Abstract
Microorganisms are commonly exposed to fluoride in acidic environments, which can induce its intracellular accumulation, and lead to the inhibition of essential cellular processes such as nucleic acid production. However some unicellular organisms possess fluoride-specific exporting proteins of either the CLCF or the Fluc family in order to counteract the influx of toxic fluoride. Particularly, F- channels of the Fluc family allow membrane impermeable fluoride to passively diffuse from the cell to sub-toxic levels. Recently, these extremely F- specific proteins have been determined to adopt an antiparallel homodimeric assembly in the lipid bilayer, likely forming two antiparallel pores. Interestingly, these interior regions of Fluc are comprised of residues with highly conserved polarity that are predicted to form the ion conduction pathway and contribute to the channel’s unusual F- selectivity. Mutagenesis removing or modifying the polar characteristic of these residues in two homologues of Fluc revealed different responses of channel functionality in proteoliposome efflux experiments. Some residue mutations had no observable changes in F- efflux rates, while others lead to dramatic effects on ion channel function even when mildly mutated to other polar residues. The more sensitive polar residues are thus believed to be specifically integral to the structural and functional composition of this channel. A crystal structure of a functionally inactive mutant channel was also obtained and provides insight on future investigations of Fluc’s pore-lining regions.