Reconstitution of the RIG-I pathway reveals a signaling role of unanchored polyubiquitin chains in innate immunity.
RIG-I detects invading viral RNA and activates the transcription factors NF-kappaB and IRF3 through the mitochondrial protein MAVS. Here we show that RNA bearing 5'-triphosphate strongly activates the RIG-I-IRF3 signaling cascade in a reconstituted system composed of RIG-I, mitochondria, and cytosol. Activation of RIG-I requires not only RNA but also polyubiquitin chains linked through lysine 63 (K63) of ubiquitin. RIG-I binds specifically to K63-polyubiquitin chains through its tandem CARD domains in a manner that depends on RNA and ATP. Mutations in the CARD domains that abrogate ubiquitin binding also impair RIG-I activation. Remarkably, unanchored K63-ubiquitin chains, which are not conjugated to any target protein, potently activate RIG-I. These ubiquitin chains function as an endogenous ligand of RIG-I in human cells. Our results delineate the mechanism of RIG-I activation, identify CARD domains as a ubiquitin sensor, and demonstrate that unanchored K63-polyubiquitin chains are signaling molecules in antiviral innate immunity.
Pubmed ID: 20403326 RIS Download
Adenosine Triphosphate | Cell Line | DEAD-box RNA Helicases | Humans | I-kappa B Kinase | Immunity, Innate | Interferon Regulatory Factor-3 | Polyphosphates | Polyubiquitin | RNA, Double-Stranded | RNA, Viral | Signal Transduction | Transcription Factors | Ubiquitin-Conjugating Enzymes | Ubiquitin-Protein Ligases