Binding of cellular polypeptides to human adenovirus type 5 E1A proteins produced in Escherichia coli

Characterization of Adenovirus 5 E1A Exon 1 Deletion Mutants in the Viral Replicative Cycle

Human adenovirus infection is driven by Early region 1A (E1A) proteins, which are the first proteins expressed following the delivery of the viral genome to the cellular nucleus. E1A is responsible for reprogramming the infected cell to support virus replication alongside the activation of expression of all viral transcriptional units during the course of the infection. Although E1A has been extensively studied, most of these studies have focused on understanding the conserved region functions outside of a full infection. Here, we investigated the effects of small deletions in E1A exon 1 on the viral replicative cycle. Almost all deletions were found to have a negative impact on viral replication with the exception of one deletion found in the mutant dl1106, which replicated better than the wild-type E1A expressing dl309. In addition to growth, we assessed the virus mutants for genome replication, induction of the cytopathic effect, gene and protein expression, sub-cellular localization of E1A mutant proteins, induction of cellular S-phase, and activation of S-phase specific cellular genes. Importantly, our study found that virus replication is likely limited by host-specific factors, rather than specific viral aspects such as the ability to replicate genomes or express late proteins, after a certain level of these has been expressed. Furthermore, we show that mutants outside of the conserved regions have significant influence on viral fitness. Overall, our study is the first comprehensive evaluation of the dl1100 series of exon 1 E1A deletion mutants in viral fitness and provides important insights into the contribution that E1A makes to viral replication in normal human cells.

The Human Adenovirus Type 5 E4orf6/E1B55K E3 Ubiquitin Ligase Complex Enhances E1A Functional Activity

Following our demonstration that adenovirus E3 ubiquitin ligase formed by the viral E4orf6 and E1B55K proteins is able to mimic the activation of E2F by E1A, we conducted a series of studies to determine if this complex might also promote the ability of E1A to do so. We found that the complex both significantly stabilizes E1A proteins and also enhances their ability to activate E2F. This finding is of significance because it represents an entirely new function for the ligase in regulating adenovirus replication by enhancing the action of E1A products.

Human adenovirus (Ad) E1A proteins have long been known as the central regulators of virus infection as well as the major source of adenovirus oncogenic potential. Not only do they activate expression of other early viral genes, they make viral replication possible in terminally differentiated cells, at least in part, by binding to the retinoblastoma (Rb) tumor suppressor family of proteins to activate E2F transcription factors and thus viral and cellular DNA synthesis. We demonstrate in an accompanying article (F. Dallaire et al., mSphere 1:00014-15, 2016) that the human adenovirus E3 ubiquitin ligase complex formed by the E4orf6 and E1B55K proteins is able to mimic E1A activation of E2F transactivation factors. Acting alone in the absence of E1A, the Ad5 E4orf6 protein in complex with E1B55K was shown to bind E2F, disrupt E2F/Rb complexes, and induce hyperphosphorylation of Rb, leading to induction of viral and cellular DNA synthesis, as well as stimulation of early and late viral gene expression and production of viral progeny. While these activities were significantly lower than those exhibited by E1A, we report here that this ligase complex appeared to enhance E1A activity in two ways. First, the E4orf6/E1B55K complex was shown to stabilize E1A proteins, leading to higher levels in infected cells. Second, the complex was demonstrated to enhance the activation of E2F by E1A products. These findings indicated a new role of the E4orf6/E1B55K ligase complex in promoting adenovirus replication.

IMPORTANCE Following our demonstration that adenovirus E3 ubiquitin ligase formed by the viral E4orf6 and E1B55K proteins is able to mimic the activation of E2F by E1A, we conducted a series of studies to determine if this complex might also promote the ability of E1A to do so. We found that the complex both significantly stabilizes E1A proteins and also enhances their ability to activate E2F. This finding is of significance because it represents an entirely new function for the ligase in regulating adenovirus replication by enhancing the action of E1A products.

