Generation of giant unilamellar liposomes containing biomacromolecules at physiological intracellular concentrations using hypertonic conditions

Artificial cells, particularly cell-sized liposomes, serve as tools to improve our understanding of the physiological conditions of living cells. However, such artificial cells typically contain a more dilute solution of biomacromolecules than that found in living cells (300 mg mL^-1). Here, we reconstituted the intracellular biomacromolecular conditions in liposomes using hyperosmotic pressure. Liposomes encapsulating 80 mg mL^-1 of macromolecules of BSA or a protein mixture extracted from Escherichia coli were immersed in hypertonic sucrose. The concentration of macromolecules in BSA-containing liposomes was increased in proportion to the initial osmotic pressure ratio between internal and external media. On the other hand, the concentration of the protein mixture in liposomes could be saturated to reach the physiological concentration of macromolecules in cells. Furthermore, membrane transformation after the hypertonic treatment differed between BSA- and protein mixture-containing liposomes. These results strongly suggested that the crowded environment in cells is different from that found in typical single-component systems.