Blocking the thiol at cysteine-322 destabilizes tau protein and prevents

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Letter

Blocking the thiol at cysteine-322 destabilizes tau protein and prevents its oligomer formation Hui Chen, Simu Liu, Shuiming Li, Jierui Chen, Jiazuan Ni, and Qiong Liu ACS Chem. Neurosci., Just Accepted Manuscript • DOI: 10.1021/acschemneuro.8b00003 • Publication Date (Web): 01 May 2018 Downloaded from http://pubs.acs.org on May 3, 2018

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Blocking the thiol at cysteine-322 destabilizes tau protein and

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prevents its oligomer formation

3 Hui Chen1,2,*,§, Simu Liu3,4,*, Shuiming Li4, Jierui Chen1, Jiazuan Ni1, Qiong Liu1,§

4 5 6

1

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College of Life Sciences and Oceanography, Shenzhen University, Shenzhen 518060,

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China.

9

2

Shenzhen Key Laboratory of Marine Bioresource and Eco-environmental Sciences,

Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education and

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Guangdong Province, College of Optoelectronic Engineering, Shenzhen University,

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Shenzhen 518060, China.

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3

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and Oceanography, Shenzhen University, Shenzhen 518060, China.

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4

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and Oceanography, Shenzhen University, Shenzhen 518060, China.

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*H.C. and S.M.Liu contributed equally to this work.

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§

Guangdong Provincial Key Laboratory for Plant Epigenetics, College of Life Sciences

Shenzhen Key Laboratory of Microbial Genetic Engineering, College of Life Sciences

Corresponding Authors.

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ABSTRACT: Abnormal accumulation of tau protein into oligomers contributes to

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neuronal dysfunction. Reduction of tau level is potentially able to prevent its accumulation.

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Here we uncover a critical role of the free thiol at Cys-322 in determining tau stability. We

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found that the application of thiol-blocking agents like NEM or MMTS blocks this thiol, by

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which it destabilizes tau protein and prevents its oligomer formation. Furthermore, we

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identified a tau-interacting protein, selenoprotein W, which attenuates tau accumulation by

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forming disulfide linkage between SelW Cys-37 and tau Cys-322. These findings provide

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a promising strategy to prevent tau accumulation and oligomer formation.

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KEYWORDS: oligomerization, selenoprotein, disulfide linkage, protein destabilization,

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Alzheimer’s disease

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INTRODUCTION

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Aggregation of microtubule-associated protein tau into neurofibrillary tangles is one of the

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defining pathological features of Alzheimer’s disease1,2. Growing evidence suggests that

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soluble tau oligomers are the main toxic species in destroying neuron functions3.

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Interaction between tau molecules by Cysteine-322 (Cys-322)-mediated disulfide linkage

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is thought to initiate tau oligomer formation and its propagation4,5. Consistent with this,

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small molecules that bind to the cysteine residues could prevent the formation of toxic tau

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oligomers by hindering tau intermolecular interaction6.

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Reduction of tau level is beneficial to inhibit neuronal dysfunction in Alzheimer’s disease

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mouse model7-9. Previous studies have identified some endoproteolytic enzymes such as

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asparagine endopeptidase10 and caspase11 which degrade tau protein by proteolytic

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cleavage. However, these cleavages are unlikely to contribute to the specific reduction of

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tau level, on the contrary, the resulting degraded tau proteins generate more oligomers

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that promote development of Alzheimer’s disease. The E3 ubiquitin ligase CHIP (carboxyl

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terminus of Hsp70 interacting protein) targets tau for ubiquitin/26S proteasome-mediated

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protein degradation to defense against tau accumulation12. However, this pathway also

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does not function due to the impairment of 26S proteasome by abnormal aggregation of

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tau proteins under pathological condition13,14. The genome editing technology would be

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efficient to delete tau gene, but complete deletion of tau protein may introduce unexpected

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disorders such as brain insulin resistance15. 2

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Thus, we hypothesized that decreasing tau protein appropriately would be helpful to

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attenuate tau accumulation-caused neurotoxicity without bringing new disorders. By

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focusing on the Cys-322 residue, we found that blocking the thiol at Cys-322 can not only

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prevent tau oligomer formation but also initiate the destabilization of tau protein, which

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provides a potential strategy to reduce excessive tau proteins effectively.

