Figure 1.

Na⁺ binding to outward-facing NCX_Mj. (a) Overall structure of native outward-facing NCX_Mj from crystals grown in 150 mM Na⁺. N- and C-terminal halves are colored yellow and cyan, respectively. Colored spheres represent the bound Na⁺ (green) and water (red). (b) Structural details and definition of the four central binding sites. Only residues flanking these sites are shown for clarity (same for all other figures). The electron density (grey mesh, 1.9 Å F_o-F_c ion omit map contoured at 4σ) at S_mid was modeled as water (red sphere) and those at S_ext, S_Ca and S_int as Na⁺ ions (green spheres). Further details are shown in Supplementary Fig. 1. (c) Concentration-dependent change in Na⁺ occupancy (see also Table 1). All F_o – F_c ion-omit maps are calculated to 2.4 Å and contoured at 3σ for comparison. The displacement of A206 reflects the [Na⁺]-dependent conformational change from the partially open to the occluded state (observed at low and high Na⁺ concentrations, respectively). At 20 mM Na⁺, both conformations co-exist. No significant changes were observed in the side-chains involved in ion or water coordination at the S_Ca, S_int and S_mid sites.