Glycine-glutamate interactions at the NMDA receptor: role of cysteine residues.

The NMDA subtype of ionotropic glutamate receptors occupation by both L-glutamate and the co-agonist glycine for efficient channel opening. To elucidate the role of disulfide bridges for the allosteric interaction of these agonists we mutated the cysteine residues in the ligand-binding NMDAR1 (NR1 or zeta) subunit of the rodent NMDA receptor and co-expressed the resulting mutants with the NR2B (epsilon 2) subunit in Xenopus oocytes. Most of the cysteine substitutions had no effect on agonist responses. However, replacement of cysteines 402 and 418 by alanine largely abolished the potentiation of glutamate currents by glycine. These cysteine residues in the putative extracellular domain of the NR1 subunit may form a disulfide bridge important for agonist interaction.