Adenovirus E1A activation domain binds the basic repeat in the TATA box transcription factor

On the Role of TATA Boxes and TATA-Binding Protein in Arabidopsis thaliana

For transcription initiation by RNA polymerase II (Pol II), all eukaryotes require assembly of basal transcription machinery on the core promoter, a region located approximately in the locus spanning a transcription start site (-50; +50 bp). Although Pol II is a complex multi-subunit enzyme conserved among all eukaryotes, it cannot initiate transcription without the participation of many other proteins. Transcription initiation on TATA-containing promoters requires the assembly of the preinitiation complex; this process is triggered by an interaction of TATA-binding protein (TBP, a component of the general transcription factor TFIID (transcription factor II D)) with a TATA box. The interaction of TBP with various TATA boxes in plants, in particular Arabidopsis thaliana, has hardly been investigated, except for a few early studies that addressed the role of a TATA box and substitutions in it in plant transcription systems. This is despite the fact that the interaction of TBP with TATA boxes and their variants can be used to regulate transcription. In this review, we examine the roles of some general transcription factors in the assembly of the basal transcription complex, as well as functions of TATA boxes of the model plant A. thaliana. We review examples showing not only the involvement of TATA boxes in the initiation of transcription machinery assembly but also their indirect participation in plant adaptation to environmental conditions in responses to light and other phenomena. Examples of an influence of the expression levels of A. thaliana TBP1 and TBP2 on morphological traits of the plants are also examined. We summarize available functional data on these two early players that trigger the assembly of transcription machinery. This information will deepen the understanding of the mechanisms underlying transcription by Pol II in plants and will help to utilize the functions of the interaction of TBP with TATA boxes in practice.

Factors Influencing Green Purchase Intention: Moderating Role of Green Brand Knowledge

The current study aims to investigate the moderating effect of green brand knowledge (GBK) on the relationship of green brand positioning (GBP), attitude towards the green brand (ATGB), environmental concern (EC) and green purchase intention (GPI) in Pakistan. For this purpose, the data was collected from the individuals who were buying organic food by using purposive sampling, using cross-sectional research design and quantitative research approach. The Partial Least Square (PLS)-Structural Equation Modeling (SEM) technique results had shown that all the direct-effect relationships, namely, GBP, ATGB, EC variables have a positive and significant relationship with the GPI. While indirect-effect relationships have shown that the relationships of ATGB, EC and GPI are significantly moderated by GBK, which indicated that the effect of GBP, and EC toward GPI would be stronger when individuals have strong knowledge about green brands. In contrast, GBK is not significantly moderating the relationship between GBP and GPI. The empirical findings of this study fill a gap in the existing body of literature regarding the effects of GPI, ATGB and EC on green brands, as well as the moderating effect of GBK. As a result, this study provides insight into the topic, which has not been thoroughly investigated in earlier studies. Therefore, we consider that understanding this moderating effect is a positive contribution to the existing body of knowledge, which could help researchers explore this relationship in the future. This study could also help the owners and managers to know about the importance of these exogenous, and moderate variables to increase their customer’s green purchase intentions.

Multiple domains in the 50 kDa form of E4F1 regulate promoter-specific repression and E1A trans-activation

The 50 kDa N-terminal product of the cellular transcription factor E4F1 (p50E4F1) mediates E1A289R trans-activation of the adenovirus E4 gene, and suppresses E1A-mediated transformation by sensitizing cells to cell death. This report shows that while both E1A289R and E1A243R stimulate p50E4F1 DNA binding activity, E1A289R trans-activation, as measured using GAL-p50E4F1 fusion proteins, involves a p50E4F1 transcription regulatory (TR) region that must be promoter-bound and is dependent upon E1A CR3, CR1 and N-terminal domains. Trans-activation is promoter-specific, as GAL-p50E4F1 did not stimulate commonly used artificial promoters and was strongly repressive when competing against GAL-VP16. p50E4F1 and E1A289R stably associate in vivo using the p50E4F1 TR region and E1A CR3, although their association in vitro is indirect and paradoxically disrupted by MAP kinase phosphorylation of E1A289R, which stimulates E4 trans-activation in vivo. Multiple cellular proteins, including TBP, bind the p50E4F1 TR region in vitro. The mechanistic implications for p50E4F1 function are discussed.

Expression and nuclear localization of the TATA-box-binding protein during baculovirus infection

The TATA-box-binding protein (TBP) plays a key role in initiating eukaryotic transcription and is used by many viruses for viral transcription. We previously reported increased TBP levels during infection with the baculovirus Autographa californica multicapsid nuclear polyhedrovirus (AcMNPV). The TBP antiserum used in that study, however, cross-reacted with a baculoviral protein. Here, we reported that increased amounts of nuclear TBP were detected upon infection of Spodoptera frugiperda and TN-368 cells with a TBP-specific antiserum. TBP levels increased until 72 h post-infection (p.i.), whilst tbp transcripts decreased by 16 h p.i., which suggested a virus-induced influence on the TBP protein levels. To address a potential modification of the TBP degradation pathway during infection, we investigated the possible role of viral ubiquitin. Infection studies with AcMNPV recombinants carrying a mutated viral ubiquitin gene revealed that the TBP increase during infection was not altered. In addition, pulse-chase experiments indicated a high TBP half-life of ~60 h in uninfected cells, suggesting that a virus-induced increase of TBP stability was unlikely. This increase in TBP correlated with a redistribution to nuclear domains resembling sites of viral DNA synthesis. Furthermore, we observed colocalization of TBP with host RNA polymerase (RNAP) II, but only until 8 h p.i., whilst TBP, but not RNAPII, was present in the enlarged replication domains late during infection. Thus, we suggested that AcMNPV adapted a mechanism to accumulate the highly stable cellular TBP at sites of viral DNA replication and transcription.

Distribution and dynamics of transcription-associated proteins during parvovirus infection

Canine parvovirus (CPV) infection leads to reorganization of nuclear proteinaceous subcompartments. Our studies showed that virus infection causes a time-dependent increase in the amount of viral nonstructural protein NS1 mRNA. Fluorescence recovery after photobleaching showed that the recovery kinetics of nuclear transcription-associated proteins, TATA binding protein (TBP), transcription factor IIB (TFIIB), and poly(A) binding protein nuclear 1 (PABPN1) were different in infected and noninfected cells, pointing to virus-induced alterations in binding dynamics of these proteins.

Cellular GCN5 is a novel regulator of human adenovirus E1A-conserved region 3 transactivation

The largest isoform of adenovirus early region 1A (E1A) contains a unique region termed conserved region 3 (CR3). This region activates viral gene expression by recruiting cellular transcription machinery to the early viral promoters. Recent studies have suggested that there is an optimal level of E1A-dependent transactivation required by human adenovirus (hAd) during infection and that this may be achieved via functional cross talk between the N termini of E1A and CR3. The N terminus of E1A binds GCN5, a cellular lysine acetyltransferase (KAT). We have identified a second independent interaction of E1A with GCN5 that is mediated by CR3, which requires residues 178 to 188 in hAd5 E1A. GCN5 was recruited to the viral genome during infection in an E1A-dependent manner, and this required both GCN5 interaction sites on E1A. Ectopic expression of GCN5 repressed transactivation by both E1A CR3 and full-length E1A. In contrast, RNA interference (RNAi) depletion of GCN5 or treatment with the KAT inhibitor cyclopentylidene-[4-(4'-chlorophenyl)thiazol-2-yl]hydrazone (CPTH2) resulted in increased E1A CR3 transactivation. Moreover, activation of the adenovirus E4 promoter by E1A was increased during infection of homozygous GCN5 KAT-defective (hat/hat) mouse embryonic fibroblasts (MEFs) compared to wild-type control MEFs. Enhanced histone H3 K9/K14 acetylation at the viral E4 promoter required the newly identified binding site for GCN5 within CR3 and correlated with repression and reduced occupancy by phosphorylated RNA polymerase II. Treatment with CPTH2 during infection also reduced virus yield. These data identify GCN5 as a new negative regulator of transactivation by E1A and suggest that its KAT activity is required for optimal virus replication.

