P2X receptors for ATP are expressed throughout the body and mediate a multitude of functions, including muscle contraction, neuronal excitability and bone formation. In the mid-1990s seven genes encoding P2X receptors (P2X(1-7)) were identified. These receptors comprised a novel family of ligand-gated ion channels with subunits that possessed intracellular N- and C-termini, two transmembrane domains and an extracellular ligand-binding loop. No crystal structures are available for these channels. Furthermore, they are distinct from the nicotinic acetylcholine (Cys-loop) and glutamate families of ion channels and have no similarity to other ATP-binding proteins, thus precluding homology modelling-based studies of their structural properties. However, molecular techniques have provided insight into the properties of P2X receptors: mutagenesis and biochemical studies have identified regions associated with ATP binding, ionic conduction, channel gating and regulation. In addition, transgenic approaches have helped to characterize the role of defined receptor subunits in native systems.
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