P2X purinoceptor plethora

Six P2X receptor subunits are currently known, encoded on different genes. The proteins deduced from their cDNAs have 379 to 472 amino acids; they are 36–48% identical. They are thought to have two transmembrane segments, with most of the protein forming a large extracellular loop. In-situ hybridization shows a widespread tissue distribution of the RNAs, with P2X₄and P2X₆being the receptors most heavily expressed in brain and P2X₃found only in sensory ganglia. P2X₁–P2X₄subunits readily form channels when expressed in mammalian cells or oocytes; the number of subunits per channel is not known, although P2X₂and P2X₃can both contribute to the same channel when co-expressed. P2X₅and P2X₆express less readily, suggesting perhaps that they normally co-assemble with other subunits. Experiments in progress seek to determine the stoichiometry of the P2X receptor channel and the parts of the molecule involved in pore formation and ATP binding.