Penaeus monodon TATA box-binding protein interacts with the white spot syndrome virus transactivator IE1 and promotes its transcriptional activity

We show here that the white spot syndrome virus (WSSV) immediate-early protein IE1 interacts with the Penaeus monodon TATA box-binding protein (PmTBP) and that this protein-protein interaction occurs in the absence of any other viral or cellular proteins or nucleic acids, both in vitro and in vivo. Mapping studies using enhanced green fluorescent protein (EGFP) fusion proteins containing truncations of IE1 and PmTBP delimited the interacting regions to amino acids (aa) 81 to 180 in IE1 and, except for aa 171 to 230, to aa 111 to 300 in PmTBP. A WSSV IE1 transactivation assay showed that large quantities (>800 ng) of the GAL4-IE1 plasmid caused "squelching" of the GAL4-IE1 activity and that this squelching effect was alleviated by the overexpression of PmTBP. Gene silencing of WSSV ie1 and PmTBP by pretreatment with double-stranded RNAs (dsRNAs) prior to WSSV challenge showed that the expression of these two target genes was specifically inhibited by their corresponding dsRNAs 72 and 96 h after dsRNA treatment. dsRNA silencing of ie1 and PmTBP expression also significantly reduced WSSV replication and the expression of the viral early gene dnapol (DNA polymerase gene). These results suggest that WSSV IE1 and PmTBP work cooperatively with each other during transcription initiation and, furthermore, that PmTBP is an important target for WSSV IE1's transactivation activity that can enhance viral gene expression and help in virus replication.

Comparison of E1A CR3-dependent transcriptional activation across six different human adenovirus subgroups

The largest E1A isoform of human adenovirus (Ad) includes a C-4 zinc finger domain within conserved region 3 (CR3) that is largely responsible for activating transcription of the early viral genes. CR3 interacts with multiple cellular factors, but its mechanism of action is modeled primarily on the basis of the mechanism for the prototype E1A protein of human Ad type 5. We expanded this model to include a representative member from each of the six human Ad subgroups. All CR3 domains tested were capable of transactivation. However, there were dramatic differences in their levels of transcriptional activation. Despite these functional variations, the interactions of these representative CR3s with known cellular transcriptional regulators revealed only modest differences. Four common cellular targets of all representative CR3s were identified: the proteasome component human Sug1 (hSug1)/S8, the acetyltransferases p300/CREB binding protein (CBP), the mediator component mediator complex subunit 23 (MED23) protein, and TATA binding protein (TBP). The first three factors appear to be critical for CR3 function. RNA interference against human TBP showed no significant reduction in transactivation by any CR3 tested. These results indicate that the cellular factors previously shown to be important for transactivation by Ad5 CR3 are similarly bound by the E1A proteins of other types. This was confirmed experimentally using a transcriptional squelching assay, which demonstrated that the CR3 regions of each Ad type could compete with Ad5 CR3 for limiting factors. Interestingly, a mutant of Ad5 CR3 (V147L) was capable of squelching wild-type Ad5 CR3, despite its failure to bind TBP, MED23, p300/CBP-associated factor (pCAF), or p300/CBP, suggestive of the possibility that an additional as yet unidentified cellular factor is required for transactivation by E1A CR3.

Identification of a second independent binding site for the pCAF acetyltransferase in adenovirus E1A

The conserved region 3 (CR3) portion of the human adenovirus (HAdV) 5 E1A protein functions as a potent transcriptional activator that induces expression of viral early genes during infection. Expression of HAdV-5 CR3 in the yeast Saccharomyces cerevisiae inhibits growth, as do the corresponding regions of the HAdV-3, 4, 9, 12 and 40 E1A proteins, which represent the remaining five HAdV subgroups. Growth inhibition is alleviated by disruption of the SAGA transcriptional regulatory complex, suggesting that CR3 targets the yeast SAGA complex. In yeast, transcriptional activation by several, but not all, of the CR3 regions requires the Gcn5 acetyltransferase component of SAGA. The CR3 regions of HAdV-3, 5, 9 and 40, but not HAdV-4 and 12 interact with the pCAF acetyltransferase, a mammalian ortholog of yeast Gcn5. Disruption of the previously described N-terminal pCAF binding site abrogates binding by the HAdV-5 243R E1A protein, but not the larger 289R E1A protein, which is otherwise identical except for the presence of CR3. RNA interference directed against pCAF decreased HAdV-5 CR3 dependent transcriptional activation in mammalian cells. Our results identify a second independent binding site for pCAF in E1A and suggest that it contributes to CR3 dependent transcriptional activation.

Requirements for E1A dependent transcription in the yeast Saccharomyces cerevisiae

BACKGROUND

The human adenovirus type 5 early region 1A (E1A) gene encodes proteins that are potent regulators of transcription. E1A does not bind DNA directly, but is recruited to target promoters by the interaction with sequence specific DNA binding proteins. In mammalian systems, E1A has been shown to contain two regions that can independently induce transcription when fused to a heterologous DNA binding domain. When expressed in Saccharomyces cerevisiae, each of these regions of E1A also acts as a strong transcriptional activator. This allows yeast to be used as a model system to study mechanisms by which E1A stimulates transcription.

RESULTS

Using 81 mutant yeast strains, we have evaluated the effect of deleting components of the ADA, COMPASS, CSR, INO80, ISW1, NuA3, NuA4, Mediator, PAF, RSC, SAGA, SAS, SLIK, SWI/SNF and SWR1 transcriptional regulatory complexes on E1A dependent transcription. In addition, we examined the role of histone H2B ubiquitylation by Rad6/Bre1 on transcriptional activation.

CONCLUSION

Our analysis indicates that the two activation domains of E1A function via distinct mechanisms, identify new factors regulating E1A dependent transcription and suggest that yeast can serve as a valid model system for at least some aspects of E1A function.

Transcriptional control by adenovirus E1A conserved region 3 via p300/CBP

The human adenovirus type 5 (HAdV-5) E1A 13S oncoprotein is a potent regulator of gene expression and is used extensively as a model for transcriptional activation. It possesses two independent transcriptional activation domains located in the N-terminus/conserved region (CR) 1 and CR3. The protein acetyltransferase p300 was previously identified by its association with the N-terminus/CR1 portion of E1A and this association is required for oncogenic transformation by E1A. We report here that transcriptional activation by 13S E1A is inhibited by co-expression of sub-stoichiometric amounts of the smaller 12S E1A isoform, which lacks CR3. Transcriptional inhibition by E1A 12S maps to the N-terminus and correlates with the ability to bind p300/CBP, suggesting that E1A 12S is sequestering this limiting factor from 13S E1A. This is supported by the observation that the repressive effect of E1A 12S is reversed by expression of exogenous p300 or CBP, but not by a CBP mutant lacking actyltransferase activity. Furthermore, we show that transcriptional activation by 13S E1A is greatly reduced by siRNA knockdown of p300 and that CR3 binds p300 independently of the well-characterized N-terminal/CR1-binding site. Importantly, CR3 is also required to recruit p300 to the adenovirus E4 promoter during infection. These results identify a new functionally significant interaction between E1A CR3 and the p300/CBP acetyltransferases, expanding our understanding of the mechanism by which this potent transcriptional activator functions.

Identification of a second CtBP binding site in adenovirus type 5 E1A conserved region 3

C-terminal binding protein (CtBP) binds to adenovirus early region 1A (AdE1A) through a highly conserved PXDLS motif close to the C terminus. We now have demonstrated that CtBP1 also interacts directly with the transcriptional activation domain (conserved region 3 [CR3]) of adenovirus type 5 E1A (Ad5E1A) and requires the integrity of the entire CR3 region for optimal binding. The interaction appears to be at least partially mediated through a sequence ((161)RRNTGDP(167)) very similar to a recently characterized novel CtBP binding motif in ZNF217 as well as other regions of CR3. Using reporter assays, we further demonstrated that CtBP1 represses Ad5E1A CR3-dependent transcriptional activation. Ad5E1A also appears to be recruited to the E-cadherin promoter through its interaction with CtBP. Significantly, Ad5E1A, CtBP1, and ZNF217 form a stable complex which requires CR3 and the PLDLS motif. It has been shown that Ad513SE1A, containing the CR3 region, is able to overcome the transcriptional repressor activity of a ZNF217 polypeptide fragment in a GAL4 reporter assay through recruitment of CtBP1. These results suggest a hitherto-unsuspected complexity in the association of Ad5E1A with CtBP, with the interaction resulting in transcriptional activation by recruitment of CR3-bound factors to CtBP1-containing complexes.