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RESULTS AND DISCUSSION

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We first examined tau oligomerization in vitro. The tau proteins tagged with 6×His were

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expressed in E.coli to perform H2O2-induced oligomerization assays in vitro. To avoid

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unnecessary confusion, we numbered tau amino acids based on the longest tau isoform

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(isoform 2, amino acids 1-441)16. though we employed tau isoform 4 (amino acids 1-352)

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containing only one cysteine residue Cys-322 in all our experiments (Figure S1A).

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Full-length tau protein could form oligomer rapidly upon H2O2 treatment (Figure S1B), in

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contrast, substitution of Cys-322 with Ala (C322A) completely abolished the formation of

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tau oligomer (Figure S1C). To further examine whether the free thiol at Cys-322 is

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required for tau oligomer formation, we employed a thiol-blocking agent N-ethylmaleimide

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(NEM) in these assays (Figure S2A) and found it completely prevented H2O2-induced tau

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oligomer formation (Figure S2B). These results suggest the critical role of free thiol at

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Cys-322 in H2O2-induced tau oligomer formation.

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To further study the effects of NEM on tau oligomerization in vivo, HEK293T cells

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expressing Flag-tau proteins were treated with NEM. NEM treatment did not cause

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detectable cell apoptosis within 20 minutes (Figure S2C). Unexpectedly, the protein level

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of Flag-tau was observed to decrease with increasing concentration of NEM (Figure 1A).

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To confirm the observation that thiol-blocking is responsible for tau destabilization, we also

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employed the other two chemicals, S-methyl methanelthiosulfonate (MMTS) and

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isoproterenol (ISO), in this study (Figure S2A). The MMTS mediates a reversible

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thiol-blocking that differs from NEM17, and ISO can bind to the cysteine residues of tau

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protein6. Different from NEM and MMTS, pretreatment with ISO in a short time (10

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minutes) could not block H2O2-induced tau oligomer formation (Figure S2B), suggesting

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that the blockage of thiol by ISO is not a fast chemical reaction. Next, the lysates of

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HEK293T cells expressing Flag-tau were incubated with these chemicals. MMTS, like 3

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NEM, could induces tau decrease, but ISO could not (Figure 1B), implying that the thiol

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group in the cysteine may contributes to tau stability.

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To test whether NEM could induce the destabilization of tau proteins that extracted from

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brain, lysates from the brain of triple transgenic model mouse of Alzheimer’s disease

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(3xTg mouse)18 were used. Results showed that tau proteins smaller than 180kDa were

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decreased by NEM treatment (Figure S3). As tau oligomers can spread between neurons

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via exosomes19. these findings suggest a potential application of thiol-blocking drugs in

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preventing tau oligomer formation and propagation.

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To provide further evidences that thiol-blocking at Cys-322 is involved in tau

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destabilization in vivo, we tried to identify the proteins, which bind to tau via disulfide

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linkage, would mimic the role of thiol-blocking agents in destabilizing tau protein. We

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identified a tau-interacting protein selenoprotein W (SelW), which is widely and highly

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expressed in brain20,21. by pull-down assay (Figure S4A). SelW contains two cysteine

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residues Cys-10 and Cys-37, and one selenocysteine (Sec, U) residue Sec-13 (Figure

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S5A). Sec is encoded by a traditional termination codon TGA, and is inserted into protein

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by a specific translational mechanism22. To efficiently express SelW proteins in E.coli and

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human cells, the Sec was substituted with Cys (SelWU13C) (Figure S5A). Intriguingly, the

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SelWU13C protein could form oligomers in the presence of H2O2 easily (Figure S5B), and

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the interactions between SelWU13C and tau could be weakened and enhanced by DTT and

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H2O2, respectively (Figure S4B).

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To analyze whether SelWU13C could form disulfide linkage with tau, purified recombinant

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His-tau and His-SelW U13C proteins were incubated with 1 mM H2O2. A protein band with an

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approximate molecular mass of 70 kilodaltons (kDa) was clearly appeared (Figure 2A),

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which we deduced to be a heterodimer formed by the combination of His-tau (~60 kDa)

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and His-SelW U13C (~10 kDa). This was supported by the observation that both His-tau and

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His-SelW U13C monomer were decreased but tau-SelW U13C dimer was increased at a

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time-dependent manner (Figure 2A).

