Nonstructural protein of severe fever with thrombocytopenia syndrome phlebovirus targets STAT2 and not STAT1 to inhibit type I interferon-stimulated JAK-STAT signaling.

The nonstructural protein NSs of severe fever with thrombocytopenia syndrome phlebovirus blocks type I interferon (IFN)-stimulated JAK-STAT signaling. However, there is continuing controversy as to whether NSs targets STAT1 or STAT2 or both for this blockade. The present study was designed to gain a further understanding of the blockade mechanism. Immunoprecipitation experiments revealed a stronger interaction of NSs with STAT2 than with any other component constituting the JAK-STAT pathway. Expression of NSs resulted in the formation of cytoplasmic inclusion bodies (IBs), and affected cytoplasmic distribution of STAT2. STAT2 was relocated to NSs-induced IBs. Consequently, NSs inhibited IFN-α-stimulated tyrosine phosphorylation and nuclear translocation of STAT2. These inhibitory effects as well as the signaling blockade activity were not observed in NSs mutant proteins lacking the STAT2-binding ability. In contrast, NSs affected neither subcellular distribution nor phosphorylation of STAT1 in response to IFN-α and IFN-γ, demonstrating that NSs has little physical and functional interactions with STAT1. Taken together, these results suggest that NSs sequesters STAT2 into NSs-induced IBs, thereby blocking type I IFN JAK-STAT signaling.