Heterogeneity of adenovirus type 5 E1A proteins: multiple serine phosphorylations induce slow-migrating electrophoretic variants but do not affect E1A-induced transcriptional activation or transformation

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

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

Acetylation of the adenovirus-transforming protein E1A determines nuclear localization by disrupting association with importin-alpha

Posttranslational modifications may alter the biochemical functions of a protein by modifying associations with other macromolecules, allosterically altering intrinsic catalytic activities, or determining subcellular localization. The adenovirus-transforming protein E1A is acetylated by its cellular targets, the co-activators CREB-binding protein, p300, and p300/CREB-binding protein-associated factor in vitro and also in vivo at a single lysine residue (Lys(239)) within a multifunctional carboxyl-terminal domain necessary for both nuclear localization and interaction with the transcriptional co-repressor carboxyl-terminal binding protein (CtBP). In contrast to a previous report, we demonstrate that acetylation of Lys(239) does not disrupt CtBP binding and that 12 S E1A-mediated repression of CREB-binding protein-dependent transcription does not require recruitment of CtBP. Instead we find that the cytoplasmic fraction of E1-transformed 293 cells is enriched for acetylated E1A with relative exclusion from the nuclear compartment. Whereas wild type 12 S E1A binds importin-alpha 3, binding affinity was markedly reduced both by single amino acid substitution mutations and acetylation at Lys(239). This is the first demonstration that acetylation may alter nuclear partitioning by direct interference with nuclear import receptor recognition. The finding that the cytoplasmic fraction of E1A is acetylated indicates that E1A may exert its pleiotropic effects on cellular transformation in part by affecting cytoplasmic processes.

cAMP-independent activation of the adenovirus type 12 E2 promoter correlates with the recruitment of CREB-1/ATF-1, E1A(12S), and CBP to the E2-CRE

Expression of the transcription unit early region 2 (E2) is of crucial importance for adenoviruses because this region encodes proteins essential for viral replication. Here, we demonstrate that the E1A(12S) protein of the oncogenic adenovirus serotype 12 activates the E2 promoter in dependence of the N terminus and the conserved region 1. Activation is mediated through a cAMP-response element that is bound by CREB-1 and ATF-1. Moreover, the Ad12 E2 promoter is inducible by protein kinase A and repressed by either a dominant-negative cAMP-response element-binding protein (CREB) mutant or the highly specific protein kinase A inhibitor protein underscoring the participation of CREB-1/ATF-1 in promoter activation. E1A(12S) binds to CREB-1 and ATF-1 in dependence of the N terminus and CR1 and is recruited to the E2 cAMP-response element through both cellular transcription factors. Most interestingly, point mutations revealed that E1A(12S) domains essential for binding to CREB-1/ATF-1 and for activation of the Ad12 E2 promoter are also essential for binding to the CREB-binding protein. Due to these data and results obtained in DNA-dependent protein-protein interaction assays, we propose a model in which the cAMP-independent activation of the Ad12 E2 promoter is mediated through a ternary complex consisting of CREB-1/ATF-1, E1A(12S), and CREB-binding protein, which assembles on the E2 cAMP-response element.

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.

Protein-protein interaction between the transcriptional repressor E4BP4 and the TBP-binding protein Dr1

We have previously mapped a repression domain from the active transcriptional repressor E4BP4 to a 65 amino acid segment near the C-terminus of the polypeptide. Here we show that the E4BP4 repression domain interacts specifically with the TBP binding repressor protein Dr1. Mutants that affect the ability of E4BP4 to bring about transcriptional repression are also deficient in their binding of Dr1. The results are discussed in the light of evidence for squelching of a 'global' repressor by a DNA binding defective E4BP4 mutant.

