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.