Promoter targeting by adenovirus E1a through interaction with different cellular DNA-binding domains

Casein kinase II interacts with the bZIP domains of several transcription factors

Casein kinase II (CKII) is thought to regulate a broad range of transcription factors, but its mode of action is not well characterized. We previously showed that CKII is co-purified with the ATF family of transcription factors using DNA-affinity latex beads. Here we report a functional and physical interaction between CKII and transcription factors. We demonstrate that CKII binds through its catalytic alpha and alpha' subunits to the basic leucine zipper (bZIP) DNA-binding domains of many transcription factors, including ATF1. Kinetic analysis using a surface plasmon resonance sensor suggests that CKII loosely associates with ATF1 in vivo . Deletion of the bZIP domain of ATF1 markedly reduces its phosphorylation by CKII, suggesting that the bZIP recruits CKII to the vicinity of the target site. ATF1-CKII complex is also formed on DNA. Using CKIIalpha fusedto a heterologous DNA-binding domain, we also demonstrate that CKII, when bound to DNA, efficiently phosphorylates its substrate, which is bound to the same DNA molecule. Taken together, CKII may regulate transcription (and possibly other events) by phosphorylating proteins on DNA.

Structural determinants present in the C-terminal binding protein binding site of adenovirus early region 1A proteins

The C-terminal binding protein (CtBP) has previously been shown to bind to a highly conserved six-amino acid motif very close to the C terminus of adenovirus early region 1A (Ad E1A) proteins. We have developed an enzyme-linked immunosorbent assay that has facilitated the screening of synthetic peptides identical or similar to the binding site on Ad E1A for their ability to bind CtBP and thus inhibit its interaction with Ad12 E1A. It has been shown that amino acids both C-terminal and N-terminal to the original proposed binding site contribute to the interaction of peptides with CtBP. Single amino acid substitutions across the binding site appreciably alter the Kd of the peptide for CtBP, indicative of a marked reduction in the affinity of the peptide for CtBP. The solution structures of synthetic peptides equivalent to the C termini of both Ad5 and Ad12 E1A and two substituted forms of these have been determined by proton NMR spectroscopy. Both the Ad12 and Ad5 peptides dissolved in trifluoroethanol/water mixtures were found to adopt regular secondary structural conformations seen as a series of beta-turns. An Ad12 peptide bearing a substitution that resulted in only very weak binding to CtBP (Ad12 L258G) was found to be random coil in solution. However, a second mutant (Ad12 V256K), which bound to CtBP rather more strongly (although not as well as the wild type), adopted a conformation similar to that of the wild type. We conclude that secondary structure (beta-turns) and an appropriate series of amino acid side chains are necessary for recognition by CtBP.

Mouse adenovirus type 1 early region 1A is dispensable for growth in cultured fibroblasts

Mouse adenovirus type 1 (MAV-1) mutants with deletions of conserved regions of early region 1A (E1A) or with point mutations that eliminate translation of E1A were used to determine the role of E1A in MAV-1 replication. MAV-1 E1A mutants expressing no E1A protein grew to titers comparable to wild-type MAV-1 titers on mouse fibroblasts (3T6 fibroblasts and fibroblasts derived from Rb+/+, Rb+/-, and Rb-/- transgenic embryos). To test the hypothesis that E1A could induce a quiescent cell to reenter the cell cycle, fibroblasts were serum starved to stop DNA replication and cellular replication and then infected with the E1A mutant and wild-type viruses. All grew to equivalent titers. Steady-state levels of MAV-1 early mRNAs (E1A, E1B, E2, E3, and E4) from 3T6 cells infected with wild-type or E1A mutant virus were examined by Northern analysis. Steady-state levels of mRNAs from the mutant-infected cells were comparable to or greater than the levels found in wild-type virus infections for most of the early regions and for two late genes. The E2 mRNA levels were slightly reduced in all mutant infections relative to wild-type infections. E1A mRNA was not detected from infections with the MAV-1 E1A null mutant, pmE109, or from infections with similar MAV-1 E1A null mutants, pmE112 and pmE113. The implications for the lack of a requirement of E1A in cell culture are discussed.

Hepatitis C virus core protein interacts with heterogeneous nuclear ribonucleoprotein K

Hepatitis C virus (HCV) core protein, a component of viral nucleocapsid, has been shown to modulate cellular and viral promoter activities. To identify potential cellular targets for HCV core protein, a human liver cDNA library was screened for core-interacting proteins using the yeast two-hybrid system. Among the proteins identified was heterogeneous nuclear ribonucleoprotein K (hnRNP K), which has been demonstrated to be a transcriptional regulator. The interaction of HCV core protein with hnRNP K was confirmed by glutathione S-transferase fusion protein binding assay, protein-protein blotting assay, and coimmunoprecipitation in vitro and in vivo. Additionally, these two proteins were shown to be partially colocalized in the nucleus. The hnRNP K-binding site in HCV core protein was mapped to the region from amino acid residues 25-91, a hydrophilic area near the N terminus. The HCV core protein-binding domain was located within amino acid residues 250 to 392, which contain the three proline-rich domains, of hnRNP K. Furthermore, HCV core protein relieved the suppression effect of hnRNP K on the activity of the human thymidine kinase gene promoter. The specific binding of HCV core protein to hnRNP K suggests that multiple functions of hnRNP K may be disrupted by the core protein during HCV infection and thus explains, in part, the pathogenesis of HCV.

Coactivation by OCA-B: definition of critical regions and synergism with general cofactors

Molecular dissection of the B-cell-specific transcription coactivator OCA-B has revealed distinct regions important, respectively, for recruitment to immunoglobulin promoters through interaction with octamer-bound Oct-1 and for subsequent coactivator function. Further analysis of general coactivator requirements showed that selective removal of PC4 from the essential USA fraction severely impairs Oct-1 and OCA-B function in a cell-free system reconstituted with partially purified factors. Full activity can be restored by the combined action of recombinant PC4 and the PC4-depleted USA fraction, thus suggesting a joint requirement for PC4 and another, USA-derived component(s) for optimal function of Oct-1/OCA-B in the reconstituted system. Indeed, USA-derived PC2 was found to act synergistically with PC4 in reproducing the function of intact USA in the assay system. Consistent with the requirement for PC4 in the reconstituted system, OCA-B was found to interact directly with PC4. Surprisingly, however, removal of PC4 from the unfractionated nuclear extract has no detrimental effect on OCA-B/Oct-1-dependent transcription. These results lead to a general model for the synergistic function of activation domains in Oct-1 and OCA-B (mediated by the combined action of the multiple USA components) and, further, suggest a functional redundancy in general coactivators.

AR1 is an integral part of the adenovirus type 2 E1A-CR3 transactivation domain

We have previously shown that the nonconserved carboxy-terminal exon of the adenovirus type 2 E1A-289R protein contains two interchangeable sequence elements, auxiliary region (AR) 1 and AR2, that are required for efficient CR3-mediated transcriptional activation of the viral E4 promoter (M. Bondesson, C. Svensson, S. Linder, and G. Akusjärvi, EMBO J. 11:3347-3354, 1992). Here we show that CR3-mediated transactivation of all adenovirus early promoters and the HSP70 promoter requires the AR1 element. We further show that AR2 can substitute for AR1 only when artificially juxtaposed to CR3. AR1 consists of six tandem glutamic acid-proline (EP) repeats and is positioned immediately downstream of CR3. Genetic dissection of AR1 showed that the number of EP repeats in AR1 is critical for CR3 function. Thus, reducing or increasing the number of EP repeats reduces the CR3 transactivation capacity. Furthermore, the introduction of amino acid substitutions into AR1 suggested that the net negative charge in AR1 is of critical importance for its function as an enhancer of CR3-mediated transcriptional activation. Using an in vitro binding approach, we showed that the AR1 element is not part of the CR3 promoter localization signal mediating contact with the Sp1, ATF-2, or c-Jun upstream-binding transcription factors. Previous studies have suggested that the 49-amino-acid sequence constituting CR3 represents the minimal domain required for E1A-induced activation of viral early promoters. Since AR1 was required for efficient CR3-mediated transcriptional activation of all tested promoters, we suggest that the carboxy-terminal boundary for the CR3 transactivation domain should be extended to include the AR1 element.

