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Previous Article | Next Article 
Molecular and Cellular Biology, March 2006, p. 1795-1805, Vol. 26, No. 5
0270-7306/06/$08.00+0 doi:10.1128/MCB.26.5.1795-1805.2006
Copyright © 2006, American Society for Microbiology. All Rights Reserved.
,
Peter R. Hoffmann,1,
Tanya Michaud,1,¶
Erin P. Forry,1
Andrea Small-Howard,1
Robert J. Stillwell,1
Nadya Morozova,2,||
John W. Harney,2 and
Marla J. Berry1*
Department of Cell and Molecular Biology, University of Hawaii at Manoa, Honolulu, Hawaii 96822,1 Thyroid Division, Brigham and Women's Hospital and Harvard Medical School, Boston, Massachusetts 021152
Received 28 June 2005/ Returned for modification 27 July 2005/ Accepted 18 November 2005
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ABSTRACT |
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INTRODUCTION |
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NMD or mRNA surveillance targets mRNAs containing premature termination codons for degradation, ensuring that they do not produce prematurely terminated polypeptides. In higher eukaryotes, a nonsense codon is usually recognized as premature if it is located more than 50 to 55 nucleotides upstream of the last intron in the pre-mRNA. Sensitivity to NMD is conferred by the deposition of an exon-junction complex upstream of exon-exon boundaries during mRNA splicing and export (23). Immunity to NMD is thought to be acquired upon removal of these proteins during the first round of translation. However, if an mRNA escapes NMD during the first round of translation, it becomes immune to subsequent decay via this pathway. While the protein synthesis machinery is thought to function primarily in the cytoplasm, mammalian NMD has been shown, in most cases, to occur on mRNAs that cofractionate or copurify with nuclei (10); this cofractionation or copurification is likely due to the association of mRNAs with the nuclear pore complex during export and concurrent initiation of the first round of translation.
For selenoprotein mRNAs to avoid NMD, it follows that the complexes necessary for decoding should assemble on selenoprotein mRNAs in the nucleus or during export. In this sense, shuttling events that determine subcellular localization of the components of the decoding complex may be as important for selenoprotein synthesis as the presence of selenium or the expression of these essential components.
To begin to investigate this possibility, we have undertaken a study of the subcellular localization of the selenoprotein synthesis factors in several cell lines. We have identified putative nuclear localization signals (NLSs) and nuclear export signals (NESs) in EFsec and putative NESs, NLSs, and a potential nucleolar localization signal (NoLS) in SBP2 and have shown that these signals are functional. We also demonstrate that the minimal functional domain of SBP2 undergoes nucleocytoplasmic shuttling and provide evidence that nuclear SBP2 may contribute to nuclear retention of EFsec. The implications of these findings for translational efficiency and NMD of selenoprotein mRNAs are discussed.
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MATERIALS AND METHODS |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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X1-4X2-3LX, where represents any hydrophobic residue and X represents any residue (16). Primary antibodies included mouse monoclonal or rabbit polyclonal anti-FLAG (Sigma-Aldrich, St. Louis, MO), rabbit polyclonal anti-HA (Santa Cruz Biotechnology, Santa Cruz, CA), mouse monoclonal anti-V5 (Invitrogen), and rabbit polyclonal anti-SBP2 (generous gift of Paul Copeland). All were titrated against isotype-control antibody or rabbit preimmune sera, found to be optimal in sensitivity and specificity at a final concentration of 1:500, and used at this dilution in subsequent experiments. Secondary anti-mouse or anti-rabbit antibodies included rhodamine, Texas Red, fluorescein isothiocyanate (FITC), and Alexa 488- and Alexa 586-conjugated (Molecular Probes, Eugene, OR) antibodies used at a final concentration of 1:500. In some experiments, cells were counterstained with DAPI (4',6'-diamidino-2-phenylindole) (Molecular Probes) at 1:1,500. For some immunofluorescence assays, rhodamine-phalloidin (Molecular Probes) was used to visualize filamentous actin.
