Structure of ligand-gated ion channels: Critical assessment of biochemicaldata supports novel topology

Rapid signaling across the synaptic junction is partially mediated by the l igand-gated ion channel superfamily (LGICS), which includes inhibitory glyc ine and GABA receptors and excitatory acetylcholine and serotonin receptors . The glycine receptor (GlyR) can assemble as homopentamers of alpha subuni ts, and baculovirus expression systems are capable of overexpressing large quantities of active receptors. Limited proteolysis coupled to mass spectro metry on reconstituted alpha1 GlyR homopentamers identified proteolytic cle avages within proposed transmembrane domains postulated to fold as bilayer- spanning alpha helices in the "classical" model and identified unexpected m embrane-associated regions in the N-terminal domain (J. F. Leite et al., 20 00, J. Biol, Chem. 275, 13683-13689). In this review, optimized sequence al ignments were used to integrate these proteolysis data with biochemical inf ormation determined in studies of all the LGICS members in order to constru ct a novel topological model.