Phototransduction mechanisms inDrosophilamicrovillar photoreceptors

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Phototransduction inDrosophilais mediated by a G-protein coupled phospholipase C (PLC) cascade leading to graded membrane depolarization. It is the preferred model for phototransduction in microvillar photoreceptors and an important model for the ubiquitous phosphoinositide signaling cascade. The photoreceptors respond sensitively to single photons 10–100× more rapidly than vertebrate rods, yet still light adapt to signal under full sunlight. Incident light is absorbed and transduced in the rhabdomere, a 1–2 μm diameter, 100 μm long waveguide composed of ˜30,000 tightly packed microvilli, which contain the visual pigment rhodopsin and all the major components of the cascade. PLC hydrolyzes phosphatidyl-inositol (4,5) bisphosphate (PIP2) to generate diacylglycerol (DAG), inositol (1,4,5) trisphosphate (InsP3), and also a proton. This results in activation of two classes of Ca2+ permeable cation channels, TRP and TRPL—the prototypical members of the transient receptor potential (TRP) superfamily of cation channels. Recent evidence suggests that the channels may be activated in a combinatorial manner by two neglected consequences of PLC activity, namely simultaneous depletion of PIP2 and acidification. Several components of the cascade, including TRP, PLC, and protein kinase C are assembled into multimolecular signaling complexes by the PDZ-domain scaffolding protein INAD. Ca2+ influx via TRP channels is essential for rapid kinetics, amplification, and light adaptation and mediates both positive and negative feedback via multiple downstream targets, including the channels, rhodopsin, and PLC. This Ca2+-dependent feedback along with the ultracompartmentalization provided by the microvillar design and molecular scaffolding is critical for the combination of sensitivity, rapid kinetics, and broad dynamic range.WIREs Membr Transp Signal 2012, 1:162–187. doi: 10.1002/wmts.20

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