Rhodopsin formation in Drosophila is dependent on the PINTA retinoid-binding protein

Retinoids participate in many essential processes including the initial event in photoreception. 11-cis-retinal binds to opsin and undergoes a light-driven isomerization to all-trans-retinal. In mammals, the all-trans-retinal is converted to vitamin A (all-trans-retinol) and is transported to the retinal pigment epithelium (RPE), where along with dietary vitamin A, it is converted into 11-cis-retinal. Although this cycle has been studied extensively in mammals, many questions remain, including the specific roles of retinoid-binding proteins. Here, we establish the Drosophila visual system as a genetic model for characterizing retinoid-binding proteins. In a genetic screen for mutations that affect the biosynthesis of rhodopsin, we identified a novel CRAL-TRIO domain protein, prolonged depolarization after potential is not apparent (PINTA), which binds to all-trans-retinol. We demonstrate that PINTA functions subsequent to the production of vitamin A and is expressed and required in the retinal pigment cells. These results represent the first genetic evidence for a role for the retinal pigment cells in the visual response. Moreover, our data implicate Drosophila retinal pigment cells as functioning in the conversion of dietary all-trans-retinol to 11-cis-retinal and suggest that these cells are the closest invertebrate equivalent to the RPE.