Biology Reference
In-Depth Information
since they are lower in abundance and more difficult to study independently. Even
though the isomers are dispensable, they are required for efficient early and late
viral gene expression (White et al. 2007). Exon 5 encodes the unique amino acid
sequence for the major IE2 gene product designated IE86 (Fig. 1). Recombinant
viruses deleted in exon 5 cannot express early or late viral genes; consequently
viral replication does not occur (Angulo et al. 2000; Marchini et al. 2001).
Therefore, the IE86 protein is essential for viral replication. Both IE72 and IE86
are key regulatory proteins in the switch from latent to lytic infections. However,
the molecular mechanisms that trigger and sustain CMV reactivation are related to
cellular differentiation and are largely unknown.
After the viral DNA enters the nucleus, subsets of viral genomes are found at
nuclear structures referred to as nuclear domain 10 (ND10) or promyelocytic
leukemia protein oncogenic domains (PODs). IE72 and IE86 proteins localize at
and adjacent to the ND10s, respectively (Ishov et al. 1997). Although IE86 does
not directly affect ND10s, it has many other functions, which occur in the
nucleus. The functions of the IE86 protein are negative autoregulation of the viral
MIE promoter, transactivation of viral and cellular promoters, inactivation of cel-
lular cytokine and chemokine promoters, and control of cell cycle progression.
While the IE86 protein is necessary for viral DNA synthesis, initiation of DNA
synthesis from oriLyt by an IE86/UL84 protein complex can occur with UL84
alone when a transactivator function in oriLyt is replaced with a constitutive pro-
moter such as SV40 (Xu et al. 2004). The effect of UL84 on the IE86 and lytic
viral DNA replication are reviewed (see the chapter by G.S. Pari, this volume).
More importantly, IE86 prepares the cell for viral DNA synthesis by activating
the expression of early viral genes and cellular genes. The IE86 protein of HCMV
is unique among viral and cellular regulatory proteins because it both negatively
and positively regulates viral and cellular promoters, and it promotes and arrests
cell cycle progression. A better understanding of this essential viral protein may
spawn novel strategies for preventing HCMV-induced disease.
Mapping the Functional Domains of the IE86 Protein
The functional domains of the IE86 protein have been studied by in vitro protein-
protein binding assays, transient transfection assays, and the construction and isola-
tion of recombinant viruses with either amino acid deletions or substitutions. The
use of bacterial artificial chromosomes (BACs) containing the HCMV genome has
been useful in confirming and extending the results of these early assays. These
studies have indicated that the IE86 protein is a homodimer with critical functional
domains located primarily toward the carboxyl end of the viral protein. Structural
analysis of the IE86 protein by X-ray crystallography has not been done to date. In
addition to tertiary structure of the IE86 protein, posttranslational modifications
such as sumoylation and phosphorylation affect the biological activity of the viral
protein. The IE86 protein of 579 amino acids is sumoylated at lysine residues 175
