Modes of Operation of the BK_Ca Channel β₂ Subunit

The β₂ subunit of the large conductance Ca²⁺- and voltage-activated K⁺ channel (BK_Ca) modulates a number of channel functions, such as the apparent Ca²⁺/voltage sensitivity, pharmacological and kinetic properties of the channel. In addition, the N terminus of the β₂ subunit acts as an inactivating particle that produces a relatively fast inactivation of the ionic conductance. Applying voltage clamp fluorometry to fluorescently labeled human BK_Ca channels (hSlo), we have investigated the mechanisms of operation of the β₂ subunit. We found that the leftward shift on the voltage axis of channel activation curves (G(V)) produced by coexpression with β₂ subunits is associated with a shift in the same direction of the fluorescence vs. voltage curves (F(V)), which are reporting the voltage dependence of the main voltage-sensing region of hSlo (S4-transmembrane domain). In addition, we investigated the inactivating mechanism of the β₂ subunits by comparing its properties with the ones of the typical N-type inactivation process of Shaker channel. While fluorescence recordings from the inactivated Shaker channels revealed the immobilization of the S4 segments in the active conformation, we did not observe a similar feature in BK_Ca channels coexpressed with the β₂ subunit. The experimental observations are consistent with the view that the β₂ subunit of BK_Ca channels facilitates channel activation by changing the voltage sensor equilibrium and that the β₂-induced inactivation process does not follow a typical N-type mechanism.