This region, sometimes called the S1 domain, is the luminal domain just upstream of the first, M1, transmembrane region of transmembrane ion-channel proteins, and it binds L-glutamate and glycine [2]. It is found in association with Lig_chan, Pfam:PF00060.

1: Molecular characterization of the family of the N-methyl-D-aspartate receptor subunits. Ishii T, Moriyoshi K, Sugihara H, Sakurada K, Kadotani H, Yokoi M, Akazawa C, Shigemoto R, Mizuno N, Masu M, et al.; J Biol Chem. 1993;268:2836-2843. PMID:8428958

2: Subunit- and site-specific pharmacology of the NMDA receptor channel. Yamakura T, Shimoji K; Prog Neurobiol. 1999;59:279-298. PMID:10465381