Mechanism of magnesium activation of calcium-activated potassium channels

Large-conductance (BK type) Ca²⁺-dependent K⁺ channels are essential for modulating muscle contraction and neuronal activities such as synaptic transmission and hearing^1-5. BK channels are activated by membrane depolarization and intracellular Ca²⁺ and Mg²⁺ (refs 6-10). The energy provided by voltage, Ca²⁺ and Mg²⁺ binding are additive in activating the channel, suggesting that these signals open the activation gate through independent pathways^{9, 11}. Here we report a molecular investigation of a Mg²⁺- dependent activation mechanism. Using a combined site-directed mutagenesis and structural analysis, we demonstrate that a structurally new Mg²⁺-binding site in the RCK/Rossman fold domain-an intracellular structural motif that immediately follows the activation gate S6 helix^12-15-is responsible for Mg²⁺-dependent activation. Mutations that impair or abolish Mg²⁺ sensitivity do not affect Ca²⁺ sensitivity, and vice versa. These results indicate distinct structural pathways for Mg²⁺- and Ca²⁺- dependent activation and suggest a possible mechanism for the coupling between Mg²⁺ binding and channel opening.