Porcine Reproductive and Respiratory Syndrome Virus Nsp1β Inhibits Interferon-Activated JAK/STAT Signal Transduction by Inducing Karyopherin-α1 Degradation

Abstract

Porcine reproductive and respiratory syndrome virus (PRRSV) inhibits the interferon-mediated antiviral response. Type I interferons (IFNs) induce the expression of IFN-stimulated genes by activating phosphorylation of both signal transducer and activator of transcription 1 (STAT1) and STAT2, which form heterotrimers (interferon-stimulated gene factor 3 [ISGF3]) with interferon regulatory factor 9 (IRF9) and translocate to the nucleus. PRRSV Nsp1␤ blocks the nuclear translocation of the ISGF3 complex by an unknown mechanism. In this study, we discovered that Nsp1␤ induced the degradation of karyopherin-␣1 (KPNA1, also called importin-␣5), which is known to mediate the nuclear import of ISGF3. Overexpression of Nsp1␤ resulted in a reduction of KPNA1 levels in a dose-dependent manner, and treatment of the cells with the proteasome inhibitor MG132 restored KPNA1 levels. Furthermore, the presence of Nsp1␤ induced an elevation of KPNA1 ubiquitination and a shortening of its half-life. Our analysis of Nsp1␤ deletion constructs showed that the N-terminal domain of Nsp1␤ was involved in the ubiquitinproteasomal degradation of KPNA1. A nucleotide substitution resulting in an amino acid change from valine to isoleucine at residue 19 of Nsp1␤ diminished its ability to induce KPNA1 degradation and to inhibit IFN-mediated signaling. Interestingly, infection of MARC-145 cells by PRRSV strains VR-2332 and VR-2385 also resulted in KPNA1 reduction, whereas infection by an avirulent strain, Ingelvac PRRS modified live virus (MLV), did not. MLV Nsp1␤ had no effect on KPNA1; however, a mutant with an amino acid change at residue 19 from isoleucine to valine induced KPNA1 degradation. These results indicate that Nsp1␤ blocks ISGF3 nuclear translocation by inducing KPNA1 degradation and that valine-19 in Nsp1␤ correlates with the inhibition.

The non-structural protein Nsp2TF of porcine reproductive and respiratory syndrome virus down-regulates the expression of Swine Leukocyte Antigen class I

“…Wang and Christopher-Hennings, 2012) by PRRSV proteins, such as nsp1α, 1β, 2 and 11 as well as N (Beura et al, 2010Subramaniam et al, 2012;Wang et al, 2013;Wongyanin et al, 2012;Yoo et al, 2010). However, few studies have attempted to identify the viral protein(s) that modulate SLA-I presentation.…”

MicroRNA-30c Modulates Type I IFN Responses To Facilitate Porcine Reproductive and Respiratory Syndrome Virus Infection by Targeting JAK1

“…Nsp1 was identified as the most potent antagonist and is able to inhibit the production and signaling of IFN-I. A previous study demonstrated that Nsp1 inhibits IFN-I signaling via inducing the degradation of karyopherin-a1 (also known as importin-a5), a nuclear transport protein involved in the import of ISG factor 3 to the nucleus (43). Different PRRSV strains exert distinct effects on the IFN-I signaling pathway.…”

“…For example, VR-2332 and VR-2385 infections decrease KPNA1 expression, whereas the MLV strain of PRRSV does not. However, MLV suppresses the activation of ISG15 and ISG56 induced by IFNs in MARC-145 cells, implying that other mechanisms might contribute to the impairment of IFN-I signaling during PRRSV infection (43). Cellular miRNAs were shown to be used by viruses to evade IFN-I-mediated antiviral responses in host cells (18,21,44,45).…”

The ORF3 protein of porcine circovirus type 2 promotes secretion of IL-6 and IL-8 in porcine epithelial cells by facilitating proteasomal degradation of regulator of G protein signalling 16 through physical interaction

“…A recent observation provides additional evidence supporting the idea that some viral proteins facilitate degradation of their binding partner in host cells through a ubiquitinmediated proteasome-dependent pathway (Wang et al, 2013).…”