benson L, Sidach S, Xiao Y, Hammond PS, James JW, Miller CH, Yohannes D. Discovery of (2S,3R)-N-[2-(Pyridin-3-ylmethyl)-1-azabicyclo[2.2.2]oct-3-yl]benzo[b]furan-2-carboxamide (TC-5619), a Selective α7 Nicotinic Acetylcholine Receptor Agonist, for the Treatment of Cognitive Disorders. J Med Chem 2012; 55:9793-809. [DOI: 10.1021/jm301048a] [Citation(s) in RCA: 41] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Affiliation(s)
- Anatoly A. Mazurov
- Targacept, Inc., Winston-Salem, North
Carolina 27101-4165,
United States
| | - David C. Kombo
- Targacept, Inc., Winston-Salem, North
Carolina 27101-4165,
United States
| | - Terry A. Hauser
- Targacept, Inc., Winston-Salem, North
Carolina 27101-4165,
United States
| | - Lan Miao
- Targacept, Inc., Winston-Salem, North
Carolina 27101-4165,
United States
| | - Gary Dull
- Targacept, Inc., Winston-Salem, North
Carolina 27101-4165,
United States
| | - John F. Genus
- Targacept, Inc., Winston-Salem, North
Carolina 27101-4165,
United States
| | - Nikolai B. Fedorov
- Targacept, Inc., Winston-Salem, North
Carolina 27101-4165,
United States
| | - Lisa Benson
- Targacept, Inc., Winston-Salem, North
Carolina 27101-4165,
United States
| | - Serguei Sidach
- Targacept, Inc., Winston-Salem, North
Carolina 27101-4165,
United States
| | - Yunde Xiao
- Targacept, Inc., Winston-Salem, North
Carolina 27101-4165,
United States
| | - Philip S. Hammond
- Targacept, Inc., Winston-Salem, North
Carolina 27101-4165,
United States
| | - John W. James
- Targacept, Inc., Winston-Salem, North
Carolina 27101-4165,
United States
| | - Craig H. Miller
- Targacept, Inc., Winston-Salem, North
Carolina 27101-4165,
United States
| | - Daniel Yohannes
- Targacept, Inc., Winston-Salem, North
Carolina 27101-4165,
United States
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Akdemir A, Edink E, Thompson AJ, Lummis SCR, Kooistra AJ, de Graaf C, de Esch IJP. Identification of novel α7 nicotinic receptor ligands by in silico screening against the crystal structure of a chimeric α7 receptor ligand binding domain. Bioorg Med Chem 2012; 20:5992-6002. [PMID: 22959526 PMCID: PMC3460237 DOI: 10.1016/j.bmc.2012.06.054] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/21/2012] [Revised: 06/28/2012] [Accepted: 06/29/2012] [Indexed: 11/24/2022]
Abstract
A hierarchical in silico screening procedure using the crystal structure of an agonist bound chimeric α7/Ls-AChBP protein was successfully applied to both proprietary and commercial databases containing drug-like molecules. An overall hit rate of 26% (pKi ⩾5.0) was obtained, with an even better hit rate of 35% for the commercial compound collection. Structurally novel and diverse ligands were identified. Binding studies with [3H]epibatidine on chimeric α7/5-HT3 receptors yielded submicromolar inhibition constants for identified hits. Compared to a previous screening procedure that utilized the wild type Ls-AChBP crystal structure, the current study shows that the recently obtained α7/Ls-AChBP chimeric protein crystal structure is a better template for the identification of novel α7 receptor ligands.
