Non-linear partitioning of client proteins to the bulk and interface of biomolecular condensates

The ability of biomolecular condensates to sequester client proteins is thought to be central to their function. For example, partitioning enables the compartmentalization of enzyme reactions. Further, recent studies have suggested that condensates can be stabilized through the localization of client proteins to the interface. However, systematic investigations of the partitioning power of condensates are limited. Here, we quantify bulk and interfacial partitioning of tubulin to FUS-polyU droplets using light microscopy. We find that classical partitioning coefficients are insufficient to characterize these phenomena. Specifically, bulk and surface concentrations increase non-linearly with tubulin concentration.