Cell culture, transfections, and subcellular fractionation. HEK-293 human embryonic kidney, HepG2 human hepatoma, and NIH 3T3 murine fibroblast cells were cultured in Dulbecco's modified Eagle's medium (GIBCO-Invitrogen)-high glucose medium with 10% fetal bovine serum (Sigma or Invitrogen) at 37°C and 5% CO2. MSTO-211H human mesothelioma cells were cultured in RPMI 1640 (GIBCO-Invitrogen) supplemented with 10% fetal bovine serum and incubated at 37°C and 5% CO2. Cells were plated on glass coverslips, and at 18 to 24 h, transfections were performed using either a standard calcium phosphate method (8) or commercially available FuGene 6 (Roche) or TransIT-LT1 reagent (Mirus Bio Corporation, Madison, WI) according to the manufacturer's instructions. All transfections involving pUHD10-3 also included pUHD15 as transcriptional activator as previously described (22). Medium was supplemented with 100 nM sodium selenite 24 h after transfection. Subcellular fractionation was carried out by lysing cell pellets in 50 mM HEPES, pH 7.5, 75 mM NaCl, 40 mM NaF, 10 mM iodoacetamide, 0.25% Triton X-100, and 0.5 mM phenylmethylsulfonyl fluoride, pelleting and washing nuclei, and lysing nuclei in the same buffer containing 350 mM NaCl. This was followed by sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis and Western blotting with the indicated antibodies. Western blot analyses with antibodies to cytoplasmic (GRB2) and nuclear (histone H1) markers were carried out to assess the cross-contamination of subcellular fractions. Labeling of endogenous selenoproteins with sodium [75Se]selenite was carried out as described previously (35).
Immunofluorescence microscopy. Immunofluorescence assays were performed 48 to 72 h after transfection as described previously (1), with modifications. Cells on slides were mounted in mounting medium with or without DAPI (Vector Laboratories, Burlingame, CA). Epifluorescence was performed using an Olympus IX71 microscope with an Olympus U-CMAD3 camera and MicroFire 1.0 software (Optronics, Goleta, CA). Confocal microscopy was performed in a Bio-Rad MRC-1024/2P system interfaced with a Zeiss Axiovert microscope.
Heterokaryon experiments. Heterokaryon experiments were carried out as described previously (29), with modifications. Briefly, human MSTO-211H cells were plated and transfected as described above. After 48 h, murine NIH-3T3 cells were added at a ratio of approximately 2:1 and allowed to adhere to MSTO-211H cells for 3 h at 37°C in the presence of 50 µg/ml cycloheximide (Sigma-Aldrich), followed by 100 µg/ml for 30 min to inhibit protein synthesis. Cell fusion was then carried out by removing the medium and adding 50% (wt/vol) polyethylene glycol for 5 min at 37°C. The fused cells were then washed five times with phosphate-buffered saline, covered in medium containing 100 µg/ml cycloheximide, and incubated at 37°C for 3 h. Cells were then washed, fixed, permeabilized, and stained as described above.
