Independent tubulin binding and polymerization by the proline-rich region of tau is regulated by tau's N-terminal domain

Tau is an intrinsically disordered, microtubule-associated protein that has a role in regulating microtubule dynamics. Despite intensive research, the molecular mechanisms of tau-mediated microtubule polymerization are poorly understood. Here, we used single-molecule fluorescence to investigate the role of tau's N-terminal domain (NTD) and proline-rich region (PRR) in regulating interactions of tau with soluble tubulin. We assayed both full-length tau isoforms and truncated variants for their ability to bind soluble tubulin and stimulate microtubule polymerization. We found that tau's PRR is an independent tubulin-binding domain that has tubulin polymerization capacity. In contrast to the relatively weak interactions with tubulin mediated by sites distributed throughout tau's microtubule-binding repeats (MTBR), resulting in heterogeneous tau:tubulin complexes, the PRR bound tubulin tightly and stoichiometrically. Moreover, we demonstrate that interactions between the PRR and MTBR are reduced by the NTD through a conserved conformational ensemble. On the basis of these results, we propose that tau's PRR can serve as a core tubulin-binding domain, whereas the MTBR enhances polymerization capacity by increasing the local tubulin concentration. Moreover, the NTD appears to negatively regulate tubulin-binding interactions of both of these domains. The findings of our study draw attention to a central role for the PRR in tau function and provide mechanistic insight into tau-mediated polymerization of tubulin.