Novel Ca²⁺ channels underlying transduction in Drosophila photoreceptors: implications for phosphoinositide-mediated Ca²⁺ mobilization

Drosophila photoreceptors are excellent models for studies of the ubiquitous phosphoinositide signalling cascade. Recent studies suggest that light-induced phosphoinositide hydrolysis in Drosophila leads to the activation of two classes of channels. One is selective for Ca²⁺ and absent in the transient receptor potential mutant trp. The trp gene product, which shows some structural similarity to vertebrate voltage-gated Ca²⁺ channels, may thus define a novel family of second-messenger-operated Ca²⁺ channels generally responsible for the widespread but poorly understood phenomenon of phosphoinositide-mediated Ca²⁺ entry. The other channel is a non-selective cation channel that requires Ca²⁺ for activation. As well as being a major charge carrier for the light-induced current, Ca²⁺ influx via the trp-dependent channels appears to be required for refilling Ca²⁺ stores sensitive to inositol 1,4,5-trisphosphate and for feedback regulation (light adaptation) of the transduction cascade.