Activation of Ligand Binding Domains of an AMPA-Type Glutamate Receptor

Abstract

Ionotropic glutamate receptors are postsynaptic tetrameric channels whose activity mediates transmission of the synaptic signal. Binding of glutamate to their ligand binding domains (LBDs) shifts the receptors from the resting to active state followed by desensitization. Here we present two different tetrameric LBD arrangements of an AMPA receptor fully bound by glutamate at 1.3 A resolution. The novel inter-dimer interfaces presented by the crystal are probed through a series of engineered histidine mutants, which are then cross-linked by zinc in outside-out patches, with biochemical measurements on full-length receptors. The functional profiles of the His mutants indicate that the most compact tetrameric LBD arrangement captured by the crystal represents an active state of the receptor. The functional and structural data are further corroborated by computational modeling showing that most of the movement in agonist-bound LBDs is mediated by two subunits further away from the overall 2-fold axis of molecular symmetry (distal subunits). The results, thus, give the first insight into the open structure of the activated receptor at the level of the LBD tetramer.