Effect of N-terminal truncation on kinetics of hERG activation. (A) Typical examples of currents recorded in response to the envelope of tails protocols shown for E518C (i) and Δ2–25 (ii) hERG channels. In these examples, the activating potential was 100 mV. (B) Peak tail currents plotted against the duration of the activating pulse for the examples shown in A. The black line is a fit of a single exponential to the activation time course. (C) Summary of the voltage dependence of rates of activation of ionic current. Error bars are SEM; n ≥ 8. (D) Summary of the voltage dependence of the rates of VSD activation/upward motion derived from single exponential fits to the fluorescent time course using the same protocol illustrated in Fig. 1 B (i). Error bars are SEM; n ≥ 7. The inset shows example fits to fluorescent traces from MTSR-labeled E518C recorded at 100 mV.