(a) Superposition of GltTksub (yellow) and GltTkapo (gray). Bound L-Asp shown as blue sticks and sodium ions as purple spheres. Arrows represent the movements during transition from apo to the substrate-bound state. Note that parts of the protein which are not directly involved in substrate-binding (for example, TMS2, TMS5) do not undergo any noticeable changes. (b) Interaction network between L-Asp (blue sticks) and sodium ions (purple spheres) mediated by N313, M314 and N405 (shown in sticks), distances are given in Å (also see Supplementary Fig. 3). (c) M314 in substrate-bound and substrate-free states with 2fo–fc electron density countered at 3σ (left) and without density (right). In the apo state two alternative conformations of the side chain are resolved, consistent with the proposed inherent mobility of this residue.