Characterization of methionine oxidation and methionine sulfoxide reduction using methionine-rich cysteine-free proteins

Background: Methionine (Met) residues in proteins can be readily oxidized by reactive oxygen species to Met sulfoxide (MetO). MetO is a promising physiological marker of oxidative stress and its inefficient repair by MetO reductases (Msrs) has been linked to neurodegeneration and aging. Conventional methods of assaying MetO formation and reduction rely on chromatographic or mass spectrometry procedures, but the use of Met-rich proteins (MRPs) may offer a more streamlined alternative. Results: We carried out a computational search of completely sequenced genomes for MRPs deficient in cysteine (Cys) residues and identified several proteins containing 20% or more Met residues. We used these MRPs to examine Met oxidation and MetO reduction by in-gel shift assays and immunoblot assays with antibodies generated against various oxidized MRPs. The oxidation of Cys-free MRPs by hydrogen peroxide could be conveniently monitored by SDS-PAGE and was specific for Met, as evidenced by quantitative reduction of these proteins with Msrs in DTT- and thioredoxin-dependent assays. We found that hypochlorite was especially efficient in oxidizing MRPs. Finally, we further developed a procedure wherein antibodies made against oxidized MRPs were isolated on affinity resins containing same or other oxidized or reduced MRPs. This procedure yielded reagents specific for MetO in these proteins, but proved to be ineffective in developing antibodies with broad MetO specificity. Conclusion: Our data show that MRPs provide a convenient tool for characterization of Met oxidation, MetO reduction and Msr activities, and could be used for various aspects of redox biology involving reversible Met oxidation

Supporting Information for: A highly efficient form of the selenocysteine insertion sequence element in protozoan parasites and its use in mammalian cells

Selenoproteins are an elite group of proteins containing a rare amino acid, selenocysteine (Sec), encoded by the codon, UGA. In eukaryotes, incorporation of Sec requires a Sec insertion sequence (SECIS) element, a stem–loop structure located in the 3\u27-untranslated regions of selenoprotein mRNAs. Here we report identification of a noncanonical form of SECIS element in Toxoplasma gondii and Neospora canine, single-celled apicomplexan parasites of humans and domestic animals. This SECIS has a GGGA sequence in the SBP2-binding site in place of AUGA previously considered invariant. Using a combination of computational and molecular techniques, we show that Toxoplasma and Neospora possess both canonical and noncanonical SECIS elements. The GGGA-type SECIS element supported Sec insertion in mammalian HEK 293 and NIH 3T3 cells and did so more efficiently than the natural mammalian SECIS elements tested. In addition, mammalian type I and type II SECIS elements mutated into the GGGA forms were functional but manifested decreased Sec insertion efficiency. We carried out computational searches for both AUGA and GGGA forms of SECIS elements in Toxoplasma and detected five selenoprotein genes, including one coding for a previously undescribed selenoprotein, designated SelQ, and two containing the GGGA form of the SECIS element. In contrast, the GGGA-type SECIS elements were not detected in mammals and nematodes. As a practical outcome of the study, we developed pSelExpress1, a vector for convenient expression of selenoproteins in mammalian cells. It contains an SBP2 gene and the most efficient tested SECIS element: an AUGA mutant of the GGGA-type Toxoplasma SelT structure

Comparative Analysis of Selenocysteine Machinery and Selenoproteome Gene Expression in Mouse Brain Identifies Neurons as Key Functional Sites of Selenium in Mammals

Although dietary selenium (Se) deficiency results in phenotypes associated with selenoprotein depletion in various organs, the brain is protected from Se loss. To address the basis for the critical role of Se in brain function, we carried out comparative gene expression analyses for the complete selenoproteome and associated biosynthetic factors. Using the Allen Brain Atlas, we evaluated 159 regions of adult mouse brain and provided experimental analyses of selected selenoproteins. All 24 selenoprotein mRNAs were expressed in the mouse brain. Most strikingly, neurons in olfactory bulb, hippocampus, cerebral cortex, and cerebellar cortex were exceptionally rich in selenoprotein gene expression, in particular in GPx4, SelK, SelM, SelW, and Sep15. Over half of the selenoprotein genes were also expressed in the choroid plexus. A unique expression pattern was observed for one of the highly expressed selenoprotein genes, SelP, which we suggest to provide neurons with Se. Cluster analysis of the expression data linked certain selenoproteins and selenocysteine machinery genes and suggested functional linkages among selenoproteins, such as that between SelM and Sep15. Overall, this study suggests that the main functions of selenium in mammals are confined to certain neurons in the brain

Comparative Analysis of Selenocysteine Machinery and Selenoproteome Gene Expression in Mouse Brain Identifies Neurons as Key Functional Sites of Selenium in Mammals

Although dietary selenium (Se) deficiency results in phenotypes associated with selenoprotein depletion in various organs, the brain is protected from Se loss. To address the basis for the critical role of Se in brain function, we carried out comparative gene expression analyses for the complete selenoproteome and associated biosynthetic factors. Using the Allen Brain Atlas, we evaluated 159 regions of adult mouse brain and provided experimental analyses of selected selenoproteins. All 24 selenoprotein mRNAs were expressed in the mouse brain. Most strikingly, neurons in olfactory bulb, hippocampus, cerebral cortex, and cerebellar cortex were exceptionally rich in selenoprotein gene expression, in particular in GPx4, SelK, SelM, SelW, and Sep15. Over half of the selenoprotein genes were also expressed in the choroid plexus. A unique expression pattern was observed for one of the highly expressed selenoprotein genes, SelP, which we suggest to provide neurons with Se. Cluster analysis of the expression data linked certain selenoproteins and selenocysteine machinery genes and suggested functional linkages among selenoproteins, such as that between SelM and Sep15. Overall, this study suggests that the main functions of selenium in mammals are confined to certain neurons in the brain

