Hindering tau fibrillization by disrupting transient precursor clusters.

Tau protein, a central player in Alzheimer's disease (AD), exhibits cytotoxicity upon fibril formation. Understanding the early stages of tau fibrillization is therefore critical for the development of effective therapeutics. Previous work [Rasmussen. et. al, J. Mol. Biol., 2023] reported the rapid formation of Thioflavin T (ThT)-inactive clusters upon mixing tau with anionic polymers, yet the functional role of these clusters remained unclear. Here, we demonstrate that these transient clusters act as obligatory precursors in the fibrillization pathway. Using small-angle X-ray scattering (SAXS) and ThT fluorescence, we show that disrupting the clusters via NaCl addition hinders fibril formation, highlighting their reversible and targetable nature. This behavior is analogous to polymer crystallization, in which disordered chains undergo structural ordering through intermediate precursor states. We propose that similar physical principles underlie the aggregation of other intrinsically disordered proteins such as α-synuclein.