<p>The G749R mutation produces gain-of-function gating changes in hERG channels assayed by two-electrode voltage clamp.</p>

Mean ± SEM are shown. Tail current amplitudes were normalized to the maximum tail current amplitude and plotted as a function of the preceding depolarization step to obtain the steady-state activation plots. The line of best fit is a Boltzmann function. hERG1a(WT), V_1/2 = −23.97 mV, slope factor = 8.34; hERG1a(G749R), V_1/2 = −35.85 mV, slope factor = 7.20.

(E) Bar graph showing half-activation potential V_1/2 derived from the Boltzmann fit of the conductance-voltage curves shown in (D). (<b>F</b>) Representative records of currents evoked using an envelope-of-tails protocol to generate the time course of activation. Channels were depolarized from a holding potential of −80 mV to +20 mV for progressively increasing durations in 25 ms increments, followed by repolarization to −105 mV to record tail currents.

(<b>G-H</b>) Activation kinetics of hERG1a(WT) and hERG1a(G749R). (G) Time course of activation was constructed using the envelope‑of‑tails protocol illustrated in (F). The normalized peak tail current was plotted as a function of pulse duration and fitted with a single‑exponential function to determine the activation time constant.

Mean ± SEM are shown. (H) Bar graph showing activation time constant derived from single-exponential fit to the time courses of activation shown in (G<i>)</i>. (<b>I-J</b>) Deactivation kinetics of hERG1a(WT) and hERG1a(G749R).

(I) Representative traces of tail currents of hERG1a(WT) and hERG1a(G749R). Tail currents recorded at −105 mV with a + 40 mV prepulse are shown. Currents were normalized to their peak current amplitude to compare deactivation kinetics.

Single exponential fits are superimposed using wider, lighter lines that match the data colors. (J) Bar graph showing deactivation time constant derived from single-exponential fit to deactivating currents. For (E, H, and J), each dot represents an independent oocyte.

Mean ± SEM are shown. *** indicates p < 0.001, **** indicates p < 0.0001. Unpaired t-test with Welch’s correction.</p>