Sequence determinants of enhanced amyloidogenicity of Alzheimer A{beta}42 peptide relative to A{beta}40

Aggregation of proteins into insoluble deposits is associated with a variety of human diseases. In Alzheimer disease, the aggregation of amyloid beta (Abeta) peptides is believed to play a key role in pathogenesis. Although the 40-mer (Abeta40) is produced in vivo at higher levels than the 42-mer (Abeta42), senile plaque in diseased brains is composed primarily of Abeta42. Likewise, in vitro, Abeta42 forms fibrils more rapidly than Abeta40. The enhanced amyloidogenicity of Abeta42 could be due simply to its greater length. Alternatively, specific properties of residues Ile(41) and Ala(42) might favor aggregation. To distinguish between these two possibilities, we constructed a library of sequences in which residues 41 and 42 were randomized. The aggregation behavior of the resulting sequences was assessed using a high throughput screen, based on the finding that fusions of Abeta42 to green fluorescence protein (GFP) prevent the folding and fluorescence of GFP, whereas mutations in Abeta42 that disrupt aggregation produce green fluorescent fusions. Correlations between the sequences of Abeta42 mutants and the fluorescence of Abeta42-GFP fusions in vivo were confirmed in vitro through biophysical studies of synthetic 42-residue peptides. The data reveal a strong correlation between aggregation propensity and the hydrophobicity and beta-sheet propensities of residues at positions 41 and 42. Moreover, several mutants containing hydrophilic residues and/or beta-sheet breakers at positions 41 and/or 42 were less prone to aggregate than Abeta40 wherein these two residues are deleted entirely. Thus, properties of the side chains at positions 41 and 42, rather than length per se, cause Abeta42 to aggregate more readily than Abeta40.