Molecular basis of IFN-γ-induced STAT3 phosphorylation stimulated by Sendai virus C protein

Sendai virus, belonging to the Respirovirus genus in the Paramyxoviridae family, possesses C protein to escape from host innate immunity by inhibiting the IFN-α/β-induced STAT1:STAT2 pathway and the interferon (IFN)-γ-induced STAT1 pathway via binding to the N-terminal domain of STAT1 (STAT1ND). In this study, a yeast two-hybrid analysis revealed that C protein also binds directly to the N-terminal domain of STAT3 (STAT3ND). The C-terminal region of C protein (named Y3) was sufficient for binding to STAT3ND, similar to STAT1ND binding. However, the affinity of Y3 for STAT3ND was significantly weaker than that for STAT1ND. Transfection experiments using 293T cells demonstrated that the introduction of C protein significantly stimulated the IFN-γ-induced phosphorylation of STAT3. Considering the results of stoichiometric and confocal analyses together, C protein likely plays a role in stabilizing the dimeric structure formed by STAT3ND, stimulating the recruitment of dimeric STAT3 to the plasma membrane. Reporter assay demonstrated the persistent activation of the STAT3 pathway in the presence of C protein after IFN-γ stimulation. The STAT1 homodimer, bound to two molecules of C protein, cannot take an active form to promote the transcription of target genes. In contrast, STAT3 can take an active form even in the presence of C protein, probably because the complex formed between them is fragile.