The human cytomegalovirus 86-kilodalton immediate-early 2 protein: synthesis as a precursor polypeptide and interaction with a 75-kilodalton protein of probable viral origin

UL84-independent replication of human cytomegalovirus strain TB40/E

The UL84 gene of human cytomegalovirus is implicated in the initiation of viral DNA replication during lytic infection. UL84 is essential for replication of a cloned viral origin of lytic replication (oriLyt) in vitro and mutants of strains AD169 or Towne with deletions or insertions in UL84 fail to grow in cells permissive for wild type virus. Here we show that UL84 is dispensable for replication of a strain TB40/E clone derived from a bacterial artificial chromosome. The genomes of the fibroblast-adapted strains AD169 and Towne are altered substantially from the consensus for strains that have not been propagated extensively in cell culture. The parental TB40/E genome conforms to the consensus genomic organization. Accordingly, natural HCMV strains may possess replication capability that extends beyond the known oriLyt-dependent replication system of laboratory strains.

Human cytomegalovirus IE2 86 and IE2 40 proteins differentially regulate UL84 protein expression posttranscriptionally in the absence of other viral gene products

It has previously been demonstrated that, during human cytomegalovirus infection, the viral IE2 86 and IE2 40 proteins are both important for the expression of an early-late viral protein, UL84. Here, we show that expression of the UL84 protein is enhanced upon cotransfection with either IE2 86 or IE2 40, although IE2 40 appears to play a more important role. The UL84 protein levels are tightly linked to the amount of IE2 40 present, but this does not appear to be true for IE2 86. RNA remains constant for all corresponding proteins, indicating posttranscriptional regulation of UL84. The first 105 amino acids of UL84 are necessary and sufficient for this phenotype, and this region is also required for an interaction with IE2 86 and IE2 40. Treatment with proteasome inhibitors shows that UL84 exhibits some proteasome-dependent degradation, and UL84 is not protected against this degradation when coexpressed with IE2 86 or IE2 40. UL84 also exhibits an inhibitory effect on IE2 86 and IE2 40 protein levels in these cotransfection assays. Further, we show that the amino acid sequence of UL84 is important for the enhancement governed by IE2 40. These results indicate that IE2 86, IE2 40, and UL84 serve to regulate protein expression in a posttranscriptional fashion and that this regulation is independent of other viral proteins.

Internal deletions of IE2 86 and loss of the late IE2 60 and IE2 40 proteins encoded by human cytomegalovirus affect the levels of UL84 protein but not the amount of UL84 mRNA or the loading and distribution of the mRNA on polysomes

The major immediate-early (IE) region of human cytomegalovirus encodes two IE proteins, IE1 72 and IE2 86, that are translated from alternatively spliced transcripts that differ in their 3' ends. Two other proteins that correspond to the C-terminal region of IE2 86, IE2 60 and IE2 40, are expressed at late times. In this study, we used IE2 mutant viruses to examine the mechanism by which IE2 86, IE2 60, and IE2 40 affect the expression of a viral DNA replication factor, UL84. Deletion of amino acids (aa) 136 to 290 of IE2 86 results in a significant decrease in UL84 protein during the infection. This loss of UL84 is both proteasome and calpain independent, and the stability of the protein in the context of infection with the mutant remains unaffected. The RNA for UL84 is expressed to normal levels in the mutant virus-infected cells, as are the RNAs for two other proteins encoded by this region, UL85 and UL86. Moreover, nuclear-to-cytoplasmic transport and the distribution of the UL84 mRNA on polysomes are unaffected. A region between aa 290 and 369 of IE2 86 contributes to the UL84-IE2 86 interaction in vivo and in vitro. IE2 86, IE2 60, and IE2 40 are each able to interact with UL84 in the mutant-infected cells, suggesting that these interactions may be important for the roles of UL84 and the IE2 proteins. Thus, these data have defined the contribution of IE2 86, IE2 60, and IE2 40 to the efficient expression of UL84 throughout the infection.

