Voltage and Ca2+ Sensor Coupling Modulation by β Subunits in the BK Channel

BK channels are modulated by β-subunits (β1-β4) in a tissue-specific manner. The β1- and β2-subunits increase in the apparent Ca2+ sensitivity of BK can be explained by a stabilization of the voltage sensor domain (VSD) in its active configuration. However, whether β1 modifies the number of charges associated with the voltage sensor activation is still a matter of controversy. The purpose of our study is to determine the effects of the presence of β-subunits on the gating charge in the presence of Ca2+, and to evaluate the coupling between Ca2+-binding and VSD activation. The number of gating charges per channel was measured in BK channels formed by α-subunit alone and with the different β-subunits in Ca2+-free internal solutions. The maximum gating charge displaced was obtained from the charge-voltage (Q-V) curve and the total number of channels in the patch was determined using noise analysis. Furthermore, we evaluated the effect of β-subunits on the interaction between Ca2+ sensors and VSD in different Ca2+ concentrations. We found that the total number of charges per channel was 4.4, 3.0 and 4.2 e0 for BKα, BKα/β1 and BKα/β3b channels respectively. Increasing intracellular [Ca2+] in BKα (100 µM) promotes a significant leftward shift (∼-140 mV) of the Q-V curve. The calcium effect on voltage sensor in BKα becomes apparent at [Ca2+] ≥ 1 µM (∼∆V −30 mV). However, the leftward shift of the Q-V curve in BKα/β1 channels becomes evident in the nanomolar [Ca2+] range (100-500 nM). BKα/β3b channels behave as BKα channels. We conclude that: a) β1-subunit not only modifies the resting-active equilibrium of the voltage sensor but also decreases the total number of apparent gating charges; b) there is a strong coupling between voltage and Ca2+ sensors, this coupling is increased in the presence of β1-subunit.