Longer and Shorter Forms of Sendai Virus C Proteins Play Different Roles in Modulating the Cellular Antiviral Response

The Sendai virus (SeV) C gene codes for a nested set of four C proteins that carry out several functions, including the modulation of viral RNA synthesis and countering of the cellular antiviral response. Using mutant C genes (and in particular a C gene with a deletion of six amino acids present only in the larger pair of C proteins) and recombinant SeV carrying these mutant C genes, we find that the nested set of C proteins carry out a nested set of functions. All of the C proteins interdict interferon (IFN) signaling to IFN-stimulated genes (ISGs) and prevent pY701-Stat1 formation. However, only the larger C proteins can induce STAT1 instability, prevent IFN from inducing an antiviral state, or prevent programmed cell death. Remarkably, interdiction of IFN signaling to ISGs and the absence of pY701-Stat1 formation did not prevent IFN-alpha from inducing an anti-Vesicular stomatitis virus (VSV) state. It is possible that IFN-alpha signaling to induce an anti-VSV state can occur independently of the well-established Jak/Stat/ISGF3 pathway and that it is this parallel pathway that is targeted by the longer C proteins.     

Role of protein phosphorylation in activation of interferon-stimulated gene factors.

Possible involvement of protein phosphorylation in interferon (IFN)-mediated activation of IFN-stimulated gene factor 3 (ISGF3) was investigated. For this purpose, in vivo experiments with specific inhibitors of protein kinases and in vitro experiments with protein phosphatases were carried out. In HeLaM cells, 2-aminopurine, an inhibitor of double-stranded RNA-dependent protein kinase, blocked the induction of ISGF3 gamma subunit but not the activation of ISGF3 alpha subunit. A series of experiments using combinations of protein and RNA synthesis inhibitors and 2-aminopurine indicated that the block elicited by 2-aminopurine was at the level of ISGF3 gamma mRNA synthesis. Activation of ISGF3 alpha, although insensitive to 2-aminopurine, was completely blocked by 10 nM staurosporine, an inhibitor of protein kinase C. On the other hand, even 500 nM staurosporine did not block the induction of ISGF3 gamma. Incubation of cytoplasmic or nuclear extracts of IFN-treated HeLaM cells in vitro with alkaline phosphatase completely eliminated their ability to form the ISGF3 complex but not the ISGF1 complex. Treatment with acid phosphatase, on the other hand, changed the electrophoretic mobility of the ISGF3 complex but did not obliterate it. Complementation experiments revealed that ISGF3 alpha was the alkaline phosphatase-sensitive component of the complex. These results suggest that a protein kinase C-mediated phosphorylation step is involved in ISGF3 alpha activation and a 2-aminopurine-sensitive component is involved in ISGF3 gamma mRNA induction.