Adenovirus E1A proteins direct subcellular redistribution of Nek9, a NimA-related kinase

A monoclonal antibody raised against adenovirus E1A-associated cellular proteins recognized Nek9, a NimA-related protein kinase. Subcellular fractionation and immunofluorescence indicated that Nek9 was primarily cytoplasmic with a small portion located in the nucleus whereas E1A was primarily nuclear. Although co-immunoprecipitation experiments indicated that nuclear Nek9 interacted, directly or indirectly, with E1A, the major effect of E1A was to diminish the amount of Nek9 in the nucleus suggesting that E1A alters the subcellular distribution of Nek9 and that the interaction is transient. A Nek9 deletion mutant lacking a central RCC1-like domain interacted stably with E1A and accumulated in the nucleus in the presence of E1A, possibly representing an intermediate stage of the normally transient Nek9/E1A interaction. The interaction of Nek9 with E1A was dependent on the N-terminal sequences of E1A. Attempts to stably overexpress either Nek9 or the kinase-inactive mutant in various cell lines were unsuccessful; however, the presence of E1A allowed stable overexpression of both proteins. These results suggest that E1A disrupts a nuclear function of Nek9.

A gas-inducible expression system in HEK.EBNA cells applied to controlled proliferation studies by expression of p27(Kip1)

We describe an efficient inducible gene expression system in HEK.EBNA cells, a well-established cell system for the rapid transient expression of research-tool proteins. The transgene control system of choice is the novel acetaldehyde-inducible regulation (AIR) technology, which has been shown to modulate transgene levels following exposure of cells to acetaldehyde. For application in HEK.EBNA cells, AlcR transactivator plasmids were constructed and co-expressed with the secreted alkaline phosphatase (SEAP) gene under the control of a chimeric mammalian promoter (P(AIR)) for acetaldehyde-regulated expression. Several highly inducible transactivator cell lines were established. Adjustable transgene induction by gaseous acetaldehyde led to high induction levels and tight repression in transient expression trials and in stably transfected HEK.EBNA cell lines. Thus, the AIR technology can be used for inducible expression of any desired recombinant protein in HEK.EBNA cells. A possible application for inducible gene expression is a controlled proliferation strategy. Clonal HEK.EBNA cell lines, expressing the fungal transactivator protein AlcR, were engineered for gas-adjustable expression of the cell-cycle regulator p27(Kip1). We show that expression of p27(Kip1) via transient or stable transfection led to a G1-phase specific growth arrest of HEK.EBNA cells. Furthermore, production pools engineered for gas-adjustable expression of p27(Kip1) and constitutive expression of SEAP showed enhanced productive capacity.

Roles for APIS and the 20S proteasome in adenovirus E1A-dependent transcription

We have determined distinct roles for different proteasome complexes in adenovirus (Ad) E1A-dependent transcription. We show that the 19S ATPase, S8, as a component of 19S ATPase proteins independent of 20S (APIS), binds specifically to the E1A transactivation domain, conserved region 3 (CR3). Recruitment of APIS to CR3 enhances the ability of E1A to stimulate transcription from viral early gene promoters during Ad infection of human cells. The ability of CR3 to stimulate transcription in yeast is similarly dependent on the functional integrity of yeast APIS components, Sug1 and Sug2. The 20S proteasome is also recruited to CR3 independently of APIS and the 26S proteasome. Chromatin immunoprecipitation reveals that E1A, S8 and the 20S proteasome are recruited to both Ad early region gene promoters and early region gene sequences during Ad infection, suggesting their requirement in both transcriptional initiation and elongation. We also demonstrate that E1A CR3 transactivation and degradation sequences functionally overlap and that proteasome inhibitors repress E1A transcription. Taken together, these data demonstrate distinct roles for APIS and the 20S proteasome in E1A-dependent transactivation.

Testifying the rice bacterial blight resistance gene xa5 by genetic complementation and further analyzing xa5 (Xa5) in comparison with its homolog TFIIAgamma1

The recessive gene xa5 for resistance to bacterial blight resistance of rice is located on chromosome 5, and evidence based on genetic recombination has been shown to encode a small subunit of the basal transcription factor IIA (Iyer and McCouch in MPMI 17(12):1348-1354, 2004). However, xa5 has not been demonstrated by a complementation test. In this study, we introduced the dominant allele Xa5 into a homozygous xa5-line, which was developed from a cross between IRBB5 (an indica variety with xa5) and Nipponbare (a japonica variety with Xa5). Transformation of Xa5 and subsequent segregation analysis confirmed that xa5 is a V39E substitution variant of the gene for TFIIAgamma on chromosome 5 (TFIIAgamma5 or Xa5). The rice has an addition gene for TFIIAgamma exists on chromosome 1 (TFIIAgamma1). Analysis of the expression patterns of Xa5 (TFIIAgamma5)/xa5 and TFIIAgamma1 revealed that both the genes are constitutively expressed in different rice organs. However, no expression of TFIIAgamma1 could be detected in the panicle by reverse transcriptase-polymerase chain reaction. To compare the structural difference between the Xa5/xa5 and TFIIAgamma1 proteins, 3-D structures were predicted using computer-aided modeling techniques. The modeled structures of Xa5 (xa5) and TFIIAgamma1 fit well with the structure of TFIIA small subunit from human, suggesting that they may all act as a small subunit of TFIIA. The E39V substitution in the xa5 protein occurs in the alpha-helix domain, a supposed conservative substitutable site, which should not affect the basal transcription function of TFIIAgamma. The structural analysis indicates that xa5 and Xa5 potentially retain their basic transcription factor function, which, in turn, may mediate the novel pathway for bacterial blight resistance and susceptibility, respectively.

Macromolecular translocation inhibitor II (Zn(2+)-binding protein, parathymosin) interacts with the glucocorticoid receptor and enhances transcription in vivo

Macromolecular translocation inhibitor II (MTI-II), which was first identified as an in vitro inhibitor of binding between the highly purified glucocorticoid receptor (GR) and isolated nuclei, is an 11.5-kDa Zn(2+)-binding protein that is also known as ZnBP or parathymosin. MTI-II is a small nuclear acidic protein that is highly conserved in rats, cows, and humans and widely distributed in mammalian tissues, yet its physiological function is unknown. To elucidate its in vivo function in relation to GR, we transiently transfected mammalian cells with an expression plasmid encoding MTI-II. Unexpectedly, we found that the expression of MTI-II enhances the transcriptional activity of GR. The magnitude of the transcriptional enhancement induced by MTI-II is comparable with that induced by the steroid receptor coactivator SRC-1. In contrast, MTI-II had little effect on the transcriptional activity of estrogen receptor. Immunoprecipitation analysis showed that in the presence of glucocorticoid hormone, GR coprecipitates with MTI-II, and, vice versa, MTI-II coprecipitates with GR. The expression of various deletion mutants of MTI-II revealed that the central acidic domain is essential for the enhancement of GR-dependent transcription. Microscopic analysis of MTI-II fused to green fluorescent protein and GR fused to red fluorescent protein in living HeLa cells showed that MTI-II colocalizes with GR in discrete subnuclear domains in a hormone-dependent manner. Coexpression of MTI-II with the coactivator SRC-1 or p300 further enhances GR-dependent transcription. Immunoprecipitation analysis showed that in the presence of glucocorticoid hormone, p300 and CREB-binding protein are coprecipitated with MTI-II. Furthermore, the knockdown of endogenous MTI-II by RNAi reduces the transcriptional activity of GR in cells. Moreover, expression of MTI-II enhances the glucocorticoid-dependent transcription of the endogenous glucocorticoid-inducible enzyme in cells. Taken together, these results indicate that MTI-II enhances GR-dependent transcription via a direct interaction with GR in vivo. Thus, MTI-II is a new member of the GR-coactivator complex.

Comprehensive sequence analysis of the E1A proteins of human and simian adenoviruses

Despite extensive study of human adenovirus type 5 E1A, surprisingly little is known about the E1A proteins of other adenoviruses. We report here a comprehensive analysis of the sequences of 34 E1A proteins. These represent all six primate adenovirus subgroups and include all human representatives of subgroups A, C, E, and F, eight from subgroup B, nine from subgroup D, and seven simian adenovirus E1A sequences. We observed that many, but not all, functional domains identified in human adenovirus type 5 E1A are recognizably present in the other E1A proteins. Importantly, we identified highly conserved sequences without known activities or binding partners, suggesting that previously unrecognized determinants of E1A function remain to be uncovered. Overall, our analysis forms a solid foundation for future study of the activities and features of the E1A proteins of different serotypes and identifies new avenues for investigating E1A function.

