Molecular Crowding Tunes Material States of Ribonucleoprotein Condensates

Ribonucleoprotein (RNP) granules are membraneless liquid condensates that dynamically form, dissolve, and mature into a gel-like state in response to changing cellular environment. RNP condensation is largely governed by the promiscuous attractive inter-chain interactions, mediated by low-complexity domains (LCDs). Using an archetypal disordered RNP, Fused in Sarcoma (FUS), here we study how molecular crowding impacts the RNP liquid condensation. We observe that the liquid-liquid coexistence boundary of FUS is lowered by polymer crowders, consistent with the excluded volume model. With increasing bulk crowder concentration, RNP partition increases and the diffusion rate decreases in the condensed phase. Furthermore, we show that RNP condensates undergo substantial hardening wherein protein-dense droplets transition from viscous fluid to a viscoelastic gel-like state in a crowder concentration-dependent manner. Utilizing two distinct LCDs that broadly represent the most commonly occurring sequence motifs driving RNP phase transition, we reveal that the impact of crowding is largely independent of LCD charge/sequence patterns. These results are consistent with a thermodynamic model of crowder-mediated depletion interaction where inter-RNP attraction is enhanced by molecular crowding. The depletion force is likely to play key roles in tuning the physical properties of RNP condensates within a crowded intracellular space.