Abstract
Fluoride ion channels of the Fluc family evolved to combat toxicity from intracellular accumulation of environmental fluoride in microorganisms. Fluc channels are built as antiparallel dimers with two individual pores running along side of each other and are extremely selective for fluoride over chloride. Although crystal structures are known, the densely packed pore region has precluded identification of the actual ion permeation pathway. With a combination of functional and structural studies, I chart out the span of the Fluc pore and characterize the biochemical requirements for F- transport. A ladder of hydrogen-bond donating residues creates a “polar track” for F- transport pathway. Surprisingly, though the polarity of this track is well-conserved among different homologues, polarity is functionally dispensable at several positions. The positions for which polarity is required are finely-tuned and brook no substitutions. A threonine at one end of the track appears to function through its β-branched methyl group rather than being a hydrogen bond donor. Two essential phenylalanines, each coordinates a F- through an edge-on fashion, presenting an unprecedented aromatic-halide coordination motif. Aromatic, polar and non-polar side-chains all fail to replace the two phenylalanines, but methionine substitution at one position generates a fully functional channel. A Crystal structure of this mutant revealed that methionine side-chain takes a twisted conformation and contacts F- through its partially positive γ-methylene in mimicry of phenylalanine’s quadrupolar interaction. Taken together, these results map out the ion permeation pathway of Fluc and demonstrate the unusual biochemical requirements for selectively transporting the strongly hydrogen bonding fluoride.