Determinants of Viscoelasticity and Flow Activation Energy in Biomolecular Condensates

Abstract

The form and function of biomolecular condensates, which are phase-separated intracellular granules of proteins and RNAs, are regulated by their material and dynamical properties. Emerging reports suggest that biomolecular condensates are viscoelastic network fluids, and the primary sequence and structure of the constituent biopolymers govern their bulk fluid phase properties. Here, we employ a multi-parametric approach to dissect the molecular determinants of condensate viscoelasticity by studying a series of condensates formed by engineered multivalent arginine-rich polypeptides and single-stranded DNA. By measuring the terminal relaxation time of the condensate network through optical tweezer-based microrheology and the activation energy of viscous flow through temperature-controlled video particle tracking, we show that condensate viscoelasticity is controlled by two distinct factors - sequence-encoded inter-chain interactions of associative polymers and entropic factors emerging from their intrinsic polymer properties such as the chain length. The biomolecular diffusion in the dense phase shows a strong dependence on the flow activation energy, indicating that the intra-condensate transport properties are primarily reaction-dominant. These results provide a glimpse of the multifaceted control of viscoelasticity and transport properties within biomolecular condensates. Flow activation energy measurement of single and multicomponent condensates by thermo-rheology provides a direct route to quantify inter-chain interactions in the dense phase and dissect the roles of chain entropy and valence in dictating the viscoelastic behavior of biomolecular condensates.