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Since substitution of tau Cys-322 with Ala (C322A) completely destroyed the formation

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of tau-SelWU13C dimer (Figure S6A), we speculated that SelWU13C should bind to tau

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through disulfide linkage. To identify which cysteines are involved in disulfide linkage

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between SelW U13C and tau, SelW U13C proteins with various substitutions at their cysteines 4

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were analyzed. Individual substituting SelWU13C Cys-10 or Cys-13 with Ser (C10S or

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C13S), and co-substituting SelWU13C Cys-10 and Cys-13 with Ser (C10S/C13S) did not

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detectably affect the formation of SelWU13C oligomer (Figure S5C) and tau-SelWU13C dimer

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(Figure S6B). However, substituting SelW U13C Cys-37 with Ser (C37S) distinctly prevented

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the formation of oligomer in all species (Figure S5C and S6B). Thus, the SelWU13C Cys-37

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is uniquely required for formation of disulfide linkage with tau Cys-322.

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To confirm the identified disulfide-linked cysteines, we performed mass spectrometry

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analysis using the tau-SelWU13C dimer samples shown in Figure 2A. An ion with a mass

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matching the theoretic mass of a disulfide linkage between peptides of tau and SelW U13C

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was detected from the sample with tryptic digestion. The expected sequences of tau and

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SelWU13C were identified from peptide fragments by MS/MS (Figure 2B). These data

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support that the disulfide linkage of tau and SelW U13C is formed through tau Cys-322 and

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SelWU13C Cys-37.

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To study whether SelW U13C could attenuate tau accumulation in human cells, plasmids

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expressing tau and SelWU13C tagged with Flag and Myc respectively were co-transfected

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into HEK293T cells. Immunoblot analysis showed that Flag-tau proteins were diminished

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with increasing expression of SelW U13C-Myc proteins (Figure 3A). As expected,

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abolishment of the disulfide linkage broke SelW U13C’s effect on Flag-tau accumulation

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(Figure 3B). Additionally, SelW U13C with Ser substitutions at both Cys-10 and Cys-13 did

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not affect the protein levels of Flag-tau (Figure 3C). Thus, formation of disulfide linkage

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between SelW U13C Cys-37 and tau Cys-322 is a key step to destabilize tau protein.

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As SelW U13C also contains thiol groups, which could be targeted by NEM like tau protein

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(Figure S7A and S7B), we examined whether NEM could also cause SelW U13C

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destabilization. Results showed that the treatment with NEM could only reduce Flag-tau

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level but not SelW U13C-Myc level (Figure S7C and S7D). Thus, protein destabilization

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resulted from thiol-blocking could only be considered as a mechanism specific for

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regulating the abundance of tau protein.

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Finally, we expressed Flag-tau and Flag-tau mutant (C322A) in human cells, and

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detected the levels of corresponding messenger RNAs (mRNA) and proteins by RT-PCR

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and immunoblot analysis, respectively. Although the mRNA levels of Flag-tau and

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Flag-tau (C322A) showed similar results, their protein levels altered significantly (Figure 5

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4A). In addition, we found that treatment with thiol-blocking agents did not lead to

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detectable change of mRNA levels of tau and tau (C322A) (Figure S8). Mutation at

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Cys-322 severely induced tau destabilization, revealing that Cys-322 is a key residue

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responsible for the stability of tau protein. These results support the observations that

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blocking the thiol at Cys-322 destabilizes tau protein.

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Previous studies have reported that ubiquitination of cysteine contributes to protein

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degradation23,24. which reveal a potential role of cysteine in regulating protein stability. Our

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data demonstrate that blocking the thiol at Cys-322 destabilizes tau protein in human cells.

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However, the mechanism involving in this is still unclear. Tau destabilization induced by

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thiol-blocking only happens in cells or cell crude extracts, suggesting that other proteins in

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human cells are also required for the reduction of tau abundance. Blocking the thiol at

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Cys-322 may cause conformational alteration25 which may possibly initiate the

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intracellular signaling for protein degradation or it may facilitate the interaction of tau with

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some proteins in vivo to induce the cleavage of tau. The precise mechanism remains to be

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investigated extensively.

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Considering that the Cys-322 residue is also required for tau oligomer formation, further

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investigations should elucidate the relationship between tau aggregation and stabilization.

