Self-organized lipid structures (protocells) have been proposed as an intermediate between nonliving material and cellular life. Synthetic production of model protocells can demonstrate the potential processes by which living cells first arose. While we have previously described a giant vesicle (GV)-based model protocell in which amplification of DNA was linked to self-reproduction, the ability of a protocell to recursively self-proliferate for multiple generations has not been demonstrated. Here we show that newborn daughter GVs can be restored to the status of their parental GVs by pH-induced vesicular fusion of daughter GVs with conveyer GVs filled with depleted substrates. We describe a primitive model cell cycle comprising four discrete phases (ingestion, replication, maturity and division), each of which is selectively activated by a specific external stimulus. The production of recursive self-proliferating model protocells represents a step towards eventual production of model protocells that are able to mimic evolution.

Giant vesicles (GVs) comprising zwitterionic and anionic phospholipids (DOPC and POPG, respectively) at a molar ratio of 90:10 formed a sparse network of aggregates that dissociated into isolated GVs reversibly depending on the pH of the dispersion (pH range, 7.02.5). This reversibility was due to the fluctuations in the composition of as-grown GVs.Amphiphiles, such as phospholipids, afford three-dimensional structures, belonging to the gyroid and hexagonal phases, under high-temperature, 1 high-concentration, 2 and high-pressure conditions.1 On the other hand, they also form vesicles characterized by closed hollow structures with a lipid bilayer. Recently, giant vesicles (GVs) with diameters larger than 1¯m have drawn considerable attention in the physical and chemical research fields due to fact that they belong to discrete (zerodimensional) systems. Aggregation and fusion of GVs, taking place at high concentrations of electrolytes or in the presence of multivalent inorganic ions, are irreversible processes. 4 Although the aggregation of GVs equipped with a specific recognition site, e.g., DNA-tag, 5 or a coordination site 6 has been extensively studied, that of phospholipid hybrid GVs of the same composition but without any specific tag has been scarcely investigated. Since a binary lipid mixture of phosphatidylcholine (PC) and phosphatidylglycerol (PG) in the lamellar phase is uniquely sensitive to the pH change, 7 we have been interested in examining whether this intravesicular response to the pH change influences the intervesicular interaction in dispersion. As a result, we conceived PC/PG hybrid GVs to study the pHsensitive reversible transformation of the aggregation states of a hybrid GV in dispersion. Here, we found that the switchable aggregation and dissociation of the PC/PG hybrid GVs can be induced simply by changing the pH of the aqueous dispersion. This phenomenon is caused by the distribution of the positive or negative surface charge of the hybrid GVs with fluctuating composition of PC and PG. The adhesion of GVs occurs around pH at which the effective surface charge of the GV membrane becomes zero.As stated above, vesicles in a colloidal dispersion easily aggregate in the presence of multivalent inorganic ions or at a high concentration of electrolytes.4 However, such aggregation often induces fusion between adhering vesicles that are difficult to dissociate reversibly. In this study, we prepared phospholipid hybrid GVs comprising zwitterionic 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) and anionic 2-oleoyl-1-palmitoyl-snglycero-3-phospho-rac-(1-glycerol) (POPG) sodium salt at a ratio of 90:10 (mol %) by a film-swelling method. The mixture of phospholipids was dissolved in chloroform. The solvent was evaporated by dry nitrogen flow. The resulting film was dried under reduced pressure overnight. Deionized water at 25°C was added to the film to obtain a dispersion of hybrid GVs with phospholipids at a concentration of 2 mM. Generally, the surface charge of the hybrid GV membrane changes from...