The nonconducting mutant Kv1.2 W366F,V381T shows slow OFF gating currents with intracellular K⁺ and Cs⁺. (A) Representative current traces recorded using K⁺_i solution were evoked by 12-ms depolarizing pulses applied in 10-mV increments from a holding potential of −100 mV. (B) Individual representative gating current traces isolated from the same recording to illustrate the onset of slowing of the Ig_OFF current recorded at −100 mV after depolarizing steps from −50 to 10 mV in 20-mV increments. (C) Q-V relationships obtained from integrating Ig_ON (Q_on-V: V_0.5 = −39.4 ± 1.7 mV, z = 4.4 ± 1.3; n = 3) and Ig_OFF (Q_off-V) for 11 ms and fit with a Boltzmann function (n = 3). (D) Plot of the normalized peak Ig_OFF amplitude against voltage with a WT G-V curve in gray for comparison. (E) Representative current traces recorded using Cs⁺_i solution. (F) Individual representative gating current traces isolated from the same recording from −50 and −10 mV. (G) Plot of the normalized peak Ig_OFF amplitude with internal Cs⁺ solution. Error bars represent SEM.