Glutamate receptors in plants

Abstract

Ionotropic glutamate receptors function in animals as glutamate-gated non-selective cation channels. Numerous glutamate receptor-like (GLR) genes have been identified in plant genomes, and plant GLRs are predicted, on the basis of sequence homology, to retain ligand-binding and ion channel activity. Non-selective cation channels are ubiquitous in plant membranes and may function in nutrient uptake, signalling and intra-plant transport. However, there is little evidence for amino acid gating of plant ion channels. Recent evidence suggests that plant GLRs do encode non-selective cation channels, but that these channels are not gated by amino acids. The functional properties of these proteins and their roles in plant physiology remain a mystery. The problems surrounding characterization and assignation of function to plant GLRs are discussed in this Botanical Briefing, and potential roles for GLR proteins as non-selective cation channels involved in metabolic signalling are described.

Fig. 1. Predicted structure and evolutionary relationships of ionotropic glutamate receptors, kainate binding proteins and a prokaryotic K⁺ channel. A, KcsA, a prokaryotic K⁺ channel subunit with M1PM2 structure. B, GluR0, a prokaryotic glutamate‐gated K⁺ channel subunit. C, Mammalian iGluR and plant GLR subunits. Consensus AtGLR splice sites are marked (the third splice is found only in group III genes). D, Kainate‐binding protein subunits, found in fish, amphibians and birds. E, Mammalian mGluR subunits. F, The putative ‘Venus flytrap’ mechanism of animal iGluR channel gating. On the left the channel subunit is in the closed state. When glutamate binds to the active site of the S1–S2 complex (right) then the conformation of the transmembrane domains is converted to an ion‐permeant state. This diagram represents a single subunit but it is predicted that four or possibly five subunits assemble to form a functional ion channel, with each subunit contributing residues to the membrane‐spanning pore. Pore conductance level appears to depend on the number of S1–S2 complexes binding glutamate. The putative membrane orientation is as indicated in A. I, Indicates cytosol; o, extracellular or other compartment; PBP, periplasmic amino acid binding protein; S1, S2, ligand binding domains; M, transmembrane domain; P, pore. Colours indicate putative homology between protein domains.

Fig. 2. Alignment of the P1 and M2 regions of the arabidopsis AtGLRs (cloned full‐length cDNAs indicated in bold) with Genbank sequences of rat GluR1 (X17184), human GluR6 (CAC67487·1), human NR1 (Q05586), rat NR2A (D13211), frog KBP (X17314), mouse δ2 (D13266) and Synechocystis GluR0 (slr1257). The ‘Q/R/N’ site of RNA editing in the pore of some AMPA and kainate receptors is indicated by an asterisk, and the pore region corresponding to the ‘GYGD’ selectivity filter of K⁺ channels is boxed. The position of the δ2 Lurcher mutation in M2 is indicated by an asterisk: the wildtype alanine (‘A’) is mutated to threonine (‘T’) in Lurcher mutants. P, Pore region; M2, second transmembrane domain. Chemical properties of amino acid side‐groups are represented by colours: red = basic; pink = histidine (pKa = 6·5); dark blue = acidic; light blue = hydrophilic; yellow = aliphatic; orange = aromatic; green = proline and glycine; and purple = cysteine.

Fig. 3. Intracellular compartmentation of glutamate and ammonium metabolism in a generalized non‐photosynthetic cell (A) and a generalized photosynthetic cell (B). The enzymes involved in glutamate (glu) and glutamine (gln) synthesis and de‐amination are shown: GS, glutamine synthetase; GOGAT, glutamate synthase; GDH, glutamate dehydrogenase; and 2‐OG, 2‐oxoglutarate. The enzymic pathways shown will not necessarily all be present in a particular cell. The synthetic capacity of GDH in planta is disputed (Miflin and Habash, 2002). Enzymes for synthesis of other amino acids are present in the cytosol, mitochondria (M), plastids (P) including chloroplasts (C), glyoxysomes (G) and peroxisomes (Pe). In non‐photosynthetic cells, NH₄⁺ derives from nitrate reduction, amino acid deamination and from the apoplast including the soil solution. In photosynthetic cells, NH₄⁺ derives from these pathways and also from photorespiration. Putative transmembrane transport of NH₄⁺ is indicated in bold lines.