Sequences in E1A proteins of human adenovirus 5 required for cell transformation, repression of a transcriptional enhancer, and induction of proliferating cell nuclear antigen

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 CtBP binding domain in the adenovirus E1A protein controls CR1-dependent transactivation

The adenovirus E1A-243R protein has the ability to force a resting cell into uncontrolled proliferation by modulating the activity of key targets in cell cycle control. Most of these regulatory mechanisms are dependent on activities mapping to conserved region 1 (CR1) and the non-conserved N-terminal region of E1A. We have previously shown that CR1 functions as a very patent transactivator when it is tethered to a promoter through a heterologous DNA binding domain. However, artificial DNA binding was not sufficient to convert full-length E1A-243R to a transactivator. Thus, an additional function(s) of the E1A-243R protein modulates the effect of CR1 in transcription regulation. Here we demonstrate that a 44 amino acid region at the extreme C-terminus of ElA inhibited transactivation by a Gal4-CR1 fusion protein. Inhibition correlated with binding of the nuclear 48 kDa C-terminal binding protein (CtBP), which has been implicated in E1A-mediated suppression of the metastazing potential of tumour cells. This might suggest that CtBP binding can regulate E1A-mediated transformation by modulating CR1-dependent control of transcription.

Repression of a matrix metalloprotease gene by E1A correlates with its ability to bind to cell type-specific transcription factor AP-2

Adenovirus E1A 243-amino acid protein can repress a variety of enhancer -linked viral and cellular promoters. This repression is presumed to be mediated by its interaction with and sequestration of p3OO, a transcriptional coactivator. Type IV 72-kDa collagenase is one of the matrix metalloproteases that has been implicated in differentiation, development, angiogenesis, and tumor metastasis. We show here that the cell type-specific transcription factor AP-2 is an important transcription factor for the activation of the type IV 72-kDa collagenase promoter and that adenovirus E1A 243-amino acid protein represses this promoter by targeting AP-2. Glutathione S-transferase-affinity chromatography studies show that the E1A protein interacts with the DNA binding/dimerization region of AP-2 and that the N-terminal amino acids of E1A protein are required for this interaction. Further, E1A deletion mutants which do not bind to p3OO can repress this collagenase promoter as efficiently as the wildtype E1A protein. Because the AP-2 element is present in a variety of viral and cellular enhancers which are repressed by E1A, these studies suggest that E1A protein can repress cellular and viral promoter/enhancers by forming a complex with cellular transcription factors and that this repression mechanism may be independent of its interaction with p3OO.

Adenovirus E1A243 disrupts the ATF/CREB-YY1 complex at the mouse c-fos promoter

The adenovirus E1A243 protein can activate transcription of the mouse c-fos gene in a manner that depends on treatment of cells with inducers or analogs of cyclic AMP (cAMP). Activation requires conserved region 1 and the N-terminal domain of E1A243 and is mediated by a 22-bp E1A response element containing a cAMP response element (CRE) at -67 and a binding site for transcription factor YY1 at -54. In the absence of E1A243, YY1 represses CRE-dependent transcription of c-fos by physically interacting with ATF/CREB proteins bound to the -67 CRE. Here we present evidence that expression of E1A243 leads to relief of YY1-mediated repression by a disruption of the ATF/CREB-YY1 complex. Addition of E1A243 to in vitro binding assays prevented binding of ATF-2 to glutathione S-transferase-YY1. Similarly, expression of E1A243 in HeLa cells prevented the association of a YY1-VP16 fusion protein with endogenous ATF/CREB proteins bound to the -67 CRE of a transfected c-fosCAT reporter plasmid. In each case, the N-terminal domain of E1A243, which mediates a direct interaction with YY1, was responsible for disruption of the ATF/CREB-YY1 complex. On the basis of these and previously published results, we present a model for the synergistic transcriptional activation of the c-fos gene by E1A243 and cAMP.

Transcriptional repression by human adenovirus E1A N terminus/conserved domain 1 polypeptides in vivo and in vitro in the absence of protein synthesis

The human adenovirus E1A 243R protein (243 residues) transcriptionally represses a set of cellular genes that regulate cellular growth and differentiation. We describe two lines of evidence that E1A repression does not require cellular protein synthesis but instead involves direct interaction with a cellular protein(s). First, E1A 243R protein represses an E1A-repressible promoter in the presence of inhibitors of protein synthesis, as shown by cell microinjection-in situ hybridization. Second, E1A 243R protein strongly represses transcription in vitro from promoters of the E1A-repressible genes, human collagenase, and rat insulin type II. Repression in vitro is promoter-specific, and an E1A polypeptide containing only the N-terminal 80 residues is sufficient for strong repression both in vivo and in vitro. By use of a series of E1A 1-80 deletion proteins, the E1A repression function was found to require two E1A sequence elements, one within the nonconserved E1A N terminus, and the second within a portion of conserved region 1 (40-80). These domains have been reported to possess binding sites for several cellular transcription regulators, including p300, Dr1, YY1, and the TBP subunit of TFIID. The in vitro transcription-repression system described here provides a powerful tool for the further analysis of molecular mechanism and the possible role of these cellular factors.

FER-1, an enhancer of the ferritin H gene and a target of E1A-mediated transcriptional repression

Ferritin, the major intracellular iron storage protein of eucaryotic cells, is regulated during inflammation and malignancy. We previously reported that transcription of the H subunit of ferritin (ferritin H) is negatively regulated by the adenovirus E1A oncogene in mouse NIH 3T3 fibroblasts (Y. Tsuji, E. Kwak, T. Saika, S. V. Torti, and F. M. Torti, J. Biol. Chem. 268:7270-7275, 1993). To elucidate the mechanism of transcriptional repression of the ferritin H gene by E1A, a series of deletions in the 5' flanking region of the mouse ferritin H gene were constructed, fused to the chloramphenicol acetyltransferase (CAT) gene, and transiently cotransfected into NIH 3T3 cells with an E1A expression plasmid. The results indicate that the E1A-responsive region is located approximately 4.1 kb 5' to the transcription initiation site of the ferritin H gene. Further analyses revealed that a 37-bp region, termed FER-1, is the target of E1A-mediated repression. This region also serves as an enhancer, augmenting ferritin H transcription independently of position and orientation. FER-1 was dissected into two component elements, i.e., a 22-bp dyad symmetry element and a 7-bp AP1-like sequence. Insertion of these DNA sequences into a ferritin H-CAT chimeric gene lacking an E1A-responsive region indicated that (i) the 22-bp dyad symmetry sequence by itself has no enhancer activity, (ii) the AP1-like sequence has moderate enhancer activity which is repressed by E1A, and (iii) the combination of the dyad symmetry element and the AP1-like sequence is required for maximal enhancer activity and repression by E1A. Gel retardation assays and cotransfection experiments with c-fos and c-jun expression vectors suggested that members of the Fos and Jun families bind to the AP1-like element of FER-1 and contribute to its regulation. In addition, gel retardation assays showed that E1A reduces the ability of nuclear proteins to bind to the AP1-like sequence without affecting the levels of nuclear factors that recognize the 22-bp dyad symmetry element. Taken together, these results demonstrate that FER-1 serves as both an enhancer of ferritin H transcription and a target for E1A-mediated repression.

