Biology Reference
In-Depth Information
shared with or occupied by a portal complex through which DNA enters and leaves
the capsid. The tegument region is approximately 50 nm thick and includes seven
relatively abundant virus-encoded protein species (see the chapter by R. Kalejta, this
volume), at least five of which are phosphorylated. The virion envelope is estimated
to be 10 nm thick and contains at least ten abundant protein species. Both the tegu-
ment and envelope contain additional less abundant virus-encoded and host-cell pro-
teins, as well as phospholipids, polyamines, and small RNAs.
It is worth noting that determinations of particle composition ultimately depend
on the nature of the starting material, which can be influenced by its source and
method of preparation. Comparisons of virus particles from different origins,
recovered by different methods, and analyzed by different and increasingly sensi-
tive procedures, are focusing even more attention on the challenge of establishing
which constituents are integral and present in all particles.
In addition to virions, five other types of virus particles have been recovered
from CMV-infected cells and characterized. Three are intracellular and nonenvel-
oped. A- and B-capsids are from nuclei prepared by treating infected cells with
NP-40 and have counterparts among the other herpesviruses. A-capsids are shells
composed of the four integral protein species and have the simplest structure.
B-capsids contain all of the A-capsid proteins and several additional internal
scaffolding species (UL80 proteins, Fig. 2). C-capsids are recovered from the
cytoplasmic fraction of infected cells treated with NP-40 (Gibson and Roizman 1972;
Gibson 1981) and are composed of the DNA genome within an A-capsid shell having
some tightly adherent tegument proteins (e.g., pUL32, pUL47, pUL48). The other
Fig. 2
Nested organization of HCMV UL80 genes and proteins. Shown here is a schematic
representation of the nested UL80 genes (
top
, DNA helix;
arrows
indicate separate promoters for
each); their 3′ co-terminal mRNAs (
dots
indicate AUG codons starting translation); and the in-frame,
carboxy-co-terminal proteins translated from each mRNA (size, name, and abbreviation
indicated).
Shading
illustrates portions of each protein shared by the others and the location of the
linker and tail portions of the scaffolding domain;
asterisk
indicates location of amino-conserved
domain. (Adapted from Gibson 2006)
