Exchange of Proteins in Liposomes through Streptolysin O Pores

Liposomes, which are vesicles surrounded by lipid membranes, can be used as biochemical reactors by encapsulating various reactions. Accordingly, they are useful for studying cellular functions under controlled conditions that mimic the environment within a cell. However, one of the shortcomings of liposomes as biochemical reactors is the difficulty of introducing or removing proteins due to the impermeability of the membrane. In this study, we established a method for exchanging proteins in liposomes by forming reversible pores in the membrane. We used the toxic protein streptolysin O (SLO); this forms pores in membranes made of phospholipids containing cholesterol that can be closed by the addition of calcium ions. After optimizing the experimental procedure and lipid composition, we observed the exchange of fluorescent proteins (transferrin Alexa Fluor 488 and 647) in 9.9 % of liposomes. We also introduced T7 RNA polymerase, a 98-kDa enzyme, and observed RNA synthesis in ∼8 % of liposomes. Our findings establish a new method for controlling the internal protein composition of liposomes, thereby increasing their utility as bioreactors.