Direct force measurements reveal that protein Tau confers short-range attractions and isoform-dependent steric stabilization to microtubules

Microtubules (MTs) are hollow cytoskeletal filaments assembled from alpha beta-tubulin heterodimers. Tau, an unstructured protein found in neuronal axons, binds to MTs and regulates their dynamics. Aberrant Tau behavior is associated with neurodegenerative dementias, including Alzheimer's. Here, we report on a direct force measurement between paclitaxel-stabilized MTs coated with distinct Tau isoforms by synchrotron small-angle X-ray scattering (SAXS) of MT-Tau mixtures under osmotic pressure (P). In going from bare MTs to MTs with Tau coverage near the physiological submonolayer regime (Tau/tubulin-dimer molar ratio; Phi(Tau) = 1/10), isoforms with longer N-terminal tails (NTTs) sterically stabilized MTs, preventing bundling up to P-B similar to 10,000-20,000 Pa, an order of magnitude larger than bare MTs. Tau with short NTTs showed little additional effect in suppressing the bundling pressure (P-B similar to 1,000-2,000 Pa) over the same range. Remarkably, the abrupt increase in P-B observed for longer isoforms suggests a mushroom to brush transition occurring at 1/13 < Phi(Tau) < 1/10, which corresponds to MT-bound Tau with NTTs that are considerably more extended than SAXS data for Tau in solution indicate. Modeling of Tau-mediated MT-MT interactions supports the hypothesis that longer NTTs transition to a polyelectrolyte brush at higher coverages. Higher pressures resulted in isoform-independent irreversible bundling because the polyampholytic nature of Tau leads to short-range attractions. These findings suggest an iso-form-dependent biological role for regulation by Tau, with longer isoforms conferring MT steric stabilization against aggregation either with other biomacromolecules or into tight bundles, preventing loss of function in the crowded axon environment.