title: Role of cross-cleft contacts in NMDA receptor gating.
abstract: In response to brief glutamate exposure, NMDA receptors produce excitatory currents that have sub-maximal amplitudes and characteristically slow kinetics. The activation sequence starts when glutamate binds to residues located on the upper lobe of extracellularly located ligand-binding domains (LBDs) and then contacts lower lobe residues to bridge the cleft between the two hinged lobes. This event stabilizes a narrow-cleft LBD conformation and may facilitate subsequent inter-lobe contacts that further stabilize the closed cleft. Agonist efficacy has been traced to the degree of agonist-induced cleft-closure and may also depend on the stability of the closed-cleft conformation. To investigate how cross-cleft contacts contribute to the amplitude and kinetics of NMDA receptor response, we examined the activation reaction of GluN1/GluN2A receptors that had single-residue substitutions at the interface between LBD lobes. We found that side-chain truncations at residues of putative contact between lobes increased glutamate efficacy through independent additive mechanisms in GluN1 and GluN2A subunits. In contrast, removing side-chain charge with isosteric substitutions at the same sites decreased glutamate efficacy. These results support the view that in GluN1/GluN2A receptors' natural interactions between residues on opposing sides of the ligand-binding cleft encode the stability of the glutamate-bound closed-cleft conformations and limit the degree of cleft closure, thus contributing to the sub-maximal response and emblematically slow NMDA receptor deactivation after brief stimulation.
authors.affiliation: Neuroscience Program, University at Buffalo, Buffalo, New York, United States of America.
authors.forename: Meaghan A
authors.initials: MA
authors.lastname: Paganelli
authors.forename: Cassandra L
authors.initials: CL
authors.lastname: Kussius
authors.forename: Gabriela K
authors.initials: GK
authors.lastname: Popescu
mesh_terms: D000200:Action Potentials; D000818:Animals; D002460:Cell Line; D006801:Humans; D007700:Kinetics; D008024:Ligands; D008958:Models, Molecular; D011485:Protein Binding; D011487:Protein Conformation; D021122:Protein Subunits; D051381:Rats; D016194:Receptors, N-Methyl-D-Aspartate / Q000737:chemistry* / Q000378:metabolism*
chemical_list: D008024:Ligands; D021122:Protein Subunits; D016194:Receptors, N-Methyl-D-Aspartate