Reduction of adenovirus E1A mRNA by RNAi results in enhanced recombinant protein expression in transiently transfected HEK293 cells

Human embryonic kidney 293 (HEK293) cells, a widely used host for large-scale transient expression of recombinant proteins, are transformed with the adenovirus E1A and E1B genes. Because the E1A proteins function as transcriptional activators or repressors, they may have a positive or negative effect on transient transgene expression in this cell line. Suspension cultures of HEK293 EBNA (HEK293E) cells were co-transfected with a reporter plasmid expressing the GFP gene and a plasmid expressing a short hairpin RNA (shRNA) targeting the E1A mRNAs for degradation by RNA interference (RNAi). The presence of the shRNA in HEK293E cells reduced the steady state level of E1A mRNA up to 75% and increased transient GFP expression from either the elongation factor-1alpha (EF-1alpha) promoter or the human cytomegalovirus (HCMV) immediate early promoter up to twofold. E1A mRNA depletion also resulted in a twofold increase in transient expression of a recombinant IgG in both small- and large-scale suspension cultures when the IgG light and heavy chain genes were controlled by the EF-1alpha promoter. Finally, transient IgG expression was enhanced 2.5-fold when the anti-E1A shRNA was expressed from the same vector as the IgG light chain gene. These results demonstrated that E1A has a negative effect on transient gene expression in HEK293E cells, and they established that RNAi can be used to enhance recombinant protein expression in mammalian cells.

Transcriptional regulation by the retinoblastoma protein

The retinoblastoma protein (RB) plays a key role in the control of cell proliferation and mediates the terminal differentiation of certain cell types. Increasing evidence suggests that RB functions by contacting and modifying the behaviour of transcription factors. RB can form complexes with E2F and MyoD in vivo, and complexes with a number of other transcription factors have also been demonstrated in vitro. The interaction of regulatory transcription factors with RB may be explained by sequence similarity between RB and two general transcription factors: TBP and TFIIB. Here I review the evidence for a role of RB in the regulation of transcription and highlight some of the likely mechanisms of RB function.

The multifunctional role of E1A in the transcriptional regulation of CREB/CBP-dependent target genes

Oncoproteins encoded by the early region 1A (E1A) of adenoviruses (Ads) have been shown to be powerful tools to study gene regulatory mechanisms. As E1A proteins lack a sequence-specific DNA-binding activity, they modulate viral and cellular gene expression by interacting directly with a diverse array of cellular factors, among them sequence-specific transcription factors, proteins of the general transcription machinery, co-activators and chromatin-modifying enzymes. By making use of these factors, E1A affects major cellular events such as cell cycle control, differentiation, apoptosis, and oncogenic transformation. In this review we will focus on the interaction of E1A with cellular components involved in the cAMP/PKA signal transduction pathway and we will discuss the consequences of these interactions in respect to the activation of CREB/CBP-dependent target genes.

TFIIA abrogates the effects of inhibition by HMGB1 but not E1A during the early stages of assembly of the transcriptional preinitiation complex

Successful assembly of the transcriptional preinitiation complex (PIC) is prerequisite to transcriptional initiation. At each stage of PIC assembly, regulation may occur as repressors and activators compete with and influence the incorporation of general transcription factors (GTFs). Both TFIIA and HMGB1 bind individually to the TATA-binding protein (TBP) to increase the rate of binding and to stabilize TBP binding to the TATA element. The competitive binding between these two cofactors for TBP/TATA was examined to show that TFIIA binds preferentially to TBP and inhibits HMGB1 binding. TFIIA can also readily dissociate HMGB1 from the preestablished HMGB1/TBP/TATA complex. This suggests that TFIIA and HMGB1 may bind to the same or overlapping sites on TBP and/or compete for similar DNA sites that are 5' to the TATA element. In addition, EMSA studies show that adenovirus E1A(13S) oncoprotein is unable to disrupt either the preestablished TFIIA/TBP/TATA or TFIIA/TFIIB/TBP/TATA complexes, but does inhibit complex formation when all transcription factors were simultaneously added. The inhibitory effect of E1A(13S) on the assembly of the PIC is overcome when excess TBP is added back in the reaction, while addition of either excess TFIIA or TFIIB were ineffective. This shows that the main target for E1A(13S) is free TBP and emphasizes the primary competition between E1A and the TATA-element for unbound TBP. This may be the principal point, if not the only point, at which E1A can target TBP to exert its inhibitory effect. This work, coupled with previous findings in our laboratory, indicates that TFIIA is much more effective than TFIIB in reversing the inhibitory effect of HMGB1 binding in the early stages of PIC assembly, which is consistent with the in vitro transcription results.

Caspase-mediated cleavage of adenovirus early region 1A proteins

Adenovirus 2 and 12 early region 1A (Ad2 and Ad12 E1A) proteins were cleaved during cisplatin-induced apoptosis of Ad-transformed rat and human cells. Cleavage was inhibited in the presence of caspase inhibitors such as Z-VAD-FMK. In Ad12 transformants both 13S and 12S E1A proteins were cleaved at a similar rate. In Ad2 transformants the E1A 13S component was appreciably less stable than the 12S component. In in vitro studies Ad2 and Ad12 E1A 13S and Ad2 12S proteins were rapidly cleaved by caspase 3 whereas Ad12 12S E1A and Ad12 13S E1A were rapidly degraded by caspase 7. Cleavage sites in Ad12 13S proteins for caspase 3 have been determined. Initial cleavage occurred at D24 and D150; this was followed by cleavage at D204 and D242. Caspase-3-mediated cleavage of Ad12 13S E1A destroyed its ability to bind to CBP and TBP but interaction between C terminal E1A polypeptides and CtBP was observed. During viral infection Ad5 and Ad12 E1A 12S proteins were markedly more stable than 13S proteins but no difference was observed in Ad E1A levels in the absence or presence of the caspase inhibitors Z-VAD-FMK or Z-D(OMe)-E(OMe)-V-D(OMe)-CH(2)F. Limited caspase 3 and 10 activation occurred during infection with the E1B 19K(-) virus Ad2 pm1722 but little or no activation of caspase 3 was observed during wt virus infection. Examination of protein cleavage during viral infection of A549 cells showed proteolysis of lamin B and PARP in response to Ad5 wt and Ad2 pm1722. Protein degradation in response to both viruses was partially inhibited by Z-VAD-FMK. Following infection of human skin fibroblasts lamin B was degraded, although only limited changes in PARP levels were observed. We have concluded that Ad E1A is cleaved by caspases during apoptosis but not during viral infection. However, some of the processes commonly associated with apoptosis occur during viral infection, particularly with E1B 19K(-) mutants, although apoptosis per se is not evident.

Transgenic expression in mouse lung reveals distinct biological roles for the adenovirus type 5 E1A 243- and 289-amino-acid proteins

Little is known about the biological significance of human adenovirus type 5 (Ad5) E1A in vivo. However, Ad5 E1A is well defined in vitro and can be detected frequently in the lungs of patients with pulmonary disease. Transgenic expression of the Ad5 E1A gene targeted to the mouse lung reveals distinct biological effects caused by two Ad5 E1A products. Either of two Ad5 E1A proteins was preferentially expressed in vivo in the transgenic lungs. The preferential expression of the Ad5 E1A 243-amino-acid (aa) protein at a moderate level was associated with cellular hyperplasia, nodular lesions of proliferating lymphocyte-like cells, and a low level of p53-dependent apoptosis in the lungs of transgenic mice. In contrast, the preferential expression of the Ad5 E1A 289-aa protein at a moderate level resulted in a proapoptotic injury and an acute pulmonary proinflammation in the lungs of transgenic mice, mediated by multiple apoptotic pathways, as well as an enhancement of the host immune cell response. Expression of the Ad5 E1A 243-aa protein resulted in proliferation-stimulated p53 upregulation, while expression of the Ad5 E1A 289-aa protein led to DNA damage-induced p53 activation. These data suggest that the Ad5 E1A 243- and 289-aa proteins lead to distinct biological roles in vivo.