Cyclin-dependent kinases phosphorylate the adenovirus E1A protein, enhancing its ability to bind pRb and disrupt pRb-E2F complexes

The adenovirus E1A protein of 243 amino acids has been shown to affect a variety of cellular functions, most notably the immortalization of primary cells and the promotion of quiescent cells into S phase. The activity of E1A is derived, in part, from its association with various cellular proteins, many of which play important roles in regulating cell cycle progression. E1A is known to have multiple sites of phosphorylation. It has been suggested that cell cycle-dependent phosphorylation may also control some of E1A's functions. We find now that immune complexes of cyclin-dependent kinases such as cdk4, cdk2, and cdc2 are all capable of phosphorylating E1A in vitro. Additionally, the sites on E1A phosphorylated by these kinases in vitro are similar to the E1A sites phosphorylated in vivo. We have also found that a phosphorylated E1A is far more efficient than an unphosphorylated E1A in associating with pRB and in disrupting E2F/DP-pRB complexes as well. On the basis of our findings and the differences in timing and expression levels of the various cyclins regulating cdks, we suggest that E1A functions at different control points in the cell cycle and that phosphorylation controls, to some extent, its biological functions.

Analysis of ATF2 gene expression during early Xenopus laevis development

Activating transcription factor 2 (ATF2) is a cellular sequence-specific DNA-binding protein that mediates transcriptional activation by the adenovirus (Ad) E1A protein. In injected Xenopus laevis oocytes, E1A-induced transactivation requires an ATF2 recognition sequence within the responding promoters, thereby suggesting that ATF2 is present in oocytes and perhaps has a developmentally important function. As a first step in assessing this, an ATF2 cDNA was cloned and sequenced. The protein encoded by this cDNA contains 486 amino acids and is 92% identical to mammalian ATF2. ATF2 RNA and protein levels are very low in oocytes, but rise dramatically during blastulation. These high levels are maintained through gastrulation, but return to low levels during neurulation. In the blastula, ATF2 RNA and protein are virtually completely confined to cells of the animal pole. The temporal and spatial regulation of ATF2 suggests that it has an important function in early development.

Phosphorylation of E2F-1 modulates its interaction with the retinoblastoma gene product and the adenoviral E4 19 kDa protein

The transcription factor E2F is regulated through its cyclical interaction with a spectrum of cellular proteins. One such protein is the product of the retinoblastoma gene (Rb); association of E2F with Rb inhibits its transactivation potential. However, in adenovirus-infected cells, E2F is complexed to the 19 kDa product of the adenovirus E4 gene. We have studied the interaction of E2F-1 with the Rb and adenovirus E4 proteins and show that phosphorylation of E2F-1 on serine residues 332 and 337 prevented its interaction with Rb but was a prerequisite for interaction with E4. These residues were phosphorylated in vivo and by p34cdc2 kinase in vitro. Upon stimulation of serum-starved cells, phosphorylation was induced in the late G1 phase of the cell cycle. These observations suggest that phosphorylation of E2F-1 is important in the regulation of its activity during the cell cycle and during infection of cells by adenovirus.

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

A puzzling property of the transcriptional activators encoded by several animal viruses is their ability to function promiscuously. The adenovirus E1a protein, for example, stimulates transcription of adenoviral genes as well as a wide variety of other viral and cellular genes. We show that E1a can interact with several classes of cellular DNA-binding domains and thereby be recruited to diverse promoters. Our results explain how a single protein can regulate transcription of multiple genes that lack a common promoter element.

Identification and characterization of a protein kinase gene in the Lymantria dispar multinucleocapsid nuclear polyhedrosis virus

The Lymantria dispar multinucleocapsid nuclear polyhedrosis virus (LdMNPV) gene encoding vPK has been cloned and sequenced. LdMNPV vPK shows a 24% amino acid identity to the catalytic domains of the eucaryotic protein kinases nPKC from rabbits, HSPKCE from humans, APLPKCB from Aplysia californica, and dPKC98F from Drosophila melanogaster, and homology to several other protein kinases from yeasts, mice, and bovines. The homology suggests that vPK is a serine/threonine protein kinase as defined by Hanks et al. (S.K. Hanks, A.M. Quinn, and T. Hunter, Science 241:42-52, 1988). Temporal expression studies indicate that vPK is expressed throughout the infection cycle beginning at 4 h postinfection, first as a delayed-early gene and subsequently as a late gene. Sequence analysis and primer extension reactions confirm the presence of distinct early and late transcription initiation regions. Expression of vPK with a rabbit reticulocyte system generated a 31-kDa protein, which is in close agreement with the predicted size of 32 kDa from the amino acid sequence. Phosphorylation activity of in vitro-expressed vPK was demonstrated by using calf thymus histones.

