Cysteines are critical determinants of spontaneous and seeded tau aggregation in cells.

The frontotemporal dementia-linked S320F mutation in the microtubule-associated protein tau promotes spontaneous aggregation, yet the structural basis of its amyloidogenesis remains unclear. Using cryo-electron microscopy, we determined the structure of an S320F295-330 tau fibril composed of parallel chains stabilized by the 306VQIVYK311 amyloid motif, with S320F buried in the fibril core and a C322-C322 disulfide linking two protofilaments. Although cysteines are dispensable for fibril formation by isolated peptide fragments in vitro, tau repeat domain constructs containing both C291 and C322 generate more potent seeds in cellular assays. In contrast, the C322S mutation suppresses spontaneous aggregation of S320F tau in cells, and combined C291S and C322S mutations inhibit seeded aggregation in both wild-type and S320F contexts. Systematic alanine mutagenesis coupled with seeding by tauopathy-derived material identifies cysteine residues as critical determinants of tau seeding, comparable in importance to core amyloid motifs. Together, these findings establish cysteines as central chemical regulators of tau aggregation and propagation.