Fig. 5.

The conformational changes in SthK upon activation. a Left: SthK in the resting state (cartoon based on the high-resolution cryo-EM structure, PDB 6CJQ). Right: Model of SthK in the activated state: Upon activation, the CNBDs rotate by ~25° clockwise (when viewed from the intracellular side) and move by ~6 Å towards the membrane and by ~4 Å outwards from the four-fold axis (note, the activated state is a cartoon assembled using domains of the SthK structure repositioned according to the displacements found with HS-AFM). Key residues in and around S3 that are evidenced to undergo conformational changes are highlighted in white. b Close-up view onto the SthK C-linker (yellow) from the intracellular side in the resting state. The CNBDs are attached at the periphery to the C-linker disk with a radius ~3 nm from the central axis. Rotation of the CNBDs by ~25° clockwise induces a torque that is transmitted to the CNBD-S6 attachment. c Clockwise rotation of the C-linker will pull the right-handed S6 bundle-crossing open like a diaphragm. d Simple mechanical model where the C-linker is a type-2 lever with two attachment points to the S6 helix and to the CNBD. (S6) and (CNBD) indicate the respective attachment sites, r(S6) and r(CNBD) are the radial distance from the central axis, F(S6) and F(CNBD) the forces acting on the attachment sites, and ∆x(S6) and ∆x(C-linker) are the displacements, of the respective domains