Unanchored Lysine48-linked polyubiquitin chains positively regulate the type I IFN-mediated antiviral response (P1391)

Abstract

Abstract Type-I interferons (IFN-I) are essential antiviral cytokines produced upon microbial infection. IFN-I elicits this activity through the upregulation of 100s of IFN-I stimulated genes (ISGs), many of which have known antiviral activity. The full breadth of ISG induction requires activation of a number of cellular factors including the IκB kinase epsilon (IKKϵ). However, the mechanism of IKKϵ activation upon viral infection or IFN receptor signaling remains elusive. Here we show that TRIM6, a member of the E3-ubiquitin ligase tripartite motif (TRIM) family of proteins, interacts with IKKϵ and promotes induction of IKKϵ-dependent ISGs. Some studies have suggested a role for unanchored K63-linked polyubiquitin chains, which are not conjugated to any protein, in regulation of kinase activity. However, no role has yet been established for unanchored K48-linked polyubiquitin chains in kinase activation. We show that TRIM6 and the E2-ubiquitin conjugase UbE2K cooperate in the synthesis of unanchored K48-linked polyubiquitin chains, which activate IKKϵ for subsequent STAT1 phosphorylation. Our work defines a previously unrecognized activating role of K48-linked unanchored polyubiquitin chains in kinase activation and identifies the UbE2K-TRIM6-ubiquitin axis as critical for IFN signaling and antiviral response.