The viral protein N is unique to the genus within the family . After autocatalytic cleavage from the nascent polyprotein, N suppresses type I IFN (IFN-α/β) induction by mediating proteasomal degradation of IFN regulatory factor 3 (IRF-3). Previous studies found that the N-mediated IRF-3 degradation was dependent of a TRASH domain in the C-terminal half of N coordinating zinc by means of the amino acid residues C112, C134, D136 and C138. Interestingly, four classical swine fever virus (CSFV) isolates obtained from diseased pigs in Thailand in 1993 and 1998 did not suppress IFN-α/β induction despite the presence of an intact TRASH domain. Through systematic analyses, it was found that an amino acid mutation at position 40 or mutations at positions 17 and 61 in the N-terminal half of N of these four isolates were related to the lack of IRF-3-degrading activity. Restoring a histidine at position 40 or both a proline at position 17 and a lysine at position 61 based on the sequence of a functional N contributed to higher stability of the reconstructed N compared with the N from the Thai isolate. This led to enhanced interaction of N with IRF-3 along with its degradation by the proteasome. The results of the present study revealed that amino acid residues in the N-terminal domain of N are involved in the stability of N, in interaction of N with IRF-3 and subsequent degradation of IRF-3, leading to downregulation of IFN-α/β production.


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