Reaction mechanism, evolutionary analysis, and role of zinc in Drosophila methionine-R-sulfoxide reductase

Selenium-Binding Protein 1 (SBP1): A New Putative Player of Stress Sensing in Plants

Selenium-binding proteins (SBPs) represent a ubiquitous and conserved protein family with yet unclear biochemical and molecular functions. The importance of the human homolog has been extensively studied as it is implicated in many cancer types and other diseases. On the other hand, little is known regarding plant homologs. In plants, there is evidence that SBP participates in developmental procedures, oxidative stress responses, selenium and cadmium binding, and pathogenic tolerance. Moreover, recent studies have revealed that SBP is a methanethiol oxidase (MTO) catalyzing the conversion of methanethiol into formaldehyde, H2S, and H2O2. The two later products emerge as key signal molecules, playing pivotal roles in physiological processes and environmental stress responses. In this review, we highlight the available information regarding plants in order to introduce and emphasize the importance of SBP1 and its role in plant growth, development, and abiotic/biotic stress.

The association of protein-bound methionine sulfoxide with proteomic basis for aging in beech seeds

BACKGROUND

European beech (Fagus sylvatica L.) trees produce seeds irregularly; therefore, it is necessary to store beech seeds for forestation. Despite the acquisition of desiccation tolerance during development, beech seeds are classified as intermediate because they lose viability during long-term storage faster than typical orthodox seeds. In this study, beech seeds stored for short (3 years) or long (20 years) periods under optimal conditions and displaying 92 and 30% germination capacity, respectively, were compared.

RESULTS

Aged seeds displayed increased membrane damage, manifested as electrolyte leakage and lipid peroxidation levels. Analyses have been based on embryonic axes, which contained higher levels of reactive oxygen species (ROS) and higher levels of protein-bound methionine sulfoxide (MetO) in aged seeds. Using label-free quantitative proteomics, 3,949 proteins were identified, of which 2,442 were reliably quantified pointing to 24 more abundant proteins and 35 less abundant proteins in beech seeds under long-term storage conditions. Functional analyses based on gene ontology annotations revealed that nucleic acid binding activity (molecular function), ribosome organization or biogenesis and transmembrane transport (cellular processes), translational proteins (protein class) and membranous anatomical entities (cellular compartment) were affected in aged seeds. To verify whether MetO, the oxidative posttranslational modification of proteins that can be reversed via the action of methionine sulfoxide reductase (Msr) enzymes, is involved in the aging of beech seeds, we identified and quantified 226 MetO-containing proteins, among which 9 and 19 exhibited significantly up- and downregulated MetO levels, respectively, in beech seeds under long-term storage conditions. Several Msr isoforms were identified and recognized as MsrA1-like, MsrA4, MsrB5 and MsrB5-like in beech seeds. Only MsrA1-like displayed decreased abundance in aged seeds.

CONCLUSIONS

We demonstrated that the loss of membrane integrity reflected in the elevated abundance of membrane proteins had a higher impact on seed aging progress than the MetO/Msr system. Proteome analyses enabled us to propose protein Sec61 and glyceraldehyde-3-phosphate dehydrogenase as potential longevity modulators in beech seeds.

Dynamic proximity interaction profiling suggests that YPEL2 is involved in cellular stress surveillance

YPEL2 is a member of the evolutionarily conserved YPEL family involved in cellular proliferation, mobility, differentiation, senescence, and death. However, the mechanism by which YPEL2, or YPEL proteins, mediates its effects is largely unknown. Proteins perform their functions in a network of proteins whose identities, amounts, and compositions change spatiotemporally in a lineage-specific manner in response to internal and external stimuli. Here, we explored interaction partners of YPEL2 by using dynamic TurboID-coupled mass spectrometry analyses to infer a function for the protein. Our results using inducible transgene expressions in COS7 cells indicate that proximity interaction partners of YPEL2 are mainly involved in RNA and mRNA metabolic processes, ribonucleoprotein complex biogenesis, regulation of gene silencing by miRNA, and cellular responses to stress. We showed that YPEL2 interacts with the RNA-binding protein ELAVL1 and the selective autophagy receptor SQSTM1. We also found that YPEL2 localizes stress granules in response to sodium arsenite, an oxidative stress inducer, which suggests that YPEL2 participates in stress granule-related processes. Establishing a point of departure in the delineation of structural/functional features of YPEL2, our results suggest that YPEL2 may be involved in stress surveillance mechanisms.

Synergistic outcomes of Chlorella-bacterial cellulose based hydrogel as an ethylene scavenger

Increasing the freshness of vegetables requires the elimination of ethylene, which can be done through chemical methods. However, the development of eco-friendly approaches is required for environmental reasons. Chlorella vulgaris (C. vulgaris) was selected as a new biological material for demonstrating an excellent performance in ethylene removal. To support C. vulgaris, bacterial cellulose (BC) produced by Gluconacetobacter hansenii (G. hansenii) was chosen due to its high water content and biodegradability. To increase BC productivity, UV-induced mutant G. hansenii was isolated, and they produced high yields of BC (9.80 ± 0.52 g/L). Furthermore, comparative transcriptome analysis revealed metabolic flux changes toward UDP-glucose accumulation and enhanced BC production. BC-based hydrogels (BC hydrogels) were successfully prepared using a 2.4 % carboxymethyl cellulose (CMC) and 1 % agar mixture. We used Chlorella-BC hydrogels as an ethylene scavenger, which reduced 90 % of ethylene even when the immobilized C. vulgaris was preserved for 14 days at room temperature without media supplementation. We demonstrated for the first time the potential of BC hydrogels to integrate C. vulgaris as a sustainable ethylene absorber for green food packaging and biomass technology.

Evolutionary Aspects of Selenium Binding Protein (SBP)

Selenium-binding proteins represent a ubiquitous protein family and recently SBP1 was described as a new stress response regulator in plants. SBP1 has been characterized as a methanethiol oxidase, however its exact role remains unclear. Moreover, in mammals, it is involved in the regulation of anti-carcinogenic growth and progression as well as reduction/oxidation modulation and detoxification. In this work, we delineate the functional potential of certain motifs of SBP in the context of evolutionary relationships. The phylogenetic profiling approach revealed the absence of SBP in the fungi phylum as well as in most non eukaryotic organisms. The phylogenetic tree also indicates the differentiation and evolution of characteristic SBP motifs. Main evolutionary events concern the CSSC motif for which Acidobacteria, Fungi and Archaea carry modifications. Moreover, the CC motif is harbored by some bacteria and remains conserved in Plants, while modified to CxxC in Animals. Thus, the characteristic sequence motifs of SBPs mainly appeared in Archaea and Bacteria and retained in Animals and Plants. Our results demonstrate the emergence of SBP from bacteria and most likely as a methanethiol oxidase.

Methionine sulfoxide reductase B5 plays a key role in preserving seed vigor and longevity in rice (Oryza sativa)

Oxidation of methionine leads to the formation of methionine S-sulfoxide and methionine R-sulfoxide, which can be reverted by two types of methionine sulfoxide reductase (MSR): MSRA and MSRB. Though the role of MSR enzymes has been elucidated in various physiological processes, the regulation and role of MSR in seeds remains poorly understood. In this study, through molecular, biochemical, and genetic studies using seed-specific overexpression and RNAi lines of OsMSRB5 in Oryza sativa, we demonstrate the role of OsMSRB5 in maintaining seed vigor and longevity. We show that an age-induced reduction in the vigor and viability of seeds is correlated with reduced MSR activity and increased methionine sulfoxide (MetSO) formation. OsMSRB5 expression increases during seed maturation and is predominantly localized to the embryo. Further analyses on transgenic lines reveal the role of OsMSRB5 in modulating reactive oxygen species (ROS) homeostasis to preserve seed vigor and longevity. We show that ascorbate peroxidase and PROTEIN l-ISOASPARTYL METHYLTRANSFERASE undergo MetSO modification in seeds that affects their functional competence. OsMSRB5 physically interacts with these proteins and reverts this modification to facilitate their functions and preserve seed vigor and longevity. Our results thus illustrate the role of OsMSRB5 in preserving seed vigor and longevity by modulating ROS homeostasis in seeds.

