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

Biomolecular condensates form via macromolecular phase separation. Here, we report results from our characterization of synthetic condensates formed by phase separation of mixtures comprising two types of RNA molecules and the biocompatible polymer polyethylene glycol. Purine-rich RNAs are scaffolds that drive phase separation via heterotypic interactions. Conversely, pyrimidine-rich RNA molecules are adsorbents defined by weaker heterotypic interactions. They adsorb onto and wet the interfaces of coexisting phases formed by scaffolds. Lattice-based simulations reproduce the phenomenology observed in experiments and these simulations predict that scaffolds and adsorbents have different non-random orientational preferences at interfaces. Dynamics at interfaces were probed using single-molecule tracking of fluorogenic probes bound to RNA molecules. These experiments revealed dynamical anisotropy at interfaces whereby motions of probe molecules parallel to the interface are faster than motions perpendicular to the interface. Taken together, our findings have broad implications for designing synthetic condensates with tunable interfacial properties.