Selenoprotein mRNA quantitation. For comparison of the selenoprotein mRNA levels in HepG and HEK-293 cells, two 60-nucleotide-long oligonucleotide probes (37 to 60% GC) were designed for each gene, with positioning as close as possible to the 3' end. Sequences were analyzed for the absence of secondary structure and cross-hybridization elsewhere in the genome. Probes were spotted onto GeneScreen Plus nylon membranes by using a V&P Scientific (San Diego, CA) 1,536-pin replicator and immobilized by alkali treatment. Total RNA (2 µg) was labeled via oligoT-directed first-strand cDNA synthesis using 400 U of murine leukemia virus reverse transcriptase (Invitrogen, Carlsbad, CA) and alpha-33P-dCTP (40 µCi). cDNA was purified using QIAquick PCR columns (QIAGEN, Valencia, CA), heat denatured, and hybridized in triplicate to arrays in MicroHyb buffer (Research Genetics, Huntsville, AL) overnight at 60°C. Arrays were washed with 2x SSC (1x SSC is 0.15 M NaCl plus 0.015 M sodium citrate) and 0.5% SDS at 50°C, followed by 1 to 2x SSC and 0.5% SDS at 65°C, and then exposed to phosphor storage screens and signals quantified using a Molecular Dynamics PhosphorImager (Sunnyvale, CA). Signal readings were taken for each spot, and background readings were taken at empty spots. Raw data was further automatically processed using Microsoft Excel. Spot readings that failed to exceed the average background value by more than 3 standard deviations were excluded from the analysis. The remaining readings were scaled by the average signal in selected steadily expressed genes and then averaged among triplicate measurements. For comparison of GPX1 and selenoprotein P mRNAs in the presence versus the absence of SBP2 transfection, quantitative real-time PCR was performed on a LightCycler 2 (Roche) by using cDNA prepared from HEK-293 cells transfected with an either full-length or truncated version of SBP2 or empty pcDNA DEST40 vector. Student's unpaired t test was used to compare means of groups by using GraphPad Prism4 (San Diego, CA).
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RESULTS |
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X1-4X2-3LX, where represents any hydrophobic residue and X represents any residue (16). A putative NES was identified in the amino-terminal elongation factor domain of the murine EFsec sequence and was found to be conserved in the ortholog sequences from Homo sapiens, Drosophila melanogaster, Anopheles gambiae, and Ciona intestinalis but not from Caenorhabditis elegans or Archaea. A putative NLS was found in the carboxy-terminal region of the murine EFsec sequence, overlapping the region we previously identified as the SBP2 interaction domain (39). This NLS was also found to be conserved among the human, A. gambiae, and C. intestinalis ortholog sequences but not among the D. melanogaster, C. elegans, or Archaea sequences.
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Subcellular localization of EFsec and SBP2. To determine the subcellular localization of full-length EFsec, an epitope tag was introduced at the amino terminus of the coding region and localization was assessed by immunofluorescence, following transient transfection. Intriguingly, EFsec localization varied depending on the cell line, being predominantly cytoplasmic with faint nuclear staining in HEK-293 cells (Fig. 2A), cytoplasmic and nuclear in MSTO-211H cells (Fig. 2B), and predominantly nuclear in HepG2 cells (Fig. 2C). The expression and subcellular localization of endogenous SBP2 were assessed in the same three cell lines by using antisera prepared against bacterially expressed SBP2. Strikingly, SBP2 exhibited patterns of localization similar to those of EFsec in MSTO-211H and HepG2 cells, i.e., nuclear and cytoplasmic staining in the former (Fig. 2E) and predominantly nuclear staining in the latter (Fig. 2F). As confirmation of the specificity of the anti-SBP2 antisera, staining was found to be completely blocked by preincubation of the antisera with bacterially expressed, purified SBP2 but not following incubation with an unrelated bacterially expressed, purified protein (data not shown). In contrast to the patterns in MSTO-211 and HepG2 cells, endogenous SBP2 was barely detectable in HEK-293 cells (Fig. 2D). Previous studies have shown that endogenous expression levels of SBP2 mRNA are low in most tissues (13) and in several cell lines (unpublished results). Localization of the factors by subcellular fractionation and Western blotting revealed a pattern similar to that observed with immunofluorescence for SBP2 in MSTO-211 cells (Fig. 2J, center panel), but a significant fraction of EFsec was detected in the nucleus in HEK-293 cells (Fig. 2J, left panel), suggesting differential sensitivity of the two methods. Western blotting of subcellular fractions with antibodies to cytoplasmic (GRB2) and nuclear (histone H1) markers showed no detectable cross-contamination of subcellular fractions.
