Fig.1 Schematic representation of NoV. (Campillay-Véliz, et al., 2020)
Norovirus (previously known as Norwalk-like virus) is a Caliciviridae family nonenveloped, single-stranded, positive-sense RNA virus. They are highly contagious and have the potential to cause severe gastroenteritis in humans. NoVs are the causative agents of epidemic and sporadic acute viral gastroenteritis in humans, causing 684 million illnesses and 212,000 deaths each year. Controlling the spread of norovirus is difficult because only ten virions are required to infect an adult.
Norovirus is a 7.6 kb nonenveloped, single-stranded, positive-sense RNA. The virion is a 27-nm round particle with a 'ragged' outer edge but no discernible surface structure. ORF-1, ORF-2, and ORF-3 are three open reading frames (ORFs) that encode eight viral proteins. ORF1 encodes nonstructural proteins (p48, NTPase, p22, VPg, Protease, and RdRp), while ORF2 and ORF3 encode a major structural protein (referred to as VP1) and a minor structural protein (referred to as VP2). The shell (S) and protruding (P) domains of the VP1 protein are separated.
Fig.2 Genomic organization and structure of human NoVs. (Pogan, et al., 2018)
The shell (S) and protruding (P) domains of VP1 are distinct substructures linked by a flexible hinge. The S domain is required for the icosahedral capsid shell to form. The P domain is divided into two subdomains: P1 and P2. The P domain helps to regulate the size and stability of the NoV capsid. P2's hypervariable region is thought to be important in receptor binding and immune reactivity.
VP2 is not required for virion production, but it is important for virion stability and genome encapsidation. VP2 also protects VP1 from disassembly and protease degradation.
There are at least seven different genogroups in the norovirus family (GI, GII, GIII, GIV, GV, GVI, and GVII), which can be further subdivided into different genetic clusters or genotypes. In humans, GI and GII are the most commonly associated with enteric disease. Human stools frequently contain GII noroviruses, particularly the GII.4 genotype followed by the GII.3 genotype. In the environment, the detection rate of GI is higher. GII.4 is the most common cause of acute gastroenteritis outbreaks worldwide.
Fig.3 Phylogenetic tree of whole-genome sequences of NoVs. (Charoenkul, et al., 2020)
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