Alterations in the GAL4 DNA-binding domain can affect transcriptional activation independent of DNA binding

The GAL4 protein belongs to a large class of fungal transcriptional activator proteins encoding within their DNA-binding domains (DBD) six cysteines that coordinate two atoms of zinc (the Zn2Cys6 domain). In an effort to characterize the interactions between the Zn2Cys6 class transcriptional activator proteins and their DNA-binding sites, we have replaced in the full-length GAL4 protein small regions of the Zn2Cys6 domain with the analogous regions of another Zn2Cys6 protein called PPR1 an activator of pyrimidine biosynthetic genes. Alterations between the first and third cysteines abolished binding to GAL4 (upstream activation sequence of GAL (UASG)) or PPR1 (upstream acitvation sequence of UAS) DNA-binding sites and severely reduced transcriptional activation in yeast. In contrast, alterations between the third and fourth cysteines had only minor effects on binding to UASG but led to substantial decreases in activation in both yeast and a mammalian cell line. In the crystal structure of the GAL4 DBD-UASG complex (Marmorstein, R., Carey, M., Ptashne, M., and Harrison, S. C. (1992) Nature 356, 408-414), this region is facing away from the DNA, making it likely that there exists within the GAL4 DBD an accessible domain important in activation.

UTF1, a novel transcriptional coactivator expressed in pluripotent embryonic stem cells and extra-embryonic cells

We have obtained a novel transcriptional cofactor, termed undifferentiated embryonic cell transcription factor 1 (UTF1), from F9 embryonic carcinoma (EC) cells. This protein is expressed in EC and embryonic stem cells, as well as in germ line tissues, but could not be detected in any of the other adult mouse tissues tested. Furthermore, when EC cells are induced to differentiate, UTF1 expression is rapidly extinguished. In normal mouse embryos, UTF1 mRNA is present in the inner cell mass, the primitive ectoderm and the extra-embryonic tissues. During the primitive streak stage, the induction of mesodermal cells is accompanied by the down-regulation of UTF1 in the primitive ectoderm. However, its expression is maintained for up to 13.5 days post-coitum in the extra-embryonic tissue. Functionally, UTF1 boosts the level of transcription of the adenovirus E2A promoter. However, unlike the pluripotent cell-specific E1A-like activity, which requires the E2F sites of the E2A promoter for increased transcriptional activation, UTF1-mediated activation is dependent on the upstream ATF site of this promoter. This result indicates that UTF1 is not a major component of the E1A-like activity present in pluripotent embryonic cells. Further analyses revealed that UTF1 interacts not only with the activation domain of ATF-2, but also with the TFIID complex in vivo. Thus, UTF1 displays many of the hallmark characteristics expected for a tissue-specific transcriptional coactivator that works in early embryogenesis.

E1A represses apolipoprotein AI enhancer activity in liver cells through a pRb- and CBP-independent pathway

The apolipoprotein AI (apoAI) promoter/enhancer contains multiple cis -acting elements on which a variety of hepatocyte-enriched and ubiquitous transcription factors function synergistically to regulate liver-specific transcription. Adenovirus E1A proteins repress tissue-specific gene expression and disrupt the differentiated state in a variety of cell types. In this study expression of E1A 12Sor 13S in hepatoblastoma HepG2 cells repressed apoAI enhancer activity 8-fold. Deletion mapping analysis showed that inhibition by E1A was mediated by the apoAI promoter site B. E1A selectively inhibited the ability of HNF3beta and HNF3alpha to transactivate reporter genes controlled by the apoAI site B and the HNF3 binding site from the transthyretin promoter. The E1A-mediated repression of HNF3 activity was not reversed by overexpression of HNF3beta nor did E1A alter nuclear HNF3beta protein levels or inhibit HNF3 binding to DNA in mobility shift assays. Overexpression of two cofactors known to interact with E1A, pRb and CBP failed to overcome inhibition of HNF3 activity. Similarly, mutations in E1A that disrupt its interaction with pRb or CBP did not compromise its ability to repress HNF3beta transcriptional activity. These data suggest that E1A inhibits HNF3 activity by inactivating a limiting cofactor(s) distinct from pRb or CBP.

Association of activating transcription factor 2 (ATF2) with the ubiquitin-conjugating enzyme hUBC9. Implication of the ubiquitin/proteasome pathway in regulation of ATF2 in T cells

Activating transcription factor 2 (ATF2) is regulated by phosphorylation via the Jun N-terminal kinase, and its binding activity is markedly induced at late stages of T and B lymphocyte activation (Feuerstein, N., Firestein, R., Aiyer, N., Xiao, H., Murasko, D., and Cristofalo, V. (1996) J. Immunol. 156, 4582-4593). To identify proteins that interact specifically with ATF2 in lymphocytes, the yeast two-hybrid interaction system was employed using ATF2 cDNA as a "bait." In two separate screenings, a clone was identified that revealed a novel sequence with homology to several members of the ubiquitin-conjugating enzyme family. An identical sequence was recently reported as the human homolog of the yeast UBC9, hUBC9. Northern blot analysis revealed a 1.3-kilobase RNA transcript, which showed differential levels of expression in various human tissues and a moderate induction after a 48-h stimulation of peripheral blood T lymphocytes. An antibody that was generated against the bacterially expressed glutathione S-transferase-hUBC9 detected a approximately 19-kDa protein, which localizes predominantly in the nuclei of T cells. Further quantitative assays using the yeast two-hybrid system confirmed a high and specific level of interaction of hUBC9 with ATF2 and lack of interaction with lamin or control vectors. Two other cyclic AMP-responsive element-binding transcription factors, CREB and ATF1, also showed significant levels of interaction with hUBC9. However, this interaction was severalfold lower as compared with ATF2. Far Western blot analysis confirmed the specific binding of ATF2 and hUBC9 also in vitro. Evidence is presented that indicates a physiological significance for the interaction of hUBC9 with ATF2. (a) We show that ATF2 is ubiquitinated in vivo and in vitro, and (b) ATF2 ubiquitination in vitro is facilitated by addition of purified hUBC9. (c) ATF2 is shown to undergo a proteolytic process, which is rapidly regulated upon T cell activation concomitant with induction of ATF2 phosphorylation. (d) A proteasome inhibitor delays the down-regulation of ATF2 phophorylation after T cell activation. Taken collectively, these results implicate a role for hUBC9 and the ubiquitin/proteasome pathway in regulation of ATF2 in T cells.

Identification of glucocorticoid- and adenovirus E1A-regulated genes in lung epithelial cells by differential display

Adenovirus infection has been implicated in the pathogenesis of lung inflammatory diseases for which glucocorticoids provide effective antiinflammatory treatment. In this study, the differential display assay was used to identify messenger RNAs (mRNAs) differentially expressed in dexamethasone (1 microM for 24 h)-treated A549 lung epithelial cells compared to A549 cells transfected with the adenoviral E1A gene. Thirty-seven complimentary DNAs (cDNAs) (15 glucocorticoid-regulated, 22 adenovirus E1A-regulated) were isolated. DNA sequence analysis showed that 35 of these were unique, 2 were identical with each other, and 3 were common to the glucocorticoid- and E1A-regulated groups. Genes identified included those involved in transcription/translation, cytoskeletal/contractile element genes, metabolic enzyme genes, and genes associated with cell regulation/signal transduction. After further analysis of the isolated clones by Northern blotting, ribonuclease protection, and semiquantitative RT-PCR (reverse transcriptase-polymerase chain reaction), 10 of the 14 glucocorticoid-regulated and one of the three common to both the adenovirus E1A- and glucocorticoid-regulated cDNAs were confirmed for this control of their expression. We conclude that the strategy of identifying cDNAs regulated by both adenovirus E1A and glucocorticoids provides a promising approach for identifying genes that may be important in the pathogenesis of lung inflammation and therefore targets for glucocorticoid treatment.