Metabolic benefits of methionine restriction in adult mice do not require functional methionine sulfoxide reductase A (MsrA)

Methionine restriction (MR) extends lifespan and improves several markers of health in rodents. However, the proximate mechanisms of MR on these physiological benefits have not been fully elucidated. The essential amino acid methionine plays numerous biological roles and limiting its availability in the diet directly modulates methionine metabolism. There is growing evidence that redox regulation of methionine has regulatory control on some aspects of cellular function but interactions with MR remain largely unexplored. We tested the functional role of the ubiquitously expressed methionine repair enzyme methionine sulfoxide reductase A (MsrA) on the metabolic benefits of MR in mice. MsrA catalytically reduces both free and protein-bound oxidized methionine, thus playing a key role in its redox state. We tested the extent to which MsrA is required for metabolic effects of MR in adult mice using mice lacking MsrA. As expected, MR in control mice reduced body weight, altered body composition, and improved glucose metabolism. Interestingly, lack of MsrA did not impair the metabolic effects of MR on these outcomes. Moreover, females had blunted MR responses regardless of MsrA status compared to males. Overall, our data suggests that MsrA is not required for the metabolic benefits of MR in adult mice.

Thiol Reductases in Deinococcus Bacteria and Roles in Stress Tolerance

Deinococcus species possess remarkable tolerance to extreme environmental conditions that generate oxidative damage to macromolecules. Among enzymes fulfilling key functions in metabolism regulation and stress responses, thiol reductases (TRs) harbour catalytic cysteines modulating the redox status of Cys and Met in partner proteins. We present here a detailed description of Deinococcus TRs regarding gene occurrence, sequence features, and physiological functions that remain poorly characterised in this genus. Two NADPH-dependent thiol-based systems are present in Deinococcus. One involves thioredoxins, disulfide reductases providing electrons to protein partners involved notably in peroxide scavenging or in preserving protein redox status. The other is based on bacillithiol, a low-molecular-weight redox molecule, and bacilliredoxin, which together protect Cys residues against overoxidation. Deinococcus species possess various types of thiol peroxidases whose electron supply depends either on NADPH via thioredoxins or on NADH via lipoylated proteins. Recent data gained on deletion mutants confirmed the importance of TRs in Deinococcus tolerance to oxidative treatments, but additional investigations are needed to delineate the redox network in which they operate, and their precise physiological roles. The large palette of Deinococcus TR representatives very likely constitutes an asset for the maintenance of redox homeostasis in harsh stress conditions.

An ethylene-hypersensitive methionine sulfoxide reductase regulated by NAC transcription factors increases methionine pool size and ethylene production during kiwifruit ripening

Ethylene plays an important role in regulating fruit ripening by triggering dynamic changes in expression of ripening-associated genes, but the functions of many of these genes are still unknown. Here, a methionine sulfoxide reductase gene (AdMsrB1) was identified by transcriptomics-based analysis as the gene most responsive to ethylene treatment in ripening kiwifruit. The AdMsrB1 protein exhibits a stereospecific activity toward the oxidative stress-induced R enantiomer of methionine sulfoxide (MetSO), reducing it to methionine (Met). Stable overexpression of AdMsrB1 in kiwifruit significantly increased the content of free Met and 1-aminocyclopropane-1-carboxylic acid (ACC), the immediate precursor of ethylene, and increased ethylene production. Dual-luciferase assays indicated that the AdMsrB1 promoter was not directly upregulated by ethylene treatment but was modulated by two ethylene-inducible NAM/ATAF/CUC transcription factors (AdNAC2 and AdNAC72) that bind directly to the AdMsrB1 promoter. Overexpression of AdNAC72 in kiwifruit not only enhanced AdMsrB1 expression, but also increased free Met and ACC content and ethylene production rates. This finding establishes an unexpected regulatory loop that enhances ethylene production and the concentration of its biosynthetic intermediates.

Differential Responses of Methionine Sulfoxide Reductases A and B to Anoxia and Oxidative Stress in the Freshwater Turtle Trachemys scripta

Oxidative stress has been acknowledged as a major factor in aging, senescence and neurodegenerative conditions. Mammalian models are susceptible to these stresses following the restoration of oxygen after anoxia; however, some organisms including the freshwater turtle Trachemys scripta can withstand repeated anoxia and reoxygenation without apparent pathology. T. scripta thus provides us with an alternate vertebrate model to investigate physiological mechanisms of neuroprotection. The objective of this study was to investigate the antioxidant methionine sulfoxide reductase system (Msr) in turtle neuronal tissue. We examined brain transcript and protein levels of MsrA and MsrB and examined the potential for the transcription factor FOXO3a to regulate the oxygen-responsive changes in Msr in vitro. We found that Msr mRNA and protein levels are differentially upregulated during anoxia and reoxygenation, and when cells were exposed to chemical oxidative stress. However, while MsrA and MsrB3 levels increased when cell cultures were exposed to chemical oxidative stress, this induction was not enhanced by treatment with epigallocatechin gallate (EGCG), which has previously been shown to enhance FOXO3a levels in the turtle. These results suggest that FOXO3a and Msr protect the cells from oxidative stress through different molecular pathways, and that both the Msr pathway and EGCG may be therapeutic targets to treat diseases related to oxidative damage.

Functional characterization of methionine sulfoxide reductases from Leptospira interrogans

BACKGROUND

Methionine (Met) oxidation leads to a racemic mixture of R and S forms of methionine sulfoxide (MetSO). Methionine sulfoxide reductases (Msr) are enzymes that can reduce specifically each isomer of MetSO, both free and protein-bound. The Met oxidation could change the structure and function of many proteins, not only of those redox-related but also of others involved in different metabolic pathways. Until now, there is no information about the presence or function of Msrs enzymes in Leptospira interrogans.

METHODS

We identified genes coding for putative MsrAs (A1 and A2) and MsrB in L. interrogans serovar Copenhageni strain Fiocruz L1-130 genome project. From these, we obtained the recombinant proteins and performed their functional characterization.

RESULTS

The recombinant L. interrogans MsrB catalyzed the reduction of Met(R)SO using glutaredoxin and thioredoxin as reducing substrates and behaves like a 1-Cys Msr (without resolutive Cys residue). It was able to partially revert the in vitro HClO-dependent inactivation of L. interrogans catalase. Both recombinant MsrAs reduced Met(S)SO, being the recycle mediated by the thioredoxin system. LinMsrAs were more efficient than LinMsrB for free and protein-bound MetSO reduction. Besides, LinMsrAs are enzymes involving a Cys triad in their catalytic mechanism. LinMsrs showed a dual localization, both in cytoplasm and periplasm.

CONCLUSIONS AND GENERAL SIGNIFICANCE

This article brings new knowledge about redox metabolism in L. interrogans. Our results support the occurrence of a metabolic pathway involved in the critical function of repairing oxidized macromolecules in this pathogen.

