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 beta(1) subunit with the alpha subunit (mSlo) of BK channels increases the apparent Ca2+ sensitivity of die channel. This study investigates whether the mechanism underlying the increased Ca2+ sensitivity requires Ca2+, by comparing the gating in 0 Ca-i(2+) of BK channels composed of alpha subunits to those composed of alpha+beta(1) subunits. The beta(1) subunit increased burst duration similar to 20-fold and the duration of gaps between bursts similar to 3-fold, giving an similar to 10-fold increase in open probability (P-o) in 0 Ca-i(2+). The effect of the beta(1) subunit on increasing burst duration was little changed over a wide range of P-o achieved by varying either Ca-i(2+) or depolarization. The effect of the beta(1) subunit on increasing the durations of the gaps between bursts in 0 Ca-i(2+) was preserved over a range of voltage, but was switched off as Ca-i(2+), was increased into the activation range. The Ca2+-independent, beta(1) subunit-induced increase ill burst duration accounted for 80% of the leftward shift in the P-o vs. Ca-i(2+) curve that reflects the increased Ca2+ sensitivity induced by the beta(1) subunit. The Ca2+-dependent effect of the beta(1) subunit on the gaps between bursts accounted for the remaining 20% of the leftward shift. Our observation that the major effects of the beta(1) subunit are independent of Ca-i(2+) suggests that the beta(1) subunit mainly alters the energy barriers of Ca2+-independent transitions. The changes in gating induced by the beta(1) subunit differ from those induced by depolarization, as increasing P-o by depolarization or by the beta(1) subunit gave different gating kinetics. The complex gating kinetics for both alpha and alpha+beta(1) channels in 0 Ca-i(2+) 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 Ca-i(2+).