Solvent-Dependent Conformation of the Third Repeat Fragment in the Microtubule-Binding Domain of Tau Protein, Analyzed by 1H-NMR Spectroscopy and Molecular Modeling Calculation

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Abstract
The third repeat fragment (3MBD, 31 residues) in the four-repeat microtubule-binding domain of tau protein is considered to be responsible for the formation of the neuropathological filament. To clarify the structural function of 3MBD in the filamentous assembly, solution structures in water and 2,2,2-trifluoroethanol (TFE) were investigated by CD measurement and the combination of two-dimensional 1H-NMR measurement and a molecular modeling calculation. The CD spectra suggested the solvent-dependent conformation of 3MBD, i.e., random and α-helical structures in water and TFE solvents, respectively. All protons were assigned by various 2D NMR spectral measurements. The NOE pattern characteristic of a typical helical structure was observed in TFE solution. Using NOE and JHNCαH data observed in both solutions, possible 3D structures were generated by the dynamical simulated annealing method. The constructed NMR conformers in water consisted of an equilibrium mixture of extended and folded conformers, although the N-terminal Val1–Lys6 and middle fragments formed well-refined extended (Val1–Lys6) and α-helical (Leu10–Leu20) structures in TFE, respectively. Interestingly, the helical structure showed an amphipathic distribution of the respective side chains. This amphipathic behaviour of the 3MBD structure is advantageous for self-association through the alternating hydrophilic and hydrophobic interactions into a helical filament of the tau MBD domain. Based on the situation-dependent conformational flexibility of 3MBD, the possibility of filament formation of tau protein via self-assembly of the α-helical structure under neuropathological conditions is discussed.