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Interactions of divalent cations with calcium binding sites of BK channels reveal independent motions within the gating ring

Significance Large-conductance voltage- and calcium-activated K+ (BK) channels are allosterically regulated by intracellular Ca2+ and voltage. The Ca2+ sensor is a large tetrameric structure called the “gating ring.” Each subunit of this structure contains two distinct regulator of conductance of potassium (RCK) domains, each contributing one Ca2+ binding site. Both sites have a different selectivity for Ca2+, Mg2+, Cd2+, and Ba2+. Using FRET, we monitored the conformational changes of different regions of the gating ring upon binding of divalent ions. Our results reveal the existence of additive and independent motions by the two RCK domains that form the gating ring. These results indicate that the gating ring is a flexible structure capable of complex conformational changes that can be triggered specifically by different divalent ions. Large-conductance voltage- and calcium-activated K+ (BK) channels are key physiological players in muscle, nerve, and endocrine function by integrating intracellular Ca2+ and membrane voltage signals. The open probability of BK channels is regulated by the intracellular concentration of divalent cations sensed by a large structure in the BK channel called the “gating ring,” which is formed by four tandems of regulator of conductance for K+ (RCK1 and RCK2) domains. In contrast to Ca2+ that binds to both RCK domains, Mg2+, Cd2+, or Ba2+ interact preferentially with either one or the other. Interaction of cations with their binding sites causes molecular rearrangements of the gating ring, but how these motions occur remains elusive. We have assessed the separate contributions of each RCK domain to the cation-induced gating-ring structural rearrangements, using patch-clamp fluorometry. Here we show that Mg2+ and Ba2+ selectively induce structural movement of the RCK2 domain, whereas Cd2+ causes motions of RCK1, in all cases substantially smaller than those elicited by Ca2+. By combining divalent species interacting with unique sites, we demonstrate that RCK1 and RCK2 domains move independently when their specific binding sites are occupied. Moreover, binding of chemically distinct cations to both RCK domains is additive, emulating the effect of fully occupied Ca2+ binding sites.