Biomolecular condensates are protein-dense regions in cells that often arise from liquid-liquid phase separation. Interfacial tension is a key determinant of biomolecular condensate behavior, influencing condensate size and interactions with intracellular structures. Certain proteins and RNAs are known to selectively localize to the interface of condensates, where they can regulate condensate function in cells. Previously, we designed amphiphilic proteins that preferentially adsorb to the surface of condensates. These proteins contain one condensate-philic domain (RGG) and one condensate-phobic domain (MBP or GST). Here, we demonstrate through direct quantification that these amphiphilic proteins act as surfactants, reducing the interfacial tension of RGG-RGG condensates from ∼260 to ∼100 μN/m in a concentration-dependent manner. Notably, the GST-based surfactant protein exhibits a 10-fold greater efficacy in lowering interfacial tension compared with the MBP-based surfactant. We show that this increased efficacy is due to its higher surface density, driven by GST's ability to oligomerize. We also show that these surfactant proteins slow droplet fusion and reduce the average droplet size, as would be expected of a typical surfactant. Our findings quantitatively show how surfactant proteins can play a critical role in regulating the behavior of biomolecular condensates by modulating their interfacial tension.
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