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Phototransduction in Drosophila melanogaster

Roger C. Hardie^*

Cambridge University, Department of Anatomy, Downing Street, Cambridge CB2 3DY, UK

View larger version (56K): [in a new window]   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 Gq protein via GTP–GDP exchange, releasing the Gq subunit. Genes and mutants for both (dgq) and ß (gbe) subunits have been identified. (2) Gq 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).

View larger version (10K): [in a new window]   Fig. 2. Voltage-clamped responses from dissociated Drosophila melanogaster photoreceptors. (A) Quantum bumps in a vertebrate [top, toad rod, courtesy of Professor T. Lamb; data from (Whitlock and Lamb, 1999)] compared with Drosophila melanogaster (below). Note the different scales; the kinetics of the toad bump is approximately 100 times slower than that of Drosophila melanogaster. (B) Drosophila melanogaster quantum bumps recorded in normal (1.5 mmol l-1), intermediate (0.1 mmol l-1) and ‘0’ (<100 nmol l-1) Ca2+. The amplitude is reduced and the kinetics greatly slowed in low external [Ca2+]. In intermediate concentrations, a slow rising phase appears to trigger a full-sized bump after a brief delay [adapted from (Henderson et al., 2000)]. (C) Responses to 1 s steps of light of increasing intensity (maximum approximately 50 000 effectively absorbed photons per second); the rapid peak-to-plateau transitions are a direct manifestation of Ca2+-dependent light adaptation [adapted from (Raghu et al., 2000a)].