Family - Caliciviridae

Abstract

This chapter focuses on Caliciviridae family whose member genuses are Vesivirus, Lagovirus, Norovirus, Sapovirus, and Nebovirus. The virions of this family are nonenveloped with icosahedral symmetry and are of 27–40 nm in diameter by negative-stain electron microscopy and 35–40 nm by cryo-electron microscopy. The capsid is composed of 90 dimers of the major structural protein VP1 arranged on a T = 3 icosahedral lattice. In noroviruses, the VP1 forms a subunit composed of a shell and two protruding domains. A characteristic feature of calicivirus capsid architecture is the 32 cup-shaped depressions at each of the icosahedral five-fold and three-fold axes. In negative-stain virus preparations, some cup-shaped depressions appear distinct and well defined, while in others these depressions are less prominent. Generally, caliciviruses are stable in the environment and many strains are resistant to inactivation by heat and certain chemicals such as ether, chloroform, and mild detergents. The caliciviruses have conserved nucleotide motifs at the genomic 5´-terminus and at the junction of the coding sequences for the nonstructural/structural proteins. All the nonstructural proteins are encoded by a large polyprotein and expressed in the same order for each genus. The virions are predominantly composed of one major capsid protein, VP1 (58–60 kDa) and a second minor structural protein named VP2 (8.5–23 kDa) has been found in association with FCV and RHDV virions, and Norwalk virus-like particles. Some nonstructural proteins have sequence and motif homology with those of the family Picornaviridae replicative enzymes and include 2C (NTPase), 3C (cysteine protease) and 3D (RNA-dependent RNA polymerase) domains. Caliciviruses have single stranded, positive sense genomic RNA (positive-strand RNA) organized into either two or three major ORFs.