Molecular Basis for Differential Anion Binding and Proton Coupling in the Cl–/H+ Exchanger ClC-ec1

By Tao Jiang, Wei Han, Merritt C. Maduke, and Emad Tajkhorshid.

Published in Journal of the American Chemical Society 2016 Mar 9;138(9):3066-75. PMID: 26880377. PMID: 24379362. PMCID: PMCID3918786 Link to publication page.

Projects: The Transport Cycle in Neurotransmitter Uptake Systems, Conformational Dynamics in the CLC Channel/Transporter Family . Core Facility: Computational Modeling.

Abstract

Cl/H+ transporters of the CLC superfamily form a ubiquitous class of membrane proteins that catalyze stoichiometrically coupled exchange of Cl and H+ across biological membranes. CLC transporters exchange H+ for halides and certain polyatomic anions, but exclude cations, F, and larger physiological anions, such as PO43 and SO42. Despite comparable transport rates of different anions, the H+ coupling in CLC transporters varies significantly depending on the chemical nature of the transported anion. Although the molecular mechanism of exchange remains unknown, studies on bacterial ClC-ec1 transporter revealed that Cl binding to the central anion-binding site (Scen) is crucial for the anion-coupled H+ transport. Here, we show that Cl, F, NO3, and SCN display distinct binding coordinations at the Scen site and are hydrated in different manners. Consistent with the observation of differential bindings, ClC-ec1 exhibits markedly variable ability to support the formation of the transient water wires, which are necessary to support the connection of the two H+ transfer sites (Gluin and Gluex), in the presence of different anions. While continuous water wires are frequently observed in the presence of physiologically transported Cl, binding of F or NO3 leads to the formation of pseudo-water-wires that are substantially different from the wires formed with Cl. Binding of SCN, however, eliminates the water wires altogether. These findings provide structural details of anion binding in ClC-ec1 and reveal a putative atomic-level mechanism for the decoupling of H+ transport to the transport of anions other than Cl.