The amyloid stretch hypothesis: recruiting proteins toward the dark side

Abstract

A detailed understanding of the molecular events underlying the conversion and self-association of normally soluble proteins into amyloid fibrils is fundamental to the identification of therapeutic strategies to prevent or cure amyloid-related disorders. Recent investigations indicate that amyloid fibril formation is not just a general property of the polypeptide backbone depending on external factors, but that it is strongly modulated by amino acid side chains. Here, we propose and address the validation of the premise that the amyloidogenicity of a protein is indeed localized in short protein stretches (amyloid stretch hypothesis). We demonstrate that the conversion of a soluble nonamyloidogenic protein into an amyloidogenic prone molecule can be triggered by a nondestabilizing six-residue amyloidogenic insertion in a particular structural environment. Interestingly enough, although the inserted amyloid sequences clearly cause the process, the protease-resistant core of the fiber also includes short adjacent sequences from the otherwise soluble globular domain. Thus, short amyloid stretches accessible for intermolecular interactions trigger the self-assembly reaction and pull the rest of the protein into the fibrillar aggregate. The reliable identification of such amyloidogenic stretches in proteins opens the possibility of using them as targets for the inhibition of the amyloid fibril formation process.

Fig. 1.

Ribbon representation of the α-SH3. The sites selected for insertion of amyloidogenic sequences are indicated by arrows.

Fig. 2.

Far-UV (Left) and near-UV (Right) CD spectra recorded for the α-SH3 with insertions at the N terminus and distal loop (A) and the C terminus (B). Spectra have been recorded at t = 0, pH 2.6, and c ≈ 300 μM, except for 2-SH and AmyBergerac (t = 0, pH 2.6, and c ≈ 100 μM). The spectrum of the WT protein is displayed as a reference. Arrows in the far-UV CD plots indicate the inflexion point at 221 nm. The arrow in the near-UV CD plot (A Right) indicates the new peak at 280 nm exhibited by 1-SH.

Fig. 3.

Electron micrographs of the α-SH3 variants that form amyloid fibrils (pH 2.6, c ≈ 300 μM, t = 3 months, and room temperature). 2-SH has been assayed only at the lowest concentration (c ≈ 100 μM) because of solubility problems.