Fluoride resistance and transport by riboswitch-controlled CLC antiporters

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 “CLCF” clade protectEscherichia colifrom F-toxicity, and that the purified proteins catalyze transport of F-in liposomes. Sequence alignments and membrane transport experiments using19F NMR, osmotic response assays, and planar lipid bilayer recordings reveal four mechanistic traits that set CLCFproteins apart from all other known CLCs. First, CLCFs lack conserved residues that form the anion binding site in canonical CLCs. Second, CLCFs exhibit high anion selectivity for F-over Cl-. Third, at a residue thought to distinguish CLC channels and transporters, CLCFs 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.