Chemistry and conformation of the ligand-binding domain of GluR2 subtype of glutamate receptors

In the present report, using vibrational spectroscopy we have probed the ligand-protein interactions for full agonists (glutamate and alpha-amino-5-methyl-3-hydroxy-4-isoxazole propionate (AMPA)) and a partial agonist (kainate) in the isolated ligand-binding domain of the GluR2 subunit of the glutamate receptor. These studies indicate differences in the strength of the interactions of the alpha-carboxylates for the various agonists, with kainate having the strongest interactions and glutamate having the weakest. Additionally, the interactions at the alpha-amine group of the agonists have also been probed by studying the environment of the non-disulfide-bonded Cys-425, which is in close proximity to the alpha-amine group. These investigations suggest that the interactions at the alpha-amine group are stronger for full agonists such as glutamate and AMPA as evidenced by the increase in the hydrogen bond strength at Cys-425. Partial agonists such as kainate do not change the environment of Cys-425 relative to the apo form, suggesting weak interactions at the alpha-amine group of kainate. In addition to probing the ligand environment, we have also investigated the changes in the secondary structure of the protein. Results clearly indicate that full agonists such as glutamate and AMPA induce similar secondary structural changes that are different from those of the partial agonist kainate; thus, a spectroscopic signature is provided for identifying the functional consequences of a specific ligand binding to this protein.