Figure 8. Gating model for SthK channels.

(A) Two adjacent subunits of SthK (green and blue) illustrating the molecular motions needed to transition from the resting to the ‘activated’ CNBD conformation seen in the crystal structure. The numbers and arrows are used to describe the motions and directionality. (B) Specific interactions between the C-linker and the S4-S5-linker from the adjacent subunit indicated in the dashed circle in A. Left: same orientation as in A; right: rotated by 180°. Potential interactions between protein regions are shown with dotted lines. (C) Overlay between apo SthK (green) and the open TAX-4 (red) structures showing the upward displacement of the C-linker and the CNBD (especially the C-helix) during channel activation. Only two opposite subunits are shown for clarity. (D) The overlay of the same two structures viewed from the extracellular side showing only the C-linker/CNBD and two helical turns of S6 reveals an additional counterclockwise rotation of the C-linker that pulls on the ends of the S6 helices resulting in enlargement of the base of the pore. (E) Cartoon of two gating models. Blue squares represent the resting and blue circles the activated states. The MWC-model (top) proposes concerted conformational changes from closed to open states (all squares become all circles). The red dot is the ligand. Dashed green circles highlight the resting, closed SthK states solved with cryo-EM and presented here. Dashed red circle indicates the open, activated TAX-4-like state. The KNF model (bottom) proposes sequential changes in individual subunits as ligands bind on the path towards open states (one square becomes a circle when red ligand binds and an octagon when green ligand binds). (F) Cartoon of activation depicting domain rearrangements from the resting/closed state (left) to the activated/open state (right). Colors as in Figure 2B. Curved arrows indicate the C-linker rotation shown in D.