Complementary dimerization of microtubule-associated tau protein: Implications for microtubule bundling and tau-mediated pathogenesis

Abstract

Tau is an intrinsically unstructured microtubule (MT)-associated protein capable of binding to and organizing MTs into evenly spaced parallel assemblies known as ''MT bundles.'' How tau achieves MT bundling is enigmatic because each tau molecule possesses only one MT-binding region. To dissect this complex behavior, we have used a surface forces apparatus to measure the interaction forces of the six CNS tau isoforms when bound to mica substrates in vitro. Two types of measurements were performed for each isoform: symmetric configuration experiments measured the interactions between two tau-coated mica surfaces, whereas ''asymmetric'' experiments examined tau-coated surfaces interacting with a smooth bare mica surface. Depending on the configuration (of which there were 12), the forces were weakly adhesive, strongly adhesive, or purely repulsive. The equilibrium spacing was determined mainly by the length of the tau projection domain, in contrast to the adhesion force/energy, which was determined by the number of repeats in the MT-binding region. Taken together, the data are incompatible with tau acting as a monomer; rather, they indicate that two tau molecules associate in an antiparallel configuration held together by an electrostatic ''zipper'' of complementary salt bridges composed of the N-terminal and central regions of each tau monomer, with the C-terminal MT-binding regions extending outward from each end of the dimeric backbone. This tau dimer determines the length and strength of the linker holding two MTs together and could be the fundamental structural unit of tau, underlying both its normal and pathological action.bridging interaction ͉ intrinsically unstructured proteins ͉ protein dimerization ͉ surface forces ͉ bioadhesion T he neural microtubule (MT)-associated protein (MAP) tau is essential for the proper development and maintenance of the nervous system. Among other functions, tau promotes the assembly of MTs into well organized, evenly spaced bundles in neuronal axons (1-6) and regulates the growing and shortening dynamics of individual MTs (7-11). Tau dysfunction has long been correlated with many neurodegenerative diseases, including Alzheimer's and related dementias. In the past decade, mutational analyses have demonstrated a direct cause-and-effect relationship between tau dysfunction and/or misregulation and the dramatic neuronal cell death underlying many of these dementias [for example,]. Some mutations cause amino acid substitutions in tau, whereas others are regulatory, causing aberrant patterns of tau RNA splicing without affecting the tau amino acid sequence.As a result of alternative RNA splicing, there are six naturally occurring isoforms of tau expressed in the CNS (Fig. 1). Based on sequence analysis and structure-function dissection (5,8,9,(15)(16)(17)(18), tau can be viewed as possessing four distinct regions. The C-terminal tail contains both basic and acidic subregions and serves to indirectly regulate tau binding to MTs, at least in part via regulated phosphorylation. On the...

Intracellular transport driven by cytoskeletal motors: General mechanisms and defects

“…Some protein called tau (see section 8.4) has a bundling function and leads to a parallel arrangement of the axonal MTs in an imperfect hexagonal lattice (see Fig. 28) with spacing of 26.4 ± 9.5 nm [72,312]. An electron micrograph showing the arrangement of MTs under the influence of tau can be found in [72].…”

“…It is still debated whether these MAPs are "crosslinkers or spacers of microtubules" [248]. On the one hand, the hydrophobic parts of bound tau molecules are able to dimerize and to induce bundling of MTs [312]. Cross-links have been observed both in vitro [1] and in vivo [159,226].…”

The fuzzy coat of pathological human Tau fibrils is a two-layered polyelectrolyte brush

“…Thus, the polyelectrolyte brush of Tau on MTs may show quite different properties, especially if further modified by the flexible acidic Cterminal tubulin tails that also protrude from the MT surface (47) and can be regarded as a disordered polymer brush. Nevertheless, a pH and ion concentration-dependent softening of Tau termini brushes on MTs could have a considerable impact on MT stability, stiffness, interactions, and MT bundling (48)(49)(50).…”

Low Micromolar Zinc Accelerates the Fibrillization of Human Tau via Bridging of Cys-291 and Cys-322

“…There are two distinct domains of Tau protein, the projection domain and the microtubule binding domain. The microtubule binding domain mainly contains either three or four imperfect repeats (18 amino acids in length) separated from one another by inter repeats (13-14 amino acids in length) (5). In its normal state, Tau facilitates and stabilizes the assembly of microtubules.…”