Fig. 4: Atomic structures of amyloid-like association of NCAP segments revealed by crystallography.

a Quality of the fit of each atomic model to its corresponding simulated annealing composite omit maps⁹². The maps are contoured at the 1.0 sigma level. All structural features are well defined by the density. The view is down the fibril axis. Each chain shown here corresponds to one strand in a β-sheet. Thousands of identical strands stack above and below the plane of the page making ~100 micron-long β-sheets. The face of each β-sheet of AALALL (PDB 7LTU) [https://doi.org/10.2210/pdb7LTU/pdb] (form 1) is symmetric with its back. However, GSQASS (PDB 7LV2) [https://doi.org/10.2210/pdb7LV2/pdb] and GQTVTK (PDB 7LUZ) [https://doi.org/10.2210/pdb7LUZ/pdb] each reveal two distinct sheet–sheet interfaces: face-to-face and back-to-back. The tighter associated pair of sheets is shown in this figure. b 18 strands from each of the steric zippers at a view nearly perpendicular to the fibril axis. GSQASS and GQTVTK are parallel, in-register sheets, mated with Class 1 zipper symmetry. The AALALL zippers are antiparallel, in register sheets, mated with Class 7 zipper symmetry. Trifluoroacetic acid (TFA) appears in the AALALL-form 1 steric zipper, and polyethylene glycol (PEG) binds form 2 (PDB 7LUX [https://doi.org/10.2210/pdb7LUX/pdb] (form 2); Supplementary Fig. 4). Carbon atoms in a and b are shown in orange and heteroatoms are colored by atom type. Water molecules are shown as red dots. c Steric zipper structures (same order as in a) viewed down the fibril axis with residues colored according to the Kyte and Doolittle hydrophobicity scale (bottom right) shown with semi-transparent surface representation to emphasize the shape complementarity and tight fit between the β-sheets. Images in c were rendered with UCSF Chimera⁹⁰. A stereo view of all structures is given in Supplementary Fig. 5.