Model of the Glt_Ph transport cycle for a monomer based on available crystal structures and MD simulations on the binding and release order of the ligands. Protein data bank (PDB) codes are in parentheses. The helical HP structure in red is HP2 and the blue one is HP1. The purple circles represent Na⁺ ions binding at Na1, Na2 and Na3. The grey ellipse represents aspartate. Starting from the upper left corner, in the outward-facing apo structure, Na⁺ ion binding at Na1 triggers structural changes in the transport domain and HP2, which opens the aspartate and Na2 sites to conformations similar to that in the holo transporter [30]. After aspartate and Na⁺ ion bind to their corresponding binding sites, there is a further, unknown conformational change linked to the binding of Na3 before movement across the membrane. Once the transport domain reaches the intracellular side, through opening of the intercellular gate, the substrates release into cytoplasm. The transport domain stays compacted with collapsed ligand-binding sites, which make it suitable for TM movement, thus completing the transport cycle [30]. There are as yet no experimental data on the position of the third Na⁺ ion-binding site or the binding order of the ligands.