Conserved quaternary structure of ligand-gated ion channels: the postsynaptic glycine receptor is a pentamer

The postsynaptic glycine receptor of rat spinal cord is a glycosylated membrane protein that, after affinity purification, contains membrane-spanning subunits of Mr 48,000 and 58,000 and an associated peripheral polypeptide of Mr 93,000. Here, the quaternary structure of the transmembrane core of the receptor was investigated by chemically crosslinking its subunits. Upon treatment with crosslinking reagents of different side-chain specificities and lengths, a consistent set of adducts up to Mr 260,000 was detected after separation by NaDodSO4/PAGE. The observed pattern of adducts was similar irrespective of whether purified receptor protein or synaptosomal membranes were crosslinked. Compositional analysis revealed that the crosslinked adducts contained the Mr 48,000 and 58,000 subunits in varying ratios but not the peripheral Mr 93,000 polypeptide. Thus adducts of intermediate molecular weight represent dimers, trimers, and tetramers of the transmembrane subunits, whereas the major adduct of Mr 260,000 corresponds to a pentameric assembly of subunits forming the ion channel of the glycine receptor. This subunit arrangement is similar to that reported for the nicotinic acetylcholine receptor of fish electric organ and skeletal muscle. Hence, we suggest that the different ligand-gated ion channels of excitable membranes share a similar quaternary structure.