Alignment of the apolipophorin-III α-helices in complex with dimyristoylphosphatidylcholine: A unique spatial orientation

Abstract

Apolipophorin-III (apoLp-III) from Manduca sexta can exist in two alternate states: as a globular, lipid-free helix bundle or a lipid surface-associated apolipoprotein. Previous papers (Ryan R. O., Oikawa K., and Kay C. M. (1993) J. Biol. Chem. 268, 1525-1530; Wientzek M., Kay C. M., Oikawa K., and Ryan R. O. (1994) J. Biol. Chem. 269, 4605-4612) have investigated the structures and properties of apolipophorin-III from M. sexta in the lipid-free state and associated to lipids. Association of apoLp-III with dimyristoylphosphatidylcholine vesicles leads to the formation of uniform lipid discs with an average diameter and thickness of 18.5 ± 2.0 and 4.8 ± 0.8 nm, respectively. These discs contain six molecules of apoLp-III. Geometrical calculations based on these data, together with x-ray crystallographic data from the homologous L. migratoria apoLp-III (Breiter D. R., Kanost M. R., Benning M. M., Wesenberg G., Law J. H., Wells M. A., Rayment L, and Holden H. M. (1991) Biochemistry 30, 603-608), have allowed the presentation of a model of lipid-protein interaction, in which the α-helices of the apoLp-III orient perpendicular to the phospholipid chains and surround the lipid disc. Here, using polarized Fourier transform-attenuated total reflection infrared spectroscopy, we provide the first experimental evidence of a unique perpendicular orientation of the α-helices with respect to the fatty acyl chains of the phospholipids in the disc.