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Cellular Selenoprotein mRNA Tethering via Antisense Interactions with Ebola and HIV-1 mRNAs May Impact Host Selenium Biochemistry

View Article: PubMed Central - PubMed

ABSTRACT

Regulation of protein expression by non-coding RNAs typically involves effects on mRNA degradation and/or ribosomal translation. The possibility of virus-host mRNA-mRNA antisense tethering interactions (ATI) as a gain-of-function strategy, via the capture of functional RNA motifs, has not been hitherto considered. We present evidence that ATIs may be exploited by certain RNA viruses in order to tether the mRNAs of host selenoproteins, potentially exploiting the proximity of a captured host selenocysteine insertion sequence (SECIS) element to enable the expression of virally-encoded selenoprotein modules, via translation of in-frame UGA stop codons as selenocysteine. Computational analysis predicts thermodynamically stable ATIs between several widely expressed mammalian selenoprotein mRNAs (e.g., isoforms of thioredoxin reductase) and specific Ebola virus mRNAs, and HIV-1 mRNA, which we demonstrate via DNA gel shift assays. The probable functional significance of these ATIs is further supported by the observation that, in both viruses, they are located in close proximity to highly conserved in-frame UGA stop codons at the 3′ end of open reading frames that encode essential viral proteins (the HIV-1 nef protein and the Ebola nucleoprotein). Significantly, in HIV/AIDS patients, an inverse correlation between serum selenium and mortality has been repeatedly documented, and clinical benefits of selenium in the context of multi-micronutrient supplementation have been demonstrated in several well-controlled clinical trials. Hence, in the light of our findings, the possibility of a similar role for selenium in Ebola pathogenesis and treatment merits serious investigation.

No MeSH data available.


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Proposed mechanism of selenocysteine (Sec) incorporation into viral proteins via hijacking of a SECIS element from a tethered host selenoprotein mRNA. Both panels show schematic ribosomes with bound tRNAs (blue), one carrying the rare selenium-containing amino acid Sec, the other a growing peptide chain (colored beads). A. During biosynthesis of mammalian selenoproteins, Sec is inserted at UGA codons, normally a STOP signal for protein synthesis. This mechanism requires a specialized RNA stem-loop structure, the SECIS element, generally located in the 3′ untranslated region (3′-UTR) of the selenoprotein mRNA [4]. By recruiting various protein factors, including SECIS binding protein 2 (SBP2) and elongation factor Sec (EF), the SECIS element enables delivery of tRNASec to the ribosome for Sec incorporation at the UGA codon, preventing it from acting as a stop signal. B. Using HIV-1 as an example, the lower panel shows how a viral mRNA, via antisense tethering interactions (ATI), could hijack a host SECIS element for decoding viral selenoprotein modules, such as the HIV-encoded glutathione peroxidase (viral GPx) [7-8, 13]. ATI-1 is the interaction shown as structure B in Fig. 1, and spans the highly conserved 3′-UGA codon of the nef gene; ATI-2 is a second shorter antisense region, consisting of 13 consecutive Watson-Crick base pairs near the end of the viral mRNA (bases 9111-9123, CAGCUGCUUUUUG). Similarly, in the Ebola nucleoprotein mRNA, less than 350 bases from the ATI shown as structure A in Fig. 1, there is also a secondary ATI region downstream of the conserved 3′-UGA stop codon (bases 2719-2734, CGACAAAUAGCUAACA), with only one mismatch to TR3 over 16 base pairs (A, opposite a G in TR3).
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Figure 3: Proposed mechanism of selenocysteine (Sec) incorporation into viral proteins via hijacking of a SECIS element from a tethered host selenoprotein mRNA. Both panels show schematic ribosomes with bound tRNAs (blue), one carrying the rare selenium-containing amino acid Sec, the other a growing peptide chain (colored beads). A. During biosynthesis of mammalian selenoproteins, Sec is inserted at UGA codons, normally a STOP signal for protein synthesis. This mechanism requires a specialized RNA stem-loop structure, the SECIS element, generally located in the 3′ untranslated region (3′-UTR) of the selenoprotein mRNA [4]. By recruiting various protein factors, including SECIS binding protein 2 (SBP2) and elongation factor Sec (EF), the SECIS element enables delivery of tRNASec to the ribosome for Sec incorporation at the UGA codon, preventing it from acting as a stop signal. B. Using HIV-1 as an example, the lower panel shows how a viral mRNA, via antisense tethering interactions (ATI), could hijack a host SECIS element for decoding viral selenoprotein modules, such as the HIV-encoded glutathione peroxidase (viral GPx) [7-8, 13]. ATI-1 is the interaction shown as structure B in Fig. 1, and spans the highly conserved 3′-UGA codon of the nef gene; ATI-2 is a second shorter antisense region, consisting of 13 consecutive Watson-Crick base pairs near the end of the viral mRNA (bases 9111-9123, CAGCUGCUUUUUG). Similarly, in the Ebola nucleoprotein mRNA, less than 350 bases from the ATI shown as structure A in Fig. 1, there is also a secondary ATI region downstream of the conserved 3′-UGA stop codon (bases 2719-2734, CGACAAAUAGCUAACA), with only one mismatch to TR3 over 16 base pairs (A, opposite a G in TR3).

