Transcription and replication of the influenza A virus are carried out in the nucleus of the infected cells in the context of viral ribonucleoproteins (RNPs). The viral polymerase responsible for these processes is a protein complex composed by the PB1, PB2 and PA proteins. We previously identified a set of polymerase-associated cellular proteins by proteomic analysis of polymerase-containing intracellular complexes expressed and purified from human cells. Here we characterize the role of NXP2/MORC3 in the infection cycle. NXP2/MORC3 is a member of the Microrchidia (MORC) family that is associated to the nuclear matrix and has RNA-binding activity. Influenza virus infection led to a slight increase in NXP2/MORC3 expression and partial relocalization to the cytoplasm. Co-immunoprecipitation and immunofluorescence experiments indicated an association of NXP2/MORC3 with viral polymerase and RNPs during infection. Down-regulation of NXP2/MORC3 with two independent shRNAs reduced virus titers in low-multiplicity infections. Consistent with these findings, analysis of virus-specific RNA in high-multiplicity infections indicated a reduction of vRNA and mRNA after NXP2/MORC3 down-regulation. Silencing of NXP2/MORC3 in a recombinant mini-replicon system, where virus transcription and replication are uncoupled, showed a reduction in cat mRNA and CAT protein accumulation, but no alterations in cat vRNA levels, suggesting that NXP2/MORC3 is important for influenza virus transcription.

IMPORTANCE:

Influenza infections appear as yearly epidemics and occasional pandemics of respiratory disease with large morbidity and occasional mortality. Influenza viruses are intracellular parasites that replicate and transcribe their genomic ribonucleoproteins in the nucleus of infected cells, in a complex interplay with host cell factors. Here we characterized the role of human NXP2/MORC3 protein, a member of the Microrchidia family that is associated to the nuclear matrix, during virus infection. NXP2/MORC3 associates to the viral ribonucleoproteins in infected cells. Down-regulation of NXP2/MORC3 reduced virus titers and the accumulations of viral genomic and mRNAs. Silencing of NXP2/MORC3 in an influenza CAT mini-replicon system diminished CAT protein and cat mRNA but not genomic RNA. We propose that NXP2/MORC3 plays a role in influenza virus transcription