Constitutive activity of ionotropic glutamate receptors via hydrophobic substitutions in the ligand-binding domain

• Published in: Structure (London) Vol. 32; no. 7; pp. 966 - 978.e6
• Main Authors: Seljeset, Sandra, Sintsova, Oksana, Wang, Yuhong, Harb, Hassan Y., Lynagh, Timothy
• Format: Journal Article
• Language: English
• Published: United States Elsevier Inc 11.07.2024
• Subjects: binding site; constitutive activity; glutamate; iGluR; ion channels; LBD; ligand-gated ion channels; neurotransmitter receptors; NMDA receptor; Placozoa; neurotransmitter receptors; iGluR; ligand-gated ion channels; constitutive activity; Placozoa; NMDA receptor; glutamate; ion channels; binding site; LBD
• ISSN: 0969-2126; 1878-4186; 1878-4186
• DOI: 10.1016/j.str.2024.04.001

Neurotransmitter ligands electrically excite neurons by activating ionotropic glutamate receptor (iGluR) ion channels. Knowledge of the iGluR amino acid residues that dominate ligand-induced activation would enable the prediction of function from sequence. We therefore explored the molecular determinants of activity in rat N-methyl-D-aspartate (NMDA)-type iGluRs (NMDA receptors), complex heteromeric iGluRs comprising two glycine-binding GluN1 and two glutamate-binding GluN2 subunits, using amino acid sequence analysis, mutagenesis, and electrophysiology. We find that a broadly conserved aspartate residue controls both ligand potency and channel activity, to the extent that certain substitutions at this position bypass the need for ligand binding in GluN1 subunits, generating NMDA receptors activated solely by glutamate. Furthermore, we identify a homomeric iGluR from the placozoan Trichoplax adhaerens that has utilized native mutations of this crucial residue to evolve into a leak channel that is inhibited by neurotransmitter binding, pointing to a dominant role of this residue throughout the iGluR superfamily. [Display omitted] •Mutagenesis in co-liganded rat NMDA receptors highlights a crucial carboxylate residue•Hydrophobic substitutions here generate solely glutamate-gated NMDA receptors•A receptor in placozoan animals evolved into a leak channel via similar substitutions Seljeset et al. show that a carboxylate amino acid residue in the ligand-binding domain of ionotropic glutamate receptors profoundly determines receptor activity. Artificial substitutions here convert co-liganded rat NMDA receptors to solely glutamate-gated receptors, and natural substitutions here during evolution have given rise to leak channels in placozoan animals.