Order - Herpesvirales
Abstract
The virions of Herpesvirales order have complex and characteristic structures consisting of both symmetrical and nonsymmetrical components. The spherical virion is composed of the core, capsid, tegument and envelope. The core consists of the viral genome packaged as a single, linear, dsDNA molecule into a preformed capsid and the DNA is packed in a liquid–crystalline array that fills the entire internal volume of the capsid. The capsids assemble by cocondensation around a protein scaffold to form a procapsid in which the subunits are weakly connected. Proteolytic cleavage of the scaffolding protein triggers loss of scaffold and reorganization of the shell into the characteristic capsid form. The structure of the tegument is poorly defined, with evidence of symmetry only in the region immediately adjacent to the capsid. The tegument contains various proteins, not all of which are required for the formation of virions. The genomes are composed of linear dsDNA ranging from 125 to 295 kbp in size and from 32 to 75% in G + C content. The polypeptide composition of the mature virion varies greatly among herpesviruses. The mature capsid is composed of four major and several minor proteins, while the tegument contains at least 15 different polypeptides, many of which are dispensable in vitro and are therefore not required for virion morphogenesis. The viral envelope contains at least 10 integral membrane proteins, a subset of which is required for adsorption and penetration of the host cell. The virion envelopes contain multiple proteins that carry N-linked and O-linked glycans. Mature, cell-free virions contain complex glycans, whereas a proportion of intracellular virions contain N-linked glycans of the immature high mannose type.