Naturally Occurring Substitutions in the P/V Gene Convert the Noncytopathic Paramyxovirus Simian Virus 5 into a Virus That Induces Alpha/Beta Interferon Synthesis and Cell Death

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Abstract
The V protein of the paramyxovirus simian virus 5 (SV5) is responsible for targeted degradation of STAT1 and the block in alpha/beta interferon (IFN-α/β) signaling that occurs after SV5 infection of human cells. We have analyzed the growth properties of a recombinant SV5 that was engineered to be defective in targeting STAT1 degradation. A recombinant SV5 (rSV5-P/V-CPI−) was engineered to contain six naturally occurring P/V protein mutations, three of which have been shown in previous transfection experiments to disrupt the V-mediated block in IFN-α/β signaling. In contrast to wild-type (WT) SV5, human cells infected with rSV5-P/V-CPI− had STAT1 levels similar to those in mock-infected cells. Cells infected with rSV5-P/V-CPI− were found to express higher-than-WT levels of viral proteins and mRNA, suggesting that the P/V mutations had disrupted the regulation of viral RNA synthesis. Despite the inability to target STAT1 for degradation, single-step growth assays showed that the rSV5-P/V-CPI− mutant virus grew better than WT SV5 in all cell lines tested. Unexpectedly, cells infected with rSV5-P/V-CPI− but not WT SV5 showed an activation of a reporter gene that was under control of the IFN-β promoter. The secretion of IFN from cells infected with rSV5-P/V-CPI− but not WT SV5 was confirmed by a bioassay for IFN. The rSV5-P/V-CPI− mutant grew to higher titers than did WT rSV5 at early times in multistep growth assays. However, rSV5-P/V-CPI− growth quickly reached a final plateau while WT rSV5 continued to grow and produced a final titer higher than that of rSV5-P/V-CPI− by late times postinfection. In contrast to WT rSV5, infection of a variety of cell lines with rSV5-P/V-CPI− induced cell death pathways with characteristics of apoptosis. Our results confirm a role for the SV5 V protein in blocking IFN signaling but also suggest new roles for the P/V gene products in controlling viral gene expression, the induction of IFN-α/β synthesis, and virus-induced apoptosis.

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