Transport of Carotenoids by a Carotenoid-Binding Protein in the Silkworm - Carotenoids: Physical, Chemical, and Biological Functions and Properties - page 518

Midgut lumen Intestinal mucosal cell Hemolymph Silk gland cell Liquid silk

Lipophorin

CBP CBP

Lutein

Membrane

LTP?

protein?

CBP

Lipophorin
receptor

Lipophorin

FIGURE 24.6 Model of the molecular function of CBP. CBP moves in the cytosol and relays carotenoid in
combination with the lipophorin in the hemolymph. At the membrane, other factors might be involved in the
carotenoid transport (magnii cation, see the text for explanation).

Although precise mapping is needed to draw a strict conclusion, all of the data strongly suggest
that the Y gene corresponds directly to the CBP gene. Furthermore, CBP is the i rst intracellular
molecule involved in carotenoid transport to be genetically characterized. Figure 24.6 presents a
“gondola” model for CBP function in which CBP picks up a load (carotenoid) on one bank (apical
membrane), moves it into the lake (cytosol) in a boatlike manner, and sets it down on the other bank
(basal membrane). Immunohistochemical and confocal microscopic analysis (Alpy et al. 2001), or
live imaging of mutants tagged with l uorescent protein (Zhang et al. 2002) might provide insights
into the validity of this model.
Analysis of lipid composition differences in the hemolymph, midgut, fat body, and silk gland
between the Y and

Y allele strains suggested that the Y gene does not affect the metabolism of
general lipids, hydrocarbon, triacylglycerol, diacylglycerol, cholesterol, or phospholipid (Tsuchida
et al. 2004b). This specii city is expected to be determined by the ligand specii city of CBP for
carotenoids.

+

24.4 MEMBRANE-SPANNING ISOFORM OF CBP
Considering the function of its homologous genes, CBP can be expected to have a function in cho-
lesterol transport: StAR and MLN64, the closest mammalian homologs of CBP (Figure 24.2b), are
known to bind and transport cholesterol. Furthermore, DmStart1, the closest homolog of CBP, StAR,
and MLN64 in D. melanogaster , is highly expressed in the prothoracic gland cells where ecdysone,
the molting hormone that plays a pivotal role in the control of the insect developmental schedule,
is synthesized from cholesterol (Roth et al. 2004). DmStart1 contains a MENTAL domain in its
N-terminus and a START domain in its C-terminus, as does MLN64. Its developmental expression
pattern in the larval stage correlates with the humoral ecdysone level. No other homologs of StAR,