Competing Interactions Stabilize Pro- and Anti-aggregant Conformations of Human Tau

Abstract

Aggregation of Tau into amyloid-like fibrils is a key process in neurodegenerative diseases such as Alzheimer. To understand how natively disordered Tau stabilizes conformations that favor pathological aggregation, we applied single-molecule force spectroscopy. Intramolecular interactions that fold polypeptide stretches of ϳ19 and ϳ42 amino acids in the functionally important repeat domain of full-length human Tau (hTau40) support aggregation. In contrast, the unstructured N terminus randomly folds long polypeptide stretches >100 amino acids that prevent aggregation. The pro-aggregant mutant hTau40⌬K280 observed in frontotemporal dementia favored the folding of short polypeptide stretches and suppressed the folding of long ones. This trend was reversed in the anti-aggregant mutant hTau40⌬K280/PP. The aggregation inducer heparin introduced strong interactions in hTau40 and hTau40⌬K280 that stabilized aggregation-prone conformations. We show that the conformation and aggregation of Tau are regulated through a complex balance of different intra-and intermolecular interactions.Amyloid forming proteins such as ␣-synuclein, the prion protein (1), and Tau (2) contain unstructured domains or belong to the family of natively unfolded or intrinsically disordered proteins (IDPs) 3 (3). The aggregation of Tau into amyloid-like fibers, known as paired helical filaments (4, 5), is a key process in human protein aggregation diseases that are summarized as tauopathies. In vivo, Tau binds and stabilizes microtubules (MTs) to regulate the cellular MT network. The dissociation of Tau from MTs is controlled by the phosphorylation of Tau at multiple sites (6, 7). The longest human Tau isoform, hTau40 (441 amino acids (aa)), contains a ϳ250-aa long N terminus of unknown function, whereas the C terminus comprises the Tau repeat domain, which encompasses four ϳ31-aa long semi-conserved repeats (R1 to R4) flanked by proline-rich stretches (Fig. 1A). Both, binding to MTs and fibril assembly are mediated through the Tau repeat domain (8, 9).As most IDPs, Tau shows a high content of charged aa residues and a low hydrophobicity, which result in an extended solution conformation with a large radius of gyration (10). In solution, Tau has no stable secondary and tertiary structure, as judged by CD and Fourier transform infrared spectroscopy (10). The Stokes radius of Tau increases upon chemical denaturation with urea or guanidine hydrochloride (11, 12) indicating some limited folding. NMR experiments revealed transient secondary structures in hTau40 that partially interact with other polypeptide regions (13). Two hexapeptide motifs, PHF6* in R2 and PHF6 in R3, can adopt ␤-strand conformation and are predominantly responsible for Tau aggregation into fibrils (9, 14). Using Förster resonance energy transfer (11), the transient "paper clip"-like folding of the C and N termini onto the repeat domain was detected in hTau40. After removing the N-and C-terminal domains, the Tau repeat domain exhibits faster aggregation than full-length Tau (15). Th...

Common Features at the Start of the Neurodegeneration Cascade

“…Δ L c and F were found to be uncorrelated (Figure S10A) as recently reported for the NP tau [29]. Interestingly, this incipient conformational polymorphism included conformers with extremely high mechanical stability ( F ≥400 pN).…”

Identification of an Aggregation-Prone Structure of Tau

“…Interactions with heparin are generally electrostatic in nature 49 and we find that NaCl is able to block the effects of heparin on the MTBR in a concentration-dependent manner (Figure S3). High salt has been shown to inhibit heparin-induced aggregation 33 and to disrupt heparin-induced structure 42 and our data suggest that such observations may derive mechanistically from altered interactions between heparin and the MTBR.…”

Single-molecule Force Spectroscopy Reveals the Individual Mechanical Unfolding Pathways of a Surface Layer Protein

“…A double-peak of about 87 pN that occurs prior to the mechanical unfolding events related to the I27 domains serves as a mechanical fingerprint for the individual unfolding of certain domains of SbsB, revealing a rather complex unfolding pathway through distinct, well defined intermediates where different initial extensibilities are observed. Such a phenomenon has been observed previously for the mechanical unfolding of polyQ chains (63), the human tau protein (64), and ␣-synuclein (65), where these proteins are all linked to the formation of self-assembled aggregates involved in the pathology of neurodegenerative diseases. On the basis of single-molecule force spectroscopy measurements, the aggregation propensity of these proteins could be related to certain conformational properties, where an ensemble of mechanically resilient, highly stable collapsed states could be observed.…”