RIG-I

RIG-I (Retinoic Acid-Inducible Gene I, encoded in humans by the DDX58 [or RIGI] gene on chromosome 9p21.1) is the prototypical cytosolic RIG-I-like receptor (RLR) of the innate branch of immunology, serving as a frontline intracellular sensor of invading RNA viruses. First identified as an essential viral RNA receptor in 2004 by Mitsutoshi Yoneyama, Takashi Fujita, and colleagues, human RIG-I is a 925-amino-acid (~106.6 kDa) superfamily-2 (SF2) DExD/H-box RNA helicase-ATPase that discriminates viral RNA from abundant host cytosolic RNAs by recognizing short, blunt-ended double-stranded RNA (>10–20 bp, typically <300–500 bp) bearing an uncapped 5′-triphosphate (5′-ppp) or 5′-diphosphate (5′-pp) moiety and lacking 2′-O-methylation at the first nucleotide. In resting cells, including epithelial cells and macrophages, RIG-I resides in an autoinhibited conformation in which its N-terminal tandem caspase activation and recruitment domains (2CARD) are sequestered against the helicase insertion domain (Hel2i). High-affinity binding of a viral 5′-ppp dsRNA panhandle by the C-terminal domain (CTD) and helicase ring ejects the 2CARD module, which undergoes TRIM25- and RIPLET-mediated K63-linked polyubiquitination to assemble a tetrameric lock-washer that nucleates MAVS filaments on the outer membrane of mitochondria, driving Type I and Type III interferon expression.

Five-Domain Modular Structure of the 925-Amino-Acid Polypeptide

Human DDX58 comprises 18 exons at chromosome 9p21.1 and encodes a 925-amino-acid polypeptide organized into five functional domains:

1. Tandem N-Terminal Caspase Activation and Recruitment Domains (CARD1: aa 1–92; CARD2: aa 102–188): Two rigid six-helix bundle death-fold domains arranged head-to-tail that serve as the downstream signaling effector module.
2. Flexible CARD–Helicase Linker (aa 189–238): Connects CARD2 to the helicase core and restricts random exposure of the signaling domains.
3. Central DExD/H-Box Helicase-ATPase Core (aa 239–794): Divided into two RecA-like ATPase lobes (Hel1 [aa 239–454] and Hel2 [aa 609–744]) separated by an α-helical bundle insertion domain (Hel2i [aa 455–608]) and followed by a V-shaped α-helical pincer / bridging domain (aa 745–794) that couples ATP hydrolysis to the C-terminal domain.
4. C-Terminal Domain (CTD / Regulatory Domain; aa 795–925): A globular β-sheet fold stabilized by a structural Zn²⁺ ion coordinated by four invariant cysteine residues (C810, C813, C864, C869), containing a deep, positively charged basic cleft responsible for 5′-ppp/5′-pp recognition.

Structural Basis of Autoinhibition in Uninfected Cells

Crystallographic studies of ligand-free (apo) RIG-I by Cusack, Pyle, and Hur (Kowalska et al., 2011; Jiang et al., 2011; Luo et al., 2011) revealed how RIG-I prevents spontaneous interferon toxicity in the absence of viral infection. In the apo state, the helicase ring adopts an open, flexible conformation, while CARD2 binds directly against a conserved α-helical patch on the Hel2i domain. This intramolecular CARD2–Hel2i interaction buries the signaling surface of the 2CARD module and sterically hinders non-specific binding of cellular single-stranded RNAs to the helicase channel. Furthermore, in resting cells, conventional protein kinase C isoforms (PKC-α and PKC-β) constitutively phosphorylate Ser8 (in CARD1) and Thr170 (in CARD2), preventing premature TRIM25 binding and K63 polyubiquitin assembly.

The 5′-Triphosphate / 5′-Diphosphate Basic Cleft and Blunt-End Requirement

RIG-I acts as a coincidence detector requiring two simultaneous structural features on the same RNA molecule:

