Drosophila ommatidium, the cell cluster of eight photoreceptor cells surrounded by pigment cells. Intact ommatidium and cross-section through the distal region of the ommatidium. The first event in the phototransduction cascade is the absorption of a photon by the protein rhodopsin, which carries the light-absorbing chromophore. The all-trans isomer is called metarhodopsin, the activated state of rhodopsin. Both IP3 and DAG are intracellular messengers that can individually carry out further biochemical signalling cascades.
DAG activates protein kinase C in the R-cells.
All of this takes place in the R-cells' rhabdomere. Schematic model of the proposed phototransduction cascade in Drosophila. In some as yet unidentified way, these cascades cause the opening of ion channels in the plasmamembrane and depolarization of the cell. Termination of photoreceptor activation requires the inactivation of all steps in this pathway. Because of the universal importance of cellular signalling in living cells, and because Drosophila retina is amenable to various methodologies, these pathways are under intense investigation.