Diversity of Protein and mRNA Forms of Mammalian Methionine Sulfoxide Reductase B1 Due to Intronization and Protein Processing

Background: Methionine sulfoxide reductases (Msrs) are repair enzymes that protect proteins from oxidative stress by catalyzing stereospecific reduction of oxidized methionine residues. MsrB1 is a selenocysteine-containing cytosolic/nuclear Msr with high expression in liver and kidney. Principal Findings: Here, we identified differences in MsrB1 gene structure among mammals. Human MsrB1 gene consists of four, whereas the corresponding mouse gene of five exons, due to occurrence of an additional intron that flanks the stop signal and covers a large part of the 3′-UTR. This intron evolved in a subset of rodents through intronization of exonic sequences, whereas the human gene structure represents the ancestral form. In mice, both splice forms were detected in liver, kidney, brain and heart with the five-exon form being the major form. We found that both mRNA forms were translated and supported efficient selenocysteine insertion into MsrB1. In addition, MsrB1 occurs in two protein forms that migrate as 14 and 5 kDa proteins. We found that each mRNA splice form generated both protein forms. The abundance of the 5 kDa form was not influenced by protease inhibitors, replacement of selenocysteine in the active site or mutation of amino acids in the cleavage site. However, mutation of cysteines that coordinate a structural zinc decreased the levels of 5 and 14 kDa forms, suggesting importance of protein structure for biosynthesis and/stability of these forms. Conclusions: This study characterized unexpected diversity of protein and mRNA forms of mammalian selenoprotein MsrB1

A highly efficient form of the selenocysteine insertion sequence element in protozoan parasites and its use in mammalian cells

Selenoproteins are an elite group of proteins containing a rare amino acid, selenocysteine (Sec), encoded by the codon, UGA. In eukaryotes, incorporation of Sec requires a Sec insertion sequence (SECIS) element, a stem–loop structure located in the 3’-untranslated regions of selenoprotein mRNAs. Here we report identification of a noncanonical form of SECIS element in Toxoplasma gondii and Neospora canine, single-celled apicomplexan parasites of humans and domestic animals. This SECIS has a GGGA sequence in the SBP2-binding site in place of AUGA previously considered invariant. Using a combination of computational and molecular techniques, we show that Toxoplasma and Neospora possess both canonical and noncanonical SECIS elements. The GGGA-type SECIS element supported Sec insertion in mammalian HEK 293 and NIH 3T3 cells and did so more efficiently than the natural mammalian SECIS elements tested. In addition, mammalian type I and type II SECIS elements mutated into the GGGA forms were functional but manifested decreased Sec insertion efficiency. We carried out computational searches for both AUGA and GGGA forms of SECIS elements in Toxoplasma and detected five selenoprotein genes, including one coding for a previously undescribed selenoprotein, designated SelQ, and two containing the GGGA form of the SECIS element. In contrast, the GGGA-type SECIS elements were not detected in mammals and nematodes. As a practical outcome of the study, we developed pSelExpress1, a vector for convenient expression of selenoproteins in mammalian cells. It contains an SBP2 gene and the most efficient tested SECIS element: an AUGA mutant of the GGGA-type Toxoplasma SelT structure

Analysis of recombinant MsrB1.

<p>Top panel: Cys mutant of MsrB1 (MsrB1-Cys) (lane 2) and its form with the engineered factor Xa protease cleavage site (MsrB1-Cys-Xa) (lane 3) were analyzed by SDS-PAGE followed by Coomassie Blue staining. MsrB1-Cys-Xa protein was digested with factor Xa and the digestion products (lane 4) were subjected to SDS-PAGE analysis. Molecular weight markers are shown in lane 1. Arrows indicate migration of the uncleaved (full length) protein and cleaved protein fragments (cleaved form). Bottom panel: Sequence of MsrB1-Cys-Xa. The engineered factor Xa site is shown in blue, U95C in orange, and the factor Xa cleavage site is shown by “∧”.</p

Immunoblot analysis of MsrB2 in mouse tissues.

<p>MsrB2 expression was analyzed in C57BL/6 mouse kidney (K), liver (L), heart (H), and brain (B) by western blot using antibodies specific for mouse MsrB2. Migration of MsrB2 is shown by an arrow, molecular weight markers are shown on the right, and the control for protein loading (β-Actin) is also shown.</p

Analysis of MsrB1 mRNA forms in mice.

<p>MsrB1 mRNA expression in indicated organs of two C57BL/6 mice was analyzed by real-time PCR. (A) Agarose gel electrophoresis analysis of MsrB1 PCR products. Real-time PCR products derived from mouse MsrB1 mRNA (Forms 1 and 2) and MsrB1 Form 2 mRNA were subjected to 2% agarose gel electrophoresis. L, liver; K, kidney; H, heart; B, brain; M, molecular markers, The size of the band running fastest is 100 bp, and that of the adjacent band is 200 bp. (B) MsrB1 mRNA expression. Expression levels of total MsrB1 mRNA (Forms 1 and 2) and Form 2 mRNA were normalized to the levels of GAPDH mRNA, and the value for Form 2 in the brain was set as 1. The data represent means±SD. (B) Relative expression of MsrB1 Form 2 mRNA. Expression of MsrB1 Form 2 mRNA was normalized to total MsrB1 mRNA.</p