Transcriptional repression of the c-fos gene by YY1 is mediated by a direct interaction with ATF/CREB

Transcriptional activation of the mouse c-fos gene by the adenovirus 243-amino-acid E1A protein requires a binding site for transcription factor YY1 located at -54 of the c-fos promoter. YY1 normally represses transcription of c-fos, and this repression depends on the presence of a cyclic AMP (cAMP) response element located immediately upstream of the -54 YY1 DNA-binding site. This finding suggested that the mechanism of transcriptional repression by YY1 might involve a direct interaction with members of the ATF/CREB family of transcription factors. In vitro and in vivo binding assays were used to demonstrate that YY1 can interact with ATF/CREB proteins, including CREB, ATF-2, ATFa1, ATFa2, and ATFa3. Structure-function analyses of YY1 and ATFa2 revealed that the C-terminal zinc finger domain of YY1 is necessary and sufficient for binding to ATFa2 and that the basic-leucine zipper region of ATFa2 is necessary and sufficient for binding to YY1. Overexpression of YY1 in HeLa cells resulted in repression of a mutant c-fos chloramphenicol acetyltransferase reporter that lacked binding sites for YY1, suggesting that repression can be triggered through protein-protein interactions with ATF/CREB family members. Consistent with this finding, repression was relieved upon removal of the upstream cAMP response element. These data support a model in which YY1 binds simultaneously to its own DNA-binding site in the c-fos promoter and also to adjacent DNA-bound ATF/CREB proteins in order to effect repression. They further suggest that the ATF/CREB-YY1 complex serves as a target for the adenovirus 243-amino-acid E1A protein.

Suppression of mutations in two Saccharomyces cerevisiae genes by the adenovirus E1A protein

The protein products of the adenoviral E1A gene are implicated in a variety of transcriptional and cell cycle events, involving interactions with several proteins present in human cells, including parts of the transcriptional machinery and negative regulators of cell division such as the Rb gene product and p107. To determine if there are functional homologs of E1A in Saccharomyces cerevisiae, we have developed a genetic screen for mutants that depend on E1A for growth. The screen is based on a colony color sectoring assay which allows the identification of mutants dependent on the maintenance and expression of an E1A-containing plasmid. Using this screen, we have isolated five mutants that depend on expression of the 12S or 13S cDNA of E1A for growth. A plasmid shuffle assay confirms that the plasmid-dependent phenotype is due to the presence of either the 12S or the 13S E1A cDNA and that both forms of E1A rescue growth of all mutants equally well. The five mutants fall into two classes that were named web1 and web2 (for "wants E1A badly"). Plasmid shuffle assays with mutant forms of E1A show that conserved region 1 (CR1) is required for rescue of the growth of the web1 and web2 E1A-dependent yeast mutants, while the N-terminal 22 amino acids are only partially required; conserved region 2 (CR2) and the C terminus are dispensable. The phenotypes of mutants in both the web1 and the web2 groups are due to a single gene defect, and the yeast genes that fully complement the mutant phenotypes of both groups were cloned. The WEB1 gene sequence encodes a 1,273-amino-acid protein that is identical to SEC31, a protein involved in the budding of transport vesicles from the endoplasmic reticulum. The WEB2 gene encodes a 1,522-amino-acid protein with homology to nucleic acid-dependent ATPases. Deletion of either WEB1 or WEB2 is lethal. Expression of E1A is not able to rescue the lethality of either the web1 or the web2 null allele, implying allele-specific mutations that lead to E1A dependence.

Relief of YY1 transcriptional repression by adenovirus E1A is mediated by E1A-associated protein p300

YY1 represses transcription when bound upstream of transcriptional initiation sites. This repression can be relieved by adenovirus E1A. Here, we present genetic evidence that the ability of E1A to relieve YY1 repression was impaired by mutations that affect E1A binding to its associated protein p300. This suggests that E1A may modulate the repressor activity of YY1 by binding to p300, which may be physically complexed with YY1. A YY1/p300 protein complex in vivo was demonstrated by several independent approaches, and the YY1-interacting domain was mapped to the carboxy-terminal region of p300, distinct from the E1A-binding site. Unlike E2F/RB, the YY1/p300 complex is not disrupted by E1A. Functional studies using recombinant p300 demonstrated unequivocally that p300 is capable of mediating E1A-induced transcriptional activation through YY1. Taken together, these results reveal, for the first time, a YY1/p300 complex that is targeted by E1A and demonstrate a function for p300 in mediating interactions between YY1 and E1A. Our data thus identify YY1 as a partner protein for p300 and uncover a molecular mechanism for the relief of YY1-mediated repression by E1A.

Repression of the c-Jun trans-activation function by the adenovirus type 12 E1A 52R protein correlates with the inhibition of phosphorylation of the c-Jun activation domain

The early region 1A 52R polypeptide, a protein expressed exclusively by the in vivo oncogenic adenovirus subtype 12, represses the trans-activating function of the cellular transcription factor complex AP-1 consisting of c-Jun-c-Jun homodimers. In this report we demonstrate that the repression in vivo correlates with a direct physical interaction of the adenovirus protein with c-Jun in vitro. Interestingly, the 52R protein binds to the bZIP domain of c-Jun essential for dimerization and DNA binding but not to the c-Jun activation domain. This interaction does not prevent the promoter binding of c-Jun/AP-1. Moreover, the physical association between c-Jun and the TATA box-binding protein TBP is not disturbed by the 52R polypeptide. In fact, we show evidence that down-regulation of c-Jun activity by the adenoviral protein is due to the inhibition of phosphorylation of the c-Jun trans-activation domain. In vivo phosphorylation of the c-Jun activation domain is necessary for the interaction of c-Jun with specific cofactors such as CBP and therefore a prerequisite for the activation of target genes. Due to these results we propose a model in which the 52R protein represses the trans-activating function of c-Jun by preventing its phosphorylation through a specific kinase necessary for the activation of the cellular transcription factor.

