Sharma, B.; Wang, J.; Retana, P. C.; Baum, J.; Shi, Z.

Tau and -Synuclein (Syn) frequently co-aggregate in various neurodegenerative disorders. Recently, Tau has been shown to form dynamic, liquid-like condensates that can recruit Syn, and potentially serve as a precursor to pathological aggregation. However, the quantitative impact of Syn on the material properties of these condensates remains elusive. Here, we measure the viscosity and interfacial tension of Tau condensates and determine how these properties are modulated by Syn monomers and fibril seeds. We find that while both forms of Syn partition efficiently into Tau condensates, they exert vastly different effects on the condensate's material state. The viscosity of Tau condensates remains unchanged in the presence of Syn monomers at concentrations up to 200 M, accompanied by a moderate reduction in the condensates' interfacial tension. In contrast, the addition of only 5 M Syn fibril seeds triggers rapid solidification of Tau condensates, manifested by a nearly 100-fold increase in condensate viscosity within one hour. These findings provide quantitative insights into condensate mechanics, highlighting the unique capacity of Syn seeds to drive the liquid-to-solid transition of Tau condensates that may underlie the formation of pathological aggregates.