Abstract：Phase-separated biomolecular condensates must respond agilely to biochemical and environmental cues in performing their wide-ranging cellular functions, but our understanding of condensate dynamics is lagging. Ample evidence now indicates biomolecular condensates as viscoelastic fluids, where shear stress relaxes at a finite rate, not instantaneously as in viscous liquids. Yet the fusion dynamics of condensate droplets has mostly been modeled based on viscous liquids, with fusion time given by the viscocapillary ratio (viscosity over interfacial tension). To directly test the applicability of the viscocapillary model, we used optical tweezers to measure the viscoelasticity, interfacial tension, and fusion speed of four biomolecular condensates. The results demonstrate the breakdown of the viscocapillary model: the measured fusion speeds are either faster or slower than predicted by the model, indicating shear thinning or thickening. Shear thinning occurs in condensates formed by disordered chains, likely arising from chain alignment, whereas shear thickening occurs in condensates formed by proteins with folded domains, possibly due to shear-induced stabilization of clusters. Enormous shear thinning, ~100-fold, is observed in condensates formed by short peptides and bridged by ATP, reflecting rapid rupture and reformation of molecular networks.

Bio:

Huan-Xiang Zhou received his Ph.D. from Drexel University in 1988 and did postdoctoral work at the National Institutes of Health. After faculty appointments at Hong Kong University of Science and Technology, Drexel University, and Florida State University, he moved in 2017 to the University of Illinois Chicago, where he is Professor of Chemistry and Physics and holds an LAS Endowed Chair in the Natural Sciences. He was elected a fellow of the American Association for the Advancement of Science, a fellow of the American Physical Society, and very recently a fellow of the Biophysical Society. His group does theoretical, computational, and experimental research on molecular and cellular biophysics. Current interests include thermodynamic and dynamic properties of biomolecular condensates; membrane association of intrinsically disordered proteins; structures and aggregation pathways of amyloid-beta and other amyloidogenic proteins; structures and interactions of M. tuberculosis divisome proteins; and functional mechanisms of glutamate-receptor ion channels. He has published over 300 peer-reviewed papers and has an H-index of 85.