Adenovirus E1A inhibits SCF(Fbw7) ubiquitin ligase

The SCF(Fbw7) ubiquitin ligase complex plays important roles in cell growth, survival, and differentiation via the ubiquitin-proteasome-mediated regulation of protein stability. Fbw7 (also known as Fbxw7, Sel-10, hCdc4, or hAgo), a substrate recognition subunit of SCF(Fbw7) ubiquitin ligase, facilitates the degradation of several proto-oncogene products by the proteasome. Given that mutations in Fbw7 are found in various types of human cancers, Fbw7 is considered to be a potent tumor suppressor. In the present study, we show that E1A, an oncogene product derived from adenovirus, interferes with the activity of the SCF(Fbw7) ubiquitin ligase. E1A interacted with SCF(Fbw7) and attenuated the ubiquitylation of its target proteins in vivo. Furthermore, using in vitro purified SCF(Fbw7) component proteins, we found that E1A directly bound to Roc1/Rbx1 and CUL1 and that E1A inhibited the ubiquitin ligase activity of the Roc1/Rbx1-CUL1 complex but not that of another RING-type ubiquitin ligase, Mdm2. Ectopically expressed E1A interacted with cellular endogenous Roc1/Rbx1 and CUL1 and decelerated the degradation of several protooncogene products that were degraded by SCF(Fbw7) ubiquitin ligase. Moreover, after wild-type adenovirus infection, adenovirus-derived E1A interacted with endogenous Roc1/Rbx1 and decelerated degradation of the endogenous target protein of SCF(Fbw7). These observations demonstrated that E1A perturbs protein turnover regulated by SCF(Fbw7) through the inhibition of SCF(Fbw7) ubiquitin ligase. Our findings may help to explain the mechanism whereby adenovirus infection induces unregulated proliferation.

How the Rb tumor suppressor structure and function was revealed by the study of Adenovirus and SV40

The review recounts the history of how the study of the DNA tumor viruses including polyoma, SV40 and Adenovirus brought key insights into the structure and function of the Retinoblastoma protein (Rb). Knudsen's model of the two-hit hypothesis to explain patterns of hereditary and sporadic retinoblastoma provided the foundation for the tumor suppressor hypothesis that ultimately led to the cloning of the Rb gene. The discovery that SV40 and Adenovirus could cause tumors when inoculated into animals was startling not only because SV40 had contaminated the poliovirus vaccine and Adenovirus was a common cause of viral induced pneumonia but also because they provided an opportunity to study the genetics and biochemistry of cancer. Studies of mutant forms of these viruses led to the identification of the E1A and Large T antigen (LT) oncogenes and their small transforming elements including the Adenovirus Conserved Regions (CR), the SV40 J domain and the LxCxE motif. The immunoprecipitation studies that initially revealed the size and ultimately the identity of cellular proteins that could bind to these transforming elements were enabled by the widespread development of highly specific monoclonal antibodies against E1A and LT. The identification of Rb as an E1A and LT interacting protein quickly led to the cloning of p107, p130, p300, CBP, p400 and TRRAP and the concept that viral transformation was due, at least in part, to the perturbation of the function of normal cellular proteins. In addition, studies on the ability of E1A to transactivate the Adenovirus E2 promoter led to the cloning of the heterodimeric E2F and DP transcription factor and recognition that Rb repressed transcription of cellular genes required for cell cycle entry and progression. More recent studies have revealed how E1A and LT combine the activity of Rb and the other cellular associated proteins to perturb expression of many genes during viral infection and tumor formation.

Novel mechanisms of E2F induction by BK virus large-T antigen: requirement of both the pRb-binding and the J domains

E2F activity is regulated in part by the retinoblastoma family of tumor suppressor proteins. Viral oncoproteins, such as simian virus 40 (SV40) large-T antigen (TAg), adenovirus E1A, and human papillomavirus E7, can disrupt the regulation of cellular proliferation by binding to pRb family members and dissociating E2F-pRb family protein complexes. BK virus (BKV), which infects a large percentage of the human population and has been associated with a variety of human tumors, encodes a TAg homologous to SV40 TAg. It has been shown that BKV TAg, when expressed at low levels, does not detectably bind to pRb family members, yet it induces a serum-independent phenotype and causes a decrease in the overall levels of pRb family proteins. The experiments presented in this report show that, despite the lack of TAg-pRb interactions, BKV TAg can induce transcriptionally active E2F and that this induction does in fact require an intact pRb-binding domain as well as an intact J domain. In addition, E2F-pRb family member complexes can be detected in both BKV and SV40 TAg-expressing cells. These results suggest the presence of alternate cellular mechanisms for the release of E2F in addition to the well-established model for TAg-pRb interactions. These results also emphasize a role for BKV TAg in the deregulation of cellular proliferation, which may ultimately contribute to neoplasia.

