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

摘要

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 PO_4~(3-) and SO_4~(2-). 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 CIC-ecl transporter revealed that Cl~ binding to the central anion-binding site (S_(cen)) is crucial for the anion-coupled H~+ transport. Here, we show that Cl~-, F~-, NO_3~-, and SCN~- display distinct binding coordinations at the S_(cen) site and are hydrated in different manners. Consistent with the observation of differential bindings, CIC-ecl 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 (Glu_(in) and Glu_(ex)), in the presence of different anions. While continuous water wires are frequently observed in the presence of physiologically transported Cl~-, binding of F~- or NO_3~- 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 CIC-ecl and reveal a putative atomic-level mechanism for the decoupling of H~+ transport to the transport of anions other than Cl~-.