Repression in vitro, by human adenovirus E1A protein domains, of basal or Tat-activated transcription of the human immunodeficiency virus type 1 long terminal repeat

Human adenovirus E1A proteins can repress the expression of several viral and cellular genes. By using a cell-free transcription system, we demonstrated that the gene product of the E1A 12S mRNA, the 243-residue protein E1A243R, inhibits basal transcription from the human immunodeficiency virus type 1 (HIV-1) long terminal repeat (LTR). The HIV-1 transactivator protein Tat greatly stimulates transcription from the viral promoter in vitro. However, E1A243R can repress Tat-activated transcription in vitro. Strong repression of both basal and Tat-activated transcriptions requires only E1A N-terminal amino acid residues 1 to 80. Deletion analysis showed that E1A N-terminal amino acids 4 to 25 are essential for repression, whereas amino acid residues 30 to 49 and 70 to 80 are dispensable. Transcriptional repression by E1A in the cell-free transcription system is promoter specific, since under identical conditions, transcription of the adenovirus major late promoter and the Rous sarcoma virus LTR promoter was unaffected. The repression of transcription by small E1A peptides in vitro provides an assay for investigation of molecular mechanisms governing E1A-mediated repression of both basal and Tat-activated transcriptions of the HIV-1 LTR promoter.

The N-terminal region of the adenovirus type 5 E1A proteins can repress expression of cellular genes via two distinct but overlapping domains

The transforming E1A 12S and E1A 13S proteins of human adenovirus type 5 (Ad5) contain two and three conserved regions, respectively. In the present study, the contribution of sequences in the nonconserved N-terminal region of the E1A proteins to morphological transformation and to down-regulation of a number of mitogen-inducible genes was investigated. As described previously, transformation of NRK cells (an established normal rat kidney cell line) results in denser cell growth and a cuboidal cellular morphology. None of the cells expressing N-terminally mutated E1A proteins, however, show these transformed properties, which suggests an important role for sequences in that domain. The decrease in cyclin D1 levels requires exactly the same sequences. The ability to transform NRK cells and to reduce cyclin D1 levels does not correlate with the presence in the E1A proteins of binding domains for p300, CBP, p107, pRb, cyclin A, or cdk2. In contrast, down-regulation of expression of the JE gene in NRK cells and repression of transcription of the collagenase gene in human HeLa cells does correlate with the presence in the E1A proteins of an intact binding domain for p300 and CBP. The results suggest that the N-terminal domain of the E1A proteins can repress expression of cellular genes by at least two different mechanisms.

Identification of a novel E1A response element in the mouse c-fos promoter

Transcriptional activation of the c-fos gene in mouse S49 cells by the adenovirus 243-amino-acid E1A protein depends on domains of E1A that are also required for transformation and that bind the cellular protein p300. Activation additionally depends on stimulation of endogenous cyclic AMP (cAMP)-dependent protein kinase by analogs or inducers of cAMP. Transient transfection assays were used to analyze the c-fos promoter for sequences that confer responsiveness to E1A. Linker substitution and point mutants revealed that transcriptional activation by E1A depended on a cAMP response element (CRE) located at -67 relative to the start site of transcription and a neighboring binding site for transcription factor YY1 located at -54. A 22-bp sequence containing the -67 CRE and the -54 YY1 site was sufficient to confer responsiveness to a minimal E1B promoter and was termed the c-fos E1A response element (ERE). Function of the c-fos ERE depended on both the CRE and the YY1 site, since mutation of either site resulted in a loss of responsiveness to E1A. These results imply a specific functional interaction between CRE-binding proteins, transcription factor YY1, and E1A in the regulation of the c-fos gene.

Adenovirus E1A proteins interact with the cellular YY1 transcription factor

The adenovirus 12S and 13S E1A proteins have been shown to relieve repression mediated by the cellular transcription factor YY1. The 13S E1A protein not only relieves repression but also activates transcription through YY1 binding sites. In this study, using a variety of in vivo and in vitro assays, we demonstrate that both E1A proteins can bind to YY1, although the 13S E1A protein binds more efficiently than the 12S E1A protein. Two domains on the E1A proteins interact with YY1: an amino-terminal sequence (residues 15 to 35) that is present in both E1A proteins and a domain that includes at least a portion of conserved region 3 (residues 140 to 188) that is present in the 13S but not the 12S E1A protein. Two domains on YY1 interact with E1A proteins: one is contained within residues 54 to 260, and the other is contained within the carboxy-terminal domain of YY1 (residues 332 to 414). Cotransfection of a plasmid expressing carboxy-terminal amino acids 332 to 414 of YY1 fused to the GAL4 DNA-binding domain can inhibit expression from a reporter construct with GAL4 DNA binding sites in its promoter, and inclusion of a third plasmid expressing E1A proteins can relieve the repression. Thus, we find a correlation between the ability of E1A to interact with the carboxy-terminal domain of YY1 and its ability to relieve repression caused by the carboxy-terminal domain of YY1. We propose that E1A proteins normally relieve YY1-mediated transcriptional repression by binding directly to the cellular transcription factor.

Functional importance of complex formation between the retinoblastoma tumor suppressor family and adenovirus E1A proteins as determined by mutational analysis of E1A conserved region 2

Adenovirus early region 1A (E1A) products induce DNA synthesis, transform primary rodent cells, and activate transcription factor E2F through complex formation with an array of cellular proteins via the E1A amino terminus and conserved regions 1 and 2 (CR1 and CR2). Interactions with the retinoblastoma tumor suppressor, pRb, and related proteins p107 and p130 rely somewhat on CR1 but largely on CR2, which contains a core binding sequence Leu-122-X-Cys-X-Glu. We introduced point mutations in CR2 to define such interactions more precisely. In human cells, alteration of any of the conserved residues within the binding core eliminated complex formation with pRb. Conversion of nonconserved Thr-123 to Pro (but not to either Ala or Ser) disrupted binding of pRb, presumably because of conformational changes in the binding core. No single E1A point mutant was completely defective in binding p107, suggesting that molecular interactions between E1A proteins and p107 clearly differ from those with pRb and p130. In general, the patterns of complex formation by E1A mutants in rat, monkey, and human cells were quite similar. All mutants which failed to bind significant amounts of pRb also failed to transform primary rat cells. Several mutants demonstrated selective binding to pRb, p107, and p130, but transforming activity corresponded largely with complex formation with pRb, regardless of the levels of interactions with p107 and p130. Mutants defective for binding of both pRb and p107 failed to induce the activity of transcription factor E2F; however, quite high levels were activated by E1A mutants that interacted with p107 alone. These results suggested that both pRb and p107 are important regulators of E2F activity but that complex formation with and activation of E2F by p107 are insufficient for cell transformation.