Viral transactivating proteins

Many viruses utilize the cellular transcription apparatus to express their genomes, and they encode transcriptional regulatory proteins that modulate the process. Here we review the current understanding of three viral regulatory proteins. The adenovirus E1A protein acts within the nucleus to regulate transcription through its ability to bind to other proteins. The herpes simplex type 1 virus VP16 protein acts within the nucleus to control transcription by binding to DNA in conjunction with cellular proteins. The human T-cell leukemia virus Tax protein influences transcription through interactions with cellular proteins in the nucleus as well as the cytoplasm.

p300 and ATF-2 are components of the DRF complex, which regulates retinoic acid- and E1A-mediated transcription of the c-jun gene in F9 cells

Transcriptional activation of the c-jun gene is a critical event in the differentiation of F9 cells. In our previous studies we characterized an element [differentiation response element (DRE)] in the c-jun promoter that is both necessary and sufficient to confer the capacity for differentiation-dependent up-regulation. This element binds the differentiation regulatory factor (DRF) complex, of which one component is the adenovirus E1A-associated protein p300. We have now identified activation transcription factor-2 (ATF-2) as a DNA-binding subunit of the DRF complex. p300 and ATF-2 interact with each other in vivo and in vitro. The bromodomain and the C/H2 domain of p300 mediate the binding to ATF-2, which in turn requires a proline-rich region between amino acids 112 and 350 for its interaction with p300. The phosphorylation of the serine residue at position 121 of ATF-2 appears to be induced by protein kinase C alpha (PKC alpha) after treatment of cells with retinoic acid (RA) or induction with E1A. In cotransfection assays, wild-type ATF-2 enhanced the transcription of an E2/tk-luciferase construct, in conjunction with p300-E2. However, a mutant form of ATF-2 with a mutation at position 121 (pCMVATF-2(Ser121-Ala)) did not. These results suggest that ATF-2 and p300 cooperate in the control of transcription by forming a protein complex that is responsive to differentiation-inducing signals, such as RA or E1A, and moreover, that the phosphorylation of ATF-2 by PKC alpha is probably a signaling event in the pathway that leads to the transactivation of the c-jun gene in F9 cells.

The human papillomavirus E7 oncoprotein functionally interacts with the S4 subunit of the 26 S proteasome

Human papillomaviruses (HPV) have been etiologically linked to human cervical cancer. More than 90% of cervical cancer tissues express two HPV-encoded oncoproteins E6 and E7. Both E6 and E7 proteins possess transformation activity. and together they cooperate to transform primary human keratinocytes, fibroblasts. and epithelial cells. The transforming activity of E7 is associated with its ability to bind the retinoblastoma tumor suppressor protein (Rb). However, the carboxyl-terminal mutants of E7 are also defective for transformation, suggesting that other cellular targets for E7 might exist. We screened a human placenta cDNA library by yeast two-hybrid assay using HPV 16 E7 as a bait and identified the subunit 4 (S4) ATPase of the 26 S proteasome as a novel E7-binding protein. E7 binds to S4 through the carboxyl-terminal zinc binding motif, and the binding is independent of E7 sequences involved in binding to Rb. The interaction between S4 and E7 can be easily detected by in vitro protein binding assays. Moreover, we found that E7 increases the ATPase activity of S4. A recent study has shown that, in epithelial cells, E7 degrades Rb through the 26 S proteasome pathway. We hypothesize that E7 might target Rb for degradation by 26 S proteasome through its interaction with the subunit 4 of the proteasome.

Interactions between Tat of HIV-2 and transcription factor Sp1

Tat of HIV-2 (Tat-2) requires host cellular factors for optimal function. We show that transactivation by Tat-2 of the HIV promoter requires cis-acting binding sites for Sp1 or Sp1 brought to the promoter via a heterologous system. We demonstrate that an activation domain in Tat-2 consists of one of two potential alpha-helices in the amino-terminal region, the cysteine-rich region, and the core region and that this independent activation domain requires cis-acting Sp1-binding sites for function. Tat-2 interacts with Sp1 in in vitro binding assays, and these interactions require basic residues outside of the Tat-2 activation domain. The regions in Sp1 sufficient for functional synergy with Tat are the Sp1 activation domains, while the DNA-binding region is dispensable. Substitution mutations of a glutamine-rich region in one Sp1 activation domain, which eliminate interactions with a TBP-associated factor, also significantly decrease synergy with Tat. Thus, the functional synergy between Tat-2 and Sp1 localizes to domains in each activator that interact with components of the transcription complex. We suggest that these interactions, rather than direct Tat/Sp1 binding, result in highly processive RNA polymerase II complexes and full-length viral transcripts.

Transforming region of 243R E1A contains two overlapping but distinct transactivation domains

Conserved regions 1 and 2 as well as the amino terminus of E1A are required for the transforming activity of the E1A oncoprotein. We show here that the amino terminus of 243R E1A has transactivation activity when brought to a promoter in yeast. Recruitment to a specific promoter is essential. Mutagenesis studies correlated the transactivation function with the extreme amino terminus and the conserved region 1 of E1A. Cotransfection assays in rodent cells confirmed that two overlapping but distinguishable domains, amino acids 1-65 and 37-80, can transactivate independently when targeted to a promoter. We also observed that when recruited to the proliferating cell nuclear antigen (PCNA) promoter, the amino-terminal region was sufficient to transactivate the PCNA promoter. On the other hand, deletion of the amino terminus of E1A resulted in failure to induce PCNA expression. Fusion of VP16 with the amino-terminal-deleted E1A mutant was able to restore the ability to induce the PCNA promoter. We further show that the amino-terminal region also is required for 243R E1A to repress the transactivation mediated by a universal transactivator DBD.VP16 and DBD.E1A. This repression could be specifically relieved by overexpression of TBP but not TFIIB. In addition, we show that the amino terminus of E1A is involved in in vitro interaction with the TATA binding protein (TBP). Thus the amino-terminal transforming region of E1A may regulate cellular gene expression in species that are distant in evolution via a common mechanism, functionally targeting TBP.

Induction of apoptosis by DPC4, a transcriptional factor regulated by transforming growth factor-beta through stress-activated protein kinase/c-Jun N-terminal kinase (SAPK/JNK) signaling pathway

Many of the actions of serine/threonine kinase receptors for the transforming growth factor-beta (TGFbeta) are mediated by DPC4, a human MAD-related protein identified as a tumor suppressor gene in pancreatic carcinoma. Overexpression of DPC4 is sufficient to induce the activation of gene expression and cell cycle arrest, characteristic of the TGFbeta response. The stress-activated protein kinase/c-Jun N-terminal kinase (SAPK/JNK) is also one of the downstream targets required for TGFbeta-mediated signaling. Here we report that expression of the dominant-interfering mutant of various components of the SAPK/JNK cascade specifically blocked both TGFbeta and DPC4-induced gene expression. These dominant-interfering mutants also inhibited TGFbeta-stimulated DPC4 transcriptional activity. Moreover, we find that overexpression of DPC4 causes transfected cells to undergo the morphological changes typical of apoptosis. These findings define a mechanism whereby TGFbeta signals mediated by DPC4 and SAPK/JNK cascade are integrated in the nucleus to activate gene expression and identify a new cellular function for DPC4.

The transactivation domain of adenovirus E1A interacts with the C terminus of human TAF(II)135

The CR3 activation domain of the human adenovirus E1A protein stimulates transcription by forming protein-protein interactions with DNA sequence-specific binding factors and components of the TFIID complex. Here, we demonstrate that CR3 can complex with the extreme C-terminal 105 amino acids of the human TATA box binding-factor-associated protein, hTAF(II)135. Furthermore, the C-terminal region of hTAF(II)135 can block transcriptional stimulation from an E1A-inducible promoter in vivo. This ability of the C terminus of hTAF(II)135 to bind CR3 and to inhibit E1A-inducible activation is highly specific. These results demonstrate for the first time that a discrete fragment of a mammalian TBP-associated factor which targets a specific activator can impair the stimulation of transcription.

