STUDIES ON INTERACTIONS BETWEEN PHOSPHATIDYLCHOLINE AND CASEIN

The interactions between casein and either egg phosphatidylcholine or dipalmitoylphosphatidylcholine were studied using dynamic light scattering and fluorescence polarization. Casein was found not to adsorb to a suspension of phospholipid vesicles, but a stable complex was formed when casein and phospholipid were homogenized together to form the vesicles. The hydrodynamic thickness of the layer of casein associated with vesicles was close to twice that of the casein layer adsorbed to the surfaces of oil droplets in oil-in-water emulsions. The breakdown of incorporated casein molecules by trypsin caused aggregation or fusion of the vesicles, in contrast to the behavior of emulsions, which remain stable under such treatment. Changes in the fluorescence polarization demonstrated that the association of casein with DPPC vesicles made the lipid bilayer more rigid in its liquid crystalline state, but little difference was detected in the gel state. The topological response of the vesicles to temperature change was significantly modified by the presence of casein in the bilayer. The results suggest that the casein associated with the lipid bilayer is not simply adsorbed to the surface but penetrates deeper into the inner core of the bilayer, so that caseins appear to behave as transmembrane proteins.