The phase and charge of milk polar lipid membrane bilayers govern their selective interactions with proteins as demonstrated with casein micelles

The biological membrane surrounding fat globules in milk (milk fat globule membrane; MFGM) is an interface involved in many biological functions and interactions with the surrounding proteins or lipolytic enzymes in the gastro-intestinal tract during digestion. The MFGM exhibits lateral heterogeneities resulting from the different phase states and/or head-group charge of the polar lipids, which were both hypothesized to drive interaction with the casein micelles that is the major milk protein assembly. Atomic force microscopy (AFM) imaging was used to track the interactions of casein micelles with hydrated supported lipid bilayers of different composition, phase state and charge. Zeta-potential and Langmuir isotherms of the different polar lipids offered additional information necessary to interpret AFM observations. We showed that the negatively-charged casein micelles did not interact with milk sphingomyelin in the gel or liquid-ordered phases but did interact with polar lipids in the liquid-disordered phase (unsaturated polar lipids and milk sphingomyelin above its melting point). A wide intermolecular distance between polar lipids allowed protein adsorption on the membranes. However, the presence of the anionic polar lipids phosphatidylserine and phosphatidylinositol prevented any interaction with the casein micelles, probably due to electrostatic repulsion. These results open perspectives for the preparation of tailored emulsions covered by polar lipids able to modulate the interfacial interactions with proteins.