Solution structure of the ATF-2 recognition site and its interaction with the ATF-2 peptide

The effect of leucine zipper proteins binding to the DNA recognition site is controversial. Results from crystallography, gel and solution methods have led to opposite conclusions about the conformation of the DNA in the complex. The role of the DNA binding site in the recognition process and in the gene induction mediated by transcription factors needs to be investigated further. In this article the self-complementary 16 bp oligodeoxynucleotide (CATGTGACGTCACATG)2, which contains the cAMP response element recognised by numerous transcription factors of the leucine zipper family, has been examined free from proteins and in its interaction with the mammalian activating transcription factor 2. The recognition process has been investigated by circular dichroism analysis, which has revealed conformational changes in both DNA and protein upon binding. The solution structure of the 16mer, important in order to define the effects induced by binding of leucine zipper proteins and the intrisic bending properties of DNA, has been determined from NMR data using direct refinement against NOE intensities, analysis of scalar coupling constants and restrained molecular dynamics calculations. Final structures starting from the A and B forms of DNA agreed to a pairwise root mean square deviation (r.m.s.d.) of 1.04 +/- 0.3 A (0.7 +/- 0.2 A to the average) for all atoms. The terminal base pairs were less well determined, and the pairwise deviation of the 12 core bp was 0.83 +/- 0.27 A (0.55 +/- 0.19 A to the average). The final structures are within the B-family with an average helical twist of 36+/-2 degrees. No significant intrinsic DNA bend is shown in the activating transcription factor regulatory site. However, there are substantial deviations from the canonical B-DNA (r.m.s.d. = 3.6 A) in the core of the molecule, associated with relatively large base inclinations.

xnf7 functions in dorsal-ventral patterning of the Xenopus embryo

Xenopus nuclear factor 7 (xnf7) is a maternally expressed nuclear protein that is retained in the cytoplasm from oocyte maturation until the midblastula transition (MBT). Mutations of the xnf7 phosphorylation sites to glutamic acids (dnxnf7) resulted in the retention of the endogenous protein in the cytoplasm past the MBT, indicating that cytoplasmic retention is a phosphorylation dependent process. In addition, dnxnf7 acted as a dominant negative mutant by keeping the endogenous xnf7 protein in the cytoplasm past the MBT. Overexpression of dnxnf7 in future dorsal blastomeres resulted in a ventralized or posteriorized phenotype in which the embryos lacked anterior structures, while overexpression in ventral blastomeres resulted in dorsalized embryos. dnxnf7 also affected the expression of both dorsal and ventral mesodermal markers. These data suggest that xnf7 functions in dorsal/ventral patterning and that the movement of the protein from the cytoplasm to the nucleus at the MBT is critical for the execution of a genetic program conferring a dorsal or ventral identity to the mesoderm.

Characterization of human activating transcription factor 4, a transcriptional activator that interacts with multiple domains of cAMP-responsive element-binding protein (CREB)-binding protein

We demonstrate that human activating transcription factor 4 (hATF4), a member of the activating transcription factor/cAMP-responsive element-binding protein (ATF/CREB) family of transcription factors, is a potent transcriptional activator in both mammalian cells and yeast. The N-terminal 113 amino acids of hATF4 activate transcription efficiently, and unexpectedly, the C-terminal bZip DNA binding domain of hATF4 also activates transcription, albeit weakly. Our results indicate that hATF4 interacts with several general transcription factors: TATA-binding protein, TFIIB, and the RAP30 subunit of TFIIF. In addition, hATF4 interacts with the coactivator CREB-binding protein (CBP) at four regions: 1) the KIX domain, 2) a region that contains the third zinc finger and the E1A-interacting domain, 3) a C-terminal region that contains the p160/SRC-1-interacting domain, and 4) the recently identified histone acetyltransferase domain. Interestingly, both the N-terminal and C-terminal regions of hATF4 interact with the above general transcription factors and CBP, providing a mechanistic explanation for their ability to activate transcription. Consistent with its role as a coactivator, CBP potentiates the ability of hATF4 to activate transcription. The potential significance of the interaction between hATF4 and multiple factors is discussed.

Mutational analysis of transcriptional activation by the bovine papillomavirus type 1 E6

While the bovine papillomavirus type 1 (BPV-1) E6 induces tumorigenic transformation of murine C127 cells, it does not bind or promote the degradation of p53. We recently showed the cellular protein ERC-55/E6BP binds BPV-1 E6 as well as the cancer-related human papillomavirus (HPV) E6 proteins. BPV-1 E6 also binds E6-AP, a ubiquitin ligase necessary for HPV E6-induced p53 degradation. We previously reported that the transforming activity of a set of BPV-1 E6 mutants correlated with their E6BP-binding ability. Another function of BPV-1 E6 is stimulation of transcription when targeted to a promoter, although cellular promoters responsive to BPV-1 E6 have not been identified. To examine whether its transcriptional function is required for oncogenic activity, or is related to its interactions with E6-AP or E6BP, a series of BPV-1 E6 mutants were analyzed as fusions to a sequence-specific DNA binding domain for activity in yeast and in mammalian cells. We show that some transformation defective mutants retained substantial levels of transcriptional activation activity. These mutants also distinguish transcriptional activation from E6-AP and E6BP binding. These results suggest the transcriptional activation function of BPV-1 E6 is not sufficient for cell transformation.

Regulation of inosine-5'-monophosphate dehydrogenase type II gene expression in human T cells. Role for a novel 5' palindromic octamer sequence

Expression of the gene encoding human inosine- 5'-monophosphate dehydrogenase (IMPDH) type II, an enzyme catalyzing the rate-limiting step in the generation of guanine nucleotides, is increased more than 10-fold in activated peripheral blood T lymphocytes and is required for T cell activation. We have examined the 5'-regulatory sequences that are important for the transcriptional regulation of this gene in T cells. DNase I mapping of genomic DNA identified a hypersensitive element near the transcription initiation site. Fine mapping by in vivo footprinting demonstrated five transcription factor binding sites that are occupied in both resting and activated peripheral blood T lymphocytes; these are tandem CRE motifs, a Sp1 site, an overlapping Egr-1/Sp1 site, and a novel palindromic octamer sequence (POS). The tandem CRE and POS sites are of major functional importance as judged by mutational and electrophoretic mobility shift analyses. These data provide evidence that expression of the human IMPDH type II gene is predominantly regulated by the nuclear factors ATF-2 and an as yet unidentified POS-binding protein. Additional major protein-DNA interactions do not occur within the promoter region after T lymphocyte activation, indicating a requirement for additional protein-protein interactions and/or post-translational modifications of pre-bound transcription factors to account for the observed increase in IMPDH type II gene expression.

Binding of SP1 to the immediate-early protein-responsive element of the human cytomegalovirus DNA polymerase promoter

Human cytomegalovirus (HCMV), a member of the herpesvirus family of DNA viruses, encodes two major immediate-early (IE) transcription factors, IE72 and IE86, that are important for regulated expression of the viral genome. The purpose of this study was to identify the host cellular components required for regulation of the HCMV DNA polymerase promoter (UL54) by HCMV IE proteins. Extensive mutagenesis defined a DNA element located between -54 and -43 relative to the transcription start site that was required for both basal transcriptional activity and transactivation by viral IE proteins. A single copy of the UL54 -54/-43 sequence enhanced the responsiveness of a heterologous minimal promoter to HCMV IE proteins. Fractionation of extracts prepared from uninfected cells led to the isolation of two cellular proteins with apparent molecular masses of 95 and 105 kDa that bound specifically to the UL54 -54/-43 element. Biochemical and immunochemical analyses identified this protein as the transcription factor SP1. Although initial inspection of the UL54 -54/-43 sequence did not predict an SP1 binding site, subsequent analyses indicated that it is indeed a nonconsensus GC box. We propose that SP1 is required to direct basal levels of promoter activity and that SP1-regulated transcription complexes allow the entry of HCMV IE proteins into the transcription cycle.

The transcriptional co-activator proteins p300 and CBP stimulate adenovirus E1A conserved region 1 transactivation independent of a direct interaction

p300 and CBP are two related transcriptional co-activator proteins required by many cellular transcription factors for activity. The adenovirus E1A protein binds p300 and CBP through its amino-terminus and conserved region (CR) 1. Fusing CR1 to a heterologous DNA-binding domain creates a potent transcriptional activator, suggesting that CR1 might activate transcription by recruiting p300/CBP to the promoter. We show that both p300 and CBP enhances CR1-dependent transactivation. However, this enhancement occurs independently of a direct interaction with E1A and does not correlate with the CR1 activator function.

