nmdarThe NMDA receptor (NMDAR) is an ionotropic receptor for glutamate (NMDA (N-methyl D-aspartate) is a name of its selective specific agonist). Activation of NMDA receptors results in the opening of an ion channel that is nonselective to cations. This allows flow of Na+ and small amounts of Ca2+ ions into the cell and K+ out of the cell.
Calcium flux through NMDARs is thought to play a critical role in synaptic plasticity, a cellular mechanism for learning and memory. The NMDA receptor is distinct in that it is both ligand-gated and voltage-dependent.
The NMDA receptor forms a heterotetramer between two NR1 and two NR2 subunits, which explains why NMDA receptors contain two obligatory NR1 subunits and two regionally localized NR2 subunits.[6] A related gene family of NR3 A and B subunits have an inhibitory effect on receptor activity. Multiple receptor isoforms with distinct brain distributions and functional properties arise by selective splicing of the NR1 transcripts and differential expression of the NR2 subunits.
Each receptor subunit has modular design and each structural module also represents a
functional unit:
The extracellular domain contains two globular structures: a modulatory domain and a ligand binding domain. NR1 subunits bind the co-agonist glycine and NR2 subunits bind the neurotransmitter glutamate.
The agonist-binding module links to a membrane domain, which consists of three trans-membrane segments and a re-entrant loop reminiscent of the selectivity filter of potassium channels.
The membrane domain contributes residues to the channel pore and is responsible for the receptor’s high-unitary conductance, high-calcium permeability, and voltage-dependent magnesium block.
Each subunit has an extensive cytoplasmic domain, which contain residues that can be directly modified by a series of protein kinases and protein phosphatases, as well as residues that interact with a large number of structural, adaptor, and scaffolding proteins.
The glycine-binding module of the NR1 subunit and the glutamate-binding module of the NR2A subunit have been expressed as soluble proteins, and their three-dimensional structure has been solved at atomic resolution by x-ray crystallography. This has revealed a common fold with amino acid-binding bacterial proteins and with the glutamate-binding module of AMPA-receptors and kainate-receptors
Tags: AMPA-receptors and kainate-receptors, Calcium flux, Glycine-binding, Ion channel, Magnesium block, Potassium channels, Structure of NMDA