Default assembly of early adenovirus chromatin

In adenovirus particles, the viral nucleoprotein is organized into a highly compacted core structure. Upon delivery to the nucleus, the viral nucleoprotein is very likely to be remodeled to a form accessible to the transcription and replication machinery. Viral protein VII binds to intra-nuclear viral DNA, as do at least two cellular proteins, SET/TAF-Ibeta and pp32, components of a chromatin assembly complex that is implicated in template remodeling. We showed previously that viral DNA-protein complexes released from infecting particles were sensitive to shearing after cross-linking with formaldehyde, presumably after transport of the genome into the nucleus. We report here the application of equilibrium-density gradient centrifugation to the analysis of the fate of these complexes. Most of the incoming protein VII was recovered in a form that was not cross-linked to viral DNA. This release of protein VII, as well as the binding of SET/TAF-Ibeta and cellular transcription factors to the viral chromatin, did not require de novo viral gene expression. The distinct density profiles of viral DNA complexes containing protein VII, compared to those containing SET/TAF-Ibeta or transcription factors, were consistent with the notion that the assembly of early viral chromatin requires both the association of SET/TAF-1beta and the release of protein VII.

Adenovirus type 5 DNA-protein complexes from formaldehyde cross-linked cells early after infection

We report here the properties of viral DNA-protein complexes that purify with cellular chromatin following formaldehyde cross-linking of intact cells early after infection. The cross-linked viral DNA fractionated into shear-sensitive (S) and shear- resistant (R) components that were separable by sedimentation, which allowed independent characterization. The R component had the density and sedimentation properties expected for DNA-protein complexes and contained intact viral DNA. It accounted for about 50% of the viral DNA recovered at 1.5 h after infection but less than 20% by 4.5 h. The proportion of R component was independent of multiplicity of infection, even at less than one particle per cell. Viral hexon and protein VII, but not protein VI, were detected in the fractions containing the R component. These properties are consistent with those of partially uncoated virions associated with the nuclear envelope. A substantial proportion of the S component viral DNA had the same density as cellular chromatin. Protein VII was the most abundant viral protein present in gradient fractions that contained the S component. Complexes containing USF transcription factor cross-linked to the adenovirus major late promoter were detected by viral chromatin immunoprecipitation of the fractions containing S component. The S component probably contained uncoated nuclear viral DNA that assembles into early viral transcription complexes.

Activation of transcription of the human cytomegalovirus early UL4 promoter by the Ets transcription factor binding element

The human cytomegalovirus (HCMV) early UL4 promoter has served as a useful model for studying the activation of early viral gene expression. Previous transient-transfection experiments detected cis-acting elements (the NF-Y site and site 2) upstream of the transcriptional start site (L. Huang and M. F. Stinski, J. Virol. 69:7612-7621, 1995). The roles of two of these sites, the NF-Y site and site 2, in the context of the viral genome were investigated further by comparing mRNA levels from the early UL4 promoter in human foreskin fibroblasts infected by recombinant viruses with either wild-type or mutant cis-acting elements. Steady-state mRNA levels from the UL4 promoter with a mutation in the NF-Y site were comparable to that of wild type. A mutation in an Elk-1 site plus putative IE86 protein binding sites decreased the steady-state mRNA levels compared to the wild type at early times after infection. Electrophoretic mobility shift assays and antibody supershifts detected the binding of cellular transcription factor Elk-1 to site 2 DNA with infected nuclear extracts but not with mock-infected nuclear extracts. The role of cellular transcription factors activated by the mitogen activated protein kinase/extracellular signal-regulated kinase pathway in activating transcription from early viral promoters is discussed.

The human cytomegalovirus IE86 protein can block cell cycle progression after inducing transition into the S phase of permissive cells

Human cytomegalovirus (HCMV) infection of permissive cells has been reported to induce a cell cycle halt. One or more viral proteins may be involved in halting progression at different stages of the cell cycle. We investigated how HCMV infection, and specifically IE86 protein expression, affects the cell cycles of permissive and nonpermissive cells. We used a recombinant virus that expresses the green fluorescent protein (GFP) to determine the effects of HCMV on the cell cycle of permissive cells. Fluorescence by GFP allowed us to select for only productively infected cells. Replication-defective adenovirus vectors expressing the IE72 or IE86 protein were also used to efficiently transduce 95% or more of the cells. The adenovirus-expressed IE86 protein was determined to be functional by demonstrating negative autoregulation of the major immediate-early promoter and activation of an early viral promoter in the context of the viral genome. To eliminate adenovirus protein effects, plasmids expressing GFP for fluorescent selection of only transfected cells and wild-type IE86 protein or a mutant IE86 protein were tested in permissive and nonpermissive cells. HCMV infection induced the entry of U373 cells into the S phase. All permissive cells infected with HCMV were blocked in cell cycle progression and could not divide. After either transduction or transfection and IE86 protein expression, the number of all permissive or nonpermissive cell types in the S phase increased significantly, but the cells could no longer divide. The IE72 protein did not have a significant effect on the S phase. Since IE86 protein inhibits cell cycle progression, the IE2 gene in a human fibroblast IE86 protein-expressing cell line was sequenced. The IE86 protein in these retrovirus-transduced cells has mutations in a critical region of the viral protein. The locations of the mutations and the function of the IE86 protein in controlling cell cycle progression are discussed.

