Composite structure and ligand binding of the NMDAR ATD. (A) The isolated GluN2B ATD (PDB: 3JPW) adopts a clamshell-like structure consisting of an upper (R1, dark red) and lower (R2, pink) lobe. Zinc (purple) binds near the hinge between the two lobes and allosterically inhibits GluN2A, and to a lesser extent GluN2B, receptors. (B) Contrary to early work, the allosteric antagonist ifenprodil (yellow) binds between the GluN1b/GluN2B ATD subunits (blues, R1 and R2 lobes, and reds, R1 and R2 lobes, respectively) stabilizing dimer formation (PDB: 3QEL). While the R1 lobes of the GluN1b and GluN2B ATDs are closely apposed, there is practically no interaction between the R2 lobes in the dimer. Conversely, the AMPAR ATD homodimer (PDB: 3H5V) exhibits extensive interactions between both R1 and R2 lobes.