The β subunit determines the ligand binding properties of synaptic glycine receptors

Inhibitory glycine receptors (GlyRs) regulate motor coordination and sensory signal processing in spinal cord and other brain regions. GlyRs are pentameric proteins composed of membrane-spanning alpha and beta subunits. Here, site-directed mutagenesis combined with homology modeling based on the crystal structure of the acetylcholine binding protein identified key ligand binding residues of recombinant homooligomeric alpha1beta and heterooligomeric alpha1beta GlyRs. This disclosed two highly conserved, oppositely charged residues located on adjacent subunit interfaces as being crucial for agonist binding. In addition, the P subunit was found to determine the ligand binding properties of heterooligomeric GlyRs. Expression of an alp tandem construct and affinity purification of metabolically labeled GlyRs confirmed a subunit stoichionnetry of 2alpha3beta. Because the beta subunit anchors GlyRs at synaptic sites, our results have important implications for the biosynthesis, clustering, and pharmacology of synaptic GlyRs.