TRIM25 RING-finger E3 ubiquitin ligase is essential for RIG-I-mediated antiviral activity

Retinoic-acid-inducible gene-I (RIG-I; also called DDX58) is a cytosolic viral RNA receptor that interacts with MAVS (also called VISA, IPS-1 or Cardif) to induce type I interferon-mediated host protective innate immunity against viral Infection. Furthermore, members of the tripartite motif (TRIM) protein family, which contain a cluster of a RING-finger domain, a B box/coiled-coil domain and a SPRY domain, are involved in various cellular processes, including cell proliferation and Antiviral activity. Here we report that the amino-terminal Caspase recruitment domains (CARDs) of RIG-I undergo robust ubiquitination induced by TRIM25 in mammalian cells. The carboxy-terminal SPRY domain of TRIM25 interacts with the N-terminal CARDs of RIG-I; this interaction effectively delivers the Lys 63-linked ubiquitin moiety to the N-terminal CARDs of RIG-I, resulting in a marked increase in RIG-I downstream signalling activity. The Lys 172 residue of RIG-I is critical for efficient TRIM25-mediated ubiquitination and for MAVS binding, as well as the ability of RIG-I to induce Antiviral signal transduction. Furthermore, gene targeting demonstrates that TRIM25 is essential not only for RIG-I ubiquitination but also for RIG-I-mediated interferon- production and Antiviral activity in response to RNA virus Infection. Thus, we demonstrate that TRIM25 E3 ubiquitin Ligase induces the Lys 63-linked ubiquitination of RIG-I, which is crucial for the cytosolic RIG-I signalling pathway to elicit host Antiviral innate immunity.