How does visual signal transduction take place?
Drosophila visual signal transduction, the process by which incoming light is converted to neural signals that can be passed to the brain, provides an ideal system for the molecular dissection the process by which extracellular signals are transduced across the plasma membrane leading to neuron activation. In this signal transduction pathway, light stimulates rhodopsin, which activates an eye-specific G protein (Galphaq). Six genes have been identified that code for different Rhodopsins expressed in different sets of photoreceptor cells in the visual system (FlyBase, 1998). The phototransduction cascade is one of the fastest known G protein receptor coupling systems. In Drosophila, the high temporal resolution is evidenced by the less than 20 milli-second latency between photon excitation and photoreceptor cell depolarization (Ranganathan, 1991). Activated Galphaq triggers NorpA (no receptor potential A), a phospholipase C-beta to catalyze the breakdown of phospholipids to generate ino
