Eukaryotic resistance to fluoride toxicity mediated by a widespread family of fluoride export proteins

Abstract

Fluorine is an abundant element and is toxic to organisms from bacteria to humans, but the mechanisms by which eukaryotes resist fluoride toxicity are unknown. The Escherichia coli gene crcB was recently shown to be regulated by a fluoride-responsive riboswitch, implicating it in fluoride response. There are >8,000 crcB homologs across all domains of life, indicating that it has an important role in biology. Here we demonstrate that eukaryotic homologs [renamed FEX (fluoride exporter)] function in fluoride export. FEX KOs in three eukaryotic model organisms, Neurospora crassa, Saccharomyces cerevisiae, and Candida albicans, are highly sensitized to fluoride (>200-fold) but not to other halides. Some of these KO strains are unable to grow in fluoride concentrations found in tap water. Using the radioactive isotope of fluoride, 18 F, we developed an assay to measure the intracellular fluoride concentration and show that the FEX deletion strains accumulate fluoride in excess of the external concentration, providing direct evidence of FEX function in fluoride efflux. In addition, they are more sensitive to lower pH in the presence of fluoride. These results demonstrate that eukaryotic FEX genes encode a previously unrecognized class of fluoride exporter necessary for survival in standard environmental conditions. toxicity resistance | environmental toxin | ion transport | dual membrane topology

A Single Nucleotide Change in the Promoter mutp Enhances Fluoride Resistance of Streptococcus mutans

“…Due to the extensive use of fluoride, many microorganisms have developed resistance to fluoride (3). Recent studies revealed that a conserved fluoride antiporter protein (CrcB) mediates microbial fluoride resistance by exporting fluoride ions (F Ϫ ) to maintain a low F Ϫ level in the cell (4,5). The expression of the fluoride antiporter genes can be regulated by fluoride-sensing riboswitches (4).…”

Yeast Fex1p Is a Constitutively Expressed Fluoride Channel with Functional Asymmetry of Its Two Homologous Domains

“…CLC F genes encode F Ϫ /H ϩ antiporters in eubacteria and are generally under the control of F Ϫ -sensitive riboswitches (8,14). The second, much more broadly distributed family is called Fluc in bacteria (formerly crcB) (15) and FEX 2 in eukaryotes (16). Several Fluc proteins have been demonstrated to function as bona fide ion channels that can discriminate against the chloride ion with Ͼ10,000-fold selectivity (15).…”

“…Several Fluc proteins have been demonstrated to function as bona fide ion channels that can discriminate against the chloride ion with Ͼ10,000-fold selectivity (15). Among eukaryotic systems, Saccharomyces, Candida, and Neurospora have been shown to depend upon FEX proteins for resistance to fluoride (16), but neither the mode of transport nor the anion selectivity of those proteins has been established. S. cerevisiae contains two nearly identical FEX proteins (Fex1p and Fex2p) that maintain a low fluoride concentration in the cell, even under external fluoride assault.…”

“…S. cerevisiae contains two nearly identical FEX proteins (Fex1p and Fex2p) that maintain a low fluoride concentration in the cell, even under external fluoride assault. Without these proteins, yeast are 1000-fold more sensitive to the toxic effects of fluoride and would be * This work was supported, in whole or in part, by National Institutes of Health unable to survive in many environmental conditions, including most municipal tap water (16).…”

In silico analysis of fluoride transport proteins from prokaryotic and eukaryotic organisms

“…Fluc proteins protect bacteria from fluoride toxicity [1,2]. Fluc channels are also being investigated as novel targets for antibiotics [3,4] as bacterial growth can be retarded when they are shut down. In the present study, fluoride transport proteins from different prokaryotic and eukaryotic organisms are analyzed by in silico methods and the results are discussed and communicated.…”