Invertebrate photoreceptors use the ubiquitous inositol-lipid signaling pathway for phototransduction. This pathway depends on Ca 2+ release from internal stores and on Ca 2+ entry via light-activated channels to replenish the loss of Ca 2+ in those stores. The Drosophila transient receptor potential (TRP) protein in essential for the high Ca 2+ permeability and other biophysical properties of these light-activated channels, which affect both excitation and adaptation in photoreceptor cells. Physiological and heterologous expression studies indicate that TRP is a putative subunit of a surface membrane channel that can be activated by depletion of internal Ca 2+ stores. Furthermore, trp is an archetypal member of a multigene family whose products share a structure that is highly conserved throughout evolution, from worms to humans.
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