Adenovirus E1A inhibits cardiac myocyte-specific gene expression through its amino terminus

Adenovirus E1A oncoproteins inhibit muscle-specific gene expression and myogenic differentiation by suppressing the transcriptional activating functions of basic helix-loop-helix proteins. As one approach to identifying cardiac-specific gene regulatory proteins, we analyzed the functional regions of E1A proteins that are required for muscle gene repression in cardiac cells. Myocyte-specific promoters, including the alpha-actins and alpha-myosin heavy chain, were selectively and potently inhibited (>90%) by E1A, while the ubiquitously expressed beta-actin promoter was only partially ( approximately 30%) repressed; endogenous gene expression was also affected. Distinct E1A protein binding sites mediated repression of muscle-specific and ubiquitous actin promoters. E1A-mediated inhibition of beta-actin required both an intact binding site for the tumor repressor proteins pRb and p107 and a second E1A domain (residues 15-35). In contrast, cardiac-specific promoter repression required the E1A amino-terminal residues 2-36. The proximal skeletal actin promoter (3' to base pair -153) was a target for repression by E1A. Although E1A binding to p300 was not required for inhibition of either promoter, co-expression of p300 partially reversed E1A-mediated transcriptional repression. We conclude that cardiac-specific and general promoter inhibition by E1A occurs by distinct mechanisms and that cardiac-specific gene expression is modulated by cellular factors interacting with the E1A p300/CBP-binding domain.

Rapid induction of the Grp78 gene by cooperative actions of okadaic acid and heat-shock in 9L rat brain tumor cells--involvement of a cAMP responsive element-like promoter sequence and a protein kinase A signaling pathway

We have demonstrated that treatment with 200 nM okadaic acid (OA) for 1 h followed by a 15-min heat shock (HS) at 45 degrees C (termed OA-->HS treatment) leads to a rapid transactivation of grp78, the gene for the 78-kDa glucose-regulated protein, in 9L rat brain tumor cells. The level of Grp78 mRNA rose 15-fold in 60 min after the combined treatment. Nuclear extracts from cells subjected to OA-->HS treatment, compared to those of treatment with OA or HS alone, exhibited an increased binding activity toward an oligonucleotide probe containing the cAMP-responsive element-like (CRE-like, TGACGTGA) regulatory element in electrophoretic mobility shift assays (EMSA). The binding resulted in the formation of two protein-EMSA probe complexes exhibiting different association and dissociation rates in kinetic studies. The protein factors in the upper band (complex I) and lower band (complex II) were identified as the activating transcription factor-2 (ATF-2) and the CRE binding factor 1 (CREB-1), respectively, by antibody interference assays. In addition, the identity of CREB-1 was confirmed by supershift analysis. The binding activity, as well as the transactivation of the grp78 gene, can be abolished by a 1-h treatment with the cAMP-dependent protein kinase (PKA) inhibitor but not with protein kinase C or Ca2+/calmodulin-dependent protein kinase II inhibitors. Accumulation of steady-state level of ATF-2 was observed and was also modulated by treatment with H-89, a PKA inhibitor. From these results, we conclude that the CRE-like element plays an important role in the rapid transactivation of the grp78 gene and that the PKA signaling pathway is involved. In addition, PKA-mediated transcriptional regulation of grp78 in OA-->HS treatment is through regulation of protein phosphorylation as well as de novo synthesis of ATF-2.

Activating transcription factor 2 (ATF2) down-regulates hepatitis B virus X promoter activity by the competition for the activating protein 1 binding site and the formation of the ATF2-Jun heterodimer

The hepatitis B viral X promoter is known to be positively autoregulated by its own HBx protein, which also interacts with many cellular regulatory proteins. We investigated the effect of activating transcription factor 2 (ATF2) on the activity of the X promoter. Cotransfection of the ATF2 expression vector with a X promoter-chloramphenicol acetyltransferase plasmid repressed the X promoter activity in HepG2 cells. HBx activated activating protein 1 (AP-1)-mediated transcription through the hepatitis B virus E element by 35-fold, while its activation activity was inhibited in the presence of ATF2, suggesting that ATF2 inhibited the autoactivation of X promoter by HBx and basal transcription mediated by AP-1. Since the binding sites of AP-1 and ATF2 in the hepatitis B virus E element overlap, the repression of X promoter activity by ATF2 is exerted by the competition for the AP-1 binding site and the formation of the ATF2-Jun heterodimer as in the case of the consensus AP-1 element. However, the small X promoter had a ATF2 binding site and was activated by ATF2. These results suggest that the syntheses of X proteins are differentially regulated by ATF2.

The hepatitis B virus X-associated protein, XAP3, is a protein kinase C-binding protein

The hepatitis B virus X protein induces transcriptional activation of a wide variety of viral and cellular genes. In addition to its ability to interact directly with many nuclear transcription factors, several reports indicate that the X protein stimulates different cytoplasmic kinase signal cascades. Using the yeast two-hybrid screen, we have isolated a clone designated X-associated protein 3 (XAP3) that encodes a human homolog of the rat protein kinase C-binding protein. One of the activation domains of X (amino acids 90-122) is required for binding to XAP3, while the NH2-terminal part of XAP3 is necessary for binding to X. Both X and XAP3 bound specifically to the eta PKC isoenzyme synthesized in rabbit reticulocyte lysates. Overexpression of XAP3 enhanced X transactivation activity. These results support earlier findings that one of the mechanisms of transactivation by X is through involvement with the cellular protein kinase C pathway.

Casein kinase II-mediated phosphorylation of the C terminus of Sp1 decreases its DNA binding activity

We have previously observed that Sp1, a ubiquitous zinc finger transcription factor, is phosphorylated during terminal differentiation in the whole animal, and this results in decreased DNA binding activity (Leggett, R. W., Armstrong, S. A., Barry, D., and Mueller, C. R. (1995) J. Biol. Chem. 270, 25879-25884). In this study, we demonstrate that casein kinase II (CKII) is able to phosphorylate the C terminus of Sp1 and results in a decrease in DNA binding activity. This suggests that CKII may be responsible for the observed regulation of Sp1. Mutation of a consensus CKII site at amino acid 579, within the second zinc finger, eliminates phosphorylation of this site and the CKII-mediated inhibition of Sp1 binding. Phosphopeptide analysis confirms the presence of a CKII site at Thr-579 as well as additional sites within the C terminus. No gross changes in CKII subunit levels were seen during de-differentiation associated with liver regeneration. The serine/threonine phosphatase PP1 was identified as the endogenous liver nuclear protein able to dephosphorylate Sp1 but again no gross changes in activity were observed in the regenerating liver. Okadaic acid treatment of K562 cells increases Sp1 phosphorylation and inhibits its DNA binding activity suggesting that steady state levels of Sp1 phosphorylation are established by a balance between kinase and phosphatase activities.

Phosphorylation within the transactivation domain of adenovirus E1A protein by mitogen-activated protein kinase regulates expression of early region 4

A critical role of the 289-residue (289R) E1A protein of human adenovirus type 5 during productive infection is to transactivate expression of all early viral transcription. Sequences within and proximal to conserved region 3 (CR3) promote expression of these viral genes through interactions with a variety of transcription factors requiring the zinc binding motif in CR3 and in some cases a region at the carboxy-terminal end of CR3, including residues 183 to 188. It is known that 3',5' cyclic AMP (cAMP) reduces the level of phosphorylation of the 289R E1A protein through the activation of protein phosphatase 2A by the E4orf4 protein. This study was designed to identify the E1A phosphorylation sites affected by E4orf4 expression and to determine their importance in regulation of E1A activity. We report here that two previously unidentified sites at Ser-185 and Ser-188 are the targets for decreased phosphorylation in response to cAMP. At least one of these sites, presumably Ser-185, is phosphorylated in vitro by purified mitogen-activated protein kinase (MAPK), and both are hyperphosphorylated in cells which express a constitutively active form of MAPK kinase. Analysis of E1A-mediated transactivation activity indicated that elevated phosphorylation at these sites increased expression of the E4 promoter but not that of E3. We have recently shown that one or more E4 products induce cell death due to p53-independent apoptosis, and thus it seems likely that one role of the E4orf4 protein is to limit production of toxic E4 products by limiting expression of the E4 promoter.

