The minimal size of liposome-based model cells brings about a remarkably enhanced entrapment and protein synthesis.

The question of the minimal size of a cell that is still capable of endorsing life has been discussed extensively in the literature, but it has not been tackled experimentally by a synthetic-biology approach. This is the aim of the present work; in particular, we examined the question of the minimal physical size of cells using liposomes that entrapped the complete ribosomal machinery for expression of enhanced green fluorescence protein, and we made the assumption that this size would also correspond to a full fledged cell. We found that liposomes with a radius of about 100 nm, which is the smallest size ever considered in the literature for protein expression, are still capable of protein expression, and surprisingly, the average yield of fluorescent protein in the liposomes was 6.1-times higher than in bulk water. This factor would become even larger if one would refer only to the fraction of liposomes that are fully viable, which are those that contain all the molecular components (about 80). The observation of viable liposomes, which must contain all macromolecular components, indeed represents a conundrum. In fact, classic statistical analysis would give zero or negligible probability for the simultaneous entrapment of so many different molecular components in one single 100 nm radius spherical compartment at the given bulk concentration. The agreement between theoretical statistical predictions and experimental data is possible with the assumption that the concentration of solutes in the liposomes becomes larger by at least a factor twenty. Further investigation is required to understand the over-concentration mechanism, and to identify the several biophysical factors that could play a role in the observed activity enhancement. We conclude by suggesting that these entrapment effects in small-sized compartments, once validated, might be very relevant in the origin-of-life scenario.