Single-Molecule FRET Reveals an Extended Structure of Tau Bound to Tubulin Heterodimers

Abstract

Tau is an abundant neuronal protein that promotes tubulin polymerization and stabilizes microtubules (MTs) in the axons of neurons. The disruption of its normal biological function as well as its aggregation and deposition into neurofibrillary tangles has been proposed to contribute to several neurodegenerative disorders known as tauopathies, including Alzheimer's disease. Although taus interaction with MTs has been extensively studied, a detailed structural characterization of the complex is lacking. This deficit is, in part, due to the fact that tau is intrinsically disordered in solution and remains largely so upon binding to MTs, making its structural characterization challenging. Here, we identified conformational changes in tau underlying its binding to tubulin. To do so, we used intramolecular single-molecule Forster Resonance Energy Transfer (smFRET) measurements of the full-length tau protein upon binding to soluble tubulin heterodimers under non-assembly conditions. Tau was labeled with donor and acceptor fluorophores at multiple pairs of positions to cover all domains of interest. We observed that tau undergoes significant structural rearrangements when binding to tubulin. Specifically, it results in a loss of long-range contacts between the two termini and the microtubule binding region (MTBR) that characterizes taus compact solution conformation, leading to an overall expansion of its structure. Moreover, we contrast the structures of two isoforms of tau which differ in the number of MTBR (2N3R and 2N4R). From this work, we propose a topological model of the structure of tau bound to tubulin, which provides insight to the molecular mechanism of tau-mediated tubulin polymerization into MTs.