The Nuclear Export Protein of H5N1 Influenza A Viruses Recruits Matrix 1 (M1) Protein to the Viral Ribonucleoprotein to Mediate Nuclear Export*

Background: NEP stimulates viral RNA synthesis and nuclear vRNP export. Results: NEP is required to stabilize M1-vRNP binding for nuclear export. Deletion of the last three amino acids of NEP abrogates nuclear export and the polymerase-enhancing function of NEP. Conclusion: The polymerase-enhancing function and the nuclear export function of NEP are linked functionally. Significance: This study provides new insights into the assembly of the nuclear export complex. In influenza A virus-infected cells, replication and transcription of the viral genome occurs in the nucleus. To be packaged into viral particles at the plasma membrane, encapsidated viral genomes must be exported from the nucleus. Intriguingly, the nuclear export protein (NEP) is involved in both processes. Although NEP stimulates viral RNA synthesis by binding to the viral polymerase, its function during nuclear export implicates interaction with viral ribonucleoprotein (vRNP)-associated M1. The observation that both interactions are mediated by the C-terminal moiety of NEP raised the question whether these two features of NEP are linked functionally. Here we provide evidence that the interaction between M1 and the vRNP depends on the NEP C terminus and its polymerase activity-enhancing property for the nuclear export of vRNPs. This suggests that these features of NEP are linked functionally. Furthermore, our data suggest that the N-terminal domain of NEP interferes with the stability of the vRNP-M1-NEP nuclear export complex, probably mediated by its highly flexible intramolecular interaction with the NEP C terminus. On the basis of our data, we propose a new model for the assembly of the nuclear export complex of Influenza A vRNPs.