OAS3

OAS3 is the dominant 2'-5'-oligoadenylate synthetase isoform in human cells, functioning as a cytoplasmic pattern recognition receptor that senses viral double-stranded RNA and synthesizes 2-5A to activate the endoribonuclease RNase L for antiviral RNA degradation (PMID:26858407, PMID:31719180). Its catalytic activity, mediated by active-site residues D816, D818, D888, and K950, is essential for antiviral restriction of diverse viruses including West Nile virus, Sindbis virus, influenza virus, Chikungunya virus, and enterovirus 71, and it cooperates with ZAP in restricting viruses with elevated CpG/UpA dinucleotide content (PMID:35934228, PMID:31276592, PMID:19056102). Basal OAS3 transcription is maintained by IRF2 binding its promoter and amplified by STAT1/STAT2 during interferon signaling, while protein turnover is controlled by TRIM21-mediated K48-linked polyubiquitination at K1079, targeting OAS3 for proteasomal degradation; viral evasion strategies include direct proteolytic cleavage at Q982-G983 by the EV71 3C protease (PMID:39475651, PMID:39494334, PMID:35504537). Beyond canonical antiviral defense, OAS3 partners with DMRT3 to direct RNase L-mediated degradation of specific mRNAs such as ESR1, and negatively regulates RIG-I/MDA5-dependent chemokine induction in macrophages (PMID:32553473, PMID:30078389).

1. 2008 Medium

   Establishing that OAS3 restricts an alphavirus and that its C-terminal region is functionally required resolved that the largest OAS isoform possesses autonomous antiviral activity beyond a presumed redundancy with OAS1/OAS2.

   Evidence Inducible expression of OAS3 and truncation mutant OAS3-R844X in HeLa cells with Chikungunya virus replication assays

   PMID:19056102

   Open questions at the time

   • Mechanism of early-stage restriction not defined
   • Catalytic activity of truncation mutant not measured directly
2. 2012 Medium

   Demonstration that a single viral envelope mutation (E2-E166K) confers escape from OAS3 restriction by enhancing entry efficiency pinpointed the step at which OAS3 blocks Chikungunya virus.

   Evidence Luciferase reporter molecular clones, viral entry assays, and site-directed mutagenesis of E2 glycoprotein

   PMID:22889614

   Open questions at the time

   • Whether OAS3/RNase L degrades incoming viral RNA at entry is not directly shown
   • Generalizability to other alphaviruses unknown
3. 2016 High

   CRISPR knockout of individual OAS family members established OAS3 — not OAS1 or OAS2 — as the dominant isoform responsible for 2-5A synthesis and RNase L activation in human cells, fundamentally redefining the hierarchy within the OAS family.

   Evidence CRISPR-Cas9 KO of OAS1/2/3 in A549 and other human cell lines; 2-5A quantification, rRNA cleavage, and infection with WNV, Sindbis, influenza, and vaccinia

   PMID:26858407

   Open questions at the time

   • Structural basis for OAS3's higher dsRNA affinity in cells not resolved
   • Whether OAS3 dominance holds in all human cell types is untested
4. 2019 High

   Multiple studies expanded OAS3's functional scope: it synergizes with ZAP to restrict CpG/UpA-enriched viruses, it negatively regulates RIG-I/MDA5-dependent chemokine responses independently of type I IFN, and its dominance is conserved in bat cells acting independently of MAVS signaling.

   Evidence CRISPR KOs of OAS3, RNase L, ZAP, and MAVS in human A549 and bat RoNi/7 cells; echovirus replicon systems; transcriptomic and cytokine profiling in macrophages

   PMID:30078389 PMID:31276592 PMID:31719180

   Open questions at the time

   • Molecular basis of OAS3-ZAP synergy undefined
   • How OAS3 cross-regulates RIG-I/MDA5 pathway mechanistically is unclear
   • Conservation of dominance across mammalian orders beyond bats and humans unknown
5. 2020 Medium

   Discovery that OAS3 partners with DMRT3 to target ESR1 mRNA for RNase L-dependent degradation revealed a non-canonical, gene-specific mRNA decay role for the OAS3/RNase L axis beyond pan-antiviral defense.

   Evidence Co-immunoprecipitation, in vitro RNA binding assay, exome sequencing identifying OAS3 R869H, patient testis tissue validation

   PMID:32553473

   Open questions at the time

   • How DMRT3-OAS3 selects specific mRNA targets is unknown
   • Whether the R869H variant affects canonical antiviral function is untested
   • Finding from single lab awaiting independent replication
6. 2022 High

   Identification of EV71 3C protease cleavage of OAS3 at Q982-G983 and definition of catalytic residues D816/D818/D888/K950 as essential for 2-5A synthesis established the first viral immune evasion mechanism targeting OAS3 and mapped its enzymatic active site.

   Evidence In vitro cleavage assays with 3C catalytic-dead mutants, active-site mutagenesis of OAS3, STAT1 promoter binding analysis, viral replication assays

   PMID:35504537 PMID:35934228

   Open questions at the time

   • Whether other viral proteases similarly cleave OAS3 is unexplored
   • No crystal structure of full-length OAS3 to confirm active-site architecture
7. 2024 High

   A trio of discoveries resolved OAS3's transcriptional and post-translational regulation: IRF2 maintains basal OAS3 expression (cooperating with STAT2 under IFN), TRIM21 ubiquitinates K1079 for proteasomal turnover, and HNRNPU positively regulates OAS3 expression to restrict HBV.

   Evidence Genome-wide CRISPR screen with promoter binding and epistasis (IRF2/STAT2); ubiquitination assays with K1079 mutagenesis and CLP mouse model (TRIM21); HNRNPU knockdown/overexpression with HBV transcription measurement

   PMID:39011773 PMID:39475651 PMID:39494334

   Open questions at the time

   • Full set of transcription factors regulating OAS3 not mapped
   • Whether TRIM21-OAS3 axis is exploited by specific viruses unknown
   • HNRNPU-OAS3 link tested only in single lab
8. 2024 Medium

   Discovery that METTL3-mediated m6A modification of OAS3 mRNA upregulates its expression in tumor-associated macrophages, driving M2d polarization and cytokine secretion, extended OAS3's role to the tumor microenvironment.

   Evidence m6A modification analysis, METTL3 knockdown, OAS3 KO macrophages, cytokine measurement, humanized mouse model of pancreatic cancer

   PMID:39738657

   Open questions at the time

   • Which m6A sites on OAS3 mRNA are functionally important is undefined
   • Whether the tumor-promoting effect requires OAS3 catalytic activity or is RNase L-independent is unknown

• No full-length OAS3 crystal or cryo-EM structure exists; the structural basis for its preferential dsRNA binding, allosteric activation across its three OAS domains, and selective mRNA targeting in partnership with DMRT3 remain unresolved.
• No structural model of full-length OAS3
• Mechanism of substrate selectivity in non-canonical mRNA decay unknown
• In vivo physiological significance during natural human infection not established by genetic studies