Mobility of transport domains helices. (a) Superimposition of transport domains from Data S1. A1 is salmon, A3 is teal, A6 is yellow, and A7 is magenta. (b) Superimposition of crystal structures of substrate-bound (PDB accession no. 2NWX; light yellow), TBOA-bound (PDB accession no. 2NWW; dark blue), and Na⁺-bound (PDB accession no. 7AHK; maroon) Glt_Ph. The domains were superimposed on HP1 and TM7a (residues 258–309). The views are from the intracellular (left) and extracellular (right) sides of the transport domain. (c) Cartoon representation of the transport domain resolved in the equilibrated Data S2 after refinement in C3. Resolved waters are shown as blue spheres. Sodium ions are purple spheres, the substrate is green, HP1 is yellow, and HP2 is red.