Sendai virus C protein impairs both phosphorylation and dephosphorylation processes of Stat1

Sendai virus expresses C protein that blocks interferon (IFN) signaling. We previously reported suppression of IFN-stimulated tyrosine phosphorylation of signal transducers and activators of transcription (Stats) in infected cells. However this conclusion has remained controversial. To settle it, we re-examined the effect of C protein expression on phosphorylation of Stat1 in detail. IFN-stimulated tyrosine phosphorylation of Stat1 was doubtlessly suppressed early in infection, but the suppression was incomplete, suggesting the importance of the unknown blocking mechanism that inactivates the tyrosine-phosphorylated (pY)-Stat1 generated as the signaling leak. Interestingly, the dephosphorylation process of pY-Stat1 was also impaired. These effects on both phosphorylation and dephosphorylation processes were attributable to the function of the C protein.