A charged residue in S4 regulates coupling among the activation gate, voltage, and Ca²⁺ sensors in BK channels

Coupling between the activation gate and sensors of physiological stimuli during ion channel activation is an important, but not well-understood, molecular process. One difficulty in studying sensor–gate coupling is to distinguish whether a structural perturbation alters the function of the sensor, the gate, or their coupling. BK channels are activated by membrane voltage and intracellular Ca²⁺ via allosteric mechanisms with coupling among the activation gate and sensors quantitatively defined, providing an excellent model system for studying sensor–gate coupling. By studying BK channels expressed in Xenopus oocytes, here we show that mutation E219R in S4 alters channel function by two independent mechanisms: one is to change voltage sensor activation, shifting voltage dependence, and increase valence of gating charge movements; the other is to regulate coupling among the activation gate, voltage sensor, and Ca²⁺ binding via electrostatic interactions with E321/E324 located in the cytosolic side of S6 in a neighboring subunit, resulting in a shift of the voltage dependence of channel opening and increased Ca²⁺ sensitivity. These results suggest a structural arrangement of the inner pore of BK channels differing from that in other voltage gated channels.