New structural motif for ligand-gated ion channels defined by an ionotropic ATP receptor

THE adenosine-5'-triphosphate (ATP) molecule is an extracellular messenger in neural and non-neural tissues, where it activates several cell-surface-receptor subtypes, including G-protein-coupled receptors and ligand-gated ion channels¹. ATP-gated channels (termed P_2X receptors) have been characterized on smooth muscle cells and autonomic and sensory neurons, where they mediate membrane depolarization and, in some cases, Ca²⁺ entry². P_2x receptors are functionally heterogeneous, but resemble acetylcholine-and serotonin-gated channels with respect to ion selectivity and kinetic parameters of channel gating. We report here that despite such close functional similarities, the deduced sequence of a cloned P_2x receptor predicts an unusual subunit structure resembling voltage-insensitive cation channels. Thus, the P_2x receptor provides a striking example of convergent evolution, whereby proteins have been fashioned with similar functional properties from subunits having very different structural characteristics. There is sequence similarity between the ATP receptor and RP-2, a gene activated in thymocytes undergoing programmed cell death³. RP-2 may encode a receptor for ATP or another metabolite released during apoptosis.