Structural and Biochemical Characterization of Thioredoxin-2 from Deinococcus radiodurans

Thioredoxin (Trx), a ubiquitous protein showing disulfide reductase activity, plays critical roles in cellular redox control and oxidative stress response. Trx is a member of the Trx system, comprising Trx, Trx reductase (TrxR), and a cognate reductant (generally reduced nicotinamide adenine dinucleotide phosphate, NADPH). Bacterial Trx1 contains only the Trx-fold domain, in which the active site CXXC motif that is critical for the disulfide reduction activity is located. Bacterial Trx2 contains an N-terminal extension, which forms a zinc-finger domain, including two additional CXXC motifs. The multi-stress resistant bacterium Deinococcus radiodurans encodes both Trx1 (DrTrx1) and Trx2 (DrTrx2), which act as members of the enzymatic antioxidant systems. In this study, we constructed Δdrtrx1 and Δdrtrx2 mutants and examined their survival rates under H2O2 treated conditions. Both drtrx1 and drtrx2 genes were induced following H2O2 treatment, and the Δdrtrx1 and Δdrtrx2 mutants showed a decrease in resistance toward H2O2, compared to the wild-type. Native DrTrx1 and DrTrx2 clearly displayed insulin and DTNB reduction activity, whereas mutant DrTrx1 and DrTrx2, which harbors the substitution of conserved cysteine to serine in its active site CXXC motif, showed almost no reduction activity. Mutations in the zinc binding cysteines did not fully eliminate the reduction activities of DrTrx2. Furthermore, we solved the crystal structure of full-length DrTrx2 at 1.96 Å resolution. The N-terminal zinc-finger domain of Trx2 is thought to be involved in Trx-target interaction and, from our DrTrx2 structure, the orientation of the zinc-finger domain of DrTrx2 and its interdomain interaction, between the Trx-fold domain and the zinc-finger domain, is clearly distinguished from those of the other Trx2 structures.

Compensatory Protection of Thioredoxin-Deficient Cells from Etoposide-Induced Cell Death by Selenoprotein W via Interaction with 14-3-3

Selenoprotein W (SELENOW) is a 9.6 kDa protein containing selenocysteine (Sec, U) in a conserved Cys-X-X-Sec (CXXU) motif. Previously, we reported that SELENOW regulates various cellular processes by interacting with 14-3-3β at the U of the CXXU motif. Thioredoxin (Trx) is a small protein that plays a key role in the cellular redox regulatory system. The CXXC motif of Trx is critical for redox regulation. Recently, an interaction between Trx1 and 14-3-3 has been predicted. However, the binding mechanism and its biological effects remain unknown. In this study, we found that Trx1 interacted with 14-3-3β at the Cys32 residue in the CXXC motif, and SELENOW and Trx1 were bound at Cys191 residue of 14-3-3β. In vitro binding assays showed that SELENOW and Trx1 competed for interaction with 14-3-3β. Compared to control cells, Trx1-deficient cells and SELENOW-deficient cells showed increased levels of both the subG1 population and poly (ADP-ribose) polymerase (PARP) cleavage by etoposide treatment. Moreover, Akt phosphorylation of Ser473 was reduced in Trx1-deficient cells and was recovered by overexpression of SELENOW. These results indicate that SELENOW can protect Trx1-deficient cells from etoposide-induced cell death through its interaction with 14-3-3β.

Identification of a Thiol-Disulfide Oxidoreductase (SdbA) catalyzing Disulfide Bond Formation in the Superantigen SpeA in Streptococcus pyogenes