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As SelW directly binds to tau through disulfide linkage and facilitates tau destabilization, it

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will be interesting to examine whether SelW could control tau abundance in brain.

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CONCLUSION

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In summary, we present a strategy to prevent tau accumulation and oligomer formation

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by blocking the thiol at tau Cys-322 using the thiol-blocking agents NEM and MMTS.

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Moreover, we identified a tau-interacting protein SelW, which attenuates tau accumulation

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in human cells by forming disulfide linkage between SelW Cys-37 and tau Cys-322. As

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both NEM and MMTS are general thiol-blocking agents, those provide a new way to

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design anti-tauopathy drugs via blocking the thiol at tau Cys-322. In addition, exploring the

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precise biochemical mechanism underlying tau destabilization induced by thiol-blocking

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will be helpful to discover new targets for treating tauopathy-caused disease including

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Alzheimer’s disease.

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METHODS

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Plasmid construction. For recombinant protein expression, full-length tau and SelW

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were amplified by PCR and introduced into the vector pET28a to generate pET8a-tau and

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pET28a-SelW expression plasmids using the EcoRI/XhoI sites. For protein expression in

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human cells, full-length tau and SelW was amplified by PCR and introduced into the

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modified vector pCDNA3.1 (Invitrogen) to generate pCDNA-tau and pCDNA-SelW

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expression plasmids using the EcoRI/XhoI sites. Substitution mutations were generated

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by PCR-mediated site-directed mutagenesis. All constructs were confirmed by DNA

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sequencing. PCR primers used in this study were listed in Table S1.

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Preparation of recombinant proteins. The pET28a-based expression plasmids were

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transformed into E.coli BL21 (DE3). The bacteria were cultured in liquid medium at 37ºC

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till the OD600 reaches to 0.5-0.6, then isopropyl-β-D-thiogalactopyranoside (IPTG) was

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added into the culture at a final concentration of 0.3 mM. The culture was incubated for

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additional 4 hours at 37ºC to induce the expression of recombinant proteins. The bacteria

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were collected then lysed by ultrasonic cell crusher and cleared by filtration. Recombinant

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proteins with His-tags were purified using Ni Sepharose (GE Healthcare) following the

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manufacturer’s instructions.

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Pull down assay. Crude proteins were extracted from E.coli cells expressing

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recombinant His-tau proteins using pull down buffer [PBS containing 0.1% Triton X-100

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and 1 mM phenylmethyl sulfonyl fluoride (PMSF; Sigma)]. The H2O2 or DTT with indicated

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concentration was added into pull down buffer to test their effects. For GST pull down

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assay, GST-SelW U13C proteins bound to Glutathione Sepharose bead were added into the

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crude extracts and incubated at 4ºC for 2 hours with gentle agitation. Sepharose beads

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were collected by centrifugation at 500g for 5 minutes. After washing five times with PBS

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buffer, the beads were resuspended in 2× SDS-PAGE sample buffer without reducing

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agent. The samples were heated at 95ºC for 5 minutes, and then separated by

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electrophoresis on 10% SDS-PAGE gels, followed by immunoblot using anti-His antibody

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(Proteintech). After immunoblotting, the PVDF membrane was stained using ponceau.

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In vitro H2O2-induced protein oligomerization. After His tagged recombinant proteins

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(100 µM) mixing in buffer containing 50 mM NaH2PO4, 300 mM NaCl (pH7.4), H2O2 (1 mM

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or indicated concentration) was added and incubated at 37ºC for 30 minutes. The reaction 7

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was stopped by adding 5×SDS-PAGE sample buffer without reducing agent. The samples

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were heated at 95ºC for 5 minutes, and then separated by electrophoresis on 12%

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SDS-PAGE gels, followed by coomassie staining.

