Figure 9.

Reversal potentials comparing WT with that seen in mutated channels S621I and H562W/S621I. Representative tail current traces of WT (A), S621I (B), and H562W/S621I (C) at different potentials in 1 mMol/L extracellular K⁺ solution. (D) The amplitudes of tail currents were plotted against the repolarizing membrane potentials. At an extracellular K⁺ of 5.4 mMol/L, the S621I did not show positive outward deactivation tails; therefore, it was impossible to measure the reverse potential of S621I and H562W/S621I. To record the outward tail current out of the range of voltages showing inactivation, the external K⁺ concentration was reduced to 1 mM. In WT the reverse potential negatively shifted from −81 mV to −115 mV, as predicted by the Nernst equation for a highly selective potassium channel (the predicted E_K for a channel with virtually 100% potassium selectivity would be −125 mV under these conditions). However, in S621I (n = 3) and H562W/S621I, the polarity of the tail current did not change at all. They still showed clear inward tails at potentials as high as −80 mV. Accordingly, for S621I and H562W/S621I K⁺, the selectivity filter characteristics are substantially altered compared to WT. The same results were obtained in the five S621I and seven H562W/S621I experiments.