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 (By similarity). 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 (By similarity). Belongs to the 2-5A synthase family.