Biology Reference
In-Depth Information
et al. 2007). Within mature virions, the HCMV genome is housed in an icosahedral
protein capsid that is surrounded by a layer of proteins called the tegument,
which in turn is enclosed within a lipid membrane termed the envelope. Virally
encoded glycoproteins in the envelope function as mediators of viral entry
through a membrane fusion event (see the chapter by M.K. Isaacson et al., this
volume) that releases both the DNA-containing capsids and the tegument
proteins into the cell (Fig. 1).
As many as 59 viral proteins have been found in the viral tegument, although
only about 35 are incorporated at significant levels (Baldick et al. 1996; Varnum
et al. 2004). Virions also contain a sampling of cellular proteins (Varnum et al.
2004), as well as viral and cellular RNA molecules (Terhune et al. 2004).
Bioinformatic and experimental approaches have failed to detect a tegument locali-
zation signal (i.e., a sequence necessary and sufficient to direct macromolecules
into the tegument) on either proteins or RNAs. The process of assembling the tegu-
ment upon viral egress, as well as the disassembly of the tegument upon viral entry
into cells, is poorly understood. Likewise, the structure of the tegument within the
virion is not known. Although mostly amorphous, there appears to be some struc-
turing of tegument proteins that are closely associated with the capsid (Chen et al.
1999; Trus et al. 1999). Many tegument proteins are phosphorylated (Irmiere and
Fig. 1
Postfusion, preimmediate early events during lytic replication of human cytomegalovirus.
Schematic representation of delivery of viral genomes and tegument proteins to the nucleus
(1.1-1.4), the generation of a silencing complex (PML-NB) on infecting viral genomes (1.5-1.8),
and the initial step in the destruction of that complex by tegument-delivered pp71 (1.9-1.11). See
the text for further details
