Interferon-induced, dsRNA-activated antiviral enzyme which plays a critical role in cellular innate antiviral response. In addition, it may also play a role in other cellular processes such as apoptosis, cell growth, differentiation and gene regulation. Synthesizes higher 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. Monomer (By similarity). Homotetramer (By similarity). Interacts with OAS1D; the interaction inhibits OAS1A catalytic activity (PubMed:15899864). Associated with different subcellular fractions such as mitochondrial, nuclear, and rough/smooth microsomal fractions. Expressed in oocytes and granulosa cells of ovary, in intestine, stomach, spleen and uterus (at protein level) (PubMed:15899864). Expressed at high levels in the digestive tract and lymphoid organs (PubMed:12396720). Expressed in ovary and spleen (PubMed:27663720). Detected 5 weeks after birth in developing ovary and spleen (PubMed:27663720). Expressed in fully grown, germinal vesicle (GV)-intact oocytes, in oocytes at the metaphase II stage and in the 2- to 8-cell embryos (PubMed:15899864). Up-regulated in response to the type I interferon IFNB1, in a STAT1 and STAT2-dependent manner (PubMed:1709495, PubMed:22305621). Induced by polyinosinic:polycytidylic acid (poly I:C) (PubMed:15899864, PubMed:27663720). C-terminal prenylated. Belongs to the 2-5A synthase family.