An adenovirus E1A transcriptional repressor domain functions as an activator when tethered to a promoter

The adenovirus E1A protein contains three well conserved regions, designated conserved region (CR) 1, 2 and 3, which are important for the multiple activities ascribed to E1A. The CR3 domain constitutes a prototypic transcription activator, consisting of a promoter targeting region and a transactivating region. Here we demonstrate the existence of a second transactivating region located within amino acids 28 to 90 (essentially the CR1 domain) of the E1A protein. A fusion protein, containing the Gal4 DNA binding domain linked to CR1, was as efficient as the classical CR3 transactivator in activating transcription from a reporter plasmid containing Gal4 binding sites. However, competition experiments suggest that Gal/CR1 and Gal/CR3 work through different cellular targets. The E1A-243R protein has previously been extensively characterized as a repressor of transcription. Here we show that a Gal4 fusion protein expressing the CR1 domain is indeed sufficient for repression of SV40 enhancer activity. Collectively, our results suggest that CR1 functions as an activator if tethered to a promoter and as a repressor in the absence of promoter association.

Complementary functions of E1a conserved region 1 cooperate with conserved region 3 to activate adenovirus serotype 5 early promoters

The amino-terminal region of the adenovirus type 5 E1a protein including conserved regions (CRs) 1 and 2 binds the 105-kDa retinoblastoma protein and a second, 300-kDa, cellular protein. We show that mutant viruses with deletions of CR1 which release the binding of either p105 or p300 still activate early promoters and infect cells productively. However, mutations which disrupt binding of both proteins disrupt early promoter activity and block the viral life cycle. Ela CR3, which has an established role in early promoter activation, can act in trans to the amino-terminal functions. This suggests that the amino terminus provides distinct, redundant functions related to p300 and Rb binding that synergize with CR3 to transactivate early genes.

Multiple pathways for activation of E2A expression in human KB cells by the 243R E1A protein of adenovirus type 5

Adenovirus type 5 (Ad5) mutant dl520, which produces only the smaller 243 residue (243R) E1A protein, induced efficient production of the viral E2A 72-kDa DNA binding protein (DBP) in human KB cells, but not in human WI38, 143, or HeLa cells. In transient expression assays, the 243R E1A protein induced transcription from the E2 early promoter in KB but not in HeLa cells; there was no transcription from the E3 promoter in either cell line. In KB cells, truncation of the E2 promoter from -285 to -97 basepairs dramatically reduced transactivation by the 243R E1A product but not by wt E1A, suggesting that the 243R protein acts through factors binding in this region. Multiple deletions in both exon 1 and exon 2 of the 243R E1A protein failed to disrupt its ability to induce DBP expression. The possible redundant pathways for this induction are discussed. The multiplicity of these pathways and the fact that they are all inactivated in the WI38 and 143 lines are surprising.

Modulation of transcriptional activation of the proliferating cell nuclear antigen promoter by the adenovirus E1A 243-residue oncoprotein depends on proximal activators

Previous analyses defined a proliferating cell nuclear antigen (PCNA) E1A-responsive element (PERE) in the PCNA promoter that is essential for transactivation by the 243-residue product of the adenovirus type 2 E1A 12S mRNA (E1A 243R). In this report, we show that the PERE activates a heterologous basal promoter and confers susceptibility to transactivation by E1A 243R, indicating that the PERE is both necessary and sufficient for the response of the PCNA promoter to this oncoprotein. Insertion of linker sequences between the PERE and the site of transcription initiation in the PCNA promoter severely impairs the promoter's response to E1A 243R transactivation. GAL4 sites can replace the function of the PERE in the E1A 243R response of the PCNA basal promoter if transcriptional activators of suitable strength are supplied as GAL4 fusion proteins. Weak transcriptional activators render the PCNA basal promoter subject to transactivation by E1A 243R but do not endow the adenovirus E1B basal promoter with a similar response. Strong transcriptional activators do not support transactivation by E1A 243R, however; instead, E1A reduces the ability of the strong activators to activate both the PCNA and E1B basal promoters. Although other mechanistic differences might determine the response, the data imply a relationship between the activation strength of promoter-proximal effectors and the response of the PCNA basal promoter to E1A 243R. These experiments indicate that the PERE can function autonomously in mediating transactivation by E1A 243R and that the PCNA basal promoter is configured in a manner that permits modulation by E1A 243R of transcriptional activation by promoter-proximal effectors.

Modulation of transcriptional activation of the proliferating cell nuclear antigen promoter by the adenovirus E1A 243-residue oncoprotein depends on proximal activators

Previous analyses defined a proliferating cell nuclear antigen (PCNA) E1A-responsive element (PERE) in the PCNA promoter that is essential for transactivation by the 243-residue product of the adenovirus type 2 E1A 12S mRNA (E1A 243R). In this report, we show that the PERE activates a heterologous basal promoter and confers susceptibility to transactivation by E1A 243R, indicating that the PERE is both necessary and sufficient for the response of the PCNA promoter to this oncoprotein. Insertion of linker sequences between the PERE and the site of transcription initiation in the PCNA promoter severely impairs the promoter's response to E1A 243R transactivation. GAL4 sites can replace the function of the PERE in the E1A 243R response of the PCNA basal promoter if transcriptional activators of suitable strength are supplied as GAL4 fusion proteins. Weak transcriptional activators render the PCNA basal promoter subject to transactivation by E1A 243R but do not endow the adenovirus E1B basal promoter with a similar response. Strong transcriptional activators do not support transactivation by E1A 243R, however; instead, E1A reduces the ability of the strong activators to activate both the PCNA and E1B basal promoters. Although other mechanistic differences might determine the response, the data imply a relationship between the activation strength of promoter-proximal effectors and the response of the PCNA basal promoter to E1A 243R. These experiments indicate that the PERE can function autonomously in mediating transactivation by E1A 243R and that the PCNA basal promoter is configured in a manner that permits modulation by E1A 243R of transcriptional activation by promoter-proximal effectors.

