Molecular mechanism of exchange coupling in CLC chloride/proton antiporters.

The ubiquitous CLC membrane transporters are unique in their ability to exchange anions for cations. Despite extensive study, there is no mechanistic model that fully explains their 2:1 Cl/H stoichiometric exchange mechanism. Here, we provide such a model. Using differential hydrogen-deuterium exchange mass spectrometry, cryo-EM structure determination, and molecular dynamics simulations, we uncovered new conformational dynamics in CLC-ec1, a bacterial CLC homolog that has served as a paradigm for this family of transporters. Simulations based on a cryo-EM structure at pH 3 revealed critical steps in the transport mechanism, including release of Cl ions to the extracellular side, opening of the inner gate, and novel water wires that facilitate H transport. Surprisingly, these water wires occurred independently of Cl binding, prompting us to reassess the relationship between Cl binding and Cl/H coupling. Using isothermal titration calorimetry and quantitative flux assays on mutants with reduced Cl binding affinity, we conclude that, while Cl binding is necessary for coupling, even weak binding can support Cl/H coupling. By integrating our findings with existing literature, we establish a complete and efficient CLC 2:1 Cl/H exchange mechanism.