Two forms of VPg on poliovirus RNAs

The mammalian orthoreovirus bicistronic M3 mRNA initiates translation using a 5' end-dependent, scanning mechanism that does not require interaction of 5'-3' untranslated regions

Mammalian orthoreovirus mRNAs possess short 5' UTR, lack 3' poly(A) tails, and may lack 5' cap structures at late times post-infection. As such, the mechanisms by which these viral mRNAs recruit ribosomes remain completely unknown. Toward addressing this question, we used bicistronic MRV M3 mRNA to analyze the role of 5' and 3' UTRs during MRV protein synthesis. The 5' UTR was found to be dispensable for translation initiation; however, reducing its length promoted increased downstream initiation. Modifying start site Kozak context altered the ratio of upstream to downstream initiation, whereas mutations in the 3' UTR did not. Moreover, an M3 mRNA lacking a 3' UTR was able to rescue MRV infection to WT levels in an siRNA trans-complementation assay. Together, these data allow us to propose a model in which the MRV M3 mRNA initiates translation using a 5' end-dependent, scanning mechanism that does not require the viral mRNA 3' UTR or 5'-3' UTRs interaction.

Visualisation of direct interaction of MDA5 and the dsRNA replicative intermediate form of positive strand RNA viruses

The innate immune system is a vital part of the body's defences against viral pathogens. The proteins retinoic acid-inducible gene-I (RIG-I) and melanoma differentiation associated gene 5 (MDA5) function as cytoplasmic pattern recognition receptors that are involved in the elimination of actively replicating RNA viruses. Their location and their differential responses to RNA viruses emphasises the complexity of the innate detection system. Despite the wealth of information on the types of RNA that trigger RIG-I, much less is known about the nature of the RNAs that act as agonists for MDA5. In order to identify which RNA species triggers MDA5 activation during infection, we isolated viral ssRNA and replicative intermediates of RNA from positive sense ssRNA viruses. We reveal that MDA5 recognises not the genomic ssRNA but the dsRNA generated by the replication of these viruses. Furthermore, using fluorescent imaging we present the first report of the visualisation of dsRNA and MDA5, which provides unique evidence of the relationship between viral dsRNA and MDA5 and proves without a doubt that MDA5 is the key sensor for the dsRNA replicative intermediate form of positive sense ssRNA viruses.

Two forms of poliovirus VPg result from amino acid modification of a single viral protein

The protein (VPg) covalently attached to the 5' terminus of poliovirus RNA has been reported to resolve into two forms, separable by electrofocusing, yet the viral gene sequence predicts only one apparent gene for VPg. The two VPg species were separated and analyzed for structural differences. The protein contains no phosphorylated amino acid residues other than the junction tyrosine linked to the nucleic acid moiety. After isolation, the acidic form of VPg remains stable, but the more basic form again generates a distribution of both species. This suggests some lability or modification of at least one amino acid residue postsynthesis. The position of the alteration in the protein was localized to the amino-terminal tryptic peptide, containing nine amino acids.

GDVII and DA isolates of Theiler's virus: proteins attached to the 5' end of the RNA are bound covalently to the same nucleotides

GDVII and DA picornaviruses, which represent two biologically distinct subgroups of Theiler's murine encephalomyelitis viruses, were analyzed to detect the presence of a RNA-linked protein, VPg. It was found that the single-stranded RNA genomes isolated from the virions of both viruses contain a protein of approximately 7,000 daltons, which is covalently linked to the 5' end of the RNA. Like in other picornaviruses (e.g., poliovirus and encephalomyocarditis virus), the terminal nucleotide adjacent to the protein was found to be pUp. The possibility that differently charged VPg species exist is also mentioned.