Mg2+ enhances voltage sensor/gate coupling in BK channels

BK (Slo1) potassium channels are activated by millimolar intracellular Mg2+ as well as micromolar Ca2+ and membrane depolarization. Mg2+ and Ca2+ act in an approximately additive manner at different binding sites to shift the conductance-voltage (G(K)-V) relation, suggesting that these ligands might work through functionally similar but independent mechanisms. However, we fi nd that the mechanism of Mg2+ action is highly dependent on voltage sensor activation and therefore differs fundamentally from that of Ca2+. Evidence that Ca2+ acts independently of voltage sensor activation includes an ability to increase open probability (P-O) at extreme negative voltages where voltage sensors are in the resting state; 2 mu M Ca2+ increases PO more than 15- fold at - 120 mV. However 10 mM Mg2+, which has an effect on the G(K)-V relation similar to 2 mu M Ca2+, has no detectable effect on PO when voltage sensors are in the resting state. Gating currents are only slightly altered by Mg2+ when channels are closed, indicating that Mg2+ does not act merely to promote voltage sensor activation. Indeed, channel opening is facilitated in a voltage- independent manner by Mg2+ in a mutant (R210C) whose voltage sensors are constitutively activated. Thus, 10 mM Mg2+ increases P-O only when voltage sensors are activated, effectively strengthening the allosteric coupling of voltage sensor activation to channel opening. Increasing Mg2+ from 10 to 100 mM, to occupy very low affi nity binding sites, has additional effects on gating that more closely resemble those of Ca2+. The effects of Mg2+ on steady- state activation and I-K kinetics are discussed in terms of an allosteric gating scheme and the state-dependent interactions between Mg2+ and voltage sensor that may underlie this mechanism.