| OAS1 is a type I interferon‑induced 2′–5′‑oligoadenylate synthetase that belongs to the OAS family of double‑stranded RNA–sensing enzymes and functions as a central effector in innate antiviral defense by linking viral RNA recognition to regulated RNA decay and translational shutdown. The catalytic core adopts the nucleotidyl‑transferase fold characteristic of OAS proteins and contains the conserved acidic residues that coordinate divalent cations for ATP polymerization, while C‑terminal regions generated by alternative splicing create multiple human isoforms with distinct tail sequences that influence subcellular distribution, enzymatic activity, and interaction with membranes or partner proteins. Double‑stranded RNA binding induces an allosteric rearrangement that aligns active‑site residues for synthesis of 2′–5′‑linked oligoadenylates from ATP, and these 2′–5′ oligomers act as second messengers that bind latent monomeric RNase L, promote its dimerization, and trigger endonucleolytic cleavage of viral and cellular RNA, leading to broad inhibition of protein synthesis and restriction of viral replication. OAS1 activity integrates into the interferon‑stimulated gene network, where induced expression and dsRNA‑dependent activation operate in parallel with RIG‑I–like receptors, PKR, and cGAS–STING pathways, using distinct ligand recognition and catalytic mechanisms but converging on translational control, RNA turnover, and amplification of antiviral signaling. The isoform repertoire of human OAS1 includes variants with different C‑terminal extensions and prenylation motifs, which position subsets of isoforms at intracellular membranes and replication organelles and define which dsRNA substrates are sensed, how efficiently 2′–5′ oligomers are produced, and how strongly RNase L–dependent antiviral programs are engaged. Isoform‑specific differences in 2′–5′ oligoadenylate output and dsRNA responsiveness give OAS1 the capacity to tune the magnitude and duration of translational arrest and RNA degradation in response to particular viral contexts, while preserving basal RNA metabolism in uninfected conditions. Genetic variation in the OAS1 locus alters splice site usage and protein sequence and thereby modifies isoform abundance, enzymatic properties, and antiviral capacity, and these changes associate with human susceptibility or resistance to viral infections and with autoimmune or inflammatory phenotypes where interferon signaling and RNA sensing are chronically engaged. |
