A unique feature of influenza A virus (IAV) life cycle is replication of the viral genome in the host cell nucleus. The nuclear import of IAV genome is an indispensable step in establishing virus infection. IAV nucleoprotein (NP) is known to mediate the nuclear import of viral genome via its nuclear localization signals. Here, we demonstrate that cellular heat shock protein 40 (Hsp40/DnaJB1) facilitates the nuclear import of incoming IAV viral ribonucleoproteins (vRNPs) and is important for efficient IAV replication. Hsp40 was found to interact with NP component of IAV RNPs during early stages of infection. This interaction is mediated by the J domain of Hsp40 and N-terminal region of NP. Drug or RNAi mediated inhibition of Hsp40 resulted in reduced nuclear import of IAV RNPs, diminished viral polymerase function and attenuates overall viral replication. Hsp40 was also found to be required for efficient association between NP and importin alpha, which is crucial for IAV RNP nuclear translocation. These studies demonstrate an important role for cellular chaperone Hsp40/DnaJB1 in influenza A virus life cycle by assisting nuclear trafficking of viral ribonucleoproteins.
Reassortant influenza A viruses bearing the H1 subtype of hemagglutinin (HA) and the N2 subtype of neuraminidase (NA) were isolated from humans in the United States, Canada, Singapore, Malaysia, India, Oman, Egypt, and several countries in Europe during the 2001-2002 influenza season. The HAs of these H1N2 viruses were similar to that of the A/New Caledonia/20/99(H1N1) vaccine strain both antigenically and genetically, and the NAs were antigenically and genetically related to those of recent human H3N2 reference strains, such as A/Moscow/10/99(H3N2). All 6 internal genes of the H1N2 reassortants examined originated from an H3N2 virus. This article documents the first widespread circulation of H1N2 reassortants on 4 continents. The current influenza vaccine is expected to provide good protection against H1N2 viruses, because it contains the A/New Caledonia/20/99(H1N1) and A/Moscow/10/99(H3N2)-like viruses, which have H1 and N2 antigens that are similar to those of recent H1N2 viruses.
Influenza virus non-structural protein 1 (NS1) counteracts host antiviral innate immune responses by inhibiting Retinoic acid inducible gene-I (RIG-I) activation. However, whether NS1 also specifically regulates RIG-I transcription is unknown. Here, we identify a CCAAT/Enhancer Binding Protein beta (C/EBPβ) binding site in the RIG-I promoter as a repressor element, and show that NS1 promotes C/EBPβ phosphorylation and its recruitment to the RIG-I promoter as a C/EBPβ/NS1 complex. C/EBPβ overexpression and siRNA knockdown in human lung epithelial cells resulted in suppression and activation of RIG-I expression respectively, implying a negative regulatory role of C/EBPβ. Further, C/EBPβ phosphorylation, its interaction with NS1 and occupancy at the RIG-I promoter was associated with RIG-I transcriptional inhibition. These findings provide an important insight into the molecular mechanism by which influenza NS1 commandeers RIG-I transcriptional regulation and suppresses host antiviral responses.