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

Abstract

Several genes of IFN-mediated pathways in vertebrates, among them the genes that participate in the 2ʹ,5ʹ-oligoadenylate synthetase (OAS)/RNase L pathway, have been identified in C. gigas. In the present study, we identified genes, which encode proteins having 2ʹ,5ʹ-oligoadenylate degrading activity in C. gigas. These proteins belong to the 2H phosphoesterase superfamily and have sequence similarity to the mammalian A kinase anchoring protein 7 (AKAP7) central domain, which is responsible for the 2ʹ,5ʹ-phosphodiesterase (2ʹ,5ʹ-PDE) activity. Comparison of the genomic structures of C. gigas proteins with that of AKAP7 suggests that these enzymes originate from a direct common ancestor. However, the identified nucleases are not typical 2ʹ,5ʹ-PDEs. The found enzymes catalyse the degradation of 2ʹ,5ʹ-linked oligoadenylates in a metal-ion-independent way, yielding products with 2ʹ,3ʹ -cyclic phosphate and 5ʹ-OH termini similarly to the 3ʹ−5ʹ bond cleavage in RNA, catalyzed by metal-independent ribonucleases. 3ʹ,5ʹ-linked oligoadenylates are not substrates for them. The preferred substrates for the C. gigas enzymes are 5ʹ-triphosphorylated 2ʹ,5ʹ-oligoadenylates, whose major cleavage reaction results in the removal of the 5ʹ-triphosphorylated 2ʹ,3ʹ-cyclic phosphate derivative, leaving behind the respective unphosphorylated 2ʹ,5ʹ-oligoadenylate. Such a cleavage reaction results in the direct inactivation of the biologically active 2–5A molecule. The 2ʹ,5ʹ-ribonucleases (2ʹ,5ʹ-RNases) from C. gigas could be members of the ancient group of ribonucleases, specific to 2ʹ−5ʹ phosphodiester bond, together with the enzyme that was characterized previously from the marine sponge Tethya aurantium. The novel 2ʹ,5ʹ-RNases may play a role in the control of cellular 2–5A levels, thereby limiting damage to host cells after viral infection.