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 12-14 proteins depending on strains. 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 alternately spliced, giving rise to mRNA coding for M2 and NEP proteins. Some strains can express M42 from the M segment . PB1 mRNA encode three proteins by leaky scanning: PB1, N40 and PB1-F2 . Pa-X protein is translated by ribosomal frameshift on the PA gene. Leaky scanning may produces N-truncated PA protein, namely Aa-N155 and PA-N182 .
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- Virus attaches to sialic acid receptor through HA protein and penetrates into the host cell by clathrin-mediated endocytosis in the host cell.
- Endosome acidification induces fusion of virus membrane with the vesicle membrane and encapsidated RNA segments migrate to and penetrate into the nucleus.
- Viral RdRp transcribes all the genomic genes to produce mRNAs and express viral proteins .
- Viral RdRp performs genomic (-)RNA replication through antigenomic (+)RNA intermediate.
- High level of M1 protein induces genomes segments export from nucleus by NEP protein.
- Virus assembly and budding occurs at the plasma membrane.