[Selenium and zinc: "antioxidants" for healthy aging?]

Selenium and zinc are essential trace elements and an inadequate dietary intake has been implicated in the decline of immune and cognitive functions in aged persons and in the pathogenesis of age-related disorders. Both micronutrients are often marketed as "antioxidants" in mineral supplements; however, neither selenium nor zinc are antioxidants per se but they may exert beneficial effects as components of enzymes and other proteins that catalyze redox reactions and/or are involved in the maintenance of redox homeostasis. According to epidemiological data older individuals have an increased risk of developing deficiencies in the selenium and zinc status; however, such statistical correlations in epidemiological studies do not imply a causal association. Intervention trials are scarce and have yielded inconsistent and sometimes even adverse results. It should also be noted that the observed deficiencies in micronutrients may not necessarily be attributable to inadequate dietary intake as the absorption and distribution within the body might also be influenced by factors such as medications or interaction with other food ingredients. Thus, any dietary supplementation should be implemented with caution and persons who wish to take mineral supplements should first seek medical advice. This article discusses the role of selenium and zinc in biological antioxidant systems, summarizes findings on the supply and supplementation of aged persons with these trace elements and on the influence they may exert on aging-related health issues, such as cognitive decline and type 2 diabetes mellitus.

Methionine sulfoxide reductase A (MsrA) modulates cells and protects against Mycoplasma genitalium induced cytotoxicity

Methionine sulfoxide reductase A (MsrA) is a ubiquitous antioxidant repair enzyme which specifically reduces the oxidized methionine (Met-O) in proteins to methionine (Met). Previous studies have shown that lack of or overexpression of MsrA in cells affects the function of proteins and can lead to altered cellular processes. Interestingly, some pathogenic bacteria secrete and/or carry MsrA on their surface, suggesting some key roles for this enzyme in the modulation of host cellular processes. Therefore, we investigated how exogenously added MsrA affects the ability of the host cells in combating infection by using an in vitroMycoplasma genitalium cytotoxicity model. HeLa cells pretreated with MsrA and infected with M. genitalium showed significantly lower necrosis (cytotoxicity) than untreated cells infected with M. genitalium. Intriguingly, necrotic cell death pathway specific real time RT-PCR revealed that M. genitalium infection upregulates the expression of the TNF gene in HeLa cells and that MsrA pretreatment of the cells downregulates its expression significantly. Consistent with this, enzyme linked immunosorbent assay (ELISA) results showed that HeLa cells pretreated with MsrA secreted reduced levels of TNF-α following M. genitalium infection. Also, our study demonstrates that MsrA treatment of cells affects the phosphorylation status of transcriptional regulators such as NF-кB, JNK and p53 that regulate different cytokines. Further, fluorescent microscopy showed the cellular uptake of exogenously added MsrA fused with red fluorescent protein (MsrA-RFP). Altogether, our results suggest that secreted MsrA may help pathogens to modulate host cellular processes.

Methionine sulfoxide reductase B from Corynebacterium diphtheriae catalyzes sulfoxide reduction via an intramolecular disulfide cascade

Corynebacterium diphtheriae is a human pathogen that causes diphtheria. In response to immune system-induced oxidative stress, C. diphtheriae expresses antioxidant enzymes, among which are methionine sulfoxide reductase (Msr) enzymes, which are critical for bacterial survival in the face of oxidative stress. Although some aspects of the catalytic mechanism of the Msr enzymes have been reported, several details still await full elucidation. Here, we solved the solution structure of C. diphtheriae MsrB (Cd-MsrB) and unraveled its catalytic and oxidation-protection mechanisms. Cd-MsrB catalyzes methionine sulfoxide reduction involving three redox-active cysteines. Using NMR heteronuclear single-quantum coherence spectra, kinetics, biochemical assays, and MS analyses, we show that the conserved nucleophilic residue Cys-122 is S-sulfenylated after substrate reduction, which is then resolved by a conserved cysteine, Cys-66, or by the nonconserved residue Cys-127. We noted that the overall structural changes during the disulfide cascade expose the Cys-122-Cys-66 disulfide to recycling through thioredoxin. In the presence of hydrogen peroxide, Cd-MsrB formed reversible intra- and intermolecular disulfides without losing its Cys-coordinated Zn²⁺, and only the nonconserved Cys-127 reacted with the low-molecular-weight (LMW) thiol mycothiol, protecting it from overoxidation. In summary, our structure-function analyses reveal critical details of the Cd-MsrB catalytic mechanism, including a major structural rearrangement that primes the Cys-122-Cys-66 disulfide for thioredoxin reduction and a reversible protection against excessive oxidation of the catalytic cysteines in Cd-MsrB through intra- and intermolecular disulfide formation and S-mycothiolation.

Methionine sulfoxide reductase (Msr) dysfunction in human brain disease

Extensive research has shown that oxidative stress is strongly associated with aging, senescence and several diseases, including neurodegenerative and psychiatric disorders. Oxidative stress is caused by the overproduction of reactive oxygen species (ROS) that can be counteracted by both enzymatic and nonenzymatic antioxidants. One of these antioxidant mechanisms is the widely studied methionine sulfoxide reductase system (Msr). Methionine is one of the most easily oxidized amino acids and Msr can reverse this oxidation and restore protein function, with MsrA and MsrB reducing different stereoisomers. This article focuses on experimental and genetic research performed on Msr and its link to brain diseases. Studies on several model systems as well as genome-wide association studies are compiled to highlight the role of MSRA in schizophrenia, Alzheimer's disease, and Parkinson's disease. Genetic variation of MSRA may also contribute to the risk of psychosis, personality traits, and metabolic factors.

Effect of Exogenous Zinc on MsrB1 Expression and Protein Oxidation in Human Lens Epithelial Cells

Aging has been related to zinc deficiency, resulting in protein oxidation and age-related decline of methionine sulfoxide reductase (Msr) activity. This study was designed to investigate the levels of methionine sulfoxide reductase B1 (MsrB1) mRNA and oxidized proteins in human lens epithelial (hLE) cells after treatment with exogenous zinc. The role of exogenous zinc in regulation of MsrB1 gene expression and protein oxidation in hLE cells was studied by MTT assay, oxidized protein measurement kit, and real-time PCR. The results showed that hLE cell viability was significantly decreased by MsrB1 gene knockdown or peroxynitrite (ONOO^-) treatment, while it was significantly increased after treatment with exogenous zinc (P < 0.05). Protein carbonyl content in hLE cell by MsrB1 gene knockdown or ONOO^- treatment was significantly decreased after treatment with ZnSO₄ (P < 0.01). And exogenous zinc could increase the level of MsrB1 in hLE cell under normal (P < 0.001) and oxidative stress (P < 0.01) conditions. In conclusion, exogenous zinc could protect hLE cells against MsrB1 gene knockdown or ONOO^--induced cell death by upregulation of MsrB1 involved in the elimination of reactive oxygen species (ROS) and oxidized proteins.

