Acknowledgements: We thank C. Charlier and F. Ferrage for providing the Mathematica script for IMPACT analysis; H. Wang and T. P. J. Knowles for helpful discussions; J. Grimmett, T. Darling and I. Clayson for help with high-performance computing; and M. Wilkinson and R. A. Crowther for critical reading of the manuscript. This work was supported by the facilities for biophysics, electron microscopy, NMR and scientific computing of the MRC Laboratory of Molecular Biology, and by the Francis Crick Institute through provision of access to the MRC Biomedical NMR Centre. The Francis Crick Institute receives its core funding from Cancer Research UK (CC1078), the UK MRC (CC1078) and the Wellcome Trust (CC1078). This work was supported by the MRC, as part of United Kingdom Research and Innovation (MC_U105184291 to M.G. and MC_UP_A025-1013 to S.H.W.S.), and a Marshall scholarship to D.L.Intermediate species in the assembly of amyloid filaments are believed to play a central role in neurodegenerative diseases and may constitute important targets for therapeutic intervention1,2. However, structural information about intermediate species has been scarce and the molecular mechanisms by which amyloids assemble remain largely unknown. Here we use time-resolved cryogenic electron microscopy to study the in vitro assembly of recombinant truncated tau (amino acid residues 297-391) into paired helical filaments of Alzheimer's disease or into filaments of chronic traumatic encephalopathy3. We report the formation of a shared first intermediate amyloid filament, with an ordered core comprising residues 302-316. Nuclear magnetic resonance indicates that the same residues adopt rigid, β-strand-like conformations in monomeric tau. At later time points, the first intermediate amyloid disappears and we observe many different intermediate amyloid filaments, with structures that depend on the reaction conditions. At the end of both assembly reactions, most intermediate amyloids disappear and filaments with the same ordered cores as those from human brains remain. Our results provide structural insights into the processes of primary and secondary nucleation of amyloid assembly, with implications for the design of new therapies
