iGluRs share a common architecture that consists of the following layers: an upper extracellular layer of amino-terminal domains (ATDs), which are involved in receptor assembly and regulation a layer of transmembrane domains (TMDs), which form an ion-conducting channel a layer of LBDs sandwiched between ATD and TMD layers and a layer of intracellular domains (ICDs), which have not been structurally resolved 3. IGluRs are tetrameric ion channels that mediate the majority of excitatory neurotransmission in the central nervous system 1. The lack of subunit independence and low efficiency coupling of glutamate binding to channel opening underlie the gating of synaptic complexes to submaximal conductance levels, which provide a potential for upregulation of synaptic activity. Conversely, agonist binding to all four LBDs does not guarantee maximal channel conductance and favours subconductance states O1 and O2, with O3 and O4 being rare and not captured structurally. Our structures combined with single-channel recordings, molecular dynamics simulations and machine-learning analysis suggest that channel opening requires agonist binding to at least two LBDs. We show that glutamate binds to LBDs of subunits B and D only after it is already bound to at least the same number of LBDs that belong to subunits A and C. Here we determine structures of a synaptic complex of AMPA-subtype iGluR and the auxiliary subunit γ2 in non-desensitizing conditions with various occupancy of the LBDs by glutamate.
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Higher conductance levels have been associated with an increased number of agonist molecules bound to four individual ligand-binding domains (LBDs) 6, 7, 8, 9, 10. An ionic current through a single iGluR channel shows up to four discrete conductance levels (O1–O4) 4, 5, 6.
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Ionotropic glutamate receptors (iGluRs) are tetrameric ligand-gated ion channels that open their pores in response to binding of the agonist glutamate 1, 2, 3.