Mutagenesis of the central hydrophobic cluster in Aβ42 Alzheimer's pepticle -: Side-chain properties correlate with aggregation propensities

Protein misfolding and deposition underlie an increasing number of debilitating human disorders. Alzheimer's disease is pathologically characterized by the presence of numerous insoluble amyloid plaques in the brain, composed primarily of the 42 amino acid human beta-amyloid peptide (A beta 42). Disease-linked mutations in A beta 42 occur in or near a central hydrophobic cluster comprising residues 17-21. We exploited the ability of green fluorescent protein to act as a reporter of the aggregation of upstream fused A beta 42 variants to characterize the effects of a large set of single-point mutations at the central position of this hydrophobic sequence as well as substitutions linked to early onset of the disease located in or close to this region. The aggregational properties of the different protein variants clearly correlated with changes in the intrinsic physicochemical properties of the side chains at the point of mutation. Reduction in hydrophobicity and beta-shcet propensity resulted in an increase of in vivo fluorescence indicating disruption of aggregation, as confirmed by the in vitro analysis of synthetic A beta 42 variants. The results confirm the key role played by the central hydrophobic stretch on A beta 42 deposition and support the hypothesis that sequence tunes the aggregation propensities of polypeptides.