A strong negative transcriptional regulatory region between the human cytomegalovirus UL127 gene and the major immediate-early enhancer

The region of the human cytomegalovirus (CMV) genome between the UL127 open reading frame and the major immediate-early (MIE) enhancer is referred to as the unique region. DNase I protection analysis with human cell nuclear extracts demonstrated multiple protein binding sites in this region of the viral genome (P. Ghazal, H. Lubon, C. Reynolds-Kohler, L. Hennighausen, and J. A. Nelson, Virology 174:18-25, 1990). However, the function of this region in the context of the viral genome is not known. In wild-type human CMV-infected human fibroblasts, cells permissive for viral replication, there is little to no transcription from UL127. We determined that the unique region prevented transcription from the UL127 promoter but had no effect on the divergent MIE promoter. In transient-transfection assays, the basal level of expression from the UL127 promoter increased significantly when the wild-type unique sequences were mutated. In recombinant viruses with similar mutations in the unique region, expression from the UL127 promoter occurred only after de novo viral protein synthesis, typical of an early viral promoter. A 111-bp deletion-substitution of the unique sequence caused approximately a 20-fold increase in the steady-state level of RNA from the UL127 promoter and a 245-fold increase in the expression of a downstream indicator gene. This viral negative regulatory region was also mutated at approximately 50-bp regions proximal and distal to the UL127 promoter. Although some repressive effects were detected in the distal region, mutations of the region proximal to the UL127 promoter had the most significant effects on transcription. Within the proximal and distal regions, there are potential cis sites for known eucaryotic transcriptional repressor proteins. This region may also bind unknown viral proteins. We propose that the unique region upstream of the UL127 promoter and the MIE enhancer negatively regulates the expression from the UL127 promoter in permissive human fibroblast cells. This region may be a regulatory boundary preventing the effects of the very strong MIE enhancer on this promoter.

Binding of the human cytomegalovirus 80-kDa immediate-early protein (IE2) to minor groove A/T-rich sequences bounded by CG dinucleotides is regulated by protein oligomerization and phosphorylation

The 80-kDa immediate-early regulatory protein IE2 of human cytomegalovirus (HCMV) functions as an essential positive transactivator of downstream viral promoters, but it also specifically down-regulates transcription from the major immediate-early promoter through a 14-bp DNA target motif known as the cis-repression signal (CRS) located at the transcription start site. The IE2 protein purified from bacteria as a fusion product of either staphylococcal Protein A/IE2(290-579) or glutathione-S-transferase (GST)/IE2(346-579) bound specifically to a [32P]-labeled CRS oligonucleotide probe in an in vitro electrophoretic mobility shift assay (EMSA). In contrast, no direct interaction with the CRS probes could be detected with IE2 wild-type protein in extracts from infected or transfected mammalian cells or when synthesized by in vitro translation. However, in vitro phosphorylation of GST/IE2(346-579) by incubation with either the catalytic subunit of protein kinase A (PKA) or a HeLa cell nuclear extract strongly inhibited its DNA-binding activity. This process required ATP hydrolysis and could be reversed by subsequent incubation with bacterial alkaline phosphatase. Importantly, dephosphorylation of the constitutively expressed native IE2 protein present in a nuclear extract from the U373(A45) cell line unmasked a specific CRS DNA-binding activity that could be supershifted with anti-IE2 monoclonal antibody (mAb). A series of high-molecular-weight hetero-oligomeric DNA-bound structures of intermediate mobility were formed in EMSA assays when a mixture of staphylococcal Protein A/IE2 and GST/IE2 was coincubated with the CRS probe. Coincubation with a DNA-binding negative but dimerization-competent GST/IE2 deletion mutant competitively inhibited DNA-binding by staphylococcal Protein A/IE2, whereas coincubation with a GST/IE2 deletion mutant that lacked the ability to both dimerize and bind to DNA failed to influence the mobility of the DNA-bound staphylococcal Protein A/IE2 protein. Therefore, IE2 appears to bind to DNA as a higher-order oligomer in which the presence of subunits with mutant DNA-binding domains interferes with the overall DNA-binding function. A series of point mutations introduced into each of nine conserved motifs throughout the DNA-binding and dimerization domain, all of which abolish the ability of the transfected intact IE2 protein to autoregulate the MIE promoter, also all lacked the ability to bind to CRS sequences as GST/IE2(346-379) fusion proteins. Detailed analysis of point mutations in the 14-bp CRS target DNA binding motif revealed that IE2 binds in a relatively sequence-independent manner to 10-bp-long A/T-rich DNA elements bounded on each side by CG dinucleotides. Moreover, the A/T-rich minor groove binding agent distamycin, but not the G/C-rich minor groove binding agent chromomycin-A3, actively competed with IE2 for binding to the CRS motif in a dose-dependent fashion. In conclusion, IE2 binds preferentially as multimerized dimers to A/T-rich sequences in the minor groove that are flanked on both sides by appropriately spaced CG dinucleotides, and inhibition of the DNA-binding or oligomerization activity by PKA phosphorylation probably accounts for the inactivity of the mammalian and in vitro translated forms of the protein.

