Effects of the nuclear localization of the N super(pro) protein of classical swine fever virus on its virulence in pigs

The N super(pro) protein of classical swine fever virus (CSFV) is localized in the cytoplasm and nucleus. However, it is unknown whether the nuclear localization of N super(pro) correlates with the virulence of CSFV in the host. Previously, we showed that the N super(pro) protein fused with interferon regulatory factor 3 (IRF3) was present only in the cytoplasm. Here, we generated and evaluated a recombinant CSFV vSM-IRF3 harboring the IRF3 gene inserted into the N super(pro) gene of the highly virulent CSFV Shimen strain. Compared to the even nuclear and cytoplasmic distribution of the enhanced green fluorescent protein (EGFP)-N super(pro) fusion expressed by the recombinant CSFV EGFP-CSFV, vSM-IRF3 expressed an IRF3-N super(pro) fusion protein that only was localized in the cytoplasm. vSM-IRF3 was markedly attenuated in vitro and in vivo, and the inoculated pigs were completely protected from lethal CSFV challenge, whereas the parental virus as well as EGFP-CSFV exhibited a typical virulent phenotype. Taken together, the nuclear localization of N super(pro) plays a significant role in the CSFV replication and virulence.