Mechanisms of disulfide bond formation in the human pathogen Streptococcus pyogenes is currently unknown. To date, no disulfide bond forming thiol-disulfide oxidoreductase (TDOR) has been described and at least one disulfide bonded protein is known in S. pyogenes . This protein is the superantigen SpeA, which contains 3 cysteine residues (Cys 87, Cys90, and Cys98), and has a disulfide bond is formed between Cys87 and Cys98. In this study, candidate TDORs were identified from the genome seuence of S. pyogenes MGAS8232. Using mutational and biochemical approaches, one of the candidate proteins, SpyM18_2037 (named here SdbA), was shown to be the catalyst that introduces the disulfide bond in SpeA. SpeA in the culture supernatant remained reduced when sdbA was inactivated and restored to the oxidized state when a functional copy of sdbA was returned to the sdbA -knockout mutant. SdbA has a typical C 46 XXC 49 active site motif commonly found in TDORs. Site-directed mutagenesis experiments showed that the cysteines in the CXXC motif were required the disulfide bond in SpeA to form. Interactions between SdbA and SpeA were examined using cysteine variant proteins. The results showed that SdbA C49A formed a mixed disulfide with SpeA C87A , suggesting that the N-terminal Cys46 of SdbA and the C-terminal Cys98 of SpeA participated in the initial reaction. SpeA oxidized by SdbA displayed biological activities suggesting that SpeA was properly folded following oxidation by SdbA. In conclusion, formation of the disulfide bond in SpeA is catalyzed by SdbA and the findings represent the first report of disulfide bond formation in S. pyogenes . IMPORTANCE Here, we reported the first example of disulfide bond formation in Streptococcus pyogenes . The results showed that a thiol-disulfide oxidoreductase, named SdbA, is responsible for introducing the disulfide bond in the superantigen SpeA. The cysteine residues in the CXXC motif of SdbA are needed for catalyzing the disulfide bond in SpeA. The disulfide bond in SpeA and neighboring amino acids form a disulfide loop that is conserved among many superantigens, including those from Staphylococcus aureus . SpeA and staphylococcal enterotoxins lacking the disulfide bond are biologically inactive. Thus, the discovery of the enzyme that catalyzes the disulfide bond in SpeA is important to understanding the biochemistry of SpeA production and presents a target for mitigating the virulence of S. pyogenes .

EDEM2 stably disulfide-bonded to TXNDC11 catalyzes the first mannose trimming step in mammalian glycoprotein ERAD

Sequential mannose trimming of N-glycan (Man9GlcNAc2 -> Man8GlcNAc2 -> Man7GlcNAc2) facilitates endoplasmic reticulum-associated degradation of misfolded glycoproteins (gpERAD). Our gene knockout experiments in human HCT116 cells have revealed that EDEM2 is required for the first step. However, it was previously shown that purified EDEM2 exhibited no α1,2-mannosidase activity toward Man9GlcNAc2 in vitro. Here, we found that EDEM2 was stably disulfide-bonded to TXNDC11, an endoplasmic reticulum protein containing five thioredoxin (Trx)-like domains. C558 present outside of the mannosidase homology domain of EDEM2 was linked to C692 in Trx5, which solely contains the CXXC motif in TXNDC11. This covalent bonding was essential for mannose trimming and subsequent gpERAD in HCT116 cells. Furthermore, EDEM2-TXNDC11 complex purified from transfected HCT116 cells converted Man9GlcNAc2 to Man8GlcNAc2(isomerB) in vitro. Our results establish the role of EDEM2 as an initiator of gpERAD, and represent the first clear demonstration of in vitro mannosidase activity of EDEM family proteins.

Identification and Characterisation of a Novel Antioxidant Activity for the BCAT1 Cxxc Motif: Implications for Myeloid Leukaemia Development

