Figure 2: The architecture of MloK1.

(a) Cross-section through the superimposed cryo-EM maps; the regions of interest (ROI) indicated by the frames are shown in (b–e). MloK1 with cAMP: mesh, blue; MloK1 without cAMP: solid, yellow. The K⁺ ions (purple) in a as well as the ribbon models shown in b–e are from the X-ray structure (PDB ID 2ZD9, purple) and fitted to the cryo-EM map of MloK1 without ligand. (b) Detailed view of the VSDs shows that the X-ray structure agrees with the densities of the cryo-EM maps of MloK1. The tilt of helix S2 with respect to the X-ray structure is evident. (c) Detailed view of the selectivity filter region. The ion densities determined in both cryo-EM maps (red dashed ellipse) are in the selectivity filter region; their positions correspond well to the X-ray structure. (d) Detailed view of the intracellular pore permeation pathway region. Both maps of MloK1 show a significantly wider opening at the helix bundle crossing than the X-ray structure. Residues that form the inner helix bundle crossing (A207) and a constriction region from the X-ray structure (Y215)¹⁸ are shown as sticks for the X-ray structure. These are clearly outside the cryo-EM maps. (e) Detailed view of the CNBD–VSD interaction region. Upon binding of cAMP, the CNBD undergoes conformational changes resulting in a 3-Å shift towards the lipid bilayer (orange arrows), establishing an interaction of the CNBDs with helix S1 of the same monomer (blue arrow) and helix S3 (red arrow) of the adjacent monomer.