Figure 3.

Ca²⁺-dependent conformations of MthK and BK gating rings. (A) Conformations of the MthK gating ring in (left to right) unliganded, singly liganded, and fully liganded states. N termini, which are tethered to pore-lining helices of the channel, are represented by black spheres; purple spheres represent Ba²⁺ ions in the singly liganded ring, and green spheres represent Ca²⁺ ions. RCK dimers comprising the unliganded gating ring can assume two different conformations, thus breaking fourfold symmetry. The singly liganded (Ba²⁺ bound) state stabilizes a single, symmetric conformation that facilitates channel opening, and opening is further facilitated by additional liganding. In the fully liganded state, distances between all N termini are uniformly greater than in the unliganded or singly liganded states, consistent with movements that could open the transmembrane pore. (B) Conformations of the BK gating ring in (left to right) the unliganded and Ca²⁺-bound states, in which Ca²⁺ (green spheres) is bound at the Ca²⁺ bowl. As with the MthK gating ring, binding of Ca²⁺ increases distances between the RCK domain N termini, which in turn underlies channel opening.