Oligonucleotides can act as superscaffolds that enhance liquid-liquid phase separation of intracellular mixtures

Intracellular liquid-liquid phase separation (LLPS) enables the formation of biomolecular condensates, which play a crucial role in the spatiotemporal organisation of biomolecules (proteins, oligonucleotides). While LLPS of biopolymers has been demonstrated in both experiments and computer simulations, the physical determinants governing phase separation of protein-oligonucleotide systems are not fully understood. Here, we introduce a minimal coarse-grained model to investigate concentration-dependent features of protein-oligonucleotide mixtures. We demonstrate that adding oligonucleotides to biomolecular condensates composed of oligonucleotide-binding scaffold proteins enhances LLPS; since oligonucleotides act as ultra-high-valency molecules (termed 9superscaffolds9) that increase the molecular connectivity among scaffold proteins. Importantly, we find that oligonucleotides promote protein LLPS via a seeding-type mechanism; recruiting numerous protein molecules and reducing the thermodynamic and kinetic barriers for nucleation and phase separation. By probing the conformational properties of oligonucleotides within droplets, we show that these biopolymers can undergo phase separation-driven compaction, which may be entropic in nature. Finally, we provide a quantitative comparison between mixture composition, protein valency, and protein-oligonucleotide interaction strengths. We find that superscaffolds preferentially recruit higher valency proteins to condensates, and that multiphase immiscibility within condensates can be achieved by modulating the relative protein-oligonucleotide binding strengths. These results shed light on the roles of oligonucleotides in ribonucleoprotein granule formation, heterochromatin compaction, and internal structuring of the nucleolus and stress granules.