Fig. 4: VSD-III and VSD-IV can convert.

VSD-III and VSD-IV activation at extended holding and test potentials (V_h, V_t) revealed that they also undergo conversion. Similar to VSD-I, the voltage dependence of VSD-III and VSD-IV at V_h = −80 mV (Fig. 2g, h) consisted of transitions in a mixed population of F1 and F2. a Voltage dependence of VSD-III activation in the presence of β_2a. Solid curves are the sums of two Boltzmann distributions (Eq. 3, parameters in Table S1). Error bars are S.E.M. b The first derivatives of the curves from (a) illustrate the conversion of VSD-III from F1 to F2 as V_h becomes more positive. c Apparent voltage-dependence of VSD-III conversion. Open triangles are individual data; the green surface is the 95% confidence interval of a Boltzmann fit (Eq. 4; V_0.5 = −84.2 [−87.3, −81.1] mV; z = 1.37 [1.11,1.63] e₀, n = 19 cells). d–f As in (a–c), respectively, for channels complexed with β₃. The F1-F2 transition occurs at more negative voltages: (V_0.5 = −109 [ −110, −107] mV; z = 1.91 [1.69,2.12] e₀, n = 13 cells). g–i As in (a–c), respectively, for channels with β_2a labeled in VSD-IV. In the Boltzmann fits of (i), the positive asymptote (F1_max) was a free parameter: F1_max = 57.7 [49.9,65.5] %; V_0.5 = −65.4 [−73.6, −57.2] mV; z = 1.28 [0.658,1.91] e₀, n = 26 cells. j–l As in (g–i), respectively, for VSD-IV-labeled channels with β₃. F1_max = 70.2 [64.0,76.3] %; V_0.5 = −86.3 [−90.5, −82.1] mV; z = 1.58 [1.20,1.97] e₀, n = 15 cells. Parameter values given as the mean and 95% confidence interval [lower bound, upper bound].