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Lu X, Huang J. Molecular mechanisms of Na +-driven bile acid transport in human NTCP. Biophys J 2024:S0006-3495(24)00214-5. [PMID: 38544409 DOI: 10.1016/j.bpj.2024.03.033] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/01/2024] [Revised: 02/17/2024] [Accepted: 03/25/2024] [Indexed: 04/12/2024] Open
Abstract
Human Na+ taurocholate co-transporting protein (hNTCP) is a key bile salt transporter to maintain enterohepatic circulation and is responsible for the recognition of hepatitis B and D viruses. Despite landmark cryoelectron microscopy studies revealing open-pore and inward-facing states of hNTCP stabilized by antibodies, the transport mechanism remains largely unknown. To address this knowledge gap, we used molecular dynamics and enhanced sampling metadynamics simulations to elucidate the intrinsic mechanism of hNTCP-mediated taurocholate acid (TCA) transport driven by Na+ binding. We uncovered three TCA-binding modes, including one that closely matched the limited cryoelectron microscopy density observed in the open-pore hNTCP. We also captured several key hNTCP conformations in the substrate transport cycle, particularly including an outward-facing, substrate-bound state. Furthermore, we provided thermodynamic evidence supporting that changes in the Na+-binding state drive the TCA transport by exploiting the amphiphilic nature of the substrate and modulating the protein environment, thereby enabling the TCA molecule to flip through. Understanding these mechanistic details of Na+-driven bile acid transport may aid in the development of hNTCP-targeted therapies for liver diseases.
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Affiliation(s)
- Xiaoli Lu
- Westlake AI Therapeutics Laboratory, Westlake Laboratory of Life Sciences and Biomedicine, Hangzhou, Zhejiang, China; Key Laboratory of Structural Biology of Zhejiang Province, School of Life Sciences, Westlake University, Hangzhou, Zhejiang, China
| | - Jing Huang
- Westlake AI Therapeutics Laboratory, Westlake Laboratory of Life Sciences and Biomedicine, Hangzhou, Zhejiang, China; Key Laboratory of Structural Biology of Zhejiang Province, School of Life Sciences, Westlake University, Hangzhou, Zhejiang, China.
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Abstract
Fluoride (F-) export proteins, including F- channels and F- transporters, are widespread in biology. They contribute to cellular resistance against fluoride ion, which has relevance as an ancient xenobiotic, and in more modern contexts like organofluorine biosynthesis and degradation or dental medicine. This chapter summarizes quantitative methods to measure fluoride transport across membranes using fluoride-specific lanthanum-fluoride electrodes. Electrode-based measurements can be used to measure unitary fluoride transport rates by membrane proteins that have been purified and reconstituted into lipid vesicles, or to monitor fluoride efflux into living microbial cells. Thus, fluoride electrode-based measurements yield quantitative mechanistic insight into one of the major determinants of fluoride resistance in microorganisms, fungi, yeasts, and plants.
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Affiliation(s)
- Chia-Yu Kang
- Program in Biophysics, University of Michigan, Ann Arbor, MI, United States
| | - Minjun An
- Department of Molecular, Cellular, and Developmental Biology, University of Michigan, Ann Arbor, MI, United States
| | - Randy B Stockbridge
- Program in Biophysics, University of Michigan, Ann Arbor, MI, United States; Department of Molecular, Cellular, and Developmental Biology, University of Michigan, Ann Arbor, MI, United States.
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3
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Mills KR, Torabifard H. Computational approaches to investigate fluoride binding, selectivity and transport across the membrane. Methods Enzymol 2024; 696:109-154. [PMID: 38658077 DOI: 10.1016/bs.mie.2024.01.006] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 04/26/2024]
Abstract
The use of molecular dynamics (MD) simulations to study biomolecular systems has proven reliable in elucidating atomic-level details of structure and function. In this chapter, MD simulations were used to uncover new insights into two phylogenetically unrelated bacterial fluoride (F-) exporters: the CLCF F-/H+ antiporter and the Fluc F- channel. The CLCF antiporter, a member of the broader CLC family, has previously revealed unique stoichiometry, anion-coordinating residues, and the absence of an internal glutamate crucial for proton import in the CLCs. Through MD simulations enhanced with umbrella sampling, we provide insights into the energetics and mechanism of the CLCF transport process, including its selectivity for F- over HF. In contrast, the Fluc F- channel presents a novel architecture as a dual topology dimer, featuring two pores for F- export and a central non-transported sodium ion. Using computational electrophysiology, we simulate the electrochemical gradient necessary for F- export in Fluc and reveal details about the coordination and hydration of both F- and the central sodium ion. The procedures described here delineate the specifics of these advanced techniques and can also be adapted to investigate other membrane protein systems.
