Homologous 2′,5′-phosphodiesterases from disparate RNA viruses antagonize antiviral innate immunity

Abstract

Efficient and productive virus infection often requires viral countermeasures that block innate immunity. The IFN-inducible 2′,5′-oligoadenylate (2-5A) synthetases (OASs) and ribonuclease (RNase) L are components of a potent host antiviral pathway. We previously showed that murine coronavirus (MHV) accessory protein ns2, a 2H phosphoesterase superfamily member, is a phosphodiesterase (PDE) that cleaves 2-5A, thereby preventing activation of RNase L. The PDE activity of ns2 is required for MHV replication in macrophages and for hepatitis. Here, we show that group A rotavirus (RVA), an important cause of acute gastroenteritis in children worldwide, encodes a similar PDE. The RVA PDE forms the carboxy-terminal domain of the minor core protein VP3 (VP3-CTD) and shares sequence and predicted structural homology with ns2, including two catalytic HxT/S motifs. Bacterially expressed VP3-CTD exhibited 2′,5′-PDE activity, which cleaved 2-5A in vitro. In addition, VP3-CTD expressed transiently in mammalian cells depleted 2-5A levels induced by OAS activation with poly (rI):poly(rC), preventing RNase L activation. In the context of recombinant chimeric MHV expressing inactive ns2, VP3-CTD restored the ability of the virus to replicate efficiently in macrophages or in the livers of infected mice, whereas mutant viruses expressing inactive VP3-CTD (H718A or H798R) were attenuated. In addition, chimeric viruses expressing either active ns2 or VP3-CTD, but not nonfunctional equivalents, were able to protect ribosomal RNA from RNase L-mediated degradation. Thus, VP3-CTD is a 2′,5′-PDE able to functionally substitute for ns2 in MHV infection. Remarkably, therefore, two disparate RNA viruses encode proteins with homologous 2′,5′-PDEs that antagonize activation of innate immunity.Reoviridae | Nidovirales | RNA capping enzyme | interferon-stiumulated gene T he 2′,5′-oligoadenylate (2-5A) synthetase (OAS)-ribonuclease (RNase) L pathway is among the most potent IFNinduced antiviral effectors, blocking viral infections by several mechanisms, including directly cleaving viral single-stranded RNA genomes, depleting viral and host mRNA available for translation, and enhancing type I IFN induction (1). Type I IFNs bind to the cell surface receptor IFNAR (interferon-α/β receptor), initiating JAK-STAT signaling to the OAS genes, which results in elevated levels of OAS proteins. When activated by viral doublestranded (ds)RNA, certain OAS isoforms use ATP to synthesize 5′-triphosphorylated 2-5A. Trimer and longer species of 2-5A bind with high specificity and affinity to the inactive monomeric RNase L, causing it to dimerize and become active (2) (Fig. 1A).Not surprisingly, viruses have evolved multiple mechanisms to antagonize or prevent activation of RNase L (1). One such virus is mouse hepatitis virus (MHV) (3), a member of the enveloped, positive-stranded (+)RNA coronavirus species. MHV and related 2a-betacoronaviruses (4) encode ns2, a cytoplasmic 30-kDa protein that is dispensable for virus replication in transformed cell lines but serve...

NSP1 of human rotaviruses commonly inhibits NF-κB signalling by inducing β-TrCP degradation

“…A vital arm of the innate response is the recognition of invading viruses by the host and the production of immunomodulatory and antiviral cytokines of the IFN family (Randall & Goodbourn, 2008). Rotavirus has evolved several mechanisms to counteract the host response, comprising interference with cellular proteins required for IFN induction, disruption of IFN signalling through its receptors and antagonism of specific cellular antiviral processes (Barro & Patton, 2005;Graff et al, 2009;Holloway et al, 2009Holloway et al, , 2014Sen et al, 2014;Zhang et al, 2013). A key factor in the evasion of host responses by rotavirus is its nonstructural protein 1 (NSP1), which is the least conserved rotavirus protein.…”

Rotavirus NSP1 Mediates Degradation of Interferon Regulatory Factors through Targeting of the Dimerization Domain

“…The fact that NSP1 has RNA-binding activity raises the possibility that the protein also antagonizes interactions between viral RNAs and PRRs or ISG products (64). Based on the discovery that RVA VP3 has a specialized domain that functions to suppress the antiviral oligoadenylate synthetase (OAS)/RNase L pathway, viral proteins in addition to NSP1 may play key roles in suppressing innate immune responses (65).…”

The Rotavirus Interferon Antagonist NSP1: Many Targets, Many Questions

“…In addition, uncapped rotavirus RNAs may activate other host antiviral proteins that bind to viral RNA, such as protein kinase R (PKR) or 2=-5=-oligoadenylate synthetase (OAS), but the role of these ISGs in inhibition of rotavirus is not well understood. Interestingly, the VP3 capping enzyme contains a phosphodiesterase domain that has been shown to degrade 2=,5=-oligoadenylates, thereby preventing the activation of RNase L, which would otherwise cleave viral and cellular single-stranded RNA (ssRNA) (6). Although rotavirus takes great steps to protect its genomic and (ϩ) RNAs, there are inefficiencies in these processes that the virus must overcome in order to replicate.…”