Modes of Operation of the BK(Ca) Channel β(2) Subunit

The β(2) subunit of the large conductance Ca(2+)- and voltage-activated K(+) channel (BK(Ca)) modulates a number of channel functions, such as the apparent Ca(2+)/voltage sensitivity, pharmacological and kinetic properties of the channel. In addition, the N terminus of the β(2) 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 β(2) subunit. We found that the leftward shift on the voltage axis of channel activation curves (G(V)) produced by coexpression with β(2) 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 β(2) 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 β(2) subunit. The experimental observations are consistent with the view that the β(2) subunit of BK(Ca) channels facilitates channel activation by changing the voltage sensor equilibrium and that the β(2)-induced inactivation process does not follow a typical N-type mechanism.