The β Subunit Increases the Ca²⁺ Sensitivity of Large Conductance Ca²⁺-activated Potassium Channels by Retaining the Gating in the Bursting States

Coexpression of the β subunit (K_V,_Caβ) with the α subunit of mammalian large conductance Ca²⁺- activated K⁺ (BK) channels greatly increases the apparent Ca²⁺ sensitivity of the channel. Using single-channel analysis to investigate the mechanism for this increase, we found that the β subunit increased open probability (P_o) by increasing burst duration 20–100-fold, while having little effect on the durations of the gaps (closed intervals) between bursts or on the numbers of detected open and closed states entered during gating. The effect of the β subunit was not equivalent to raising intracellular Ca²⁺ in the absence of the beta subunit, suggesting that the β subunit does not act by increasing all the Ca²⁺ binding rates proportionally. The β subunit also inhibited transitions to subconductance levels. It is the retention of the BK channel in the bursting states by the β subunit that increases the apparent Ca²⁺ sensitivity of the channel. In the presence of the β subunit, each burst of openings is greatly amplified in duration through increases in both the numbers of openings per burst and in the mean open times. Native BK channels from cultured rat skeletal muscle were found to have bursting kinetics similar to channels expressed from alpha subunits alone.