Cloning of a BrainN-Methyl-d-Aspartate- andd,l-ϵ-2-Amino-4-propyl-5-phosphono-3-pentanoic Acid (CGP 39653)-Binding Protein

Abstract A complex of four proteins isolated from neuronal membranes has ligand binding sites for N -methyl- d -aspartate (NMDA) receptor agonists and antagonists and forms NMDA-activated ion channels upon reconstitution into lipid membranes. In this study, the cDNA of a subunit of this complex containing binding sites for the competitive antagonists of NMDA receptors was cloned. The cDNA clone coded for a protein of 719 amino acids (78.9 kDa). The expressed protein had binding activity for the agonists l -[ 3 H]glutamate and [ 3 H]glycine, the antagonist (±)-[ 3 H]-( E )-2-amino-4-propyl-5-phosphonopentanoic acid ([ 3 H]CGP 39653), but not the ion channel inhibitors. The cloned cDNA had no homology to other cloned cDNAs. Northern blot analyses indicated high expression of an 3.8 kb poly(A + ) RNA in brain, but not in other tissues. These findings indicate that proteins that have recognition sites for NMDA receptor activators and inhibitors and that differ from the well-characterized NMDA receptor proteins NR1-3 are expressed in mammalian brain.