Molecular evolution of glutamate receptors: a primitive signaling mechanism that existed before plants and animals diverged.

We performed a genealogical analysis of the ionotropic glutamate receptor (iGluR) gene family, which includes the animal iGluRs and the newly isolated glutamate receptor-like genes (GLR) of plants discovered in Arabidopsis. Distance measures firmly placed the plant GLR genes within the iGluR clade as opposed to other ion channel clades and indicated that iGluRs may be a primitive signaling mechanism that predated the divergence of animals and plants. Moreover, phylogenetic analyses using both parsimony and neighbor joining indicated that the divergence of animal iGluRs and plant GLR genes predated the divergence of iGluR subtypes (NMDA vs. AMPA/KA) in animals. By estimating the congruence of the various glutamate receptor gene regions, we showed that the different functional domains, including the two ligand-binding domains and the transmembrane regions, have coevolved, suggesting that they assembled together before plants and animals diverged. Based on residue conservation and divergence as well as positions of residues with respect to functional domains of iGluR proteins, we attempted to examine structure-function relationships. This analysis defined M3 as the most highly conserved transmembrane domain and identified potential functionally important conserved residues whose function can be examined in future studies.