Mentions: The existence of ATI between the 3′ regions of viral mRNAs and host selenoprotein mRNAs suggests a new model for viral selenoprotein synthesis, as a variant of the known mechanism of eukaryotic cellular selenoprotein synthesis (Fig. 3). As shown schematically in Fig. 3B, the “tail-to-tail” antisense interaction between the 3′ ends of the TR1 and HIV-1 mRNAs spans the 3′ end of the viral nef coding sequence, which terminates in a UGA codon that is highly conserved in global HIV-1 isolates [12]. Significantly, the EBOV NP gene also terminates in a UGA codon, conserved in all 1976 through 2014 Zaire EBOV isolates, but not in the much less pathogenic Reston ebolavirus, in which the NP terminates in a UAA. In both HIV-1 nef and the EBOV NP mRNAs, recoding of the terminal UGA of these proteins as Sec via a tethered SECIS element (Fig. 3B) would produce a slightly extended isoform of the protein, containing a single selenium atom as Sec, plus a few residues encoded past the UGA. This is analogous to TR, where the UGA is also at the protein C-terminus; in HIV-1 nef, the sequence even mimics the TR redox center, with a conserved Cys immediately preceding the UGA [12].


Cellular Selenoprotein mRNA Tethering via Antisense Interactions with Ebola and HIV-1 mRNAs May Impact Host Selenium Biochemistry
Proposed mechanism of selenocysteine (Sec) incorporation into viral proteins via hijacking of a SECIS element from a tethered host selenoprotein mRNA. Both panels show schematic ribosomes with bound tRNAs (blue), one carrying the rare selenium-containing amino acid Sec, the other a growing peptide chain (colored beads). A. During biosynthesis of mammalian selenoproteins, Sec is inserted at UGA codons, normally a STOP signal for protein synthesis. This mechanism requires a specialized RNA stem-loop structure, the SECIS element, generally located in the 3′ untranslated region (3′-UTR) of the selenoprotein mRNA [4]. By recruiting various protein factors, including SECIS binding protein 2 (SBP2) and elongation factor Sec (EF), the SECIS element enables delivery of tRNASec to the ribosome for Sec incorporation at the UGA codon, preventing it from acting as a stop signal. B. Using HIV-1 as an example, the lower panel shows how a viral mRNA, via antisense tethering interactions (ATI), could hijack a host SECIS element for decoding viral selenoprotein modules, such as the HIV-encoded glutathione peroxidase (viral GPx) [7-8, 13]. ATI-1 is the interaction shown as structure B in Fig. 1, and spans the highly conserved 3′-UGA codon of the nef gene; ATI-2 is a second shorter antisense region, consisting of 13 consecutive Watson-Crick base pairs near the end of the viral mRNA (bases 9111-9123, CAGCUGCUUUUUG). Similarly, in the Ebola nucleoprotein mRNA, less than 350 bases from the ATI shown as structure A in Fig. 1, there is also a secondary ATI region downstream of the conserved 3′-UGA stop codon (bases 2719-2734, CGACAAAUAGCUAACA), with only one mismatch to TR3 over 16 base pairs (A, opposite a G in TR3).
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Figure 3: Proposed mechanism of selenocysteine (Sec) incorporation into viral proteins via hijacking of a SECIS element from a tethered host selenoprotein mRNA. Both panels show schematic ribosomes with bound tRNAs (blue), one carrying the rare selenium-containing amino acid Sec, the other a growing peptide chain (colored beads). A. During biosynthesis of mammalian selenoproteins, Sec is inserted at UGA codons, normally a STOP signal for protein synthesis. This mechanism requires a specialized RNA stem-loop structure, the SECIS element, generally located in the 3′ untranslated region (3′-UTR) of the selenoprotein mRNA [4]. By recruiting various protein factors, including SECIS binding protein 2 (SBP2) and elongation factor Sec (EF), the SECIS element enables delivery of tRNASec to the ribosome for