Abstract
Fluoride ion, ubiquitous in soil, water, and marine environments, is a chronic threat to microorganisms. Many prokaryotes, archea, unicellular eukaryotes, and plants use a recently discovered family of F(-) exporter proteins to lower cytoplasmic F(-) levels to counteract the anion's toxicity. We show here that these 'Fluc' proteins, purified and reconstituted in liposomes and planar phospholipid bilayers, form constitutively open anion channels with extreme selectivity for F(-) over Cl(-). The active channel is a dimer of identical or homologous subunits arranged in antiparallel transmembrane orientation. This dual-topology assembly has not previously been seen in ion channels but is known in multidrug transporters of the SMR family, and is suggestive of an evolutionary antecedent of the inverted repeats found within the subunits of many membrane transport proteins. DOI:http://dx.doi.org/10.7554/eLife.01084.001.
Highlights
Fluoride pervades our biosphere, appearing in groundwater, sea, and soil typically at 10–100 μM levels (Weinstein and Davison, 2004)
By expressing, purifying, and functionally reconstituting several bacterial Fluc homologs, we demonstrate that these are highly selective F−-conducting ion channels constructed as dimers of identical or homologous membrane-embedded domains arranged in an inverted-topology fashion
This type of molecular architecture is unprecedented among ion channels but is reminiscent of the dual-topology construction of small multidrug transporters (Rapp et al, 2006; Schuldiner, 2009; Morrison et al, 2012) and of the inverted structural repeats appearing in many membrane transport proteins (Forrest et al, 2010)
Summary
Fluoride pervades our biosphere, appearing in groundwater, sea, and soil typically at 10–100 μM levels (Weinstein and Davison, 2004). By expressing, purifying, and functionally reconstituting several bacterial Fluc homologs, we demonstrate that these are highly selective F−-conducting ion channels constructed as dimers of identical or homologous membrane-embedded domains arranged in an inverted-topology fashion. This type of molecular architecture is unprecedented among ion channels but is reminiscent of the dual-topology construction of small multidrug transporters (Rapp et al, 2006; Schuldiner, 2009; Morrison et al, 2012) and of the inverted structural repeats appearing in many membrane transport proteins (Forrest et al, 2010)
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