Biology Reference
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were significantly reduced, but there was not a corresponding decrease in the
mRNAs. The pUL69 matrix protein accumulated in drug-treated cells and the pro-
tein was present in a hyperphosphorylated form. pUL69 localized to intranuclear
aggregates that did not overlap with viral replication centers. The matrix protein
pp65 was also retained in the nucleus (Sanchez et al. 2007). In contrast, the levels
of capsid, envelope, and other tegument proteins were only moderately affected in
that their expression was slightly delayed. Although preliminary, our recent data
with the specific inhibitor of cdk1, RO-3306 (Vassilev et al. 2006), indicate that
only some of these effects are due to inhibition of cdk1. We find that the titers are
significantly reduced and pUL69 accumulates in a hyperphosphorylated form,
but the expression of the IE2-86 and UL32 proteins is not significantly affected
(V. Sanchez and D.H. Spector, unpublished data). Thus, there may be a later role
for cdk2, cdk7, and cdk9.
Although limited, there have been some attempts to try to decipher the contribu-
tions of specific cdks during the infection by expressing dominant-negative forms of
the catalytic subunits in infected cells. Work by Bresnahan and colleagues showed
a decrease in the levels of some capsid proteins in infected cells transfected with a
dominant-negative cdk2, suggesting that the activity of cdk2/cyclin E complexes is
essential for viral replication (Bresnahan et al. 1997). In contrast, preliminary work
by Hertel et al. suggests that cdk1 activity is dispensable for the infection, given that
virus titer was not markedly reduced in cell cultures transduced with a retrovirus
expressing a GFP-tagged dominant negative cdk1 (Hertel et al. 2007). The only
effect that the dominant negative cdk1 had was to inhibit a pseudomitosis phenotype
that they observed in some cells infected with a variant of the HCMV strain AD169
(Hertel and Mocarski 2004). As noted above, however, our preliminary results with
a specific inhibitor of cdk1 suggest that this kinase is required for production of
infectious virus. Experiments involving expression of siRNAs directed against the
individual cdks will likely provide more insight into their individual and combined
roles in the infection.
Role of Cyclin-Dependent Kinases in Viral RNA Processing
Our hypothesis for the altered pattern of expression for the IE1-72/IE2-86 and
UL37 IE RNAs in the presence of Roscovitine is based on the phosphorylation of
the C-terminal domain (CTD) of the large subunit of RNA polymerase II (RNAP
II) by cdk7/cyclin H and cdk9/cyclin T. Cdk7/cyclin H is responsible for activating
cdks1, 2, and 4 and is also a part of TFIIH, which phosphorylates the carboxyl ter-
minal domain (CTD) of the large subunit of RNA polymerase II within the 52
repeats of the heptapeptide YSPTSPS. Cdk9/cyclin T (P-TEFb) also phosphor-
ylates the CTD. The current consensus is that transcription and RNA processing are
integrated events whereby the differential phosphorylation of the CTD repeats at
Ser 2 and Ser 5 defines its affinity for various transcription factors, kinases, and
RNA processing factors. When the transcription initiation complex is formed on a
