Abstract
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.
Highlights
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
It had been previously established that light-induced membrane depolarizations in hypocotyls (Spalding and Cosgrove, 1989, 1992; Cho and Spalding, 1996; Parks et al, 1998) resembled amino acid-induced depolarizations in root cells (Dennison and Spalding, 2000; Qi et al, 2006)
An assumption at the outset was that different members of the 20-gene GLR family would be found to function in the hypocotyl and root if both organs were studied with similar techniques
Summary
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. One surprise to emerge from the first comprehensive inventory of a plant genome (Arabidopsis Genome Initiative, 2000) was the identification of a family of 20 genes unequivocally homologous to mammalian ionotropic Glu receptors (Lam et al, 1998; Lacombe et al, 2001) Such receptors function as ligand-gated ion channels in the mechanism that transmits signals between cells in the central nervous system (Madden, 2002; Mayer, 2005). Structure modeling indicates that the plant molecules may have an additional amino acid binding site not present in animal iGluRs (Acher and Bertrand, 2005) Perhaps related to these potentially atypical binding sites, Qi et al (2006) found that six very different amino acids were capable of triggering membrane depolarization and a Ca21 rise. A question raised by the surprisingly broad agonist profile is whether or not all six effective amino acids are equivalent and whether luminescence data was divided by the average pretreatment luminescence value to normalize each trace, taking into account tissue variability and differences in coelenterazine uptake
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