Non-structural protein 1 b (nsp1 b) of porcine reproductive and respiratory syndrome virus (PRRSV) contains a papain-like cysteine protease (PLPb) domain and has been identified as the main viral protein antagonizing the host innate immune response. In this study, nsp1 b was determined to suppress the expression of reporter genes as well as to suppress 'self-expression' in transfected cells, and this activity appeared to be associated with its interferon (IFN) antagonist function. To knock down the effect of nsp1 b on IFN activity, a panel of site-specific mutations in nsp1 b was analysed. Double mutations K130A/R134A (type 1 PRRSV) or K124A/R128A (type 2 PRRSV) targeting a highly conserved motif of nsp1 b, GKYLQRRLQ (in bold), impaired the ability of nsp1 b to suppress IFN-b and reporter gene expression, as well as to suppress 'self-expression' in vitro. Subsequently, viable recombinant viruses vSD01-08-K130A/R134A and vSD95-21- K124A/R128A, containing double mutations in the GKYLQRRLQ motif were generated using reverse genetics. In comparison with WT viruses, these nsp1 b mutants showed impaired growth ability in infected cells, but the PLPb cleavage function was not directly affected. The expression of selected innate immune genes was determined in vSD95-21-K124A/R128A mutant-infected cells. The results consistently showed that gene expression levels of IFN-a, IFN-b and IFN- stimulated gene 15 were upregulated in cells that were infected with the vSD95-21-K124A/ R128A compared with that of WT virus. These data suggest that PRRSV nsp1 b may selectively suppress cellular gene expression, including expression of genes involved in the host innate immune function. Modifying the key residues in the highly conserved GKYLQRRLQ motif could attenuate virus growth and improve the cellular innate immune responses.
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