Figure 6. Working hypothesis of elementary steps in MthK gating ring activation.

A, RCK domains undergo transitions from inactive (a) to partially-activated/singly-liganded (b / b*), then to fully-liganded/activated (c*). Detailed explanation provided in Discussion. B, Aligned and overlayed unliganded (1), Ba²⁺-bound (3) and Ca²⁺-bound gating rings (4). C, Expanded view of the region indicated by black dashed box in B. Spheres within the dashed box indicate the Cα of residue R116, which in turn would be linked to the transmembrane pore. Changes in position of R116 may indicate RCK domain movements coupled to channel opening. D, Expanded view of the boxed region from C. Dashed arrows indicate the sequence a→b*→c*. E, Aligned and overlayed structures, viewed normal to the plane of the membrane. F, Expanded view of the boxed region from E, indicating the sequence a→b*→c*. See also Figure S4 and Table S1.