The viral transactivator E1A regulates the mouse mammary tumor virus promoter in an isoform- and chromatin-specific manner

Proteins encoded by the adenovirus E1A gene regulate both cellular and viral genes to mediate effects on cell cycle, differentiation, and cell growth control. We have identified the mouse mammary tumor virus (MMTV) promoter as a target of E1A action and investigated the role nucleoprotein structure plays in its response to E1A. Both 12 and 13 S forms target the MMTV promoter when it has a disorganized and accessible chromatin configuration. However, whereas the 13 S form is stimulatory, the 12 S form is repressive. When the MMTV promoter adopts an organized and repressed chromatin structure, it is targeted only by the 13 S form, which stimulates it. Although evidence indicates that E1A interacts with the SWI/SNF remodeling complex, E1A had no effect on chromatin remodeling at the MMTV promoter in organized chromatin. Analysis of E1A mutants showed that stimulation of the MMTV promoter is mediated solely through conserved region 3 and does not require interaction with Rb, p300/CBP-associated factor, or CBP/p300. Imaging analysis showed that E1A colocalizes with MMTV sequences in vivo, suggesting that it functions directly at the promoter. These results indicate that E1A stimulates the MMTV promoter in a fashion independent of chromatin conformation and through a direct mechanism involving interaction with the basal transcription machinery.

The MyoD-inducible p204 protein overcomes the inhibition of myoblast differentiation by Id proteins

The murine p204 protein level is highest in heart and skeletal muscle. During the fusion of cultured myoblasts to myotubes, the p204 level increases due to transcription dependent on the muscle-specific MyoD protein, and p204 is phosphorylated and translocated from the nucleus to the cytoplasm. p204 overexpression accelerates myoblast fusion in differentiation medium and triggers this process even in growth medium. Here we report that p204 is required for the differentiation of C2C12 myoblasts. We propose that it enables the differentiation, at least in part, by overcoming the inhibition of the activities of the MyoD and E47 proteins by the Id proteins: Id1, Id2, and Id3. These are known to inhibit skeletal muscle differentiation by binding and blocking the activity of MyoD, E12/E47, and other myogenic basic helix-loop-helix (bHLH) proteins. Our hypothesis is based on the following findings. (i) A decrease in the p204 level in C2C12 myoblasts by antisense RNA (a) increased the level of the Id2; (b) inhibited the MyoD-, E12/E47-, and other bHLH protein-dependent accumulation of the muscle-specific myosin heavy-chain protein; and (c) inhibited the fusion of myoblasts to myotubes in differentiation medium. (ii) p204 bound to the Id proteins in vitro and in vivo. (iii) In the binding of p204 to Id2, the b segment of p204 and the HLH segment of Id2 were involved. (iv) Addition of p204 overcame the inhibition by the Id proteins of the binding of MyoD and E47 to DNA in vitro. (v) Overexpression of p204 in myoblasts (a) decreased the level of the Id proteins, even in a culture in growth medium, and (b) overcame the inhibition by the Id proteins of MyoD- and E47 dependent transcription and also overcame the inhibition by Id2 of the fusion of myoblasts to myotubes.

Direct interaction with contactin targets voltage-gated sodium channel Na(v)1.9/NaN to the cell membrane

The mechanisms that target various sodium channels within different regions of the neuronal membrane, which they endow with different physiological properties, are not yet understood. To examine this issue we studied the voltage-gated sodium channel Na(v)1.9/NaN, which is preferentially expressed in small sensory neurons of dorsal root ganglia and trigeminal ganglia and the nonmyelinated axons that arise from them. Our results show that the cell adhesion molecule contactin binds directly to Na(v)1.9/NaN and recruits tenascin to the protein complex in vitro. Na(v)1.9/NaN and contactin co-immunoprecipitate from dorsal root ganglia and transfected Chinese hamster ovary cell line, and co-localize in the C-type neuron soma and along nonmyelinated C-fibers and at nerve endings in the skin. Co-transfection of Chinese hamster ovary cells with Na(v)1.9/NaN and contactin enhances the surface expression of the sodium channel over that of Na(v)1.9/NaN alone. Thus contactin binds directly to Na(v)1.9/NaN and participates in the surface localization of this channel along nonmyelinated axons.

Phosphorylation of the human ubiquitin-conjugating enzyme, CDC34, by casein kinase 2

The ubiquitin-conjugating enzyme, CDC34, has been implicated in the ubiquitination of a number of vertebrate substrates, including p27(Kip1), IkappaBalpha, Wee1, and MyoD. We show that mammalian CDC34 is a phosphoprotein that is phosphorylated in proliferating cells. By yeast two-hybrid screening, we identified the regulatory (beta) subunit of human casein kinase 2 (CK2) as a CDC34-interacting protein and show that human CDC34 interacts in vivo with CK2beta in transfected cells. CDC34 is specifically phosphorylated in vitro by recombinant CK2 and HeLa nuclear extract at five sites within the carboxyl-terminal 36 amino acids of CDC34. Importantly, this phosphorylation is inhibited by heparin, a substrate-specific inhibitor of CK2. We have also identified a kinase activity associated with CDC34 in proliferating cells, and we show that this kinase is sensitive to heparin and can utilize GTP, strongly suggesting it is CK2. Phosphorylation of CDC34 by the associated kinase maps predominantly to residues 203 and 222. Mutation of CDC34 at CK2-targeted residues, Ser-203, Ser-222, Ser-231, Thr-233, and Ser-236, abolishes the phosphorylation of CDC34 observed in vivo and markedly shifts nuclearly localized CDC34 to the cytoplasm. These results suggest a potential role for CK2-mediated phosphorylation in the regulation of CDC34 cell localization and function.

Fibroblast Growth Factor Homologous Factor 1B Binds to the C Terminus of the Tetrodotoxin-resistant Sodium Channel rNav1.9a (NaN)

In this study we demonstrate a direct interaction between a cytosolic fibroblast growth factor family member and a sodium channel. A yeast two-hybrid screen for proteins that associate with the cytoplasmic domains of the tetrodotoxin-resistant sodium channel rNa(v)1.9a (NaN) led to the identification of fibroblast growth factor homologous factor 1B (FHF1B), a member of the fibroblast growth factor family, as an interacting partner of rNa(v)1.9a. FHF1B selectively interacts with the C-terminal region but not the other four intracellular segments of rNa(v)1.9a. FHF1B binds directly to the C-terminal polypeptide of rNa(v)1.9a both in vitro and in mammalian cell lines. The N-terminal 5-77 amino acid residues of FHF1B are essential for binding to rNa(v)1.9a. FHF1B did not interact with C termini of two other sodium channels hNa(v)1.7a (hNaNE) and rNa(v)1.8a (SNS), which share 50% similarity to the C-terminal polypeptide of rNa(v)1.9a. FHF1B is the first growth factor found to bind specifically to a sodium channel. Although the functional significance of this interaction is not clear, FHF1B may affect the rNa(v)1.9a channel directly or by recruiting other proteins to the channel complex. Alternatively, it is possible that rNa(v)1.9a may help deliver this factor to the cell membrane, where it exerts its function.

Adenovirus E1A down-regulates the EGF receptor via repression of its promoter

The epidermal growth factor receptor (EGF-R), after activation by its ligands, stimulates a cascade of intracellular events leading to cellular proliferation. Its expression is increased in various forms of cancer as a consequence of altered regulation. Our objective was to study potential negative regulators of EGF-R expression; we investigated the effect of adenovirus E1A proteins. E1A proteins can exert both positive and negative effects on cell growth, depending on the cell type and cellular context, and have anti-tumorigenic features on human cancer cells. We show that human cell lines stably transformed with the adenovirus E1 region show significantly reduced expression of EGF-R protein and mRNA compared to their control, non-E1A-expressing counterparts. Furthermore, the promoter activity of EGF-R can be specifically repressed by E1A in transient co-transfection analysis in multiple cell types. Transfections with deleted promoter fragments and constructs containing short fragments of the putative E1A-responsive region fused to a heterologous promoter indicate that E1A-responsive elements are contained in a promoter region (from -150 to -76). Analysis of E1A mutants showed that both E1A gene products, 12S and 13S, repress EGF-R promoter activity and that full repression requires the presence of an intact CR1 domain.

