Interferon-induced, dsRNA-activated antiviral enzyme which plays a critical role in cellular innate antiviral response. Synthesizes oligomers of 2'-5'-oligoadenylates (2-5A) from ATP which then bind to the inactive monomeric form of ribonuclease L (RNase L) leading to its dimerization and subsequent activation. Activation of RNase L leads to degradation of cellular as well as viral RNA, resulting in the inhibition of protein synthesis, thus terminating viral replication. Can mediate the antiviral effect via the classical RNase L-dependent pathway or an alternative antiviral pathway independent of RNase L. 3 ATP = 5'-triphosphoadenylyl-(2'->5')-adenylyl-(2'->5')-adenosine + 2 diphosphate Produced as a latent enzyme which is activated by dsRNA generated during the course of viral infection. The dsRNA activator must be at least 15 nucleotides long, and no modification of the 2'-hydroxyl group is tolerated. ssRNA or dsDNA do not act as activators. kcat is 0.39 sec(-1) with ATP as substrate. Strongly expressed in spleen dendritic cells, whereas, in bone marrow-derived dendritic cells, the amount increases during the maturation process. Expressed in many organs, the highest levels being in thymus, lung, and bone marrow. Belongs to the 2-5A synthase family.