Abstract
Biomolecular condensates, which assemble via the process of liquid–liquid phase separation (LLPS), are multicomponent compartments found ubiquitously inside cells. Experiments and simulations have shown that biomolecular condensates with many components can exhibit multilayered organizations. Using a minimal coarse-grained model for interacting multivalent proteins, we investigate the thermodynamic parameters governing the formation of multilayered condensates through changes in protein valency and binding affinity. We focus on multicomponent condensates formed by scaffold proteins (high-valency proteins that can phase separate on their own via homotypic interactions) and clients (proteins recruited to condensates via heterotypic scaffold–client interactions). We demonstrate that higher valency species are sequestered to the center of the multicomponent condensates, while lower valency proteins cluster towards the condensate interface. Such multilayered condensate architecture maximizes the density of LLPS-stabilizing molecular interactions, while simultaneously reducing the surface tension of the condensates. In addition, multilayered condensates exhibit rapid exchanges of low valency proteins in and out, while keeping higher valency proteins—the key biomolecules involved in condensate nucleation—mostly within. We also demonstrate how modulating the binding affinities among the different proteins in a multicomponent condensate can significantly transform its multilayered structure, and even trigger fission of a condensate into multiple droplets with different compositions.
Highlights
Liquid–liquid phase separation (LLPS) is one of the key processes employed by cells to control the spatiotemporal organization of their many components, i.e., via formation and dissolution of biomolecular condensates
To test this hypothesis, following the procedure described in [53], we evaluate the interfacial free energy of both a single-component condensate formed by the highest valency protein in our set (i.e., 4-valency/promiscuous), and our different six-component condensates, each measured below their respective critical temperatures
We find that within multicomponent condensates, high-valency proteins are placed preferentially at the core of the droplets, while low-valency species concentrate towards the interface
Summary
Liquid–liquid phase separation (LLPS) is one of the key processes employed by cells to control the spatiotemporal organization of their many components, i.e., via formation and dissolution of biomolecular condensates. These condensates are liquid-like membraneless compartments highly enriched in specific biomolecules (e.g., proteins and RNAs) and depleted in others [1,2,3,4]. Because of their ability to selectively concentrate and exclude biomolecules, these condensates have been suggested to act as reaction crucibles that speedup chemical reactions, or sequestrate unwanted components to prevent/promote reactions within the cytoplasm and nucleoplasm [4]. Elucidating the factors that tune the stability, structure, and function of condensates is highly desirable
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