Biology Reference
In-Depth Information
genome size of different herpesviruses. In contrast, the outer components
exhibit remarkable structural and organizational diversity, including the
inner tegument proteins interacting directly with the capsid. These struc-
tural results suggests that herpesviruses have evolved their tegument and
envelope substantially while adapting to their specific host cellular envi-
ronment in order to gain optimal fitness and confer unique characteristics
of different herpes infections.
Introduction
Herpesviruses are among the largest and most ubiquitous viruses.
The family
Herpesviridae
is extensive; in fact, almost every animal
species examined can be infected with at least one herpesvirus.
1
Although they vary in host range, genome size and molecular com-
position, all herpesviruses share a common virion structure consist-
ing of four basic elements: a double-stranded DNA (dsDNA) core,
an icosahedral capsid shell, a proteinaceous tegument compartment,
and a glycoprotein-decorated lipid envelope. The core of the mature
virion consists of dsDNA arranged as closely packed shells.
2
The cap-
sid is a rigid icosahedral protein shell, 1200-1300 Å in diameter,
which encloses and protects the dsDNA core. The tegument is a
poorly defined, asymmetric layer of host and viral proteins between
the capsid and the envelope. It varies in thickness and distribution
around the capsid, although some of its lower proteins are anchored
to the capsid. The envelope is a host-derived lipid bilayer containing
spikes of viral glycoprotein. The entire virion varies in diameter from
1400 to 3000 Å, depending on the thickness of the tegument and
the integrity of the envelope.
Herpesviruses have been classified into three subfamilies — alpha-,
beta- and gammaherpesviruses — based on shared biological properties,
as exemplified in Table 1 by the eight known human herpesviruses.
3
A
small number of them have also been classified into genera based on
DNA sequence homology, genome arrangement and related proteins.
The alphaherpesviruses have a variable host range, a short reproductive
cycle and rapid spread in culture. They establish latent infection prima-
rily in neurons. Human pathogens in this subfamily include the human
pathogens herpes simplex virus types 1 and 2 (HSV-1/2 or HHV-1/2)
