Isothermal calorimetry of apolipoproteins

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.

Secondary structure and orientation of the amphipathic peptide GALA in lipid structures. An infrared-spectroscopic approach

GALA, a synthetic, amphipathic 30-amino-acid peptide, based upon a Glu-Ala-Leu-Ala motive, was designed to mimic the behavior of viral fusion proteins. GALA is a water-soluble peptide with an aperiodic conformation at neutral pH, and becomes an amphipathic alpha helix as the pH is lowered to 5, where it interacts with phospholipid bilayers. Attenuated total-reflection infrared spectroscopy, using polarized light, provides information on the structure and orientation of the peptide and the lipids, which is not subject to artifacts due to light scattering with large particles. H/2H-exchange rate of the amide N-H group and analysis of the shape of the amide I' by Fourier self-deconvolution and curve fitting indicate that the alpha-helical content increases from 19% to 69%, on lowering the pH. A further increase to 100% alpha helix is observed after interaction with palmitoyloleoylglycerophosphocholine (PamOleGroPCho) vesicles. Dichroism data obtained with oriented bilayers of the PamOleGroPCho-GALA complex demonstrate that PamOleGroPCho hydrocarbon chains and the peptide alpha helical axis are essentially perpendicular (+/- 15) to the membrane plane. At neutral pH, in the presence of dimyristoylglycerophosphocholine (Myr2GroPCho), GALA is known to form discoidal structures similar to those formed under the same conditions by apolipoproteins AI and AII. In these discoidal complexes, the alpha-helical content was estimated to be 65%, with the rest of the structure being essentially unordered. No significant modification of the all-trans conformation of the hydrocarbon chain of Myr2GroPCho was detected upon disc formation. Dichroism measurements show that the alpha-helical axis is essentially parallel to the hydrocarbon chains. These data support a model in which a discoidal patch of the bilayer is surrounded by amphipathic helices which shield the hydrophobic region of the bilayer from the aqueous environment. The infrared spectrum of GALA in this complex was found to be very similar to those of apolipoproteins AI and AII which form discoidal complexes with Myr2GroPCho, but the spectrum is quite different from that of apolipoprotein B100 in low-density lipoproteins, which does not form discoidal complexes.

Mode of assembly of amphipathic helical segments in model high-density lipoproteins

The structure of discoidal apo A-I-phospholipid complexes, representing the metabolic precursors of mature high-density lipoprotein particles, was studied by a combination of both a theoretical and an experimental approach. The secondary structure of the complex was determined by circular dichroic measurements, while the relative orientation of the apo A-I helical segments and of the phospholipid acyl chains was determined by ATR infrared measurements. Fluorescence energy transfer between the tryptophan residues of apo A-I and fluorescent phospholipid probes yielded an estimation of the relative topography of the lipid and apolipoprotein components in discoidal and spherical particles. The theoretical approach consisted of the identification of the helical segments in various apo A-I species. These segments were then oriented at a lipid/water interface by minimization of their hydrophobic and hydrophilic transfer energies. The calculation of the hydrophobicity profiles along the axis of the helices leads to the identification of specific interactions between pairs of helices. The helices were further assembled together with the phospholipids by computer modelling, enabling an estimation of the dimensions of the complex. The combination of the experimental and theoretical results yielded a model for discoidal apolipoprotein-phospholipid complexes, in which the amphipathic helical segments are oriented along the edges of the discs. Such a model can be extended to the conversion of these complexes into mature spherical HDL, through the formation of a cholesteryl ester core.