Innate Immune Detection of Cardioviruses and Viral Disruption of Interferon Signaling

Figures

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  FIGURE 1. Genomes of representative members of the genus Cardiovirus. All members of the genus Cardiovirus encode a related leader (L) protein formed by the N-terminal part of the polyprotein. Theiler’s murine encephalomyelitis virus (TMEV) expresses an additional 18 kD protein called L∗ from an alternative ORF. TMEV and Saffold virus (SAFV) form the Theilovirus species. These viruses possess a frameshifting site in 2B but the 2B∗ ORF is only 14–15 codons in length. EMCV encodes a 128–129 residue-long 2B∗ protein. IRES, internal ribosome entry site; CRE, cis-replication element; FS, frameshifting site. 5′/3′UTR, 5′/3′untranslated region.

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  FIGURE 2. Detection of Cardiovirus dsRNA by MDA5. (Left) After genome delivery into cells, the 3B/VPg protein (closed circle) is cleaved from the 5′end of the RNA. The replicative form (RF) results from synthesis of the negative strand. The newly synthesized negative strand is then used as a template to generate new genomes, via the formation of replicative intermediates (RI). (Right) MDA5 was shown to detect the RF. After dsRNA recognition, MDA5 molecules oligomerize on the dsRNA molecule, exposing their N-terminal CARD domains, which oligomerize and trigger IFN transcription through MAVS activation. Several co-factors, including LGP2 and PKR, were identified that enhance Cardiovirus dsRNA recognition by MDA5, possibly by facilitating initial dsRNA recognition.

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  FIGURE 3. Interferon antagonism by the Cardiovirus 3C protease. The 3C protease is responsible for cleavage of the Cardiovirus polyprotein produced from translation of the genome. In addition, the Cardiovirus 3C protease cleaves host proteins, such as TANK and MOV10, to prevent the cell from producing IFN.

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  FIGURE 4. Model for the multiple activities of the Cardiovirus L protein. L binds 2A with low-affinity and the complex may enter the nucleus due to a NLS in 2A. Once in the nucleus, L interacts with high affinity with Ran GTPase, thus displacing 2A. The L-Ran complex would activate a kinase and trigger nucleoporin hyper-phosphorylation, thereby leading to nuclear pore complex dismantling and to nucleocytoplasmic trafficking perturbation. On the other hand, L may inhibit PKR, thus preventing translation arrest through eIF2α phosphorylation and therefore block assembly of stress granules. Inhibition of IFN gene transcription by L may result from IRF-3 trafficking perturbation and/or from the absence of PKR-enhanced dsRNA detection by MDA5.

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  FIGURE 5. Inhibition of RNase L activation by L∗ TMEV L∗ binds RNase L ankyrin repeats 1 and 2 (numbered) through a direct protein-protein interaction, thereby preventing association of 2-5A with RNase L monomers and the consequent dimerization and activation of the enzyme.