Sec incorporation at the UGA codon, preventing it from acting as a stop signal. B. Using HIV-1 as an example, the lower panel shows how a viral mRNA, via antisense tethering interactions (ATI), could hijack a host SECIS element for decoding viral selenoprotein modules, such as the HIV-encoded glutathione peroxidase (viral GPx) [7-8, 13]. ATI-1 is the interaction shown as structure B in Fig. 1, and spans the highly conserved 3′-UGA codon of the nef gene; ATI-2 is a second shorter antisense region, consisting of 13 consecutive Watson-Crick base pairs near the end of the viral mRNA (bases 9111-9123, CAGCUGCUUUUUG). Similarly, in the Ebola nucleoprotein mRNA, less than 350 bases from the ATI shown as structure A in Fig. 1, there is also a secondary ATI region downstream of the conserved 3′-UGA stop codon (bases 2719-2734, CGACAAAUAGCUAACA), with only one mismatch to TR3 over 16 base pairs (A, opposite a G in TR3).
Mentions: The existence of ATI between the 3′ regions of viral mRNAs and host selenoprotein mRNAs suggests a new model for viral selenoprotein synthesis, as a variant of the known mechanism of eukaryotic cellular selenoprotein synthesis (Fig. 3). As shown schematically in Fig. 3B, the “tail-to-tail” antisense interaction between the 3′ ends of the TR1 and HIV-1 mRNAs spans the 3′ end of the viral nef coding sequence, which terminates in a UGA codon that is highly conserved in global HIV-1 isolates [12]. Significantly, the EBOV NP gene also terminates in a UGA codon, conserved in all 1976 through 2014 Zaire EBOV isolates, but not in the much less pathogenic Reston ebolavirus, in which the NP terminates in a UAA. In both HIV-1 nef and the EBOV NP mRNAs, recoding of the terminal UGA of these proteins as Sec via a tethered SECIS element (Fig. 3B) would produce a slightly extended isoform of the protein, containing a single selenium atom as Sec, plus a few residues encoded past the UGA. This is analogous to TR, where the UGA is also at the protein C-terminus; in HIV-1 nef, the sequence even mimics the TR redox center, with a conserved Cys immediately preceding the UGA [12].

View Article: PubMed Central - PubMed

ABSTRACT

Regulation of protein expression by non-coding RNAs typically involves effects on mRNA degradation and/or ribosomal translation. The possibility of virus-host mRNA-mRNA antisense tethering interactions (ATI) as a gain-of-function strategy, via the capture of functional RNA motifs, has not been hitherto considered. We present evidence that ATIs may be exploited by certain RNA viruses in order to tether the mRNAs of host selenoproteins, potentially exploiting the proximity of a captured host selenocysteine insertion sequence (SECIS) element to enable the expression of virally-encoded selenoprotein modules, via translation of in-frame UGA stop codons as selenocysteine. Computational analysis predicts thermodynamically stable ATIs between several widely expressed mammalian selenoprotein mRNAs (e.g., isoforms of thioredoxin reductase) and specific Ebola virus mRNAs, and HIV-1 mRNA, which we demonstrate via DNA gel shift assays. The probable functional significance of these ATIs is further supported by the observation that, in both viruses, they are located in close proximity to highly conserved in-frame UGA stop codons at the 3′ end of open reading frames that encode essential viral proteins (the HIV-1 nef protein and the Ebola nucleoprotein). Significantly, in HIV/AIDS patients, an inverse correlation between serum selenium and mortality has been repeatedly documented, and clinical benefits of selenium in the context of multi-micronutrient supplementation have been demonstrated in several well-controlled clinical trials. Hence, in the light of our findings, the possibility of a similar role for selenium in Ebola pathogenesis and treatment merits serious investigation.

No MeSH data available.


Related in: MedlinePlus