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

The immediate-early 2 (IE2) 86-kDa polypeptide, a major immediate-early gene product of human cytomegalovirus, regulates transcription both positively and negatively. We report two new properties of the IE2 86-kDa polypeptide in infected cells. Immunoprecipitation of infected cell proteins from human embryonic lung cells by antipeptide or monoclonal antibodies specific for IE2 epitopes revealed three closely migrating polypeptide species. The slowest, p86, behaved as expected for the mature 86-kDa IE2 polypeptide. The middle species, p80, was immunoprecipitated from denatured as well as native samples and labeled to steady state rapidly. Pulse-chase analysis demonstrated directly that p80 was a metabolic precursor to p86. The fastest-migrating species, p75, was not detected by probing blots of the immunoprecipitated proteins with IE2-specific antisera; p75 was not precipitated from denatured protein samples; and the products of partial proteolysis of p75 were distinct from those of p86. These properties established p75 as an unrelated coprecipitated polypeptide complexed with p86. The p75 proteins coprecipitated from cells infected with two different strains of human cytomegalovirus, AD169 and Towne, had different mobilities. p75 was detected as early as 6 h and as late as 72 h after infection, but it was not synthesized in cells released from a cycloheximide block. Thus, it is likely that p75 is an early viral protein.

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.

Adenovirus E4orf4 protein reduces phosphorylation of c-Fos and E1A proteins while simultaneously reducing the level of AP-1

Adenovirus E1A protein and cyclic AMP cooperate to induce transcription factor AP-1 and viral gene expression in mouse S49 cells. We report that a protein encoded within the viral E4 gene region acts to counterbalance the induction of AP-1 DNA-binding activity by E1A and cyclic AMP. Studies with mutant adenoviruses demonstrated that in the absence of E4orf4 protein, AP-1 DNA-binding activity is induced to substantially higher levels than in wild-type virus-infected cells. The induction is the result of increased production of JunB and c-Fos proteins. Hyperphosphorylated forms of c-Fos and E1A proteins accumulate in the absence of functional E4orf4 protein. We propose that the E4orf4 protein acts to inhibit the activity of a cellular kinase that phosphorylates both the E1A and c-Fos proteins. Phosphorylation-dependent alterations in the activity of c-Fos, E1A, or some unidentified protein might, then, lead to decreased synthesis of AP-1 components. This E4 function likely plays an important role in natural infections, since a mutant virus unable to express the E4orf4 protein is considerably more cytotoxic than the wild-type virus.

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.

The hsp70 gene CCAAT-binding factor mediates transcriptional activation by the adenovirus E1a protein

Expression of the human hsp70 gene is cell cycle regulated and is inducible by both serum and the adenovirus E1a protein (K. Milarski and R. Morimoto, Proc. Natl. Acad. Sci. USA 83:9517-9521, 1986; M. C. Simon, K. Kitchener, H.-T. Kao, E. Hickey, L. Weber, R. Voellmy, N. Heintz, and J. R. Nevins, Mol. Cell. Biol. 7:2884-2890, 1987; B. Wu, H. Hurst, N. Jones, and R. Morimoto, Mol. Cell. Biol. 6:2994-2999, 1986; B. Wu and R. Morimoto, Proc. Natl. Acad. Sci. USA 82:6070-6074, 1985). This regulated expression is predominantly controlled by the CCAAT element at position -70 relative to the transcriptional initiation site (G. Williams, T. McClanahan, and R. Morimoto, Mol. Cell. Biol. 9:2574-2587, 1989; B. Wu, H. Hurst, N. Jones, and R. Morimoto, Mol. Cell. Biol. 6:2994-2999, 1986). A corresponding CCAAT-binding factor (CBF) of 999 amino acids has recently been cloned and shown to stimulate transcription selectively from the hsp70 promoter in a CCAAT element-dependent manner (L. Lum, L. Sultzman, R. Kaufman, D. Linzer, and B. Wu, Mol. Cell. Biol. 10:6709-6717, 1990). We report here that the first 192 residues of CBF, when fused to the DNA-binding domain of the heterologous activator GAL-4, are necessary and sufficient to mediate E1a-dependent transcriptional activation. E1a and CBF exhibit complex formation in vitro, suggesting that an in vivo interaction between these proteins may be relevant to the well-characterized E1a-induced transcriptional activation of the hsp70 promoter.