A cis repression sequence adjacent to the transcription start site of the human cytomegalovirus US3 gene is required to down regulate gene expression at early and late times after infection

Human cytomegalovirus has two enhancer-containing immediate-early (IE) promoters with a cis repression sequence (CRS) positioned immediately upstream of the transcription start site, designated the major IE (MIE) promoter and the US3 promoter. The role of the CRS upstream of the US3 transcription start site in the context of the viral genome was determined by comparing the levels of transcription from these two enhancer-containing promoters in recombinant viruses with a wild-type or mutant CRS. Upstream of the CRS of the US3 promoter was either the endogenous enhancer (R2) or silencer (R1). The downstream US3 gene was replaced with the indicator gene chloramphenicol acetyltransferase (CAT). Infected permissive human fibroblast cells or nonpermissive, undifferentiated monocytic THP-1 cells were analyzed for expression from the US3 promoter containing either the wild-type or mutant CRS. With the wild-type CRS, the maximum level of transcription in permissive cells was detected within 4 to 6 h after infection and then declined. With the mutant CRS and the R2 enhancer upstream, expression from the US3 promoter continued to increase throughout the viral replication cycle to levels 20- to 40-fold higher than for the wild type. In nonpermissive or permissive monocytic THP-1 cells, expression from the US3 promoter was also significantly higher when the CRS was mutated. Less expression was obtained when only the R1 element was present, but expression was higher when the CRS was mutated. Thus, the CRS in the enhancer-containing US3 promoter appears to allow for a short burst of US3 gene expression followed by repression at early and late times after infection. Overexpression of US3 may be detrimental to viral replication, and its level of expression must be stringently controlled. The role of the CRS and the viral IE86 protein in controlling enhancer-containing promoters is discussed.

Soluble expression and complex formation of proteins required for HCMV DNA replication using the SFV expression system

Several of the viral proteins required for human cytomegalovirus (HCMV) DNA replication have been difficult to study due to their low abundance in infected cells and low solubility in bacterial or insect-cell expression systems. Therefore we used the Semliki Forest virus expression system to express these proteins in mammalian cells. All of the recombinant proteins were soluble, on the basis of ultracentrifugation properties and their ability to be immunoprecipitated from solution with specific antibodies. Pulse-chase analysis of the 86-kDa major immediate-early protein (IE86) revealed two expressed forms-a precursor and a product-indicating that this recombinant protein, like the native HCMV protein, is posttranslationally processed. The recombinant proteins (polymerase core and accessory as well as the IE86 and pUL84) formed stable complexes similar to those known to form in HCMV-infected cells. The recombinant DNA polymerase holoenzyme also exhibited enzyme activity that was phosphonoformic acid sensitive, as is the infected-cell DNA polymerase activity. This expression system offers many advantages for the expression and study of the HCMV replication proteins, including the expression of soluble, active proteins that are able to interact to form complexes. Additionally, the relative ease with which SFV recombinants can be made lends itself to the construction and evaluation of mutants.

