Interaction of the alpha-helices of apolipophorin III with the phospholipid acyl chains in discoidal lipoprotein particles: a fluorescence quenching study

pH-Dependent Interactions of Apolipophorin-III with a Lipid Disk

Apolipophorin-III (ApoLp-III) is required for stabilization of molecular shuttles of lipid fuels in insects and is found to contribute to the insect immune reaction. Rearrangement of its five [Formula: see text]-helices enables ApoLp-III to reversibly associate with lipids. We investigate computationally the conformational changes of ApoLp-III and the pH-dependence of the binding free energy of ApoLp-III association with a lipid disk. A dominant binding mode along with several minor, low population, modes of the ApoLp-III binding to a lipid disk was identified. The pH-dependence of the binding energy for ApoLp-III with the lipid disk is predicted to be significant, with the pH-optimum at pH[Formula: see text]. The calculations suggest that there are no direct interactions between the lipid head groups and titratable residues of ApoLp-III. In the physiological pH range from 6.0 to 9.0, the binding free energy of ApoLp-III with the lipid disk decreases significantly with respect to its optimal value at pH 8.0 (at pH[Formula: see text], it is 1.02[Formula: see text]kcal/mol and at pH[Formula: see text] it is 0.23[Formula: see text]kcal/mol less favorable than at the optimal pH[Formula: see text]), indicating that the pH is an important regulator of ApoLp-III lipid disk association.

Apolipoprotein-induced conversion of phosphatidylcholine bilayer vesicles into nanodisks

Apolipoprotein mediated formation of nanodisks was studied in detail using apolipophorin III (apoLp-III), thereby providing insight in apolipoprotein-lipid binding interactions. The spontaneous solubilization of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) vesicles occured only in a very narrow temperature range at the gel-liquid-crystalline phase transition temperature, exhibiting a net exothermic interaction based on isothermal titration calorimetry analysis. The resulting nanodisks were protected from proteolysis by trypsin, endoproteinase Glu-C, chymotrypsin and elastase. DMPC solubilization and the simultaneous formation of nanodisks were promoted by increasing the vesicle diameter, protein to lipid ratio and concentration. Inclusion of cholesterol in DMPC dramatically enhanced the rate of nanodisk formation, presumably by stabilization of lattice defects which form the main insertion sites for apolipoprotein α-helices. The presence of fully saturated acyl chains with a length of 13 or 14 carbons in phosphatidylcholine allowed the spontaneous vesicle solubilization upon apolipoprotein addition. Nanodisks with C13:0-phosphatidylcholine were significantly smaller with a diameter of 11.7 ± 3.1nm compared to 18.5 ± 5.6 nm for DMPC nanodisks determined by transmission electron microscopy. Nanodisk formation was not observed when the phosphatidylcholine vesicles contained acyl chains of 15 or 16 carbons. However, using very high concentrations of lipid and protein (>10mg/ml), 1,2,-dipalmitoyl-sn-glycero-3-phosphocholine nanodisks could be produced spontaneously although the efficiency remained low.

The helix bundle: a reversible lipid binding motif

Apolipoproteins are the protein components of lipoproteins that have the innate ability to inter convert between a lipid-free and a lipid-bound form in a facile manner, a remarkable property conferred by the helix bundle motif. Composed of a series of four or five amphipathic alpha-helices that fold to form a helix bundle, this motif allows the en face orientation of the hydrophobic faces of the alpha-helices in the protein interior in the lipid-free state. A conformational switch then permits helix-helix interactions to be substituted by helix-lipid interactions upon lipid binding interaction. This review compares the apolipoprotein high-resolution structures and the factors that trigger this switch in insect apolipophorin III and the mammalian apolipoproteins, apolipoprotein E and apolipoprotein A-I, pointing out the commonalities and key differences in the mode of lipid interaction. Further insights into the lipid-bound conformation of apolipoproteins are required to fully understand their functional role under physiological conditions.

Purification and characterization of recombinant lipid storage protein-2 from Drosophila melanogaster

Lipid storage protein 2 (Lsd 2) is a conserved insect protein that belongs to the small PAT family of proteins. PAT proteins are found associated to the lipid droplets of adipocytes and play significant roles in the regulation of triacylglycerides metabolism. Here we describe the expression and purification of Lsd2, its reconstitution in lipoprotein particles, the location of putative lipid binding sites and its secondary structure. This study provides the starting point for future studies on the mechanism of function of Lsd2. The similarities and differences between Lsd1 and Lsd2, the only PAT proteins found in insects, are discussed.

On the structure and function of apolipoproteins: more than a family of lipid-binding proteins

Exchangeable apolipoproteins have been the subject of intense biomedical investigation for decades. However, only in recent years the elucidation of the three-dimensional structure reported for several members of the apolipoprotein family has provided insights into their functions at a molecular level for the first time. Moreover, the role of exchangeable apolipoproteins in several cellular events distinct from lipid metabolism has recently been described. This review summarizes these contributions, which have not only allowed the identification of the apolipoprotein domains that determine substrate binding specificity and/or affinity but also the plausible molecular mechanism(s) involved.

Structure of apolipophorin-III in discoidal lipoproteins. Interhelical distances in the lipid-bound state and conformational change upon binding to lipid

The structure of apolipophorin III in the lipid-bound state and the extent of the conformational change that takes place when the five-helix bundle apolipoprotein binds to a lipoprotein lipid surface were investigated by fluorescence resonance energy transfer in discoidal lipoproteins. Four intramolecular interhelical distances between helix pairs 1-4, 2-4, 3-4, and 5-4 were estimated by fluorescence resonance energy transfer in both the lipid-free and the lipid-bound states. Depending on the helices pairs, the intramolecular interhelical distances increased between 15 and > or = 20 A upon binding of the apolipoprotein to lipid, demonstrating for the first time that binding to lipid is accompanied by a major change in interhelical distances. Using discoidal lipoproteins made with a combination of apolipophorin III molecules containing donor and acceptor groups and apolipophorin III molecules containing neither donor nor acceptor groups, it was possible to obtain information about intermolecular interhelical distances between the helix 4 of one apolipoprotein and the helices 1, 2, 3, and 5 of a second apolipoprotein residing in the same discoidal lipoprotein. Altogether, the estimated intermolecular and intramolecular interhelical distances suggest a model in which the apolipoprotein arranges in pairs of antiparallel and fully extended polypeptide chains surrounding the periphery of the bilayer disc.