Effect of [H⁺]_ext on ClC-Ka WT and its mutants E261Q, D278N, E261Q/D278N, and E261Q/D278N/H497M. (A) The current at 60 mV, shown in color, of the triple mutant E261Q/D278N/H497M as a function of time (color code as in Fig. 6). Inset in A shows representative current traces of the mutant in standard solution. Horizontal bars, 100 ms; vertical bars, 1 µA. Capacitive transients have been blanked for clarity. (B) Dose–response relationship of the modulation by protons of ClC-Ka WT (black circles; n = 14) and the mutants E261Q (yellow squares; n = 4), D278N (red triangles; n = 5), E261Q/D278N (light blue rhombi; n = 5), and E261Q/D278N/E261Q (green hexagons; n = 5). Currents at 60 mV were normalized to values measured in standard solution and plotted versus pH. Data for WT are different compared with Fig. 2 because they were obtained from different oocytes as control measurements in the mutagenic screen.