Degradation of CREB-binding protein and modulation of type I interferon induction by the zinc finger motif of the porcine reproductive and respiratory syndrome virus nsp1α subunit

Non-structural protein (nsp) 1 of PRRS virus is a viral antagonist for type I interferons (IFNs), and in cells expressing nsp1, CREB-binding protein (CBP) is degraded. nsp1 is auto-processed into nsp1α and nsp1β subunits and in the present study we show that the nsp1α subunit was responsible for CBP degradation. The nsp1α subunit contains three distinct functional motifs; a papain-like cysteine protease α (PCPα) motif, an N-terminal zinc finger motif (ZF1), and a newly reported C-terminal zinc finger motif (ZF2). To study the structure function of nsp1α and its IFN antagonism, these motifs were individually mutated and the mutants were examined for their IFN suppression ability. The mutations that destroyed the PCPα activities (C76S, H146Y, and C76S/H146Y) did not affect the IFN suppressive activity of nsp1α, indicating that the cysteine protease activity did not participate in IFN suppression. The mutations of C70S, C76S, H146Y, and/or M180I which coordinated the ZF2 motif also did not alter IFN suppression. However, the mutations of C8S, C10S, C25S, and/or C28S for the ZF1 motif impaired the IFN antagonism of nsp1α, demonstrating that ZF1 was the essential element of nsp1α for IFN suppression. Wild-type nsp1α localized in the both nucleus and cytoplasm, but the ZF1 mutants that lost the IFN suppressive activity did not localize in the nucleus and remained in the cytoplasm. Consistent with their cytoplasmic distribution, CBP was not degraded by these mutants. Our results indicate that the ZF1 motif of nsp1α plays an important role for IFN regulation and further demonstrate that the CBP degradation is likely the key mechanism for IFN suppression mediated by the nsp1α subunit protein of PRRS virus.