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Mass spectrometric analysis of disulfide linkage. Proteins in gel slices were

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digested with trypsin. The peptide samples were resuspended in buffer (0.1% formic acid,

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2% acetonitrile), then was subjected to Triple-TOF 5600 mass spectrometry equipped with

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Eksigent nano LC-Ultra 1D plus system. Chromatographic separations were conducted

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on a reverse-phase capillary column (75 µm×15 cm C18-3µm 120 Å, ChromXP Eksigent)

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with a gradient from starting with 5% of buffer B (acetonitrile containing 0.1% formic acid

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and 2% water) in 5 min, 35% of buffer B in 35 min, to final 80% of buffer B in 5 min. Mass

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spectra were obtained in Information Dependent Acquisition mode (IDA), survey scan of

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TOF-MS was obtained in 250 ms, and MS/MS spectra were obtained in high sensitivity

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mode with accumulation time of 80 ms per spectra. A maximum of 30 precursor ions was

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allowed within each cycle. The threshold intensity of the precursor ions was 200 cps with a

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charge state from +2 to +5. Dynamic exclusion was set for 10 s. The ionization

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parameters were set as follows, ion spray voltage of 2300 V, curtain gas of 35 psi,

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nebulizer gas of 5 psi, and interface heat temperature of 150ºC. For protein identification,

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the MS/MS raw data were analyzed using Protein Pilot software v.4.5 (AB Sciex Inc., USA)

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to search against UniProt human proteome database. Disulfide-linked peptides were

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annotated by PEAKS AB software (Bioinformatics Solutions Inc., Waterloo, ON, Canada).

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Cell culture and plasmid transfection. HEK293T cells were routinely cultured at 37ºC

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in Dulbecco’s Modified Eagle’s Medium (DMEM; Life Technologies) supplemented with 10%

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(v/v) fetal bovine serum (Biological Industries), 100 U/ml of penicillin and 100 µg/ml

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streptomycin, in humidified 5% (v/v) CO2 air. HEK293T cells were seeded at a density of

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approximately 2.5×105 cells/well on a 6-well culture plate and transfected with

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Lipofectamine 3000 (Invitrogen) following the manufacturer’s protocol. Briefly, plasmid

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(1~2 µg) diluted in DMEM medium and mixed well with P3000 reagent (2 µl/µg DNA) was

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added into DMEM-diluted Lipofectamine 3000 reagent (1:1 ratio), and incubated for 15

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min at room temperature before applying to host cells. 2 ml fresh medium containing 200

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µg/ml G418 was added to each well. For pCDNA-tau and pCDNA-SelW co-transfection, 6

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hours after pCDNA-tau transfection, plasmid pCDNA-SelW was transfected into the same 8

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culture plate. After cultured overnight, the medium was replaced with 2 ml fresh medium

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without G418. The cells were usually harvested 48 hr later for RT-PCR or immunoblot

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analyses.

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Chemical treatments. For analyzing chemical effects on H2O2-induced protein

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oligomerization,

100

µM

N-ethylmaleimide

(NEM;

Sangon),

or

S-methyl

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methanethiosulfonate (MMTS; Sigma), or isoproterenol (ISO; Sigma) was added into the

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His tagged recombinant proteins mixing in buffer containing 50 mM NaH2PO4, 300 mM

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NaCl (pH7.4). After pretreated at 37ºC for 10 minutes, H2O2 (1 mM) was added and

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incubated at 37ºC for 30 minutes. For analyzing chemical effects on protein stability, the

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2.5×106 transfected HEK293T cells were incubated for 10 min with NEM in indicated

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concentration followed by protein extraction, or crude extracts from the cells were

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incubated for 10 min in 100 µM NEM, MMTS or ISO. Proteins were detected by

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immunoblot analysis. For analyzing chemical effects on gene expression, the 2.5×106

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transfected HEK293T cells were incubated for 10 min with 100 µM NEM or MMTS

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followed by RNA extraction. mRNA levels were detected by RT-PCR analysis.

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Annexin V-EGFP apoptosis detection. Apoptotic HEK293T cells were identified using

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the Anaexin V-EGFP Apoptosis Detection Kit (Beyotime) following the manufacturer’s

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instructions. Briefly, after cells were incubated with Annexin V-EGFP for 10 minutes, 100

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µM NEM was supplied to treat cells at indicated time points. The cells were imaged by

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confocal laser-scanning microscopy (CLSM). The apoptosis inducer Kit (C0005; Beyotime)

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was purchased for positive control assays.

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RNA extraction and RT-PCR. Total RNA was extracted using RNAiso Plus (Takara)

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following the manufacturer’s instructions. Reverse transcription reaction was carried out

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using PrimeScript RT reagent Kit with gDNA Eraser (Takara) following the manufacturer’s

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instructions. PCR was performed using the primers listed in Table S2. The relative

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expression level of tau gene was normalized to Actinβ expression level. Three

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independent experiments were performed.

