Crystallization and preliminary analysis of crystals of apolipophorin III isolated from Locusta migratoria

Molecular characterization of an Apolipophorin-III gene from the Chinese oak silkworm, Antheraea pernyi (Lepidoptera: Saturniidae)

Apolipophorin-III (ApoLp-III) acts in lipid transport, lipoprotein metabolism, and innate immunity in insects. In this study, an ApoLp-III gene of Antheraea pernyi pupae (Ap-ApoLp-III) was isolated and characterized. The full-length cDNA of Ap-ApoLp-III is 687 bp, including a 5'-untranslated region (UTR) of 40 bp, 3'-UTR of 86 bp and an open reading frame of 561 bp encoding a polypeptide of 186 amino acids that contains an Apolipophorin-III precursor domain (PF07464). The deduced Ap-apoLp-III protein sequence has 68, 59, and 23% identity with its orthologs of Manduca sexta, Bombyx mori, and Aedes aegypti, respectively. Phylogenetic analysis showed that the Ap-apoLp-III was close to that of Bombycoidea. qPCR analysis revealed that Ap-ApoLp-III expressed during the four developmental stages and in integument, fat body, and ovaries. After six types of microorganism infections, expression levels of the Ap-ApoLp-III gene were upregulated significantly at different time points compared with control. RNA interference (RNAi) of Ap-ApoLp-III showed that the expression of Ap-ApoLp-III was significantly downregulated using qPCR after injection of E. coli. We infer that the Ap-ApoLp-III gene acts in the innate immunity of A. pernyi.

Hydrogen/deuterium exchange kinetics of apolipophorin-III in lipid-free and phospholipid-bound states. An analysis by Fourier transform infrared spectroscopy

Attenuated total reflection Fourier transform infrared spectroscopy was used to probe the kinetics of hydrogen/deuterium exchange in Manduca sexta apolipophorin-III (apoLp-III). ApoLp-III is an exchangeable apolipoprotein that is made up of five elongated amphipathic alpha-helices in a helical bundle conformation in the monomeric lipid-free form. Upon interaction with phospholipids, it is postulated to undergo a large conformational change whereby the hydrophobic interior is exposed, facilitating binding to the lipid surfaces. We have used the lipid-free and dimyristoylphosphatidylcholine-bound apoLp-III to study the dynamically variable domains in the two forms. Three populations of amide protons varying in their hydrogen/deuterium exchange rates were found to exist: slow, intermediate, and fast exchanging, which could correspond to completely buried, partially buried, and solvent-exposed domains on the protein in both the states. In lipid-free apoLp-III, 36, 12, and 52% of the total residues contributed to the slow, intermediate, and fast exchanging populations, respectively. In the dimyristoylphosphatidylcholine-bound form, the corresponding distribution was 20, 16, and 64%, representing a 12% increase in the number of exposed residues. The results are discussed in terms of increased solvent accessibility due to gross tertiary structural reorganization.

Alignment of the apolipophorin-III alpha-helices in complex with dimyristoylphosphatidylcholine. A unique spatial orientation

Apolipophorin-III (apoLp-III) from Manduca sexta can exist in two alternate states: as a globular, lipid-free helix bundle or a lipid surface-associated apolipoprotein. Previous papers (Ryan R.O., Oikawa K., and Kay C. M. (1993) J. Biol. Chem. 268, 1525-1530; Wientzek M., Kay C.M., Oikawa K., and Ryan R.O. (1994) J. Biol. Chem. 269, 4605-4612) have investigated the structures and properties of apolipophorin-III from M. sexta in the lipid-free state and associated to lipids. Association of apoLp-III with dimyristoylphosphatidylcholine vesicles leads to the formation of uniform lipid discs with an average diameter and thickness of 18.5 +/- 2.0 and 4.8 +/- 0.8 nm, respectively. These discs contain six molecules of apoLp-III. Geometrical calculations based on these data, together with x-ray crystallographic data from the homologous L. migratoria apoLp-III (Breiter D. R., Kanost M.R., Benning M.M., Wesenberg G., Law J.H., Wells M.A., Rayment I., and Holden H.M. (1991) Biochemistry 30, 603-608), have allowed the presentation of a model of lipid-protein interaction, in which the alpha-helices of the apoLp-III orient perpendicular to the phospholipid chains and surround the lipid disc. Here, using polarized Fourier transform-attenuated total reflection infrared spectroscopy, we provide the first experimental evidence of a unique perpendicular orientation of the alpha-helices with respect to the fatty acyl chains of the phospholipids in the disc.

