Overall, these results show that motifs in the microtubule-binding domain of the tau protein control the pathogenic changes caused by overexpression of human tau in vivo. By facilitating the transition towards a β-sheet conformation, the P301S mutation associated with FTDP-17 dramatically enhances tau misfolding and aggregation, despite low levels of phosphorylation, and leads to a rapidly progressing pathology. In contrast, WT 4R0N tau is more extensively hyperphosphorylated. However, the protein does not accumulate into highly aggregated forms, and the behavioral effects of the induced pathology do not progress over time. Nevertheless, it is possible to prevent these behavioral defects by inserting β-sheet-breaking proline residues in the microtubule-binding domain. This modification of the tau protein enhances interaction with microtubules and prevents hyperphosphorylation on the residues that are abundantly phosphorylated in WT tau. This region of the tau protein has therefore a critical role both in the phosphorylation and aggregation of the tau protein, and might be centrally implicated in the pathogenic mechanisms that confer toxic properties to the tau protein.