The aggregation of the microtubule-associated tau protein and formation of "neurofibrillary tangles" is one of the hallmarks of Alzheimer disease. The mechanisms underlying the structural transition of innocuous, natively unfolded tau to neurotoxic forms and the detailed mechanisms of binding to microtubules are largely unknown. Here we report the high-resolution characterization of the repeat domain of soluble tau using multidimensional NMR spectroscopy. NMR secondary chemical shifts detect residual beta-structure for 8-10 residues at the beginning of repeats R2-R4. These regions correspond to sequence motifs known to form the core of the cross-beta-structure of tau-paired helical filaments. Chemical shift perturbation studies show that polyanions, which promote paired helical filament aggregation, as well as microtubules interact with tau through positive charges near the ends of the repeats and through the beta-forming motifs at the beginning of repeats 2 and 3. The high degree of similarity between the binding of polyanions and microtubules supports the hypothesis that stable microtubules prevent paired helical filament formation by blocking the tau-polyanion interaction sites, which are crucial for paired helical filament formation.