Structural and binding characteristics of the carboxyl terminal fragment of apolipophorin III from Manduca sexta

The molecular basis of the interaction of apolipophorin III (apoLp-III), an exchangeable apolipoprotein from hemolymph of the sphinx moth. Manduca sexta, with lipoprotein surfaces and phospholipids was studied by investigating the structural and binding properties of the C-terminal fragment of the native protein. A 4K peptide, corresponding to the terminal helical segment of the native protein, was generated by cyanogen bromide treatment, purified by gel filtration and reverse-phase HPLC, and characterized by N-terminal sequencing and amino acid and mass spectrometric analysis. Circular dichroism (CD) spectroscopy of the peptide in buffer indicated a predominantly unstructured state while addition of trifluoroethanol (TFE), a helix-inducing agent, resulted in an alpha-helical structure. Sedimentation equilibrium studies revealed that the 4K peptide was monomeric in buffer. The 4K peptide assumed an alpha-helical conformation in the presence of sodium dodecyl sulfate (SDS) and lysolecithin, but was unstructured in the presence of dimyristoylphosphatidylcholine, either when added to preformed vesicles or upon cosonication, indicating an ability to bind to detergent micelles but not to phospholipid bilayers. Unlike native apoLp-III, the 4K peptide did not confer protection against turbidity development to human low density lipoprotein upon incubation with phospholipase C, indicating an inability to interact with the surface of lipoproteins. Upon interaction with SDS micelles, both the 4K peptide and apoLp-III were resistant to urea-induced denaturation when compared to free apoLp-III, as evaluated by CD spectroscopy. The structural stability conferred upon interaction with detergents was similar for both the peptide and the native protein.(ABSTRACT TRUNCATED AT 250 WORDS)

Calorimetric and spectroscopic studies of the interaction of Manduca sexta apolipophorin III with zwitterionic, anionic, and nonionic lipids

The nature of the interaction of apolipophorin III (apoLp-III) from the insect Manduca sexta with a variety of zwitterionic and anionic phospholipids and with several nonionic glycolipids was investigated by differential scanning calorimetry (DSC) and 31P-NMR spectroscopy. Monoglucosyldiacylglycerol, phosphatidylethanolamine, and phosphatidic acid (pH 7.2) appear to exhibit the weakest interaction with apoLp-III. DSC studies revealed that the gel/liquid-crystalline phase transition of these lipids is little affected by the binding of apoLp-III. Diglucosyldiacylglycerol, phosphatidylcholine (PC), phosphatidylserine, and phosphatidic acid (pH congruent to 8.8) seem to exhibit somewhat stronger interactions with apoLp-III. The binding of apoLp-III to these lipids induces the formation of lipid domains which melt less cooperatively and at higher temperatures than do the pure lipid dispersions, while having little effect on the melting enthalpy of lipid hydrocarbon chains. Phosphatidylglycerol (PG) and phosphatidic acid (pH > 9.3) appear to exhibit the strongest interactions with apoLp-III. The binding of apoLp-III to these lipids severely disrupts their bilayer structure, resulting in marked reductions in the cooperativity and enthalpy of the gel/liquid-crystalline phase transition of the lipids. Studies of binary mixtures of PC and PG indicate that such bilayer-disrupting interactions only occur in the presence of nonphysiologically high concentrations of PG. The binding of apoLp-III to binary mixtures of diacylglycerol and zwitterionic phospholipid has little effect on the chain-melting or the bilayer/nonbilayer phase transitions of these lipids, but it appears to promote the retention of water at the surface of the lipid aggregate. Our results indicate that the binding of apoLp-III to lipid bilayers is mediated primarily through polar and/or ionic interactions at the lipid bilayer surface. Our results also suggest that the interaction of apoLp-III with lipid bilayers promotes the hydration of their surfaces, a property which is consistent with the proposed in vivo functions of this protein.

Trimethyllead acetate: a first-choice heavy atom derivative for protein crystallography

The three-dimensional conformation of a protein provides a wealth of biochemical information and with the advent of cloning techniques that allow the preparation of proteins almost at will, a renewed interest has arisen in the crystallographic determination of protein structures. As in any research technique, however, there are often many difficulties encountered in an X-ray crystallographic investigation. One of these is the "phase problem." Although in recent years there has been considerable progress in the development of techniques for phase determination, including the use of molecular replacement and multiple wavelength measurements, the multiple isomorphous replacement method is still the most successful method for obtaining a three-dimensional structure. Here we report the use of trimethyllead acetate as a heavy atom compound of first choice in the preparation of an isomorphous heavy atom derivative.