Charge and steric hindrance in the glycocalyx govern membrane interactions

The outer leaflet of the plasma membrane, the cell's interface with exogenous biomolecules and particles, is largely electrostatically neutral and highly viscous, conditions that are generally unfavorable for fusion of external membrane-bound objects such as liposomes. Yet living cells efficiently interact and fuse with charged liposomes, revealing a discrepancy between synthetic and biological membranes. Here, we identify the glycocalyx, the carbohydrate-rich layer that coats cells, as the component that explains this mismatch. Using synthetic membranes reconstituted with an artificial glycocalyx, we resolve how electrostatics and steric hindrance regulate the interactions of cationic liposomes with synthetic membranes. Charged glycolipids are found to be sufficient to convert non-fusogenic neutral membranes into fusion-competent membranes. In contrast, large polymers in the glycocalyx suppress fusion. Steric hindrance acts primarily at the docking stage, while fusion proceeds through a long-lived hemifusion intermediate. Strikingly, these large differences occur without measurable changes in membrane viscosity or phase state, demonstrating that glycocalyx-mediated control is independent of the membrane mechanics. Selected glycocalyx components can also promote endocytic budding and cargo internalization. We thus show that the glycocalyx is a tunable physicochemical regulator of liposome docking, fusion, and internalization. The findings provide a framework for understanding how biological agents and therapeutics engage cell membranes and they offer key design principles for synthetic cells.