Background: Soluble A␤42 oligomers, rather than insoluble amyloid fibrils, are toxic species in Alzheimer's disease. Results: We obtained structural restraints at all 42 residue positions in A␤42 oligomers and performed structural modeling. Conclusion: In oligomers, each A␤42 protein forms a single ␤-sheet with three antiparallel ␤-strands. Significance: Our novel structural model provides new structural framework for understanding oligomer-fibril interconversion and designing oligomer-targeted therapeutics.

Formation of amyloid fibrils is involved in a range of fatal human disorders including Alzheimer, Parkinson, and prion diseases. Yeast prions, despite differences in sequence from their mammalian counterparts, share similar features with mammalian prions including infectivity, prion strain phenomenon, and species barrier and thus are good model systems for human prion diseases. Yeast prions normally have long prion domains that presumably form multiple ␤ strands in the fibril, and structural knowledge about the yeast prion fibrils has been limited. Here we use site-directed spin labeling and electron paramagnetic resonance (EPR) spectroscopy to investigate the structures of amyloid fibrils of Ure2 prion domain. We show that 15 spin-labeled Ure2 mutants, with spin labels at every 5th residue from position 5 to position 75, show a single-line or nearly single-line feature in their EPR spectra as a result of strong spin exchange interactions. These results suggest that a parallel in-register ␤ structure exists at these spin-labeled positions. More interestingly, we also show that residues in the segment 30 -65 have stronger spin exchange interactions, higher local stability, and lower solvent accessibility than segments 5-25 and 70 -75, suggesting different local environment at these segments. We propose a hierarchical organization in the amyloid core of Ure2, with the segment 30 -65 forming an inner core and the segments 5-25 and 70 -75 forming an outer core. The hierarchical organization in the amyloid core may be a structural origin for polymorphism in fibrils and prion strains.

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Background: A␤ monomer is the building block of neurotoxic oligomers in Alzheimer disease. Results: EPR studies of A␤40 monomers tethered on solid support show lower spin label mobility at residues 14 -18, 29 -30, and 38 -40. Conclusion: A␤40 monomer adopts a structured state with three ordered segments at 14 -18, 29 -30, and 38 -40. Significance: Structural information of A␤40 monomer is crucial for understanding the mechanism of A␤ oligomerization.