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Affiliation(s)
- Kira R Mills
- Department of Chemistry & Biochemistry, The University of Texas at Dallas, Richardson, TX, United States
| | - Hedieh Torabifard
- Department of Chemistry & Biochemistry, The University of Texas at Dallas, Richardson, TX, United States.
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4
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Ernst M, Orabi EA, Stockbridge RB, Faraldo-Gómez JD, Robertson JL. Dimerization mechanism of an inverted-topology ion channel in membranes. Proc Natl Acad Sci U S A 2023; 120:e2308454120. [PMID: 37956279 PMCID: PMC10666096 DOI: 10.1073/pnas.2308454120] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/19/2023] [Accepted: 09/28/2023] [Indexed: 11/15/2023] Open
Abstract
Many ion channels are multisubunit complexes where oligomerization is an obligatory requirement for function as the binding axis forms the charged permeation pathway. However, the mechanisms of in-membrane assembly of thermodynamically stable channels are largely unknown. Here, we demonstrate a key advance by reporting the dimerization equilibrium reaction of an inverted-topology, homodimeric fluoride channel Fluc in lipid bilayers. While the wild-type channel is a long-lived dimer, we leverage a known mutation, N43S, that weakens Na+ binding in a buried site at the interface, thereby unlocking the complex for reversible association in lipid bilayers. Single-channel recordings show that Na+ binding is required for fluoride conduction while single-molecule microscopy experiments demonstrate that N43S Fluc exists in a dynamic monomer-dimer equilibrium in the membrane, even following removal of Na+. Quantifying the thermodynamic stability while titrating Na+ indicates that dimerization occurs first, providing a membrane-embedded binding site where Na+ binding weakly stabilizes the complex. To understand how these subunits form stable assemblies while presenting charged surfaces to the membrane, we carried out molecular dynamics simulations, which show the formation of a thinned membrane defect around the exposed dimerization interface. In simulations where subunits are permitted to encounter each other while preventing protein contacts, we observe spontaneous and selective association at the native interface, where stability is achieved by mitigation of the membrane defect. These results suggest a model wherein membrane-associated forces drive channel assembly in the native orientation while subsequent factors, such as Na+ binding, result in channel activation.
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Affiliation(s)
- Melanie Ernst
- Department of Biochemistry and Molecular Biophysics, Washington University School of Medicine, St. Louis, MO63110
| | - Esam A. Orabi
- Theoretical Molecular Biophysics Section, National Heart, Lung, and Blood Institute, NIH, Bethesda, MD20894
| | - Randy B. Stockbridge
- Department of Molecular, Cellular, and Developmental Biology, University of Michigan, Ann Arbor, MI48109
| | - José D. Faraldo-Gómez
- Theoretical Molecular Biophysics Section, National Heart, Lung, and Blood Institute, NIH, Bethesda, MD20894
| | - Janice L. Robertson
- Department of Biochemistry and Molecular Biophysics, Washington University School of Medicine, St. Louis, MO63110
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5
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Zhang J, Song D, Schackert FK, Li J, Xiang S, Tian C, Gong W, Carloni P, Alfonso-Prieto M, Shi C. Fluoride permeation mechanism of the Fluc channel in liposomes revealed by solid-state NMR. Sci Adv 2023; 9:eadg9709. [PMID: 37611110 PMCID: PMC10446490 DOI: 10.1126/sciadv.adg9709] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 02/02/2023] [Accepted: 07/21/2023] [Indexed: 08/25/2023]
Abstract
Solid-state nuclear magnetic resonance (ssNMR) methods can probe the motions of membrane proteins in liposomes at the atomic level and propel the understanding of biomolecular processes for which static structures cannot provide a satisfactory description. In this work, we report our study on the fluoride channel Fluc-Ec1 in phospholipid bilayers based on ssNMR and molecular dynamics simulations. Previously unidentified fluoride binding sites in the aqueous vestibules were experimentally verified by 19F-detected ssNMR. One of the two fluoride binding sites in the polar track was identified as a water molecule by 1H-detected ssNMR. Meanwhile, a dynamic hotspot at loop 1 was observed by comparing the spectra of wild-type Fluc-Ec1 in variant buffer conditions or with its mutants. Therefore, we propose that fluoride conduction in the Fluc channel occurs via a "water-mediated knock-on" permeation mechanism and that loop 1 is a key molecular determinant for channel gating.
