Bunyamwera Virus Nonstructural Protein NSs Counteracts Interferon Regulatory Factor 3-Mediated Induction of Early Cell Death

Abstract

The genome of Bunyamwera virus (BUN; family Bunyaviridae, genus Orthobunyavirus) consists of three segments of negative-sense RNA. The smallest segment, S, encodes two proteins, the nonstructural protein NSs, which is nonessential for viral replication and transcription, and the nucleocapsid protein N. Although a precise role in the replication cycle has yet to be attributed to NSs, it has been shown that NSs inhibits the induction of alpha/beta interferon, suggesting that it plays a part in counteracting the host antiviral defense. A defense mechanism to limit viral spread is programmed cell death by apoptosis. Here we show that a recombinant BUN that does not express NSs (BUNdelNSs) induces apoptotic cell death more rapidly than wild-type virus. Screening for apoptosis pathways revealed that the proapoptotic transcription factor interferon regulatory factor 3 (IRF-3) was activated by both wild-type BUN and BUNdelNSs infection, but only wild-type BUN was able to suppress signaling downstream of IRF-3. Studies with a BUN minireplicon system showed that active replication induced an IRF-3-dependent promoter, which was suppressed by the NSs protein. In a cell line (P2.1) defective in double-stranded RNA signaling due to low levels of IRF-3, induction of apoptosis was similar for wild-type BUN and BUNdelNSs. These data suggest that the BUN NSs protein can delay cell death in the early stages of BUN infection by inhibiting IRF-3-mediated apoptosis.

Inhibition of RNA Polymerase II Phosphorylation by a Viral Interferon Antagonist

“…6B, panel 3). This result is in agreement with our previous finding that IRF-3 activation is not affected by NSs (20). Taken together, these data suggest that RNAP II can be fully phosphorylated and recruited to the IFN-␤ promoter even in the absence of virus infection but requires IRF-3 for efficient transcriptional activity.…”

“…However, in mice with a defective IFN system this NSs-deficient virus was as virulent as the wild-type virus (wt BUNV). Further analyses revealed that NSs inhibited transcriptional activation of the IFN-␤ gene without interfering with the activation of IRF-3 (18,20). These findings suggested that NSs blocks a transcriptional step downstream of IRF-3.…”

Interaction of Bunyamwera Orthobunyavirus NSs Protein with Mediator Protein MED8: a Mechanism for Inhibiting the Interferon Response

“…Previously, we characterized the role of Bunyamwera virus NSs protein in host cell protein shutoff and inhibition of the interferon pathway (12,29,47,50). Further studies suggested that anti-interferon activity was a consequence of the ability of NSs to inhibit phosphorylation of the CTD of RNA polymerase II in mammalian cells (49).…”

Effects of Nonstructural Proteins NS1 and NS2 of Human Respiratory Syncytial Virus on Interferon Regulatory Factor 3, NF-κB, and Proinflammatory Cytokines

“…Activated IRF-3 also can directly up-regulate transcription of additional genes, such as the proinflammatory chemokine RANTES (regulated on activation normal T-cell expressed and secreted) (22). A number of viruses have been shown to encode proteins that interfere with activation of IRF-3, including Bunyamwera (18), influenza A (32), and Ebola (2) viruses. Recently, the NS1 and NS2 proteins of BRSV were implicated in inhibiting phosphoryla-tion of IRF-3 (6).…”