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Correspondence to Michael Pusch: pusch{at}ge.ibf.cnr.it
Some CLC proteins function as passive Cl– ion channels whereas others are secondary active chloride/proton antiporters. Voltage-dependent gating of the model Torpedo channel ClC-0 is modulated by intracellular and extracellular pH, possibly reflecting a mechanistic relationship with the chloride/proton coupling of CLC antiporters. We used inside-out patch clamp measurements and mutagenesis to explore the dependence of the fast gating mechanism of ClC-0 on intracellular pH and to identify the putative intracellular proton acceptor(s). Among the tested residues (S123, K129, R133, K149, E166, F214L, S224, E226, V227, C229, R305, R312, C415, H472, F418, V419, P420, and Y512) only mutants of E166, F214, and F418 qualitatively changed the pHint dependence. No tested amino acid emerged as a valid candidate for being a pH sensor. A detailed kinetic analysis of the dependence of fast gate relaxations on pHint and [Cl–]int provided quantitative constraints on possible mechanistic models of gating. In one particular model, a proton is generated by the dissociation of a water molecule in an intrapore chloride ion binding site. The proton is delivered to the side chain of E166 leading to the opening of the channel, while the hydroxyl ion is stabilized in the internal/central anion binding site. Deuterium isotope effects confirm that proton transfer is rate limiting for fast gate opening and that channel closure depends mostly on the concentration of OH– ions. The gating model is in natural agreement with the finding that only the closing rate constant, but not the opening rate constant, depends on pHint and [Cl–]int.
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