Recent progress in molecular genetic studies on the carotenoid transport system using cocoon-color mutants of the silkworm

The existence of tissue-specific delivery for certain carotenoids is supported by genetic evidence from the silkworm Bombyx mori and the identification of cocoon color mutant genes, such as Yellow blood (Y), Yellow cocoon (C), and Flesh cocoon (F). Mutants with white cocoons are defective in one of the steps involved in transporting carotenoids from the midgut lumen to the middle silk gland via the hemolymph lipoprotein, lipophorin, and the different colored cocoons are caused by the accumulation of specific carotenoids into the middle silk gland. The Y gene encodes carotenoid-binding protein (CBP), which is expected to function as the cytosolic transporter of carotenoids across the enterocyte and epithelium of the middle silk gland. The C and F genes encode the C locus-associated membrane protein, which is homologous to a mammalian high-density lipoprotein receptor-2 (Cameo2) and scavenger receptor class B member 15 (SCRB15), respectively; these membrane proteins are expected to function as non-internalizing lipophorin receptors in the middle silk gland. Cameo2 and SCRB15 belong to the cluster determinant 36 (CD36) family, with Cameo2 exhibiting specificity not only for lutein, but also for zeaxanthin and astaxanthin, while SCRB15 seems to have specificity toward carotene substrates such as α-carotene and β-carotene. These findings suggest that Cameo2 and SCRB15 can discriminate the chemical structure of lutein and β-carotene from circulating lipophorin during uptake. These data provide the first evidence that CD36 family proteins can discriminate individual carotenoid molecules in lipophorin.