Sequence determinants of enhanced amyloidogenicity of Alzheimer Aβ42 peptide relative to Aβ40

Aggregation of proteins into insoluble deposits is associated with a variety of human diseases. In Alzheimer disease, the aggregation of amyloid beta(A beta) peptides is believed to play a key role in pathogenesis. Although the 40-mer (A beta 40) is produced in vivo at higher levels than the 42-mer (A beta 42), senile plaque in diseased brains is composed primarily of A beta 42. Likewise, in vitro, A beta 42 forms fibrils more rapidly than A4 beta 0. The enhanced amyloidogenicity of A beta 42 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 A beta 42 to green fluorescence protein (GFP) prevent the folding and fluorescence of GFP, whereas mutations in A beta 42 that disrupt aggregation produce green fluorescent fusions. Correlations between the sequences of A beta 42 mutants and the fluorescence of A beta 42-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 A beta 40 wherein these two residues are deleted entirely. Thus, properties of the side chains at positions 41 and 42, rather than length per se, cause A beta 42 to aggregate more readily than A beta 40.