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Overexpression of SBP2 results in predominantly cytoplasmic localization. To begin to assess the function of the predicted NLS and NES sequences, we introduced epitope tags into full-length SBP2 expression constructs and carried out expression by transient transfection, followed by immunofluorescence staining. In either HEK-293 or MSTO-211H cells transfected with an SBP2 expression vector, SBP2 staining was predominantly cytoplasmic, which was in agreement with recent results in SBP2-transfected rat hepatoma cells (32). This pattern was observed with untagged SBP2 (not shown), SBP2 bearing an N-terminal HA tag (Fig. 2G), SBP2 bearing a C-terminal V5 tag (Fig. 2H and I), or SBP2 fused at the C terminus to green fluorescent protein (GFP) (data not shown), indicating that localization was not influenced by the identity of the epitope tag or its location in the protein. The difference in the localization of endogenous versus transiently expressed SBP2 does suggest, however, that localization may be affected by expression levels, possibly due to changes in stoichiometry relative to other cellular factors (see below). The localization pattern in transfected HEK-293 was also confirmed by subcellular fractionation and Western blotting, but in this case, a faint SBP2 band was also detected in the nuclear fraction (Fig. 2J, right panel).
EFsec contains functional nuclear export and nuclear localization signals.
We next examined the localization of truncated versions of EFsec to delineate the functions of putative localization signals. The deletion of the N-terminal elongation factor domain, removing the putative NES (amino acids 95 to 103), resulted in the nearly exclusive nuclear localization of the C-terminal SBP2-interaction domain of EFsec in HEK-293 and MSTO-211H cells (Fig. 3A, B, and K) and the elimination of the faint cytoplasmic staining in HepG2 cells observed for full-length EFsec (data not shown). Thus, the deletion of the putative NES and the retention of the putative NLS in EFsec resulted in nuclear localization. NLS function was further confirmed by the mutation of two amino acids in the putative NLS, amino acids K536
E and K537E, resulting in a reversion to cytoplasmic localization in HEK-293 (Fig. 3C) and MSTO-211H cells (Fig. 3D). Intriguingly, these two amino acids were previously shown to be essential for interaction of EFsec with SBP2 (39). The fact that the NLS and SBP2 interaction domains of EFsec overlap suggests that their functions may be mutually exclusive. In this view, the NLS would be masked when EFsec and SBP2 are associated but available when EFsec was free of SBP2 (see Discussion).
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SBP2 contains a potential nucleolar localization signal. Strikingly, SBP2 399-777 also exhibited nucleolar localization (Fig. 3E and F), as did SBP2 517-777 to a lesser extent (Fig. 3G and H), whereas SBP2 399-517 did not (Fig. 3I and J). This suggested the presence of a functional NoLS within the region of amino acids 517 to 777 of SBP2. These signals are not well defined but typically consist of runs of basic residues, predominantly arginine, lysine, and glutamine. A motif fitting this pattern is found at amino acids 524 to 540 (Fig. 1B). A role for ribosomal protein L30 in selenoprotein synthesis has recently been described (9), providing a potential explanation for nucleolar localization of SBP2, where recruitment of and/or interaction with L30 might take place. Intriguingly, a patient homozygous for a point mutation in this putative NoLS has been identified and found to express decreased levels of several selenoproteins, providing support for the functional importance of this region (18).
SBP2 minimal functional domain undergoes nucleocytoplasmic shuttling. The presence of multiple NESs and NLSs in SBP2, its role in cytoplasmic selenocysteine incorporation and its potential role in circumventing nuclear-associated NMD suggest that this protein might undergo nucleocytoplasmic shuttling. To assess potential shuttling, we carried out a heterokaryon assay in which the SBP2 minimal functional domain was transfected into human MSTO-211H cells, followed by fusion with mouse NIH-3T3 cells. Following 3 h of fusion, SBP2 shuttling from the human to mouse nuclei was clearly apparent (Fig. 4A and B).
