Structure of the Dimyristoylphosphatidylcholine Vesicle and the Complex Formed by Its Interaction with Apolipoprotein C-III: X-ray Small-Angle Scattering Studies

Single bilayer vesicles of dimyristoylphosphatidylcholine have been investigated by small-angle X-ray scattering at 28 °C. The results indicate that these vesicles are hollow spherical shell structures with an outer radius of approximately 12 nm and a molecular weight of (3.2 ± 0.5) × 106. The shell was found to be 4.4 ± 0.2 nm thick with a cross-sectional electron-density profile characteristic for a single phospholipid bilayer. Upon interaction of these vesicles with apolipoprotein C-III from human very low density lipoproteins at a protein/lipid ratio greater than 0.08 (g/g), a complex containing 0.25 g of protein/g of lipid, with molecular weight of (3.9 ± 0.4) × 105 is formed. The shape analysis indicates a highly asymmetric particle with an internal partition of low and high electron density resembling that produced by a bilayer structure. Model calculations and curve-fitting procedures show good agreement between the experimental scattering curve and that computed for an oblate ellipsoidal structure with dimensions of 17 × 17 × 5 nm and a 1 nm thick shell of high electron density surrounding the core of low electron density.