Biology Reference
In-Depth Information
Many of the core replication proteins remain poorly characterized at the
biochemical level with the exception of UL44 and UL54, which are the best studied
of the DNA replication enzymes. UL44 in infected cells is very abundant and found
in replication compartments in early and late time of infection. UL44 encodes the
polymerase accessory factor and appears to form a C-shaped clamp that interacts
with DNA with increased specificity when bound to UL54 (Loregian et al. 2004;
Appleton et al. 2006). The C-terminal end of UL54 appears to be the point of inter-
action with UL44 and a small peptide corresponding to a portion of the UL54 protein
efficiently interferes with binding to UL44 in vitro (Loregian et al. 2003). UL44 was
shown to interact with UL54 in the cellular environment and this interaction can
influence subcellular localization (Alvisi et al. 2006). The crystal structure of UL44
is known and its interaction with UL54 was shown to enhance the affinity of
UL44 for DNA (Appleton et al. 2006). UL44 is a substrate for the HCMV-encoded
protein kinase UL97 (Krosky et al. 2003) as well as other cellular encoded kinases.
This suggests that UL44 activity or interaction with other viral/cellular proteins may
be regulated by both viral as well as cellular kinases (Krosky et al. 2003).
UL54 was initially purified from insect cells along with UL44 and found to syn-
thesize DNA in vitro (Ertl and Powell 1992; Ertl et al. 1994). Expression of UL54
is regulated by several different promoter elements depending on the time of infec-
tion. In addition, it appears that several regulatory proteins contribute to efficient
expression of UL54 in the context of the virus genome (Kerry et al. 1994, 1996).
The helicase-primase complex, composed of UL105, UL70 and UL102, has
been studied using recombinant expression systems as well as limited characteriza-
tion and identification from infected cells (Smith and Pari 1995; Smith et al. 1996).
Although this group of proteins comprises the proposed counterparts of the herpes
simplex helicase primase proteins, no data has been published demonstrating heli-
case or primase activity in vitro. Elucidation of the transcription units for UL102
and UL105 revealed that these transcripts are unspliced and approximately 2.7 and
3.4 kb, respectively (Smith and Pari 1995; Smith et al. 1996). The UL105 protein
product is a 110-kDa species that is present at early times postinfection. It was
demonstrated that all three proteins form a complex and each member of the com-
plex is in contact with every other protein (McMahon and Anders 2002).
HCMV UL57 encodes the single-stranded DNA-binding protein. Initial studies
show that UL57 encodes a 140-kDa protein that is confined to the nucleus and asso-
ciates with replication compartments (Penfold and Mocarski 1997). It is assumed
that UL57 acts to facilitate strand separation during DNA synthesis. Interestingly,
the HSV-1 counterpart, UL29, was shown to exhibit RNA-binding and aid in the
formation of R-loops and participate in helix destabilization (Boehmer 2004).
The proteins encoded by the UL112-113 locus were required in the initial
cotransfection replication assay and appear to act as transcriptional enhancers. It
was recently demonstrated that the UL112-113 proteins may influence the intranu-
clear targeting of UL44 to foci involved in DNA replication (Park et al. 2006).
The IRS1/TRS1 locus encode proteins that are part of the HCMV virion. At least
one locus is required for viral growth and a deletion of TRS1 results in a decrease
in production of infectious virus (Romanowski et al. 1997; Blankenship and Shenk
