Figure 7.

The β1 subunit does not alter the voltage dependence of the closed-to-open conformational change. Macroscopic ionic currents were recorded from BK_α (A) and BK_α+β1 (B) channels in excised oocyte macropatches. Voltage steps were varied as indicated, and the time courses of relaxation were fitted with single-exponential functions. Time constants (τ) from the fits are plotted in C. For voltages of +100 mV and greater, activation time constants are plotted. Otherwise deactivation time courses are plotted. The τ–V curves in C were fitted (solid lines) to a function that approximates the kinetics of Scheme I in the limit that voltage sensor movement is fast relative to channel opening and closing (see MATERIALS AND METHODS). The fit parameters were as follows: BK_α: held V_hc=151 mV, L = 2.2 × 10⁻⁶, z_J = 0.58 e, z_L = 0.41 e, fitting yielded D = 12.6 ± 0.43, z_γ = 0.10 ± 0.002 e, γ0(0) = 7452.3 s⁻¹, γ1(0) = 4121.4 s⁻¹, γ2(0) = 5645.8 s⁻¹, γ3(0) = 851 s⁻¹, γ4(0) = 1025 s⁻¹, δ0(0) = 0.016 s⁻¹, δ1(0) = 0.114 s⁻¹, δ2(0) = 1.98 s⁻¹, δ3(0) = 3.76 s⁻¹, δ4(0) = 57.12 s⁻¹; BK_α+β1: held V_hc = 80 mV, z_J = 0.57 e, fitting yielded L = 3.3 × 10⁻⁶ ± 6.4 × 10⁻⁶, z_L = 0.46 e, D = 10.4 ± 6.4, z_γ = 0.17 e, γ0(0) = 931.7 s⁻¹, γ1(0) = 213.2 s⁻¹, γ2(0) = 547.8 s⁻¹, γ3(0) = 333.5 s⁻¹, γ4(0) = 126.7 s⁻¹; δ0(0) = 0.003 s⁻¹, δ1(0) = 0.007 s⁻¹, δ2(0) = 0.198 s⁻¹, δ3(0) = 1.251 s⁻¹, δ4(0) = 4.934 s⁻¹. Both curves are also fitted (dashed lines) at far negative voltages with the following function:

τ=.

Fit parameters are as follows: BK_α (voltages <−180 mV) γ(0) = 6940.8 s⁻¹, z = 0.11 e; BK_α+β1 (voltages <−280 mV) γ(0) = 1808.7 s⁻¹, z_γ = 0.11 e.