The Npro product of classical swine fever virus interacts with I?B?, the NF-?B inhibitor

Classical swine fever virus (CSFV) belongs to the genus Pestivirus and is the causative agent of classical swine fever, a haemorrhagic disease of pigs. The virus replicates in host cells without activating interferon (IFN) production and has been reported to be an antagonist of double-stranded RNA-induced apoptosis. The N-terminal protease (Npro) of CSFV is responsible for this evasion of the host innate immune response. In order to identify cellular proteins that interact with the Npro product of CSFV, a yeast two-hybrid screen of a human library was carried out, which identified I?B?, the inhibitor of NF-?B, a transcription factor involved in the control of apoptosis, the immune response and IFN production. The Npro–I?B? interaction was confirmed using yeast two-hybrid analysis and additional co-precipitation assays. It was also shown that Npro localizes to both the cytoplasmic and nuclear compartments in stably transfected cells and in CSFV-infected cells. Following stimulation by tumour necrosis factor alpha, PK-15 cell lines expressing Npro exhibited transient nuclear accumulation of pI?B?, but no effect of CSFV infection on I?B? localization or NF-?B p65 activation was observed.