Local thermodynamics governs the formation and dissolution of protein condensates in living cells

The spatial organization of cells is characterized in part by chemically distinct membraneless compartments known as condensates. A well-studied example of condensates are P granules in the roundworm C. elegans which play an important role in the determination of the germ line. P granules are RNA-rich protein condensates which share the key properties of liquid droplets such as the spherical shape, the ability to fuse, fast diffusion of molecular components. A remaining question is to what extend an equilibrium thermodynamic picture is appropriate to describe the formation of condensates in an active cytoplasm. To address this question, we investigate the response of P granule condensates in living cells to temperature changes as a thermodynamic perturbation. We observe that P granules dissolve upon increasing the temperature and recondense upon lowering the temperature in a reversible manner. Strikingly, this temperature response can be captured by a thermodynamic Flory-Huggins model. Together with the previously characterized droplet properties of P granules our findings provide strong evidence that P granules assembly and disassembly are governed by phase separation based on local thermal equilibria embedded in larger scale non-equilibrium conditions of a living cell.