Identification of a cellular repressor of transcription of the adenoviral late IVa(2) gene that is unaltered in activity in infected cells

The gene encoding the adenovirus type 2 IVa(2) protein, a sequence-specific activator of transcription from the viral major late promoter, is itself transcribed only during the late phase of infection. We previously identified a cellular protein (IVa(2)-RF) that binds specifically to an intragenic sequence of the IVa(2) transcription unit. We now report that precise substitutions within the IVa(2)-RF-binding site that decreased binding affinity increased the efficiency of IVa(2) transcription in in vitro reactions containing IVa(2)-RF. Consistent with the conclusion that this cellular protein represses IVa(2) transcription, mutations that led to more efficient transcription in the presence of IVa(2)-RF were without effect in reactions lacking this cellular protein. No change in the concentration or activity of IVa(2)-RF could be detected in adenovirus-infected cells during the period in which the IVa(2) gene is transcribed. We therefore propose that restriction of IVa(2) transcription to the late phase is the result of titration of this cellular repressor as the number of copies of the IVa(2) promoter increases upon replication of the viral genome.

Structural determinants in adenovirus 12 E1A involved in the interaction with C-terminal binding protein 1

The interaction between the C-terminal binding protein 1 (CtBP-1) and purified Ad12 E1A protein has been examined through the use of a combination of biophysical techniques. A fragment equivalent to the 77 C-terminal amino acids of Ad12 E1A (Ad12 77-a.a. E1A) was generated by limited proteolysis of Ad12 266-a.a. E1A at Phe(187) and/or Tyr(189) using chymotrypsin. The impact of deletion of the 189 N-terminal amino acids from E1A on the equilibrium dissociation constant K(d) for binding to CtBP was assessed using ELISA in vitro binding assays and intrinsic fluorescence spectroscopy. Values of K(d) of 4.0 and 38 nM were determined for full-length and truncated forms of E1A, respectively. Circular dichroism spectroscopic studies revealed that the conformation adopted by these polypeptides is dependent on the surrounding environment, which is predominately randomly folded when free in solution, but adopting a more ordered alpha-helical secondary structure in the presence of trifluoroethanol. Using nuclear magnetic resonance (NMR) spectroscopy to examine the interaction between Ad E1A and CtBP it was observed that the chemical shift positions of individual backbone amide nitrogen atoms were well resolved in (15)N-(1)H-HSQC NMR spectra performed on samples of isotopically (15)N-labeled Ad12 77-a.a. E1A. In the presence of CtBP, signals of backbone amide nitrogen atoms displayed increased linewidth consistent with an increase in molecular mass upon binding CtBP. In addition, some signals that have been attributed to Val(254/256) and Leu(259), and reside within the binding site for CtBP on E1A, are shifted in the (15)N- and/or (1)H-dimensions, defining specific contacts between E1A and CtBP. These data suggest that structural determinants in the C-terminal PXDLS binding motif in the rest of exon 2 and in exon 1 all contribute to optimizing the conformation of the binding site on Ad12 E1A for CtBP. However, no interaction was observed between CtBP and truncated Ad12 E1A, which no longer contained the C-terminal binding motif.

Influence of HMG-1 and adenovirus oncoprotein E1A on early stages of transcriptional preinitiation complex assembly

The TATA-binding protein (TBP) in the TFIID complex binds specifically to the TATA-box to initiate the stepwise assembly of the preinitiation complex (PIC) for RNA polymerase II transcription. Transcriptional activators and repressors compete with general transcription factors at each step to influence the course of the assembly. To investigate this process, the TBP.TATA complex was titrated with HMG-1 and the interaction monitored by electrophoretic mobility shift assays. The titration produced a ternary HMG-1.TBP. TATA complex, which exhibits increased mobility relative to the TBP. TATA complex. The addition of increasing levels of TFIIB to this complex results in the formation of the TFIIB.TBP.TATA complex. However, in the reverse titration, with very high mole ratios of HMG-1 present, TFIIB is not dissociated off and a complex is formed that contains all factors. The simultaneous addition of E1A to a mixture of TBP and TATA; or HMG-1, TBP, and TATA; or TFIIB, TBP, and TATA inhibits complex formation. On the other hand, E1A added to the pre-established complexes shows a significantly reduced capability to disrupt the complex. In add-back experiments with all complexes, increased levels of TBP re-established the complexes, indicating that the primary target for E1A in all complexes is TBP.

Zinc finger and carboxyl regions of adenovirus E1A 13S CR3 are important for transactivation of the cytomegalovirus major immediate early promoter by adenovirus

Reactivation of latent cytomegalovirus (CMV) is an important cause of disease in susceptible patients. We previously demonstrated that an adenovirus early gene product can transactivate the CMV major immediate early (IE) promoter in inflammatory cells. This effect was due to the conserved region 3 (CR3) of the adenovirus E1A 13S gene product. There are two domains in the CR3 region, a zinc finger (aa 147-177) and a carboxyl (aa 180-188) domain. Both are crucial for transactivation of downstream promoter elements of adenovirus in E1A 13S. We sought to determine if either or both of these specific domains is also necessary for transactivation of the CMV IE promoter by the adenovirus E1A 13S gene product. We cotransfected T-lymphocyte Jurkat cells and monocyte/macrophage-like THP-1 cells with plasmids expressing wild-type (WT) or CR3 mutant E1A 13S and a CMV IE chloramphenicol acetyltransferase (CAT) reporter construct. With extracts of cells coinfected with E1A WT set to 100%, mutation in the zinc finger domain, the carboxyl domain, or both domains decreased CMV IE CAT activity by >/= 96%. In contrast, a mutation in the region between the zinc finger and carboxyl domains reduced CMV IE CAT activity by only 24 to 26%. Mixing studies in Jurkat cells confirmed the importance of these domains. We also evaluated the active site of the CMV IE promoter involved in transactivation in THP-1 cells using CMV IE promoter deletions and single promoter element constructs. These studies showed that progressive deletion of the 19-bp CMV IE repeats containing cyclic AMP response element binding protein/activating transcription factor (CREB/ATF) sites resulted in progressive loss of activity. The importance of this element was confirmed using single promoter elements containing CMV IE 16-, 18-, 19-, and 21-bp repeats. Finally, using a 19-bp single promoter element construct and the CR3 mutants we demonstrated that mutations in the zinc finger (C171S) carboxyl region (S185N) or both regions (C171S/ S185N) resulted in significant (83, 94, and 85%) loss of activity. We conclude that the zinc finger and carboxyl domains of the CR3 region of E1A 13S are necessary for transactivation of the CMV promoter and that this occurs mainly through activation of the 19-bp CREB/ATF site of the promoter.

The E4-6/7 protein functionally compensates for the loss of E1A expression in adenovirus infection

The E1A gene products are required and sufficient for activation of adenovirus gene expression in cultured cells. The E4-6/7 gene product induces the binding of the cellular transcription factor E2F to the viral E2a promoter region. The induction of E2F binding to the E2a promoter in vitro is directly correlated with transcriptional activation of the E2a promoter in vivo. The E2 region encodes the viral replication proteins, yet adenoviruses lacking E4-6/7 function demonstrate no defective phenotype in infected cells. Here we show that the E4-6/7 protein can functionally compensate for E1A expression in virus infection. In the absence of the E1A gene products, expression of the E4-6/7 protein is sufficient to displace retinoblastoma protein family members from E2Fs, activate expression of early region 2 via induction of E2F DNA binding to the E2a promoter region, and significantly enhance replication of an E1A-defective adenovirus. These results have implications in the regulation of viral gene expression and for the development of recombinant adenovirus vectors.

The retinoblastoma-interacting zinc-finger protein RIZ is a downstream effector of estrogen action

Co-immunoprecipitation experiments in cell extract from cultured cells or target tissues indicated that estrogen receptor was complexed with the retinoblastoma binding protein RIZ in a ligand-dependent manner. Mapping of interaction sites indicated that in both proteins the same regions and motifs responsible for the interaction of transcriptional co-activator and nuclear receptors were involved. In cultured cells, estradiol induced a redistribution of RIZ protein within the nucleus and in the cytoplasm. A similar effect was produced in vivo, in prepuberal rat endometrium, by administration of a physiological dose of estradiol. Therefore, RIZ protein could be a specific effector of estrogen action downstream of the hormone-receptor interaction, presumably involved in proliferation control.