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Coexpression of SBP2 and EFsec results in partial nuclear retention of EFsec. To further test the possibility that EFsec localization is influenced by its interaction with SBP2, we carried out cotransfections of EFsec plus tRNA[Ser]Sec with either the full-length SBP2 (predominantly cytoplasmic) or the SBP2 minimal functional domain (predominantly nuclear) in HEK-293 cells. The tRNA[Ser]Sec plasmid was included, as we have recently shown that its presence greatly enhances EFsec-SBP2 interaction (39). Cotransfection of full-length EFsec and SBP2 in the presence of tRNA[Ser]Sec resulted in the cytoplasmic localization of both factors (Fig. 5A to D), as seen above when either factor was expressed alone. However, upon the cotransfection of full-length EFsec with the minimal functional domain of SBP2 in the presence of tRNA[Ser]Sec, a significant fraction of EFsec was seen colocalizing with SBP2 in the nucleus (Fig. 5E and F). Thus, the localization of SBP2 to the nucleus apparently results in the redistribution of EFsec, possibly due to increased nuclear retention in a complex with SBP2 and/or other factors.
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150 kDa in the nuclear lysate (Fig. 6, right lane), a position consistent with its existence in a heterodimer with SBP2. Gradient fractionation of cytoplasmic extracts revealed EFsec in both free mRNP and polysome fractions (33a), but complexes in association with ribosomes would be excluded from entering the nondenaturing gel.
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7- to 90-fold higher in HepG2 (high endogenous SBP2) cells than that in HEK-293 cells (low endogenous SBP2) when normalized to glyceraldehyde-3-phosphate dehydrogenase (GAPDH) mRNA (Fig. 7A). Transfection of either full-length SBP2 or the minimal functional domain (amino acids 399 to 777) into HEK-293 cells increased the levels of endogenous selenoprotein mRNAs encoding GPX1 and selP by 1.4- to 2.8-fold (Fig. 7B). Similar but less pronounced effects were seen with selenoprotein S and GPX4 mRNAs (not shown). In contrast, the effect of transfecting the SECIS binding domain alone (amino acids 517 to 777) on selenoprotein mRNA levels was not statistically different from the effect of transfecting empty vector. Thus, either full-length SBP2 or the truncated, nuclearly localized SBP2 399-777 fragment is capable of increasing the steady-state levels of multiple, distinct selenoprotein mRNAs.
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We also assessed the effects of expressing the SBP2 full-length and truncated fragments on selenoprotein synthesis. 75Se labeling of endogenous selenoproteins increased 1.5- to 2.2-fold following transfection of full-length SBP2 or the SBP2 399-777 region. Incorporation of label was minimally affected by transfection with the SBP2 399-517 region and was decreased upon transfection of amino acids 517 to 777 of the SECIS binding domain (Fig. 7C and D).
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DISCUSSION |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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Numerous studies have reported on the differential sensitivity of different selenoprotein mRNAs or even the same mRNAs in different tissues to degradation upon selenium depletion (12, 25, 27). For example, classical cytoplasmic GPX1 mRNA levels fall to 5 to 10% in liver following selenium depletion in rats (11, 33, 38). The decline in GPX1 mRNA upon selenium depletion has been shown to exhibit all the hallmarks of NMD (33), resulting in the down-regulation of the levels of this mRNA when selenium is not abundant. In contrast, phospholipid hydroperoxide glutathione peroxidase is highly resistant to the effects of selenium depletion (3, 27, 34, 37). The selenoprotein mRNA and tissue-specific hierarchy have long been speculated to be results of the differential retention of selenium in different organs (2). However, the studies reported herein suggest that other factors, including SBP2 levels and subcellular localization, SBP2-SECIS interactions, and sensitivity to NMD, may also play a significant role in the hierarchy of selenoprotein synthesis.
Comparative analysis of the EFsec and SBP2 sequences in higher and lower eukaryotes as well as archaea sheds additional light on the importance of the localization signals in selenoprotein synthesis and NMD. Localization signals were not detected in EFsec of C. elegans, and the only homology to SBP2 we could identify in this organism was found in a protein comprising only the SECIS binding domain. In accord with this, the sole selenoprotein in C. elegans, a thioredoxin reductase, is predicted to be insensitive to NMD, as the selenocysteine and termination codons are both located in the last exon. The NLS in EFsec was also not conserved in D. melanogaster. Intriguingly, recent studies have indicated that the identification of termination codons as premature in this organism does not occur via exon junction complexes as is the case in vertebrates (21), suggesting that NMD may not occur in association with the nuclear fraction in insects. Localization signals are also lacking in the archaeal homolog of EFsec, which is not surprising, given the absence of nuclei. No SBP2-like factor has been identified in archaea to date, but a recent report indicates that this function may be served by the C-terminal extension of archaeal EFsec (28), suggesting that recruitment of the archaeal elongation factor may occur in a manner that combines features of prokaryotes and eukaryotes.
