Structural Research of Caliciviridae

The Caliciviridae is a group of small, enveloped viruses with 7.4-8.3 kb linear positive-sense RNA genomes that are major pathogens of gastroenteritis and respiratory infections in humans and animals, which is of increasing concern in immunocompromised individuals. The Caliciviridae contains four recognised genera, Norovirus, Sapovirus, Lagovirus, and Vesivirus. Of these, the most clinically essential representative is human norovirus, which is the main cause of acute gastroenteritis. In recent years, sequence analysis and structural biology analyses have determined the three-dimensional structures of a range of caliciviruses, which have been used to understand their replication strategies, thus providing a basis for the design of novel antiviral compounds.

Figure 1. Cryo-image (left) of the Norwalk virus-like particle and X-ray structure (right) of its capsid. (Vinjé J, et al., 2019)

Genome Structure of the Calicivirus

The calicivirus genome consists of 7.4-8.3 kb of linear positive ssRNA molecules containing 2-4 open reading frames, ORF1, ORF2, ORF3, and ORF4. The viral proteogenome linkage (VPg) is covalently bound to the 5'-terminal and 3'-poly(A)-tails. The 5'-proximal ORF1 encodes a large polyprotein, which is subsequently co-translated and cleaved by the protease NS6 pro into at least six non-structural proteins, NS1/2, NS3, NS4, NS5, NS6, and NS7. The 2.2-2.4 kb subgenomic RNA (sgRNA) is synthesized intracellularly and ligated by VPg.

Characteristic Viral Particle Morphology of Caliciviruses

In recent years, researchers have used cryo-electron microscopy (cryo-EM), X-ray crystallography, and 3D image reconstruction to resolve the structure of caliciviruses at sub-nanometre resolution. The structure shows that the virus particles are unenveloped, with a diameter of about 35-40 nm and icosahedral symmetry. The capsid consists of 90 dimers of the major structural protein VP1 arranged on a T=3 icosahedral lattice. The VP1 is divided into two structural domains, S and P. The S domain constitutes the continuous bottom of the capsid and has a β-barrel motif. The P structural domain protrudes from the surface of the capsid to form an arch, which is further divided into the P1 and P2 sub-structural domains. P2 is an insertion fragment in P1, located in the outermost layer of the capsid, and is the site of several neutralization epitopes.

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References

1. Vinjé J, et al. ICTV Virus Taxonomy Profile: Caliciviridae. J Gen Virol. 2019. 100(11): 1469-1470.
2. Lee JH, et al. Structure and Function of Caliciviral RNA Polymerases. Viruses. 2017. 9(11): 329.
3. Chen R, et al. X-ray structure of a native calicivirus: structural insights into antigenic diversity and host specificity. Proc Natl Acad Sci U S A. 2006. 103(21): 8048-8053.