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Protein extraction and immunoblot analysis. HEK293T cells or mouse brain tissues

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were washed once with PBS and lysed in cell lysis buffer [20 mM Tris (pH7.5), 150 mM

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NaCl, 1% Triton X-100, 1 mM PMSF (Sigma), 1×Protease inhibitor cocktail (Roche),

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1×phosphatase inhibitor cocktail (Roche)]. Cell lysates were placed on ice for 20 min, 9

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quickly frozen in liquid N2 once, and kept on ice for 30 min. After centrifuging at 13,000g at

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4ºC for 10 min, the supernatant was collected and heated at 95ºC for 5 minutes with

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sample loading buffer, and then separated by electrophoresis on 10% SDS-PAGE gels.

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Mouse monoclonal anti-tau5 (1:5000; Abcam) and anti-Myc (1:2000; Proteintech)

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antibodies were used to detect Flag-tau and SelW-Myc proteins, respectively. Actin used

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for loading control was detected by anti-Actinβ (1:1000; Proteintech) antibody. Bands in

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immunoblots were visualized by Pierce ECL Western Blotting Substrate (Thermo Fisher

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Scientific) and quantified by densitometry using ImageJ software program.

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ASSOCIATED CONTENT

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Supporting Information is available.

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AUTHOR INFORMATION

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Corresponding Authors

16

§

Email: [email protected].

17

§

Email: [email protected].

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ORCID

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Hui Chen: 0000-0001-8879-2185

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Simu Liu: 0000-0002-0374-3454

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Author Contributions

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H.C. conceived this study. H.C. and S.M.Liu designed the research and analyzed the data.

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Q.L. and J.Z.N. supervised the project. H.C. performed most of the experiments, assisted

24

by S.M.Liu and J.R.C. S.M.Li performed mass spectrometry. H.C. interpreted the results.

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S.M.Liu, Q.L. and J.Z.N. discussed the results. H.C. and Q.L. wrote the manuscript. All

26

authors read and commented on the manuscript.

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Notes

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The authors declare no competing interest.

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ACKNOWLEDGMENTS

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We thank K.Q.Ye (Emory University) for providing pET28c-tau plasmids; N.Jin and

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L.M.Huang (Shenzhen University) for providing 3×Tg mouse materials. This work was

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supported by National Natural Science Foundation of China (31470804) and Shenzhen

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Bureau of Science, Technology and Information (JCYJ20150529164656093).

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FIGURE LEGENDS

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Figure 1. Blocking the thiol at Cys-322 destabilizes tau protein. (A) Treatment with NEM

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reduces Flag-tau abundance within HEK293T cells. (B) Thiol-blocking agents reduce

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Flag-tau abundance in cell-free system. Protein crude extracts from HEK293T cells

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expressing Flag-tau were used in these assays. The Flag-tau was detected using anti-tau

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(tau 5) antibody, and the Flag-tau protein level in the absence of chemical was set as

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100%. Actin was used as a loading control. Error bars represent SD. Three biological

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repeats were performed and analyzed.

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Figure 2. SelW binds to tau protein via disulfide linkage. (A) Analysis of the interaction

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between tau and SelWU13C in the presence of H2O2 (1 mM) over the indicated time course.

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The SDS-PAGE gel was stained using coomassie. (B) Identification of the cysteines

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involved in the disulfide linkage between tau and SelW U13C by mass spectrometric

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analysis. The protein of tau-SelW U13C dimer shown in (A) was digested for MS analysis.

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Figure 3. SelW U13C attenuates tau accumulation by forming disulfide linkage. Protein

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crude extracts from HEK293T cells co-transfected with Flag-tau and SelWU13C-Myc

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plasmids with indicated quantity were analyzed in these assays. (A) The levels of Flag-tau

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protein is decreased with increasing expression of SelW U13C-Myc proteins. (B) The

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increasing SelW U13C (C37S)-Myc proteins does not significantly affect tau protein levels.

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(C) Mutants of SelW U13C-Myc with various cysteine substitutions except Cys-37 does not

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alter Flag-tau protein levels. The Flag-tau was detected using anti-tau (tau 5) antibody,

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and the level of Flag-tau protein in the absence of SelW U13C-Myc was set as 100%. Actin

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was used as a loading control. Error bars represent SD. *** P