Subunit arrangement in N-methyl-D-aspartate (NMDA) receptors.

N-Methyl-d-aspartate (NMDA) receptors, the main mediators of excitatory synaptic transmission, are heterotetrameric receptors. Typically, glycine binding NR1 subunits co-assemble with glutamate binding NR2 subunits to form a functional receptor. Here we have used luminescence resonance energy transfer (LRET) investigations to establish the specific configuration in which these subunits assemble to form the functional tetramer and show that the dimer of dimers structure is formed by the NR1 subunits assembling diagonally to each other. The distances measured by LRET are consistent with the NMDA structure predicted based on cross-linking investigations and on the structure of the full-length alpha-amino-5-methyl-3-hydroxy-4-isoxazole propionic acid (AMPA) receptor structure (1). Additionally, the LRET distances between the NR1 and NR2A subunits within a dimer measured in the desensitized state of the receptor are longer than the distances in the previously published crystal structure of the isolated ligand binding domain of NR1-NR2A. Because the dimer interface in the isolated ligand binding domain crystallizes in the open channel structure, the longer LRET distances would be consistent with the decoupling of the dimer interface in the desensitized state. This is similar to what has been previously observed for the AMPA subtype of the ionotropic glutamate receptors, suggesting a similar mechanism for desensitization in the two subtypes of the glutamate receptor.