The third repeat fragment (3MBD, 31 residues) in the four-repeat microtubule-binding domain of water-soluble tau protein has been considered to be responsible for the formation of the neuropathological filament. To clarify the structural requisite of 3MBD for the filamentous assembly, the solution structures in water and trifluoroethanol (TFE) were investigated by a combination of two-dimensional H-1-NMR measurements and molecular modeling calculations. All protons were assigned by various 2D NMR spectral measurements. The NOE patterns characteristic to the typical helical structure were observed in TFE solution, as was expected from the CD spectra. Using 273 NOE and 23 (3)J(NHalphaH) data, possible 3D structures were generated by the dynamical simulated annealing method. The constructed NMR conformers showed that the N-terminal Vall-Lys6 and Leu10-Leu20 fragments form the well-refined extended and alpha-helical structures, respectively, whereas the C-terminal moiety is highly flexible. Interestingly, the helical structure showed amphipathic distribution of the respective side chains. This amphipathic behavior of the 3MBD structure would be necessary for self-associating into a helical filament of the tau MBD domain, because such a filament is stabilized by the alternating hydrophilic and hydrophobic interactions between the 3MBD fragments. (C) 2002 Elsevier Science (USA). All rights reserved.