Cap-snatching mechanism in yeast L-A double-stranded RNA virus

Instituto de Biología Funcional y Genómica, Consejo Superior de Investigaciones Científicas/Universidad de Salamanca, Edificio Departamental, Avenida del Campo Charro, Salamanca 37007, Spain.
Proceedings of the National Academy of Sciences (Impact Factor: 9.67). 10/2011; 108(43):17667-71. DOI: 10.1073/pnas.1111900108
Source: PubMed


The 5' cap structure (m(7)GpppX-) is an essential feature of eukaryotic mRNA required for mRNA stability and efficient translation. Influenza virus furnishes its mRNA with this structure by a cap-snatching mechanism, in which the viral polymerase cleaves host mRNA endonucleolytically 10-13 nucleotides from the 5' end and utilizes the capped fragment as a primer to synthesize viral transcripts. Here we report a unique cap-snatching mechanism by which the yeast double-stranded RNA totivirus L-A furnishes its transcript with a cap structure derived from mRNA. Unlike influenza virus, L-A transfers only m(7)Gp from the cap donor to the 5' end of the viral transcript, thus preserving the 5' α- and β-phosphates of the transcript in the triphosphate linkage of the final product. This in vitro capping reaction requires His154 of the coat protein Gag, a residue essential for decapping of host mRNA and known to form m(7)Gp-His adduct. Furthermore, the synthesis of capped viral transcripts in vivo and their expression were greatly compromised by the Arg154 mutation, indicating the involvement of Gag in the cap-snatching reaction. The overall reaction and the structure around the catalytic site in Gag resemble those of guanylyltransferase, a key enzyme of cellular mRNA capping, suggesting convergent evolution. Given that Pol of L-A is confined inside the virion and unable to access host mRNA in the cytoplasm, the structural protein Gag rather than Pol catalyzing this unique cap-snatching reaction exemplifies the versatility as well as the adaptability of eukaryotic RNA viruses.

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Available from: Tsutomu Fujimura, Jul 21, 2015
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    • "The family (Totiviridae) that contains the totiviruses is ancient with a broad eukaryotic host range and extensive co-evolution in the fungi (Liu et al., 2012). Totiviruses ‘snatch’ the hosts’ mRNA caps (modified guanines at the 5′ end) with a unique binding mechanism involving at least five proposed residues of the coat protein (Fujimura & Esteban, 2011). Frequently, totiviruses in fungi are associated with a satellite killer dsRNA virus that codes for a toxin (Bostian et al., 1984). "
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    • "It has been suggested that this vital 5′ cap could be pirated from host RNA by a unique mechanism dubbed “cap-snatching”. Here, the 5′ m7Gp of host mRNA is transferred onto the diphosphorylated 5′ end of the viral transcripts (Fujimura and Esteban, 2011). The viral capsid plays a central role in this theft, where a histidine at position 154 in the protein has been deemed essential. "
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    ABSTRACT: Eukaryotic mRNA bears a cap structure (m(7)GpppX-) at the 5' terminus crucial for efficient translation and stability. The yeast L-A double-stranded RNA virus furnishes its mRNA with this structure by a novel cap-snatching mechanism in which the virus transfers an m(7)Gp moiety from host mRNA to the diphosphorylated 5' terminus of the viral transcript, thus forming on it an authentic cap structure (referred to as cap0) in the budding yeast. This capping reaction is essential for efficient viral expression. His-154 of the capsid protein Gag is involved in the cap transfer. Here we show that the virus can utilize an externally added viral transcript as acceptor in the capping reaction. The acceptor needs to be 5' diphosphorylated, consistent with the fact that the viral transcript bears diphosphate at the 5' terminus. A 5' triphosphorylated or monophosphorylated transcript does not function as acceptor. N7 methylation at the 5' cap guanine of mRNA is essential for cap donor activity. We also demonstrate that the capping reaction requires the viral polymerase actively engaging in transcription. Because the cap-snatching site of Gag is located at the cytoplasmic surface of the virion, whereas Pol is confined inside the virion, the result indicates coordination between the cap-snatching and polymerization sites. This will allow L-A virus to efficiently produce capsid proteins to form new virions when Pol is actively engaging in transcription. The coordination may also minimize the risk of accidental capping of nonviral RNA when Pol is dormant.
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