Introduction of a Trp highlights functional differences to potassium channel VSDs. (A) Chemical structure of Phe and Trp. (B) Sample traces for currents recorded from WT or mutants in which Trp has been introduced in the S2 HC (DI, Tyr168Trp; DII, Phe621Trp; DIII, Phe1076Trp; DIV, Phe1396Trp). The inset below DI Trp shows normalized currents recorded from WT (black) and the DI Trp mutant (red) in response to a depolarization to +15 mV. Bars: horizontal, 5 ms; vertical, 500 nA. Voltage steps were from −40 to +20 mV in 10-mV increments. (C and D) G-V (C) and SSI curves (D) for WT and mutants in which the S2 aromatic was replaced by Trp; the inset in D shows a bar graph representing the average time constants for fast inactivation (τ) for a depolarizing voltage step to −15 mV for WT and the Trp mutants in DI–DIV. *, statistical difference to WT values in an unpaired t test (P < 0.01). Note that the corresponding value for Tyr168Trp could not be determined because of the lack of significant ionic current at −15 mV.