Regulation of Interferon-β by MAGI-1 and Its Interaction with Influenza A Virus NS1 Protein with ESEV PBM

The NS1 protein from avian influenza A viruses contains a PDZ binding motif (PBM) at its carboxyl terminus with the consensus sequence ESEV. The ESEV PBM confers binding to several cellular PDZ proteins, including Dlg1, MAGI-1 and Scribble. The interaction between NS1 and Scribble protects infected cells from apoptosis, while the interaction between NS1 and both Dlg1 and Scribble disrupts tight junctions. In this study, we examined the MAGI-1 protein. We made the unexpected observation that siRNA depletion of MAGI-1 activates IRF3 and induces the IFN-β promoter. We found that the ESEV NS1 protein sequesters MAGI-1 away from the plasma membrane in infected cells. Using plasmid vectors to express NS1 proteins, we observed that the ESEV PBM elicits an IFN-β induction signal as indicated by activation of IRF3 and a relative deficiency in NS1 inhibition of induction of the IFN-β promoter by dsRNA or RIG-I. Taken together, our data suggest that disruption of MAGI-1 by the ESEV PBM activates an IFN-β induction signal. During viral infection, however, induction of the IFN-β gene does not occur presumably because other anti-IFN functions dominate over the IFN-activation activity of the ESEV PBM. We postulate that the ESEV PBM's broad binding activity for PDZ proteins may allow NS1 to bind to some PDZ proteins such as MAGI-1 that confer no benefit or may even be detrimental to viral replication. However, the advantage of binding to key PDZ proteins such as Dlg1 and Scribble may dominate and therefore provide an overall benefit for the virus to encode the ESEV PBM.