Voltage-Dependent F−/Cl− Selectivity in a Subclass of CLC Antiporters

A newly discovered class of F− riboswitch-controlled CLC F−/H+ antiporters (CLCFs) preferentially transport F− over Cl−, providing bacteria with a mechanism to protect themselves against F− toxicity. F− selectivity is essential for function, and how these proteins are capable of selecting F− over the much more abundant Cl− is a mystery. It has recently been shown that the preferential transport of F− over Cl− by CLCFs is influenced by the ionic environment the protein experiences. Reconstituted liposomes containing CLCFs are completely selective for F− over Cl− in symmetrical K+ conditions. However, with K+ inside and Na+ outside the liposomes, the protein now transports Cl− as well. These initial observations have led to the discovery that the altered selectivity is due to the presence of voltage across the liposomal membrane. As the membrane potential hyperpolarizes from 0 mV to −200 mV, the rate of F− transport increases 2-fold, while that of Cl− transport is steeply voltage-dependent, increasing much more dramatically over the same range. We aim to explore this finding using the CLCF homologue, CLC-eca, by monitoring anion fluxes in reconstituted liposomes.