Structural Mechanisms Underlying Activity Changes in an AMPA-type Glutamate Receptor Induced by Substitutions in Its Ligand-Binding Domain.

α-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-type glutamate receptors produce postsynaptic current by transmitting an agonist-induced structural change in the ligand-binding domain (LBD) to the transmembrane channel. Receptors carrying T686S/A substitutions in their LBDs produce weaker glutamate-evoked currents than wild-type (WT) receptors. However, the substitutions induce little differences in the crystal structures of their LBDs. To understand the structural mechanism underlying reduced activities of these AMPAR variants, we analyzed the structural dynamics of WT, T686S, and T686A variants of LBD using nuclear magnetic resonance. The HD exchange studies of the LBDs showed that the kinetic step where the ligand-binding cleft closes was changed by the substitutions, and the substitution-induced population shift from cleft-closed to cleft-open structures is responsible for the reduced activities of the variants. The chemical shift analyses revealed another structural equilibrium between cleft-locked and cleft-partially-open conformations. The substitution-induced population shift in this equilibrium may be related to slower desensitization observed for these variants.