Direct binding of divalent metal ion, especially Zn , have been shown to increase the rate of tau aggregation and enhance tau toxicity in cells. Hence, understanding the molecular basis of the Zn -accelerated tau aggregation can potentially determine the molecular interactions modulating tau aggregation. Herein, we show that Zn coordinates through the cysteine in R3 repeat and significantly accelerates the aggregation rate of the three repeat tau constructs (K19) but that the coordination is incapable of increasing the aggregation rate of the 20 amino acid peptide derived from the R3 repeat (R3) of tau. The NMR characterization of the binding of Zn to K19, together with the aggregation studies with K19, R3 and R4 peptides, reveal the presence of an aggregation-inhibitory interaction between the R3 and R4 repeat of K19. Our data show that binding of Zn to R3 repeat of tau, weaken the aggregation-inhibiting influence between R3 and R4 repeats, leading to faster aggregation of tau protein.

In tau proteins, the hexapeptides in the R2 and R3 repeats are known to initiate tau fibril formation, which causes a class of neurodegenerative diseases called the taupathies. We show that in R3, in addition to the presence of the hexapeptides, the correct turn conformation upstream to it is also essential for producing prion-like fibrils that are capable of propagation. A time-dependent NMR aggregation assay of a slow fibril forming R3-S316P peptide revealed a trans to cis equilibrium shift in the peptide-bond conformation preceding P316 during the growth phase of the aggregation process. S316 was identified as the key residue in the turn that confers templating capacity on R3 fibrils to accelerate the aggregation of the R3-S316P peptide. These results on the specific interactions and conformational changes responsible for tau aggregation could prove useful for developing an efficient therapeutic intervention in Alzheimer's disease.

In tau proteins,t he hexapeptides in the R2 and R3 repeats are knowntoinitiate tau fibril formation, whichcauses aclass of neurodegenerative diseases called the taupathies.W e show that in R3, in addition to the presence of the hexapeptides, the correct turn conformation upstream to it is also essential for producing prion-like fibrils that are capable of propagation. A time-dependent NMR aggregation assayo faslow fibril forming R3-S316P peptide revealed at rans to cis equilibrium shift in the peptide-bond conformation preceding P316 during the growth phase of the aggregation process.S 316 was identified as the key residue in the turn that confers templating capacity on R3 fibrils to accelerate the aggregation of the R3-S316P peptide.T hese results on the specific interactions and conformational changes responsible for tau aggregation could prove useful for developing an efficient therapeutic intervention in Alzheimersd isease.The abnormal aggregation of tau proteins to form the paired helical filaments (PHFs) that constitute the major part of neurofibrillary tangles (NFTs) [1] is involved in many neuronal diseases,i ncluding chronic traumatic encephalopathy (CET) [2] and Alzheimersdisease. [3] Tauproteins are predominantly unfolded in solution and slowly aggregate to form long fibrils that constitute the PHFs. [4] In the different isoforms of tau protein, 3o r4repeat domains of around 32 amino acid residues are located at the C-terminal part. [5] Studies with protease digestion as well as fluorescence spectroscopy have revealed that the cores of tau fibrils from different isoforms are made up of the R2 and R3 repeat for the four-repeat isoform and only the R3 repeat in the case of the three-repeat isoform. [6] TheR 2a nd R3 repeats contain ah exapeptide ( 306 VQIVYK 311 )m otif that is known to form fibrils on its own. [7] Mutations in the hexapeptide lead to the abrogation of tau fibril formation. [8] Furthermore,the X-ray structure of the hexapeptides in R3 reveals as teric zipper arrangement consistent with the model of fibril initiation in many of the amyloid proteins. [9] However,f rom solid-state NMR (ssNMR) studies on tau fibrils,a tl east two additional bstrand regions in R3, upstream to the hexapeptide,have also been identified to form the core of the fibril. [10] These strands are 1) V313-L315, which is sandwiched between ak ink at P312 and the second kink at S316, and 2) K317-K321, which is found after the kink at S316.Nevertheless,t he exact arrangement of b-strands within the monomeric unit of the fibrils was undetermined. Moreover, the structural importance of the kink residues in promoting aggregation is also unknown. This is crucial for understanding the early stages of fibril formation. [11] In this work, we engineered peptides from the fibril-forming core of tau (R3;F igure 1) that encompass the hexapeptide but have different aggregation properties depending on the presence of the key kink residue S316 in the peptide sequence.Inthe slow fibril forming R3-S316P peptide,w es how compelling e...