Hybrid Protocells based on Coacervate-Templated Fatty Acid Vesicles combine Improved Membrane Stability with Functional Interior Protocytoplasm

Prebiotically-plausible compartmentalization mechanisms include membrane vesicles formed by amphiphile self-assembly and coacervate droplets formed by liquid-liquid phase separation. Both types of structures form spontaneously and can be related to cellular compartmentalization motifs in today’s living cells. As prebiotic compartments, they have complementary capabilities, with coacervates offering excellent solute accumulation and membranes providing superior boundaries. Herein, we describe protocell models constructed by spontaneous encapsulation of coacervate droplets by mixed fatty acid/phospholipid and by purely fatty acid membranes. Coacervate-supported membranes formed over a range of coacervate and lipid compositions, with membrane properties impacted by charge-charge interactions between coacervates and membranes. Vesicles formed by coacervate-templated membrane assembly exhibited profoundly different permeability than traditional fatty acid or blended fatty acid/phospholipid membranes without coacervate interiors, particularly in the presence of Mg2+ ions. While fatty acid and blended membrane vesicles were disrupted by addition of 25 mM MgCl2, the corresponding coacervate-supported membranes remained intact and impermeable to externally-added solutes even in the presence of MgCl2. With the more robust membrane, fluorescein diacetate (FDA) hydrolysis, which is commonly used for cell viability assays, could be performed inside the protocell model due to the simple diffusion of FDA and then following with the coacervate-mediated abiotic hydrolysis to fluorescein.