Fountain et al . report the identification and characterization of an ATP receptor in Dictyostelium discoideum that is homologous to vertebrate P2X receptors. When heterologously expressed in the human embryonic kidney (HEK) cell line, the protein was localized to the plasma membrane, was activated by ATP or ATP analogs, was a cation channel permeable to calcium and smaller cations, and appeared to form a trimer. Furthermore, mutagenesis studies showed that certain residues critical for ATP binding and channel activity in the vertebrate P2X receptors were conserved in the DdP2X. One key difference between the known vertebrate receptors and the DdP2X receptors was that in vivo, the DdP2X receptor was localized to the membrane of the contractile vacuole instead of the plasma membrane. Thus far, all vertebrate P2X receptors have been reported to act at the plasma membrane. Pharmacological blockade or genetic knockout of DdP2X function produced Dictyostelium that could not osmoregulate properly under hypotonic conditions. They swelled and, although filling of the contractile vacuole was normal, contraction and emptying of the vacuole, which is critical for osmoregulation, was impaired. S. J. Fountain, K. Parkinson, M. T. Young, L. Cao, C. R. L. Thompson, R. A. North, An intracellular P2X receptor required for osmoregulation in Dictyostelium discoideum . Nature 448 , 200-203 (2007). [PubMed]