Apolipoprotein-alanine (ApoLP-Ala) from human plasma very low density lipoproteins has been shown previously to interact with phosphatidylcholine (PC) as determined by ultracentrifugal flotation, circular dichroism, and intrinsic tryptophan fluorescence. This protein also causes spherical bilamellar vesicles of phosphatidylcholine to become aligned as linear stacks or rouleaux when negatively stained and viewed by electron microscopy. The complex formed by interaction of PC vesicles and ApoLP-Ala in solution has been studied further by analytical gel filtration, analytical ultracentrifugation, and quasi-elastic light scattering. The complex eluted from a Sepharose 6B column at approximately the same volume as PC vesicles alone, but well ahead of ApoLP-Ala. Upon titration of PC vesicles with the apoprotein, the observed sedimentation coefficient increased from 1.19 S to a limiting value of 4.93 S, which was first reached when the protein-lipid ratio was 0.23 g/g. This weight ratio corresponds to a molar ratio of about 1:53 or about 46 ApoLP-Ala molecules/PC vesicle. As determined by light scattering, the average translational diffusion coefficient for a vesicle-apoprotein complex of this stoichiometry was D20,w = (2.08 ± 0.03) × 10-7 cm2 sec-1. These data provide direct evidence that ApoLP-Ala and PC vesicles form stable complexes which do not exist in solution as large multivesicular aggregates. In addition, the hydrodynamic data lead to the conclusions that both the hydration shell around the vesicle as well as the volume occupied by the nonpolar portion of the bilayer were changed upon complex formation with apoLP-Ala. This is consistent with a model where the apolipoprotein is bound to hydrophobic sites within the vesicle, with parts of the protein protruding into the polar head region so that interactions with hydrophilic sites on the surface of the bilayer can occur.
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