AMPA-type glutamate receptors (AMPARs), central mediators of rapid neurotransmission and synaptic plasticity, predominantly exist as heteromers of the GluA1-4 subunits. Here we report first AMPAR heteromer structures, which deviate substantially from existing GluA2 homomers. Crystal structures of the GluA2/3 and GluA2/4 N-terminal domains reveal a novel compact conformation with an alternating arrangement of the four subunits around a central axis. This organization is confirmed by cysteine crosslinking in full-length receptors and permitted us to determine the structure of an intact GluA2/3 receptor by cryo-EM. Two models in the ligand-free state, at 8.25 Å and 10.3 Å resolution, exhibit a substantial vertical compression and close associations between domain layers, reminiscent of NMDA receptors. Model 1 resembles a resting state, model 2 a desensitized state, providing snapshots of gating transitions in the nominal absence of ligand. Our data reveal organizational features of heteromeric AMPARs and provide a framework to decipher AMPAR architecture and signaling.Ionotropic glutamate receptors (iGluRs) are tetrameric cation channels that mediate fast excitatory signal transmission upon binding presynaptically released glutamate (1). They are essential for brain development and experience-dependent synaptic plasticity, which underlies learning. iGluR dysfunction is implicated in a number of neurological disorders including dementia, mood disorders and epilepsy (2). Three major subtypes, the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid, N-methyl-D-aspartate and kainate receptors (AMPARs, NMDARs and KARs), each contribute a different component to the synaptic signal (1). AMPARs mediate the initial depolarization of the postsynaptic membrane, triggering NMDAR activation and the generation of an excitatory postsynaptic potential. The rapid kinetics of AMPARs permit moment-to-moment signaling and their trafficking to synapses is central to synaptic plasticity (3).Gating kinetics, ion permeation and trafficking are set by the subunit stoichiometry and ultimately shape the synaptic response. AMPAR tetramers are composed of the GluA1-* Correspondence to: ig@mrc-lmb.cam.ac.uk. GluA4 subunits with the vast majority containing GluA2, which renders the channel Ca 2+ -impermeable, lowers its conductance and alters its voltage dependence (1, 4). Europe PMC Funders GroupStructures of GluA2 homomers have been instrumental in clarifying AMPAR modular architecture (5). The receptor is arranged in domain layers (Fig. 1A) -a two-fold symmetrical extracellular region (ECR), composed of the N-terminal domain (NTD) and ligand-binding domain (LBD), is attached to the transmembrane ion channel domain (TMD) of approximately four-fold symmetry (Fig. 1A), while a cytoplasmic tail mediates trafficking and anchorage at synapses. The four chains (A-D) in a tetramer are conformationally nonequivalent and likely contribute differently to gating: the 'AC' pair is positioned closer to the ion conduction pore axis (at the level of...

AMPA-type glutamate receptors (AMPARs) mediate excitatory neurotransmission, and are central regulators of synaptic plasticity, a molecular mechanism underlying learning and memory. Although AMPARs act predominantly as heteromers, structural studies have focused on homomeric assemblies. Here we present a cryo-EM structure of the heteromeric GluA1/2 receptor associated with two TARP γ8 auxiliary subunits, the principal AMPAR complex at hippocampal synapses. Within the receptor, the core subunits arrange to give the GluA2 subunit dominant control of gating. This structure reveals the geometry of the Q/R-site controlling calcium flux, suggests association of TARP-stabilized lipids, and demonstrates that the extracellular loop of γ8 modulates gating by selectively interacting with the GluA2 ligand-binding domain. Collectively, this structure provides a blueprint for deciphering the signal transduction mechanisms of synaptic AMPARs.

The apoptosis-inducing factor (AIF) is a mitochondrial-flavoprotein that, after cell death induction, is distributed to the nucleus to mediate chromatinolysis. In mitochondria, AIF is present in a monomer–dimer equilibrium that after reduction by NADH gets displaced toward the dimer. The crystal structure of the human AIF (hAIF):NAD(H)-bound dimer revealed one FAD and, unexpectedly, two NAD(H) molecules per protomer. A 1:2 hAIF:NAD(H) binding stoichiometry was additionally confirmed in solution by using surface plasmon resonance. The here newly discovered NAD(H)-binding site includes residues mutated in human disorders, and accommodation of the coenzyme in it requires restructuring of a hAIF portion within the 509–560 apoptogenic segment. Disruption of interactions at the dimerization surface by production of the hAIF E413A/R422A/R430A mutant resulted in a nondimerizable variant considerably less efficiently stabilizing charge-transfer complexes upon coenzyme reduction than WT hAIF. These data reveal that the coenzyme-mediated monomer–dimer transition of hAIF modulates the conformation of its C-terminal proapoptotic domain, as well as its mechanism as reductase. These observations suggest that both the mitochondrial and apoptotic functions of hAIF are interconnected and coenzyme controlled: a key information in the understanding of the physiological role of AIF in the cellular life and death cycle.