Cell-Sized Confinement Initiates Phase Separation of Polymer Blends and Promotes Fractionation upon Competitive Membrane Wetting

Biomolecular condensates driven by liquid–liquid phase separation (LLPS) have received attention as novel activity regulators of living organisms. In intracellular LLPS, an important question is what type of biomolecules form condensates under what conditions. Recently, LLPS condensates have been reported to regulate the membrane structure upon wetting. However, the opposite possibility, i.e., whether membrane wetting regulates the LLPS, remains unexplored. Using variously sized droplets of short polyethylene glycol (PEG) and long dextran blends encapsulated with a lipid membrane, we demonstrate that membrane wetting regulates LLPS in cell-sized spaces and alters the equilibrium state. In small droplets, the two-phase region expands beyond the bulk, and the degree of fractionation increases as the droplet size decreases. We explain the space-size-dependent LLPS from the higher wettability of short PEG than dextran. This shows that cell-sized confinement can regulate LLPS upon competition for membrane wettability among various molecules, rendering this LLPS principle feasible in living cells.