Mg2+-dependent regulation of BK channels: importance of electrostatics

Mg(2+) binding to open and closed states can activate BK channels provided that the voltage sensors are elevated

BK channels are activated by intracellular Ca²⁺ and Mg²⁺ as well as by depolarization. Such activation is possible because each of the four subunits has two high-affinity Ca²⁺ sites, one low-affinity Mg²⁺ site, and a voltage sensor. This study further investigates the mechanism of Mg²⁺ activation by using single-channel recording to determine separately the action of Mg²⁺ on the open and closed states of the channel. To limit Mg²⁺ action to the Mg²⁺ sites, the two high-affinity Ca²⁺ sites are disabled by mutation. When the voltage is stepped from negative holding potentials to +100 mV, we find that 10 mM Mg²⁺ decreases the mean closed latency to the first channel opening 2.1-fold, decreases the mean closed interval duration 8.7-fold, increases mean burst duration 10.1-fold, increases the number of openings per burst 4.4-fold, and increases mean open interval duration 2.3-fold. Hence, Mg²⁺ can bind to closed BK channels, increasing their opening rates, and to open BK channels, decreasing their closing rates. To explore the relationship between Mg²⁺ action and voltage sensor activation, we record single-channel activity in macropatches containing hundreds of channels. Open probability (P_o) is dramatically increased by 10 mM Mg²⁺ when voltage sensors are activated with either depolarization or the mutation R210C. The increased P_o arises from large decreases in mean closed interval durations and moderate increases in mean open interval durations. In contrast, 10 mM Mg²⁺ has no detectable effects on P_o or interval durations when voltage sensors are deactivated with very negative potentials or the mutation R167E. These observations are consistent with a model in which Mg²⁺ can bind to and alter the gating of both closed and open states to increase P_o, provided that one or more voltage sensors are activated.

Molecular mechanisms of BK channel activation

Large conductance, Ca(2+)-activated potassium (BK) channels are widely expressed throughout the animal kingdom and play important roles in many physiological processes, such as muscle contraction, neural transmission and hearing. These physiological roles derive from the ability of BK channels to be synergistically activated by membrane voltage, intracellular Ca(2+) and other ligands. Similar to voltage-gated K(+) channels, BK channels possess a pore-gate domain (S5-S6 transmembrane segments) and a voltage-sensor domain (S1-S4). In addition, BK channels contain a large cytoplasmic C-terminal domain that serves as the primary ligand sensor. The voltage sensor and the ligand sensor allosterically control K(+) flux through the pore-gate domain in response to various stimuli, thereby linking cellular metabolism and membrane excitability. This review summarizes the current understanding of these structural domains and their mutual interactions in voltage-, Ca(2+)- and Mg(2+)-dependent activation of the channel.