Role of Tau as a Microtubule-Associated Protein: Structural and Functional Aspects

Abstract

Microtubules (MTs) play a fundamental role in many vital processes such as cell division and neuronal activity. They are key structural and functional elements in axons, supporting neurite differentiation and growth, as well as transporting motor proteins along the axons, which use MTs as support tracks. Tau is a stabilizing MT associated protein, whose functions are mainly regulated by phosphorylation. A disruption of the MT network, which might be caused by Tau loss of function, is observed in a group of related diseases called tauopathies, which includes Alzheimer's disease (AD). Tau is found hyperphosphorylated in AD, which might account for its loss of MT stabilizing capacity. Since destabilization of MTs after dissociation of Tau could contribute to toxicity in neurodegenerative diseases, a molecular understanding of this interaction and its regulation is essential.

Keywords: Alzheimer’s disease; biophysical methods; intrinsically disordered proteins; neurodegenerative diseases; post-translational modifications.

Figure 1

Assembly of tubulins into Microtubules (MTs). Models of a depolymerizing (left) and polymerized (right) MTs. The guanosine 5′ triphosphate (GTP) cap or labile domain is composed mostly of tyrosinated GTP-tubulin. The stable domain is mostly composed of detyrosinated guanosine 5′ diphosphate (GDP)-tubulin. β-tubulin subunits are represented as orange cubes in their GTP-bound states and green cubes in their GDP-bound states. α-tubulin subunits are represented as blue cubes. Red sticks planted on the cubes represent the C-terminal tail of α- and β-tubulin subunits. Tyrosination is represented by a red dot on the C-terminal tail. Two MT regions are distinguished: a labile MT region mostly composed of tyrosinated GTP-tubulins, including the GTP cap, and a stable MT region mostly consisting of assembled detyrosinated GDP-tubulins. MT depolymerization is characterized by curled protofilaments at MT ends (left).

Figure 2

Tau protein sequence and domain organization. The sequence numbering is according to the longest Tau isoform (441 amino acid residues). (A) Amino acid sequence of the microtubule binding region (MTBR) and flanking regions, P2 in the PRR and R’ in the C-terminal domain. (B) General scheme of the full-length Tau protein and of TauF4 fragment. (A,B) The MTBR region of Tau consists of four partially repeated sequences, R1 to R4, highlighted in dark and light brown. PHF6* and PHF6 hexapeptides, in R1 and R2 repeats respectively, are highlighted in green and the KXGS motifs in yellow. Phosphorylation sites mentioned in the text are indicated with a v sign, cysteine residues with a star sign and acetylation with a circle. Segments of R1 and R2 shown in Figure 3 are indicated with dashed lines.

Figure 3

Electron microscopy (EM) models of Tau/MTs interaction. α-tubulin subunit is represented as a blue cube and β-tubulin subunit as a brown cube. H12 C-terminal helix is schematized, the C-terminal tail prolongs the H12 helix. Model based on cryo-EM coupled to Rosetta modeling: contacts of the R1 repeat with the α-tubulin subunit, at the inter-dimer interface: S258 and S262 of R1 make hydrogen bonds with E434; K259 of R1 interacts with an acidic patch formed by E420, E423 and D424; I260 of R1 is in a hydrophobic pocket formed by residues I265, V435 and Y262; K267 of R1 is in contact with the acidic C-terminal tail. Additional contacts for the R2 repeat with β-tubulin subunit, at the intra-dimer interface: K274 of R2 interacts with an acidic patch formed by D427 and S423; K281 of R2 is in contact with the acidic C-terminal tail of β-tubulin subunit. The PHF6* peptide (highlighted green) is close to this tail and localize at the intra-dimer interface. Additional contacts for the R2 repeat with α-tubulin subunit: K294 and K298 are in contact with the acidic C-terminal tail of α-tubulin subunit. Finally, H299 of R2 is buried in a cleft formed by residues F395 and F399 of β-tubulin subunit.

Figure 4

Electron-paramagnetic resonance (EPR) models of the interaction of Tau F4 with MTs. C291 of Tau in R2 is proposed to interact with C347 of α-tubulin, and the C322 of Tau in R3 with C131 of β-subunit. Note that C322 could actually interact either with the β-subunit of the same protofilament or with the one of an adjacent protofilament (as depicted here).

Figure 5

Nuclear magnetic resonance (NMR) model of the interaction of TauF4 with stathmin-like domain (SLD)-stabilized tubulins. (A) In interaction with a single tubulin, the IGSTENL peptide of TauF4 is proposed to adopt a turn conformation and is not bound to tubulin. The PRR is mainly detected in proximity to the β-subunit. (B) In interaction with two tubulins, the IGSTENL peptide of TauF4 would be straight and overlapping two consecutive α- and β-subunits.