ID: 90: The pestiviral IFN antagonist E(rns) cleaves dsRNA as nicking endoribonuclease

Bovine virus diarrhea virus (BVDV), a pestivirus in the Flaviviridae family, causes persistent infection (PI) in cattle by entering the fetus early in gestation prior to development of adaptive immunity. In addition, the inhibition of interferon (IFN) type I synthesis plays an important role in escaping innate immunity and is a prerequisite for the birth of PI calves. Thus, pestiviruses express two IFN antagonists: the non-structural protein N(pro) that inhibits IFN induction in infected cells by degradation of IRF-3, and the soluble glycoprotein E(rns), which cleaves extracellular dsRNA prior to activation of non-infected cells. E(rns), which forms covalently-linked homodimers, belongs to the T2 family of ribonucleases that preferably degrade ssRNA. Accordingly, the crystallographic structure of the catalytic domain of E(rns) did not provide evidence for dsRNA to fit into the active site. Thus, it may well be envisaged that E(rns) independently cleaves both strands of dsRNA, which would be in accordance with the fact that ssRNA is degraded by this viral RNase more efficiently than dsRNA. Here we show that wild-type, but not an RNase-inactive mutant form of E(rns), was able to degrade RNA in a RNA/DNA hybrid with the DNA strand being resistant to degradation. Together with the fact that monomeric E(rns) is similarly able to cleave dsRNA, these results support our model that E(rns) might be a ’nicking endoribonuclease’ degrading ssRNA within double-stranded substrates. This efficiently prevents the activation of patter recognition receptors (PRRs) and helps to maintain a state of innate immunotolerance in PI animals.