Manduca sexta lipid transfer particle acts upon a lipoprotein to catalyze lipid and apoprotein disproportionation

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.

A discoidal lipoprotein from the coelomic fluid of the polychaete Nereis virens

A discoidal lipoprotein was isolated from the coelomic fluid of the polychaete, Nereis virens, by density gradient centrifugation. The lipoprotein was present in both sexes and moved as a uniform band in an agarose gel. The average diameter of the lipoprotein particles determined by electron microscopy was 42 nm with a thickness of 10 nm. SDS electrophoresis showed two apoprotein subunits with molecular masses of 247 and 85 kDa, respectively. In lectin blots, both apoproteins were reactive with Concanavalin A indicating the presence of N-glycans. The small subunit was also reactive with peanut lectin, indicating additional O-glycosylation. The total lipid content was 48% and consisted mainly of phospholipids and some diglycerides as judged by thin layer chromatography. The estimated native molecular mass of N. virens lipoprotein ( approximately 675 kDa) lies in the range of vertebrate high-density lipoprotein and insect lipophorins. The size of the apoproteins is similar to those found in insects, while the composition of the lipid fraction is more similar to that of crustacean lipoproteins.

Lipid storage and mobilization in insects: current status and future directions

In this paper we review the current status of research on fatty acid absorption and conversion to diacylglycerol in the midgut. We further discuss how diacylglycerol may leave the midgut and associate with lipophorin in hemolymph. We review the present understanding of the role of the lipid transfer particle and lipophorin receptors in lipid delivery between lipophorin and tissues. Finally, we discuss recent studies on the mobilization of diacylglycerol from the fat body in response to adipokinetic hormone. Several suggestions for exciting areas of future research are described.

Displacement of apolipophorin III from the surface of low density lipophorin by human apolipoprotein A-I

A hybrid low density lipophorin particle (LDLp) was prepared by incubation with human apolipoprotein (apo) A-I in vitro. ApoA-I associated with LDLp in a concentration dependent, saturable manner which was accompanied by dissociation of apolipophorin III (apoLp-III). The apoA-I hybrid LDLp had the same lipid composition, density and morphology as native LDLp indicating that displacement of apoLp-III by apoA-I did not affect its structural properties. The molar ratio of apoLp-I:apoLp-II:apoLp-III was maximally reduced from 1:1:16 to 1:1:2 in native versus hybrid LDLp with the latter particle binding 7 molecules of apoA-I. The inability of apoA-I to displace the remaining 2 apoLp-III supports the concept that these apoLp-III molecules are not equivalent to the other fourteen. Native and hybrid LDLp particles were both metabolized to high density lipophorin in vivo. The displacement reaction represents a novel method for the production of apolipoprotein hybrids of LDLp and the results indicate that apoA-I has an inherently higher affinity for lipid surfaces than apoLp-III.

Role of lipid transfer particle in transformation of lipophorin in insect oocytes

Oocyte development in Manduca sexta involves the deposition of large amounts of lipoprotein-derived lipid. One source of this lipid is hemolymph high-density lipophorin-adult (HDLp-A) which, upon entry into the oocyte, is transformed into a lipid and apolipophorin III deficient product particle, egg very-high- density lipophorin (VHDLp-E; density = 1.24 g/ml; Kawooya et al. (1988) J. Biol. Chem. 263, 8740-8747). An in vitro model of this transformation has been established using human low-density lipoprotein (LDL) as acceptor of HDLp-A associated lipid in a reaction catalyzed by isolated M. sexta hemolymph lipid transfer particle (LTP). Facilitated vectorial net transfer of lipid from HDLp-A to LDL resulted in formation of a very-high-density lipophorin (VHDLp) product with a density and apolipoprotein content similar to that of VHDLp-E. Lipid was found to comprise 25% of the VHDLp particle mass, whereas over 50% of HDLp-A mass is lipid. Based on these observations it was hypothesized that a lipid transfer factor may be present in M. sexta oocytes and function in the transformation of HDLp-A to VHDLp-E in vivo. Transfer activity was present in the buffer soluble fraction of oocyte homogenates and purification of the active material revealed a catalyst with electrophoretic and immunological properties identical to hemolymph LTP. Incubation of 125I-HDLp-A with an M. sexta oocyte homogenate resulted in transformation of the radiolabeled lipoprotein to a density corresponding to that of VHDLp-E. When the incubation media was preincubated with anti-LTP IgG this conversion was inhibited to a large extent. Inhibition was relieved, however, by addition of exogenous LTP. The results provide the first demonstration of M. sexta LTP in a tissue other than hemolymph and support the concept that LTP-catalyzed lipid transfer plays an integral role in the conversion of HDLp-A to VHDLp-E in vivo.