Evaluation of lipid-binding properties of the N-terminal helical segments in human apolipoprotein A-I using fragment peptides.

Although the N-terminal region in human apolipoprotein (apo) A-I is thought to stabilize the lipid-free structure of the protein, its role in lipid binding is unknown. Using synthetic fragment peptides, we examined the lipid-binding properties of the first 43 residues (1-43) of apoA-I in comparison with residues 44-65 and 220-241, which have strong lipid affinity in the molecule. Circular dichroism measurements demonstrated that peptides corresponding to each segment have potential propensity to form alpha-helical structure in trifluoroethanol. Spectroscopic and thermodynamic measurements revealed that apoA-I (1-43) peptide has the strong ability to bind to lipid vesicles and to form alpha-helical structure comparable to apoA-I (220-241) peptide. Substitution of Tyr-18 located at the center of the most hydrophobic region in residues 1-43 with a helix-breaking proline resulted in the impaired lipid binding, indicating that the alpha-helical structure in this region is required to trigger the lipid binding. In contrast, apoA-I (44-65) peptide exhibited a lower propensity to form alpha-helical structure upon binding to lipid, and apoA-I (44-65/S55P) peptide exhibited diminished, but not completely impaired, lipid binding, suggesting that the central region of residues 44-65 is not pivotally involved in the formation of the alpha-helical structure and lipid binding. These results indicate that the most N-terminal region of apoA-I molecule, residues 1-43, contributes to the lipid interaction of apoA-I through the hydrophobic helical residues.