Genetically Engineered, Biarsenically Labeled Influenza Virus Allows Visualization of Viral NS1 Protein in Living Cells

Real-time fluorescence imaging of viral proteins in living cells provides a valuable means to study virus-host interactions. The challenge of generating replication-competent fluorescent influenza A virus is that the segmented genome does not allow fusion of a fluorescent protein gene to any viral gene. Here, we introduced the tetracysteine (TC) biarsenical labeling system into influenza virus in order to fluorescently label viral protein in the virus life cycle. We generated infectious influenza A viruses bearing a small TC tag (CCPGCC) in the loop/linker regions of the NS1 proteins. In the background of A/Puerto Rico/8/34 (H1N1) (PR8) virus, the TC tag can be inserted into NS1 after amino acid 52 (AA52) (PR8-410), AA79 (PR8-412), or AA102 (PR8-413) or the TC tag can be inserted and replace amino acids 79 to 84 (AA79-84) (PR8-411). Although PR8-410, PR8-411, and PR8-412 viruses are attenuated than the wild-type (WT) virus to some extent in multiple-cycle infection, their growth potential is similar to that of the WT virus during a single cycle of infection, and their NS1 subcellular localization and viral protein synthesis rate are quite similar to those of the WT virus. Furthermore, labeling with membrane-permeable biarsenical dye resulted in fluorescent NS1 protein in the context of virus infection. We could exploit this strategy on NS1 protein of A/Texas/36/91 (H1N1) (Tx91) by successfully rescuing a TC-tagged virus, Tx91-445, which carries the TC tag replacement of AA79-84. The infectivity of Tx91-445 virus was similar to that of WT Tx91 during multiple cycles of replication and a single cycle of replication. The NS1 protein derived from Tx91-445 can be fluorescently labeled in living cells. Finally, with biarsenical labeling, the engineered replication-competent virus allowed us to visualize NS1 protein nuclear import in virus-infected cells in real time.