Figure 3.

Divalent cation binding and apo structure of NCX_Mj. (a) A single Sr²⁺ (dark blue sphere) binds at S_Ca in crystals titrated with 10 mM Sr²⁺ and 2.5 mM Na⁺ (see also Supplementary Fig. 2). Residues involved in Sr²⁺ coordination are labeled. There are no significant changes in the side-chains involved in ion coordination, relative to the Na⁺-bound state. T50 and T209 (labeled in red) coordinate Sr²⁺ through their backbone carbonyl-oxygen atoms. High Na⁺ concentration (100 mM) completely displaces Sr²⁺ and reverts NCX_Mj to the occluded state (right panel). The contour level of the F_o – F_c omit map of the structure at high Na⁺ concentration was lowered (to 4σ) so as to visualize the density from Na⁺ ions and H₂O. (b) Ca²⁺ (tanned spheres) binds either to S_Ca or S_mid in crystals titrated with 10 mM Ca²⁺ and 2.5 mM Na⁺ (see also Supplementary Fig. 2). The relative occupancies are 55% and 45%, respectively. (c) Superimposition of NCX_Mj structures obtained at low Na⁺ concentration (10 mM) and pH 6.5 (brown) and in the absence of Na⁺ and pH 4 (light green), referred to as apo state. (d) Close-up view of the ion-binding sites in the apo (or high H⁺) state. The side chains of E54 and E213 from the low Na⁺ structure are also shown (light brown) for comparison. White spheres indicate the location S_int, S_mid S_Ca. (e) Extracellular solvent accessibility of the ion-binding sites in apo NCX_Mj.