A Conserved Mechanism for Gating in an Ionotropic Glutamate Receptor

Ionotropic glutamate receptor (iGluR) channels control synaptic activity. The crystallographic structure of GluA2, the prototypical iGluR, reveals a clamshell-like ligand-binding domain (LBD) that closes in the presence of glutamate to open a gate on the pore lining α-helix. How LBD closure leads to gate opening remains unclear. Here, we show that bending the pore helix at a highly conserved alanine residue (A621) below the gate is responsible for channel opening. Substituting A621 to the smaller more flexible glycine resulted in a constitutively active, non-desensitizing channel with ~ 36-fold increase in glutamate potency without affecting surface expression or binding. On GluA2(A621G), the partial agonist kainate showed efficacy similar to a full agonist, and the competitive antagonist CNQX acted as a partial agonist. Met629 in GluA2 sits above the channel gate and is critical in transmitting LBD closure to the gate. Substituting M629 with the flexible glycine resulted in reduced channel activity and glutamate potency. The pore regions in potassium channels are structurally similar to iGluRs. While potassium channels typically use glycines as a hinge for gating, iGluRs use the less flexible alanine as a hinge at a similar position in order to maintain low basal activity allowing for ligand mediated gating.