Biology Reference
In-Depth Information
2002). TRS1 was shown to interact with dsRNA and both IRS1 and TRS1 can
block the host protein shutoff response mediated by HSV-1 (Child et al. 2004;
Cassady 2005; Hakki and Geballe 2005). The role of IRS1/TRS1 in the initial
cotransfection replication assay was assumed to be that of a transcriptional activator
based on the findings that TRS1 and IE2 enhanced the expression of UL44 in tran-
sient assays (Stasiak and Mocarski 1992; Colberg-Poley 1996).
UL84 and IE2
Since the initial elucidation of the
trans-
acting factors required for oriLyt-depend-
ent DNA replication, the most complex interaction is that of UL84 and IE2. This
interaction has become the focus of much of the research with respect to the char-
acterization of a dual assignment for UL84 as a replication and regulatory factor for
gene expression. UL84 was first described as the product of a 1,761-bp ORF encod-
ing a 65-kDa protein (He et al. 1992). The mRNA transcript encoding UL84 can be
detected as early as 2.5 h postinfection and the protein product is clearly detectable
at about 20 h postinfection. Later studies showed that UL84 was found to be associ-
ated with IE2 in infected cells (Spector and Tevethia 1994). This association can be
found throughout the virus infectious cycle and the only time point where UL84 is
not found bound to IE2 is at very early times after infection before the production
of UL84. At this time, it is not clear how much of the UL84 or IE2 within infected
cells is part of the complex. This is an important point since UL84 has a regulatory
effect on IE2-mediated transactivation. Transient assays show that at least some
IE2-responsive promoters can be efficiently silenced by the addition of a UL84
expression plasmid (Gebert et al. 1997). The overexpression of UL84 prior to
HCMV infection leads to the complete shut down of virus replication (Gebert et al.
1997). This phenomenon is thought to arise from the ability of UL84 to suppress
the transactivation function of IE2; however, other mechanisms for inhibition of
virus replication are possible.
In the context of the virus genome, UL84 is required for efficient viral replica-
tion and regulated gene expression (Xu et al. 2004a). A recombinant HCMV BAC
with UL84 deleted results in an aberrant gene expression pattern, especially with
respect to late gene transcription. This suggests that UL84 may regulate the effects
of IE2, as implicated by the results of the transient reporter assays. In addition, lack
of UL84 expression resulted in failure of the formation of replication compartments
in infected cells, also consistent with what was observed in transient assays (Sarisky
and Hayward 1996; Xu et al. 2004a).
UL84 was shown be a phosphorylated protein and to exhibit UTPase activity in
vitro. This activity occurred independent of the presence of DNA or RNA (Colletti
et al. 2005). This utilization of UTP suggests the use of an energy-generating
system for an as yet unidentified enzymatic activity for UL84. UL84 contains
amino acid motifs that are consistent with it belonging to the DExH/D Box family
of proteins. This class of proteins has a broad range of activities. DExH/D Box
