Abstract

Jack O. Law1+, Carl M. Jones1,3+, Thomas Stevenson1, Thomas A. Williamson2, Matthew S. Turner4, Halim Kusumaatmaja2, Sushma N. Grellscheid1,3. 1 Computational Biology Unit and Dept. of Biological Sciences, University of Bergen, Norway 2 Dept. of Physics, University of Durham, United Kingdom 3 Dept. of Biosciences, University of Durham, United Kingdom 4 Dept. of Physics, University of Warwick, United Kingdom + These authors contributed equally.

• Corresponding authors’ Emails:  halim.kusumaatmaja@durham.ac.uk, sushma.grellscheid@uib.no Abstract Surface tension plays a significant role in governing the dynamics of droplet coalescence and determining how condensates interact with and deform lipid membranes and biological filaments. However, approaches for studying them in-situ in living cells under physiologically relevant conditions are not yet well established. Through an interdisciplinary collaboration between cell biology and theoretical soft matter physics groups, we developed a high-throughput flicker spectroscopy approach to calculate the surface tension of thousands of condensates inside living cells. Demonstrating this approach on stress granules, we discovered firstly that a surface tension-only model is inadequate for describing stress granules in live cells. We find that the measured fluctuation spectra require an additional bending rigidity parameter, which had previously not been described for any biomolecular condensate. Secondly, we show that stress granules do not have a spherical base-shape but fluctuate around a more irregular geometry. Taken together, these results demonstrate quantitatively that the mechanics of stress granules clearly suggest that stress granules are viscoelastic droplets with a structured interface.   The approach can distinguish between stress granules induced by different chemicals or under different stoichiometries of constituent proteins, based on their characteristic distributions of surface tension and bending rigidity values. The measured surface tensions and bending rigidities span a range of several orders of magnitude. As such, different types of stress granules (and more generally, other biomolecular condensates) can only be differentiated via large-scale surveys.