The C-terminal helix of human apolipoprotein AII promotes the fusion of unilamellar liposomes and displaces apolipoprotein AI from high-density lipoproteins

To assess the functional properties of apolipoprotein (ape) Ail and to investigate the mechanism leading to the displacement of apo Al from native and reconstituted high-density lipoproteins (HDL and r-HDL) by apo AII, wild-type and variant apo All peptides were synthesized. The wild-type peptides, residues 53-70 and 58-70, correspond to the C-terminal helix of apo AII and art predicted to insert at a tilted angle into a lipid bilayer. We demonstrate that both the apo AII-(53-70) peptide, and to a lesser extent the apo AII-(58-70) peptide are able to induce fusion of unilamellar lipid vesicles together with membrane leakage, and to displace apo Al from HDL and r-HDL. Two variants of the apo AII-(53-70)wild-type (WT) peptide, designed either to be parallel to the water/lipid interface [apo AII-(53-70)-0 degrees] or to retain an oblique orientation [ape AII-(53-70)-30 degrees], were synthesized in order to test the influence of the obliquity on their fusogenic properties and ability to displace apo Al from HDL. The parallel variant did not bind lipids, due to its self-association properties. However, the ape AII-(53-70)-30 degrees variant was fusogenic and promoted the displacement of apo AI from HDL. Moreover, the extent of fusion of the apo AII-(53-70)-WT, apo AII-(58-70)-WT and apo AII-(53-70)-30 degrees peptides was related to the a-helical content of the lipid-bound peptides measured by infrared spectroscopy. infrared measurements using polarized light also confirmed the oblique orientation of the helical component of the three peptides. in native and r-HDL, the tilted insertion of the C-terminal helix of ape AII resulting in a partial destabilization of the HDL external lipid layer might contribute to the displacement of apo AI by apo Ail.