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.

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.

BK virus large T antigen: interactions with the retinoblastoma family of tumor suppressor proteins and effects on cellular growth control

BK virus (BKV) is a polyomavirus which infects a large percentage of the human population. It is a potent transforming agent and is tumorigenic in rodents. BKV DNA has also been found in human brain, pancreatic islet, and urinary tract tumors, implicating this virus in neoplastic processes. BKV T antigen (TAg) is highly homologous to simian virus 40 TAg, particularly in regions required for mitogenic stimulation and binding to tumor suppressor proteins, The experiments presented in this report show that BKV TAg can bind the tumor suppressor protein p53. BKV TAg also has the ability to bind to members of the retinoblastoma (pRb) family of tumor suppressor proteins both in vivo and in vitro. However, these interactions are detected only when large amounts of total protein are used, because the levels of BKV TAg normally produced from viral promoter-enhancer elements are too low to bind a significant amount of the pRb family proteins in the cell. The low levels of BKV TAg produced by the viral promoter elements are sufficient to affect the levels and the phosphorylation patterns of these proteins and to induce serum-independent growth in these cells. Additional events, however, are required for full transformation. These data further support the notion that BKV TAg can affect cellular growth control mechanisms and may in fact be involved in neoplastic processes.

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.

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.

Independent transformation activity by adenovirus-5 E1A-conserved regions 1 or 2 mutants

Two conserved regions (CR1 and CR2) on the adenovirus E1A proteins have previously been shown to be required for cooperation with the ras oncogene in the transformation of primary rodent cells. Sequences within these regions are essential for the ability of E1A to associate with the 105K product of the retinoblastoma susceptibility gene, p105-RB, as well as with other cellular proteins, including a 107K (p107) and a 300K (p300) species. In this paper, we show that CR1 mutants deficient in p300 binding and CR2 mutants with lost or reduced binding of p105-RB and/or p107 have a low, but not abolished focus formation activity. In contrast, CR1/CR2 double mutants were deficient in focus formation, suggesting that the transformation activities displayed by the single CR1 or CR2 mutants were due to an independent transformation activity by both CR1 and CR2. No strict correlation between p105-RB binding and E1A-mediated transformation was observed. The E1A enhancer repression function was found to correlate with the binding of p300 but not with E1A-mediated transformation. Complex formation between E1A and p107, similar to the p105-RB binding, required sequences within both CR1 and CR2. The CR2 sequences required for binding of p107K or p105-RB were overlapping, but not identical. Finally, a larger segment of CR2 was required for stable complex formation between E1A and phosphorylated forms of p105-RB or p107 compared to corresponding unphosphorylated species.

Retinoblastoma growth suppressor and a 300-kDa protein appear to regulate cellular DNA synthesis

Previous work has suggested that oncogenic transformation by the E1A gene products of adenovirus type 5 may be mediated through interactions with at least two cellular proteins, the 105-kDa product of the retinoblastoma growth suppressor gene (p105-Rb) and a 300-kDa protein (p300). By using viral mutants, we now show that the induction of cellular DNA synthesis in quiescent cells by E1A differs from transformation in that E1A products induce synthesis if they are able to bind to either p105-Rb or p300, and only mutant products that bind to neither are extremely defective. These results suggest that p105-Rb and p300 (or cellular proteins with similar E1A-binding properties) provide parallel means by which DNA synthesis can be regulated.

