Lipid-binding properties of synthetic peptide fragments of human apolipoprotein A-II.

Human apolipoprotein A-II (apo A-II) consists of three potential amphipathic helices of 17 residues each, which contribute to the lipid-binding properties of this apolipoprotein. The conformation and lipid-binding properties of these peptides, either as single-helix or as two-helix peptides, were investigated by turbidity, fluorescence, electron-microscopy and circular-dichroism measurements, and are compared in this article. The lipid affinity of shorter C-terminal segments of apo A-II was compared with those of the single-helix or two-helix peptides, to define the minimal peptide length required for stable complex formation. The properties of the apo-A-II-(13-48)-peptide were further compared with those of the same segment after deletion of the Ser31 and Pro32 residues, because the deleted apo-A-II-(13-30)-(33-48)-peptide, is predicted to form a long uninterrupted helix. The single helices of apo A-II could not form stable complexes with phospholipids, and the helix-turn-helix segment spanning residues 13-48 was not active either. The apo-A-II-(37-77)-peptide and the apo-A-II-(40-73)-peptide could form complexes with lipids, which appear as discoidal particles by negative-staining electron microscopy. The shortest C-terminal domain of apo A-II able to associate with lipids to form stable complexes was the apo-A-II-(40-73)-peptide, which consisted of the C-terminal helix, a beta-turn and part of the preceding helix. The shorter apo-A-II-(49-77)-peptide, and the helical apo-A-II-(13-30)-(33-48)-peptide, could also associate with phospholipids. The complexes formed were, however, less stable, as they dissociated outside the transition temperature range of the phospholipid. These data suggest that the C-terminal pair of helices of apo A-II, which is the most hydrophobic pair, is responsible for the lipid-binding properties of the entire protein. The N-terminal pair of helices of apo A-II at residues 13-48 does not associate tightly with lipids. The degree of internal similarity and the cooperativity between the helical segments of apo A-II is thus less pronounced than in apo A-I or apo A-IV. The N-terminal and C-terminal domains of apo A-II appear to behave as two distinct entities with regard to lipid-protein association.