Gating pore currents from other HypoPP mutations at site R666. Steady-state gating pore currents were recorded from R666S, -C, and -H mutant channels and compared with currents recorded in WT channels (denoted at top). Representative current traces, after leak correction and normalization to the corresponding maximal gating charge displacement, are shown for recordings made in bath and internal solutions approximating the normal mammalian physiological cation gradient (A) or in bath and internal solutions containing NMDG (B). Scale bars for all traces are shown in the inset to A. The mean I-V relationships of gating pore currents seen in each mutant under these conditions is shown in C. For each panel, the filled circles represent currents recorded in the physiological cation gradient, whereas open circles represent currents recorded when NMDG was present in both internal and external compartments. Number of samples recorded for each condition is denoted in parenthesis in the inset legend for each figure. Gating pore currents presumably carried by monovalent cations and abolished by NMDG substitution are seen in R666S and R666C mutant channels. In contrast, inward gating pore currents of similar magnitude are seen in R666H channels in both ionic conditions, consistent with the notion that the charge carriers of this gating pore current are protons rather than larger monovalent cations.