Figure 4.

Schematic representation of the molecular components of the signal transduction cascade of Drosophila. Upon absorption of a photon, rhodopsin (R) is converted into metarhodopsin (M). This photoconversion leads to the activation of heterotrimeric G-protein (G_qα) by promoting the GDP to GTP exchange. In turn, this leads to activation of phospholipase Cβ (PLCβ), which hydrolyzes PIP₂ into the soluble InsP₃ and the membrane-bound DAG. Subsequently, two classes of light-sensitive channels, the TRP and TRPL open by a still unknown mechanism. PLC also promotes hydrolysis of the bound GTP, resulting in G_qα bound to GDP and this ensures the termination of G_qα activity. The TRP and TRPL channel openings lead to elevation of cellular Ca²⁺. Elevation of DAG and Ca²⁺ promote eye-specific protein kinas C activity, which regulates channel activity. PLC, PKC and the TRP ion channel form a supramolecular complex with the scaffolding protein INAD.