Selective Cation Binding to the Gating-Ring Triggers Independent RCK Motions in the BK Channel

Abstract

Large conductance voltage and calcium dependent potassium channels (BK channels) are important controllers of cell excitability. Functional BK channels are formed as tetramers of alpha subunits. Their open probability is regulated synergistically by changes in transmembrane voltage and intracellular concentration of divalent cations including Ca2+, Mg2+, Cd2+ and Ba2+. The voltage sensor resides within the transmembrane region of the alpha subunit, whereas a large C-terminal intracellular region called ‘gating ring’ senses divalent cation binding. The gating ring assembles as a large tetrameric structure that contains a total of eight Regulator of Conductance of K (RCK) domains, two from each subunit. In each RCK domain resides one high-affinity Ca2+ binding site. Even though the two RCK domains of each subunit (RCK1 and RCK2) are similar, their Ca2+ binding sites are not, particularly in their selectivity to divalent ions other than Ca2+. For example, Cd2+ effect is mediated exclusively through the RCK1 domains, while Ba2+ and Mg2+ (at low concentrations) prefer solely the site from the RCK2 domains. Using patch clamp fluorometry, we have shown previously that large rearrangements of the gating-ring occur in response to Ca2+ and voltage. By themselves, Cd2+, Mg2+ and Ba2+ induce movements of the gating-ring that are substantially smaller than those triggered by Ca2+. Full extent motions are achieved once binding sites from RCK1 and RCK2 domains are occupied by Cd2+ and Ba2+, or Cd2+ and Mg2+. These results suggest that the gating ring structure is flexible, potentially allowing independent movements of the RCK1 and RCK2 domains.