Architecture of fully occupied GluA2 AMPA receptor – TARP complex elucidated by single particle cryo-electron microscopy

Fast excitatory neurotransmission in the mammalian central nervous system is largely carried out by AMPA-sensitive ionotropic glutamate receptors. Localized within the postsynaptic density of glutamatergic spines, AMPA receptors are composed of heterotetrameric receptor assemblies associated with auxiliary subunits, the most common of which are transmembrane AMPA-receptor regulatory proteins (TARPs). The association of TARPs with AMPA receptors modulates the kinetics of receptor gating and pharmacology, as well as trafficking. Here we report the cryo-EM structure of the homomeric GluA2 AMPA receptor saturated with TARP γ2 subunits, showing how the TARPs are arranged with four-fold symmetry around the ion channel domain, making extensive interactions with the M1, M2 and M4 TM helices. Poised like partially opened 'hands' underneath the twofold symmetric ligand binding domain (LBD) 'clamshells', one pair of TARPs are juxtaposed near the LBD dimer interface, while the other pair are near the LBD dimer-dimer interface. The extracellular 'domains' of TARP are positioned to not only modulate LBD 'clamshell' closure, but also to affect conformational rearrangements of the LBD layer associated with receptor activation and desensitization, while the TARP transmembrane (TM) domains buttress the ion channel pore. Fast excitatory neurotransmission at chemical synapses of the brain underpins a spectrum of activities ranging from memory and learning, to speech and hearing, to movement and coordination. Ionotropic glutamate receptors (iGluRs) are a family of transmitter-gated ion channels comprised of three related subfamilies – AMPA, kainate and NMDA receptors – that mediate the majority of ionotropic excitatory signaling 1. Neuronal AMPA and kainate receptors, by contrast with NMDA receptors, are associated with auxiliary membrane protein subunits that, in turn, modulate receptor gating, trafficking, and pharmacology 2. Stargazin is the founding member of the transmembrane AMPA receptor regulatory proteins (TARP) 3 , a family of membrane proteins related in amino acid sequence to claudin, a four-helix transmembrane protein 4. Coexpression of recombinant AMPA receptors with TARPs largely recapitulates native receptor gating kinetics, ion channel properties, and pharmacology, consistent with the notion that TARPs are fundamental components of neuronal AMPA receptor signaling complexes 5 , yet with a heterogeneous stoichiometry ranging from 1 to 4 TARPs per receptor 6. Stargazin, also known as TARP γ2, modulates AMPA receptor gating by slowing deactivation and desensitization, accelerating the recovery from desensitization, increasing the efficacy of partial agonists such as kainate, and attenuating polyamine block of calcium-permeable AMPA receptors 7,8. Despite progress in visualization of the AMPA receptor – …