EQQ single-channel conductance is reduced by high external potassium. (A) Schematic illustration of EQQ channels (left), representative current traces recorded from oocytes expressing EQQ at 5 and 97 mM K⁺_o (middle), and corresponding statistical analysis of peak current at +60 mV (right); n = 7, error bars represent ±SEM, traces correspond to linear fit of the data. (B) Sample single channel records of EQQ at low and high external K⁺. Cells were pulsed for 4 s to +60 mV from a holding potential of −80 mV. (C) All-point histograms of 11 active EQQ single channel sweeps each at 5 mM (blue) and 97 mM (red) external K⁺ with an activation potential of +60 mV. (D) Current–voltage data for EQQ single channels at low and high external K⁺. Solid traces correspond to the fits of GHK equation to the data to determine the absolute permeability (P) of the channel, P_{5 mM} = 5.82 × 10⁻⁴ ± 2.9 × 10⁻⁵ cm/s; P_{97 mM} = 2.47 × 10⁻⁴ ± 4.73 × 10⁻⁶ cm/s (P = 0.013); n = 3–9 at each voltage. Dashed curve corresponds to a hypothetical channel without external K⁺ sensitivity. Error bars represent ±SEM. (E) Sample current traces from single-channel records of wild type and F339A mutant using identical conditions as for EQQ for 5 mM K⁺ and a holding potential of −10 mV for the 97 mM K⁺_o to confirm this was a K⁺ current. (F) Current–voltage data for F339A mutant and corresponding fits to GHK equation. Absolute permeability values are as follows: P_{5 mM} = 1.89 × 10⁻⁴ ± 2.01 × 10⁻⁵ cm/s; P_{97 mM} = 1.80 × 10⁻⁴ ± 3.13 × 10⁻⁶ cm/s (P = 0.038); n = 3–4 at each voltage. The theoretical curve for 97 mM K⁺ corresponding to GHK is shown dashed.