Tau is a microtubule-associated protein which stabilizes microtubules and stimulates their assembly. Current descriptions of the tubulin-interacting regions of Tau involve microtubules as the target and result mainly from deletions of Tau domains based on sequence analysis and from NMR spectroscopy experiments. Here, instead of microtubules, we use the complex of two tubulin heterodimers with the stathmin-like domain of the RB3 protein (T2R) to identify interacting Tau fragments generated by limited proteolysis. We show that fragments in the proline rich region and in the microtubule binding repeats domain each interact on their own not only with T2R but also with microtubules, albeit with moderate affinity. NMR analysis of the interaction with T2R of constructs in these two regions leads to a fragment composed of adjacent parts of the microtubule binding repeat domain and of the proline rich region that binds tightly to stabilized microtubules. This demonstrates the synergy of the two Tau regions we identified in the Tau-microtubule interaction. Moreover, we show that this fragment binds two tubulin heterodimers in a complex that incorporates in microtubules more readily than a single heterodimer and in this way stimulates microtubule assembly.