Rhodobacter sphaeroides methionine sulfoxide reductase P reduces R- and S-diastereomers of methionine sulfoxide from a broad-spectrum of protein substrates

Methionine (Met) is prone to oxidation and can be converted to Met sulfoxide (MetO), which exists as R- and S-diastereomers. MetO can be reduced back to Met by the ubiquitous methionine sulfoxide reductase (Msr) enzymes. Canonical MsrA and MsrB were shown to be absolutely stereospecific for the reduction of S-diastereomer and R-diastereomer, respectively. Recently, a new enzymatic system, MsrQ/MsrP which is conserved in all gram-negative bacteria, was identified as a key actor for the reduction of oxidized periplasmic proteins. The haem-binding membrane protein MsrQ transmits reducing power from the electron transport chains to the molybdoenzyme MsrP, which acts as a protein-MetO reductase. The MsrQ/MsrP function was well established genetically, but the identity and biochemical properties of MsrP substrates remain unknown. In this work, using the purified MsrP enzyme from the photosynthetic bacteria Rhodobacter sphaeroides as a model, we show that it can reduce a broad spectrum of protein substrates. The most efficiently reduced MetO is found in clusters, in amino acid sequences devoid of threonine and proline on the C-terminal side. Moreover, R. sphaeroides MsrP lacks stereospecificity as it can reduce both R- and S-diastereomers of MetO, similarly to its Escherichia coli homolog, and preferentially acts on unfolded oxidized proteins. Overall, these results provide important insights into the function of a bacterial envelop protecting system, which should help understand how bacteria cope in harmful environments.

Abscisic acid-dependent nitric oxide pathway and abscisic acid-independent nitric oxide routes differently modulate NaCl stress induction of the gene expression of methionine sulfoxide reductase A and B in rice roots

The methionine residues of proteins are the preferred targets of oxidation by reactive oxygen species resulting in the formation of methionine sulfoxide (MetSO), which impairs protein function. Methionine sulfoxide reductase A and B (MSR) catalyze the reduction of the MetSO S and R epimers back to Met residues, respectively. The roles of abscisic acid (ABA) and nitric oxide (NO) on the transcript levels of methionine sulfoxide reductase (MSR; EC 1.8.4.6) in the roots of 2-d-old etiolated rice (Oryza sativa L.) seedlings exposed to NaCl were examined. The OsMSR transcript levels increased upon exposure to NaCl, which increased as the NaCl concentrations increased. Fluridone (Flu) pretreatment inhibited the increases in ABA and NO contents and the OsMSRA4, OsMSRA5, OsMSRB1.1, OsMSRB3 and OsMSRB5 transcripts by NaCl, while ABA application reversed the effects of Flu. Flu did not affect the OsMSRA2 and OsMSRB1.2 transcripts. The application of the NO scavenger, 2-(4-carboxyphenyl)-4,4,5,5,-tetramethylimidazoline-1-oxyl-3-oxide (cPTIO), to NaCl-treated roots inhibited the increase in all of the OsMSRs transcripts with the exception of OsMSRB1.2. Treatment with the NO donor sodium nitroprusside (SNP) increased all the OsMSRs transcripts. The inhibitory effect of Flu on the increase of the OsMSRA4, OsMSRA5, OsMSR1.1, OsMSRB 3, and OsMSRB5 transcripts in the NaCl-treated roots was reversed by SNP. cPTIO inhibited the expression of all the OsMSR genes. The OsMSRA2.1 and OsMSRB1.2 transcripts can be increased by SNP. The Flu-inhibited internal ABA contents cannot be recovered by treatment with cPTIO or SNP. In addition, NaCl-induced NO production can be divided into ABA-dependent and ABA-independent routes. Therefore, the ABA-dependent NO route regulated the expression of OsMSRA4, OsMSRA5, OsMSRB1.1, OsMSRB 3, and OsMSRB5 in the NaCl-treated rice roots, while the ABA-independent NO pathway modulated OsMSRA2.1, and the ABA-independent and NO-independent pathway modulated OsMSRB1.2 expression in response to NaCl treatment.

In Vivo Effects of Methionine Sulfoxide Reductase Deficiency in Drosophila melanogaster

The deleterious alteration of protein structure and function due to the oxidation of methionine residues has been studied extensively in age-associated neurodegenerative disorders such as Alzheimer's and Parkinson's Disease. Methionine sulfoxide reductases (MSR) have three well-characterized biological functions. The most commonly studied function is the reduction of oxidized methionine residues back into functional methionine thus, often restoring biological function to proteins. Previous studies have successfully overexpressed and silenced MSR activity in numerous model organisms correlating its activity to longevity and oxidative stress. In the present study, we have characterized in vivo effects of MSR deficiency in Drosophila. Interestingly, we found no significant phenotype in animals lacking either methionine sulfoxide reductase A (MSRA) or methionine sulfoxide reductase B (MSRB). However, Drosophila lacking any known MSR activity exhibited a prolonged larval third instar development and a shortened lifespan. These data suggest an essential role of MSR in key biological processes.

Methionine Sulfoxide Reductases of Archaea

Methionine sulfoxide reductases are found in all domains of life and are important in reversing the oxidative damage of the free and protein forms of methionine, a sulfur containing amino acid particularly sensitive to reactive oxygen species (ROS). Archaea are microbes of a domain of life distinct from bacteria and eukaryotes. Archaea are well known for their ability to withstand harsh environmental conditions that range from habitats of high ROS, such as hypersaline lakes of intense ultraviolet (UV) radiation and desiccation, to hydrothermal vents of low concentrations of dissolved oxygen at high temperature. Recent evidence reveals the methionine sulfoxide reductases of archaea function not only in the reduction of methionine sulfoxide but also in the ubiquitin-like modification of protein targets during oxidative stress, an association that appears evolutionarily conserved in eukaryotes. Here is reviewed methionine sulfoxide reductases and their distribution and function in archaea.

Methionine sulfoxide reductase B3 requires resolving cysteine residues for full activity and can act as a stereospecific methionine oxidase

The oxidation of methionine residues in proteins occurs during oxidative stress and can lead to an alteration in protein function. The enzyme methionine sulfoxide reductase (Msr) reverses this modification. Here, we characterise the mammalian enzyme Msr B3. There are two splice variants of this enzyme that differ only in their N-terminal signal sequence, which directs the protein to either the endoplasmic reticulum (ER) or mitochondria. We demonstrate here that the enzyme can complement a bacterial strain, which is dependent on methionine sulfoxide reduction for growth, that the purified recombinant protein is enzymatically active showing stereospecificity towards R-methionine sulfoxide, and identify the active site and two resolving cysteine residues. The enzyme is efficiently recycled by thioredoxin only in the presence of both resolving cysteine residues. These results show that for this isoform of Msrs, the reduction cycle most likely proceeds through a three-step process. This involves an initial sulfenylation of the active site thiol followed by the formation of an intrachain disulfide with a resolving thiol group and completed by the reduction of this disulfide by a thioredoxin-like protein to regenerate the active site thiol. Interestingly, the enzyme can also act as an oxidase catalysing the stereospecific formation of R-methionine sulfoxide. This result has important implications for the role of this enzyme in the reversible modification of ER and mitochondrial proteins.

Structural aspects of protein kinase ASK1 regulation

Apoptosis signal-regulating kinase 1 (ASK1, also known as MAP3K5), a member of the mitogen-activated protein kinase kinase kinase (MAP3K) family, activates the p38 mitogen-activated protein kinase and the c-Jun N-terminal kinase (JNK) signaling cascades in response to various stressors. ASK1 activity is tightly regulated through phosphorylation and interaction with various binding partners. However, the mechanistic details underlying the ASK1 regulation are still not fully understood. This review focuses on recent advances in structural studies of protein kinase ASK1 and on the insights they provide into its mechanism of regulation. In addition, we also discuss protein-protein interactions between ASK1 and its binding partners thioredoxin (TRX) and 14-3-3 protein.

