Plant thioredoxin CDSP32 regenerates 1-cys methionine sulfoxide reductase B activity through the direct reduction of sulfenic acid

Atypical thioredoxins in poplar: the glutathione-dependent thioredoxin-like 2.1 supports the activity of target enzymes possessing a single redox active cysteine

Plant thioredoxins (Trxs) constitute a complex family of thiol oxidoreductases generally sharing a WCGPC active site sequence. Some recently identified plant Trxs (Clot, Trx-like1 and -2, Trx-lilium1, -2, and -3) display atypical active site sequences with altered residues between the two conserved cysteines. The transcript expression patterns, subcellular localizations, and biochemical properties of some representative poplar (Populus spp.) isoforms were investigated. Measurements of transcript levels for the 10 members in poplar organs indicate that most genes are constitutively expressed. Using transient expression of green fluorescent protein fusions, Clot and Trx-like1 were found to be mainly cytosolic, whereas Trx-like2.1 was located in plastids. All soluble recombinant proteins, except Clot, exhibited insulin reductase activity, although with variable efficiencies. Whereas Trx-like2.1 and Trx-lilium2.2 were efficiently regenerated both by NADPH-Trx reductase and glutathione, none of the proteins were reduced by the ferredoxin-Trx reductase. Only Trx-like2.1 supports the activity of plastidial thiol peroxidases and methionine sulfoxide reductases employing a single cysteine residue for catalysis and using a glutathione recycling system. The second active site cysteine of Trx-like2.1 is dispensable for this reaction, indicating that the protein possesses a glutaredoxin-like activity. Interestingly, the Trx-like2.1 active site replacement, from WCRKC to WCGPC, suppresses its capacity to use glutathione as a reductant but is sufficient to allow the regeneration of target proteins employing two cysteines for catalysis, indicating that the nature of the residues composing the active site sequence is crucial for substrate selectivity/recognition. This study provides another example of the cross talk existing between the glutathione/glutaredoxin and Trx-dependent pathways.

The emerging roles of protein glutathionylation in chloroplasts

Reactive oxygen species play important roles in redox signaling mainly through a set of reversible post-translational modifications of cysteine thiol residues in proteins, including glutathionylation and dithiol/disulfide exchange. Protein glutathionylation has been extensively studied in mammals but emerging evidence suggests that it can play important roles in plants and in chloroplast in particular. This redox modification involves protein thiols and glutathione and is mainly controlled by glutaredoxins, oxidoreductases belonging to the thioredoxin superfamily. In this review, we first present the possible mechanisms of protein glutathionylation and then introduce the chloroplast systems of glutaredoxins and thioredoxins, in order to pinpoint the biochemical properties that make some glutaredoxin isoforms the master enzymes in deglutathionylation. Finally, we discuss the possible roles of glutathionylation in thiol protection, protein regulation, reactive oxygen species scavenging and redox signaling in chloroplasts, with emphasis on the crosstalk between thioredoxin- and glutaredoxin-mediated signaling pathways.

Two highly homologous methionine sulfoxide reductase A from tomato (Solanum lycopersicum), exhibit distinct catalytic properties

E4, which is a fruit-ripening gene that is strongly induced by ethylene, has been reported to be a member of the methionine sulfoxide reductase A (MSRA) gene. In the present study, we determined for the first time the enzymatic activity and delineated the catalytic mechanism of the E4 protein via site-directed mutagenesis. The disulfide intermolecular cross-linking, kinetics parameter, thiol content titration analysis of wild-type and mutated E4 proteins revealed that the cysteine at position 37 (Cys-37) was the key catalytic residue, and Cys-194, but not Cys-180 served as the first recycling Cys in the thioredoxin (Trx)-dependent regeneration system. In addition, the SlMSRA2 protein, which was encoded by another MSRA gene, shared high similarity with the E4 protein and was truncated at the C-terminus. The wild-type and mutated SlMSRA2 enzymes had similar activities compared to the E4 protein using DTT as a reductant, but showed extremely low activities in the Trx-dependent reduction system. Our results indicated that E4 and SlMSRA2 proteins might exhibit distinct catalytic mechanisms.