Induction of gene expression by exon 2 of the major E1A proteins of adenovirus type 5

We have constructed an adenovirus type 5 (Ad5) E1A mutant, dl1119/520, that produces essentially only exon 2 of the major E1A proteins. In infected primary baby rat kidney cells, this mutant induced expression of the E1B 55-kDa protein, and in infected human KB cells, it induced expression of this protein, the E2A 72-kDa protein, and hexon. In KB cells, this mutant grew substantially better than Ad5 dl312, which lacks E1A, and as well as Ad5 dl520, an E1A mutant producing only the 243-residue protein. These results suggest that exon 2 of E1A proteins on its own was able to activate gene expression. We also constructed mutants of dl1119/520, containing small deletions in regions of exon 2 that others found to be associated with effects on the properties of E1A transformants. None of these deletions destroyed gene activation completely, indicating that there may be some redundancy among sequences in exon 2 for inducing gene expression. The two deletions that decreased induction the most, residues 224 to 238 and 255 to 270, were in regions reported to be associated with the expression of a metalloprotease and with enhanced transformation, suggesting that exon 2 may regulate expression of genes governing cell growth. It is remarkable that all sections of E1A proteins, exon 1, the unique region, and exon 2, have now been found to affect gene expression.

Identification of distinct roles for separate E1A domains in disruption of E2F complexes

The adenovirus E1A protein can disrupt protein complexes containing the E2F transcription factor in association with cellular regulatory proteins such as the retinoblastoma gene product (Rb) and the Rb-related p107 protein. Previous experiments have shown that the CR1 and CR2 domains of E1A are required for this activity. We now demonstrate that the CR2 domain is essential for allowing E1A to interact with the E2F-Rb or the E2F-p107-cyclin A-cdk2 complex. Multimeric complexes containing E1A can be detected when the CR1 domain has been rendered inactive by mutation. In addition, the E1A CR1 domain, but not the CR2 domain, is sufficient to prevent the interaction of E2F with Rb or p107. On the basis of these results, we suggest a model whereby the CR2 domain brings E1A to the E2F complexes and then, upon a normal equilibrium dissociation of Rb or p107 from E2F, the E1A CR1 domain is able to block the site of interaction on Rb or p107, thereby preventing the re-formation of the complexes.

Identification of distinct roles for separate E1A domains in disruption of E2F complexes

The adenovirus E1A protein can disrupt protein complexes containing the E2F transcription factor in association with cellular regulatory proteins such as the retinoblastoma gene product (Rb) and the Rb-related p107 protein. Previous experiments have shown that the CR1 and CR2 domains of E1A are required for this activity. We now demonstrate that the CR2 domain is essential for allowing E1A to interact with the E2F-Rb or the E2F-p107-cyclin A-cdk2 complex. Multimeric complexes containing E1A can be detected when the CR1 domain has been rendered inactive by mutation. In addition, the E1A CR1 domain, but not the CR2 domain, is sufficient to prevent the interaction of E2F with Rb or p107. On the basis of these results, we suggest a model whereby the CR2 domain brings E1A to the E2F complexes and then, upon a normal equilibrium dissociation of Rb or p107 from E2F, the E1A CR1 domain is able to block the site of interaction on Rb or p107, thereby preventing the re-formation of the complexes.

E1A-mediated inhibition of myogenesis correlates with a direct physical interaction of E1A12S and basic helix-loop-helix proteins

The observation that adenovirus E1A gene products can inhibit differentiation of skeletal myocytes suggested that E1A may interfere with the activity of myogenic basic helix-loop-helix (bHLH) transcription factors. We have examined the ability of E1A to mediate repression of the muscle-specific creatine kinase (MCK) gene. Both the E1A12S and E1A13S products repressed MCK transcription in a concentration-dependent fashion. In contrast, amino-terminal deletion mutants (d2-36 and d15-35) of E1A12S were defective for repression. E1A12S also repressed expression of a promoter containing a multimer of the MCK high-affinity E box (the consensus site for myogenic bHLH protein binding) that was dependent, in C3H10T1/2 cells, on coexpression of a myogenin bHLH-VP16 fusion protein. A series of coprecipitation experiments with glutathione S-transferase fusion and in vitro-translated proteins demonstrated that E1A12S, but not amino-terminal E1A deletion mutants, could bind to full-length myogenin and E12 and to deletion mutants of myogenin and E12 that spare the bHLH domains. Thus, the bHLH domains of myogenin and E12, and the high-affinity E box, are targets for E1A-mediated repression of the MCK enhancer, and domains of E1A required for repression of muscle-specific gene transcription also mediate binding to bHLH proteins. We conclude that E1A mediates repression of muscle-specific gene transcription through its amino-terminal domain and propose that this may involve a direct physical interaction between E1A and the bHLH region of myogenic determination proteins.

Proteins with transcription regulatory properties encoded by human adenoviruses

Of the more than 30 genes encoded by the adenovirus genome, no less than six have been shown to encode proteins that have transcription regulatory properties. None of them is a sequence-specific DNA-binding protein. They act to modulate the activity of cellular transcription factors by causing their phosphorylation or dephosphorylation, by physically interacting with them, or by dissociating transcription factor inhibitory protein complexes.

E1A-mediated inhibition of myogenesis correlates with a direct physical interaction of E1A12S and basic helix-loop-helix proteins

The observation that adenovirus E1A gene products can inhibit differentiation of skeletal myocytes suggested that E1A may interfere with the activity of myogenic basic helix-loop-helix (bHLH) transcription factors. We have examined the ability of E1A to mediate repression of the muscle-specific creatine kinase (MCK) gene. Both the E1A12S and E1A13S products repressed MCK transcription in a concentration-dependent fashion. In contrast, amino-terminal deletion mutants (d2-36 and d15-35) of E1A12S were defective for repression. E1A12S also repressed expression of a promoter containing a multimer of the MCK high-affinity E box (the consensus site for myogenic bHLH protein binding) that was dependent, in C3H10T1/2 cells, on coexpression of a myogenin bHLH-VP16 fusion protein. A series of coprecipitation experiments with glutathione S-transferase fusion and in vitro-translated proteins demonstrated that E1A12S, but not amino-terminal E1A deletion mutants, could bind to full-length myogenin and E12 and to deletion mutants of myogenin and E12 that spare the bHLH domains. Thus, the bHLH domains of myogenin and E12, and the high-affinity E box, are targets for E1A-mediated repression of the MCK enhancer, and domains of E1A required for repression of muscle-specific gene transcription also mediate binding to bHLH proteins. We conclude that E1A mediates repression of muscle-specific gene transcription through its amino-terminal domain and propose that this may involve a direct physical interaction between E1A and the bHLH region of myogenic determination proteins.