The hsp70 gene CCAAT-binding factor mediates transcriptional activation by the adenovirus E1a protein

Expression of the human hsp70 gene is cell cycle regulated and is inducible by both serum and the adenovirus E1a protein (K. Milarski and R. Morimoto, Proc. Natl. Acad. Sci. USA 83:9517-9521, 1986; M. C. Simon, K. Kitchener, H.-T. Kao, E. Hickey, L. Weber, R. Voellmy, N. Heintz, and J. R. Nevins, Mol. Cell. Biol. 7:2884-2890, 1987; B. Wu, H. Hurst, N. Jones, and R. Morimoto, Mol. Cell. Biol. 6:2994-2999, 1986; B. Wu and R. Morimoto, Proc. Natl. Acad. Sci. USA 82:6070-6074, 1985). This regulated expression is predominantly controlled by the CCAAT element at position -70 relative to the transcriptional initiation site (G. Williams, T. McClanahan, and R. Morimoto, Mol. Cell. Biol. 9:2574-2587, 1989; B. Wu, H. Hurst, N. Jones, and R. Morimoto, Mol. Cell. Biol. 6:2994-2999, 1986). A corresponding CCAAT-binding factor (CBF) of 999 amino acids has recently been cloned and shown to stimulate transcription selectively from the hsp70 promoter in a CCAAT element-dependent manner (L. Lum, L. Sultzman, R. Kaufman, D. Linzer, and B. Wu, Mol. Cell. Biol. 10:6709-6717, 1990). We report here that the first 192 residues of CBF, when fused to the DNA-binding domain of the heterologous activator GAL-4, are necessary and sufficient to mediate E1a-dependent transcriptional activation. E1a and CBF exhibit complex formation in vitro, suggesting that an in vivo interaction between these proteins may be relevant to the well-characterized E1a-induced transcriptional activation of the hsp70 promoter.

The adenovirus E3-14.5K protein which is required for prevention of TNF cytolysis and for down-regulation of the EGF receptor contains phosphoserine

The E3-14.5K and E3-10.4K proteins form a complex and function to down-regulate the epidermal growth factor receptor and to prevent tumor necrosis factor cytolysis in adenovirus-infected cells. Both 14.5K and 10.4K are cytoplasmic membrane proteins with a Ccyt orientation in the membrane. We show here that 14.5K is phosphorylated on serine residues in cells infected by adenoviruses that synthesize both 14.5K and 10.4K. 14.5K is phosphorylated on both serine and threonine in cells infected by a mutant that does not synthesize 10.4K; thus, the presence or absence of 10.4K affects the phosphorylation of 14.5K. Phosphotyrosine was not detected. 14.5K is also phosphorylated when translated in vitro in a rabbit reticulocyte extract. Both in vivo and in vitro, at least one of the phosphorylation sites is near the C-terminus, in the cytoplasmic domain of 14.5K. This C-terminal region of 14.5K is the most conserved among Ad5, Ad2, Ad3, and Ad7, and it is essential for 14.5K to prevent tumor necrosis factor cytolysis.

trans-dominant mutants of E1A provide genetic evidence that the zinc finger of the trans-activating domain binds a transcription factor

The 289R E1A protein of adenovirus stimulates transcription of early viral and certain cellular genes. trans-Activation requires residues 140 to 188, which encompass a zinc finger. Several studies have indicated that trans-activation by E1A is mediated through cellular transcription factors. In particular, the ability of the trans-dominant E1A point mutant hr5 (Ser-185 to Asn) to inhibit wild-type E1A trans-activation was proposed to result from the sequestration of a cellular factor. Using site-directed mutagenesis, we individually replaced every residue within and flanking the trans-activating domain with a conservative amino acid, revealing 16 critical residues. Six of the individual substitutions lying in a contiguous stretch C terminal to the zinc finger (carboxyl region183-188) imparted a trans-dominant phenotype. trans-Dominance was even produced by deletion of the entire carboxyl region183-188. Conversely, an intact finger region147-177 was absolutely required for trans-dominance, since second-site substitution of every critical residue in this region abrogated the trans-dominant phenotype of the hr5 protein. These data indicate that the finger region147-177 bind a limiting cellular transcription factor and that the carboxyl region183-188 provides a separate and essential function. In addition, we show that four negatively charged residues within the trans-activating domain do not comprise a distinct acidic activating region. We present a model in which the trans-activating domain of E1A binds to two different cellular protein targets through the finger and carboxyl regions.