Redox regulation in photosynthetic organisms: focus on glutathionylation

SIGNIFICANCE

In photosynthetic organisms, besides the well-established disulfide/dithiol exchange reactions specifically controlled by thioredoxins (TRXs), protein S-glutathionylation is emerging as an alternative redox modification occurring under stress conditions. This modification, consisting of the formation of a mixed disulfide between glutathione and a protein cysteine residue, can not only protect specific cysteines from irreversible oxidation but also modulate protein activities and appears to be specifically controlled by small disulfide oxidoreductases of the TRX superfamily named glutaredoxins (GRXs).

RECENT STUDIES

In recent times, several studies allowed significant progress in this area, mostly due to the identification of several plant proteins undergoing S-glutathionylation and to the characterization of the molecular mechanisms and the proteins involved in the control of this modification.

CRITICAL ISSUES

This article provides a global overview of protein glutathionylation in photosynthetic organisms with particular emphasis on the mechanisms of protein glutathionylation and deglutathionylation and a focus on the role of GRXs. Then, we describe the methods employed for identification of glutathionylated proteins in photosynthetic organisms and review the targets and the possible physiological functions of protein glutathionylation.

FUTURE DIRECTIONS

In order to establish the importance of protein S-glutathionylation in photosynthetic organisms, future studies should be aimed at delineating more accurately the molecular mechanisms of glutathionylation and deglutathionylation reactions, at identifying proteins undergoing S-glutathionylation in vivo under diverse conditions, and at investigating the importance of redoxins, GRX, and TRX, in the control of this redox modification in vivo.

Structural and biochemical analysis of mammalian methionine sulfoxide reductase B2

Methionine sulfoxide reductases are antioxidant enzymes that repair oxidatively damaged methionine residues in proteins. Mammals have three members of the methionine-R-sulfoxide reductase family, including cytosolic MsrB1, mitochondrial MsrB2, and endoplasmic reticulum MsrB3. Here, we report the solution structure of reduced Mus musculus MsrB2 using high resolution nuclear magnetic resonance (NMR) spectroscopy. MsrB2 is a β-strand rich globular protein consisting of eight antiparallel β-strands and three N-terminal α-helical segments. The latter secondary structure elements represent the main structural difference between mammalian MsrB2 and MsrB1. Structural comparison of mammalian and bacterial MsrB structures indicates that the general topology of this MsrB family is maintained and that MsrB2 more resembles bacterial MsrBs than MsrB1. Structural and biochemical analysis supports the catalytic mechanism of MsrB2 that, in contrast to MsrB1, does not involve a resolving cysteine (Cys). pH dependence of catalytically relevant residues in MsrB2 was accessed by NMR spectroscopy and the pK(a) of the catalytic Cys162 was determined to be 8.3. In addition, the pH-dependence of MsrB2 activity showed a maximum at pH 9.0, suggesting that deprotonation of the catalytic Cys is a critical step for the reaction. Further mobility analysis showed a well-structured N-terminal region, which contrasted with the high flexibility of this region in MsrB1. Our study highlights important structural and functional aspects of mammalian MsrB2 and provides a unifying picture for structure-function relationships within the MsrB protein family.

Investigating the plant response to cadmium exposure by proteomic and metabolomic approaches

Monitoring molecular dynamics of an organism upon stress is probably the best approach to decipher physiological mechanisms involved in the stress response. Quantitative analysis of proteins and metabolites is able to provide accurate information about molecular changes allowing the establishment of a range of more or less specific mechanisms, leading to the identification of major players in the considered pathways. Such tools have been successfully used to analyze the plant response to cadmium (Cd), a major pollutant capable of causing severe health issues as it accumulates in the food chain. We present a summary of proteomics and metabolomics works that contributed to a better understanding of the molecular aspects involved in the plant response to Cd. This work allowed us to provide a finer picture of general signaling, regulatory and metabolic pathways that appeared to be affected upon Cd stress. In particular, we conclude on the advantage of employing different approaches of global proteome- and metabolome-wide techniques, combined with more targeted analysis to answer molecular questions and unravel biological networks. Finally, we propose possible directions and methodologies for future prospectives in this field, as many aspects of the plant-Cd interaction remain to be discovered.