Phosphorylation of the human cytomegalovirus 86-kilodalton immediate-early protein IE2

We have investigated the phosphorylation state of the human cytomegalovirus 86-kDa immediate-early (IE) protein IEP86 from transfected and infected cells. We show that multiple domains of IEP86 are phosphorylated by cellular kinases, both in vitro and in vivo. Our data suggest that serum-inducible kinases play a significant role in cell-mediated IE protein phosphorylation and that a member of the mitogen-activated protein (MAP) kinase (MAPK) family, extracellular regulated kinase 2 (ERK2), phosphorylates several domains of IEP86 in vitro. Alanine substitution mutagenesis was performed on specific serines or threonines (T27, S144, T233/S234, and T555) found in consensus MAP kinase motifs. Analysis of these mutations showed that T27 and T233/S234 are the major sites for serum-inducible kinases and are the major ERK2 sites in vitro. S144 appeared to be phosphorylated in a serum-independent manner in vitro. All of the mutations except T555 eliminated specific phosphorylation in vivo. In transient transfection analyses, IEP86 isoforms containing mutations in S144 and, especially, T233/S234 displayed increased transcriptional activation relative to the wild type, suggesting that phosphorylation at these sites in wild-type IEP86 may result in reduction of its transcriptional activation ability.

Identification of domains within the human cytomegalovirus major immediate-early 86-kilodalton protein and the retinoblastoma protein required for physical and functional interaction with each other

The human cytomegalovirus major immediate-early 86-kDa protein (IE2 86) plays an important role in the trans activation and regulation of HCMV gene expression. Previously, we demonstrated that IE2 86 contains three regions (amino acids [aa] 86 to 135, 136 to 290, and 291 to 364) that can independently bind to in vitro-translated Rb when IE2 86 is produced as a glutathione S-transferase fusion protein (M. H. Sommer, A. L. Scully, and D. H. Spector, J. Virol. 68:6223-6231, 1994). In this report, we have elucidated the regions of Rb involved in binding to IE2 86 and have further analyzed the functional nature of the interaction between these two proteins. We find that two domains on Rb, the A/B pocket and the carboxy terminus, can each independently form a complex with IE2 86. In functional assays, we demonstrate that IE2 86 and another IE protein, IE1 72, can counter the enlarged flat cell phenotype, but not the G1/S block, which results from expression of wild-type Rb in the human osteosarcoma cell line Saos-2. Mutational analysis reveals that there are two domains on IE2 86 that can independently affect Rb function. One region (aa 241 to 369) includes the major Rb-binding domain, while the second maps to the amino-terminal region (aa 1 to 85) common to both IE2 86 and IE1 72. These data show that Rb and IE2 86 physically and functionally interact with each other via at least two separate domains and provide further support for the hypothesis that IE2 86 may exert its pleiotropic effects through the formation of multimeric protein complexes.

The UL84 protein of human cytomegalovirus acts as a transdominant inhibitor of immediate-early-mediated transactivation that is able to prevent viral replication

The 86-kilodalton immediate-early (IE) 2 protein (IE2-p86) of human cytomegalovirus (HCMV) is a multifunctional regulator of HCMV gene expression which appears to be essential for triggering the lytic replicative cycle. IE2-p86 functions as a promiscuous transactivator of both viral and cellular gene expression and can repress transcription from its own promoter. In this study we demonstrate that a viral early protein, termed pUL84, which is able to interact with IE2-p86 both in vivo and in vitro, modulates IE2-p86 in a specific manner. First, pUL84 acts as a transdominant inhibitor of IE2-p86-mediated transactivation of both homologous and heterologous promoters. Second, negative autoregulation by IE2-p86 is augmented in the presence of pUL84. Using two in vivo assays, we obtained evidence that expression of pUL84 during the IE phase of the viral replicative cycle leads to an inhibition of viral early gene expression which prevents replication of HCMV and results in a persistent infection of UL84-positive cell lines. Transdominant inhibition of a viral IE function by a protein expressed during the later phases of replication appears to be a novel principle used by herpesviruses which could account for the slow replication of HCMV and may be useful in the development of new antiviral strategies.

