Interplay Between Membrane Adhesion and Distribution of Lipid Rafts

Adhesion of cell membranes is relevant to many biological processes and arises from the specific binding of membrane-anchored receptor proteins to their ligands present in the apposing membrane. Here, we employ a statistical–mechanical model and perform Monte Carlo simulations to study a system of adhered membranes in which the receptor and ligand proteins exhibit affinity for association with so-called lipid rafts, which are fluctuating nanoscale molecular clusters enriched in sphingolipid and cholesterol. We focus on equilibrium properties of the adhered membranes in the mixed phase, where both the membrane-anchored proteins and lipid rafts are distributed more-or-less uniformly within the membranes. Our simulation results show that lateral attraction between lipid rafts enhances the receptor–ligand binding, affecting the adhesion of the membranes. On the other hand, the receptor–ligand binding causes lipid rafts to be distributed less uniformly within the membranes and, simultaneously, leads to an increased co-localization of the membrane-anchored proteins with lipid rafts. We quantify and discuss all these effects, providing a detailed picture of the complex interplay between the adhesion of the membranes and the lateral distribution of the membrane-anchored proteins and lipid rafts. Our results broaden the understanding of the physical mechanisms that determine the supra-molecular organization of lipid rafts and membrane receptors in cell membranes. This understanding may help to elucidate how lipid rafts function in biological processes such as cell signaling and immune responses.