Reconstitution of the RIG-I Pathway Reveals a Signaling Role of Unanchored Polyubiquitin Chains in Innate Immunity

Department of Molecular Biology, University of Texas Southwestern Medical Center, Dallas, TX 75390-9148, USA.
Cell (Impact Factor: 33.12). 04/2010; 141(2):315-30. DOI: 10.1016/j.cell.2010.03.029
Source: PubMed

ABSTRACT 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.

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