Out-of-Register Parallel ß-Sheets and Antiparallel ß-Sheets Coexist in 150-kDa Oligomers Formed by Amyloid-ß(1-42).

ABSTRACT

We present solid-state NMR measurements of ß-strand secondary structure and inter-strand organization within a 150-kDa oligomeric aggregate of the 42-residue variant of the Alzheimer's amyloid-ß peptide (Aß(1-42)). We build upon our previous report of a ß-strand spanned by residues 30-42, which arranges into an antiparallel ß-sheet. New results presented here indicate that there is a second ß-strand formed by residues 11-24. Contrary to expectations, NMR data indicate that this second ß-strand is organized into a parallel ß-sheet despite the co-existence of an antiparallel ß-sheet in the same structure. In addition, the in-register parallel ß-sheet commonly observed for amyloid fibril structure does not apply to residues 11-24 in the 150-kDa oligomer. Rather, we present evidence for an inter-strand registry shift of three residues that likely alternate in direction between adjacent molecules along the ß-sheet. We corroborated this unexpected scheme for ß-strand organization using multiple two-dimensional NMR and 13C-13C dipolar recoupling experiments. Our findings indicate a previously unknown assembly pathway and inspire a suggestion as to why this aggregate does not grow to larger sizes.