Characterization of two VQIXXK motifs for tau fibrillization in vitro

Tau proteins are building blocks of the filaments that form neurofibrillary tangles of Alzheimer's disease (AD) and related neurodegenerative tauopathies. It was recently reported that two VQIXXK motifs in the microtubule (MT) binding region, named PHF6 and PHF6*, are responsible for tau fibrillization. However, the exact role each of these motifs plays in this process has not been analyzed in detail. Using a recombinant human tau fragment containing only the four MT-binding repeats (K18), we show that deletion of either PHF6 or PHF6* affected tau assembly but only PHF6 is essential for filament formation, suggesting a critical role of this motif. To determine the amino acid residues within PHF6 that are required for tau fibrillization, a series of deletion and mutation constructs targeting this motif were generated. Deletion of VQI in either PHF6 or PHF6* lessened but did not eliminate K18 fibrillization. However, removal of the single K311 residue from PHF6 completely abrogated the fibril formation of K18. K311D mutation of K18 inhibited tau filament formation, while K311A and K311R mutations had no effect. These data imply that charge change at position 311 is important in tau fibril formation. A similar requirement of nonnegative charge at this position for fibrillization was observed with the full-length human tau isoform (T40), and data from these studies indicate that the formation of fibrils by T40K311D and T40K311P mutants is repressed at the nucleation phase. These findings provide important insights into the mechanisms of tau fibrillization and suggest targets for AD drug discovery to ameliorate neurodegeneration mediated by filamentous tau pathologies.