This was a fun collaboration with Dr Randy Stockbridge (University of Michigan) and Professor Chris Miller (Brandeis University).
It was the first time monobodies had been used as crystallisation chaperones in the lipid cubic phase and the first dual toplogy ion channel structure!
To contend with hazards posed by environmental fluoride, microorganisms export this anion through F(-)-specific ion channels of the Fluc family. Since the recent discovery of Fluc channels, numerous idiosyncratic features of these proteins have been unearthed, including strong selectivity for F(-) over Cl(-) and dual-topology dimeric assembly. To understand the chemical basis for F(-) permeation and how the antiparallel subunits convene to form a F(-)-selective pore, here we solve the crystal structures of two bacterial Fluc homologues in complex with three different monobody inhibitors, with and without F(-) present, to a maximum resolution of 2.1 Å. The structures reveal a surprising ‘double-barrelled’ channel architecture in which two F(-) ion pathways span the membrane, and the dual-topology arrangement includes a centrally coordinated cation, most likely Na(+). F(-) selectivity is proposed to arise from the very narrow pores and an unusual anion coordination that exploits the quadrupolar edges of conserved phenylalanine rings.