We have searched for a minimal interaction motif in protein that supports the aggregation into Alzheimer-like paired helical fila-ments. Digestion of the repeat domain with different proteases yields a GluC-induced fragment comprising 43 residues (termed PHF43), which represents the third repeat of plus some flanking residues. This fragment self assembles readily into thin filaments without a paired helical appearance, but these filaments are highly competent to nucleate bona fide PHFs from full-length . Probing the interactions of PHF43 with overlapping peptides derived from the full sequence yields a minimal hexapeptide interaction motif of 306 VQIVYK 311 at the beginning of the third internal repeat. This motif coincides with the highest predicted ␤-structure potential in . CD and Fourier transform infrared spectroscopy shows that PHF43 acquires pronounced ␤ structure in conditions of self as-sembly. Point mutations in the hexapeptide region by proline-scanning mutagenesis prevent the aggregation. The data indicate that PHF assembly is initiated by a short fragment containing the minimal interaction motif forming a local ␤ structure embedded in a largely random-coil protein.
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