Molecular Determinants of Ligand Selectivity in Diverse Ionotropic Glutamate Receptors

Ionotropic glutamate receptors (iGluRs) mediate rapid signals between neurons by binding extracellular neurotransmitters and conducting ionic current across the cell membrane. Animal iGluRs can be divided into clades of phylogenetically related receptors, only some of which are present in mammals. Despite their name, many receptors are selectively activated by ligands other than glutamate, and despite high-resolution structural data on most mammalian iGluRs, the molecular basis for ligand selectivity in most clades is not clear. We questioned whether closely related iGluRs within phylogenetic clades share similar ligand selectivity profiles by testing several receptors from bilaterians, cnidarians, a placozoan, and a ctenophore, and by probing the molecular basis of ligand selectivity in iGluRs from different clades with site-directed mutagenesis. We identified glutamate-, glycine-, and GABA-gated channels from both the AKDF clade and the Epsilon clade, and a selectively aspartate-gated channel from the Epsilon clade. We show that in some cases, even very closely related channels from within the same clade and from the same animal species have diverged functionally, leading to channels gated by different agonists. Although we find special roles in ligand selectivity for glycine, threonine, and leucine residues in the "αF-GST" and "β10/αI-LE" motifs (mammalian AMPA receptor nomenclature), our results in diverse iGluRs warn that the effects of mutations, and thus the roles of these residues, are not always easy to predict.