Identification of a novel member of 2H phosphoesterases, 2',5'-oligoadenylate degrading ribonuclease from the oyster Crassostrea gigas

A protein inhibitor of activated STAT (CgPIAS) negatively regulates the expression of ISGs by inhibiting STAT activation in oyster Crassostrea gigas

The protein inhibitor of activated STAT (PIAS) family proteins act as the important negative regulators in janus kinase (JAK)/signal transducer and activator of transcription (STAT) signaling pathway, which can be also involved in regulating the expression of interferon-stimulated genes (ISGs). In the present study, a PIAS homologue (designated as CgPIAS) was identified from oyster Crassostrea gigas. The open reading frame (ORF) of CgPIAS cDNA was of 1887 bp encoding a peptide of 628 amino acid residues. The CgPIAS protein contains a conserved scaffold attachment factor A/B/acinus/PIAS (SAP) domain, a Pro-Ile-Asn-Ile-Thr (PINIT) motif, a RING-finger-like zinc-binding domain (RLD) and two SUMO-interaction Motifs (SIMs). The deduced amino acid sequence of CgPIAS shared 74.58-81.36% similarity with other PIAS family members in the RLD domain. The mRNA transcripts of CgPIAS were detected in all the tested tissues with highest level in haemocytes (32.98-fold of mantles, p < 0.001). After poly (I:C) and recombinant Interferon-like protein (rCgIFNLP) stimulation, the mRNA expression of CgPIAS in haemocytes significantly up-regulated to the highest level at 48 h (7.38-fold, p < 0.001) and at 24 h (13.08-fold, p < 0.01), respectively. Moreover, the nuclear translocation of CgPIAS was observed in haemocytes after poly (I:C) stimulation. Biolayer Interferometry (BLI) assay revealed that the recombinant protein CgPIAS-RLD could interact with the recombinant protein CgSTAT in vitro with the K_D value of 3.88 × 10^-8 M. In the CgPIAS-RNAi oysters, the green signals of CgSTAT protein in nucleus of haemocytes increased compared with that in NC-RNAi group, and the mRNA expression of myxovirus resistance (CgMx1), oligoadenylate synthase-like proteins (CgOASL), CgViperin and IFN-induced protein 44-like (CgIFI44L-1) in haemocytes significantly increased at 12 h after poly (I:C) stimulation, which were 2.39-fold (p < 0.05), 2.18-fold (p < 0.001), 1.74-fold (p < 0.05), and 2.89-fold (p < 0.01) of that in control group, respectively. The above results indicated that CgPIAS negatively regulated the ISG expression by inhibiting STAT activation in oyster C. gigas.

Specificity and Mechanism of Coronavirus, Rotavirus, and Mammalian Two-Histidine Phosphoesterases That Antagonize Antiviral Innate Immunity

The 2′,5′-oligoadenylate (2-5A)-dependent endoribonuclease, RNase L, is a principal mediator of the interferon (IFN) antiviral response. Therefore, the regulation of cellular levels of 2-5A is a key point of control in antiviral innate immunity. Cellular 2-5A levels are determined by IFN-inducible 2′,5′-oligoadenylate synthetases (OASs) and by enzymes that degrade 2-5A. Importantly, many coronaviruses (CoVs) and rotaviruses encode 2-5A-degrading enzymes, thereby antagonizing RNase L and its antiviral effects. A-kinase-anchoring protein 7 (AKAP7), a mammalian counterpart, could possibly limit tissue damage from excessive or prolonged RNase L activation during viral infections or from self-double-stranded RNAs that activate OAS. We show that these enzymes, members of the two-histidine phosphoesterase (2H-PE) superfamily, constitute a subfamily referred here as 2′,5′-PEs. 2′,5′-PEs from the mouse CoV mouse hepatitis virus (MHV) (NS2), Middle East respiratory syndrome coronavirus (MERS-CoV) (NS4b), group A rotavirus (VP3), and mouse (AKAP7) were investigated for their evolutionary relationships and activities. While there was no activity against 3′,5′-oligoribonucleotides, they all cleaved 2′,5′-oligoadenylates efficiently but with variable activity against other 2′,5′-oligonucleotides. The 2′,5′-PEs are shown to be metal ion-independent enzymes that cleave trimer 2-5A (2′,5′-p₃A₃) producing mono- or diadenylates with 2′,3′-cyclic phosphate termini. Our results suggest that the elimination of 2-5A might be the sole function of viral 2′,5′-PEs, thereby promoting viral escape from innate immunity by preventing or limiting the activation of RNase L.