Norovirus (taxid:142786)

VIRION

Non-enveloped, capsid of about 38-40 nm in diameter, with T=3 icosahedral symmetry. The capsid is composed of 180 VP1 proteins. Small empty virions are about 23nm in diameter, and would be of T=1 icosahedral symmetry, composed of 60 VP1 proteins.

GENOME

Monopartite, linear ssRNA(+) genome of 7.3 to 8.3 kb. The 5'-terminus is linked to a VPg protein and the 3'-terminus has a poly(A) tract.n (VPg)is covalently linked to genome, whereas 3'-terminus is polyadenylated.

GENE EXPRESSION

The virion RNA is infectious and serves as both genome and viral messenger RNA. Cleavage of ORF1 polyprotein by the virus-encoded 3C-like cysteine proteinase yields the mature nonstructural proteins. Subgenomic RNA encodes the capsid protein and VP2 minor structural protein which is expressed through RNA termination-reinitiation. Murine norovirus express an alternative ORF from the CP mRNA by leaky scanning.

ENZYMES

• RNA-dependent RNA polymerase [RdRp Q83883]
• VPG-type capping [VPg Q83883]
• NTPase-helicase [NTPase Q83883]
• Polyprotein major protease (Peptidase C37) [3CLpro Q83883]

REPLICATION

CYTOPLASMIC

1. Attachement to host receptors mediates clathrin- and caveolin-independent endocytosis of the virus.
2. Uncoating, and release of the viral genomic RNA into the cytoplasm.
3. VPg is removed from the viral RNA, which is then translated into a processed ORF1 polyprotein to yield the replication proteins.
4. Replication occurs in viral factories. A dsRNA genome is synthesized from the genomic ssRNA(+).
5. The dsRNA genome is transcribed/replicated thereby providing viral mRNAs/new ssRNA(+) genomes.
6. Subgenomic RNA translation gives rise to the capsid protein and VP2.
7. Assembly of new virus particles and release by cell lysis.

Host-virus interaction

Apoptosis modulation

Murine norovirus induces host cell apoptosis .

Host gene expression shutoff by virus

Noroviruses mediate host translation shutoff by cleaving PABP1 .