Further studies of the selenocysteine incorporation process and the roles, localization, and levels of the factors involved will provide exciting new insights into the cellular mechanisms involved when an mRNA faces the "UGA, stop or selenocysteine" molecular dilemma.
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ACKNOWLEDGMENTS |
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This study was supported by NIH grants DK47320, DK52963, and RR16467.
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FOOTNOTES |
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These authors contributed equally to this work.
Present address: Proteonik, Inc., Gyeonggi Technopark, Rm. 911, Ansan 425-170, South Korea.
¶ Present address: Albany Medical College, 47 New Scotland Ave., Albany, NY 12203.
|| Present address: Department of Biological Sciences, University of Illinois at Chicago, 900 South Ashland Avenue, Chicago, IL 60607.
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REFERENCES |
|---|
|
TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
|---|
2. Behne, D., H. Hilmert, S. Scheid, H. Gessner, and W. Elger. 1988. Evidence for specific selenium target tissues and new biologically important selenoproteins. Biochim. Biophys. Acta 966:12-21.[Medline]
3. Bermano, G., J. R. Arthur, and J. E. Hesketh. 1996. Role of the 3' untranslated region in the regulation of cytosolic glutathione peroxidase and phospholipid-hydroperoxide glutathione peroxidase gene expression by selenium supply. Biochem. J. 320:891-895.[Medline]
4. Berry, M. J. 2000. Recoding UGA as selenocysteine, p. 763-783. In N. Sonenberg, J. W. B. Hershey, and M. B. Mathews (ed.), Translational control of gene expression. Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y.
5. Berry, M. J., L. Banu, Y. Y. Chen, S. J. Mandel, J. D. Kieffer, J. W. Harney, and P. R. Larsen. 1991. Recognition of UGA as a selenocysteine codon in type I deiodinase requires sequences in the 3' untranslated region. Nature 353:273-276.[CrossRef][Medline]
6. Berry, M. J., L. Banu, J. W. Harney, and P. R. Larsen. 1993. Functional characterization of the eukaryotic SECIS elements which direct selenocysteine insertion at UGA codons. EMBO J. 12:3315-3322.[Medline]
7. Berry, M. J., J. W. Harney, T. Ohama, and D. L. Hatfield. 1994. Selenocysteine insertion or termination: factors affecting UGA codon fate and complementary anticodon:codon mutations. Nucleic Acids Res. 22:3753-3759.
8. Brent, G. A., J. W. Harney, Y. Chen, R. L. Warne, D. D. Moore, and P. R. Larsen. 1989. Mutations of the rat growth hormone promoter which increase and decrease response to thyroid hormone define a consensus thyroid hormone response element. Mol. Endocrinol. 3:1996-2004.[Abstract]
9. Chavatte, L., B. A. Brown, and D. M. Driscoll. 2005. Ribosomal protein L30 is a component of the UGA-selenocysteine recoding machinery in eukaryotes. Nat. Struct. Mol. Biol. 12:408-416.[CrossRef][Medline]
10. Cheng, J., and L. E. Maquat. 1993. Nonsense codons can reduce the abundance of nuclear mRNA without affecting the abundance of pre-mRNA or the half-life of cytoplasmic mRNA. Mol. Cell. Biol. 13:1892-1902.
11. Christensen, M. J., and K. W. Burgener. 1992. Dietary selenium stabilizes glutathione peroxidase mRNA in rat liver. J. Nutr. 122:1620-1626.