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Affiliation(s)
- Jin Zhang
- Department of Neurosurgery, The First Affiliated Hospital of USTC, Center for BioAnalytical Chemistry, Hefei National Research Center for Interdisciplinary Sciences at the Microscale, University of Science and Technology of China, 230027 Hefei, P. R. China
| | - Dan Song
- Department of Neurosurgery, The First Affiliated Hospital of USTC, Center for BioAnalytical Chemistry, Hefei National Research Center for Interdisciplinary Sciences at the Microscale, University of Science and Technology of China, 230027 Hefei, P. R. China
| | - Florian Karl Schackert
- Institute for Advanced Simulations (IAS-5) and Institute of Neuroscience and Medicine (INM-9), Computational Biomedicine, Forschungszentrum Jülich, 52428 Jülich, Germany
- Department of Physics, RWTH Aachen University, 52074 Aachen, Germany
| | - Juan Li
- Department of Neurosurgery, The First Affiliated Hospital of USTC, Center for BioAnalytical Chemistry, Hefei National Research Center for Interdisciplinary Sciences at the Microscale, University of Science and Technology of China, 230027 Hefei, P. R. China
| | - Shengqi Xiang
- Department of Neurosurgery, The First Affiliated Hospital of USTC, Center for BioAnalytical Chemistry, Hefei National Research Center for Interdisciplinary Sciences at the Microscale, University of Science and Technology of China, 230027 Hefei, P. R. China
| | - Changlin Tian
- Department of Neurosurgery, The First Affiliated Hospital of USTC, Center for BioAnalytical Chemistry, Hefei National Research Center for Interdisciplinary Sciences at the Microscale, University of Science and Technology of China, 230027 Hefei, P. R. China
| | - Weimin Gong
- Department of Neurosurgery, The First Affiliated Hospital of USTC, Center for BioAnalytical Chemistry, Hefei National Research Center for Interdisciplinary Sciences at the Microscale, University of Science and Technology of China, 230027 Hefei, P. R. China
| | - Paolo Carloni
- Institute for Advanced Simulations (IAS-5) and Institute of Neuroscience and Medicine (INM-9), Computational Biomedicine, Forschungszentrum Jülich, 52428 Jülich, Germany
- Department of Physics, RWTH Aachen University, 52074 Aachen, Germany
| | - Mercedes Alfonso-Prieto
- Institute for Advanced Simulations (IAS-5) and Institute of Neuroscience and Medicine (INM-9), Computational Biomedicine, Forschungszentrum Jülich, 52428 Jülich, Germany
| | - Chaowei Shi
- Department of Neurosurgery, The First Affiliated Hospital of USTC, Center for BioAnalytical Chemistry, Hefei National Research Center for Interdisciplinary Sciences at the Microscale, University of Science and Technology of China, 230027 Hefei, P. R. China
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Mills KR, Torabifard H. Uncovering the Mechanism of the Proton-Coupled Fluoride Transport in the CLC F Antiporter. J Chem Inf Model 2023; 63:2445-2455. [PMID: 37053383 DOI: 10.1021/acs.jcim.2c01228] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 04/15/2023]
Abstract
Fluoride is a natural antibiotic abundantly present in the environment and, in micromolar concentrations, is able to inhibit enzymes necessary for bacteria to survive. However, as is the case with many antibiotics, bacteria have evolved resistance methods, including through the use of recently discovered membrane proteins. One such protein is the CLCF F-/H+ antiporter protein, a member of the CLC superfamily of anion-transport proteins. Though previous studies have examined this F- transporter, many questions are still left unanswered. To reveal details of the transport mechanism used by CLCF, we have employed molecular dynamics simulations and umbrella sampling calculations. Our results have led to several discoveries, including the mechanism of proton import and how it is able to aid in the fluoride export. Additionally, we have determined the role of the previously identified residues Glu118, Glu318, Met79, and Tyr396. This work is among the first studies of the CLCF F-/H+ antiporter and is the first computational investigation to model the full transport process, proposing a mechanism which couples the F- export with the H+ import.
