Identification of amino acids in the N-methyl-D-aspartate receptor NR1 subunit that contribute to the glycine binding site.

The mammalian N-methyl-D-aspartate (NMDA) receptor complex is though to consist of an NR1 subunit in combination with one or more of the four NR2 subunits (A, B, C, and D). When corresponding cDNAs are expressed in Xenopus oocytes, ion channels with the characteristic profile of NMDA receptors are formed. The receptor is unique in requiring two coagonists, glutamate and glycine, for activation of the channel. We have used site-directed mutagenesis to study amino acids in the human NR1 subunit that contribute to the glycine binding site of the NMDA receptor without affecting the agonist site for glutamate. Mutations to D481 and K483 produced receptors with up to 160-fold lower affinities for glycine, as well as other agonists and partial agonists, without affecting maximum current size or the degree of agonist efficacy. The D481A mutation also led to 40-50-fold lower affinities for two structurally diverse glycine site antagonists. From these data we propose that the carboxyl group of this aspartate interacts with the amino moiety of glycine and the equivalent group contained in other agonists and antagonists.