Repression of RNA polymerase III transcription by adenovirus E1A

Adenovirus E1A encodes two major proteins of 289 and 243 amino acids (289R and 243R), which both have transcription regulatory properties. E1A-289R is a transactivator whereas E1A-243R primarily functions as a repressor of transcription. Here we show that E1A repression is not restricted to RNA polymerase II genes but also includes the adenovirus virus-associated (VA) RNA genes. These genes are transcribed by RNA polymerase III and have previously been suggested to be the target of an E1A-289R-mediated transactivation. Surprisingly, we found that during transient transfection both E1A proteins repressed VA RNA transcription. E1A repression of VA RNA transcription required both conserved regions 1 and 2 and therefore differed from the E1A-mediated inhibition of simian virus 40 enhancer activity which primarily required conserved region 1. The repression was counteracted by the E1B-19K protein, which also, in the absence of E1A, enhanced the accumulation of VA RNA. Importantly, we show that efficient VA RNA transcription requires expression of both E1A and the E1B-19K protein during virus infection.

The adenovirus E1A-associated kinase consists of cyclin E-p33cdk2 and cyclin A-p33cdk2

The adenovirus E1A oncoproteins form stable complexes with several cellular proteins. Association of E1A with these proteins has been shown to be important for the oncogenic potential of E1A. Several of these proteins have been identified and include the product of the retinoblastoma gene and a key cell cycle regulatory protein, cyclin A. E1A also associates with a potent histone H1 kinase. The two major components of this activity are the cyclin E-p33cdk2 and cyclin A-p33cdk2 complexes. Both the cyclin E-p33cdk2 and cyclin A-p33cdk2 complexes have been implicated in regulatory events controlling entry into or passage through DNA synthesis. Although the architecture of such interactions remains unclear, it is likely that by targeting such complexes, adenovirus is affecting some aspect of cell cycle control.

Induction of the cell cycle in baby rat kidney cells by adenovirus type 5 E1A in the absence of E1B and a possible influence of p53

From previous studies on the induction of DNA synthesis in quiescent primary baby rat kidney cells by adenovirus type 5 (Ad5) E1A deletion mutants, we concluded that induction is prevented only when cellular proteins p300 and pRb are both uncomplexed with E1A (J.A. Howe, J.S. Mymryk, C. Egan, P.E. Branton, and S.T. Bayley, Proc. Natl. Acad. Sci. USA 87:5883-5887, 1990). We have now examined induction by these same mutants in virus lacking the E1B region, so that cellular p53 was no longer complexed to the E1B 55-kDa protein. E1A mutants that fail to bind pRb induced DNA synthesis at a significantly lower level in Ad5 lacking E1B than in Ad5 containing E1B. Apparently, therefore, uncomplexed p53 can partially replace p300 in cooperating with pRb to suppress DNA synthesis in baby rat kidney cells.

Adenovirus-E1A proteins transform cells by sequestering regulatory proteins

Cell transformation by adenovirus-E1A proteins is mediated by binding to cellular proteins whose functions are thereby inactivated or altered. The various properties of the E1A proteins are reviewed in relation to their binding to cellular proteins. A number of the cellular proteins which associate to E1A have been identified: the retinoblastoma-susceptibility protein (Rb), the p107 protein, cyclin A and the p33cdk2 kinase. Recent data have shown that those proteins are also able to bind to transcription factor E2F. Binding of Rb to E2F represses the transcription-activating potential of E2F. E1A can sequester the regulatory proteins, like Rb, and thereby release free, active E2F. The domains in E1A that are essential for this transcriptional regulation are also required for the transforming properties of E1A.

Identification of specific adenovirus E1A N-terminal residues critical to the binding of cellular proteins and to the control of cell growth

Adenovirus early region 1A (E1A) oncogene-encoded sequences essential for transformation- and cell growth-regulating activities are localized at the N terminus and in regions of highly conserved amino acid sequence designated conserved regions 1 and 2. These regions interact to form the binding sites for two classes of cellular proteins: those, such as the retinoblastoma gene product, whose association with the E1A products is specifically dependent on region 2, and another class which so far is known to include only a large cellular DNA-binding protein, p300, whose association with the E1A products is specifically dependent on the N-terminal region. Association between the E1A products and either class of cellular proteins can be disrupted by mutations in conserved region 1. While region 2 has been studied intensively, very little is known so far concerning the nature of the essential residues in the N-terminal region, or about the manner in which conserved region 1 participates in the binding of two distinct sets of cellular proteins. A combination of site-directed point mutagenesis and monoclonal antibody competition experiments reported here suggests that p300 binding is dependent on specific, conserved residues in the N terminus, including positively charged residues at positions 2 and 3 of the E1A proteins, and that p300 and pRB bind to distinct, nonoverlapping subregions within conserved region 1. The availability of precise point mutations disrupting p300 binding supports previous data linking p300 with cell cycle control and enhancer function.

The adenovirus E1A 12S product displays functional redundancy in activating the human proliferating cell nuclear antigen promoter

The adenovirus E1A 243R oncoprotein stimulates expression from the promoter of the human proliferating cell nuclear antigen (PCNA). To gain insight into the mechanism of activation, we analyzed deletion and point mutations of the 243R protein for their abilities to activate PCNA promoter-directed reporter gene expression upon cotransfection into HeLa cells. Large deletions that in combination span the entire protein severely impaired the ability of E1A 243R to induce PCNA expression. Smaller deletions and specific point mutations that target specific E1A-binding proteins were less deleterious to PCNA induction. The data suggest that E1A activates transcription of the PCNA gene by multiple mechanisms and that, of the known 243R-associated proteins, p300 and p107-cyclin A can mediate the response while p105-RB does not appear to participate. Presumably, the functional redundancy ensures that 243R can activate expression of this essential DNA replication protein regardless of cell type and physiological conditions.

