(A) Diagram of the BK channel-gating scheme. Both voltage (R↔A) and Ca²⁺ (X↔X·Ca²⁺) can activate the channels (C→O). H⁺ modulates the Ca²⁺-binding transition (X↔X·Ca²⁺) via the RCK1 domain. (B) With substitution into the 314 position by amino acids with ionizable side chains, charging the 314 side chains could bias the C↔O transition toward O (open). H⁺ is also involved in this process (X314, neutral↔charged). (C) In X314:H365R:H394R channels, H⁺ no longer affects the gating process via the RCK1-mediated mechanism.