Vibrational spectroscopic investigation of the ligand binding domain of kainate receptors

Fourier transform infrared spectroscopy has been used to probe the agonist‐protein interactions in the ligand binding domain of the GluR6 subunit, one subunit of the kainate subtype of glutamate receptors. In order to study the changes in the interactions over a range of activations the investigations were performed using the wild type, N690S, and T661E mutations. These studies show that the strength of the interactions at the α‐amine group of the agonist, as probed by studying the environment of the nondisulphide bonded Cys 432, acts as a switch with weaker interactions at lower activations and stronger interactions at higher activations. The α‐carboxylate interactions of the agonist, however, are not significantly different over the wide range of activations, as measured by the maximum currents mediated by the receptors at saturating concentrations of agonists. Previous investigations of AMPA receptors show a similar dependence of the α‐amine interactions on activation indicating that the roles of the α‐amine interactions in mediating receptor activation are similar for both subtypes of receptors; however, in the case of the AMPA receptors a tug of war type of change was observed between the α‐amine and α‐carboxylate interactions and this is not observed in kainate receptors. This decoupling of the two interactions could arise due to the larger cleft observed in kainate receptors, which allows for a more flexible interaction for the α‐amine and α‐carboxylate groups of the agonists.