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Affiliation(s)
- Atilla Akdemir
- Division of Pharmacology, Faculty of Pharmacy, Bezmialem Vakif University, Istanbul, Turkey
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Person AM, Wells GB. Characterizing low affinity epibatidine binding to α4β2 nicotinic acetylcholine receptors with ligand depletion and nonspecific binding. BMC BIOPHYSICS 2011; 4:19. [PMID: 22112852 PMCID: PMC3287110 DOI: 10.1186/2046-1682-4-19] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 06/07/2011] [Accepted: 11/23/2011] [Indexed: 11/11/2022]
Abstract
Background Along with high affinity binding of epibatidine (Kd1≈10 pM) to α4β2 nicotinic acetylcholine receptor (nAChR), low affinity binding of epibatidine (Kd2≈1-10 nM) to an independent binding site has been reported. Studying this low affinity binding is important because it might contribute understanding about the structure and synthesis of α4β2 nAChR. The binding behavior of epibatidine and α4β2 AChR raises a question about interpreting binding data from two independent sites with ligand depletion and nonspecific binding, both of which can affect equilibrium binding of [3H]epibatidine and α4β2 nAChR. If modeled incorrectly, ligand depletion and nonspecific binding lead to inaccurate estimates of binding constants. Fitting total equilibrium binding as a function of total ligand accurately characterizes a single site with ligand depletion and nonspecific binding. The goal of this study was to determine whether this approach is sufficient with two independent high and low affinity sites. Results Computer simulations of binding revealed complexities beyond fitting total binding for characterizing the second, low affinity site of α4β2 nAChR. First, distinguishing low-affinity specific binding from nonspecific binding was a potential problem with saturation data. Varying the maximum concentration of [3H]epibatidine, simultaneously fitting independently measured nonspecific binding, and varying α4β2 nAChR concentration were effective remedies. Second, ligand depletion helped identify the low affinity site when nonspecific binding was significant in saturation or competition data, contrary to a common belief that ligand depletion always is detrimental. Third, measuring nonspecific binding without α4β2 nAChR distinguished better between nonspecific binding and low-affinity specific binding under some circumstances of competitive binding than did presuming nonspecific binding to be residual [3H]epibatidine binding after adding a large concentration of cold competitor. Fourth, nonspecific binding of a heterologous competitor changed estimates of high and low inhibition constants but did not change the ratio of those estimates. Conclusions Investigating the low affinity site of α4β2 nAChR with equilibrium binding when ligand depletion and nonspecific binding are present likely needs special attention to experimental design and data interpretation beyond fitting total binding data. Manipulation of maximum ligand and receptor concentrations and intentionally increasing ligand depletion are potentially helpful approaches.
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Affiliation(s)
- Alexandra M Person
- Department of Molecular and Cellular Medicine, Texas A&M University System Health Science Center, College Station, TX 77843-1114, USA.
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Kasheverov IE, Zhmak MN, Khruschov AY, Tsetlin VI. Design of new α-conotoxins: from computer modeling to synthesis of potent cholinergic compounds. Mar Drugs 2011; 9:1698-1714. [PMID: 22072993 PMCID: PMC3210602 DOI: 10.3390/md9101698] [Citation(s) in RCA: 23] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/01/2011] [Revised: 08/29/2011] [Accepted: 09/16/2011] [Indexed: 11/16/2022] Open
Abstract
A series of 14 new analogs of α-conotoxin PnIA Conus pennaceus was synthesized and tested for binding to the human α7 nicotinic acetylcholine receptor (nAChR) and acetylcholine-binding proteins (AChBP) Lymnaea stagnalis and Aplysia californica. Based on computer modeling and the X-ray structure of the A. californica AChBP complex with the PnIA[A10L, D14K] analog, single and multiple amino acid substitutions were introduced in α-conotoxin PnIA aimed at compounds of higher affinity and selectivity. Three analogs, PnIA[L5H], PnIA[A10L, D14K] and PnIA[L5R, A10L, D14R], have high affinities for AChBPs or α7 nAChR, as found in competition with radioiodinated α-bungarotoxin. That is why we prepared radioiodinated derivatives of these α-conotoxins, demonstrated their specific binding and found that among the tested synthetic analogs, most had almost 10-fold higher affinity in competition with radioactive α-conotoxins as compared to competition with radioactive α-bungarotoxin. Thus, radioiodinated α-conotoxins are a more sensitive tool for checking the activity of novel α-conotoxins and other compounds quickly dissociating from the receptor complexes.
