Differential interactions determine anisotropies at interfaces of RNA-based biomolecular condensates

Abstract Biomolecular condensates form via macromolecular phase separation, giving rise to coexisting phases delineated by interfaces. Here, we characterize the structures of interfaces formed by phase separation driven by heterotypic interactions in ternary mixtures of two types of RNA molecules and polyethylene glycol. We find that purine-rich RNAs are scaffolds that drive phase separation via strong heterotypic interactions. Conversely, pyrimidine-rich RNA molecules are defined by weaker heterotypic interactions. They function as adsorbents that accumulate at and wet the interfaces of coexisting phases formed by phase separation of scaffolds. Our computations predict that scaffolds and adsorbents have different non-random orientational preferences at interfaces. We tested these predictions using single-molecule super resolution imaging that tracks the motions of fluorogenic probes that are bound to RNA molecules. Motions parallel to the interface were found to be faster than motions perpendicular to the interface. These findings support previous predictions regarding anisotropies of motions at interfaces.