Novel function of pigment granules in the pigment cells of retina

果蝇视网膜的色素细胞含有大量的色素颗粒。我们纯化了这些颗粒，并鉴定了两种可能与视觉周期有关的蛋白质(脂肪酸结合蛋白(FBP)和光感受器/色素细胞脱氢酶(PDH))。为了研究FBP和PDH的功能，我们首先对这些蛋白进行了免疫组织化学定位。结果表明，这两种蛋白均定位于色素颗粒表面和视网膜色素细胞的细胞质中。接下来，我们利用在大肠杆菌中表达的蛋白检测了FBP与全反式视黄醇的结合活性。结果表明，视黄醇能与FBP特异性结合。此外，我们还发现FBP可以与PDH免疫共沉淀，这表明FBP和PDH可能参与了类维A酸代谢的共同途径，并直接相互作用。接下来，我们通过过度表达野生型FBP或用dsRNA抑制它来修改FBP的体内表达。突变果蝇体内维甲酸成分的高效液相色谱分析表明，FBP参与了11-顺式视黄醇暗异构化为全反式异构体的过程。这些结果表明，FBP和PDH参与了果蝇视网膜色素细胞中11-顺式视黄醇的暗异构化和氧化为全反式视黄醇，这可能与脊椎动物的视网膜色素上皮细胞一样，在视觉周期中起着重要作用。

Pigment cells of the Drosophila retina contain numerous pigment granules. We purified these granules and identified 2 proteins (fatty acid-binding protein (FBP) and photoreceptor/pigment cell dehydrogenase (PDH)) possibly involved in the visual cycle. To investigate the function of FBP and PDH, we first carried out immunohistochemical localization of these proteins. The results demonstrated that both proteins are localized on the surface of pigment granules as well as in the cytoplasm of retinal pigment cells. We next examined the binding activity of FBP to all-trans retinol with the use of the protein expressed in E. coli cells. The results demonstrated that retinol was specifically bound to FBP. Furthermore, we showed that FBP could be coimmunoprecipitated with PDH, suggesting that FBP and PDH would be involved in a common pathway of retinoid metabolism and directly interact with each other. We next modified the in vivo expression of FBP by overexpressing wild-type FBP or suppressing it with dsRNA. The HPLC analysis of retinoid composition in the mutant flies demonstrated that FBP is involved in the dark isomerization of 11-cis retinol to all-trans isomer. These results thus suggest that FBP and PDH are involved in the dark isomerization and oxidation of 11-cis retinol to all-trans retinal in the Drosophila retinal pigment cells, which may play a crucial role in the visual cycle like retinal pigment epithelium cells in the vertebrate eye.