Evidence that the UL84 gene product of human cytomegalovirus is essential for promoting oriLyt-dependent DNA replication and formation of replication compartments in cotransfection assays

The protein products of 11 viral genomic loci cooperate in a transient cotransfection assay to mediate lytic-phase DNA replication of oriLyt, the human cytomegalovirus (HCMV) origin of replication. Six of these genes have homology with the well-characterized herpes simplex virus replication genes and encode core replication machinery proteins that are typically essential for DNA synthesis. The remaining five HCMV gene loci, initially referred to as auxiliary components, include several known immediate-early (IE) transcriptional regulatory proteins as well as genes encoding functionally uncharacterized polypeptides. Some or all of the auxiliary components may be necessary in trans to replicate the HCMV oriLyt only because they are required for efficient expression or transactivation of the native early promoters and 3' processing elements included in the genomic clones. Therefore, we reassessed the requirements for the auxiliary components by adding constitutive heterologous promoters and control signals to the coding regions and carrying out transient DpnI replication assays in cotransfected Vero cells. The results revealed that in the presence of the UL69 posttranscriptional activator and the remaining auxiliary polypeptides, UL84 was the only auxiliary component that could not be omitted to obtain oriLyt-dependent DNA replication. Nevertheless, in human diploid fibroblasts, some additional auxiliary loci as well as UL84 were critical. There was also an obligatory requirement for UL84, in cooperation with two other auxiliary factors, UL112-113 and IE2, and the core machinery, to constitute the minimal HCMV proteins necessary to direct oriLyt-dependent DNA amplification. However, the Epstein-Barr virus core replication genes could substitute for the HCMV core genes, and in these circumstances, UL84 alone directed amplification of HCMV oriLyt. Moreover, there was also an absolute requirement for UL84 along with the core and other auxiliary factors for the formation of intranuclear replication compartments as assayed by immunofluorescence in transient DNA cotransfection assays. These compartments were typical of those associated with active viral DNA replication in HCMV-infected cells, they incorporated pulse-labeled bromodeoxyuridine, and their formation was both phosphonoacetic acid sensitive and oriLyt dependent. These results demonstrate that UL84 is obligatory for both intranuclear replication compartment formation and origin-dependent DNA amplification and suggest that it is a key viral component in promoting the initiation of HCMV oriLyt-directed DNA replication.

Four of eleven loci required for transient complementation of human cytomegalovirus DNA replication cooperate to activate expression of replication genes

As previously shown, 11 loci are required to complement human cytomegalovirus (HCMV) DNA replication in a transient-transfection assay (G. S. Pari and D. G. Anders, J. Virol. 67:6979-6988, 1993). Six of these loci encode known or candidate replication fork proteins, as judged by sequence and biochemical similarities to herpes simplex virus homologs of known function; three encode known immediate early regulatory proteins (UL36-38, IRS1/TRS1, and the major immediate early region spanning UL122-123); and two encode early, nucleus-localized proteins of unknown functions (UL84 and UL112-113). We speculated that proteins of the latter five loci might cooperate to promote and regulate expression of the six replication fork proteins. To test this hypothesis we made luciferase reporter plasmids for each of the replication fork gene promoters and measured their activation by the candidate effectors, expressed under the control of their respective native promoters, using a transient-cooperativity assay in which the candidate effectors were subtracted individually from a transfection mixture containing all five loci. The combination of UL36-38, UL112-113, IRS1, or TRS1 and the major immediate early region produced as much as 100-fold-higher expression than the major immediate early region alone; omitting any one of these four loci from complementing mixtures produced a significant reduction in expression. In contrast, omitting UL84 had insignificant (less than twofold), promoter-dependent effects on reporter activity, and these data do not implicate UL84 in regulating HCMV early-gene expression. Most of the effector interactions showed significant positive cooperativity, producing synergistic enhancement of expression. Similar responses to these effectors were observed for the each of the promoters controlling expression of replication fork proteins. However, subtracting UL112-113 had little if any effect on expression by the UL112-113 promoter or by the simian virus 40 promoter-enhancer under the same conditions. Several lines of evidence argue that the cooperative interactions observed in our transient-transfection assays are important to viral replication in permissive cells. Therefore, the data suggest a model in which coordinate expression of multiple essential replication proteins during permissive infection is vitally dependent upon the cooperative regulatory interactions of proteins encoded by multiple loci and thus have broad implications for our understanding of HCMV biology.

Protein-protein interactions between human cytomegalovirus IE2-580aa and pUL84 in lytically infected cells

The human cytomegalovirus immediate-early protein IE2-580aa (ppUL122a) activates transcription of viral and cellular genes and represses its own transcription through sequence-specific binding to the major immediate-early promoter. In lytically infected cells, IE2-580aa interacts with a 75-kDa viral protein (p75), an early protein that is also synthesized at late times after infection. Here we show that p75 is the product of the UL84 gene. Its association with IE2-580aa in infected cells suggests that pUL84 is involved in transcription control.