Figure 9.

EM pictures and a diagram showing structural features of the signaling compartment and the localization of the signaling molecules. Top, TEM showing the microvilli composing dark-adapted WT rhabdomere. A, TEM of a cross section of a rhabdomere, which is composed of tightly packed microvilli. The elongated membrane vesicles virtually touching the base of the microvilli (arrows) are extensions of the smooth ER called SMC and constitute the IP₃-sensitive Ca²⁺ stores. B, Higher magnification of the rhabdomere at the base of the microvilli, showing the SMC (arrows). C, A diagram showing the molecular components of the signal transduction cascade of Drosophila:,1, Upon absorption of a photon (hν), rhodopsin (R) is converted into metarhodopsin (M). 2, The R-to-M photoconversion leads to the activation of heterotrimeric G-protein(G_qα,β,γ) by promoting the GDP-to-GTP exchange. 3, The GDP-to-GTP exchange in turn leads to activation of PLCβ, which hydrolyzes PIP₂ into the soluble IP₃ and the membrane-bound DAG. 4, Subsequently, IP₃ molecules (blue dots) diffuse along the microvillus and bind to the IP₃ receptor (IP₃R) located at the SMC. 5, Binding of IP₃ to the IP₃R causes release of Ca²⁺ (red dots) from the SMC and its diffusion back into the microvillus followed by binding of Ca²⁺ to both PLCβ and the TRP channel. 6, This binding either facilitates the catalytic activity of PLCβ (?) or reduces the threshold of TRP channel activation (?, see Fig. 8). 7, Two classes of light-sensitive channels, the TRP and TRPL, open by a still unknown mechanism (?) following PLCβ activation. The TRP and TRPL channel openings lead to elevation of cellular Ca²⁺. Elevation of DAG and Ca²⁺ promote eye-specific PKC activity, which regulates channel activity. PLCβ, PKC, and the TRP ion channel form a supramolecular complex with the scaffolding protein INAD, which is bound to the F-actin cytoskeleton via the NINAC protein. Bottom, Magnification of the box marked by dotted lines in the top diagram.