Figure 5:

X-ray crystal structure of the KdpFABC complex in an E₁-like conformation (PDB accession code 5MRW). A: Overview of the structure, which had a resolution of 2.9 Å. The KdpF subunit (cyan) consists in a single membrane spanning helix at the interface of KdpA (green) and KdpB (transmembrane helices brown, P domain blue, A domain yellow, N domain red). KdpC (purple) consists of a single transmembrane helix and a periplasmic domain near the entrance to the selectivity filter of KdpA. B: A cavity (blue surface) runs between the K⁺ ion (purple sphere) in the selectivity filter of KdpA and the water molecule in the canonical ion binding site in KdpB (occupied by a water molecule, red sphere). The cavity has a diameter ≥ 1.4 Å, which is consistent with its occupancy by water.