A Conformational Intermediate in Glutamate Receptor Activation

Ionotropic glutamate receptors (iGluRs) transduce the chemical signal of neurotransmitter release into membrane depolarization at excitatory synapses in the brain. The opening of the transmembrane ion channel of these ligand-gated receptors is driven by conformational transitions that are induced by the association of glutamate molecules to the ligand-binding domains (LBDs). Here, we describe the crystal structure of a GluA2 LBD tetramer in a configuration that involves an ∼30° rotation of the LBD dimers relative to the crystal structure of the full-length receptor. The configuration is stabilized by an engineered disulfide crosslink. Biochemical and electrophysiological studies on full-length receptors incorporating either this crosslink or an engineered metal bridge show that this LBD configuration corresponds to an intermediate state of receptor activation. GluA2 activation therefore involves a combination of both intra-LBD (cleft closure) and inter-LBD dimer conformational transitions. Overall, these results provide a comprehensive structural characterization of an iGluR intermediate state. •The crystal structure of a GluA2 LBD tetramer captures an activation intermediate•Glutamate-binding domain dimers rotate by ?30° during activation•A disulfide bond between subunits traps a partially activated state•Trapping by engineered metal bridges confirms the conformational arrangement Lau et al. show that an intermediate state in the activation pathway of AMPARs, a class of ionotropic glutamate receptors, involves an ∼30° rotation between the two ligand-binding domain dimers.