Vibrio cholerae ensures function of host proteins required for virulence through consumption of luminal methionine sulfoxide

Vibrio cholerae is a diarrheal pathogen that induces accumulation of lipid droplets in enterocytes, leading to lethal infection of the model host Drosophila melanogaster. Through untargeted lipidomics, we provide evidence that this process is the product of a host phospholipid degradation cascade that induces lipid droplet coalescence in enterocytes. This infection-induced cascade is inhibited by mutation of the V. cholerae glycine cleavage system due to intestinal accumulation of methionine sulfoxide (MetO), and both dietary supplementation with MetO and enterocyte knock-down of host methionine sulfoxide reductase A (MsrA) yield increased resistance to infection. MsrA converts both free and protein-associated MetO to methionine. These findings support a model in which dietary MetO competitively inhibits repair of host proteins by MsrA. Bacterial virulence strategies depend on functional host proteins. We propose a novel virulence paradigm in which an intestinal pathogen ensures the repair of host proteins essential for pathogenesis through consumption of dietary MetO.

The Anti-Oxidant Defense System of the Marine Polar Ciliate Euplotes nobilii: Characterization of the MsrB Gene Family

Organisms living in polar waters must cope with an extremely stressful environment dominated by freezing temperatures, high oxygen concentrations and UV radiation. To shed light on the genetic mechanisms on which the polar marine ciliate, Euplotes nobilii, relies to effectively cope with the oxidative stress, attention was focused on methionine sulfoxide reductases which repair proteins with oxidized methionines. A family of four structurally distinct MsrB genes, encoding enzymes specific for the reduction of the methionine-sulfoxide R-forms, were identified from a draft of the E. nobilii transcriptionally active (macronuclear) genome. The En-MsrB genes are constitutively expressed to synthesize proteins markedly different in amino acid sequence, number of CXXC motifs for zinc-ion binding, and presence/absence of a cysteine residue specific for the mechanism of enzyme regeneration. The En-MsrB proteins take different localizations in the nucleus, mitochondria, cytosol and endoplasmic reticulum, ensuring a pervasive protection of all the major subcellular compartments from the oxidative damage. These observations have suggested to regard the En-MsrB gene activity as playing a central role in the genetic mechanism that enables E. nobilii and ciliates in general to live in the polar environment.

Arabidopsis thaliana methionine sulfoxide reductase B8 influences stress-induced cell death and effector-triggered immunity

Reactive oxygen species (ROS) oxidize methionine to methionine sulfoxide (MetSO) and thereby inactivate proteins. Methionine sulfoxide reductase (MSR) enzyme converts MetSO back to the reduced form and thereby detoxifies the effect of ROS. Our results show that Arabidopsis thaliana MSR enzyme coding gene MSRB8 is required for effector-triggered immunity and containment of stress-induced cell death in Arabidopsis. Plants activate pattern-triggered immunity (PTI), a basal defense, upon recognition of evolutionary conserved molecular patterns present in the pathogens. Pathogens release effector molecules to suppress PTI. Recognition of certain effector molecules activates a strong defense, known as effector-triggered immunity (ETI). ETI induces high-level accumulation of reactive oxygen species (ROS) and hypersensitive response (HR), a rapid programmed death of infected cells. ROS oxidize methionine to methionine sulfoxide (MetSO), rendering several proteins nonfunctional. The methionine sulfoxide reductase (MSR) enzyme converts MetSO back to the reduced form and thereby detoxifies the effect of ROS. Though a few plant MSR genes are known to provide tolerance against oxidative stress, their role in plant-pathogen interaction is not known. We report here that activation of cell death by avirulent pathogen or UV treatment induces expression of MSRB7 and MSRB8 genes. The T-DNA insertion mutant of MSRB8 exaggerates HR-associated and UV-induced cell death and accumulates a higher level of ROS than wild-type plants. The negative regulatory role of MSRB8 in HR is further supported by amiRNA and overexpression lines. Mutants and overexpression lines of MSRB8 are susceptible and resistant respectively, compared to the wild-type plants, against avirulent strains of Pseudomonas syringae pv. tomato DC3000 (Pst) carrying AvrRpt2, AvrB, or AvrPphB genes. However, the MSRB8 gene does not influence resistance against virulent Pst or P. syringae pv. maculicola (Psm) pathogens. Our results altogether suggest that MSRB8 function is required for ETI and containment of stress-induced cell death in Arabidopsis.

Methionine Sulfoxide Reductase-B3 (MsrB3) Protein Associates with Synaptic Vesicles and its Expression Changes in the Hippocampi of Alzheimer's Disease Patients

Genome-wide association studies (GWAS) identified susceptibility loci associated with decreased hippocampal volume, and found hippocampal subfield-specific effects at MSRB3 (methionine sulfoxide reductase-B3). The MSRB3 locus was also linked to increased risk for late onset Alzheimer's disease (AD). In this study, we uncovered novel sites of MsrB3 expression in CA pyramidal layer and arteriolar walls by using automated immunohistochemistry on hippocampal sections from 23 individuals accompanied by neuropathology reports and clinical dementia rating scores. Controls, cognitively intact subjects with no hippocampal neurofibrillary tangles, exhibited MsrB3 signal as distinct but rare puncta in CA1 pyramidal neuronal somata. In CA3, however, MsrB3-immunoreactivity was strongest in the neuropil of the pyramidal layer. These patterns were replicated in rodent hippocampi where ultrastructural and immunohistofluorescence analysis revealed MsrB3 signal associated with synaptic vesicles and colocalized with mossy fiber terminals. In AD subjects, the number of CA1 pyramidal neurons with frequent, rather than rare, MsrB3-immunoreactive somatic puncta increased in comparison to controls. This change in CA1 phenotype correlated with the occurrence of AD pathological hallmarks. Moreover, the intensity of MsrB3 signal in the neuropil of CA3 pyramidal layer correlated with the signal pattern in neurons of CA1 pyramidal layer that was characteristic of cognitively intact individuals. Finally, MsrB3 signal in the arteriolar walls in the hippocampal white matter decreased in AD patients. This characterization of GWAS-implicated MSRB3 protein expression in human hippocampus suggests that patterns of neuronal and vascular MsrB3 protein expression reflect or underlie pathology associated with AD.

Crystal structure of the zinc-bound HhoA protease from Synechocystis sp. PCC 6803

The high temperature requirement A (HtrA) proteases are oligomeric serine proteases essential for protein quality control. HtrA homolog A (HhoA) from the photosynthetic cyanobacterium Synechocystis sp. PCC 6803 assembles into a proteolytically active hexamer. Herein, we present the crystal structure of the hexameric HhoA in complex with the copurified peptide. Our data indicate the presence of three methionines in close proximity to the peptide-binding site of the PDZ domain. Unexpectedly, we observed that a zinc ion is accommodated within the central channel formed by a HhoA trimer. However, neither calcium nor magnesium showed affinity for HhoA. The role of the zinc ion in HhoA was tested in an in vitro proteolytic assay against the nonspecific substrate β-casein and was found to be inhibitory. Our findings provide insights into the regulation of HhoA by a redox-related mechanism involving methionine residues and by zinc ion-binding within the central channel.

Porcine methionine sulfoxide reductase B3: molecular cloning, tissue-specific expression profiles, and polymorphisms associated with ear size in Sus scrofa

Background

In Sus scrofa, methionine sulfoxide reductase B3 (MSRB3) is a crucial candidate gene for ear size, and an important conformational trait of pig breeds. However, challenges in MSRB3 cDNA amplification have prevented further identification of MSRB3 allelic variants influencing pig ear size.