Recently the human cytosolic branched-chain amino transferase (BCAT1) has been implicated in the development of myeloid leukaemia. Previously studies identified a redox active CXXC motif for BCAT1 analogous to the mitochondrial isoform (BCAT2). Oxidation of this CXXC motif can regulate aminotransferase activity in both isoforms. Interestingly the reduction mid-point potential (Em) of the BCAT1 CXXC motif is around 80mV lower compared with BCAT2. This suggests that the BCAT1 CXXC motif is more reducing in nature i.e. has antioxidant capacity. Further evidence to support this notion comes from the observation that BCAT1 is significantly less sensitive to inactivation mediated by hydrogen peroxide (H2O2). This is an important observation, since H2O2 is a reactive oxygen species (ROS) implicated in myeloid leukaemia development. Here we provide the first evidence to show that the BCAT1 CXXC can metabolise H2O2. By using CXXC motif mutant constructs, we also evaluate the role of the BCAT1 CXXC motif in myeloid leukaemia cells. To investigate the antioxidant capacity of the BCAT1 CXXC motif, cDNA (accession NM_001178094.1) was cloned into a pET28a vector for overexpression and purification in E.coli BL21(DE3) cells. Site directed mutagenesis of the CXXC motif subsequently followed creating three constructs: 1) BCAT1-WT (unmutated), 2) BCAT1-CXXS (C-terminal Cys → Ser mutant) and 3) BCAT1-SXXS (N/C-terminal Cys → Ser double mutant). Mutation of the BCAT1 CXXC motif was confirmed at the protein level by 5,5'-ditho-bis-(2-nitrobenzoic acid) titration. WT and CXXC motif mutant BCAT1 protein was added to 5mM H2O2 & monitored at λ240nm for the disappearance of H2O2. This was compared with 1U of catalase as a positive control. Our data show that 15μg of purified BCAT1-WT could metabolise H2O2 at a rate of 0.14±0.02 μmol/min. The BCAT1 CXXC motif mutants lacked capacity to do this. To verify our findings, BCAT1-WT treated with N-ethylmaleimide also lost the capacity to metabolise H2O2. This confirms that the antioxidant activity observed is Cys mediated. This novel finding for the BCAT1 CXXC motif may therefore be important in myeloid leukaemia development. To evaluate the BCAT1 CXXC motif in myeloid leukaemia cells, WT and CXXS mutant BCAT1 was subcloned into a pLENTI-C-Myc-DDK lentiviral vector prior to transduction of U937 cells. Empty vector (EV) control transgenic U937 cells were also generated. Stable expression of BCAT1 was achieved and confirmed by western-blot and qPCR; BCAT1-WT (6.5±1.7 fold increase) & BCAT1-CXXS (5.1±2.6 fold increase). The BCAT1 and EV transgenic U937 cells were subjected to H2O2 mediated oxidative stress and monitored for cell viability after 24h. A significant difference in the LD50 between BCAT1-WT, BCAT1-CXXS and EV control cells was observed; 620±28mM, 306±37mM, 251±26mM respectively (p<0.05). This finding suggests that the BCAT1 CXXC provides protection against H2O2 mediated cell death. To corroborate these findings, the same cells treated with rotenone observed a similar pattern in LD50; 1.05mM, 0.06mM, 0.12mM respectively. ROS are implicated in many cellular processes, including differentiation. Thus, we next asked whether the BCAT1 CXXC motif could suppress U937 cell differentiation mediated by phorbol 12-myristate 13-acetate (PMA). This compound was selected since ROS feature in PMA mediated differentiation. U937 cells were monitored for the expression of CD11b and CD36 following 48h PMA incubation. Our data show a significant reduction in the frequency of CD11b+/CD36+ U937 cells for BCAT1-WT (17.5±6.9%) compared with EV controls (54.0±10.5%, p<0.05). BCAT1-CXXS also showed a reduction (30.6±4.9%), but not to the same extent as BCAT1-WT. This finding was mirrored by the expression level of CD11b, where the MFI was significantly less for BCAT1-WT (88.0±16.3) compared with EV control (251.5±54.7, p<0.001) and BCAT1-CXXS (152.5±6.3, p<0.05). Analysis of intracellular ROS using 2′,7′-Dichlorodihydrofluorescein diacetate revealed a reduction in cellular ROS for BCAT1-WT cells (5929±13.8 MFI), compared with EV controls (8527±196 MFI) and BCAT1-CXXS (7879±18.4 MFI). This shows that the BCAT1 CXXC motif may control cellular ROS levels in myeloid leukaemia cells. In summary, this study identifies a novel antioxidant role for the BCAT1 CXXC which confers protection against ROS mediated cell death and differentiation of myeloid leukaemia cells. Disclosures No relevant conflicts of interest to declare.

Sequence Analysis of a Subset of Plasma Membrane Raft Proteome Containing CXXC Metal Binding Motifs

In an attempt to identify the metal sensing proteins localized to mammalian plasma membrane, the authors screened a list of 300 raft associated proteins that are involved in cellular signaling mechanisms by searching the presence of metal thionin (CXXC) motifs. 50 proteins were found to possess CXXC motifs that could act as potential metal sensing proteins. The authors determined membrane topologies of the above CXXC motif containing proteins using TM-pred and analyzed the positions of their transmembrane (TM) domains using Bio-edit software. Based on the topology of CXXC domains, the authors classified all the raft-associated metal sensing proteins into six categories. They are (i) Exoplasmic tails with CXXC motif, (ii) Exoplasmic loops with CXXC motif, (iii) Cytosolic tails with CXXC motif, (iv) Cytosolic loop with CXXC motif, (v) TM domains with CXXC motifs, (vi) Proteins with multiple topologies of CXXC motif. The authors' study will lead to understanding of the raft-mediated mechanism of heavy metal sensing and signaling in mammalian cells.