The N-methyl-D-aspartate (NMDA) receptor is a ligand-gated ion channel that requires both glutamate and glycine for efficient activation. Here, a strategy combining cysteine scanning mutagenesis and affinity labeling was used to investigate the glycine binding site located on the NR1 subunit. Based on homology modeling to the crystal structure of the glutamate binding site of the 2-amino-3-(3-hydroxy-5-methyl-4-isoxazolyl)-propionic acid receptor GluR2, cysteines were introduced into the NR1 subunit as chemical sensors for three thiolreactive derivatives of the competitive antagonist L-701,324. After coexpressing the mutant NR1 with wildtype NR2B subunits in Xenopus oocytes, agonist-induced currents were recorded to monitor irreversible receptor inactivation by the reactive antagonists. For each derivative, glycine site-specific inactivations were observed with a distinct subset of cysteine-substituted receptors. Together these inactivating substitutions identified seven NR1 residues (Ile-385, Gln-387, Glu-388, Thr-500, Asn-502, Ala-696, and Val-717) that undergo proximity-induced covalent coupling with specific regions of the bound antagonist and disclose its mode of docking in the glycine binding pocket of the NMDA receptor. Our approach may help to unravel the structural basis of distinct NMDA receptor subtype pharmacologies.The N-methyl-D-aspartate (NMDA) 1 subtype of glutamate receptors is a ligand-gated cation channel characterized by a high Ca 2ϩ /Na ϩ permeability ratio (1, 2). It requires both glutamate and the co-agonist glycine for efficient channel gating (3, 4) and is composed of two NR1 and two NR2 subunits, each (5). The NR1 subunit binds glycine with submicromolar affinity, whereas the NR2 subunit harbors the glutamate binding site (6, 7). Various splice variants of the NR1 subunit and four isoforms of the NR2 subunit (A-D) generate different subtypes of tetrameric NMDA receptors, which differ in their pharmacologies (8) and expression patterns (9, 10).NMDA receptors are crucially implicated in synaptic plasticity and memory formation as well as in many neurological and psychiatric disorders (11). Classic NMDA receptor antagonists have therefore been examined for their therapeutic potential; in most cases, however, severe side-effects (reviewed by Kornhuber and Weller, Ref. 12), have prevented their clinical use. An alternative strategy focuses on glycine site antagonists that display considerable subtype selectivity (13). Structureactivity studies have defined a pharmacophore that delineates different ligand interactions within the glycine binding site of NMDA receptors (reviewed in Ref. 14). However, the structure of this site and the amino acid residues responsible for these interactions have only been deduced by indirect approaches.Mutational analysis has revealed residues whose substitution reduces the efficacies of glycine site agonists and antagonists (15)(16)(17)(18)(19). Notably, these residues are located within ϳ200 positions N-terminal to the first putative transmembrane domain (segme...