Glutamate receptor subtypes evidenced by differences in desensitization and dependence on the GLR3.3 and GLR3.4 genes

Ionotropic glutamate (Glu) receptors in the central nervous system of animals are tetrameric ion channels that conduct cations across neuronal membranes upon binding Glu or another agonist. Plants possess homologous molecules encoded by GLR genes. Previous studies of Arabidopsis thaliana root cells showed that the amino acids alanine (Ala), asparagine (Asn), cysteine (Cys), Glu, glycine (Gly), and serine trigger transient Ca(2+) influx and membrane depolarization by a mechanism that depends on the GLR3.3 gene. This study of hypocotyl cells demonstrates that these six effective amino acids are not equivalent agonists. Instead, they grouped into hierarchical classes based on their ability to desensitize the response mechanism. Sequential treatment with two different amino acids separated by a washout phase demonstrated that Glu desensitized the depolarization mechanism to Gly, but Gly did not desensitize the mechanism to Glu. All 36 possible pairs of agonists were tested to characterize the desensitization hierarchy. The results could be explained by a model in which one class of channels contained a subunit that was activated and therefore desensitized only by Glu, while a second class could be activated and desensitized by Ala, Cys, Glu, or Gly. A third class could be activated and desensitized by any of the six effective amino acids. Analysis of knockout mutants indicated that GLR3.3 was a required component of all three classes of channels, while the related GLR3.4 molecule specifically affected only two of the classes. The resulting model is an important step toward understanding the biological roles of these enigmatic ion channels.