Nucleotidyltransferase (NT) domain of 2'5'-oligoadenylate (2-5A)synthetase (2-5OAS) and class I CCA-adding enzyme.
In vertebrates, 2-5OASs are induced by interferon during the innate immune response to protect against RNA virus infections. In the presence of an RNA activator, 2-5OASs catalyze the oligomerization of ATP into 2-5A. 2-5A activates endoribonuclease L, which leads to degradation of the viral RNA. 2-5OASs are also implicated in cell growth control, differentiation, and apoptosis. This family includes human OAS1, -2, -3, and OASL. CCA-adding enzymes add the sequence [cytidine(C)-cytidine-adenosine (A)], one nucleotide at a time, onto the 3' end of tRNA, in a template-independent reaction. This class I group includes the archaeal Sulfolobus shibatae and Archeoglobus fulgidus CCA-adding enzymes. It belongs to the Pol beta-like NT superfamily. In the majority of enzymes in this superfamily, two carboxylates, Dx[D/E], together with a third more distal carboxylate, coordinate two divalent metal cations involved in a two-metal ion mechanism of nucleotide addition. These carboxylate residues are conserved in this family.