Results

We cloned a full-length cDNA sequence of porcine MSRB3 by rapid-amplification of cDNA ends. The 3,765-bp gene contained a 5’-untranslated region (UTR) (190 bp), a coding region (552 bp), and a 3’-UTR (3,016 bp) and shared 84 %, 84 %, 87 %, 86 %, and 70 % sequence identities with human, orangutan, mouse, chicken, and zebrafish, respectively. The gene encoded a 183-amino acid protein, which shared 88 %, 91 %, 89 %, 86 %, and 67 % identities with human, orangutan, mouse, chicken, and zebrafish, respectively. Tissue expression analysis using qRT-PCR revealed that MSRB3 was expressed in the heart, liver, lung, kidney, spleen, ear, muscle, fat, lymph, skeletal, and hypothalamic tissues. Three single nucleotide polymorphisms (SNPs) were identified in MSRB3: c.-735C > T in the 5’ flanking region, c.2571 T > C in the 3’-UTR, and a synonymous mutation of c.484 T > C in the coding region. The SNPs c.-735C > T and c.2571 T > C were significantly associated with ear size in a Large White × Minzhu F2 population other than in Beijing Black pigs. Subsequently, at SNP c.-735C > T, the mRNA of MSRB3 was significantly higher expressed in ears of individuals with the TT genotype (Minzhu) than those with CC (Large White).

Conclusions

The porcine MSRB3 owned a 3,765-bp full-length cDNA sequence and was detected to express in ear tissue. Two SNPs of this gene were shown to be significantly associated with ear size in a Large White × Minzhu intercross population instead of Beijing Black pig population. What’s more, the individuals with higher mRNA expression of MSRB3 have larger ear sizes. These results provide useful information for further functional analyses of MSRB3 influencing ear size in pigs.

Electronic supplementary material

The online version of this article (doi:10.1186/s40104-015-0060-x) contains supplementary material, which is available to authorized users.

One- and two-electron oxidation of thiols: mechanisms, kinetics and biological fates

The oxidation of biothiols participates not only in the defense against oxidative damage but also in enzymatic catalytic mechanisms and signal transduction processes. Thiols are versatile reductants that react with oxidizing species by one- and two-electron mechanisms, leading to thiyl radicals and sulfenic acids, respectively. These intermediates, depending on the conditions, participate in further reactions that converge on different stable products. Through this review, we will describe the biologically relevant species that are able to perform these oxidations and we will analyze the mechanisms and kinetics of the one- and two-electron reactions. The processes undergone by typical low-molecular-weight thiols as well as the particularities of specific thiol proteins will be described, including the molecular determinants proposed to account for the extraordinary reactivities of peroxidatic thiols. Finally, the main fates of the thiyl radical and sulfenic acid intermediates will be summarized.

The discovery of methionine sulfoxide reductase enzymes: An historical account and future perspectives

L-Methionine (L-Met) is the only sulphur-containing proteinogenic amino acid together with cysteine. Its importance is highlighted by it being the initiator amino acid for protein synthesis in all known living organisms. L-Met, free or inserted into proteins, is sensitive to oxidation of its sulfide moiety, with formation of L-Met sulfoxide. The sulfoxide could not be inserted into proteins, and the oxidation of L-Met in proteins often leads to the loss of biological activity of the affected molecule. Key discoveries revealed the existence, in rats, of a metabolic pathway for the reduction of free L-Met sulfoxide and, later, in Escherichia coli, of the enzymatic reduction of L-Met sulfoxide inserted in proteins. Upon oxidation, the sulphur atom becomes a new stereogenic center, and two stable diastereoisomers of L-Met sulfoxide exist. A fundamental discovery revealed the existence of two unrelated families of enzymes, MsrA and MsrB, whose members display opposite stereospecificity of reduction for the two sulfoxides. The importance of Msrs is additionally emphasized by the discovery that one of the only 25 selenoproteins expressed in humans is a Msr. The milestones on the road that led to the discovery and characterization of this group of antioxidant enzymes are recounted in this review.

Global quantitative proteomics reveals novel factors in the ecdysone signaling pathway in Drosophila melanogaster

The ecdysone signaling pathway plays a major role in various developmental transitions in insects. Recent advances in the understanding of ecdysone action have relied to a large extent on the application of molecular genetic tools in Drosophila. Here, we used a comprehensive quantitative SILAC MS-based approach to study the global, dynamic proteome of a Drosophila cell line to investigate how hormonal signals are transduced into specific cellular responses. Global proteome data after ecdysone treatment after various time points were then integrated with transcriptome data. We observed a substantial overlap in terms of affected targets between the dynamic proteome and transcriptome, although there were some clear differences in timing effects. Also, downregulation of several specific mRNAs did not always correlate with downregulation of their corresponding protein counterparts, and in some cases there was no correlation between transcriptome and proteome dynamics whatsoever. In addition, we performed a comprehensive interactome analysis of EcR, the major target of ecdysone. Proteins copurified with EcR include factors involved in transcription, chromatin remodeling, ecdysone signaling, ecdysone biosynthesis, and other signaling pathways. Novel ecdysone-responsive proteins identified in this study might link previously unknown proteins to the ecdysone signaling pathway and might be novel targets for developmental studies. To our knowledge, this is the first time that ecdysone signaling is studied by global quantitative proteomics. All MS data have been deposited in the ProteomeXchange with identifier PXD001455 (http://proteomecentral.proteomexchange.org/dataset/PXD001455).

Effects of ionizing radiation on biological molecules--mechanisms of damage and emerging methods of detection

SIGNIFICANCE

The detrimental effects of ionizing radiation (IR) involve a highly orchestrated series of events that are amplified by endogenous signaling and culminating in oxidative damage to DNA, lipids, proteins, and many metabolites. Despite the global impact of IR, the molecular mechanisms underlying tissue damage reveal that many biomolecules are chemoselectively modified by IR.

RECENT ADVANCES

The development of high-throughput "omics" technologies for mapping DNA and protein modifications have revolutionized the study of IR effects on biological systems. Studies in cells, tissues, and biological fluids are used to identify molecular features or biomarkers of IR exposure and response and the molecular mechanisms that regulate their expression or synthesis.

CRITICAL ISSUES

In this review, chemical mechanisms are described for IR-induced modifications of biomolecules along with methods for their detection. Included with the detection methods are crucial experimental considerations and caveats for their use. Additional factors critical to the cellular response to radiation, including alterations in protein expression, metabolomics, and epigenetic factors, are also discussed.

FUTURE DIRECTIONS

Throughout the review, the synergy of combined "omics" technologies such as genomics and epigenomics, proteomics, and metabolomics is highlighted. These are anticipated to lead to new hypotheses to understand IR effects on biological systems and improve IR-based therapies.

Over-expression of methionine sulfoxide reductase A in the endoplasmic reticulum increases resistance to oxidative and ER stresses

MsrA and MsrB catalyze the reduction of methionine-S-sulfoxide and methionine-R-sulfoxide, respectively, to methionine in different cellular compartments of mammalian cells. One of the three MsrBs, MsrB3, is an endoplasmic reticulum (ER)-type enzyme critical for stress resistance including oxidative and ER stresses. However, there is no evidence for the presence of an ER-type MsrA or the ER localization of MsrA. In this work, we developed an ER-targeted recombinant MsrA construct and investigated the potential effects of methionine-S-sulfoxide reduction in the ER on stress resistance. The ER-targeted MsrA construct contained the N-terminal ER-targeting signal peptide of human MsrB3A (MSPRRSLPRPLSLCLSLCLCLCLAAALGSAQ) and the C-terminal ER-retention signal sequence (KAEL). The over-expression of ER-targeted MsrA significantly increased cellular resistance to H2O2-induced oxidative stress. The ER-targeted MsrA over-expression also significantly enhanced resistance to dithiothreitol-induced ER stress; however, it had no positive effects on the resistance to ER stresses induced by tunicamycin and thapsigargin. Collectively, our data suggest that methionine-S-sulfoxide reduction in the ER compartment plays a protective role against oxidative and ER stresses.

