Linking modulation of bio-molecular phase behaviour with collective interactions

Bio-molecular condensates formed in the cytoplasm of cells are increasingly recognised as key spatiotemporal organisers of living matter, and are implicated in a wide range of functional or pathological processes. This discovery opens up a new avenue for condensate-based applications and a crucial step in controlling this process is to understand the underlying interactions driving condensate formation or dissolution. However, these condensates are highly multi-component assemblies and many inter-component interactions are present, rendering it difficult to identify key promoters of phase separation. In this work, we extend the recently formulated dominance analysis to modulations of condensate formation. By carrying out dilute phase concentration measurements of a single target solute, the theoretical framework allows one to deduce whether the modulator acts on the target solute or another unspecified, auxiliary solute, as well as the attractive/repulsive nature of the added interaction. This serve as a general guide towards deducing possible modulation mechanisms on the molecular level, which can be complemented by orthogonal measurements. As a case study, we investigate the modulation of G3BP1/RNA condensates by the small molecule suramin, and the dominance measurements point towards a dissolution mechanism where suramin acts on G3BP1 to disrupt G3BP1/RNA interactions, as confirmed by a diffusional sizing assay. Our approach and the dominance framework have a high degree of adaptability and can be applied in many other condensate-forming systems.