Coupling of Ca2+ and voltage activation in BK channels through the αB helix/voltage sensor interface

Abstract

Large conductance Ca2+ and voltage activated K+ (BK) channels control membrane excitability in many cell types. BK channels are tetrameric. Each subunit is comprised of a voltage sensor domain (VSD), a central pore gate domain, and a large cytoplasmic domain (CTD) that contains the Ca2+ sensors. While it is known that BK channels are activated by voltage and Ca2+, and that voltage and Ca2+ activations interact, less is known about the mechanisms involved. We now explore mechanism by examining the gating contribution of an interface formed between the VSDs and the B helices located at the top of the CTDs. Proline mutations in the B helix greatly decreased voltage activation while having negligible effects on gating currents. Analysis with the HCA model indicated a decreased coupling between voltage sensors and pore gate. Proline mutations decreased Ca2+ activation for both Ca2+ bowl and RCK1 Ca2+ sites, suggesting that both high affinity Ca2+ sites transduce their effect, at least in part, through the B helix. Mg2+ activation was also decreased. The crystal structure of the CTD with proline mutation L390P showed a flattening of the first helical turn in the B helix compared to WT, without other notable differences in the CTD, indicating structural change from the mutation was confined to the B helix. These findings indicate that an intact B helix/VSD interface is required for effective coupling of Ca2+ binding and voltage depolarization to pore opening, and that shared Ca2+ and voltage transduction pathways involving the B helix may be involved. SignificanceLarge conductance BK (Slo1) K+ channels are activated by voltage, Ca2+, and Mg2+ to modulate membrane excitability in neurons, muscle, and other cells. BK channels are of modular design, with pore-gate and voltage sensors as transmembrane domains and a large cytoplasmic domain CTD containing the Ca2+ sensors. Previous observations suggest that voltage and Ca2+ sensors interact, but less is known about this interaction and its involvement in the gating process. We show that a previously identified structural interface between the CTD and voltage sensors is required for effective activation by both voltage and Ca2+, suggesting that these processes may share common allosteric activation pathways. Such knowledge should help explain disease processes associated with BK channel dysfunction.