A short peptide at the amino terminus of the Sendai virus C protein acts as an independent element that induces STAT1 instability

The Sendai virus C protein acts to dismantle the interferon-induced cellular antiviral state in an MG132-sensitive manner, in part by inducing STAT1 instability. This activity of C maps to the first 23 amino acids (C1-23) of the 204-amino-acid (aa)-long protein (C1-204). C1-23 was found to act as an independent viral element that induces STAT1 instability, since this peptide fused to green fluorescent protein (C1-23/GFP) is at least as active as C1-204 in this respect. This peptide also induces the degradation of C1-23 /GFP and other proteins to which it is fused. Most of C1-204, and particularly its amino-terminal half, is predicted to be structurally disordered. C1-23 as a peptide was found to be disordered by circular dichroism, and the first 11 aa have a strong potential to form an amphipathic alpha-helix in low concentrations of trifluoroethanol, which is thought to mimic protein-protein interaction. The critical degradation-determining sequence of C1-23 was mapped by mutation to eight residues near its N terminus: (FLKKILKL11)-F-4. All the large hydrophobic residues of (FLK)-F-4 KILKL11, plus its ability to form an amphipathic alpha-helix, were found to be critical for STAT1 degradation. In contrast, C1-23/GFP self-degradation did not require (ILKL11)-I-8, nor the ability to form an alpha-helix throughout this region. Remarkably, C1-23/GFP also stimulated C1-204 degradation, and this degradation in trans required the same peptide determinants as for STAT1. Our results suggest that C1-204 coordinates its dual activities of regulating viral RNA synthesis and counteracting the host innate antiviral response by sensing both its own intracellular concentration and that of STAT1.