Adenovirus E1B 55-kilodalton oncoprotein inhibits p53 acetylation by PCAF

Acetylation of p53 augments its site-specific DNA binding both in vitro and in vivo

p53 promotes tumor suppression through its ability to function as a transcriptional factor and is activated by posttranslational modifications that include acetylation. Our earlier study demonstrated that p53 acetylation can enhance its sequence-specific DNA binding in vitro, and this notion was later confirmed in several other studies. However, a recent study has reported that in vitro acetylation of p53 fails to stimulate its DNA binding to large DNA fragments, raising an important issue that requires further investigation. Here, we show that unacetylated p53 is able to bind weakly to its consensus site within the context of large DNA fragments, although it completely fails to bind the same site within short oligonucleotide probes. Strikingly, by using highly purified and fully acetylated p53 proteins obtained from cells, we show that acetylation of the C-terminal domain can dramatically enhance site-specific DNA binding on both short oligonucleotide probes and long DNA fragments. Moreover, endogenous p53 apparently can be fully acetylated in response to DNA damage when both histone deacetylase complex 1 (HDAC1)- and Sir2-mediated deacetylation are inhibited, indicating dynamic p53 acetylation and deacetylation events during the DNA damage response. Finally, we also show that acetylation of endogenous p53 indeed significantly augments its ability to bind an endogenous target gene and that p53 acetylation levels correlate well with p53-mediated transcriptional activation in vivo. Thus, our results clarify some of the confusion surrounding acetylation-mediated effects on p53 binding to DNA and suggest that acetylation of p53 in vivo may contribute, at least in part, to its transcriptional activation functions.

The diverse superfamily of lysine acetyltransferases and their roles in leukemia and other diseases

Acetylation of the epsilon-amino group of lysine residues, or N(epsilon)-lysine acetylation, is an important post-translational modification known to occur in histones, transcription factors and other proteins. Since 1995, dozens of proteins have been discovered to possess intrinsic lysine acetyltransferase activity. Although most of these enzymes were first identified as histone acetyltransferases and then tested for activities towards other proteins, acetyltransferases only modifying non-histone proteins have also been identified. Lysine acetyltransferases form different groups, three of which are Gcn5/PCAF, p300/CBP and MYST proteins. While members of the former two groups mainly function as transcriptional co-activators, emerging evidence suggests that MYST proteins, such as Esa1, Sas2, MOF, TIP60, MOZ and MORF, have diverse roles in various nuclear processes. Aberrant lysine acetylation has been implicated in oncogenesis. The genes for p300, CBP, MOZ and MORF are rearranged in recurrent leukemia-associated chromosomal abnormalities. Consistent with their roles in leukemogenesis, these acetyltransferases interact with Runx1 (or AML1), one of the most frequent targets of chromosomal translocations in leukemia. Therefore, the diverse superfamily of lysine acetyltransferases executes an acetylation program that is important for different cellular processes and perturbation of such a program may cause the development of cancer and other diseases.

Resistance to UV-induced apoptosis in human keratinocytes during accelerated senescence is associated with functional inactivation of p53

Compared to proliferating keratinocytes (KCs), growth-arrested KCs are relatively resistant to UV-light induced apoptosis. When KCs undergo confluency, or following exposure to anti-proliferative agents such as IFN-gamma plus a phorbol ester-12-O-tetradecanoylyphorbol-13-acetate (TPA), they convert from a proliferative to a nonproliferative state resembling senescence. Since p53 regulates UV-induced apoptosis of KCs, this report further characterizes p53 half-life, post-translational modifications, and transcriptional activity using cultured human KCs and living epidermal equivalents. The half-life of p53 in KCs was longer than fibroblasts (greater than approximately 3 h vs. 30 min). Exposure of proliferating KCs to UV-light induces post-translational modifications of p53 including acetylation of lysine-382 residues. By contrast, KCs undergoing irreversible growth arrest following confluency, or exposure to IFN-gamma plus TPA, were resistant to UV-induced apoptosis, and failed to undergo the acetylation modification of p53. Exposure of KCs to IFN-gamma plus TPA reduced total cellular p53 levels and reduced the transcriptional activity of p53. Addition of Trichostatin A (TSA), an inhibitor of de-acetylation, increased acetylation of lysine-382 in confluent KCs, thereby enhancing susceptibility of confluent cultures to UV-induced apoptosis. Pre-treatment of epidermal equivalents with IFN-gamma plus TPA also blocked UV-light induced increase in p53 levels, and reduced apoptosis. In conclusion, these studies demonstrate that growth arrested KCs may resist UV-light induced apoptosis by inactivating the pro-apoptotic function of p53.

Sequestration of p53 in the cytoplasm by adenovirus type 12 E1B 55-kilodalton oncoprotein is required for inhibition of p53-mediated apoptosis

