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kDa
50
37
CBP
25
(a)
Lumen
G
(b)
FIGURE 24.3
Distribution of CBP. (a) Analysis of tissue distribution by Western blotting. (b)
Immunohistochemistry of the midgut epithelium. CBP was not detected in the goblet cells (G) of the midgut
epithelium, which are thought to be involved in ion transport.
of the developmental proi le of CBP expression in the midgut revealed that CBP reached the high-
est concentration on days 4 and 5 of the i fth larval instar, which coincides with the highest food
consumption (Tsuchida et al. 2004a). Immunohistochemistry demonstrated that CBP was located
in the epithelial cells of the midgut and silk gland that are in contact with the intestinal lumen and
hemolymph, respectively (Figure 24.3b) (Tsuchida et al. 2004a). In the midgut, CBP was uniformly
expressed along the brush border of the columnar cells, providing a large surface for carotenoid
absorption. These data on the distribution of CBP are consistent with the view that CBP is involved
in the cellular uptake of carotenoids from the diet or lipophorin at the apical membrane and that
CBP functions as a key determinant of the concentration of carotenoids in each tissue.
24.3 LINK BETWEEN CBP AND A GENETIC LOCUS RESPONSIBLE FOR
CAROTENOID TRANSPORT
24.3.1 S
ILKWORM
G
ENES
R
ESPONSIBLE
FOR
C
AROTENOID
T
RANSPORT
The expression proi le of CBP suggests its involvement in carotenoid transport, which led to the
investigation of the relationship between CBP and the genetic loci for carotenoid transport. The
genetic loci for carotenoid transport are summarized in Figure 24.4a. Among them, we describe the
Y
(
yellow blood
),
I
(
yellow inhibitor
), and
C
(
yellow cocoon
) genes. The functions of these three
loci are illustrated diagrammatically in Figure 24.4b. The
Y
gene (Toyama 1906, 1912) controls the
uptake of carotenoids from the midgut lumen into the intestinal mucosal cells and from the hemo-
lymph into the cells in the middle division of the silk gland (Nakajima 1963). Larvae of mutants
with the
+
Y
/
+
Y
genotype (
+
Y
indicates a recessive allele of the
Y
gene) inadequately absorb dietary
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