Volume properties of mixtures of lipophilin and dimyristoylphosphatidylcholine.

The hydrophobic myelin protein, lipophilin, has been incorporated into bilayers of dimyristoylphosphatidylcholine by dialysis from 2-chloroethanol. The protein was shown to be incorporated into a protein-lipid complex of uniform density by density gradient sedimentation equilibrium. The volume properties of the resulting complexes were studied by densitometry. It was found that a molecule of the protein could prevent an increase in volume at the phase transition temperature of 19 lipid molecules. The remaining lipid underwent its phase transition over a broader temperature range, resulting in a decrease in the volume coefficient of expansion in the region of the phase transition. The protein has little effect on this parameter at higher or lower temperatures. The partial specific volume of the lipid alone was similar to what has been previously determined using freshly prepared suspensions. The partial specific volume of the protein alone was similar to the value calculated based on the amino acid composition. The partial specific volume of the lipid-protein complex, however, was less than the weighted average of the components, indicating that lipophilin could induce an increase in the density of the lipid. This condensing effect of lipophilin was observed both above and below the phase transition and may be a general property of proteins incorporated into lipid bilayers.