Family - Reoviridae

Abstract

This chapter focuses on Reoviridae family that has two subfamilies, Spinareovirinae and Sedoreovirinae. The member genuses of this family include Orthoreovirus, Aquareovirus, Oryzavirus, Orbivirus, Rotavirus, and Seadornavirus. The virus particles of members of the family, collectively called reoviruses, have icosahedral symmetry but may appear spherical in shape. The protein capsid is organized as one, two, or three concentric layers of capsid proteins, which surround the linear dsRNA segments of the viral genome, with an overall diameter of 60–80 nm. The subfamily Spinareovirinae contains viruses that have relatively large spikes or turrets situated at the 12 icosahedral vertices of either the virus or core particle. The subfamily Sedoreovirinae includes viruses that do not have large surface projections on their virions or core particles, thus giving them an almost spherical or smooth appearance. The innermost protein layer of reovirus particles has an internal diameter of approximately 50–60 nm and surrounds the 9, 10, 11, or 12 linear dsRNA genome segments. In the smooth-cored genera, the enzymatically active minor proteins of the virion are attached to the inner surface of the central space at the five-fold axes of symmetry. These include the RNA-dependent RNA polymerase, NTPase, helicase, and capping, and transmethylase enzymes. Particles of some genera can leave infected cells by budding or can bud into the endoplasmic reticulum during morphogenesis, acquiring an envelope derived from cellular membranes. Mature virions lack a lipid envelope, and depending on the genus, a myristyl residue may be covalently attached to one of the virion proteins. Coltiviruses, rotaviruses, and orbiviruses have an intermediate in virus morphogenesis or release, which may have a lipid envelope that is subsequently lost or removed.