Toxicological and pathological applications of proliferating cell nuclear antigen (PCNA), a novel endogenous marker for cell proliferation

A major stimulus to study cell proliferation, particularly in rodent carcinogenicity assays and human tumors, has been the belief that the quantification of this fundamental biological process will provide the toxicologist and pathologist with objective data allowing a better understanding of the mechanisms involved in the toxicity and/or carcinogenicity of certain compounds as well as guiding more effective management of patients afflicted with neoplasia. Among the markers used for cell proliferation measurement, PCNA has recently gained much attention and holds much promise as it is intricately involved in the cell replication processes. It not only could allow measurement of the replication rates without necessitating pretreatment of the animal/tissue in prospective studies, but also would allow retrospective assessment of the proliferative rates in archival tissues due to the conservation of this marker in fixed and paraffin-embedded tissues. Finally, knowledge of the function of PCNA in the cell cycle and its regulation by other factors may help us understand the advantages and limitations of PCNA as a cell proliferation marker in its application in toxicology and as a prognostic marker in human tumors.

Homologous sequences in adenovirus E1A and human papillomavirus E7 proteins mediate interaction with the same set of cellular proteins

Studies of adenovirus E1A oncoprotein mutants suggest that the association of E1A with the retinoblastoma protein (pRB) is necessary for E1A-mediated transformation. Mutational analysis of E1A indicates that two regions of pRB are required for E1A to form stable complexes with the retinoblastoma protein. In addition to pRB binding, these regions are necessary for E1A association with several other cellular proteins, including p130, p107, cyclin A, and p33cdk2. Here we show that short synthetic peptides containing the pRB-binding sequences of E1A are sufficient for interaction with p107, cyclin A, and p130. The E7 protein of human papillomavirus type 16 contains an element that binds to pRB and appears to be functionally homologous to the E1A sequences. Peptides containing this region of the E7 protein were able to interact with p107, cyclin A, and p130 in addition to pRB. These findings suggest that the common mechanism of transformation used by these viral oncogenes involves their association with a set of polypeptides.

Repression of liver-specific hepatitis B virus enhancer 2 activity by adenovirus E1A proteins

Two regions of the hepatitis B virus (HBV) genome have been shown to display properties of a transcriptional enhancer. Enhancer 1 is active in most hepatoma lines examined as well as in some non-hepatocyte-derived cell lines. In contrast, enhancer 2 activity is strictly liver specific. In this study, we show that adenovirus E1A expression in the highly differentiated human hepatoma line Huh6 strongly inhibits HBV enhancer 2-stimulated transcription while having no effect on HBV enhancer 1 activity. A sequence motif in HBV enhancer 2 which is essential for its enhancer function is the target for E1A-mediated repression. The repression of HBV enhancer 2 activity is mediated through the N-terminal region of the E1A proteins known to bind a 300-kDa cellular protein. Our results suggest that HBV enhancer function may be modulated by a cellular mechanism similar to E1A-mediated transcriptional repression.

Promoter-specific trans-activation by the adenovirus E1A12S product involves separate E1A domains

Recent studies have shown that the adenovirus E1A12S product can trans-activate transcription by activating the transcription factor E2F. However, E2F cannot be the only target for the E1A12S product, since several cellular promoters have been found to be activated by the E1A12S protein even though they lack E2F sites. Indeed, we now show that activation of the hsp70 promoter by the E1A12S product requires the TATAA sequence. Moreover, activation of the hsp70 promoter requires the N-terminal domain of the E1A protein and does not require the conserved region 2 sequences which are required for the E2F-dependent activation of transcription. We conclude that the targeting of distinct transcription factors, leading to trans-activation of transcription of multiple promoters, involves distinct domains of the E1A proteins that are also required for oncogenic activity.

Ability of adenovirus 5 E1A proteins to suppress differentiation of BC3H1 myoblasts correlates with their binding to a 300 kDa cellular protein

We have used deletion mutants to define the regions in Ad5 E1A proteins necessary to suppress differentiation of mouse BC3H1 myoblasts. We examined the differentiation of cells infected at a low multiplicity with viruses containing the E1A deletions and constructed so as to produce only the smaller of the two major E1A proteins. Only four of the mutant viruses containing deletions within the N-terminal 69 residues failed to suppress differentiation as judged by changes in morphology and in levels of muscle-specific alpha-actin mRNA and creatine kinase activity. The results were confirmed by analyses of lines of cells stably transfected with representative E1A mutants. The mouse cellular proteins to which mutant E1A proteins bound were identified by immunoprecipitating E1A proteins specifically from infected BC3H1 cells and by analyzing the precipitates on denaturing gels. Bands of proteins of 300, 130, 107, 105 (the retinoblastoma product), and 60 kDa (cyclin A) were distinguished. Failure to suppress differentiation correlated with loss of binding to the 300-kDa protein but not to any of the others. The regions of E1A defined in this way have been shown to be required for several other activities, including enhancer repression and transformation. One function of the 300-kDa protein appears to be to facilitate the action of transcriptional enhancers of differentiation-specific genes.

Expression, purification, and functional characterization of adenovirus 5 and 12 E1A proteins produced in insect cells

The 12 S and 13 S E1A cDNAs from both the Adenovirus (Ad) nononcogenic type 5 and the oncogenic type 12 were overexpressed in an insect cell/baculovirus system. Upon infection of Spodoptera frugiperda cells, the production of E1A proteins reached a level of about 15 micrograms/10(6) cells. The E1A proteins are highly soluble and apparently are processed authentically. They are readily recognized by various antibodies and display phosphorylation patterns similar to those of E1A proteins synthesized in mammalian cells. Single-step immunoaffinity chromatography was used to purify the Ad5 E1A proteins to near homogeneity under nondenaturing conditions. The Ad5 and Ad12 E1A proteins are able to form complexes with the retinoblastoma susceptibility gene product (Rb) and other cellular proteins. Interestingly, the presence of a cellular extract seems to be a prerequisite for association between highly purified E1A and Rb polypeptides.

