A crucial role of N-terminal domain of influenza A virus M1 protein in interaction with swine importin α1 protein

The matrix 1 (M1) protein is a multifunctional protein in the life cycle of influenza virus. It plays an important role in virus budding and intracellular trafficking of viral ribonucleoproteins (vRNPs). The M1 protein consists of three domains based on the structure: N-terminal domain, Middle domain, and C-terminal domain. However, the functions of different domains of the M1 protein remain largely unclear. In this study, using bimolecular fluorescence complementation assays (BIFC) we demonstrated that swine importin alpha 1 interacts with the M1 protein and transports it to the nucleus. Interestingly, M1 with mutated nuclear localization signal (NLS; 101-RKLKR-105 to 101-AALAA-105) still interacts with swine importin alpha 1 and is localized in the nucleus, suggesting that the NLS located at residues 101-105 is not the only NLS within M1 recombinant protein containing 1-160 residues of M1 with mutated nuclear localization signal is able to interact with swine importin alpha 1, but M1/60-252 domains cannot bind importin alpha 1. Further mapping showed that the deletion of residues 1-20 impaired the interaction between N terminus of M1 and importin alpha 1. Collectively, our data suggested that the N-terminal domain of M1 protein is critical for binding swine importin alpha 1 and for nuclear localization.