AN ALANINE RESIDUE IN THE M3-M4 LINKER LINES THE GLYCINE BINDING POCKET OF THE N-METHYL-D-ASPARTATE RECEPTOR

While attempting to map a central region in the M3-M4 linker of the N- methyl-D-aspartate receptor NR1 subunit, we found that mutation of a s ingle position, Ala-714, greatly reduced the apparent affinity for gly cine, Proximal N-glycosylation localized this region to the extracellu lar space. Glycine affinities of additional Ala-714 mutations correlat ed with side chain volume. Substitution of alanine 714 with cysteine d id not alter glycine sensitivity, although this mutant was rapidly inh ibited by dithionitrobenzoate. Glycine protected the A714C mutant from modification by dithionitrobenzoate, whereas the co-agonist L-glutama te was ineffective. These experiments place Ala-714 in the glycine bin ding pocket of the N-methyl-D-aspartate receptor, a determination not predicted by previous structural models based on bacterial periplasmic binding protein homology.