Biology Reference
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which results in a 100-fold reduction in viral titers after infections at either high or
low multiplicities (Bechtel and Shenk 2002). The UL48 gene has been classified as
either augmenting (Yu et al. 2003) or essential (Dunn et al. 2003).
The UL82 gene encodes the pp71 tegument protein that localizes to the
nucleus in both HCMV-infected and UL82-transfected cells (Hensel et al. 1996).
Although pp71 is not absolutely essential, it is required for efficient viral replica-
tion (Bresnahan and Shenk 2000) because of its ability to facilitate viral IE gene
expression (see Sect. 4 below). Other tegument proteins such as pUL26 and
pUL35 may assist in the pp71-mediated activation of IE gene expression (see
Sect. 4 below). pp71 also targets the hypophosphorylated forms of the Rb family
of tumor suppressors for proteasome-dependent, ubiquitin-independent degrada-
tion, leading to cell cycle stimulation (Kalejta et al. 2003; Kalejta and Shenk
2003), and decreases the cell surface expression of MHC class I proteins by
slowing their intracellular transport (Trgovcich et al. 2006).
Delivery of the Genome to the Nucleus
Once in the cytoplasm, HCMV genome-containing capsids and some tegument
proteins must make their way to the nucleus. Although a seemingly simple task,
this journey is a difficult one due to the size of the viral particle and the density of
the cytoplasm. HCMV overcomes these obstacles using strategies that are also
employed by other viruses (Dohner et al. 2005; Greber and Way 2006), namely
hijacking the intracellular transport machinery. Cells contain an organized network
of microtubules (MTs) that extend from the microtubule-organizing center (MTOC)
near the nucleus all the way to the periphery, ending near the cell membrane. This
network, along with other mechanisms, allows for the temporal and spatial control
of the transport of large cargoes to help establish and maintain cell polarity as well
as the uneven distributions of proteins, RNAs, and organelles (Welte 2004). MTs
are composed of ordered, head-to-tail associations of tubulin monomers, and thus
have a distinct polarity, with their negative ends near the MTOC and their positive
ends near the cell surface. Cytoplasmic dynein is a minus-end-directed motor pro-
tein, which, along with dynactin, uses power generated from ATP hydrolysis to
transport cargo along microtubules toward the MTOC (Malik and Gross 2004).
An intact microtubule network is required for the transport to the nucleus of
capsids deposited in cells upon HCMV infection (Ogawa-Goto et al. 2003).
Nocodazole, a drug that de-polymerizes microtubules, inhibits IE gene expression,
likely by preventing infecting HCMV from depositing its DNA in the nucleus.
Entering capsids in transit to the nucleus can be localized by detecting the tightly
associated tegument protein pp150 through indirect immunofluorescence (Sinzger
et al. 2000). In the absence of nocodazole, incoming pp150 is found associated
with MTs and concentrated near the nucleus, but is diffusely distributed in the
cytoplasm in the presence of the drug (Ogawa-Goto et al. 2003). Transmission
electron microscopy (TEM) also showed entering DNA-containing capsids in the
