Abstract
A subclass of bacterial CLC anion-transporting proteins, phylogenetically distant from long-studied CLCs, was recently shown to be specifically up-regulated by F ⁻. We establish here that a set of randomly selected representatives from this “CLC F” clade protect Escherichia coli from F ⁻ toxicity, and that the purified proteins catalyze transport of F ⁻ in liposomes. Sequence alignments and membrane transport experiments using ¹⁹F NMR, osmotic response assays, and planar lipid bilayer recordings reveal four mechanistic traits that set CLC F proteins apart from all other known CLCs. First, CLC Fs lack conserved residues that form the anion binding site in canonical CLCs. Second, CLC Fs exhibit high anion selectivity for F ⁻ over Cl ⁻. Third, at a residue thought to distinguish CLC channels and transporters, CLC Fs bear a channel-like valine rather than a transporter-like glutamate, and yet are F ⁻/H ⁺ antiporters. Finally, F ⁻/H ⁺ exchange occurs with 1∶1 stoichiometry, in contrast to the usual value of 2∶1.