Dynamically arrested condensate fusion creates complex structures with varying material properties

The cell nucleus and cytosol contain numerous biomolecular condensates which dynamically reshape, fuse and split to accomplish precise compartmentalization of the cell material. While it has been observed that some condensates rapidly coalesce, some others only attach to each other, or do not establish persistent interactions over time. Here, we explain these observations through optical tweezers and Molecular Dynamics simulations focusing on two condensate-forming, RNA-binding proteins--FUS and G3BP1--strongly involved in RNA metabolism and stress responses. We find that the fusion of pure droplets formed by these proteins can give rise to multiphase single-component condensates exhibiting notably different densities, architectures, and material properties. Such behaviour is dictated by the relative timescales of condensate fusion and protein internal mixing. A critical parameter controlling this interplay is the extent of ageing that condensates display; e.g., their progressive hardening driven by the accumulation of inter-protein {beta}-sheet assemblies over time. Strikingly, different degrees of ageing in fusing droplets can lead single-component condensates to form diverse architectures including concentric drops or two-sided condensates. Overall, our results highlight a mechanism, based on the temporal coupling between ageing, fusion, and mixing rate, by which biomolecular condensates form multiphasic structures with markedly different material properties, and hence potentially distinct biological roles.