The adenovirus E1A repression domain disrupts the interaction between the TATA binding protein and the TATA box in a manner reversible by TFIIB

The human adenovirus E1A 243 amino acid oncoprotein possesses a transcription repression function that appears to be linked with its ability to induce cell cycle progression and to inhibit cell differentiation. The molecular mechanism of E1A repression has been poorly understood. Recently, we reported that the TATA binding protein (TBP) is a cellular target of E1A repression. Here we demonstrate that the interaction between TBP and the E1A repression domain is direct and specific. The TBP binding domain within E1A 243R maps to E1A N-terminal residues approximately 1 to 35 and is distinct from the TBP binding domain within conserved region 3 unique to the E1A 289R transactivator. An E1A protein fragment consisting of only the E1A N-terminal 80 amino acids (E1A 1-80) and containing the E1A repression function was found to block the interaction between TBP and the TATA box element as shown by gel mobility and DNase protection analysis. Interestingly, a preformed TBP-TATA box promoter complex can be dissociated by E1A 1-80. Further, TFIIB can prevent E1A disruption of TBP-TATA box interaction. TFIIB, like TBP, can overcome E1A repression of transcription in vitro. The ability of the E1A repression domain to block TBP interaction with the TATA box and the ability of TFIIB to reverse E1A disruption of the TBP-TATA box complex implies a mechanism for E1A repression distinct from those of known cellular repressors that target TBP.

Mutant forms of the enhancer-binding protein NtrC can activate transcription from solution

Activators of the sigma54-holoenzyme catalyze the isomerization of closed complexes between this polymerase and a promotor to open complexes in a reaction that depends upon hydrolysis of a nucleoside triphosphate. The activators normally bind to DNA sites with the properties of transcriptional enhancers and contact the polymerase by means of DNA loop formation. Here, we demonstrate that mutant forms of the activator nitrogen regulatory protein C (NtrC) that lack one helix of the helix-turn-helix (HTH) DNA-binding motif or the entire motif retain residual capacity to activate transcription from solution, despite the fact that they are largely unable to dimerize and have greatly decreased ability to hydrolyze ATP. We show that substitution of alanine for three hydrophilic residues in the second helix of the HTH yields a stable, dimeric form of NtrC defective in DNA-binding. Like mutant forms with deletions of one or both helices, the NtrC3ala protein failed to bind DNA in a sensitive affinity co-electrophoresis assay, indicating that its affinity for a strong enhancer was reduced by at least 5000-fold. (The assay detected enhancer-binding by two mutant forms of NtrC with single amino acid substitutions in the HTH and non-specific DNA-binding by the wild-type protein.) The phosphorylated NtrC3ala protein had normal ATPase activity in solution but, unlike the activity of the phosphorylated wild-type protein, which could be stimulated at least tenfold by an oligonucleotide carrying a strong enhancer, the ATPase activity of the phosphorylated NtrC3ala protein was not stimulated. At concentrations of 100 nM or greater, the phosphorylated NtrC3ala protein activated transcription from the major glnA promoter. In agreement with the fact that it did not show detectable DNA-binding in other assays, its ability to activate transcription was no greater on templates carrying the glnA enhancer than on templates lacking an enhancer. The results indicate that both roles of the glnA enhancer, tethering and facilitation of the formation of an active oligomer of NtrC, can be bypassed if the protein is present at high concentrations in solution.

Basal transcription factors TBP and TFIIB and the viral coactivator E1A 13S bind with distinct affinities and kinetics to the transactivation domain of NF-kappaB p65

Transactivation domains (TADs) are able to contact several components of the basal transcription apparatus and co-activator molecules. In order to study these interactions in biophysical detail, binding of the well-characterized TAD from the human transcription factor NF-kappaB p65 (RelA) to the basal transcription factors TBP and TFIIB and the viral co-activator protein E1A 13S was chosen as a model system to investigate the kinetics and affinities of such protein-protein interactions by surface plasmon resonance analysis. The TAD of NF-kappaB p65 showed remarkably different affinities and kinetics in binding to the various proteins. The real-time kinetic measurements revealed an association rate constant (kass) of 2.3 x 10(6)/M/s for the interaction between the p65 TAD and TBP. The association rate constants of the p65 TAD were much weaker for TFIIB (6.8 x 10(4)/M/s) and for the E1A 13S protein (4.9 x 10(4)/M/s). The dissociation rate constants (kdiss) were determined to be 7.9 x 10(-4)/s for TBP, 1.6 x 10(-3)/s for TFIIB and 1.3 x 10(-3)/s for the E1A protein. Accordingly, the calculated dissociation constants (Kd) differed between 3.4 x 10(-10)M for the strongly binding TBP protein and 2.3 x 10(-8)M and 2.6 x 10(-8)M for the weaker binding TFIIB and E1A 13S proteins respectively. Non-linear analysis of the appropriate part of the sensorgrams revealed monophasic association and dissociation kinetics for binding between the p65 TAD and all three interaction partners. The remarkable differences in protein affinities add another aspect to a more detailed understanding of formation of the transcription preinitiation complex. The co-transfection of TBP and E1A 13S stimulated NF-kappaB p65-dependent gene expression, showing the biological significance of these interactions.

Distinct domains of adenovirus E1A interact with specific cellular factors to differentially modulate human immunodeficiency virus transcription

Transcription of human immunodeficiency virus (HIV) type 1 and other viruses is regulated by the transcription factor NF-kappaB, which interacts with the multifunctional cellular protein p300. p300, originally identified by its ability to bind adenovirus early region 1A (E1A), has also been shown to regulate HIV transcription through its interaction with NF-kappaB. The 13S form of E1A activates HIV gene expression, while the 12S form represses its transcription. In this report, we have investigated whether these divergent effects of E1A are dependent upon common or distinct cellular cofactors, including p300, pRb, and the TATA box-binding protein (TBP). Unlike activation in the absence of E1A, cooperative stimulation of HIV gene expression by 13S E1A and RelA was independent of the ability of E1A to bind p300 but was critically dependent on the E1A CR3 region which associates with TBP. In contrast, inhibition of basal HIV gene expression by the 12S form of E1A was dependent on p300 but not pRb or TBP. Interestingly, mutation of the CR2 region of 12S E1A responsible for pRb binding abolished the repression of HIV transcription stimulated by tumor necrosis factor alpha, suggesting that repression of cytokine-activated transcription involves cofactors different from those used in unstimulated cells. Repression and activation of HIV transcription by different forms of E1A are mediated by distinct sets of cellular cofactors. These findings suggest that E1A has evolved to interact by alternative mechanisms with a transcriptional coactivator and its associated cofactors to differentially modulate cellular and viral gene expression.

Extreme N terminus of E1A oncoprotein specifically associates with a new set of cellular proteins

By interacting with key regulatory proteins such as the pRb family, cyclins, cyclin-dependent kinases and p300/CBP of host cells, adenoviral E1A interferes with various cellular processes to provide a suitable environment for the replication of viruses. E1A may promote DNA synthesis and cell cycle progression, immortalize rodent cells in culture and transform cultured cells in cooperation with E1B, Ras, or other oncoproteins. Both extreme N terminus and conserved region 1 of E1A are required for the immortalization and the transformation of rodent cells, transcriptional repression and specific induction of the expression of cellular genes such as the proliferating cell nuclear antigen (PCNA) and heat shock protein 70 (HSP70). Although the molecular mechanisms of these functions of E1A are not fully understood, it is believed that protein-protein interactions may play essential roles. In this communication, we report that a new set of cellular proteins with apparent molecular weight of 200, 90, 45, 30, and 28 specifically associate with the extreme N terminus of E1A. Further analysis demonstrate that these associations do not depend on E1A's association with p300 or pRB. Neither the 30 kDa nor the 28 kDa polypeptide is identical to Cdc2 or Cdk2. The region of E1A required for the protein interaction is also required for the recently identified N-terminal transactivation activity of E1A. Our observations suggest that in addition to p300/CBP, the new set of cellular proteins may be involved in the functional complexity of the N terminus of E1A, thus predicting a p300/CBP independent pathway.

