Conformational Changes Underlying Glutamate Receptor Gating

Abstract

The gating mechanism of glutamate receptor ion channels involves conformational changes triggered by the binding of glutamate that open the ion channel, followed within milliseconds by nearly complete desensitization. In a collaboration with the Subramaniam Lab at NCI, cryo-EM structures of full length AMPA (GluA2) receptors, stabilized in the active state by allosteric modulators, reveal concerted conformational changes in the ligand binding domain that produce a vertical contraction accompanied by a corkscrew like rotation that leads to expansion of the linkers leading to the M3 helix bundle; in the desensitized state the ATD dimer pairs adopts multiple conformations with evidence for substantial reorganization of the ligand binding domain tetramer. By contrast, for the GluK2 kainate receptor the ATD dimer assembly remains intact in the desensitized state, while the LBD rearranges into a 4-fold symmetric structure in which the linkers leading to the M3 helix bundle adopt a contracted conformation that is however different from the closed state. Measurements of ATD tetramer association by analytical ultracentrifugation reveal a 1000-fold high higher affinity for GluK2 versus GluA2 that underlies the different behavior of the ATD in the desensitized state, movements of which however are secondary to and play no major role in desensitization.