trans-dominant mutants of E1A provide genetic evidence that the zinc finger of the trans-activating domain binds a transcription factor

The 289R E1A protein of adenovirus stimulates transcription of early viral and certain cellular genes. trans-Activation requires residues 140 to 188, which encompass a zinc finger. Several studies have indicated that trans-activation by E1A is mediated through cellular transcription factors. In particular, the ability of the trans-dominant E1A point mutant hr5 (Ser-185 to Asn) to inhibit wild-type E1A trans-activation was proposed to result from the sequestration of a cellular factor. Using site-directed mutagenesis, we individually replaced every residue within and flanking the trans-activating domain with a conservative amino acid, revealing 16 critical residues. Six of the individual substitutions lying in a contiguous stretch C terminal to the zinc finger (carboxyl region183-188) imparted a trans-dominant phenotype. trans-Dominance was even produced by deletion of the entire carboxyl region183-188. Conversely, an intact finger region147-177 was absolutely required for trans-dominance, since second-site substitution of every critical residue in this region abrogated the trans-dominant phenotype of the hr5 protein. These data indicate that the finger region147-177 bind a limiting cellular transcription factor and that the carboxyl region183-188 provides a separate and essential function. In addition, we show that four negatively charged residues within the trans-activating domain do not comprise a distinct acidic activating region. We present a model in which the trans-activating domain of E1A binds to two different cellular protein targets through the finger and carboxyl regions.

Inhibition of the cellular response to interferons by products of the adenovirus type 5 E1A oncogene

Expression of the E1A oncogene of adenovirus type 5 inhibits the response of interferon (IFN)-inducible constructs to Type I (alpha,beta) and II (gamma) IFNs in transient transfection assays. In human cell lines stably expressing E1A mRNA and protein acquisition of an antiviral state and the induction of a number of genes in response to alpha- and gamma-IFNs is inhibited. A short IFN-stimulable response element (ISRE) present in the 5' flanking region of a number of genes mediates induction by alpha- and gamma-IFNs. In cells expressing E1A there is a substantial reduction in the levels of the ISRE-binding factors E and M, inducible by alpha-IFN, and of factor G, inducible by gamma-IFN. In E1A-expressing cells the E alpha subunit of factor E is activated normally in response to alpha-IFN; the defect is in the production or activation of the E gamma subunit. The inhibitory activity of E1A is lost upon deletion of the CR1 domain. The induction of HLA class II genes by gamma-IFN, which involves a different DNA response element(s), and of beta-IFN mRNA in response to double-stranded RNA are also inhibited by E1A. An essential component(s) of a number of signalling pathways must, therefore, be subject, directly or indirectly, to inhibition by E1A.

Expression and interactions of human adenovirus oncoproteins

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The degradation sequence of adenovirus E1A consists of the amino-terminal tetrapeptide Met-Arg-His-Ile

The adenovirus E1A gene product is a potent transcriptional activator and nuclear oncoprotein. Like other regulatory proteins, E1A has a short half-life, in the range of 30 to 120 min. This short half-life, which was measured in cells synthesizing E1A, is not observed in cells injected with E1A protein made in bacteria or in vitro. In these cases, E1A is essentially refractory to degradation. In an attempt to reconcile this apparent paradox, we suggested that E1A was marked for degradation during its synthesis. Furthermore, we showed that a domain in the amino terminus of E1A was required for rapid degradation in cells translating E1A mRNA (J. M. Slavicek, N. C. Jones, and J. D. Richter, EMBO J. 7:3171-3180, 1988). In this study, we have used Xenopus laevis oocytes injected with mRNAs encoding altered E1A proteins to show that the amino-terminal tetrapeptide Met-Arg-His-Ile is required for E1A degradation. Even conservative amino acid substitutions in this degradation sequence render it nonfunctional. This degradation sequence can function as a transferable signal, since it induces instability when fused to another normally stable protein. Furthermore, the degradation sequence requires a proximity of no more than six residues from the amino terminus for activity. These data suggest that a trans-acting factor recognizes the amino terminus of E1A during the translation of its message to mark the protein for subsequent destruction.