12. Christensen, M. J., P. M. Cammack, and C. D. Wray. 1995. Tissue specificity of selenoprotein gene expression in rats. J. Nutr. Biochem. 6:367-372.[CrossRef][Medline]
13. Copeland, P. R., J. E. Fletcher, B. A. Carlson, D. L. Hatfield, and D. M. Driscoll. 2000. A novel RNA binding protein, SBP2, is required for the translation of mammalian selenoprotein mRNAs. EMBO J. 19:306-314.[CrossRef][Medline]
14. Copeland, P. R., V. A. Stepanik, and D. M. Driscoll. 2001. Insight into mammalian selenocysteine insertion: domain structure and ribosome binding properties of Sec insertion sequence binding protein 2. Mol. Cell. Biol. 21:1491-1498.
15. Curcio, C., M. M. Baqui, D. Salvatore, B. H. Rihn, S. Mohr, J. W. Harney, P. R. Larsen, and A. C. Bianco. 2001. The human type 2 iodothyronine deiodinase is a selenoprotein highly expressed in a mesothelioma cell line. J. Biol. Chem. 276:30183-30187.
16. Dostie, J., M. Ferraiuolo, A. Pause, S. A. Adam, and N. Sonenberg. 2000. A novel shuttling protein, 4E-T, mediates the nuclear import of the mRNA 5' cap-binding protein, eIF4E. EMBO J. 19:3142-3156.[CrossRef][Medline]
17. Driscoll, D. M., and P. R. Copeland. 2003. Mechanism and regulation of selenoprotein synthesis. Annu. Rev. Nutr. 23:17-40.[CrossRef][Medline]
18. Dumitrescu, A. M., X. H. Liao, M. S. Abdullah, J. Lado-Abeal, F. A. Majed, L. C. Moeller, G. Boran, L. Schomburg, R. E. Weiss, and S. Refetoff. 2005. Mutations in SECISBP2 result in abnormal thyroid hormone metabolism. Nat. Genet. 37:1247-1252.[CrossRef][Medline]
19. Fagegaltier, D., N. Hubert, K. Yamada, T. Mizutani, P. Carbon, and A. Krol. 2000. Characterization of mSelB, a novel mammalian elongation factor for selenoprotein translation. EMBO J. 19:4796-4805.[CrossRef][Medline]
20. Gasdaska, J. R., J. W. Harney, P. Y. Gasdaska, G. Powis, and M. J. Berry. 1999. Regulation of human thioredoxin reductase expression and activity by 3'-untranslated region selenocysteine insertion sequence and mRNA instability elements. J. Biol. Chem. 274:25379-25385.
21. Gatfield, D., L. Unterholzner, F. D. Ciccarelli, P. Bork, and E. Izaurralde. 2003. Nonsense-mediated mRNA decay in Drosophila: at the intersection of the yeast and mammalian pathways. EMBO J. 22:3960-3970.[CrossRef][Medline]
22. Gossen, M., and H. Bujard. 1992. Tight control of gene expression in mammalian cells by tetracycline-responsive promoters. Proc. Natl. Acad. Sci. USA 89:5547-5551.
23. Hentze, M. W., and A. E. Kulozik. 1999. A perfect message: RNA surveillance and nonsense-mediated decay. Cell 96:307-310.[CrossRef][Medline]
24. Hicks, G. R., and N. V. Raikhel. 1995. Protein import into the nucleus: an integrated view. Annu. Rev. Cell Dev. Biol. 11:155-188.[CrossRef][Medline]
25. Hill, K. E., P. R. Lyons, and R. F. Burk. 1992. Differential regulation of rat liver selenoprotein mRNAs in selenium deficiency. Biochem. Biophys. Res. Commun. 185:260-263.[CrossRef][Medline]
26. Himeno, S., H. S. Chittum, and R. F. Burk. 1996. Isoforms of selenoprotein P in rat plasma. Evidence for a full-length form and another form that terminates at the second UGA in the open reading frame. J. Biol. Chem. 271:15769-15775.
