Role of the chemical interactions of the agonist in controlling alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor activation.

Alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors are the main excitatory neurotransmitter receptors in the mammalian central nervous system. Structures of the isolated ligand binding domain of this receptor have provided significant insight into the large-scale conformational changes, which when propagated to the channel segments leads to receptor activation. However, to establish the role of specific molecular interactions in controlling fine details such as the magnitude of the functional response, we have used a multiscale approach, where changes at specific moieties of the agonists have been studied by vibrational spectroscopy, while large-scale conformational changes have been studied using fluorescence resonance energy transfer (FRET) investigations. By exploiting the wide range of activations by the agonists, glutamate, kainate, and AMPA, for the wild type and Y450F and L650T mutants of the GluR2 subtype, and by using the multiscale investigation, we show that the strength of the interactions at the alpha-amine group of the agonist with the protein in all but one case tracks the extent of activation. Since the alpha-amine group forms bridging interactions at the cusp of the ligand binding cleft, this appears to be a critical interaction through which the agonist controls the extent of activation of the receptor.