The degradation sequence of adenovirus E1A consists of the amino-terminal tetrapeptide Met-Arg-His-Ile

The adenovirus E1A gene product is a potent transcriptional activator and nuclear oncoprotein. Like other regulatory proteins, E1A has a short half-life, in the range of 30 to 120 min. This short half-life, which was measured in cells synthesizing E1A, is not observed in cells injected with E1A protein made in bacteria or in vitro. In these cases, E1A is essentially refractory to degradation. In an attempt to reconcile this apparent paradox, we suggested that E1A was marked for degradation during its synthesis. Furthermore, we showed that a domain in the amino terminus of E1A was required for rapid degradation in cells translating E1A mRNA (J. M. Slavicek, N. C. Jones, and J. D. Richter, EMBO J. 7:3171-3180, 1988). In this study, we have used Xenopus laevis oocytes injected with mRNAs encoding altered E1A proteins to show that the amino-terminal tetrapeptide Met-Arg-His-Ile is required for E1A degradation. Even conservative amino acid substitutions in this degradation sequence render it nonfunctional. This degradation sequence can function as a transferable signal, since it induces instability when fused to another normally stable protein. Furthermore, the degradation sequence requires a proximity of no more than six residues from the amino terminus for activity. These data suggest that a trans-acting factor recognizes the amino terminus of E1A during the translation of its message to mark the protein for subsequent destruction.

Activation in vitro of RNA polymerase II and III directed transcription by baculovirus produced E1A protein

The baculovirus expression system has been successfully used to overproduce a number of different protein products. In this report we describe the construction of a recombinant baculovirus containing the adenovirus E1A 13s cDNA sequence. Infection of insect cells with this virus results in the production of phosphorylated E1A protein. The phosphorylation pattern appears to be similar to the complex pattern associated with E1A protein synthesis in mammalian cells. Purified baculovirus generated E1A protein activated transcription of specific poIIII promoters both in microinjected Xenopus laevis oocytes and in HeLa cell in vitro transcription extracts. The protein also stimulates in vitro transcription of the poIIII transcribed VA1 gene.

Novel phenotype of C3H 10T1/2 fibroblasts cotransfected with the c-Ha-ras and adenovirus 5 E1A oncogenes

C3H 10T1/2 fibroblasts are converted to fully transformed phenotype following coexpression of an activated c-Ha-ras gene and either a constitutively expressed viral or cellular myc gene. In this report, we examined whether the early region 1A (E1A) of adenovirus 5, which synergizes with ras to convert primary embryonic cells to a transformed phenotype, can synergize with ras to transform the established mouse embryonic cell line, C3H 10T1/2. We demonstrate that coexpression of ras and E1A generated a transformed phenotype that could be scored by colony assays and by soft agarose assays but not by standard focus assays. The ras-E1A-transformed phenotype relies on sequences present in conserved regions 1 and 2 of the E1A proteins and, in part, on information encoded by the extreme carboxy terminus of E1A. The contrast between the transformed phenotypes generated following the transfection of C3H 10T1/2 cells with either ras and myc or ras and E1A suggests that myc and E1A cooperate with ras to transform C3H 10T1/2 cells by mechanisms that can be distinguished using this established cell line as a model system.

