Tau Mediates Widely-Spaced Microtubule Bundles through Local Polyion Attractions at the Midplane Layer: A Novel, Functional Mechanism for Intrinsically Disordered Proteins

Abstract

Dysfunction of Tau, an intrinsically disordered protein localized to neuronal axons, is associated with many neurodegenerative diseases (“Tauopathies”) including FTDP-17, Alzheimer's, and, more recently, chronic traumatic encephalopathy. However, that association is not fully understood, as the accepted, physiological functions of Tau are limited to stabilizing individual microtubules and mediating microtubule bundles. The latter conclusion is largely derived from seminal studies where transfected Tau cDNA prompted cells in culture to grow neurite-like processes with hexagonally-ordered microtubule arrays.However, ex vivo TEM of the axon shows only linear bundles of microtubules with no long-range positional ordering. Additionally, prior cell-free reconstitutions of microtubules with Tau exhibited either no bundles or bundles of dramatically different morphologies. The use of taxol in these reconstitutions further complicated matters; while making experiments tractable (stabilizing microtubules from depolymerization), taxol has been shown to weakly compete with Tau and affect microtubule assembly dynamics.Herein, we reconcile this conflicting behavior by reporting on the cell-free and taxol-free reconstitution of microtubules co-assembled with Tau at near-physiological conditions, recapitulating both hexagonally-symmetric and linear microtubule bundles. Small-angle X-ray scattering and TEM was used to obtain angstrom-resolution ensemble-averaged structural information and local morphology, respectively, of Tau-mediated MT bundles with large inter-MT spacings.Through an examination of truncated Tau and the force-response of microtubule bundles using osmotic depletants, we present a novel mechanism for long-ranged attractions between microtubules that is contingent on both the polyampholytic and intrinsically disordered nature of Tau. This proposed mechanism has heretofore been unseen in biology and would be significantly affected by Tau post-translational modifications associated with dysfunction and neurodegeneration.