Rotavirus NSP1 Inhibits NFκB Activation by Inducing Proteasome-Dependent Degradation of β-TrCP: A Novel Mechanism of IFN Antagonism

Abstract

Mechanisms by which viruses counter innate host defense responses generally involve inhibition of one or more components of the interferon (IFN) system. Multiple steps in the induction and amplification of IFN signaling are targeted for inhibition by viral proteins, and many of the IFN antagonists have direct or indirect effects on activation of latent cytoplasmic transcription factors. Rotavirus nonstructural protein NSP1 blocks transcription of type I IFNα/β by inducing proteasome-dependent degradation of IFN-regulatory factors 3 (IRF3), IRF5, and IRF7. In this study, we show that rotavirus NSP1 also inhibits activation of NFκB and does so by a novel mechanism. Proteasome-mediated degradation of inhibitor of κB (IκBα) is required for NFκB activation. Phosphorylated IκBα is a substrate for polyubiquitination by a multisubunit E3 ubiquitin ligase complex, Skp1/Cul1/F-box, in which the F-box substrate recognition protein is β-transducin repeat containing protein (β-TrCP). The data presented show that phosphorylated IκBα is stable in rotavirus-infected cells because infection induces proteasome-dependent degradation of β-TrCP. NSP1 expressed in isolation in transiently transfected cells is sufficient to induce this effect. Targeted degradation of an F-box protein of an E3 ligase complex with a prominent role in modulation of innate immune signaling and cell proliferation pathways is a unique mechanism of IFN antagonism and defines a second strategy of immune evasion used by rotaviruses.

Author Summary

Cells respond to virus infection by inducing a pattern of gene expression regulated by interferon (IFN) that modulates the host immune response. In order to successfully replicate, viruses have evolved mechanisms to block the induction or function of IFN and IFN-regulated genes. Multiple proteins are activated in the cell when virus infection is detected, and it is these steps upstream of IFN synthesis that are commonly inhibited by virally encoded proteins. In this study, we investigated the function of a rotavirus protein known to block IFN responses. Rotaviruses cause life-threatening gastroenteritis in infants and young children and are responsible for substantial morbidity and mortality worldwide. Here we show that the rotavirus protein NSP1 blocks the function of cellular protein β-TrCP by inducing its degradation. β-TrCP is a component of a multisubunit complex important in the proteasome degradation pathway and is required for activation of transcription factor NFκB, which is necessary for expression of IFN and IFN-regulated genes. This is a unique strategy for viral evasion of host immune responses. Identification of viral IFN antagonists is important to understanding how best to induce cellular responses that can override viral evasion strategies and help reduce spread of infection in the host.