Fig. 3: Conformational differences of the Glt_Tk protomers.

a Comparison of the holo (Asp) Glt_Tk cryo-EM (cornflower blue) and crystal (PDB 5E9S, light orange)¹² structures (superposition using scaffold domains (yellow)) demonstrates differences between the fully outward and intermediate outward states. The panel below shows a slice through the transport domains in surface representation with aspartate molecules shown as sticks. The arrows indicate the movement of the transport domain from the fully-outward to intermediate-outward position. b Superposition on the scaffold domains of the cryo-EM structures of Glt_Tk- holo (Asp) (cornflower blue) and Glt_Tk-apo (cyan), which are both in intermediate-outward conformations. The lower panel shows the transport domains rotated 50° relative to the upper panel to highlight structural differences. c Superposition on the scaffold domains of cryo-EM structure of Glt_Tk-TBOA (dark blue) and crystal structure Glt_Tk-Asp (PDB 5E9S, light orange). The lower panel shows the transport domains rotated 40° relative to the upper panel with an arrow indicating opening of the HP2 gate. d Superposition of the cryo-EM structure of Glt_Tk-Na⁺-only (light blue), the cryo-EM structure of ASCT2 (PDB 6RVX, light green)¹⁷, and a crystal structure of Glt_Ph-Asp in the inward-oriented state (PDB 3KBC, pink)¹⁰. The lower panel highlights opening of the HP2 gate in the inward-oriented state. Superpositions on TM2 and TM5 of the scaffold domain.