The physiological role of reversible methionine oxidation

Sulfur-containing amino acids such as cysteine and methionine are particularly vulnerable to oxidation. Oxidation of cysteine and methionine in their free amino acid form renders them unavailable for metabolic processes while their oxidation in the protein-bound state is a common post-translational modification in all organisms and usually alters the function of the protein. In the majority of cases, oxidation causes inactivation of proteins. Yet, an increasing number of examples have been described where reversible cysteine oxidation is part of a sophisticated mechanism to control protein function based on the redox state of the protein. While for methionine the dogma is still that its oxidation inhibits protein function, reversible methionine oxidation is now being recognized as a powerful means of triggering protein activity. This mode of regulation involves oxidation of methionine to methionine sulfoxide leading to activated protein function, and inactivation is accomplished by reduction of methionine sulfoxide back to methionine catalyzed by methionine sulfoxide reductases. Given the similarity to thiol-based redox-regulation of protein function, methionine oxidation is now established as a novel mode of redox-regulation of protein function. This article is part of a Special Issue entitled: Thiol-Based Redox Processes.

SelR reverses Mical-mediated oxidation of actin to regulate F-actin dynamics

Actin's polymerization properties are markedly altered by oxidation of its conserved Met 44 residue. Mediating this effect is a specific oxidation-reduction (redox) enzyme, Mical, that works with Semaphorin repulsive guidance cues and selectively oxidizes Met 44. We now find that this actin-regulatory process is reversible. Employing a genetic approach, we identified a specific methionine sulfoxide reductase (MsrB) enzyme SelR that opposes Mical redox activity and Semaphorin-Plexin repulsion to direct multiple actin-dependent cellular behaviours in vivo. SelR specifically catalyses the reduction of the R isomer of methionine sulfoxide (methionine-R-sulfoxide) to methionine, and we found that SelR directly reduced Mical-oxidized actin, restoring its normal polymerization properties. These results indicate that Mical oxidizes actin stereospecifically to generate actin Met-44-R-sulfoxide (actin(Met(R)O-44)), and also implicate the interconversion of specific Met/Met(R)O residues as a precise means to modulate protein function. Our results therefore uncover a specific reversible redox actin regulatory system that controls cell and developmental biology.

Competitive cobalt for zinc substitution in mammalian methionine sulfoxide reductase B1 overexpressed in E. coli: structural and functional insight

Expression of the mammalian enzyme methionine sulfoxide reductase B1 (MsrB1) in Escherichia coli growing in cobalt-containing media resulted in the reproducible appearance of the stable cobalt-containing protein MsrB1-Co. NMR studies and biocomputing using the programs AnisoFit and Amber allowed us to generate a structure of MsrB1-Co sharing the overall fold with the native zinc-containing protein MsrB1-Zn. Our data suggest that the N-terminus containing resolving cysteine tends to be closer to the protein’s catalytic center than was previously reported. It is argued that this proximity supports the proposed catalytic mechanism and ensures high catalytic efficiency of MsrB1. Functional studies showed that both MsrB1-Zn and MsrB1-Co exhibit similar levels of activity, in agreement with the structural studies performed. The proposed metal ion substitution approach may have a methodological significance in determining whether methionine sulfoxide reductase B proteins contain a metal ion.

Cloning, expression, and characterization of a methionine sulfoxide reductase B gene from Nicotiana tabacum

Reactive oxygen species (ROS) are generated during normal aerobic metabolism and in plants exposed to environmental stress. Methionine (Met) residues are particularly sensitive to ROS-mediated oxidation, leading to the formation of methionine sulfoxide (MetSO) under mild oxidative conditions. Methionine sulfoxide reductase (MSR) repairs oxidized Met and protects plants from oxidative damage. Herein, we identified a tobacco (Nicotiana tabacum) MSRB gene, referred to as NtMSRB3. Analysis of the sequence showed that the NtMSRB3 protein had four typical motifs in a SelR domain, which is known as the catalytic region of MSRBs. NtMSRB3 specifically reduced the R epimer of MetSO and converted either free MetSO or Dabsyl-MetSO in the presence of dithiothreitol. Escherichia coli cells harboring NtMSRB3 displayed relative high viability under H₂O₂ stress. Subcellular localization of NtMSRB3 revealed that it was a plastid-targeted protein. Furthermore, the semi-quantitative reverse transcription polymerase chain reaction assay showed that NtMSRB3 was upregulated apparently by abscisic acid, salt, cold, and methyl viologen stress within 24 h of treatment.

The methionine sulfoxide reduction system: selenium utilization and methionine sulfoxide reductase enzymes and their functions

SIGNIFICANCE

Selenium is utilized in the methionine sulfoxide reduction system that occurs in most organisms. Methionine sulfoxide reductases (Msrs), MsrA and MsrB, are the enzymes responsible for this system. Msrs repair oxidatively damaged proteins, protect against oxidative stress, and regulate protein function, and have also been implicated in the aging process. Selenoprotein forms of Msrs containing selenocysteine (Sec) at the catalytic site are found in bacteria, algae, and animals.

RECENT ADVANCES

A selenoprotein MsrB1 knockout mouse has been developed. Significant progress in the biochemistry of Msrs has been made, which includes findings of a novel reducing system for Msrs and of an interesting reason for the use of Sec in the Msr system. The effects of mammalian MsrBs, including selenoprotein MsrB1 on fruit fly aging, have been investigated. Furthermore, it is evident that Msrs are involved in methionine metabolism and regulation of the trans-sulfuration pathway.

CRITICAL ISSUES

This article presents recent progress in the Msr field while focusing on the physiological roles of mammalian Msrs, functions of selenoprotein forms of Msrs, and their biochemistry.

FUTURE DIRECTIONS

A deeper understanding of the roles of Msrs in redox signaling, the aging process, and metabolism will be achieved. The identity of selenoproteome of Msrs will be sought along with characterization of the identified selenoprotein forms. Exploring new cellular targets and new functions of Msrs is also warranted.

A second human methionine sulfoxide reductase (hMSRB2) reducing methionine-R-sulfoxide displays a tissue expression pattern distinct from hMSRB1

Peptide methionine sulfoxide reductases are important enzymes in the defense against cellular oxidative stress as they reduce methionine sulfoxide, the product of methionine oxidation by physiologically relevant reactive oxygen species. Two distinct enzyme classes, MSRA and MSRB, have evolved for selectively reducing the two epimers, methionine-S-sulfoxide and methionine-R-sulfoxide. A new human MSR enzyme (hMSRB2) specifically reducing methionine-R-sulfoxide, which showed a conversion rate for peptide-bound methionine-S-sulfoxide similar to hMSRB1, was characterized with respect to its tissue expression. As previously found for hMSRB1, expression of hMSRB2 mRNA was weak in brain, but strong in heart and skeletal muscle. In contrast to hMSRB1, its expression was high in smooth muscle-containing organs (digestive system, bladder), lung and aorta, while hMSRB1 displayed a higher expression than hMSRB2 in liver and kidney.