• An Uncapped 5′-ppp or 5′-pp Moiety: Viral RNA-dependent RNA polymerases (RdRps) initiate genome and antigenome synthesis de novo using a nucleoside triphosphate, leaving an intact 5′-triphosphate (5′-ppp) (or, in reoviruses, a 5′-diphosphate, 5′-pp) at the 5′ terminus. Within the basic cleft of the RIG-I CTD, four positively charged residues—His847, Lys858, Lys861, and Lys888—form direct electrostatic salt bridges and hydrogen bonds with the α-, β-, and γ-phosphates, while Phe853 stacks flat against the terminal base pair of a blunt duplex. By contrast, cellular tRNAs and rRNAs are processed to carry a 5′-monophosphate (5′-p), which clashes with Phe853 and fails to activate RIG-I.
• A Base-Paired Duplex Stem (>10–20 bp): Once the CTD caps the 5′-ppp blunt end with subnanomolar affinity (K_d ≈ 0.1–0.5 nM), the Hel1, Hel2i, and Hel2 domains swing closed around the adjacent 10–12 base pairs of the double-stranded RNA stem. Negative-sense single-stranded RNA viruses (Orthomyxoviridae, Rhabdoviridae, Bunyavirales) contain self-complementary 5′ and 3′ genome termini that base-pair intramolecularly to form a 5′-ppp "panhandle" promoter (12–25 bp) that serves as a physiological RIG-I agonist, amplified during infection by short defective interfering (DI) copy-back RNAs.

Steric Exclusion of Host Cap-1 (2′-O-Methylation)

Eukaryotic host mRNAs transcribed by RNA polymerase II receive a 5′ 7-methylguanosine (m⁷G) cap connected via a 5′–5′ triphosphate bridge (Cap-0: m⁷GpppN₁), followed by methylation of the ribose 2′-hydroxyl group of the first transcribed nucleotide by host cap methyltransferase 1 (CMTR1) to generate Cap-1 (m⁷GpppN₁m). Structural and functional studies (Schuberth-Wagner et al., 2015) demonstrated that while RIG-I can bind unmethylated Cap-0 dsRNA with moderate affinity, the bulky 2′-O-methyl group at nucleotide N₁ of host Cap-1 RNA sterically clashes with the invariant CTD residue His830, reducing RIG-I binding affinity by >100-fold and abolishing ATPase activation.

Clamping of the Helicase Ring and TRIM25/RIPLET Ubiquitination

When the CTD docks onto a viral 5′-ppp blunt end and the Hel1–Hel2i–Hel2 lobes clamp around the dsRNA duplex in the presence of ATP, the Hel2i domain rotates inward toward the RNA shaft. This conformational rearrangement clashes with and displaces CARD2 from its inhibitory docking site on Hel2i, ejecting the tandem 2CARD module into the cytosol while protein phosphatase 1 (PP1α / PP1γ) dephosphorylates inhibitory Ser8 and Thr170.

Ejected monomeric 2CARDs assemble into a signaling-competent homotetramer driven by two K63-linked polyubiquitin E3 ligases:

1. TRIM25 (Tripartite Motif Protein 25): Binds CARD1 (around Thr55) and conjugates covalently linked K63-polyubiquitin chains to Lys172 in CARD2 (as well as generating unanchored K63-ubiquitin chains).
2. RIPLET (RNF135): Recognizes dimeric/oligomeric RIG-I bound to viral dsRNA stems >20–40 bp and conjugates K63-ubiquitin chains to the C-terminal pincer/CTD region (Lys788, Lys849, Lys851), cross-linking adjacent RIG-I molecules.

Four 2CARD modules assemble into a single-turn helical "lock-washer" tetramer braced around its outer circumference by K63-linked ubiquitin chains. The top helical surface of this 2CARD lock-washer binds the monomeric CARD of MAVS on the outer membrane of mitochondria (and peroxisomes), templating the prion-like polymerization of MAVS into signaling filaments. Polymerized MAVS recruits TRAF2/3/5/6, activating TBK1/IKKε (phosphorylating IRF3 and IRF7) and IKKα/β/γ (activating NF-κB) to transcribe IFN-β, IFN-α subtypes, IFN-λ, and interferon-stimulated genes (ISGs).

Viral Evasion and Clinical Translation

Because RIG-I senses negative-strand RNA viruses (Orthomyxoviridae, Rhabdoviridae, Paramyxoviridae, Filoviridae) and Flaviviridae (hepatitis C virus, dengue virus, Zika virus), pathogens deploy dedicated antagonists: Influenza A NS1 binds TRIM25 and RIPLET to block K63-ubiquitination; Ebola VP35 caps 5′-ppp dsRNA; flavivirus NS5 and coronavirus nsp16–nsp10 encode viral 2′-O-methyltransferases to mimic host Cap-1 RNA; and Hepatitis C virus NS3/4A protease cleaves human MAVS at Cys508 off the mitochondrial membrane. Clinically, synthetic 5′-ppp RNA agonists (3p-hpRNA, MK-4621) are investigated as cancer immunotherapies, while germline gain-of-function mutations in DDX58 (E373A, C268F) cause Singleton-Merten syndrome type 2 (SMS2).

