Gating and Ionic Currents Reveal How the BK_Ca Channel's Ca²⁺ Sensitivity Is Enhanced by its β1 Subunit

Large-conductance Ca²⁺-activated K⁺ channels (BK_Ca channels) are regulated by the tissue-specific expression of auxiliary β subunits. β1 is predominately expressed in smooth muscle, where it greatly enhances the BK_Ca channel's Ca²⁺ sensitivity, an effect that is required for proper regulation of smooth muscle tone. Here, using gating current recordings, macroscopic ionic current recordings, and unitary ionic current recordings at very low open probabilities, we have investigated the mechanism that underlies this effect. Our results may be summarized as follows. The β1 subunit has little or no effect on the equilibrium constant of the conformational change by which the BK_Ca channel opens, and it does not affect the gating charge on the channel's voltage sensors, but it does stabilize voltage sensor activation, both when the channel is open and when it is closed, such that voltage sensor activation occurs at more negative voltages with β1 present. Furthermore, β1 stabilizes the active voltage sensor more when the channel is closed than when it is open, and this reduces the factor D by which voltage sensor activation promotes opening by ∼24% (16.8→12.8). The effects of β1 on voltage sensing enhance the BK_Ca channel's Ca²⁺ sensitivity by decreasing at most voltages the work that Ca²⁺ binding must do to open the channel. In addition, however, in order to fully account for the increase in efficacy and apparent Ca²⁺ affinity brought about by β1 at negative voltages, our studies suggest that β1 also decreases the true Ca²⁺ affinity of the closed channel, increasing its Ca²⁺ dissociation constant from ∼3.7 μM to between 4.7 and 7.1 μM, depending on how many binding sites are affected.