The KSHV Immediate-Early Transcription Factor RTA Encodes Ubiquitin E3 Ligase Activity that Targets IRF7 for Proteosome-Mediated Degradation

Abstract

Many viruses encode proteins that counteract the development of the interferon (IFN)-mediated antiviral state. Here, we report that interferon regulatory factor 7 (IRF7), a key mediator of type I IFN induction, is targeted for degradation by binding to the RTA immediate-early nuclear transcription factor encoded by Kaposi's sarcoma-associated herpesvirus (KSHV or HHV8). Cotransfection with RTA blocked IRF7-mediated IFNalpha and IFNbeta mRNA production and promoted the ubiquitination and degradation of IRF7 protein in a proteasome-dependent fashion. Addition of RTA also promoted polyubiquitination of IRF7 in an in vitro cell free assay, demonstrating that RTA itself acts as a ubiquitin E3 ligase. RTA also autoregulated its own polyubiquitination and stability, and both activities were abolished by point mutations in a Cys plus His-rich N-terminal domain. Therefore, manipulation of the stability and function of IRF7 by the KSHV RTA transcription factor provides an unexpected regulatory strategy for circumventing the innate immune defence system.

Kaposi Sarcoma-associated Herpesvirus Degrades Cellular Toll-Interleukin-1 Receptor Domain-containing Adaptor-inducing β-Interferon (TRIF)

“…However, IFN production was still inhibited by RTA as reported previously (20). We have examined the mechanism of this IRF-7 degradation-independent IFN inhibition and found that RTA degrades cellular TRIF protein, an upstream adaptor protein involved in IFN production.…”

“…Production-RTA has been shown to degrade cellular IRF-7 for the blockage of IFN production (20). We have examined whether RTA could degrade IRF-7 extensively and found that RTA was unable to degrade IRF-7 in the cell line we used (293T) (Fig.…”

“…The proteasome degradation is usually ubiquitin-dependent involving ubiquitin-activating enzyme E1, ubiquitin-conjugating enzyme E2, and ubiquitin ligase E3 (17)(18)(19). RTA was first documented as an E3 ligase for degradation of a cellular interferon regulatory factor 7 (IRF-7) (20). Two more cellular proteins, K-RBP and Hey, both are repressors of KSHV lytic replication and have been identified as targets for degradation by RTA with the same pathway (21,22).…”

“…IRF-7 was first cloned in the context of Epstein-Barr virus (EBV) latency (45) and is a master gene for IFN production (46). KSHV employs two mechanisms to nullify the function of IRF-7; RTA mediates IRF-7 degradation in certain cell lines (20), and ORF45 blocks the activation of IRF-7 to impair IFN production (47). In addition, KSHV latency-associated nuclear antigen (LANA) is able to block the activation of TBK1-mediated activation of IRF-3 (48).…”

Innate antiviral response targets HIV-1 release by the induction of ubiquitin-like protein ISG15

“…However, whereas polyubiquitination on lysine 43 targets proteins for degradation, lysine 63 polyubiquitination is a signal for kinase activation. Many viruses modulate the Ub-proteasome pathway to alter cellular signaling and the antiviral response (14,15). HIV-1 uses ubiquitination at two steps of its replication cycle.…”