ABSTRACT
2',5'-oligoadenylate(2-5A)-dependent endoribonuclease, RNase L, is a principal mediator of the interferon (IFN) antiviral response. Therefore, 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 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 these enzymes, members of the two-histidine-phosphoesterase (2H-PE) superfamily, constitute a sub-family referred here as 2',5'-PEs. 2',5'-PEs from mouse coronavirus (CoV) MHV (NS2), 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, 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 di-adenylates with 2',3'-cyclic phosphate termini. Our results suggest that 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.