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.