Susceptibility and Regulation of Biomolecular Condensates by Solutes

Biomolecular condensates compartmentalize biochemistry in living cells. While in vitro models of condensates involve only a few components, the cytoplasm is a complex mixture with thousands of components, including many small molecules. While many macromolecular drivers of phase separation have been revealed, the contributions from small molecules have received little attention. To quantify the impact of solutes on biomolecular condensates, we introduce susceptibility, a dimensionless descriptor of condensate response to solute perturbations. We measured how three model condensates, assembled by distinct cohesive mechanisms, respond to diverse solutes including amino acids and nucleotides. Generically, solutes shift condensate phase equilibria, with susceptibilities spanning over four orders of magnitude. These values reflect underlying molecular interactions, consistent with theoretical descriptions including Flory-Huggins and polyphasic linkages. As one example of the predictive power of susceptibility, we exploit enzymatic activity to induce condensation and modulate material properties. Our work establishes susceptibility as an indicator of the sensitivity of biomolecular condensates to solutes, with implications for cell physiology and therapeutic design. SIGNIFICANCE STATEMENT Cells compartmentalize biochemistry using biomolecular condensates formed through phase separation. Although cells contain thousands of small molecules, little is known about their influence on condensation. In such complex mixtures, mapping full phase diagrams is infeasible. Alternatively, we introduce susceptibility to characterize system response around a working composition. Using three distinct model condensates and over a dozen solutes, we observe susceptibilities varying over four orders of magnitude. We provide a general thermodynamic framework that clarifies the driving forces behind these responses, rationalizing their magnitude and their dependence on location in the phase diagram. Our work provides a framework for understanding and harnessing solutes to regulate biomolecular condensation, with implications for cell physiology and therapeutic design.