(a) Trimer of GltPh viewed from the extracellular side in the outward-facing state. Each monomer, in cartoon, is coloured differently. (b) Cartoon representation of a monomer of GltPh shown parallel to the membrane in the outward-facing state with aspartate and two Na+ ions bound. The TM helices and re-entrant loops are labelled. The substrate is shown as stick and the two Na+ ions are shown as purple spheres. The black dashed lines between TM3 and TM4a represent the loop connecting the helices. (c) View of the Na+-binding sites. (d) View of the aspartate-binding site. Dashed lines show the hydrogen bonds between amino acid residues and Na+ or aspartate.