High Resolution Structural Characterization of Aβ42 Amyloid Fibrils by Magic Angle Spinning NMR

The presence of amyloid plaques composed of amyloid beta (A beta) fibrils is a hallmark of Alzheimer's disease (AD). The A beta peptide is present as several length variants with two common alloforms consisting of 40 and 42 amino acids, denoted A beta(1-40) and A beta(1-42), respectively. While there have been numerous reports that structurally characterize fibrils of A beta(1-40), very little is known about the structure of amyloid fibrils of A beta(1-42), which are considered the more toxic alloform involved in AD. We have prepared isotopically C-13/N-13 labeled A beta(M01-42) fibrils in vitro from recombinant protein and examined their C-13-C-13 and C-13-N-15 magic angle spinning (MAS) NMR spectra. In contrast to several other studies of A beta fibrils, we observe spectra with excellent resolution and a single set of chemical shifts, suggesting the presence of a single fibril morphology. We report the initial structural characterization of A beta(M01-42) fibrils utilizing C-13 and N-15 shift assignments of 38 of the 43 residues, including the backbone and side chains, obtained through a series of cross-polarization based 2D and 3D C-13-C-13, C-13-N-15 MAS NMR experiments for rigid residues along with J-based 2D TOBSY experiments for dynamic residues. We find that the first similar to 5 residues are dynamic and most efficiently detected in a J-based TOBSY spectrum. In contrast, residues 16-42 are easily observed in cross-polarization experiments and most likely form the amyloid core. Calculation of psi and phi dihedral angles from the chemical shift assignments indicate that beta-strands are present in the fibril's secondary structure.