Probing the material properties of multi-component and multi-phasic condensates

The material and dynamical properties of phase-separated biomolecular condensates are key determinants of their biological functions and pathological effects inside living cells. Recent advances indicate that biomolecular condensates are viscoelastic network fluids with time-dependent material properties. Here, we aim to establish a molecular grammar of sequence-encoded and structure-specific protein-protein and protein-RNA interactions that governs the biomolecular condensate phase behavior and dynamical properties. By employing microrheology with optical tweezers, we quantify the viscoelastic dynamics of a variety of binary and ternary biomolecular condensates formed by intrinsically disordered polypeptides (IDPs). We find that at shorter timescales, IDP condensates have an elastically dominant rheological response, while at longer timescales, the same condensates behave as predominantly viscous liquids. The network reconfiguration time, viz., the timescale at which the condensate transitions from an elastically dominant behavior to a viscous behavior, is determined by the IDP sequence composition and patterning, RNA sequence, and RNA secondary structure. Further, by mapping their binary and ternary phase behavior, we show that the viscoelastic behavior and multiphasic dynamics of condensates are collectively determined by the sequence- and structure-encoded biomolecular interactions at the microscopic scale. Overall, our work reveals that condensates are network fluids, and IDP valence and interaction strengths determine the driving forces for phase separation and condensate viscoelastic properties in a predictive manner.