Crystal structures of the GluR2 ligand binding core (S1S2) have been determined in the apo state and in the presence of the antagonist DNQX, the partial agonist kainate, and the full agonists AMPA and glutamate. The domains of the S1S2 ligand binding core are expanded in the apo state and contract upon ligand binding with the extent of domain separation decreasing in the order of apo > DNQX > kainate > glutamate ≅ AMPA. These results suggest that agonist-induced domain closure gates the transmembrane channel and the extent of receptor activation depends upon the degree of domain closure. AMPA and glutamate also promote a 180° flip of a trans peptide bond in the ligand binding site. The crystal packing of the ligand binding cores suggests modes for subunit–subunit contact in the intact receptor and mechanisms by which allosteric effectors modulate receptor activity.
