Fig. 4.

Interpretation of the density. (A) Fibril density projected along the helical axis. The density exhibits a twofold rotational symmetry consisting of two protofilaments. (B) The β-sheet sandwich in the protofilament core is highlighted by the rectangle. It exhibits a local twofold symmetry and a corrugated core structure. (C) Comparison of Aβ models. The dashed lines represent disordered N-terminal residues of the peptide, whereas solid regions with arrows indicate β-strands. (Top) Petkova et al. (8). (Middle) Lührs et al (9). (Bottom) Model of the Aβ(1-40) peptide based on the present cryo-EM image reconstruction within one protofilament. Two oppositely directed polypeptide chains form a β-sheet core region and are bounded by the peptide N termini. The number of β-sheet segments making up the core cannot be determined from the cryo-EM structure and, therefore, individual β-strands are not indicated in this model.