Structural Basis of the Inhibition of STAT1 Activity by Sendai Virus C Protein

Sendai virus (SeV) C protein inhibits the signal transduction pathways of interferon alpha/beta (IFN-alpha/beta) and IFN-gamma by binding to the N-terminal domain of STAT1 (STAT1ND), thereby allowing SeV to escape from host innate immunity. Here we determined the crystal structure of STAT1ND associated with the C-terminal half of the C protein (Y3 [amino acids 99 to 204]) at a resolution of 2.0 angstrom. This showed that two molecules of Y3 symmetrically bind to each niche created between two molecules of the STAT1ND dimer. Molecular modeling suggested that an antiparallel form of the full-length STAT1 dimer can bind only one Y3 molecule and that a parallel form can bind two Y3 molecules. Affinity analysis demonstrated anticooperative binding of two Y3 molecules with the STAT1 dimer, which is consistent with the hypothetical model that the second Y3 molecule can only target the STAT1 dimer in a parallel form. STAT1 with excess amounts of Y3 was prone to inhibit the dephosphorylation at Tyr(701) by a phosphatase. In an electrophoretic mobility shift assay, tyrosine-phosphorylated STAT1 (pY-STAT1) with Y3 associated with the gamma-activated sequence, probably as high-molecular-weight complexes (HMWCs), which may account for partial inhibition of a reporter assay from IFN-gamma by Y3. Our study suggests that the full-length C protein interferes with the domain arrangement of the STAT1 dimer, leading to the accumulation of pY-STAT1 and the formation of HMWCs. In addition, we discuss the mechanism by which phosphorylation of STAT2 is inhibited in the presence of the C protein after stimulation by IFN-alpha/beta. IMPORTANCE Sendai virus, a paramyxovirus that causes respiratory diseases in rodents, possesses the C protein, which inhibits the signal transduction pathways of interferon alpha/beta (IFN-alpha/beta) and IFN-alpha by binding to the transcription factor STAT1. In virusinfected cells, phosphorylation of STAT1 at the Tyr701 residue is potently enhanced, although transcription by STAT1 is inert. Here, we determined the crystal structure of the N-terminal domain of STAT1 associated with the C-terminal half of the C protein. Molecular modeling and experiments suggested that the two C proteins bind to and stabilize the parallel form of the STAT1 dimer, which are likely to be phosphorylated at Tyr701, further inducing high-molecular-weight complex formation and inhibition of transcription by IFN-gamma. We also discuss the possible mechanism of inhibition of the IFN-alpha/beta pathways by the C protein. This is the first structural report of the C protein, suggesting a mechanism of evasion of the paramyxovirus from innate immunity.