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Affiliation(s)
- Igor E. Kasheverov
- Author to whom correspondence should be addressed; E-Mail: or ; Tel.: +7-495-330-7374; Fax: +7-495-335-5733
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Brams M, Pandya A, Kuzmin D, van Elk R, Krijnen L, Yakel JL, Tsetlin V, Smit AB, Ulens C. A structural and mutagenic blueprint for molecular recognition of strychnine and d-tubocurarine by different cys-loop receptors. PLoS Biol 2011; 9:e1001034. [PMID: 21468359 PMCID: PMC3066128 DOI: 10.1371/journal.pbio.1001034] [Citation(s) in RCA: 84] [Impact Index Per Article: 6.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/07/2010] [Accepted: 02/09/2011] [Indexed: 11/18/2022] Open
Abstract
Cys-loop receptors (CLR) are pentameric ligand-gated ion channels that mediate fast excitatory or inhibitory transmission in the nervous system. Strychnine and d-tubocurarine (d-TC) are neurotoxins that have been highly instrumental in decades of research on glycine receptors (GlyR) and nicotinic acetylcholine receptors (nAChR), respectively. In this study we addressed the question how the molecular recognition of strychnine and d-TC occurs with high affinity and yet low specificity towards diverse CLR family members. X-ray crystal structures of the complexes with AChBP, a well-described structural homolog of the extracellular domain of the nAChRs, revealed that strychnine and d-TC adopt multiple occupancies and different ligand orientations, stabilizing the homopentameric protein in an asymmetric state. This introduces a new level of structural diversity in CLRs. Unlike protein and peptide neurotoxins, strychnine and d-TC form a limited number of contacts in the binding pocket of AChBP, offering an explanation for their low selectivity. Based on the ligand interactions observed in strychnine- and d-TC-AChBP complexes we performed alanine-scanning mutagenesis in the binding pocket of the human α1 GlyR and α7 nAChR and showed the functional relevance of these residues in conferring high potency of strychnine and d-TC, respectively. Our results demonstrate that a limited number of ligand interactions in the binding pocket together with an energetic stabilization of the extracellular domain are key to the poor selective recognition of strychnine and d-TC by CLRs as diverse as the GlyR, nAChR, and 5-HT3R. Ligand-gated ion channels play an important role in fast electrochemical signaling in the brain. Cys-loop receptors are a class of pentameric ligand-gated ion channels that are activated by specific neurotransmitters, including acetylcholine (ACh), serotonin (5-HT), glycine (Gly), and γ-aminobutyric acid (GABA). Each type of cys-loop receptor contains an extracellular domain that specifically recognizes only one of these four neurotransmitters and opens an ion-conducting channel pore upon ligand binding. In this study, we investigated the poor specificity with which two potent neurotoxic inhibitors, namely strychnine and d-tubocurarine, are recognized by different cys-loop receptors. Using X-ray crystallography we solved 3-dimensional structures of strychnine or d-tubocurarine in complex with ACh binding protein (AChBP), a well-recognized structural homolog of the nicotinic ACh receptor. Based on ligand-receptor interactions observed in AChBP structures we designed mutant GlyR and α7 nAChR to identify hot spots in the binding pocket of these receptors that define potent inhibition by strychnine and d-tubocurarine, respectively. Combined, our results offer detailed understanding of the molecular recognition of antagonists that have high affinity but poor specificity for different cys-loop receptors.
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Affiliation(s)
- Marijke Brams
- Laboratory of Structural Neurobiology, KULeuven, Leuven, Belgium
| | - Anshul Pandya
- Laboratory of Neurobiology, National Institute of Environmental Health Sciences, National Institutes of Health, Department of Health and Human Services, Research Triangle Park, North Carolina, United States of America
| | - Dmitry Kuzmin
- Department of Molecular Basis of Neurosignaling, Shemyakin-Ovchinnikov Institute of Bioorganic Chemistry, Russian Academy of Sciences, Moscow, Russia
| | - René van Elk
- Department of Molecular & Cellular Neurobiology, Center for Neurogenomics and Cognitive Research, Neuroscience Campus Amsterdam, VU University, Amsterdam, The Netherlands
| | - Liz Krijnen
- Laboratory of Structural Neurobiology, KULeuven, Leuven, Belgium
| | - Jerrel L. Yakel
- Laboratory of Neurobiology, National Institute of Environmental Health Sciences, National Institutes of Health, Department of Health and Human Services, Research Triangle Park, North Carolina, United States of America
| | - Victor Tsetlin
- Department of Molecular Basis of Neurosignaling, Shemyakin-Ovchinnikov Institute of Bioorganic Chemistry, Russian Academy of Sciences, Moscow, Russia
| | - August B. Smit
- Department of Molecular & Cellular Neurobiology, Center for Neurogenomics and Cognitive Research, Neuroscience Campus Amsterdam, VU University, Amsterdam, The Netherlands
| | - Chris Ulens
- Laboratory of Structural Neurobiology, KULeuven, Leuven, Belgium
- * E-mail:
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