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The Formation of Straight and Twisted Filaments from Short Tau Peptides*

We studied fibril formation in a family of peptides based on PHF6 (VQIVYK), a short peptide segment found in the microtubule binding region of tau protein. N-Acetylated peptides AcVYK-amide (AcVYK), AcIVYK-amide (AcPHF4), AcQIVYK-amide (AcPHF5), and AcV-QIVYK-amide (AcPHF6) rapidly formed straight filaments in the presence of 0.15 m NaCl, each composed of two laterally aligned protofilaments ∼5 nm in width. X-ray fiber diffraction showed the omnipresent sharp 4.7-Å reflection indicating that the scattering objects are likely elongated along the hydrogen-bonding direction in a cross-β conformation, and Fourier transform IR suggested the peptide chains were in a parallel (AcVYK, AcPHF6) or antiparallel (AcPHF4, AcPHF5) β-sheet configuration. The dipeptide N-acetyl-YK-amide (AcYK) formed globular structures ∼200 nm to 1 μm in diameter. The polymerization rate, as measured by thioflavin S binding, increased with the length of the peptide going from AcYK → AcPHF6, and peptides that aggregated most rapidly displayed CD spectra consistent with β-sheet structure. There was a 3-fold decrease in rate when Val was substituted for Ile or Gln, nearly a 10-fold decrease when Ala was substituted for Tyr, and an increase in polymerization rate when Glu was substituted for Lys. Twisted filaments, composed of four laterally aligned protofilaments (9–19 nm width, ∼90 nm half-periodicity), were formed by mixing AcPHF6 with AcVYK. Taken together these results suggest that the core of PHF6 is localized at VYK, and the interaction between small amphiphilic segments of tau may initiate nucleation and lead to filaments displaying paired helical filament morphology.