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 P _Ca:P _Na:P _Cs= 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.

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