How does RIG-I distinguish a viral RNA molecule from the millions of normal host RNAs inside a human cell?

RIG-I inspects the 5′ end of every double-stranded RNA using three structural checkpoints in its C-terminal domain (CTD). First, viral RNA-dependent RNA polymerases start synthesis with a nucleoside triphosphate, leaving an uncapped 5′-triphosphate (5′-ppp) at the base-paired end, which locks tightly into a positively charged pocket (His847, Lys858, Lys861, Lys888) in the RIG-I CTD; host tRNAs and rRNAs have a single 5′-monophosphate and cannot bind. Second, host mRNAs receive a 7-methylguanosine cap plus a 2′-O-methyl group on the first ribose sugar (Cap-1). The invariant amino acid His830 in RIG-I collides with that 2′-O-methyl group, rejecting host mRNA.

Why do flaviviruses and coronaviruses encode their own 2′-O-methyltransferase enzymes?

Because the human RIG-I sensor uses residue His830 to reject any RNA carrying a 2′-O-methyl group at its first nucleotide (the hallmark of host Cap-1 mRNA), cytoplasmically replicating viruses cannot rely on the host cell's nuclear CMTR1 methyltransferase to disguise their transcripts. Flaviviruses (via the N-terminal domain of NS5 in dengue, Zika, and West Nile viruses) and coronaviruses (via the nsp16–nsp10 complex in SARS-CoV-2) evolved their own viral 2′-O-methyltransferases to methylate the 2′-OH position of their newly synthesized viral mRNAs in the cytoplasm. Mutant viruses engineered to lack 2′-O-methyltransferase activity are detected by RIG-I (and restricted by IFIT1) and are attenuated in vivo.

How does Hepatitis C virus shut down the RIG-I alarm to establish chronic liver infection?

When Hepatitis C virus (HCV) infects a human hepatocyte, RIG-I detects the poly(U/UC) tract and 5′-triphosphate regions of the HCV RNA genome and attempts to signal through the adapter protein MAVS. For MAVS to assemble into active signaling filaments and recruit TBK1, it must remain anchored to the outer membrane of mitochondria via its C-terminal transmembrane tail. HCV counters this by using its viral NS3/4A serine protease to cleave human MAVS at a single amino acid—cysteine-508 (Cys508)—just six residues away from the mitochondrial membrane. Dislodged into the cytosol, cleaved MAVS cannot polymerize or activate IRF3, silencing Type I interferon production in the infected liver cell.

1. Yoneyama, M., Kikuchi, M., Natsukawa, T., Shinobu, N., Imaizumi, T., Miyagishi, M., Taira, K., Akira, S., & Fujita, T. (2004). The RNA helicase RIG-I has an essential function in double-stranded RNA-induced innate antiviral responses. Nature Immunology, 5(7), 730–737. https://doi.org/10.1038/ni1087
2. Hornung, V., Ellegast, J., Kim, S., Brzózka, K., Jung, A., Kato, H., Poeck, H., Akira, S., Conzelmann, K. K., Schlee, M., Endres, S., & Hartmann, G. (2006). 5′-Triphosphate RNA is the ligand for RIG-I. Science, 314(5801), 994–997. https://doi.org/10.1126/science.1132505
3. Gack, M. U., Shin, Y. C., Joo, C. H., Urano, T., Liang, C., Sun, L., Takeuchi, O., Akira, S., Chen, Z., Inoue, S., & Jung, J. U. (2007). TRIM25 RING-finger E3 ubiquitin ligase is essential for RIG-I-mediated antiviral activity. Nature, 446(7138), 916–920. https://doi.org/10.1038/nature05732
4. Schuberth-Wagner, C., Ludwig, J., Bruder, A. K., Herzner, A. M., Zillinger, T., Goldeck, M., Schmidt, T., Schmid-Burgk, J. L., Kerber, R., Wolter, S., Stümpel, J. P., Roth, A., Bartok, E., Drosten, C., Coch, C., Hornung, V., Barchet, W., Kümmerer, B. M., Hartmann, G., & Schlee, M. (2015). A conserved histidine in the RNA sensor RIG-I controls immune tolerance to N1-2′O-methylated self RNA. Immunity, 43(1), 41–51. https://doi.org/10.1016/j.immuni.2015.06.015