27. Lei, X. G., J. K. Evenson, K. M. Thompson, and R. A. Sunde. 1995. Glutathione peroxidase and phospholipid hydroperoxide glutathione peroxidase are differentially regulated in rats by dietary selenium. J. Nutr. 125:1438-1446.
28. Leibundgut, M., C. Frick, M. Thanbichler, A. Bock, and N. Ban. 2005. Selenocysteine tRNA-specific elongation factor SelB is a structural chimaera of elongation and initiation factors. EMBO J. 24:11-22.[CrossRef][Medline]
29. Lykke-Andersen, J., M. D. Shu, and J. A. Steitz. 2000. Human Upf proteins target an mRNA for nonsense-mediated decay when bound downstream of a termination codon. Cell 103:1121-1131.[CrossRef][Medline]
30. Ma, S., K. E. Hill, R. M. Caprioli, and R. F. Burk. 2002. Mass spectrometric characterization of full-length rat selenoprotein P and three isoforms shortened at the C terminus. Evidence that three UGA codons in the mRNA open reading frame have alternative functions of specifying selenocysteine insertion or translation termination. J. Biol. Chem. 277:12749-12754.
31. Maquat, L. E. 2004. Nonsense-mediated mRNA decay: splicing, translation and mRNP dynamics. Nat. Rev. Mol. Cell Biol. 5:89-99.[CrossRef][Medline]
32. Mehta, A., C. M. Rebsch, S. A. Kinzy, J. E. Fletcher, and P. R. Copeland. 2004. Efficiency of mammalian selenocysteine incorporation. J. Biol. Chem. 279:37852-37859.
33. Moriarty, P. M., C. C. Reddy, and L. E. Maquat. 1998. Selenium deficiency reduces the abundance of mRNA for Se-dependent glutathione peroxidase 1 by a UGA-dependent mechanism likely to be nonsense codon-mediated decay of cytoplasmic mRNA. Mol. Cell. Biol. 18:2932-2939.
33. Small-Howard, A., N. Morozova, Z. Stoytcheva, E. P. Forry, J. B. Mansell, J. W. Harney, B. A. Carlson, X. M. Xu, D. L. Hatfield, and M. J. Berry. Supramolecular complexes mediate selenoystein incorporation in vivo. Mol. Cell. Biol., in press.
34. Sun, X., X. Li, P. M. Moriarty, T. Henics, J. P. LaDuca, and L. E. Maquat. 2001. Nonsense-mediated decay of mRNA for the selenoprotein phospholipid hydroperoxide glutathione peroxidase is detectable in cultured cells but masked or inhibited in rat tissues. Mol. Biol. Cell 12:1009-1017.
35. Tujebajeva, R. M., P. R. Copeland, X. M. Xu, B. A. Carlson, J. W. Harney, D. M. Driscoll, D. L. Hatfield, and M. J. Berry. 2000. Decoding apparatus for eukaryotic selenocysteine incorporation. EMBO Rep. 2:158-163.[CrossRef]
36. Tujebajeva, R. M., J. W. Harney, and M. J. Berry. 2000. Selenoprotein P expression, purification, and immunochemical characterization. J. Biol. Chem. 275:6288-6294.
37. Weiss Sachdev, S., and R. A. Sunde. 2001. Selenium regulation of transcript abundance and translational efficiency of glutathione peroxidase-1 and -4 in rat liver. Biochem. J. 357:851-858.[CrossRef][Medline]
38. Weiss, S. L., and R. A. Sunde. 1998. Cis-acting elements are required for selenium regulation of glutathione peroxidase-1 mRNA levels. RNA 4:816-827.[Abstract]
39. Zavacki, A. M., J. B. Mansell, M. Chung, B. Klimovitsky, J. W. Harney, and M. J. Berry. 2003. Coupled tRNASec dependent assembly of the selenocysteine decoding apparatus. Mol. Cell 11:773-781.[CrossRef][Medline]
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| J. Bacteriol. | J. Virol. | Eukaryot. Cell |
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| Microbiol. Mol. Biol. Rev. | Clin. Vaccine Immunol. | All ASM Journals |
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