The retinoblastoma-interacting zinc-finger protein RIZ is a downstream effector of estrogen action

Co-immunoprecipitation experiments in cell extract from cultured cells or target tissues indicated that estrogen receptor was complexed with the retinoblastoma binding protein RIZ in a ligand-dependent manner. Mapping of interaction sites indicated that in both proteins the same regions and motifs responsible for the interaction of transcriptional co-activator and nuclear receptors were involved. In cultured cells, estradiol induced a redistribution of RIZ protein within the nucleus and in the cytoplasm. A similar effect was produced in vivo, in prepuberal rat endometrium, by administration of a physiological dose of estradiol. Therefore, RIZ protein could be a specific effector of estrogen action downstream of the hormone-receptor interaction, presumably involved in proliferation control.

The activation domain of herpesvirus saimiri R protein interacts with the TATA-binding protein

The herpesvirus saimiri open reading frame (ORF) 50 produces two transcripts. The first is spliced, contains a single intron, and is detected at early times during the productive cycle, whereas the second is expressed later and is produced from a promoter within the second exon. Analysis of their gene products has shown that they function as sequence specific transactivators. In this report, we demonstrate that the carboxy terminus of ORF 50b contains an activation domain which is essential for transactivation. This domain contains positionally conserved hydrophobic residues found in a number of activation domains, including the herpes simplex virus VP16 and the Epstein-Barr virus R proteins. Mutational analysis of this domain demonstrates that these conserved hydrophobic residues are essential for ORF 50 transactivation capability. Furthermore, this domain is required for the interaction between the ORF 50 proteins and the basal transcription factor TATA-binding protein.

Identification of a DNA binding protein cooperating with estrogen receptor as RIZ (retinoblastoma interacting zinc finger protein)

Double-stranded DNA fragments were selected from a random pool by repeated cycles of estrogen receptor-specific immunoprecipitation in the presence of a nuclear extract and PCR amplification (cyclic amplification and selection of target, CAST, for multiple elements). Fragments were cloned and sequence analysis indicated the 5-nucleotide word TTGGC was the most recurrent sequence unrelated to the known estrogen responsive element. Screening a HeLa cell expression library with a probe designed with multiple repeats of this sequence resulted in the identification of a 1700-aa protein showing a complete homology with the product of the human retinoblastoma-interacting zinc-finger gene RIZ. In transfection experiments, RIZ protein was able to bestow estrogen inducibility to a promoter containing an incomplete estrogen responsive element and a TTGGC motif. RIZ protein present in MCF-7 cell nuclear extract retarded the TTGGC-containing probe in an EMSA. Estrogen receptor was co-immunoprecipitated from MCF-7 cell extract by antibodies to RIZ protein and vice versa, thus indicating an existing interaction between these two proteins.

A human TATA binding protein-related protein with altered DNA binding specificity inhibits transcription from multiple promoters and activators

The TATA binding protein (TBP) plays a central role in eukaryotic and archael transcription initiation. We describe the isolation of a novel 23-kDa human protein that displays 41% identity to TBP and is expressed in most human tissue. Recombinant TBP-related protein (TRP) displayed barely detectable binding to consensus TATA box sequences but bound with slightly higher affinities to nonconsensus TATA sequences. TRP did not substitute for TBP in transcription reactions in vitro. However, addition of TRP potently inhibited basal and activated transcription from multiple promoters in vitro and in vivo. General transcription factors TFIIA and TFIIB bound glutathione S-transferase-TRP in solution but failed to stimulate TRP binding to DNA. Preincubation of TRP with TFIIA inhibited TBP-TFIIA-DNA complex formation and addition of TFIIA overcame TRP-mediated transcription repression. TRP transcriptional repression activity was specifically reduced by mutations in TRP that disrupt the TFIIA binding surface but not by mutations that disrupt the TFIIB or DNA binding surface of TRP. These results suggest that TFIIA is a primary target of TRP transcription inhibition and that TRP may modulate transcription by a novel mechanism involving the partial mimicry of TBP functions.

The adenovirus E1A protein is a potent coactivator for thyroid hormone receptors

The thyroid hormone receptors interact with several different cofactors when activating transciption. In this study, we show that the adenovirus E1A oncoprotein functions as a strong coactivator for the thyroid hormone receptor (TR), and that TR and E1A synergistically activate transcription via direct (DR4) or palindromic (IRO) hormone-responsive sites. Cotransfection experiments using different isoforms of the chicken TR and E1A show synergistic, ligand-enhanced transactivation. This transactivation is accomplished through a direct, ligand-independent interaction between TR and E1A. The interaction domains in TR are localized to the DNA-binding domain and to the carboxy-terminal part of the ligand-binding domain. In E1A, the regions of interactions are localized to the conserved regions 1 and 3. Both of these domains in E1A are required for a 40-fold enhancement of TR-mediated activation in transfection experiments. Taken together, we show that E1A strongly enhances transcriptional activation, which suggests that it serves as a bridging factor between the receptor and other components of the transcription machinery.

Cell cycle- and Vpr-mediated regulation of human immunodeficiency virus type 1 expression in primary and transformed T-cell lines

Viral protein R (Vpr) of human immunodeficiency virus type 1 (HIV-1) transiently arrests cells in the G2 phase of the cell cycle and is a weak transcriptional transactivator. We found that Vpr increased HIV-1 long terminal repeat (LTR) activity in all cells examined but, when expressed at high levels, decreased HIV-1 LTR expression due to cytotoxic effects. Moreover, Vpr-mediated enhancement of HIV-1 LTR-driven transcription was observed in cycling primary human CD4(+) T cells but not in terminally differentiated, noncycling primary human macrophages. In single-round infection experiments using primary human CD4(+) T cells, proviral clones expressing either wild-type Vpr or Vpr mutants that retained the ability to cause a G2 arrest replicated to higher levels than proviruses lacking Vpr or expressing mutants of Vpr that did not cause an arrest. In support of the hypothesis that enhancement of HIV-1 LTR transcription by Vpr is an indirect effect of the ability of Vpr to delay cells in G2, counterflow centrifugal elutriation of cells into different phases of the cell cycle demonstrated that HIV-1 LTR expression was highest in G2. Finally, the ability of Vpr to upregulate viral transcription was dependent on a minimal promoter containing a functional TATA box and an enhancer.

Human cytomegalovirus and human herpesvirus 6 genes that transform and transactivate

This review is an update on the transforming genes of human cytomegalovirus (HCMV) and human herpesvirus 6 (HHV-6). Both viruses have been implicated in the etiology of several human cancers. In particular, HCMV has been associated with cervical carcinoma and adenocarcinomas of the prostate and colon. In vitro transformation studies have established three HCMV morphologic transforming regions (mtr), i.e., mtrI, mtrII, and mtrIII. Of these, only mtrII (UL111A) is retained and expressed in both transformed and tumor-derived cells. The transforming and tumorigenic activities of the mtrII oncogene were localized to an open reading frame (ORF) encoding a 79-amino-acid (aa) protein. Furthermore, mtrII protein bound to the tumor suppressor protein p53 and inhibited its ability to transactivate a p53-responsive promoter. In additional studies, the HCMV immediate-early protein IE86 (IE2; UL122) was found to interact with cell cycle-regulatory proteins such as p53 and Rb. However, IE86 exhibited transforming activity in vitro only in cooperation with adenovirus E1A. HHV-6 is a T-cell-tropic virus associated with AIDS-related and other lymphoid malignancies. In vitro studies identified three transforming fragments, i.e., SalI-L, ZVB70, and ZVH14. Of these, only SalI-L (DR7) was retained in transformed and tumor-derived cells. The transforming and tumorigenic activities of SalI-L have been localized to a 357-aa ORF-1 protein. The ORF-1 protein was expressed in transformed cells and, like HCMV mtrII, bound to p53 and inhibited its ability to transactivate a p53-responsive promoter. HHV-6 has also been proposed to be a cofactor in AIDS because both HHV-6 and human immunodeficiency virus type 1 (HIV-1) have been demonstrated to coinfect human CD4(+) T cells, causing accelerated cytopathic effects. Interestingly, like the transforming proteins of DNA tumor viruses such as simian virus 40 and adenovirus, ORF-1 was also a transactivator and specifically up-regulated the HIV-1 long terminal repeat when cotransfected into CD4(+) T cells. Finally, based on the interactions of HCMV and HHV-6 transforming proteins with tumor suppressor proteins, a scheme is proposed for their role in oncogenesis.

