Intermolecular reorganisation of single-component condensates during ageing promotes multiphase architectures.

Intracellular proteins can undergo phase separation to form liquid-like biomolecular condensates with a multitude of functional roles. Liquid condensates can, however, further age and progressively rigidify. In addition to single-phase systems, multiphase condensates are increasingly identified commonly within multi-component systems, where the different molecular components present sufficient physicochemical diversity to sustain separate phases. Here, we develop a multiscale modeling approach that predicts conditions under which multiphase architectures can arise also within single-component protein condensates. Such single-component condensates are initially homogeneous but become heterogeneous over time due to the gradual enhancement of interprotein interactions. We find that such enhancement could originate, for instance, from intermolecular disorder-to-order transitions within low-complexity aromatic-rich kinked segments in the prion-like domain of FUS. Our model reveals that as increasing numbers of molecules undergo a disorder-to-order transition over time, single-component protein condensates convert into either gel-core/liquid-shell or liquid-core/gel-shell multiphase structures, depending on the relative surface tension of the liquid and gel phases. Despite being formed by proteins that are chemically-identical, the different liquid and gel phases present diverse surface tensions due to their fundamentally different molecular organization. Our study highlights the regulatory role of prion-like domains in tuning condensate behavior and, more generally, suggests a new route by which multilayered compartments or hierarchically organized condensate structures can emerge.