Mutations That Replace Aromatic Side Chains Promote Aggregation of the Alzheimer's Aβ Peptide

The aggregation of polypeptides into amyloid fibrils is associated with a number of human diseases. Because these fibrils or intermediates on the aggregation pathway-play important roles in the etiology of disease, considerable effort has been expended to understand which features of the amino acid sequence promote aggregation. One feature suspected to direct aggregation is the pi-stacking of aromatic residues. Such pi-stacking interactions have also been proposed as the targets for various aromatic compounds that are known to inhibit aggregation. In the case of Alzheimer's disease, the aromatic side chains Phe19 and Phe20 in the wild-type amyloid beta (A beta) peptide have been implicated. To explicitly test whether the aromaticity of these side chains plays a role in aggregation, we replaced these two phenylalanine side chains with leucines or isoleucines. These residues have similar sizes and hydrophobicities as Phe but are not capable of pi-stacking. Thioflavin-T fluorescence and electron microscopy demonstrate that replacement of residues 19 and 20 by Leu or Ile did not prevent aggregation, but rather enhanced amyloid formation. Further experiments showed that aromatic inhibitors of aggregation are as effective against Ile- and Leu-substituted versions of A beta 42 as they are against wild-type A beta. These results suggest that aromatic pi-stacking interactions are not critical for A beta aggregation or for the inhibition of A beta aggregation.