Biology Reference
In-Depth Information
Early cell cycle studies demonstrated the stabilization of p53 in HCMV-infected
cells; p53 was sequestered into viral replication centers, thus contributing to its
inability to activate the expression of downstream cellular target genes such as p21
(Muganda et al. 1994; Bresnahan et al. 1996a; Fortunato and Spector 1998; Chen
and Fang 2001). Later work by Rosenke and co-workers described the binding of
p53 to viral promoters and showed that the DNA binding activity of the protein was
required for sequestration into the viral replication center (Rosenke et al. 2006).
HCMV infection also activates the ATM and ATR kinases that regulate the
double-stranded break and S phase checkpoints, respectively (Shen et al. 2004;
Castillo et al. 2005; Gaspar and Shenk 2006; Luo et al. 2007). However, the initia-
tion of a damage response does not appear to be necessary for the infection, as
virus replicates normally in cells deficient in ATM (Luo et al. 2007). In addition,
a fully functional damage response does not occur due to inefficient relocalization
of all of the required proteins (Gaspar and Shenk 2006; Luo et al. 2007). For
example, several proteins that are involved in nonhomologous end-joining (NHEJ)
response appear to be completely excluded from the viral replication centers (Luo
et al. 2007). This may be necessary to prevent the rejoining of ends that are gener-
ated during the replication of the virus.
Inhibition of Cellular DNA Replication
In addition to downregulation of cyclin A observed during infection, the replication
of cellular DNA is inhibited by several mechanisms during the licensing of the ori-
gins. Most importantly, the loading of MCM 2 and 7 (Biswas et al. 2003), as well
as MCM 3, 4, 5, and 6 (Wiebusch et al. 2003), onto chromatin is inhibited in
infected cells. This appears to be due to the premature accumulation of geminin,
which, as discussed above, normally accumulates in S phase to ensure that there is
no refiring of origins as the cells proceed through S and G
2
/ M.
Regulation of Cellular RNA Transcription and Protein Stability
The advent of DNA microarrays provided a means to examine the global effects of
HCMV infection on the accumulation of cellular RNAs (Zhu et al. 1997; Browne
et al. 2001; Challacombe et al. 2004; Hertel and Mocarski 2004). While increases in
the levels of RNA do not necessarily correlate with changes in the steady-state levels
of protein, these analyses provide a foundation for defining infection-associated
changes in transcription. The general picture that has emerged is that changes in the
mRNA levels of key cell cycle proteins is mirrored in the corresponding protein
levels, although there are several exceptions.
One example of an exception is where the accumulation of the protein is due to
the effects of the infection on protein stability rather than transcript level. For example,
