Mutations in matrix protein 1 and nucleoprotein caused human-specific defects in nuclear exportation and viral assembly of an avian influenza H7N1 virus

•Human-specific replication defects of an avian influenza virus were identified.•The defects might be involved in avian influenza virus adaptation to human host.•Nuclear export and RNP assembly impairments were responsible for the defects.•M1 and NP mutations were identified as the genetic determinants of the defects. Nuclear exportation of influenza ribonucleoprotein is a vital step in viral replication cycle. In this study a particular H7N1 (A/ostrich/Zimbabwe/222-E3/1996) virus showed exclusively nuclear localization of the viral nucleoprotein (NP) only in human cell lines but not in cell lines of other species suggesting a human-specific nuclear exportation defect. After 10 passages in human lung cells, an adapted strain (H7N1:P10) could efficiently replicate and export viral NP in human cells. Mutations in the NP and matrix M1 gene at position 297 and 227, respectively, were found to rescue the defect. While the NP mutant showed a comparable ratio of total to NP-associated negative-sense RNA in the cytoplasm as compared to the wild type, the M1 mutant showed an increase in free negative-sense RNA in the cytoplasm. These indicated that the NP mutation might cause a nuclear export defect, whereas the M1 mutation might cause a defect in ribonucleoprotein assembly step.