The Viruses

The family Caliciviridae is a group of small (27-40 nm) unenveloped round viruses with a linear, positive strand, unsegmented RNA genome.[1] The family includes vesiviruses that infect cats (feline calicivirus) and pigs (vesicular exanthema of swine), the lagoviruses of hares and rabbits (rabbit hemorrhagic disease), and the human caliciviruses (genera: Norovirus and Sapovirus). Hepatitis E virus has been removed from the family Caliciviridae and is currently of ''unclassified status.''[2]

Noroviruses (prototype strains Hu/NLV/Norwalk vi-rus/8FIIa/1968/95) are 38 nm in diameter by electron cryomicroscopy (Table 1). By negative stain electron microscopy they are 27-32 nm in diameter with a feathered edge (Fig. 1). The capsid is made from a single major capsid protein that can self-assemble in the absence of viral RNA to form viruslike particles (VLP). Most of our information on virus morphology comes from VLPs. The capsid comprises 90 dimers of the capsid protein which form particles with T-3 icosahedral symmetry.

There are two major capsid domains: the shell (S) and protruding (P) arm. The P-domains form archlike structures that give the virus its feathery outline. The S-domain forms a p-barrel and is at the N-terminus. The P-domain is the carboxy half of the protein and shows sequence variability. It is divided into the P1 [amino acids (aa) 226-278 and aa 406-520] and the P2 (aa 279-405) subdomains. The P2 subdomain shows the highest sequence variability. The P1 subdomain forms the sides of the archlike structure and the P2 subdomain is at the top of the arch. P2 is a major antigenic site and is involved in receptor binding.

It has thus far proved impossible to maintain norovi-ruses (or sapoviruses) in artificial culture so knowledge of the genome is derived from analysis of cDNA copies of virus genome extracted from human stool samples and by comparison with the caliciviruses that are easier to grow [feline calicivirus (FCV) and vesicular exanthema of swine virus (VESV)] in culture. The norovirus genome is a positive sense polyadenylated single-stranded RNA of around 7.6 kb. It is organized as three open reading frames (ORF). The first ORF at the 5' end encodes a large nonstructural polyprotein of predicted mass 193.5 kDa or 1738 (aa). This contains motifs with similarity to the helicase, serine protease, and RNA-dependent RNA polymerase of picornaviruses, and is thought to be post-translationally cleaved by proteolysis to the individual enzymes. ORF2 encodes a protein of 533 aa, which is the capsid protein, and can self-assemble to form VLPs when expressed using a baculovirus vector in insect cells.[3] At the 3' end ORF3 encodes a small (212 aa) basic protein, which is a minor structural protein.[4]

The sapoviruses (prototype strain Hu/SLV/Sapporovi-rus/1982/JR) are of similar size to noroviruses but have a much more distinct morphology. They have 32 cup-shaped depressions at each of the fivefold and threefold axes. These lead to their characteristic ''Star of David'' appearance (Fig. 2) on negative stain electron microscopy. The genome of sapovirus Manchester is slightly shorter than that of noroviruses at 7.4 kb, and it is arranged differently. ORF1 encodes both the nonstructural proteins and the capsid protein, which is found in-frame at the end of the nonstructural ORF.[5] ORF2 encodes a basic protein

Table 1 Norovirus genogroups


Viruses and hosts

Getting Started With Dumbbells

Getting Started With Dumbbells

The use of dumbbells gives you a much more comprehensive strengthening effect because the workout engages your stabilizer muscles, in addition to the muscle you may be pin-pointing. Without all of the belts and artificial stabilizers of a machine, you also engage your core muscles, which are your body's natural stabilizers.

Get My Free Ebook

Post a comment