A Plant Homolog of Animal Glutamate Receptors Is an Ion Channel Gated by Multiple Hydrophobic Amino Acids

Ionotropic glutamate receptors (iGluRs)are ligand-gated cation channels that mediate neuro-transmission in animal nervous systems. Homologous proteins in plants have been implicated in root development, ion transport, and several metabolic and signaling pathways. AtGLR3.4, a plant iGluR homolog fromArabidopsis thaliana, has ion channel activity and is gated by asparagine, serine, and glycine. Using heterologous expression inXenopusoocytes, we found that anotherArabidopsisiGluR homolog, AtGLR1.4, functioned as a ligand-gated, nonselective, Ca2+-permeable cation channel that responded to an even broader range of amino acids, none of which are agonists of animal iGluRs. Seven of the 20 standard amino acids-mainly hydrophobic ones-acted as agonists, with methionine being most effective and most potent. Nine amino acids were antagonists, and four, including glutamate and glycine, had no effect on channel activity. We constructed a model of this previously uncharacterized ligand specificity and used knockout mutants to show that AtGLR1.4 accounts for methionine-induced membrane depolarization inArabidopsisleaves.