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Affiliation(s)
- Kira R Mills
- Department of Chemistry and Biochemistry, The University of Texas at Dallas, Richardson, Texas 75080, United States
| | - Hedieh Torabifard
- Department of Chemistry and Biochemistry, The University of Texas at Dallas, Richardson, Texas 75080, United States
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7
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Ernst M, Orabi EA, Stockbridge RB, Faraldo-Gómez JD, Robertson JL. Dimerization mechanism of an inverted-topology ion channel in membranes. bioRxiv 2023:2023.01.27.525942. [PMID: 36789410 PMCID: PMC9928038 DOI: 10.1101/2023.01.27.525942] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/30/2023]
Abstract
Many ion channels are multi-subunit complexes with a polar permeation pathway at the oligomeric interface, but their mechanisms of assembly into functional, thermodynamically stable units within the membrane are largely unknown. Here we characterize the assembly of the inverted-topology, homodimeric fluoride channel Fluc, leveraging a known mutation, N43S, that weakens Na + binding to the dimer interface, thereby unlocking the complex. While single-channel recordings show Na + is required for activation, single-molecule photobleaching and bulk Förster Resonance Energy Transfer experiments in lipid bilayers demonstrate that N43S Fluc monomers and dimers exist in dynamic equilibrium, even without Na + . Molecular dynamics simulations indicate this equilibrium is dominated by a differential in the lipid-solvation energetics of monomer and dimer, which stems from hydrophobic exposure of the polar ion pathway in the monomer. These results suggest a model wherein membrane-associated forces induce channel assembly while subsequent factors, in this case Na + binding, result in channel activation. Teaser Membrane morphology energetics foster inverted-topology Fluc channels to form dimers, which then become active upon Na + binding.
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Yadav PR, Basha SH. Impact of F80M and F83M mutations on the functionality of fluoride ion channel elucidated in microsecond level molecular dynamic simulation. J Biomol Struct Dyn 2022; 40:10899-10904. [PMID: 34463212 DOI: 10.1080/07391102.2021.1951356] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/20/2022]
Abstract
Fluoride ion channels of the Fluc family plays a critically important role in combating environmental fluoride toxicity. As per the crystal structure of these fluoride ion channels, the pore region is densely packed with a series of hydrogen bond donating residues arranged in a ladder fashion creating an ion conducting pathway. In earlier studies, it was revealed that although the ion conducting pathway polarity is highly conserved, however the functionality of the channel protein depends on several residues at particular positions. While, a threonine at end of the pore is critically important in forming initial interactions, two phenylalanines at the central region coordinate F- transportation through the channel. It was also revealed that these two phenylalanines cannot be substituted by any other aromatic, polar or non-polar residues without hindering the functionality with exception of methionine. In another study, it was revealed that these two phenylalanines F80 and F83 when mutated with methionine; F80M lead to active state, while the F83M has lead to inactivity of F- anion conductivity. However, the exact atomic level detailing on how exactly these mutations have impacted the conductivity remained elusive. In this scenario, in this present study, we have modeled these two mutations and performed a microsecond level simulation on each mutation compared with wild type towards understanding the atomic level detailing revealing several insights on what exactly happening at these residues responsible for the selective conductivity of F- ions.Communicated by Ramaswamy H. Sarma.
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Affiliation(s)
- Pulala Raghuveer Yadav
- Department of Biotechnology, Indian Institute of Technology Hyderabad, Kandi, Telangana, India
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9
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Yue Z, Wang Z, Voth GA. Ion Permeation, Selectivity, and Electronic Polarization in Fluoride Channels. Biophys J 2022:S0006-3495(22)00143-6. [PMID: 35151630 DOI: 10.1016/j.bpj.2022.02.019] [Citation(s) in RCA: 6] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/08/2021] [Revised: 02/01/2022] [Accepted: 02/09/2022] [Indexed: 12/16/2022] Open
Abstract
Fluoride channels (Flucs) export toxic F- from the cytoplasm. Crystallography and mutagenesis have identified several conserved residues crucial for fluoride transport, but the permeation mechanism at the molecular level has remained elusive. Herein, we have applied constant-pH molecular dynamics and free-energy-sampling methods to investigate fluoride permeation through a Fluc protein from Escherichia coli. We find that fluoride is facile to permeate in its charged form, i.e., F-, by traversing through a non-bonded network. The extraordinary F- selectivity is gained by the hydrogen-bonding capability of the central binding site and the Coulombic filter at the channel entrance. The F- permeation rate calculated using an electronically polarizable force field is significantly more accurate compared with the experimental value than that calculated using a more standard additive force field, suggesting an essential role for electronic polarization in the F--Fluc interactions.
