Pulsed Electron Spin Resonance Resolves the Coordination Site of Cu2+ Ions in α1-Glycine Receptor

Herein, we identify the coordination environment of Cu2+ in the human α1-glycine receptor (GlyR). GlyRs are members of the pentameric ligand-gated ion channel superfamily (pLGIC) that mediate fast signaling at synapses. Metal ions like Zn2+ and Cu2+ significantly modulate the activity of pLGICs, and metal ion coordination is essential for proper physiological postsynaptic inhibition by GlyR in vivo. Zn2+ can either potentiate or inhibit GlyR activity depending on its concentration, while Cu2+ is inhibitory. To better understand the molecular basis of the inhibitory effect we have used electron spin resonance to directly examine Cu2+ coordination and stoichiometry. We show that Cu2+ has one binding site per α1 subunit, and that five Cu2+ can be coordinated per GlyR. Cu2+ binds to E192 and H215 in each subunit of GlyR with a 40 μM apparent dissociation constant, consistent with earlier functional measurements. However, the coordination site does not include several residues of the agonist/antagonist binding site that were previously suggested to have roles in Cu2+ coordination by functional measurements. Intriguingly, the E192/H215 site has been proposed as the potentiating Zn2+ site. The opposing modulatory actions of these cations at a shared binding site highlight the sensitive allosteric nature of GlyR.