1. Biochemistry and Chemical Biology
  2. Structural Biology and Molecular Biophysics
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The fluoride permeation pathway and anion recognition in Fluc family fluoride channels

  1. Benjamin C McIlwain
  2. Roja Gundepudi
  3. B Ben Koff
  4. Randy B Stockbridge  Is a corresponding author
  1. University of Michigan, United States
Research Article
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Cite this article as: eLife 2021;10:e69482 doi: 10.7554/eLife.69482


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 identify additional binding sites along the permeation pathway. We use this information to develop an oriented system for planar lipid bilayer electrophysiology and observe 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.

Data availability

Atomic coordinates for the Fluc-Ec2 and mutants in the presence of Br- have been deposited in the Protein Data Bank under accession numbers 7KKR (WT); 7KKA (S81A); 7KKB (S81C); 7KK8 (S81T); 7KK9 (S81A/T81A). Source data files have been provided for all figures. No custom code was used.

Article and author information

Author details

  1. Benjamin C McIlwain

    Molecular, Cellular, and Developmental Biology, University of Michigan, Ann Arbor, MI, United States
    Competing interests
    No competing interests declared.
  2. Roja Gundepudi

    Program in Biophysics, University of Michigan, Ann Arbor, MI, United States
    Competing interests
    No competing interests declared.
  3. B Ben Koff

    Molecular, Cellular, and Developmental Biology, University of Michigan, Ann Arbor, MI, United States
    Competing interests
    No competing interests declared.
  4. Randy B Stockbridge

    Molecular, Cellular, and Developmental Biology and Program in Biophysics, University of Michigan, Ann Arbor, MI, United States
    For correspondence
    Competing interests
    Randy B Stockbridge, Reviewing editor, eLife.
    ORCID icon "This ORCID iD identifies the author of this article:" 0000-0001-8848-3032


National Institutes of Health (R35-GM128768)

  • Randy B Stockbridge

The funders had no role in study design, data collection and interpretation, or the decision to submit the work for publication.

Reviewing Editor

  1. Merritt Maduke, Stanford University School of Medicine, United States

Publication history

  1. Preprint posted: April 4, 2021 (view preprint)
  2. Received: April 16, 2021
  3. Accepted: July 9, 2021
  4. Accepted Manuscript published: July 12, 2021 (version 1)
  5. Version of Record published: July 27, 2021 (version 2)


© 2021, McIlwain et al.

This article is distributed under the terms of the Creative Commons Attribution License permitting unrestricted use and redistribution provided that the original author and source are credited.


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