Synthetic inhibitors of regulatory proteins involved in the signaling pathway of the replication of human immunodeficiency virus 1

NF-kappa B, HIV-EP1, Sp1, and E1A are transcriptional proteins involved in the long terminal repeat-directed expression of HIV-1. The inhibitory effect of 18 dimethylaminopyridine-based compounds against these regulatory proteins was studied. Experiments using NF-kappa B-beads showed that histidine-pyridine-histidine compounds and their zinc complexes are inhibitory not only for the NF-kappa B-DNA binding, but also for the binding of NF-kappa B with the inhibitory protein I kappa B. Discriminative inhibition of the DNA binding of two distinct C2H2 type zinc finger proteins HIV-EP1 and Sp1 was also attempted using the synthetic compounds. Whereas some compounds inhibited the DNA binding of both HIV-EP1 and Sp1 at 300 microM, others preferentially and completely inhibited HIV-EP1 without much suppression of Sp1. Mercapto compounds were more potent and uniformly inhibitory against both HIV-EP1 and Sp1 at 30 microM. Disulfide compounds were also remarkably inhibitory against HIV-EP1 and Sp1 also at 30 microM whereas the shorter-chain disulfides 7 and 9 were effective only for HIV-EP1. S-Alkyl derivatives preferentially inhibited HIV-EP1 at 300 microM. The dimethylamino compound was the sole compound inhibitory only against Sp1, being non-inhibitory against HIV-EP1. Relevant combinations of these inhibitors would allow us to inhibit NF-kappa B, HIV-EP1, and Sp1 in any combinations. Inhibition of the TBP binding of C4 type zinc finger protein adenovirus E1A was also examined. It was found that two compounds induced, at 50 mM concentration, effective inhibition of the TBP binding of E1A, demonstrating that it is possible in principle to inhibit the protein-protein interaction of zinc finger proteins.

Susceptibility to natural killer cells and down regulation of MHC class I expression in adenovirus 12 transformed cells are regulated by different E1A domains

All human adenoviruses transform rodent cells in vitro, but only cells transformed by serotypes belonging to subgroups A (Ad12) and B (Ad3) are tumorigenic for immunocompetent animals. In these cells, the expression of MHC-class I antigens is repressed and might allow them to escape from recognition by cytotoxic T lymphocytes (CTL) and to develop in tumor. Furthermore, these cell lines appear resistant to lysis by natural killer (NK) cells. To determine the E1A domain(s) responsible for these properties several cell lines were created by transforming baby rat kidney (BRK) cells with a set of plasmids expressing different Ad2/Ad12 hybrid E1A gene products. The MHC class 1 gene expression was inhibited in cells expressing the Ad12 13S mRNA product and in cells transformed with Ad2/Ad12 hybrid E1A gene product harboring the C-terminal part of the conserved region (CR) 3 of Ad12. Susceptibility of these transformed cell lines to NK cells was determined by cytolytic assays. The results obtained suggest that two Ad12 E1A domains are required to induce resistance of the cell lines to NK cells.

mUBC9, a novel adenovirus E1A-interacting protein that complements a yeast cell cycle defect

Adenovirus E1A encodes two nuclear phosphoproteins that can transform primary rodent fibroblasts in culture. Transformation by E1A is mediated at least in part through binding to several cellular proteins, including the three members of the retinoblastoma family of growth inhibitory proteins. We report here the cloning of a novel murine cDNA whose encoded protein interacts with both adenovirus type 5 and type 12 E1A proteins. The novel E1A-interacting protein shares significant sequence homology with ubiquitin-conjugating enzymes, a family of related proteins that is involved in the proteasome-mediated proteolysis of short-lived proteins. Highest homology was seen with a Saccharomyces cerevisiae protein named UBC9. Importantly, the murine E1A-interacting protein complements a cell cycle defect of a S. cerevisiae mutant which harbors a temperature-sensitive mutation in UBC9. We therefore named this novel E1A-interacting protein mUBC9. We mapped the region of E1A that is required for mUBC9 binding and found that the transformation-relevant conserved region 2 of E1A is required for interaction.

The CCAAT-binding proteins CP1 and NF-I cooperate with ATF-2 in the transcription of the fibronectin gene

We have previously proposed a molecular interaction between the liver factors that bind to the cyclic AMP response element (CRE) and CCAAT sites of the fibronectin (FN) gene based on the following evidence: (i) the close spacing of 20 base pairs between CRE and CCAAT elements is conserved in the FN genes from rats, mice, and humans; (ii) footprinting competitions showed that CRE oligonucleotides are able to detach both liver factors; (iii) CCAAT binding and transcriptional activity of liver extracts are reduced when the distance between the CRE and CCAAT elements is increased; and (iv) CCAAT-binding is stimulated by the addition of a liver extract fraction containing the CRE-binding factor ATF-2. This report provides binding and immunochemical evidence that nuclear factor I (CTF/NF-I) and CP1 (NF-Y or CBF) are the only liver factors that bind to the -150 CCAAT element of the FN gene, forming distinct complexes. We show that these factors bind less efficiently to the CCAAT site of a FN promoter in which the -170 CRE has been disrupted by site-directed mutagenesis and that each element contributes positively to the liver transcriptional activity assessed in vitro with a G-less cassette construct and in vivo by transfection of hepatoma cells with CAT constructs. Furthermore, using a method that combines UV cross-linking and immunoprecipitation, we show that antibodies specific to ATF-2 are able to specifically precipitate protein-protein-DNA complexes containing NF-I and CP1. This simple method preserves weak macromolecular interactions, avoiding the disruptive electrophoresis conditions of gel mobility shifts assays.

Evidence for the Role of Cyclic AMP-Responsive Elements in Human Virus Replication and Disease

We review the involvement of the cyclic AMP responsive DNA element (CRE) and the ATF/CREB (activating transcription factor/CRE binding protein) family of transcription factors in the regulation and pathology of clinically important viruses that infect humans, including the herpesviridae, adenoviridae, parvoviridae, hepadnaviridae, and retroviridae families. CRE sequences found in specific regulatory elements of human viruses are listed, and the functional evidence for CRE activity, in the form of DNA binding assays, mutational studies, transfection and transcriptional activation experiments, or in vitro transcription assays, is summarized. Manipulation of cellular processes is required for virus replication in human cells following infection. A primary target of many viruses is the cellular transcription machinery, and several human viruses contain transcriptional activator and repressor proteins that affect cellular transcription. Through their effect on cellular transcription, viral genes alter the pattern of cellular gene expression, and thereby affect the differentiation state and cell cycle progression of the infected cell. We summarize evidence demonstrating that the CRE and its binding proteins are involved in the activity of the viruses, implicating their function in the pathogenesis of human diseases. The targeting of specific transcription factor pathways as a potential therapeutic approach is discussed. Copyright 1996 S. Karger AG, Basel

The CR1 and CR3 domains of the adenovirus type 5 E1A proteins can independently mediate activation of ATF-2

The adenovirus 12S E1A protein can stimulate the activity of the c-jun promoter through a conserved region 1 (CR1)-dependent mechanism. The effect is mediated by two AP-1/ATF-like elements, jun1 and jun2, that preferentially bind c-Jun-ATF-2 heterodimers. In this study, we show that the ATF-2 component of the c-Jun-ATF-2 heterodimer is the primary target for 12S E1A: 12S E1A can enhance the transactivating activity of the N terminus of ATF-2 when fused to a heterologous DNA-binding domain, whereas the transactivating activity of the c-Jun N terminus is not significantly affected. Activation of the ATF-2 N terminus by 12S E1A is dependent on CR1. In the context of the 13S E1A protein, CR1 and CR3 can both contribute to activation of ATF-2, and their relative contributions are dependent on the cell type. In contrast to activation of ATF-2 by stress-inducing agents, CR1-dependent activation of ATF-2 was found not to depend strictly on the presence of threonines 69 and 71 in the N terminus of ATF-2, which are targets for phosphorylation by stress-activated protein kinases (SAPKs). In agreement with this observation, we did not observe phosphorylation of threonines 69 and 71 or constitutively enhanced SAPK activity in E1A- plus E1B-transformed cell lines. These data suggest that CR1-dependent activation of ATF-2 by 12S E1A does not require phosphorylation of threonines 69 and 71 by SAPK.

