A cDNA encoding a novel member of the G-protein-coupled receptor (GCR) superfamily. an ATP receptor, has been isolated from an embryonic chtck whole brain cDNA hbrary by hybridizatton screenmg. The encoded protein has a sequence of 362 amino acids (41 kDa) and shares no more than 27% amino acid identity with any known GCR. When expressed as a complementary RNA (cRNA) m Xenopus oocytes a slowly-developing inward current was observed in response to application of ATP. The pharmacology of this expressed protem defines it as a PZv purinoceptor.

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The cloning and characterization of a P2X receptor (schP2X) from the parasitic blood fluke Schistosoma mansoni provides the first example of a non-vertebrate ATP-gated ion channel. A number of functionally important amino acid residues conserved throughout vertebrate P2X receptors, including 10 extracellular cysteines, aromatic and positively charged residues involved in ATP recognition, and a consensus protein kinase C site in the amino-terminal tail, are also present in schP2X. Overall, the amino acid sequence identity of schP2X with human P2X 1-7 receptors ranges from 25.8 to 36.6%. ATP evoked concentration-dependent currents at schP2X channels expressed in Xenopus oocytes with an EC 50 of 22.1 M. 2,3-O-(4-Benzoylbenzoyl)adenosine 5-triphosphate (Bz-ATP) was a partial agonist (maximum response 75.4 ؎ 4.4% that of ATP) with a higher potency (EC 50 of 3.6 M) than ATP. Suramin and pyridoxal-phosphate-6-azophenyl-2,4-disulfonic acid blocked schP2X responses to 100 M ATP with IC 50 values of 9.6 and 0.5 M, respectively. Ivermectin (10 M) potentiated currents to both ATP and Bz-ATP by ϳ60% with a minimal effect on potency (EC 50 of 18.2 and 1.6 M, respectively). The relative permeability of schP2X expressed in HEK293 cells to various cations was determined under bi-ionic conditions. schP2X has a relatively high calcium permeability (P Ca /P Na ‫؍‬ 3.80 ؎ 0.29) and an estimated minimum pore diameter similar to that of vertebrate P2X receptors. SchP2X provides a useful comparative model for the better understanding of human P2X receptor function and may also provide an alternative drug target for treatment of schistosomiasis.P2X receptors comprise a family of cation-selective ion channels gated by extracellular ATP. Mammalian species possess seven distinct P2X channel subtypes (P2X 1-7 ), each encoded by a separate gene. These subunits assemble as functional homotrimeric or heterotrimeric channels and play an important role in a wide array of physiological processes including neurotransmission, smooth muscle contraction, immune cell function, and platelet aggregation (for a recent review see Ref. 1).This remarkably diverse range of physiological roles for P2X receptors has contributed to the notion that ATP is a "primitive" extracellular signaling molecule (2, 3). Functional evidence suggests that ATP-gated ion channels exist in some lower organisms including Tetrahymena thermophilia (4), leech (5), and Amoeba proteus (6), supporting the view that the development of P2X receptors for ATP occurred relatively early in the evolution of eukaryotic organisms. However, to date, definitive molecular identification of P2X channels is restricted to vertebrate species including fish (7), amphibians (8, 9), birds (10), and mammals (1).With the recent expansion in the range of species for which genomic and EST 1 sequence databases are available, it is now possible to use a bioinformatics approach to screen a wide range of lower organisms for P2X receptor-like proteins. Using such a strategy, we have identified a P2X channel from...

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