Diversity of plant methionine sulfoxide reductases B and evolution of a form specific for free methionine sulfoxide

Methionine can be reversibly oxidized to methionine sulfoxide (MetO) under physiological conditions. Organisms evolved two distinct methionine sulfoxide reductase families (MSRA & MSRB) to repair oxidized methionine residues. We found that 5 MSRB genes exist in the soybean genome, including GmMSRB1 and two segmentally duplicated gene pairs (GmMSRB2 and GmMSRB5, GmMSRB3 and GmMSRB4). GmMSRB2 and GmMSRB4 proteins showed MSRB activity toward protein-based MetO with either DTT or thioredoxin (TRX) as reductants, whereas GmMSRB1 was active only with DTT. GmMSRB2 had a typical MSRB mechanism with Cys121 and Cys 68 as catalytic and resolving residues, respectively. Surprisingly, this enzyme also possessed the MSRB activity toward free Met-R-O with kinetic parameters similar to those reported for fRMSR from Escherichia coli, an enzyme specific for free Met-R-O. Overexpression of GmMSRB2 or GmMSRB4 in the yeast cytosol supported the growth of the triple MSRA/MSRB/fRMSR (Δ3MSRs) mutant on MetO and protected cells against H2O2-induced stress. Taken together, our data reveal an unexpected diversity of MSRBs in plants and indicate that, in contrast to mammals that cannot reduce free Met-R-O and microorganisms that use fRMSR for this purpose, plants evolved MSRBs for the reduction of both free and protein-based MetO.

Recharging oxidative protein repair: catalysis by methionine sulfoxide reductases towards their amino acid, protein, and model substrates

The sulfur-containing amino acid methionine (Met) in its free and amino acid residue forms can be readily oxidized to the R and S diastereomers of methionine sulfoxide (MetO). Methionine sulfoxide reductases A (MSRA) and B (MSRB) reduce MetO back to Met in a stereospecific manner, acting on the S and R forms, respectively. A third MSR type, fRMSR, reduces the R form of free MetO. MSRA and MSRB are spread across the three domains of life, whereas fRMSR is restricted to bacteria and unicellular eukaryotes. These enzymes protect against abiotic and biotic stresses and regulate lifespan. MSRs are thiol oxidoreductases containing catalytic redox-active cysteine or selenocysteine residues, which become oxidized by the substrate, requiring regeneration for the next catalytic cycle. These enzymes can be classified according to the number of redox-active cysteines (selenocysteines) and the strategies to regenerate their active forms by thioredoxin and glutaredoxin systems. For each MSR type, we review catalytic parameters for the reduction of free MetO, low molecular weight MetO-containing compounds, and oxidized proteins. Analysis of these data reinforces the concept that MSRAs reduce various types of MetO-containing substrates with similar efficiency, whereas MSRBs are specialized for the reduction of MetO in proteins.

Involvement of thioredoxin y2 in the preservation of leaf methionine sulfoxide reductase capacity and growth under high light

Methionine (Met) in proteins can be oxidized to two diastereoisomers of methionine sulfoxide, Met-S-O and Met-R-O, which are reduced back to Met by two types of methionine sulfoxide reductases (MSRs), A and B, respectively. MSRs are generally supplied with reducing power by thioredoxins. Plants are characterized by a large number of thioredoxin isoforms, but those providing electrons to MSRs in vivo are not known. Three MSR isoforms, MSRA4, MSRB1 and MSRB2, are present in Arabidopsis thaliana chloroplasts. Under conditions of high light and long photoperiod, plants knockdown for each plastidial MSR type or for both display reduced growth. In contrast, overexpression of plastidial MSRBs is not associated with beneficial effects in terms of growth under high light. To identify the physiological reductants for plastidial MSRs, we analyzed a series of mutants deficient for thioredoxins f, m, x or y. We show that mutant lines lacking both thioredoxins y1 and y2 or only thioredoxin y2 specifically display a significantly reduced leaf MSR capacity (-25%) and growth characteristics under high light, related to those of plants lacking plastidial MSRs. We propose that thioredoxin y2 plays a physiological function in protein repair mechanisms as an electron donor to plastidial MSRs in photosynthetic organs.

Methionine sulfoxide reduction in ciliates: characterization of the ready-to-use methionine sulfoxide-R-reductase genes in Euplotes

Genes encoding the enzyme methionine sulfoxide reductase type B, specific to the reduction of the oxidized methionine-R form, were characterized from the expressed (macronuclear) genome of two ecologically separate marine species of Euplotes, i.e. temperate water E. raikovi and polar water E. nobilii. Both species were found to contain a single msrB gene with a very simple structural organization encoding a protein of 127 (E. raikovi) or 126 (E. nobilii) amino acid residues that belongs to the group of zinc-containing enzymes. Both msrB genes are constitutively expressed, suggesting that the MsrB enzyme plays an essential role in repairing oxidative damages that appear to be primarily caused by physiological cell aging in E. raikovi and by interactions with an O(2) saturated environment in E. nobilii.

Analyses of methionine sulfoxide reductase activities towards free and peptidyl methionine sulfoxides

There have been insufficient kinetic data that enable a direct comparison between free and peptide methionine sulfoxide reductase activities of either MsrB or MsrA. In this study, we determined the kinetic parameters of mammalian and yeast MsrBs and MsrAs for the reduction of both free methionine sulfoxide (Met-O) and peptidyl Met-O under the same assay conditions. Catalytic efficiency of mammalian and yeast MsrBs towards free Met-O was >2000-fold lower than that of yeast fRMsr, which is specific for free Met-R-O. The ratio of free to peptide Msr activity in MsrBs was 1:20-40. In contrast, mammalian and yeast MsrAs reduced free Met-O much more efficiently than MsrBs. Their k(cat) values were 40-500-fold greater than those of the corresponding MsrBs. The ratio of free to peptide Msr activity was 1:0.8 in yeast MsrA, indicating that this enzyme can reduce free Met-O as efficiently as peptidyl Met-O. In addition, we analyzed the in vivo free Msr activities of MsrBs and MsrAs in yeast cells using a growth complementation assay. Mammalian and yeast MsrBs, as well as the corresponding MsrAs, had apparent in vivo free Msr activities. The in vivo free Msr activities of MsrBs and MsrAs agreed with their in vitro activities.

Glutaredoxin serves as a reductant for methionine sulfoxide reductases with or without resolving cysteine

Methionine sulfoxide reductases A and B (MsrA and MsrB) have been known to be thioredoxin (Trx)-dependent enzymes that catalyze the reduction of methionine sulfoxide in a stereospecific manner. This work reports that glutaredoxin, another major thiol-disulfide oxidoreductase, can serve as a reductant for both MsrA and MsrB. Glutaredoxins efficiently reduced 1-Cys MsrA lacking a resolving Cys, which is not reducible by Trx. Glutaredoxins also reduced 3-Cys MsrA containing two resolving Cys. The glutaredoxin-dependent activity of the 3-Cys MsrA was comparable with the Trx-dependent activity. The kinetic data suggest that 1-Cys MsrA is more efficiently reduced by glutaredoxin than 3-Cys form. Also, glutaredoxins could function as a reductant for 1-Cys MsrB lacking a resolving Cys as previously reported. In contrast to the previous report, 2-Cys MsrB containing a resolving Cys was reducible by the glutaredoxins. Collectively, this study demonstrates that glutaredoxins reduce MsrAs and MsrBs with or without resolving Cys.