Figure 4. High resolution crystal structures of the Li⁺ and Na⁺ complexes.

(A) Stereo view of the GluR5 kainate complex with Li⁺ at 1.49 Å resolution. Electron density for the pair of Li⁺ ions is shown for an Fo-Fc omit map (pink) contoured at 3.2 σ, with Li⁺ omitted from the Fc calculation. Electron density for the protein (gray), water molecules (blue), and the Cl⁻ ion (green) is shown for a 2mFo-DFc map contoured at 1.5 σ.

(B) Stereo view of the Na⁺ complex looking down the molecular two-fold axis of the dimer assembly onto the plane of the membrane; transparent spheres for the bound ions are drawn using Shannon radii; the Na⁺ and Cl⁻ ions are linked in a network formed by the side chains of Glu509 and Lys516. Numerous salt bridges and hydrogen bonds link the subunits together.

(C) Crystal structure of the GluR5 kainate complex with Li⁺ in the cation binding site showing tetrahedral coordination of the ion by the side chains of Glu509, Asp 513, the main chain carbonyl oxygen of Glu509, and a H₂O molecule, with bond distances in Å.

(D) Crystal structure of the GluR5 kainate complex at 1.72 Å resolution with Na⁺ in the cation binding site showing 5-fold coordination of the Na⁺ ion, and block of the 6^th coordination site by the side chain of Ile755.