Liquid-Liquid Phase Separation of a Surfactant-Solubilized Membrane Protein

Solubilization of membrane proteins requires surfactants, whose structural properties play a crucial role in determining the protein phase behavior. We show that ionization of a $p\mathrm{H}$-sensitive surfactant, lauryldymethylamino-N-oxide, bound to the bacterial photosynthetic Reaction Center, induces protein phase segregation in micrometric “droplets.” Liquid-liquid phase separation takes place in a narrow $p\mathrm{H}$ range, is promoted by increasing temperature, and vanishes by adding salt. After a fast initial droplet growth, the nearly arrested kinetics at a later stage leaves the system in a finely divided, long-lasting emulsified state.