Mutations that affect phosphorylation of the adenovirus DNA-binding protein alter its ability to enhance its own synthesis

The multifunctional adenovirus single-strand DNA-binding protein (DBP) is highly phosphorylated. Its phosphorylation sites are located in the amino-terminal domain of the protein, and its DNA- and RNA-binding activity resides in the carboxy-terminal half of the polypeptide. We have substituted cysteine or alanine for up to 10 of these potential phosphorylation sites by using oligonucleotide-directed mutagenesis. Alteration of one or a few of these sites had little effect on the viability of virus containing the mutated DBP. However, when eight or more sites were altered, viral growth decreased significantly. This suggests that the overall phosphorylation state of the protein was more important than whether any particular site was modified. The reduction in growth correlated with both depressed DNA replication and expression of late genes. This reduction was probably the result of lower DBP accumulation in mutant-infected cells. Interestingly, although the stability of the mutated DBP was not affected, DBP synthesis and the level of its mRNA were depressed 5- to 10-fold for the underphosphorylated protein. These results suggest that DBP enhances its own expression and imply that phosphorylation of the DBP may be important for this function. Similarities to several eucaryotic transcriptional activators, which are composed of negatively charged activating domains and separate binding domains, are discussed.

A karyophilic signal sequence in adenovirus type 5 E1A is functional in Xenopus oocytes but not in somatic cells

The carboxy-terminal five amino acids of the adenovirus type 5 E1A gene product are necessary and sufficient for this protein to become localized in the nuclei of somatic cells. In this report, we demonstrate that E1A contains a second nuclear localization signal, which resides within residues 140 to 185 and which functions in Xenopus oocytes but not Xenopus or mammalian somatic cells. These data demonstrate the cell-type-specific utilization of a nuclear localization signal.

Poly(A) elongation during Xenopus oocyte maturation is required for translational recruitment and is mediated by a short sequence element

Xenopus oocytes contain several mRNAs that are mobilized into polysomes only at the completion of meiosis (maturation) or at specific times following fertilization. To investigate the mechanisms that control translation during early development, we have focused on an mRNA, termed G10, that is recruited for translation during oocyte maturation. Coincident with its translation, the poly(A) tail of this message is elongated from approximately 90 to 200 adenylate residues. To identify the cis sequence that is required for this cytoplasmic adenylation and recruitment, we have synthesized wild-type and deletion mutant G10 mRNAs with SP6 polymerase. When injected into oocytes that subsequently were induced to mature with progesterone, wild-type G10 mRNA, but not mutant transcripts lacking a 50-base sequence in the 3'-untranslated region, was polyadenylated and recruited for translation. The 50-base sequence was sufficient to confer polyadenylation and translation when fused to globin mRNA, which does not normally undergo these processes during oocyte maturation. Further mutational analysis of this region revealed that a U-rich sequence 5' to the AAUAAA hexanucleotide nuclear polyadenylation signal, as well as the hexanucleotide itself, were both required for polyadenylation and translation. The 50-base cis element directs polyadenylation, but not translation per se, as a transcript that terminates with 3'-deoxyadenosine (cordycepin) is not recruited for translation. The available data suggest that the dynamic process of polyadenylation, and not the length of the poly(A) tail, is required for translational recruitment during oocyte maturation.

Analysis of phosphorylation sites in the exon 1 region of E1A proteins of human adenovirus type 5

Early region 1A (E1A) of human adenovirus type 5 (Ad5) produces five mRNAs that encode proteins of 55, 171, 217, 243, and 289 residues. We have shown previously that the major products of 289 and 243 residues are phosphorylated at a minimum of three sites of which one, Ser-89, is located in the amino terminal half of the protein. In the present report we show that these E1A proteins are also phosphorylated at a second site in this region located at Ser-96. The 171 and 217 residue E1A species were also tentatively identified and, as predicted, neither contained the Ser-89 or Ser-96 sites but both appeared to be phosphorylated at the same sites as 289R and 243R toward the carboxy terminus. Studies with mutants in which Ser-89 or Ser-96 were converted to alanine residues indicated that phosphorylation of Ser-89 but not Ser-96 induces the major shift in gel mobility of E1A products. However, neither site appears to be of major importance in the regulation of E1A-mediated transactivation or transformation.

