VIRION
Enveloped. Usually rounded but can be filamentous. The virions are 80-120 nm in diameter.
GENOME
segmented ssRNA(-) linear genome, encapsidated by nucleoprotein (NP) Contains 7 segments coding for 9 proteins. Segments size range from 0.9 to 2.3 kb. Genome total size is about 10 Kb
GENE EXPRESSION
Viral RNA polymerase (PB1, PB2 and PA) transcribes one mRNA from each genome segment. Transcription is primed by cap snatching. mRNA are polyadenylated by the viral polymerase stuttering on a poly U track. NS mRNA can be alternatively spliced, giving rise to mRNA coding for NEP proteins. Polyprotein p42 is cleaved in M1 and CM2 proteins.
ENZYMES
- RNA-dependent RNA polymerase [PB1]
- Cap Snatching endonuclease [PA]
- Neuraminidase [NA]
REPLICATION
NUCLEUS
- Virus attaches to sialic acid receptor though HEF protein and is endocytosed by clathrins 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 that are capped and polyadenylated by the viral polymerase.
- Replication of genomic segments.
- High level of M1 protein induces genomes segments export from nucleus by NEP protein.
- Virus assembly and budding occurs at the plasma membrane.