E1A-induced enhancer activity of the poly(dG-dT).poly(dA-dC) element (GT element) and interactions with a GT-specific nuclear factor

The alternating sequence poly(dG-dT).poly(dA-dC) is a highly repeated sequence in the eucaryotic genome. We have examined the effect of trans-acting early viral proteins on the ability of the GT element to stimulate transcription of the adenovirus major late promoter (MLP). We find that the GT element alone does not activate expression from the MLP in either the presence or absence of another enhancer element. However, in the presence of the E1A gene products of either adenovirus type 5 or 2, the GT element activated expression from the MLP. The stimulatory activity of the GT element in the presence of E1A had the properties of an enhancer element, and the trans-activating effect on the GT element was additive in conjunction with the E1A-responsive BK virus enhancer. We also have demonstrated that a specific nuclear factor(s) binds to the GT element. However, the E1A protein(s) do not affect the initial factor interaction(s) with the GT element. Overall, our data demonstrate that trans modulation of promoter activity can be mediated through the GT element.

E1A-induced enhancer activity of the poly(dG-dT).poly(dA-dC) element (GT element) and interactions with a GT-specific nuclear factor

The alternating sequence poly(dG-dT).poly(dA-dC) is a highly repeated sequence in the eucaryotic genome. We have examined the effect of trans-acting early viral proteins on the ability of the GT element to stimulate transcription of the adenovirus major late promoter (MLP). We find that the GT element alone does not activate expression from the MLP in either the presence or absence of another enhancer element. However, in the presence of the E1A gene products of either adenovirus type 5 or 2, the GT element activated expression from the MLP. The stimulatory activity of the GT element in the presence of E1A had the properties of an enhancer element, and the trans-activating effect on the GT element was additive in conjunction with the E1A-responsive BK virus enhancer. We also have demonstrated that a specific nuclear factor(s) binds to the GT element. However, the E1A protein(s) do not affect the initial factor interaction(s) with the GT element. Overall, our data demonstrate that trans modulation of promoter activity can be mediated through the GT element.

Mapping of cellular protein-binding sites on the products of early-region 1A of human adenovirus type 5

The binding sites for the 300-, 107-, and 105-kilodalton cellular proteins which associate with human adenovirus type 5 E1A products were studied with E1A deletion mutants. All appeared to bind to the amino-terminal half of E1A products in regions necessary for oncogenic transformation. These results suggest that these cellular species may be important for the biological activity of E1A products.

Mapping of cellular protein-binding sites on the products of early-region 1A of human adenovirus type 5

The binding sites for the 300-, 107-, and 105-kilodalton cellular proteins which associate with human adenovirus type 5 E1A products were studied with E1A deletion mutants. All appeared to bind to the amino-terminal half of E1A products in regions necessary for oncogenic transformation. These results suggest that these cellular species may be important for the biological activity of E1A products.

The 289-amino acid E1A protein of adenovirus binds zinc in a region that is important for trans-activation

The E1A gene of adenovirus type 5 encodes two major proteins of 289 and 243 amino acid residues, which are identical except that the larger protein has an internal stretch of 46 amino acids required for efficient trans-activation of early viral promoters. This domain contains a consensus zinc finger motif (Cys-Xaa2-Cys-Xaa13-Cys-Xaa2-Cys) in which the cysteine residues serve as postulated ligands. Atomic absorption spectrophotometry applied to bacterially expressed E1A proteins revealed that the 289-amino acid protein binds one zinc ion, whereas the 243-amino acid protein binds no zinc. Replacing individual cysteine residues of the finger with other amino acids destroyed the trans-activating ability of the 289-amino acid protein, even when structurally or functionally conserved amino acids were substituted. These results strongly suggest that the zinc finger of the 46-amino acid domain is intimately linked to the ability of the large E1A protein to stimulate transcription of E1A-inducible promoters. Furthermore, zinc binding to one of the mutant finger proteins suggests either that only a precise finger structure formed by the tetrahedral coordination of zinc to the four consensus ligands is required for trans-activation or, possibly, that one of several neighboring histidine residues in various combinations with three of the consensus cysteine residues normally coordinates zinc. How the zinc finger in E1A might interact with DNA or protein to bring about trans-activation is discussed.