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Abstract
The transport pathway of specific dietary carotenoids from the midgut lumen to the silk gland in the silkworm, Bombyx mori, is a model system for selective carotenoid transport because several genetic mutants with defects in parts of this pathway have been identified that manifest altered cocoon pigmentation. In the wild-type silkworm, which has both genes, Yellow blood (Y) and Yellow cocoon (C), lutein is transferred selectively from the hemolymph lipoprotein to the silk gland cells where it is accumulated into the cocoon. The Y gene encodes an intracellular carotenoid-binding protein (CBP) containing a lipid-binding domain known as the steroidogenic acute regulatory protein-related lipid transfer domain. Positional cloning and transgenic rescue experiments revealed that the C gene encodes Cameo2, a transmembrane protein gene belonging to the CD36 family genes, some of which, such as the mammalian SR-BI and the fruit fly ninaD, are reported as lipoprotein receptors or implicated in carotenoid transport for visual system. In C mutant larvae, Cameo2 expression was strongly repressed in the silk gland in a specific manner, resulting in colorless silk glands and white cocoons. The developmental profile of Cameo2 expression, CBP expression, and lutein pigmentation in the silk gland of the yellow cocoon strain were correlated. We hypothesize that selective delivery of lutein to specific tissue requires the combination of two components: 1) CBP as a carotenoid transporter in cytosol and 2) Cameo2 as a transmembrane receptor on the surface of the cells.
Highlights
All organisms exposed to light contain carotenoids, which are yellow to red C40 hydrophobic isoprenoid pigments
The delivery of lipids, including carotenoids, to cells can be divided into three categories: 1) enzyme-mediated processes, such as the action of lipoprotein lipase on very low density lipoproteins, which converts a lipoprotein-bound lipid, triacylglycerol, into a water-soluble product, fatty acid, which diffuses into cells and leaves behind in the blood a lipoprotein product depleted in triacylglycerol [3]; 2) receptor-mediated endocytosis, such as the uptake of low density lipoproteins by low density lipoprotein receptor, in which the entire lipoprotein particle is taken into the cell and metabolized [4]; and 3) the delivery of specific lipids to specific tissues devoid of lipoprotein degradation, called selective lipid transport, such as the delivery of cholesterol ester from high density lipoprotein (HDL)2 to the adrenal gland [5]
Membrane helix in scavenger receptor class B type I (SR-BI) homologs, CD36 family genes, has the C-linked region was further narrowed to the 375-kb range been a matter of debate [5, 47], and some of them were between two Single Nucleotide Polymorphism (SNP) markers, nsc2993-6 and C-080419-R16, predicted to have a single- or double-pass transmembrane which was on one scaffold, Bm_scaf6 (Fig. 2B)
Summary
Silkworm Strains—The c04, c05, c10, c11, c43 (Pk), e09, FL501 (Y/ϩY), and FL501 (ϩY/ϩY) strains have been preserved at the silkworm stock center of Kyushu University, Fukuoka, Japan. For Western blotting analysis of the membrane fraction, 100 pieces of the silk gland of each strain on the day 0 of the wandering stage (W0) was homogenized in ice-cold insect saline containing a protease inhibitor mixture (Protease Inhibitor Mixture Set III, EDTA-free, Calbiochem, San Diego, CA) using a Polytron homogenizer. By sib mating of the progeny, a nondiapausing strain with the phenotype of yellow hemolymph and white cocoons, termed w1-pnd-925, was established. For transgenic expression of Cameo in the w1-pnd-925 strain by the binary GAL4/upstream activating sequence (UAS) system [30], Cameo was amplified by RT-PCR from the middle silk gland of the N4 strain with Primer (5Ј-ATGCTCTAGATTCCTTGTGATAATCGCGGC-3Ј) and Primer (5ЈATGCTCTAGACATACGGACTCATTCCAATG-3Ј), both of which have an XbaI site. Experimental procedures for determination of the Cameo and Cameo cDNA sequence and Southern blotting are described under supplemental data
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