Figure 4.

β4 effects on z_L in the presence of Ca²⁺. (A, left) According to the dual-allosteric mechanism (Horrigan and Aldrich, 2002), BK channel transitions between closed (C) and open (O) conformation is allosterically regulated by the state of four independent and identical Ca²⁺ binding sites. Sub-Scheme 1c represents BK channel's gating scheme at very negative voltages, where voltage sensors remain in the resting states. Channel resides in either closed or open conformations, with 0–4 Ca²⁺ binding sites occupied. Equilibrium between the C–O transitions is allosterically regulated by the states of the Ca²⁺ binding sites. In the absence of voltage sensor activation, voltage dependence of the C–O transition is entirely dependent on z_L. (A, right) Illustration of how z_L can be estimated by logP_o-V data at high Ca²⁺ and very negative voltages. Curves are simulated logP_o-V curves in nominally 0 Ca²⁺ and 100 μM Ca²⁺ according to Scheme 1c. Gating parameters used for simulation are as follows: L₀ = 2.5 × 10⁻⁶, z_L = 0.39 e₀, z_J = 0.54 e₀, Vh_c = 173 mV, Vh_o = 25 mV, K_c = 13.9 μM, and K_o = 1.4 μM. Dashed lines represent fits for logP_o-V at limiting slopes using Eq. 8. L₀′ and z_L can be derived from the fits. (B) At 100 μM Ca²⁺, currents were recorded at very negative voltages to determine logP_o vs. V relations. Currents were low-pass filtered at 20 kHz. Representative current traces at indicated voltages in the absence and presence of β4, respectively. Traces in B were all obtained from the same patch. Currents were filtered at 5 kHz for display purposes. (C) Representative logP_o-V relations at various Ca²⁺ where limiting slopes is reached. Upper limits for z_L were estimated from the apparent limiting slopes (solid lines). (D) Estimates of z_L plotted as a function of [Ca²⁺] for α subunits alone (open symbols) and α+β4 (closed symbols) for patches where limiting slope was reached. Error bars represent SEM. The number of patches where limiting slope was reached as well as total number of recordings performed (in parenthesis) at each [Ca²⁺] are indicated.