The Mg²⁺-binding site. (A) High-resolution view of the metal-free structure in the vicinity of the Mg²⁺-binding site. Alignment was based on the S0′ segment (Tao et al., 2017) which is part of the linker between the S0 and S1 TM segments. (B) In the Mg²⁺ liganded structure, side chains of aN161 (mN172), aE328 (mV339), aE363 (mE374), and aE388 (mE399), along with the backbone carbonyl of T385 (mT396), appear to contribute to Mg²⁺ coordination, whereas residues aD86 (mD99) and aK331 (mR342) form a salt bridge that may help stabilize the bound Mg²⁺ structure. Dotted green lines reflect cytosolic loops not defined in the structure.