The blind Drosophila mutant ninaD lacks the visual chromophore. Genetic evidence that the molecular basis is a defect in carotenoid uptake which causes vitamin A deficiency exists. The ninaD gene encodes a scavenger receptor that is significantly homologous in sequence with the mammalian scavenger receptors SR-BI (scavenger receptor class B type I) and CD36 (cluster determinant 36), yet NinaD has not been characterized in functional detail. Therefore, we established a Drosophila S2 cell culture system for biochemically characterizing the ninaD gene products. We show that the two splice variant isoforms encoded by ninaD exhibit different subcellular localizations. NinaD-I, the long protein variant, is localized at the plasma membrane, whereas the short variant, NinaD-II, is localized at intracellular membranes. Only NinaD-I could mediate the cellular uptake of carotenoids from micelles in this cell culture system. Carotenoid uptake was concentration-dependent and saturable. By in vivo analyses of different mutant and transgenic fly strains, we provide evidence of an essential role of NinaD-I in the absorption of dietary carotenoids to support visual chromophore synthesis. Moreover, our analyses suggest a role of NinaD-I in tocopherol metabolism. Even though Drosophila is a sterol auxotroph, we found no evidence of a contribution of NinaD-I to the uptake of these compounds. Together, our study establishes an evolutionarily conserved connection between class B scavenger receptors and the numerous functions of fat soluble vitamins in animal physiology.
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http://dx.doi.org/10.1021/bi060701u | DOI Listing |
Int J Mol Sci
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Department of Medical, Surgical and Neurological Sciences, University of Siena, 53100 Siena, Italy.
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Division of Pharmacology, Meikai University School of Dentistry, 1-1 Keyakidai, 350-0283, Sakado, Saitama, Japan.
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