title: Direct force measurements reveal that protein Tau confers short-range attractions and isoform-dependent steric stabilization to microtubules.
abstract: Microtubules (MTs) are hollow cytoskeletal filaments assembled from αβ-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; ΦTau = 1/10), isoforms with longer N-terminal tails (NTTs) sterically stabilized MTs, preventing bundling up to PB ∼ 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 (PB ∼ 1,000-2,000 Pa) over the same range. Remarkably, the abrupt increase in PB observed for longer isoforms suggests a mushroom to brush transition occurring at 1/13 < ΦTau < 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 isoform-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.
authors.affiliation: Materials Department, University of California, Santa Barbara, CA 93106; Physics Department, University of California, Santa Barbara, CA 93106; Molecular, Cellular, and Developmental Biology Department, University of California, Santa Barbara, CA 93106;
authors.forename: Peter J
authors.initials: PJ
authors.lastname: Chung
authors.affiliation: Department of Bio and Brain Engineering, Korea Advanced Institute of Science and Technology, Daejeon 305-701, Republic of Korea;
authors.forename: Myung Chul
authors.initials: MC
authors.lastname: Choi
authors.affiliation: Molecular, Cellular, and Developmental Biology Department, University of California, Santa Barbara, CA 93106; Neuroscience Research Institute, University of California, Santa Barbara, CA 93106;
authors.forename: Herbert P
authors.initials: HP
authors.lastname: Miller
authors.affiliation: Molecular, Cellular, and Developmental Biology Department, University of California, Santa Barbara, CA 93106; Neuroscience Research Institute, University of California, Santa Barbara, CA 93106;
authors.forename: H Eric
authors.initials: HE
authors.lastname: Feinstein
authors.affiliation: Institute of Chemistry, Hebrew University of Jerusalem, Jerusalem 91904, Israel;
authors.forename: Uri
authors.initials: U
authors.lastname: Raviv
authors.affiliation: Materials Research Laboratory, University of California, Santa Barbara, CA 93106.
authors.forename: Youli
authors.initials: Y
authors.lastname: Li
authors.affiliation: Molecular, Cellular, and Developmental Biology Department, University of California, Santa Barbara, CA 93106; Neuroscience Research Institute, University of California, Santa Barbara, CA 93106;
authors.forename: Leslie
authors.initials: L
authors.lastname: Wilson
authors.affiliation: Molecular, Cellular, and Developmental Biology Department, University of California, Santa Barbara, CA 93106; Neuroscience Research Institute, University of California, Santa Barbara, CA 93106;
authors.forename: Stuart C
authors.initials: SC
authors.lastname: Feinstein
authors.affiliation: Materials Department, University of California, Santa Barbara, CA 93106; Physics Department, University of California, Santa Barbara, CA 93106; Molecular, Cellular, and Developmental Biology Department, University of California, Santa Barbara, CA 93106; safinya@mrl.ucsb.edu.
authors.forename: Cyrus R
authors.initials: CR
authors.lastname: Safinya
mesh_terms: D000818:Animals; D055592:Biophysical Phenomena; D002417:Cattle; D006801:Humans; D008870:Microtubules / Q000378:metabolism*; D008958:Models, Molecular; D009997:Osmotic Pressure; D011485:Protein Binding; D020033:Protein Isoforms / Q000378:metabolism; D016875:tau Proteins / Q000378:metabolism*
chemical_list: D020033:Protein Isoforms; D016875:tau Proteins
keywords: SAXS; Tau; force measurement; intrinsically disordered proteins; microtubule