Ionotropic glutamate receptors (GluRs) are molecular pores which facilitate the passage of ions across cell membranes and mediate excitatory signal transmission in the mammalian nervous system. Because of their essential role in normal brain function and development and increasing evidence that dysfunction of GluR activity mediates multiple CNS diseases as well as damage during stroke a substantial effort has been directed towards analysis of GluR properties. The AMPA, kainate and NMDA subtypes of ionotropic glutamate receptors are encoded by at least 7 gene families. Surprisingly, despite their role in brain function, glutamate receptors appear to have evolved directly from bacterial ion channels. Direct evidence was obtained for this from the identification and analysis of the first GluR found in a prokaryote: GluR0 from the photosynthetic bacterium syncheocystis PCC 6803. GluR0 binds glutamate, forms potassium-selective channels, and is related in amino acid sequence to both eukaryotic GluRs and potassium channels. On the basis of the amino acid sequence and functional relationships between GluR0 and eukaryotic GluRs, it seems likely that a prokaryotic GluR was the precursor to eukaryotic GluRs. With GluR0 providing evidence for the missing link between potassium channels and GluRs it is now reasonable to propose that their ion channels have a similar architecture, that GluRs are tetramers, and that the gating mechanisms of GluRs and potassium channels have essential features in common. In ongoing studies the structure of ligand bound forms of the agonist binding domain of GluR0 have been solved by x-ray diffraction. Functional studies on gating and permeation in full length GluR0 are also underway. To aid the latter a synthetic gene with eukaryotic codon optimization and a series of silent restriction sites has been assembled and expressed in Xenopus oocytes and HEK 293 cells. In related studies on eukaryotic glutamate receptors site directed mutagenesis has been employed to map in detail amino acid residues responsible for polyamine block in the kainate receptor GluR6. The approach taken has involved scanning the pore region by systematically introducing Ala, Trp or Glu into individual position in the pore forming region. Surprisingly there was good tolerance of even the large Trp and charged Glu residues with very few nonfunctional mutants. Introduction of Trp at the Q/R site in GluR6 produced sub nM affinity for polyamines. It is probable that a cage of aromatic side chains forms the high affinity polyamine binding site. Surprisingly, this was the only position at which Trp had this effect, suggesting a unique orientation of side chains is possible only at this position. In double mutants with a conserved negative charge at the +4 position following the Q/R site neutralized to Gln, introduction of -ve charges in the surrounding side chains restored high affinity polyamine block. The most effective positions were those immediately surrounding the +4 position, and polyamine block decreased as the Glu side chains were moved away from this position. It seems reasonable to propose that one component of polyamine block reflects a through space electrostatic field generated by the conserved Glu/Asp residue at the +4 position which attracts polyamines to the mouth of the channel. These studies, and those on GluR0, are beginning to reveal the pore architecture of glutamate receptors at molecular detail. Great excitement is to be anticipated from further high resolution studies.
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