Two types of methionine (Met) sulfoxide reductases (Msr) catalyze the reduction of Met sulfoxide (MetSO) back to Met. MsrA, well characterized in plants, exhibits an activity restricted to the Met-S-SO-enantiomer. Recently, a new type of Msr enzyme, called MsrB, has been identified in various organisms and shown to catalytically reduce the R-enantiomer of MetSO. In plants, very little information is available about MsrB and we focused our attention on Arabidopsis (Arabidopsis thaliana) MsrB proteins. Searching Arabidopsis genome databases, we have identified nine open reading frames encoding proteins closely related to MsrB proteins from bacteria and animal cells. We then analyzed the activity and abundance of the two chloroplastic MsrB proteins, MsrB1 and MsrB2. Both enzymes exhibit an absolute R-stereospecificity for MetSO and a higher catalytic efficiency when using protein-bound MetSO as a substrate than when using free MetSO. Interestingly, we observed that MsrB2 is reduced by thioredoxin, whereas MsrB1 is not. This feature of MsrB1 could result from the lack of the catalytical cysteine (Cys) corresponding to Cys-63 in Escherichia coli MsrB that is involved in the regeneration of Cys-117 through the formation of an intramolecular disulfide bridge followed by thioredoxin reduction. We investigated the abundance of plastidial MsrA and B in response to abiotic (water stress, photooxidative treatment) and biotic (rust fungus) stresses and we observed that MsrA and B protein levels increase in response to the photooxidative treatment. The possible role of plastidic MsrB in the tolerance to oxidative damage is discussed.Oxygen is essential to all aerobic organisms but can also lead to many harmful effects (Davies, 1995). Proteins are easily damaged by reactive oxygen species, with Met being one of the amino acid residues most susceptible to oxidation (Dann and Pell, 1989). The generation of Met sulfoxide (MetSO) is mediated by various biological oxidants such as hydrogen peroxide, hydroxyl radicals, ozone as well as by metals, and results in modifications of activity and conformation for many proteins (Gao et al., 1998;Davis et al., 2000). Oxidation of Met residues is readily reversed by the action of an enzyme initially referred as peptide MetSO reductase (PMSR), which catalyzes the thioredoxin-dependent reduction of MetSO back to Met (Brot et al., 1981). The enzyme is present in most living organisms and has been described as belonging to the minimal set of proteins sufficient for cell life (Mushegian and Koonin, 1996). PMSR has a protective role against oxidative damage (Moskovitz et al., 1997).Indeed, pmsr null mutants of Escherichia coli and Saccharomyces cerevisiae show a decreased resistance toward oxidative stress conditions (Moskovitz et al., 1995(Moskovitz et al., , 1997. Accordingly, PMSR overexpression in S. cerevisiae or in human T-cells results in an increased resistance to oxidative treatments (Moskovitz et al., 1998). The first PMSR has been renamed Met sulfoxide reductase A (MsrA) and has been sh...
Methionine oxidation to methionine sulfoxide (MetSo), which results in modification of activity and conformation for many proteins, is reversed by an enzyme present in most organisms and termed as methionine sulfoxide reductase (MSR). On the basis of substrate stereospecificity, two types of MSR, A and B, that do not share any sequence similarity, have been identified. In the present review, we first compare the multigenic MSR families in the three plant species for which the genome is fully sequenced: Arabidopsis thaliana, Oryza sativa, and Populus trichocarpa. The MSR gene content is larger in A. thaliana (five MSRAs and nine MSRBs) compared to P. trichocarpa (five MSRAs and four MSRBs) and O. sativa (four MSRAs and three MSRBs). A complete classification based on gene structure, sequence identity, position of conserved reactive cysteines and predicted subcellular localization is proposed. On the basis of in silico and experimental data originating mainly from Arabidopsis, we report that some MSR genes display organ-specific expression patterns and that those encoding plastidic MSRs are highly expressed in photosynthetic organs. We also show that the expression of numerous MSR genes is enhanced by environmental conditions known to generate oxidative stress. Thioredoxins (TRXs) constitute very likely physiological electron donors to plant MSR proteins for the catalysis of MetSO reduction, but the specificity between the numerous TRXs and methionine sulfoxide reductases (MSRs) present in plants remains to be investigated. The essential role of plant MSRs in protection against oxidative damage has been recently demonstrated on transgenic Arabidopsis plants modified in the content of cytosolic or plastidic MSRA.
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