Induction of AP-1 DNA-binding activity and c-fos mRNA by the adenovirus 243R E1A protein and cyclic AMP requires domains necessary for transformation

The 243R E1A protein can act in synergy with cyclic AMP to induce AP-1 DNA-binding activity and c-fos mRNA in mouse S49 cells. A series of deletion mutants was used to identify two domains of the 243R protein that were required for these effects. Interestingly, these domains correlated precisely with regions known to be necessary for E1A-mediated transformation. One domain was located at the N terminus of E1A. The other domain spanned residues 36 to 81, corresponding to conserved region 1 of E1A. S49 cellular proteins that associate with E1A were coimmunoprecipitated with anti-E1A antibody. These included the previously identified proteins p300, p130, p107, p105Rb, and cyclin A. In addition, proteins of 90 kDa and a series of proteins in the 120- to 170-kDa range were identified. Binding of p300, p90, and the 120- to 170-kDa proteins was abolished in cells expressing mutants of E1A that were unable to induce AP-1 DNA-binding activity and c-fos mRNA. These data strongly suggest that specific cellular E1A-binding proteins are involved in the induction of AP-1 DNA-binding activity and c-fos mRNA by the synergistic action of the 243R E1A protein and cyclic AMP and that these transcriptional events are related to the transformation process.

Promoter-specific trans-activation by the adenovirus E1A12S product involves separate E1A domains

Recent studies have shown that the adenovirus E1A12S product can trans-activate transcription by activating the transcription factor E2F. However, E2F cannot be the only target for the E1A12S product, since several cellular promoters have been found to be activated by the E1A12S protein even though they lack E2F sites. Indeed, we now show that activation of the hsp70 promoter by the E1A12S product requires the TATAA sequence. Moreover, activation of the hsp70 promoter requires the N-terminal domain of the E1A protein and does not require the conserved region 2 sequences which are required for the E2F-dependent activation of transcription. We conclude that the targeting of distinct transcription factors, leading to trans-activation of transcription of multiple promoters, involves distinct domains of the E1A proteins that are also required for oncogenic activity.

Analysis of a developmentally regulated nuclear localization signal in Xenopus

The 289 residue nuclear oncoprotein encoded by the adenovirus 5 Ela gene contains two peptide sequences that behave as nuclear localization signals (NLS). One signal, located at the carboxy terminus, is like many other known NLSs in that it consists of a short stretch of basic residues (KRPRP) and is constitutively active in cells. The second signal resides within an internal 45 residue region of E1a that contains few basic residues or sequences that resemble other known NLSs. Moreover, this internal signal functions in injected Xenopus oocytes, but not in transfected Xenopus A6 cells, suggesting that it could be regulated developmentally (Slavicek et al. 1989. J. Virol. 63:4047). In this study, we show that the activity of this signal is sensitive to ATP depletion in vivo, efficiently directs the import of a 50 kD fusion protein and can compete with the E1a carboxy-terminal NLS for nuclear import. In addition, we have delineated the precise amino acid residues that comprise the second E1a NLS, and have assessed its utilization during Xenopus embryogenesis. Using amino acid deletion and substitution analyses, we show that the signal consists of the sequence FV(X)7-20MXSLXYM(X)4MF. By expressing in Xenopus embryos a truncated E1a protein that contains only the second NLS and by monitoring its cytoplasmic/nuclear distribution during development with indirect immunofluorescence, we find that the second NLS is utilized up to the early neurula stage. In addition, there appears to be a hierarchy among the embryonic germ layers as to when the second NLS becomes nonfunctional. For this reason, we refer to this NLS as the developmentally regulated nuclear localization signal (drNLS). The implications of these findings for early development are discussed.

Phosphorylation of adenovirus E1A proteins by the p34cdc2 protein kinase

Adenovirus early region 1A (E1A) products are phosphorylated nuclear oncoproteins which appear to derive transforming activity largely through interactions with cellular proteins including the tumor suppressor p105/Rb-1 and cyclin A (p60cycA), a regulatory subunit associated with p34cdc2 and the related protein kinase p33cdk2. We have identified several sites of phosphorylation on E1A proteins previously and showed that phosphorylation at Ser-89 alters electrophoretic mobility significantly and affects E1A-mediated transforming activity to some extent. We now report that both Ser-89 and Ser-219, the major E1A phosphorylation site, were phosphorylated in vitro by p34cdc2 purified from HeLa cells. We also found that E1A proteins seemed to be phosphorylated at the highest levels in vivo in mitotic cells which express maximal levels of p34cdc2 kinase activity. Thus, in addition to forming complexes with p60cycA, a regulator of p34cdc2 and related kinases, and p105/Rb-1 which exhibits cell cycle-dependent phosphorylation, E1A proteins seem to be substrates for p34cdc2. These data suggested that a link could exist between phosphorylation, cell cycle progression, and the regulation of transforming activity of E1A proteins.

Recombinant human adenoviruses containing hybrid adenovirus type 5 (Ad5)/Ad12 E1A genes: characterization of hybrid E1A proteins and analysis of transforming activity and host range

Hybrid adenovirus type 12 (Ad12)/Ad5 E1A genes were constructed by homologous recombination in Escherichia coli, a technique which offers several advantages over conventional mutagenesis for genetic analysis of proteins. In particular, functional differences between the proteins can be mapped by correlating the replacement of specific sequences with the acquisition of new properties, and there is no requirement for common unique restriction sites or polymerase chain reaction strategies to construct the hybrids. Recombinant adenoviruses expressing these hybrid E1A proteins were capable of replicating efficiently in HeLa cells, with the exception of one construct which contained a hybrid transactivation domain. The transforming activity of the hybrid E1A constructs was assayed by DNA transfection of primary baby rat kidney cells. Plasmids containing Ad12 E1 were approximately 20-fold less efficient at transformation than those with E1 of Ad5, and it was found that two regions in exon 1 of E1A mediate this difference. No differences were found in the abilities of any hybrid E1A proteins to bind to cellular proteins previously determined to be important for transformation by E1A.

DNA-binding properties of the E1A-associated 300-kilodalton protein

One of the major E1A-associated cellular proteins is a 300-kDa product (p300) that binds to the N-terminal region of the E1A products. The p300 binding site is distinct from sequences involved in binding the retinoblastoma product and other E1A-associated cellular products such as p60-cyclin A and p107. p300 binding to E1A is linked genetically to the enhancer repression function of E1A and the other E1A-mediated gene-regulating functions as well as to the transforming functions of E1A. However, the biochemical properties of p300 have not yet been characterized. We report here that p300 has an intrinsic DNA-binding activity and shows a preferential affinity for specific DNA sequences. The sequences selectively bound by p300 are related to those of a series of enhancer elements that are recognized by NF-kappa B. The direct physical interaction of p300 with enhancer elements provides a biochemical basis for the genetic evidence linking the E1A-mediated enhancer repression function with the p300-binding activity of E1A.