Pestivirus Npro Directly Interacts with Interferon Regulatory Factor 3 Monomer and Dimer

Interferon regulatory factor 3 (IRF3) is a transcription factor involved in the activation of type I alpha/beta interferon (IFN-α/β) in response to viral infection. Upon viral infection, the IRF3 monomer is activated into a phosphorylated dimer, which induces the transcription of interferon genes in the nucleus. Viruses have evolved several ways to target IRF3 in order to subvert the innate immune response. Pestiviruses, such as classical swine fever virus (CSFV), target IRF3 for ubiquitination and subsequent proteasomal degradation. This is mediated by the viral protein N(pro) that interacts with IRF3, but the molecular details for this interaction are largely unknown. We used recombinant N(pro) and IRF3 proteins and show that N(pro) interacts with IRF3 directly without additional proteins and forms a soluble 1:1 complex. The full-length IRF3 but not merely either of the individual domains is required for this interaction. The interaction between N(pro) and IRF3 is not dependent on the activation state of IRF3, since N(pro) binds to a constitutively active form of IRF3 in the presence of its transcriptional coactivator, CREB-binding protein (CBP). The results indicate that the N(pro)-binding site on IRF3 encompasses a region that is unperturbed by the phosphorylation and subsequent activation of IRF3 and thus excludes the dimer interface and CBP-binding site.

The pestivirus N-terminal protease, N(pro), is essential for evading the host's immune system by facilitating the degradation of interferon regulatory factor 3 (IRF3). However, the nature of the N(pro) interaction with IRF3, including the IRF3 species (inactive monomer versus activated dimer) that N(pro) targets for degradation, is largely unknown. We show that classical swine fever virus N(pro) and porcine IRF3 directly interact in solution and that full-length IRF3 is required for interaction with N(pro) Additionally, N(pro) interacts with a constitutively active form of IRF3 bound to its transcriptional cofactor, the CREB-binding protein. This is the first study to demonstrate that N(pro) is able to bind both inactive IRF3 monomer and activated IRF3 dimer and thus likely targets both IRF3 species for ubiquitination and proteasomal degradation.