Biology Reference
In-Depth Information
More acetylated chromatin was found on early viral promoters in nonpermissive
undifferentiated THP-1 cells when both the IE72 and IE86 proteins were expressed
in
trans
prior to infection (Yee et al. 2007).
As shown in Fig. 2a, there is a core domain between amino acids 450 and 552
and when mutated all functions of the IE86 protein are affected (Asmar et al. 2004).
The longest stretch of conserved amino acids within the core domain (LPIYE) is
shown in Fig. 2b. Mutation of the proline (P) and tyrosine (Y) residues within this
conserved stretch of amino acids affected transactivation of early viral promoters
without affecting autoregulation of the MIE promoter (Petrik et al. 2007). Mutation
of P535 and Y537 to alanines results in a nonreplicating BAC (Table 1). The mutant
IE86 protein is found associated with the MIE promoter in ChIP assays, but fails to
transactivate early viral genes and is not found associated with viral early promoters
in a ChIP assay (Petrik et al. 2007). Therefore, recruitment to the MIE promoter and
early viral promoters by the IE86 protein occur through independent mechanisms.
The LPIYE conserved sequence may be important for protein-protein interactions.
This conserved sequence in the IE86 protein is also found in other cellular proteins
that regulate events on a DNA template such as histone methyltransferase, DNA
topisomerase III, and CHROM2. If the LPIYE sequence is mutated in the critical
proline and tyrosine residues, there is no activation of the viral early promoters. In
contrast, a mutation in the noncritical leucine (L) residue has little to no effect
(Petrik et al. 2007). An activation domain of IE86 is carboxyl to the core domain,
and it is also necessary to activate transcription of a viral early promoter (Pizzorno
et al. 1991; Yeung et al. 1993). This region is rich in acidic amino acids. Mutation
of the glutamic and aspartic acids between residues 550 and 573 to valines inacti-
vates the activation domain (Yeung et al. 1993).
E2F Promoters
After HCMV infection, gene microarray assays demonstrated a fourfold or greater
increase of at least 124 cellular mRNAs (Zhu et al. 1998). While the IE86 protein
can activate cellular promoters through the same mechanisms for viral promoters
discussed above (Hagemeier et al. 1992b), it can also activate cellular promoters by
inhibiting a repressor of transcription. The expression of genes by the transcription
factor E2F is repressed by the cellular tumor suppressor protein pRb. IE86 binds to
pRb in vitro and IE86 protein-Rb complexes can be immunoprecipitated from
HCMV-infected cells (Hagemeier et al. 1994; Sommer et al. 1994; Fortunato et al.
1997). In the HCMV-infected cell, Rb is converted from the hypophosphorylated to
hyperphosphorylated form. An analysis of gene expression using microarrays of
cellular genes indicated that HCMV or the IE86 protein strongly activated E2F
response genes (Song and Stinski 2002). These are the cellular genes that regulate
the enzymes for DNA precursor synthesis, the initiation factors of cellular DNA
synthesis, and the movement of the cell cycle. They are expressed prior to the
S phase and they include the following: DNA precursor enzymes such as