Phosphorylation of serine residue 89 of human adenovirus E1A proteins is responsible for their characteristic electrophoretic mobility shifts, and its mutation affects biological function

The shift in mobility on sodium dodecyl sulfate-polyacrylamide gel electrophoresis that is characteristic of the adenovirus E1A proteins is the result of posttranslational modification. In the present study, we demonstrate that phosphorylation of bacterially produced E1A in higher cell extracts occurs on serine and is responsible for the mobility shift. E1A protein expressed in Saccharomyces cerevisiae also undergoes the mobility shift due to serine phosphorylation. Site-directed mutagenesis was used to identify the serine residue responsible for the mobility shift. Six serine residues were altered to glycine within E1A. Substitution at serine residue 89 was shown to selectively prevent the mobility shift of both the 289R and 243R E1A proteins. We conclude that phosphorylation at serine 89 is the specific modification responsible for the mobility shift of E1A. Moreover, we demonstrate that the Ser-89-to-Gly mutation has no effect on trans activation or complementation of an E1A-deficient adenovirus. In contrast, the mutant protein does significantly reduce both the repression and transformation efficiency of E1A. The five other Ser-to-Gly mutation were also examined for functional effects. None affected trans activation, whereas repression and transformation functions were affected. One mutant affected transformation without affecting repression, suggesting that these functions are to some degree also separable. The relevance of phosphorylation to structure and activity of E1A and other nuclear oncogene proteins is discussed.

Phosphorylation at serine 89 induces a shift in gel mobility but has little effect on the function of adenovirus type 5 E1A proteins

Phosphorylation at serine 89 was shown to be the major cause of the shift in gel migration of the 289R and 243R early region 1A (E1A) proteins of human adenovirus type 5. However, conversion of Ser-89 to alanine by site-directed mutagenesis did not abolish E1A transactivating or transforming activities, suggesting that phosphorylation at this site is not necessary for these E1A functions.

The regions of the herpes simplex virus type 1 immediate early protein Vmw175 required for site specific DNA binding closely correspond to those involved in transcriptional regulation

The immediate-early (IE) protein Vmw175 (ICP4) of HSV-1 is required for the transcription of later classes of viral genes and the repression of IE gene expression. We have previously constructed a panel of plasmid-borne insertion and deletion mutants of the gene encoding Vmw175 and assayed their ability to regulate transcription in transient transfection assays. By this approach we have mapped the regions of the Vmw175 amino acid sequence that are required for transcriptional activation and repression of herpes virus promoters. This paper describes the use of nuclear extracts, made from cells transfected with these mutant plasmids, in gel retardation DNA binding assays in order to define the regions of Vmw175 involved in binding to a specific Vmw175 DNA binding site. The results show that amino acid residues 275-495 (a region which is highly conserved between Vmw175 and the varicella-zoster virus "IE" 140K protein) include structures which are critically required for specific DNA binding, transactivation and repression. This raises the interesting paradox that although the specific DNA sequence recognized by Vmw175 is not commonly found in its target promoters, the protein domain required for recognition of this sequence is required for promoter activation.

Rapid turnover of adenovirus E1A is determined through a co-translational mechanism that requires an aminoterminal domain

The product of the adenovirus E1A 13S mRNA can both stimulate and repress the expression of certain viral and cellular genes. As with several other regulatory proteins, E1A has a short half-life, approximately 40 min. Although this short half-life is observed in cells expressing the E1A gene, it is not the case with cells injected with E1A protein, where its half-life is very long, generally greater than 15 h. We have sought to reconcile these apparent differences in E1A stability. Using Xenopus oocytes, we find that E1A exhibits its characteristic short half-life when it is synthesized from injected mRNA while it has a very long half-life when it is injected as a protein synthesized originally in Escherichia coli or reticulocyte lysates. In order to delineate the amino acids responsible for rapid E1A turnover, several deletion mRNAs were constructed, injected into oocytes, and E1A half-life determined. Carboxyl-terminal deletions and an internal deletion of residues 38-86 failed to increase the half-life of E1A. In contrast, amino-terminal deletions of 70 and 14 residues resulted in very stable E1A proteins (t1/2 greater than 20 h). Furthermore, deletion of the second amino acid, an arginine, resulted in a stable E1A protein. The amino-terminal region of E1A was able to induce the rapid turnover of a normally stable protein, beta-globin, in oocytes injected with an E1A-globin chimeric mRNA. This E1A-induced instability of globin was abolished, however, when the protein was first synthesized in reticulocyte lysates and then injected into oocytes. The amino-terminal region of E1A is also important in governing halflife in adenovirus-infected HeLa cells. These results demonstrate that the half-life of E1A is established cotranslationally through a mechanism involving sequences within the amino-terminal 37 residues.