Fluoride-dependent interruption of the transport cycle of a CLC Cl-/H+ antiporter

Abstract

Cl-/H+ antiporters of the CLC superfamily transport anions across biological membranes in varied physiological contexts. These proteins are weakly selective among anions commonly studied, including Cl-, Br-, I-, NO3- and SCN-, but they seem to be very selective against F-. The recent discovery of a new CLC clade of F-/H+ antiporters, which are highly selective for F- over Cl-, led us to investigate the mechanism of Cl--over-F- selectivity by a CLC Cl-/H+ antiporter, CLC-ec1. By subjecting purified CLC-ec1 to anion transport measurements, electrophysiological recording, equilibrium ligand-binding studies and X-ray crystallography, we show that F- binds in the Cl- transport pathway with affinity similar to Cl- but stalls the transport cycle. Examination of various mutant antiporters implies a 'lock-down' mechanism of F- inhibition, in which F-, by virtue of its unique hydrogen-bonding chemistry, greatly retards a proton-linked conformational change essential for the transport cycle of CLC-ec1.