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10
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McIlwain BC, Gundepudi R, Koff BB, Stockbridge RB. The fluoride permeation pathway and anion recognition in Fluc family fluoride channels. eLife 2021; 10:69482. [PMID: 34250906 PMCID: PMC8315801 DOI: 10.7554/elife.69482] [Citation(s) in RCA: 8] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/16/2021] [Accepted: 07/09/2021] [Indexed: 11/13/2022] Open
Abstract
Fluc family fluoride channels protect microbes against ambient environmental fluoride by undermining the cytoplasmic accumulation of this toxic halide. These proteins are structurally idiosyncratic, and thus the permeation pathway and mechanism have no analogy in other known ion channels. Although fluoride-binding sites were identified in previous structural studies, it was not evident how these ions access aqueous solution, and the molecular determinants of anion recognition and selectivity have not been elucidated. Using x-ray crystallography, planar bilayer electrophysiology, and liposome-based assays, we identified additional binding sites along the permeation pathway. We used this information to develop an oriented system for planar lipid bilayer electrophysiology and observed anion block at one of these sites, revealing insights into the mechanism of anion recognition. We propose a permeation mechanism involving alternating occupancy of anion-binding sites that are fully assembled only as the substrate approaches.
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Affiliation(s)
- Benjamin C McIlwain
- Department of Molecular, Cellular, and Developmental Biology, University of Michigan, Ann Arbor, United States
| | - Roja Gundepudi
- Program in Biophysics, University of Michigan, Ann Arbor, United States
| | - B Ben Koff
- Department of Molecular, Cellular, and Developmental Biology, University of Michigan, Ann Arbor, United States
| | - Randy B Stockbridge
- Department of Molecular, Cellular, and Developmental Biology, University of Michigan, Ann Arbor, United States.,Program in Biophysics, University of Michigan, Ann Arbor, United States
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Stockbridge RB. The application of Poisson distribution statistics in ion channel reconstitution to determine oligomeric architecture. Methods Enzymol 2021; 652:321-40. [PMID: 34059289 DOI: 10.1016/bs.mie.2021.02.018] [Citation(s) in RCA: 3] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Abstract] [Key Words] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
During reconstitution, membrane proteins are randomly inserted into liposomes according to Poisson distribution statistics. When the protein to lipid ratios in the reconstitution mixture are varied systematically, the characteristics of this statistical capture permit inferences about the proteins themselves, such as the number of subunits that assemble into a single functional unit. This chapter describes the Poisson distribution as applied to the reconstitution of membrane proteins into proteoliposomes and focuses on an application whereby this statistical behavior is used to determine the number of ion channel subunits that assemble into a functional pore. Practical considerations for performing these experiments are emphasized. Harnessing Poisson dilution statistics provides a function-based method to determine ion channel oligomerization, complementing other biophysical, biochemical, or structural approaches.
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Abstract
Microorganisms contend with numerous and unusual chemical threats and have evolved a catalog of resistance mechanisms in response. One particularly ancient, pernicious threat is posed by fluoride ion (F-), a common xenobiotic in natural environments that causes broad-spectrum harm to metabolic pathways. This review focuses on advances in the last ten years toward understanding the microbial response to cytoplasmic accumulation of F-, with a special emphasis on the structure and mechanisms of the proteins that microbes use to export fluoride: the CLCF family of F-/H+ antiporters and the Fluc/FEX family of F- channels.
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Affiliation(s)
- Benjamin C McIlwain
- Department of Molecular, Cellular, and Developmental Biology, University of Michigan, Ann Arbor, Michigan 48109, USA;
| | - Michal T Ruprecht
- Department of Molecular, Cellular, and Developmental Biology, University of Michigan, Ann Arbor, Michigan 48109, USA;
| | - Randy B Stockbridge
- Department of Molecular, Cellular, and Developmental Biology, University of Michigan, Ann Arbor, Michigan 48109, USA; .,Program in Biophysics, University of Michigan, Ann Arbor, Michigan 48109, USA
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