Structural plasticity of the thioredoxin recognition site of yeast methionine S-sulfoxide reductase Mxr1

The methionine S-sulfoxide reductase MsrA catalyzes the reduction of methionine sulfoxide, a ubiquitous reaction depending on the thioredoxin system. To investigate interactions between MsrA and thioredoxin (Trx), we determined the crystal structures of yeast MsrA/Mxr1 in their reduced, oxidized, and Trx2-complexed forms, at 2.03, 1.90, and 2.70 Å, respectively. Comparative structure analysis revealed significant conformational changes of the three loops, which form a plastic "cushion" to harbor the electron donor Trx2. The flexible C-terminal loop enabled Mxr1 to access the methionine sulfoxide on various protein substrates. Moreover, the plasticity of the Trx binding site on Mxr1 provides structural insights into the recognition of diverse substrates by a universal catalytic motif of Trx.

Structure-function relationship in an archaebacterial methionine sulphoxide reductase B

Oxidation of methionine to methionine sulphoxide (MetSO) may lead to loss of molecular integrity and function. This oxidation can be 'repaired' by methionine sulphoxide reductases (MSRs), which reduce MetSO back to methionine. Two structurally unrelated classes of MSRs, MSRA and MSRB, show stereoselectivity towards the S and the R enantiomer of the sulphoxide respectively. Interestingly, these enzymes were even maintained throughout evolution in anaerobic organisms. Here, the activity and the nuclear magnetic resonance (NMR) structure of MTH711, a zinc containing MSRB from the thermophilic, methanogenic archaebacterium Methanothermobacter thermoautotrophicus, are described. The structure appears more rigid as compared with similar MSRBs from aerobic and mesophilic organisms. No significant structural differences between the oxidized and the reduced MTH711 state can be deduced from our NMR data. A stable sulphenic acid is formed at the catalytic Cys residue upon oxidation of the enzyme with MetSO. The two non-zinc-binding cysteines outside the catalytic centre are not necessary for activity of MTH711 and are not situated close enough to the active-site cysteine to serve in regenerating the active centre via the formation of an intramolecular disulphide bond. These findings imply a reaction cycle that differs from that observed for other MSRBs.

An h-type thioredoxin functions in tobacco defense responses to two species of viruses and an abiotic oxidative stress

Various thioredoxin (Trx) proteins have been identified in plants. However, many of the physiological roles played by these proteins remain to be elucidated. We cloned a TRXh-like gene predicted to encode an h-type Trx in tobacco (Nicotiana tabacum) and designated it NtTRXh3, based on the biochemical activity of the NtTRXh3 protein. Overexpression of NtTRXh3 conferred resistance to Tobacco mosaic virus and Cucumber mosaic virus, both of which showed reduced multiplication and pathogenicity in NtTRXh3-overexpressing plants compared with controls. NtTRXh3 overexpression also enhanced tobacco resistance to oxidative stress induced by paraquat, an herbicide that inhibits the production of reducing equivalents by chloroplasts. The NtTRXh3 protein localized exclusively to chloroplasts in coordination with the maintenance of cellular reducing conditions, which accompanied an elevation in the glutathione/glutathione disulfide couple ratio. NtTRXh3 gene expression and NtTRXh3 protein production were necessary for these defensive responses, because they were all arrested when NtTRXh3 was silenced and the production of NtTRXh3 protein was abrogated. These results suggest that NtTRXh3 is involved in the resistance of tobacco to virus infection and abiotic oxidative stress.