Tie-Line Analysis Reveals Interactions Driving Heteromolecular Condensate Formation

Phase separation of biomolecules gives rise to membraneless organelles that contribute to the spatiotemporal organization of the cell. In most cases, such biomolecular condensates contain multiple components, but the manner in which interactions between components control the stability of condensates have remained challenging to elucidate. Here, we develop an approach to determine tie-line gradients in ternary biomolecular phase-separation systems based on measurements of the dilute phase concentration of only one component. We show that the sign of the tie-line gradient is related to the cross-interaction energy between the polymers in the system and discriminates between associative and segregative phase separation. Using this approach, we study the interaction between protein fused in sarcoma (FUS) and polyethylene glycol (PEG) polymer chains and measure positive tie-line gradients. Our results show that PEG drives phase separation through an associative interaction with FUS, other than through acting as an inert crowder. We further study the interaction between poly(A) ribonucleic acid (RNA) (3.0 ± 0.5 kDa [kilodalton]) and the protein G3BP1, and using the tie-line gradient as a reporter for the stoichiometry of polymers in the condensate, we determine a G3BP1-to-poly(A) RNA molar ratio of 1 : 4 in the dense phase. Our framework for measuring tie-line gradients opens up a route for the characterization of interaction types and compositions in ternary phase-separation systems.