Mechanisms that enabled primitive cell membranes to self-reproduce have been discussed based on the physicochemical properties of fatty acids; however, there must be a transition to modern cell membranes composed of phospholipids [Budin I, Szostak JW (2011) Proc Natl Acad Sci USA 108:5249–5254]. Thus, a growth-division mechanism of membranes that does not depend on the chemical nature of amphiphilic molecules must have existed. Here, we show that giant unilamellar vesicles composed of phospholipids can undergo the coupled process of fusion and budding transformation, which mimics cell growth and division. After gaining excess membrane by electrofusion, giant vesicles spontaneously transform into the budded shape only when they contain macromolecules (polymers) inside their aqueous core. This process is a result of the vesicle maximizing the translational entropy of the encapsulated polymers (depletion volume effect). Because the cell is a lipid membrane bag containing highly concentrated biopolymers, this coupling process that is induced by physical and nonspecific interactions may have a general importance in the self-reproduction of the early cellular compartments.

Significance: The directed evolution of proteins in vitro has generated highly functional proteins and has contributed to elucidating the sequence–function relationship of proteins. However, the available methods consider globular proteins, not membrane proteins, despite the biological and pharmaceutical importance of the latter. We report the development of a method called liposome display, which enables the in vitro evolution of membrane proteins. We applied the method to evolve the pore-forming activity of α-hemolysin from Staphylococcus aureus and obtained a mutant with 30-fold higher activity than the WT. Given its high degree of controllability, liposome display allows for the rapid and efficient evolution of a wide range of membrane proteins, thereby improving the field of membrane protein engineering.