Characterization of mammalian homologues of Drosophila TRP proteins, which induce light-activated Ca2+ conductance in photoreceptors, has been an important clue to understand molecular mechanisms underlying receptor-activated Ca2+ influx in vertebrate cells. We have here isolated cDNA that encodes a novel TRP homologue, TRP5, predominantly expressed in the brain. Recombinant expression of the TRP5 cDNA in human embryonic kidney cells dramatically potentiated extracellular Ca2+-dependent rises of intracellular Ca2+ concentration ([Ca2+]i) evoked by ATP. These [Ca2+]i transients were inhibited by SK&F96365, a blocker of receptor-activated Ca2+ entry, and by La3+. Expression of the TRP5 cDNA, however, did not significantly affect [Ca2+]i transients induced by thapsigargin, an inhibitor of endoplasmic reticulum Ca2+-ATPases. ATP stimulation of TRP5-transfected cells pretreated with thapsigargin to deplete internal Ca2+ stores caused intact extracellular Ca2+-dependent [Ca2+]i transients, whereas ATP suppressed [Ca2+]i in thapsigargin-pretreated control cells. Furthermore, in ATP-stimulated, TRP5-expressing cells, there was no significant correlation between Ca2+ release from the internal Ca2+ store and influx of extracellular Ca2+. Whole-cell mode of patch-clamp recording from TRP5-expressing cells demonstrated that ATP application induced a large inward current in the presence of extracellular Ca2+. Omission of Ca2+ from intrapipette solution abolished the current in TRP5-expressing cells, whereas 10 nM intrapipette Ca2+ was sufficient to support TRP5 activity triggered by ATP receptor stimulation. Permeability ratios estimated from the zero-current potentials of this current were PCa:PNa:PCs = 14.3:1. 5:1. Our findings suggest that TRP5 directs the formation of a Ca2+-selective ion channel activated by receptor stimulation through a pathway that involves Ca2+ but not depletion of Ca2+ store in mammalian cells.