Voltage Sensor Activation Facilitates Magnesium-Gated Channel Opening in BK Channels

Abstract

Under physiological conditions, Mg2+ is an intracellular activator of Ca2+- and voltage-activated potassium (BK) channels. To investigate gating by Mg2+ acting through its low affinity site located under each S4 voltage sensor (Yang et al. 2007, 2008; Horrigan & Ma 2008), we studied a BK channel in which the high affinity Ca2+ sites in both the RCK1 domain and the calcium bowl were disabled by mutation. Using single-channel recording from inside-out patches to measure channel activation after voltage steps from −100 mV to +100 mV, we found that 10 mM Mg2+ reduces the latency to first opening after the voltage step and increases channel activation through an increase in the number of openings per burst and mean open duration. This suggests that Mg2+ can bind to both closed and open states of the channel, but it is not clear whether the closed-state binding occurs at the negative or positive potential. Therefore we recorded single-channel activity in macro-patches held at a constant −50 or −100 mV. At −50 mV, when the voltage sensors are occasionally activated, 10 mM Mg2+ decreases the duration of the closed intervals between bursts of activity and increases burst duration through an increase in the number of openings per burst and mean open duration. At −100 mV, when the voltages sensors are mainly deactivated, 10 mM Mg2+ has little effect on the closed intervals between bursts or mean open duration, and most of the bursts are unitary, consisting of a single brief opening. The above data are consistent with a model in which Mg2+ can bind to the BK channel in the closed conformation when the voltage sensors are activated. The bound Mg2+ then facilitates opening. Supported by NIH grant AR32805.