Homeodomain interaction with the beta subunit of the general transcription factor TFIIE

Homeodomain-containing proteins play a crucial role as transcriptional regulators in the process of cell differentiation. The homeodomain performs a dual function in this regard, acting as a DNA binding domain and participating in protein-protein interactions that enhance DNA binding specificity or regulatory activity. Here we describe a homeodomain class-specific interaction with the beta subunit of the general transcription factor TFIIE. We show that the Antennapedia and Abdominal-B homeodomains bind to TFIIEbeta, but the even-skipped homeodomain does not. Using a two-hybrid assay performed in cultured cells, we demonstrate that the homeodomain-TFIIEbeta interaction occurs in vivo. The Abdominal-B homeodomain is shown to activate transcription in vitro, and this activation can be blocked with anti-TFIIEbeta antibody without affecting basal transcription levels. Together with published data demonstrating an interaction between proteins containing even-skipped class homeodomains and the TATA-binding protein (Um, M., Li, C., and Manley, J. L. (1995) Mol. Cell. Biol. 15, 5007-5016; Zhang, H., Catron, K. M., and Abate-Shen, C. (1996) Proc. Natl. Acad. Sci. U. S. A. 93, 1764-1769), these results suggest various homeodomain containing proteins interact with different general transcription factors, an observation that may have important implications for transcriptional regulation.

A hydrophobic region within the adenovirus E1B 19 kDa protein is necessary for the transient inhibition of NF-kappaB activated by different stimuli

The early transcribed adenovirus proteins E1A and E1B display a variety of functions in the transformation of primary rodent cells and the regulation of apoptosis and transcription. We have recently shown recently that the E1B 19 kDa protein from Adenovirus 5 (Ad 5) can functionally antagonize the stimulatory effect of E1A 13S on the human transcription factor NF-kappaB. Here we show that expression of E1B 19 kDa negatively interfered with the activation of NF-kappaB by different stimuli, such as the E1A 13S protein, and treatment with phorbol ester and tumor necrosis factor alpha. This suggests that E1B 19 kDa acts on a common upstream signaling event. Band shift experiments showed that expression of E1B 19 kDa impaired the generation of the nuclear, DNA-binding form of NF-kappaB. Domain mapping experiments employing various E1B 19 kDa mutants revealed the necessity of a hydrophobic Bcl-2 homology region between amino acids 90 and 96 for NF-kappaB inhibition. Co-transfection experiments showed that the inhibitory effect of E1B 19 kDa on E1A 13S-activated NF-kappaB transcription was gradually lost in the course of time. Thus the continuous stimulatory action of E1A 13S can finally override the antagonistic effects of E1B 19 kDa on NF-kappaB activity. In contrast to E1B 19 kDa, expression of the E1B 55 kDa protein did not result in a de novo activation of NF-kappaB, but co-stimulated the transcriptional potential of activated NF-kappaB.

Mammalian Sug1 and c-Fos in the nuclear 26S proteasome

In a search for regulatory proteins that interact with the leucine zipper motif of c-Fos in the yeast two-hybrid screen, we have identified a protein (FZA-B) that has extensive sequence similarity to SUG1 of Saccharomyces cerevisiae. Here we show that FZA-B can functionally substitute for SUG1 in yeast and that FZA-B interacts with Fos proteins in vitro through their leucine zippers. In rat liver and in HeLa cells, FZA-B is present in the 26S proteasome complex, as is c-Fos. Immobilized antibody raised against an FZA-B-specific peptide depleted peptidase activity, proteasomal proteins, FZA-B, and c-Fos from a 26S proteasome preparation. FZA-B is found predominantly in the nuclear fraction of COS cells expressing an FZA-B transgene and in the nuclear 26S proteasome of HeLa cells. We conclude that FZA-B is the mammalian homolog of SUG1 (mSug1) and that it is present in the nuclear 26S proteasome of cells. Our results suggest that mSug1 may be involved in the degradation of c-Fos and other transcription factors.

Adenovirus E1A downregulates cJun- and JunB-mediated transcription by targeting their coactivator p300

Transcription factors and cofactors play critical roles in cell growth and differentiation. Alterations of their activities either through genetic mutations or by viral oncoproteins often result in aberrant cell growth and tumorigenesis. The transcriptional cofactor p300 has recently been shown to be complexed with transcription factors YY1 and CREB. Adenovirus E1A oncoproteins target these transcription complexes via physical interactions with p300, resulting in alterations of transcription mediated by these transcription factors. Here we show that p300 is also critical for repression by E1A of the activities of cJun and JunB, two members of the AP-1 transcriptional complexes. This repressive effect of E1A is dependent on the p300-binding domain of E1A and can be relieved by overexpression of p300. These results suggest that p300 serves as a mediator protein for downregulation of AP-1 activity by E1A. This hypothesis was further supported by the following observations: (i) in the absence of E1A, overexpression of p300 stimulated transcription both through an AP-1 site present in the collagenase promoter and through Jun proteins in GAL4 fusion protein-based assays; and (ii) overexpression of a mutant p300 lacking the E1A-interacting domain reduced the responsiveness of Jun-dependent transcription to E1A repression. As predicted from the functional results, p300 physically interacted with the Jun proteins. These findings thus established that p300 is a cofactor for cJun and JunB. We propose that p300 is a common mediator protein through which E1A gains control over multiple transcriptional regulatory pathways in the host cells.

Importance of the Ser-132 phosphorylation site in cell transformation and apoptosis induced by the adenovirus type 5 E1A protein

The 289-residue (289R) and 243R early region 1A (E1A) proteins of human adenovirus type 5 induce cell transformation in cooperation with either E1B or activated ras. Here we report that Ser-132 in both E1A products is a site of phosphorylation in vivo and is the only site phosphorylated in vitro by purified casein kinase II. Ser-132 is located in conserved region 2 near the primary binding site for the pRB tumor suppressor and, in 289R, just upstream of the conserved region 3 transactivation domain involved in regulation of early viral gene expression. Mutants containing alanine or glycine in place of Ser-132 interacted with pRB-related proteins at somewhat reduced efficiency; however, all Ser-132 mutants transformed primary rat cells in cooperation with E1B as well as or better than the wild type when both major E1A proteins were expressed. Such was not the case with mutants expressing only 289R. In cooperation with E1B, the Asp-132 and Gly-132 mutants yielded reduced numbers of smaller transformed foci. With activated ras, all Ser-132 mutants were significantly defective for transformation and the rare foci produced were small and contained extensive areas populated by low densities of flat cells. In the absence of E1B, all Ser-132 mutants induced p53-independent cell death more readily than virus expressing wild-type 289R. These results suggested that phosphorylation at Ser-132 may enhance the binding of pRB and related proteins and also reduce the toxicity of E1A 289R, thus increasing transforming activity.

The dual effect of adenovirus type 5 E1A 13S protein on NF-kappaB activation is antagonized by E1B 19K

The genomes of human adenoviruses encode several regulatory proteins, including the two differentially spliced gene products E1A and E1B. Here, we show that the 13S but not the 12S splice variant of E1A of adenovirus type 5 can activate the human transcription factor NF-kappaB in a bimodal fashion. One mode is the activation of NF-kappaB containing the p65 subunit from the cytoplasmic NF-kappaB-IkappaB complex. This activation required reactive oxygen intermediates and the phosphorylation of IkappaBalpha at serines 32 and 36, followed by IkappaBalpha degradation and the nuclear uptake of NF-kappaB. In addition, 13S E1A stimulated the transcriptional activity of the C-terminal 80 amino acids of p65 at a core promoter with either a TATA box or an initiator (INR) element. The C-terminal 80 amino acids of p65 were found to associate with E1A in vitro. The activation of NF-kappaB-dependent reporter gene transcription by E1A was potently suppressed upon coexpression of the E1B 19-kDa protein (19K). E1B 19K prevented both the activation of NF-kappaB and the E1A-mediated transcriptional enhancement of p65. These inhibitory effects were not found for the 55-kDa splice variant of the E1B protein. We suggest that the inductive effect of E1A 13S on the host factor NF-kappaB, whose activation is important for the transcription of various adenovirus genes, must be counteracted by the suppressive effect of E1B 19K so that the adenovirus-infected cell can escape the immune-stimulatory and apoptotic effects of NF-kappaB.