Influenza A virus use of BinCARD1 to facilitate the binding of viral NP to importin α7 is counteracted by TBK1-p62 axis-mediated autophagy

As a major component of the viral ribonucleoprotein (vRNP) complex in influenza A virus (IAV), nucleoprotein (NP) interacts with isoforms of importin α family members, leading to the import of itself and vRNP complex into the nucleus, a process pivotal in the replication cycle of IAV. In this study, we found that BinCARD1, an isoform of Bcl10-interacting protein with CARD (BinCARD), was leveraged by IAV for efficient viral replication. BinCARD1 promoted the nuclear import of the vRNP complex and newly synthesized NP and thus enhanced vRNP complex activity. Moreover, we found that BinCARD1 interacted with NP to promote NP binding to importin α7, an adaptor in the host nuclear import pathway. However, we also found that BinCARD1 promoted RIG-I-mediated innate immune signaling by mediating Lys63-linked polyubiquitination of TRAF3, and that TBK1 appeared to degrade BinCARD1. We showed that BinCARD1 was polyubiquitinated at residue K103 through a Lys63 linkage, which was recognized by the TBK1-p62 axis for autophagic degradation. Overall, our data demonstrate that IAV leverages BinCARD1 as an important host factor that promotes viral replication, and two mechanisms in the host defense system are triggered—innate immune signaling and autophagic degradation—to mitigate the promoting effect of BinCARD1 on the life cycle of IAV.