Enveloped. Usually rounded but can be filamentous. The virions are 80-120 nm in diameter.
Segmented ssRNA(-) linear genome, encapsidated by nucleoprotein (NP). Contains 8 segments coding for 11 proteins. Segments size range from 890 to 2,341nt. Genome total size is 13.5Kb
Viral RNA polymerase (PB1, PB2 and PA) transcribes one mRNA from each genome segment. Transcription is primed by cap cleaved from cellular mRNAS by Cap snatching. mRNA are polyadenylated by the viral polymerase stuttering on a poly U track. MP and NS mRNA can be alternatively spliced, giving rise to mRNA coding for M2 and NEP proteins. PB1-F2 is translated from the +1 ORF of PB1 mRNA.
- Virus attaches to sialic acid receptor though HA protein and is endocytosed in the host cell.
- endosome acidification induces fusion of virus membrane with the vesicle membrane; encapsidated RNA segments migrate to the nucleus.
- Transcription of genomic segments by the viral polymerase produces mRNAs.
- genomic (-)RNA
- High level of M1 protein induces genomes segments export from nucleus by NEP protein.
- Virus assembly and budding occurs at the plasma membrane.