Functional Coupling of the β₁ Subunit to the Large Conductance Ca²⁺-Activated K⁺ Channel in the Absence of Ca²⁺ : Increased Ca²⁺ Sensitivity from a Ca²⁺-Independent Mechanism

Coexpression of the β₁ subunit with the α subunit (mSlo) of BK channels increases the apparent Ca²⁺ sensitivity of the channel. This study investigates whether the mechanism underlying the increased Ca²⁺ sensitivity requires Ca²⁺, by comparing the gating in 0 Ca²⁺_i of BK channels composed of α subunits to those composed of α+β₁ subunits. The β₁ subunit increased burst duration ∼20-fold and the duration of gaps between bursts ∼3-fold, giving an ∼10-fold increase in open probability (P_o) in 0 Ca²⁺_i. The effect of the β₁ subunit on increasing burst duration was little changed over a wide range of P_o achieved by varying either Ca²⁺_i or depolarization. The effect of the β₁ subunit on increasing the durations of the gaps between bursts in 0 Ca²⁺_i was preserved over a range of voltage, but was switched off as Ca²⁺_i was increased into the activation range. The Ca²⁺-independent, β₁ subunit-induced increase in burst duration accounted for 80% of the leftward shift in the P_o vs. Ca²⁺_i curve that reflects the increased Ca²⁺ sensitivity induced by the β₁ subunit. The Ca²⁺-dependent effect of the β₁ subunit on the gaps between bursts accounted for the remaining 20% of the leftward shift. Our observation that the major effects of the β₁ subunit are independent of Ca²⁺_i suggests that the β₁ subunit mainly alters the energy barriers of Ca²⁺-independent transitions. The changes in gating induced by the β₁ subunit differ from those induced by depolarization, as increasing P_o by depolarization or by the β₁ subunit gave different gating kinetics. The complex gating kinetics for both α and α+β₁ channels in 0 Ca²⁺_i arise from transitions among two to three open and three to five closed states and are inconsistent with Monod-Wyman-Changeux type models, which predict gating among only one open and one closed state in 0 Ca²⁺_i.