Cell-sized water-in-oil droplet covered by a lipid layer was used to understand how lipid membranes affect biochemical systems in living cells. Here, we report a remarkable acceleration of gene expression in a cell-sized water-in-oil droplet entrapping a cell-free translation system to synthesize GFP (green fluorescent protein). The production rate of GFP (V_GFP) in each droplet remained almost constant at least for on the order of a day, which implies 0th-order reaction kinetics. Interestingly, V_GFP was inversely proportional to radius of droplets (R) when R is under 50 μm, and V_GFP in droplets with R ∼ 10 μm was more than 10 times higher than that in the bulk. The acceleration rates of GFP production in cell-sized droplets strongly depended on the lipid types. These results demonstrate that the membrane surface has the significant effect to facilitate protein production, especially when the scale of confinement is on the order of cell-size.

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