Cross-beta Order and Diversity in Nanocrystals of an Amyloid-forming Peptide

The seven-residue peptide GNNQQNY from the N-terminal region of the yeast prion protein Sup35, which forms amyloid fibers, colloidal aggregates and highly ordered nanocrystals, provides a model system for characterizing the elusively protean cross-beta conformation. Depending on preparative conditions, orthorhombic and monoclinic crystals with similar lath-shaped morphology have been obtained. Ultra high-resolution (<0.5 Å spacing) electron diffraction patterns from single nanocrystals show that the peptide chains pack in parallel cross-beta columns with ∼4.86 Å axial spacing. Mosaic striations 20–50 nm wide observed by electron microscopy indicate lateral size-limiting crystal growth related to amyloid fiber formation. Frequently obtained orthorhombic forms, with apparent space group symmetry P212121, have cell dimensions ranging from |a|=22.7–21.2 Å, |b|=39.9–39.3 Å, |c|=4.89–4.86 Å for wet to dried states. Electron diffraction data from single nanocrystals, recorded in tilt series of still frames, have been mapped in reciprocal space. However, reliable integrated intensities cannot be obtained from these series, and dynamical electron diffraction effects present problems in data analysis. The diversity of ordered structures formed under similar conditions has made it difficult to obtain reproducible X-ray diffraction data from powder specimens; and overlapping Bragg reflections in the powder patterns preclude separated structure factor measurements for these data. Model protofilaments, consisting of tightly paired, half-staggered beta strands related by a screw axis, can be fit in the crystal lattices, but model refinement will require accurate structure factor measurements. Nearly anhydrous packing of this hydrophilic peptide can account for the insolubility of the crystals, since the activation energy for rehydration may be extremely high. Water-excluding packing of paired cross-beta peptide segments in thin protofilaments may be characteristic of the wide variety of anomalously stable amyloid aggregates.