Transport of Carotenoids by a Carotenoid-Binding Protein in the Silkworm - Carotenoids: Physical, Chemical, and Biological Functions and Properties - page 519

A -r eg ion C-region

BmStart1

TM

Start domain

CBP B-region C-region

Start domain

10 0 b p

Start codon Stop codon

(a)

A-region B-region C-region

CATS

1kb

(b)

FIGURE 24.7 BmStart1, an alternative splicing isoform of CBP. (a) Schematic structure of BmStart1 and
CBP cDNA sequences. BmStart1 consists of the BmStart1-specii c A-region and the common C-region, while
CBP consists of the CBP-specii c B-region and the C-region. A-region codes the four putative TM helices. (b)
Schematic structure of the BmStart1/CBP genomic sequence in the +

Y allele strain. Connected lines indicate
the structure of the BmStart1 cDNA. A-region and C-region are not disrupted by CATS, allowing the expres-
sion of BmStart1 in the

+

Y allele strain.

MLN64, and DmStart1 other than CBP could be found in the sequence database of B. mori . Does
CBP play a role in ecdysteroidogenesis? What is the counterpart of StAR/MLN64 in the +

Y allele

strain, where CBP is absent?
It may be noteworthy that CBP has an alternative splicing isoform, BmStart1 (Figure 24.7a)
(Sakudoh et al. 2005). BmStart1 is comprised of 12 exons, i ve of which are identical to the exon
of the 3 terminus of CBP (Figure 24.7b). BmStart1 shares the START domain in the C-terminus

with CBP, and contains a MENTAL domain in its N-terminus. The exons of BmStart1 are not
disrupted in the

′

Y allele strain, and BmStart1 was expressed in various tissues including ecdys-
teroidogenic tissues, prothoracic gland, testis, and ovary in both the Y and

+

Y allele strains.
BmStart1 mRNA abundance in the prothoracic gland, the main ecdysteroidogenic tissue in
B. mori , was relatively elevated in the latter larval stage, when ecdysteroid synthesis is highly
active. Thus, BmStart1 could be a candidate for the counterpart of StAR/MLN64 in the silk-
worm, though it could also be a membrane-tethered CBP. If BmStart1 transports cholesterol and
not carotenoid, an alternative splicing could produce unique protein isoforms whose endogenous
ligands are structurally and functionally different, perhaps sterol or carotenoid. Elucidation of the
function of BmStart1 in lipid transport will provide insights into the functional evolution of the
genes homologous to CBP.

+

24.5 CONCLUDING REMARKS AND FUTURE PERSPECTIVES
CBP has been purii ed from silk gland and has been successfully shown, by utilizing the genetic
resources and technologies developed for sericulture, to have functional signii cance in carotenoid
transport. The study of CBP demonstrates that identii cation of CBPs indeed will be a powerful way
to dissect the transport system for carotenoids in each organism. To date, several CBPs have been
identii ed from various species and tissues (Bhosale and Bernstein 2007). Genetic analysis of these
proteins might also establish their functional signii cance in each transport system of carotenoids.
We consider that there must be CBPs other than CBP in the silkworm. Lipophorin binds carote-
noids (Tsuchida et al. 1998). Fujii et al. isolated a CBP of 60 kDa from the larval hemolymph, and