Ultrastructure of the 5‐Hydroxytryptamine3 Receptor

: We have determined the ultrastructure of 5‐hydroxytryptamine3 (5‐HT3) serotonin receptors purified from NG108‐15 mouse neuroblastoma × rat glioma cells by electron microscopic examination of receptor particles embedded in uranyl acetate stain and metal replicas of rapidly frozen receptors. The 5‐HT3 receptor can be modelled as a cylinder 11 nm in length and 8 nm in diameter with a closed end and a central cavity 3 nm in diameter. Analysis of the rotational symmetry of single receptor particles indicates that the 5‐HT3 receptor is composed of five subunits arranged symmetrically around a central cavity. Together with evidence obtained for related proteins in other studies using ultrastructural, biochemical, or electrophysiological methods, our observations suggest that all members of the ligand‐gated ion channel superfamily may possess a pentameric quaternary structure.