Assembly of tau protein into Alzheimer paired helical filaments depends ona local sequence motif ((306)VQIVYK(311)) forming beta structure

We have searched for a minimal interaction motif in tau protein that suppor ts the aggregation into Alzheimer-like paired helical filaments. Digestion of the repeat domain with different proteases yields a GluC-induced fragmen t comprising 43 residues (termed PHF43), which represents the third repeat of tau plus some flanking residues. This fragment self assembles readily in to thin filaments without a paired helical appearance, but these filaments are highly competent to nucleate bona fide PHFs from full-length tau. Probi ng the interactions of PHF43 with overlapping peptides derived from the ful l tau sequence yields a minimal hexapeptide interaction motif of (306)VQIVY K(311) at the beginning of the third internal repeat. This motif coincides with the highest predicted beta-structure potential in tau. CD and Fourier transform infrared spectroscopy shows that PHF43 acquires pronounced beta s tructure in conditions of self assembly. Point mutations in the hexapeptide region by proline-scanning mutagenesis prevent the aggregation. The data i ndicate that PHF assembly is initiated by a short fragment containing the m inimal interaction motif forming a local beta structure embedded in a large ly random-coil protein.