Specificity of cyclin E-Cdk2, TFIIB, and E1A interactions with a common domain of the p300 coactivator

The p300 and CREB binding protein (CBP) transcriptional coactivators interact with a variety of transcription factors and regulate their activity. Among the interactions that have been described, the COOH-terminal region of p300 binds to cyclin E-cyclin-dependent kinase 2 (cyclin E-Cdk2) and TFIIB, as well as to the E1A gene products of adenovirus. Inhibition of Cdk activity by Cdk inhibitors, such as p21 or p27, potentiates NF-kappaB activity and provides a mechanism to coordinate cell cycle progression with the transcription of genes expressed during growth arrest. In this report, we analyze the specific domains of p300 required for the binding of p300 to cyclin E-Cdk2, TFIIB, and E1A and the ability of these proteins to interact with p300, alone or in combination. 12S E1A, an inhibitor of p300-dependent transcription, reduces the binding of TFIIB, but not that of cyclin E-Cdk2, to p300. In contrast, 13S E1A, a pleiotropic transcriptional activator, does not inhibit TFIIB binding to p300, although it enhances the interaction of cyclin E-Cdk2 with p300. Modification of cyclin E-Cdk2 is most likely required for association with p300 since the interaction is observed only with cyclin E-Cdk2 purified from mammalian cells. Domain swap studies show that the cyclin homology domain of TFIIB is involved in interactions with p300, although the homologous region from cyclin E does not mediate this interaction. These findings suggest that p300 or CBP function is regulated by interactions of various proteins with a common coactivator domain.

Vitamin D represses retinoic acid-dependent transactivation of the retinoic acid receptor-beta2 promoter: the AF-2 domain of the vitamin D receptor is required for transrepression

Retinoic acid (RA)-dependent activation of the RA receptor beta2 (RARbeta2) gene in embryonal carcinoma cells is mediated by binding of retinoid receptor heterodimers (RAR/RXR) to a RA response element (RARE) located closely to the TATA box. We have analyzed the effect of vitamin D on the response of the RARbeta2 promoter to RA in pituitary GH4C1 cells that coexpress receptors for retinoids and vitamin D. Incubation with vitamin D markedly reduced the response to RA caused by transcriptional interference of the vitamin D receptor (VDR) on the RARE. This DNA element binds VDR/RXR heterodimers with high affinity, and these inactive heterodimers can displace active RAR/RXR from the RARE. Overexpression of RXR in GH4C1 cells, as well as incubation with BMS649 (a RXR-specific ligand), increased the inhibitory effect of vitamin D, suggesting that the VDR/RXR heterodimer is the repressive species and that titration of RXR is not responsible for this inhibition. Although DNA binding could be required for full potency of the inhibitory activity of VDR, it is not absolutely required because a truncated receptor (VDR delta1-111), lacking the DNA binding domain, also displays repressor activity. Furthermore, the ability to mediate transrepression by vitamin D was strongly decreased when a mutant VDR in which the last 12 C-terminal aminoacids have been deleted (VDR deltaAF-2) was used. Because this region contains the domain responsible for ligand-dependent recruitment of coactivators, titration of common coactivators for VDR and RAR could be involved in the inhibitory effect of vitamin D. In agreement with this hypothesis, overexpression of E1A, which can act as a RARbeta2 promoter-specific coactivator, significantly reversed repression by vitamin D.

Two domains within the adenovirus type 12 E1A unique spacer have disparate effects on the interaction of E1A with P105-Rb and the transformation of primary mouse cells

Transformation of primary rodent cells by functions of the adenovirus type 12 (Ad12) early region 1 (E1) is reduced severalfold compared with transformation by E1 of Ad2. We analyzed whether the unique spacer region of Ad12 E1A that borders the conserved region (CR) 2 and represents an oncogenic determinant of Ad12 E1A is involved in this impaired transformation property, putatively by modulating transformation-relevant biological E1A functions. We show that a mutant (E1ASpm1) that lacks 12 amino-terminal residues of the spacer binds p105-Rb and p130 as Ad12 E1A wild type (E1Awt), whereas a second spacer mutant (E1ASpm2) that lacks an adjacent stretch of six alanines exhibits highly reduced binding to p105-Rb. The binding of this mutant to the p130 pocket protein is, however, little impaired. E1ASpm1 diminishes the formation of the p105-Rb-E2F complex more efficiently than E1Awt or, least efficient, E1ASpm2. These properties of the spacer mutants to target and to disintegrate the p105-Rb-E2F complex correspond with their ability to transform primary mouse cells in combination with E1B: E1ASpm1 (plus Ad12 E1B)-transfected cells could be easily established as cell lines, comparable to Ad12 E1Awt- or Ad2 E1Awt-transfected cells. In contrast, cells transfected with E1ASpm2 or Ad12 E1AdelCR2 (lacking the entire CR2) died within 6-10 weeks after replating, although foci were formed in all cases. Of note, the E1ASpm1-transformed cells grow as fast as the Ad2 E1Awt-transformed cells, with a doubling rate of 15 h, whereas the doubling of the Ad12 E1Awt-transformed cells takes approximately 120 h. Moreover, in the established cell lines, the affinity of E1ASpm1 to p105-Rb was higher than with that of E1Awt. Our data suggest the presence of a transformation-suppressing domain within the carboxyl-terminal 12 residues of the Ad12 E1A-unique spacer, whereas the hydrophobic stretch of six alanines in the spacer is required for stable transformation.

Structure and function of the human transcription elongation factor DSIF

5,6-Dichloro-1-beta-D-ribofuranosylbenzimidazole (DRB) is a classic inhibitor of transcription elongation by RNA polymerase II (pol II). We have previously identified and purified a novel transcription elongation factor, termed DSIF (for DRB sensitivity-inducing factor), that makes transcription sensitive to DRB. DSIF is composed of 160- and 14-kDa subunits, which are homologs of the Saccharomyces cerevisiae transcription factors Spt5 and Spt4. DSIF may either repress or stimulate transcription in vitro, depending on conditions, but its physiological function remains elusive. Here we characterize the structure and function of DSIF p160. p160 is shown to be a ubiquitous nuclear protein that forms a stable complex with p14 and interacts directly with the pol II largest subunit. Mutation analysis of p160 is used to identify structural features essential for its in vitro activity and to map the domains required for its interaction with p14 and pol II. Finally, a p160 mutant that represses DSIF activity in a dominant-negative manner is identified and used to demonstrate that DSIF represses transcription from various promoters in vivo.

The structure of the site on adenovirus early region 1A responsible for binding to TATA-binding protein determined by NMR spectroscopy

Previous detailed mutational analysis has shown that the binding site on adenovirus (Ad) early region 1A (E1A) for TATA-binding protein (TBP) is located toward the N terminus of conserved region 3 (CR3). Here we demonstrate that synthetic peptides of between 15 and 22 amino acids, identical to amino acid sequences of CR3 present in the larger Ad5 E1A (13 S product) and in both the Ad12 E1A (13 and 12 S products) proteins that lie N-terminal to the zinc finger motif, can disrupt binding of E1A to TBP. These findings suggest that the peptides are biologically active in terms of interacting with TBP and must therefore comprise some, if not all, of the TBP binding site on E1A. The interaction between Ad12 E1A and TBP was confirmed by direct co-precipitation experiments. In 1H NMR studies of CR3 peptides, regular patterns of NOEs were observed from which their conformational preferences in aqueous solution were determined. Both Ad5 and Ad12 peptides were shown to contain regions of helical backbone structure in 50% trifluoroethanol. In each case, the type and intensities of NOE cross-peaks observed correlated best to alpha-helical turns. These helices are more extensive in larger peptides and extend from Glu141 to Val147 and from Arg144 to Pro152 in the full-length Ad5 and Ad12 13S E1A proteins, respectively. The structure of a 19-residue Ad5 CR3 peptide carrying the V147L mutation in the full-length protein that abolishes TBP binding was examined. No significant differences between the substituted and wild type peptides were observed, suggesting that this substitution in the intact protein may cause disruption of global rather than local structures.