Non-selective cation permeation in an AMPA-type glutamate receptor

Abstract

9Fast excitatory synaptic transmission in the central nervous system relies on the AMPA-type glutamate 10 receptor (AMPAR). This receptor incorporates a non-selective cation channel which is opened by the 11 binding of glutamate. Although the open pore structure has recently became available from cryo-electron 12 microscopy (Cryo-EM), the molecular mechanisms governing cation permeability in AMPA receptors are 13 not understood. Here, we combined microsecond molecular dynamics (MD) simulations on a putative 14 open state structure of GluA2 with electrophysiology on cloned channels to elucidate ion permeation 15 mechanisms. Na + , K + and Cs + permeated at physiological rates, consistent with a structure that 16represents a true open state. A single major ion binding site for Na + and K + in the pore represents the 17 simplest selectivity filter (SF) structure for any tetrameric cation channel of known structure. The 18 minimal SF comprised only Q586 and Q587, and other residues on the cytoplasmic side formed a cone-19 shaped void that lacked major interactions with ions. We observed Clinvasion of the upper pore, 20 explaining anion permeation in the edited form of GluA2. A permissive architecture of the SF 21 accommodated different alkali metals in distinct solvation states to allow rapid, non-selective cation 22 permeation, and co-permeation by water. Simulations suggested Cs + uses two equally populated ion 23 binding sites in the filter and we confirmed with electrophysiology of GluA2 that Cs + is more permeant 24 than Na + , consistent with serial binding sites preferentially driving selectivity. 25 Significance Statement 26AMPA-type glutamate receptors (AMPARs) are key actors in neurotransmission, making the final step in 27 a relay of excitability from one brain cell to another. The receptor contains an integral ion channel, 28 which, when opened by neurotransmitter binding, permits sodium and other cations to cross the cell 29 membrane. We investigated permeation of sodium, potassium and caesium in an AMPAR at the atomistic 30 level using a computational molecular dynamics approach on a structure with the ion channel pore in a 31 presumably open state. We determined that the region selecting between cations is the simplest of any 32 channel of this type. Distinct from ion channels that select single ion species, cations are never fully 33 dehydrated and have only one major ion binding site in the filter. Simulations suggested two similar 34 binding sites for caesium, and studies of AMPARs in mammalian cell membranes showed that this makes 35 caesium more permeant than sodium.Glutamate receptor ion channels are found at synapses throughout the vertebrate nervous 38 system, where they convert sub-millisecond glutamate signals into cation currents. Advances 39 in structural biology have provided molecular scale maps of their ion pores, permitting 40 comparison with a burgeoning menagerie of structures from related ion channels. It has been 41 comparatively difficult to obtain candidate open pore structures of...