Subunit-specific effect of the voltage sensor domain on Ca²⁺ sensitivity of BK channels

Large conductance Ca²⁺- and voltage-activated K⁺ (BK) channels, composed of pore-forming α-subunits and auxiliary β-subunits, play important roles in diverse physiological processes. The differences in BK channel phenotypes are primarily due to the tissue-specific expression of β-subunits (β1-β4) that modulate channel function differently. Yet, the molecular basis of the subunit-specific regulation is not clear. In our study, we demonstrate that perturbation of the voltage sensor in BK channels by mutations selectively disrupts the ability of the β1-subunit - but not that of the β2-subunit - to enhance apparent Ca²⁺ sensitivity. These mutations change the number of equivalent gating charges, the voltage dependence of voltage sensor movements, the open-close equilibrium of the channel, and the allosteric coupling between voltage sensor movements and channel opening to various degrees, indicating that they alter the conformation and movements of the voltage sensor and the activation gate. Similarly, the ability of the β1-subunit to enhance apparent Ca²⁺ sensitivity is diminished to various degrees, correlating quantitatively with the shift of voltage dependence of voltage sensor movements. In contrast, none of these mutations significantly reduces the ability of the β2-subunit to enhance Ca²⁺ sensitivity. These results suggest that the β1-subunit enhances Ca²⁺ sensitivity by altering the conformation and movements of the voltage sensor, whereas the similar function of the β2-subunit is governed by a distinct mechanism.