ABSTRACT The neuraminidase protein (NA) of influenza A viruses (IAVs) plays a role in the release of viruses from infected cells. NA on viral particles hydrolyzes sialylated glycans on the cell surface for viral budding. However, the maturation and intracellular functions of NA are poorly understood. To investigate how NA functions intracellularly, glycans displayed on IAV-infected cells were profiled by lectins, and global glycome alterations, accompanied by exposed terminal galactose and glycan recapping with α1-2 fucose, were found in the virus-infected cells. Since α1-2 fucosyltransferases are localized in the Golgi, these unique structures suggest a potential NA function in the IAV-infected cells. Functional analyses using antivirals and NA-expressing cells indicate that intracellular NA function is necessary for the glycome alterations. Time-course analyses in IAV-infected cells revealed that global desialylation and α1-2 fucosylation could be observed 5 h post-inoculation, corresponding to the timeframe of viral protein expression. These observations provide a novel theory of NA functions that NA obtains its enzymatic activity intracellularly before virus assembly and serves desialylated glycans for competitive glycosyltransferases, including α1-2 fucosyltransferases, as their acceptors, resulting in glycan recapping with α1-2 fucose. Hence, intracellular NA blocks the re-sialylation of glycans, promoting efficient virus release from infected cells by inhibiting the interaction between progeny virions and sialosides. This study further demonstrated the potential NA functions of limiting secondary IAV infection. These findings provide insights into the evolutionary strategies of IAVs for shaping the strict window of superinfection by NA functions under a balance between successful replication and reassortment. IMPORTANCE Influenza A viruses (IAVs) exploit glycans for their replication cycle. The hemagglutinin protein uses sialic acid for viral attachment, and the neuraminidase protein (NA) hydrolyzes sialosides for virus release. However, the intracellular functions of NA are not well understood. This study demonstrated that intracellular NA induces global desialylation and glycan recapping with unique structures in IAV-infected cells. This suggests a novel mode of NA function during the IAV lifecycle, where virus particles are ready to be released at the assembly, and NAs no longer need to hydrolyze the sialic acids upon egress from the cells. Therefore, the present study provides novel and significant insights into the fundamental understanding of the lifecycle of IAV. Furthermore, as NA is a primary target for anti-influenza drugs, understanding the mechanism of intracellular NA function may also support the development of antivirals.
Kobayashi et al. (Wed,) studied this question.