Fig. 4.

Atomic-resolution cross-sections of the three types of TTR(105–115) amyloid fibrils determined by combining MAS NMR and cryo-EM. Reconstructed cross-section density maps are shown as two electron density isosurfaces (1.0σ and 2.2σ above the mean density) with the secondary structure of the constituent NMR-derived protofilaments shown in a cyan ribbon representation. (A–C) The doublet (A), triplet (B), and quadruplet (C) fibril cross-sections can accommodate pairs of two, three, and four interconnected protofilaments, respectively. (D–F) All-atom representation of the doublet (D), triplet (E), and quadruplet (F) cross-sections with cryo-EM envelopes superimposed. Cryo-EM envelopes are shown as orange, yellow, and purple contours at 1.0σ above the mean density for doublet, triplet, and quadruplet fibrils, respectively. For a discussion on the resolution of the cryo-EM maps, see SI Appendix, Table S4.