In this article we show that the paramyxovirus SV5 is a poor inducer of interferon-β (IFN-β). This inefficient induction is a consequence of the expression of an intact viral V protein. In the absence of the viral V protein cysteine-rich C-terminal domain, IFN-β mRNA is strongly induced and the transcription factors NF-κB and IRF-3 are activated significantly. The V protein can work in isolation from SV5 to block intracellular dsRNA signaling. The mechanism of block to dsRNA signaling is distinct from that previously observed for blocking IFN signaling in that proteolysis of candidate factors cannot be detected, and furthermore, the respective blocks require distinct protein domains. Blocking of the induction of IFN-β by dsRNA requires the C-terminal cysteine-rich domain, a feature that is highly conserved among paramyxoviruses. We demonstrate that the V proteins from other paramyxoviruses have equivalent functions and speculate that limiting the yield of IFN-β during infection may be a general property of paramyxoviruses.
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