Formation of Multi-Compartment Structures through Aging of Protein-RNA Condensates.

Cells can dynamically organize reactions through the formation of biomolecular condensates. These viscoelastic networks exhibit complex material properties and mesoscale architectures, including the ability to form multi-phase assemblies. It was shown previously that condensates with complex architectures may arise at equilibrium in multicomponent systems or in condensates that were driven out-of-equilibrium by changes in external parameters such as temperature. In this study, we demonstrate that the aging of initially homogeneous protein-RNA condensates can spontaneously lead to the formation of kinetically arrested double-emulsion and core-shell structures without changes in external variables such as temperature or solution conditions. By combining time-resolved fluorescence-based experimental techniques with simulations based on the Cahn-Hilliard theory, we show that, as the protein-RNA condensates age, the decrease of the relative strength of protein-RNA interactions induces the release of RNA molecules from the dense phase. In condensates exceeding a critical size, aging combined with slow diffusion of the macromolecules trigger nucleation of dilute phase inside the condensates, which leads to the formation of double-emulsion structures. These findings illustrate a new mechanism of formation of multi-compartment condensates.