Fig. 1. Drosophila melanogaster phototransduction cascade. Inset: cross-section of a Drosophila melanogaster rhabdomere (electron micrograph courtesy of Dr A Polyanovsky), which is composed of some 30 000 microvilli, each approximately 1–2 µm in length and 60 nm in diameter. Each microvillus contains approximately 1000 molecules of rhodopsin and most elements of the phototransduction machinery. An enlargement of the circled area in the inset, showing the base of one microvillus with associated phototransduction machinery, is shown schematically in the main figure. Activation: (1) photoisomerization of rhodopsin to metarhodopsin (RhM, encoded by the ninaE gene) activates heterotrimeric G_q protein via GTP–GDP exchange, releasing the G_q subunit. Genes and mutants for both (dgq) and ß (gbe) subunits have been identified. (2) G_q activates phospholipase C (PLC; norpA gene), generating inositol 1,4,5-trisphosphate (InsP₃) and diacyl glycerol (DAG) from phosphatidyl inositol 4,5-bisphosphate (PIP₂). DAG is also a potential precursor for polyunsaturated fatty acids (PUFAs) via DAG lipase (gene yet to be identified in any eukaryote). (3) Two classes of light-sensitive channel (TRP and TRPL; trp and trpl genes) are activated by an unknown mechanism. Several components of the cascade, including the ion channel TRP, protein kinase C (PKC, inaC gene) and PLC are coordinated into a signalling complex by the scaffolding protein, INAD, which contains five PDZ domains. (4) At the base of the microvilli, a system of submicrovillar cisternae (SMC) has traditionally been presumed to represent Ca²⁺ stores endowed with InsP₃ receptors (InsP₃R; dip gene) and smooth endoplasmic reticulum Ca²⁺-ATPase; however, the SMC may play a more important role in phosphoinositide turnover (5): DAG is converted to phosphatidic acid (PA) via DAG kinase (rdgA gene) and to CDP-DAG via CD synthase (cds gene) in the SMC. After conversion to phosphatidyl inositol (PI) by PI synthase, PI is transported back to the microvillar membrane by a PI transfer protein (rdgB gene). PI is converted to PIP₂ via sequential phosphorylation (PI kinase and PIP kinase).