Mechanical Maneuvers Enable Fruit Fly Sight

The missing link in understanding the conversion of light to an electrical signal in Drosophila photoreceptor cells might be a mechanical contraction of cell membranes (Science, DOI: 10.1126/science.1222376). The complex path of sight starts with photo­isomerization of rhodopsin (a G-protein-coupled receptor; see page 6), goes through cleavage of the inositol head from the membrane phospholipid phosphatidylinositol 4,5-bisphos­phate (PIP₂), and ends with opening of an ion channel to import Ca²⁺ and Na⁺ into the cell. But how, exactly, hydrolysis of the phospholipid triggers opening of the ion channel was a mystery. Roger C. Hardie and Kristian Franze of Cambridge University noticed that light flashes induced physical contractions in photoreceptors, which they measured using atomic force microscopy. Cleaving the bulky inositol head from PIP₂ releases a proton and leaves behind a slimmer diacylglycerol in the membrane lipid bilayer, allowing receptor membranes to contract. The mechanical force of the contraction, combined with the released proton, triggers opening of the ion channels, the researchers propose. “The study of Hardie and Franze highlights the emerging concept that transmembrane proteins are sensitive to the membrane environment,” says University of Southern California neurobiologist Emily R. Liman in commentary about the work.