Possible role of each repeat structure of the microtubule-binding domain of the tau protein in in vitro aggregation.

Although one of the priorities in Alzheimer's research is to clarify the filament formation mechanism of the tau protein, it is currently unclear how it is transformed from a normal structure in a neuron. To examine which part and what structural change in the tau protein are involved in its transformation into a pathological entity, the initial in vitro self-aggregation features of each repeat peptide (R1-R4) constituting a three- or four-repeat microtubule-binding domain (3RMBD or 4RMBD) in the tau protein was investigated by measuring both the fluorescence and light scattering (LS) spectra on the same instrument, because these MBD domains constitute the core moiety of the tau paired helical filament (PHF) structure. The conformational features of the R1 and R4 peptides in trifluoroethanol were also investigated by (1)H-NMR and molecular modeling analyses and compared with those of the R2 and R3 peptides. The analyses of the LS spectra clarified (i) the self-aggregation rates of R1-R4, 3RMBD and 4RMBD at a fixed concentration (15 mM), (ii) their minimum concentrations for starting filament extension, and (iii) the concentration dependence of their self-aggregations. The fluorescence analyses showed that the R2 and R3 peptides have high self-aggregation abilities at the extension and nucleation steps, respectively, in their filament formation processes. It was shown that the R2 repeat exhibits a positive synergistic effect on the aggregation of 4RMBD. The R1 and R4 repeats, despite their weak self-aggregation abilities, are necessary for the intact PHF formation of tau MBD, whereas they exerted a negative effect on the R3-driven aggregation of 3RMBD. The conformational analyses showed the importance of the amphipathic conformational features of the R1 to R4 peptides, and the intermolecular disulfide bonding abilities of the R2 and R3 peptides for the PHF formation. On the basis of the present spectral and conformational results, the possible role of each repeat structure in the dimeric formation of MBD at the initial in vitro aggregation stage is discussed.