The adenovirus E1B 55-kDa protein is a potent inhibitor of p53-mediated transactivation and apoptosis. The proposed mechanisms include tethering the E1B repression domain to p53-responsive promoters via direct E1B-p53 interaction. Cytoplasmic sequestration of p53 by the 55-kDa protein would impose additional inhibition on p53-mediated effects. To investigate further the role of cytoplasmic sequestration of p53 in its inhibition by the E1B 55-kDa protein we systematically examined domains in both the Ad12 55-kDa protein and p53 that underpin their colocalization in the cytoplasmic body and show that the N-terminal transactivation domain (TAD) of p53 is essential for retaining p53 in the cytoplasmic body. Deletion of amino acids 11 to 27 or even point mutation L22Q/W23S abolished the localization of p53 to the cytoplasmic body, whereas other parts of TAD and the C-terminal domain of p53 are dispensable. This cytoplasmic body is distinct from aggresome associated with overexpression of some proteins, since it neither altered vimentin intermediate filaments nor associated with centrosome or ubiquitin. Formation of this structure is sensitive to mutation of the Ad12 55-kDa protein. Strikingly, mutation S476/477A near the C terminus of the Ad12 55-kDa protein eliminated the formation of the cytoplasmic body. The equivalent residues in the Ad5 55-kDa protein were shown to be critical for its ability to inhibit p53. Indeed, Ad12 55-kDa mutants that cannot form a cytoplasmic body can no longer inhibit p53-mediated effects. Conversely, the Ad12 55-kDa protein does not suppress p53 mutant L22Q/W23S-mediated apoptosis. Finally, we show that E1B can still sequester p53 that contains the mitochondrial import sequence, thereby potentially preventing the localization of p53 to mitochondria. Thus, cytoplasmic sequestration of p53 by the E1B 55-kDa protein plays an important role in restricting p53 activities.

PCAF is a coactivator for p73-mediated transactivation

The tumor suppressor p53-related p73 shares significant amino-acid sequence identity with p53. Like p53, p73 recognizes canonical p53 DNA-binding sites and activates p53-responsive target genes and induces apoptosis. Moreover, transcription coactivator p300/CBP binds to and coactivates with both p53 and p73 in stimulating the expression of their target genes. Here, we report that coactivator PCAF binds to p73. The N-terminal transactivation domain (TAD) and the conserved oligomerization domain (OD) of p73 are both required for its interaction with PCAF. Conversely, PCAF's HAT-domain is required for and both the N-terminal region and Bromo domain enhance binding of PCAF to p73. Significantly, PCAF stimulates p73-mediated transactivation, and binding of PCAF to p73 is necessary for p73's transactivation activity. PCAF-specific siRNA dramatically reduces p73-mediated transactivation. Stimulation of p73-mediated transactivation by PCAF requires the HAT domain of PCAF and the p53-binding site within the p21 promoter. In vivo, coexpression of wild-type, but not HAT-deficient PCAF with p73beta markedly increases p21 expression. Furthermore, cotransfection of PCAF and p73 leads to increased apoptosis and reduced colony formation. Collectively, these data suggest that p73 recruit PCAF to specific promoters to activate the transcription of p73 target genes.

Cell transformation by human adenoviruses

The last 40 years of molecular biological investigations into human adenoviruses have contributed enormously to our understanding of the basic principles of normal and malignant cell growth. Much of this knowledge stems from analyses of their productive infection cycle in permissive host cells. Also, initial observations concerning the carcinogenic potential of human adenoviruses subsequently revealed decisive insights into the molecular mechanisms of the origins of cancer, and established adenoviruses as a model system for explaining virus-mediated transformation processes. Today it is well established that cell transformation by human adenoviruses is a multistep process involving several gene products encoded in early transcription units 1A (E1A) and 1B (E1B). Moreover, a large body of evidence now indicates that alternative or additional mechanisms are engaged in adenovirus-mediated oncogenic transformation involving gene products encoded in early region 4 (E4) as well as epigenetic changes resulting from viral DNA integration. In particular, detailed studies on the tumorigenic potential of subgroup D adenovirus type 9 (Ad9) E4 have now revealed a new pathway that points to a novel, general mechanism of virus-mediated oncogenesis. In this chapter, we summarize the current state of knowledge about the oncogenes and oncogene products of human adenoviruses, focusing particularly on recent findings concerning the transforming and oncogenic properties of viral proteins encoded in the E1B and E4 transcription units.

Modulation of retinoid signaling by a cytoplasmic viral protein via sequestration of Sp110b, a potent transcriptional corepressor of retinoic acid receptor, from the nucleus

Hepatitis C virus (HCV) core protein (core) plays a significant role in the development of chronic liver diseases caused by HCV infection. We have discovered that the core sensitized all-trans-retinoic acid (ATRA)-induced cell death in MCF-7 cells. Activation of retinoic acid receptor alpha (RARalpha)-mediated transcription by the core was also seen in all the cell lines tested. By use of a yeast two-hybrid system, we identified Sp110b as a candidate for a core-interacting cellular factor. Although the function of Sp110b has remained unknown, we observed that Sp110b interacts with RARalpha and suppresses RARalpha-mediated transcription. These data suggest that Sp110b is a transcriptional cofactor negatively regulating RARalpha-mediated transcription. RNA interference-mediated reduction of endogenous Sp110b levels depressed the ability of the core to activate RARalpha-mediated transcription, suggesting an essential role for Sp110b in this pathway. The normal nuclear subcellular localization of Sp110b was altered by molecular interaction with the core to the cytoplasmic surface of the endoplasmic reticulum. This evidence suggests a model in which the core sequesters Sp110b from the nucleus and inactivates its corepressor function to activate RARalpha-mediated transcription. These findings likely describe a novel system in which a cytoplasmic viral protein regulates host cell transcription.

Adenovirus E1B 55-kilodalton oncoprotein binds to Daxx and eliminates enhancement of p53-dependent transcription by Daxx

The adenovirus E1B 55-kDa protein impairs the p53 pathway and enhances transformation, although the underlying mechanisms remain to be defined. We found that Daxx binds to the E1B 55-kDa protein in a yeast two-hybrid screen. The two proteins interact through their C termini. Mutation of three potential phosphorylation sites (S489/490 and T494 to alanine) within the E1B 55-kDa protein did not affect its interaction with Daxx, although such mutations were previously shown to inhibit E1B's ability to repress p53-dependent transcription and to enhance transformation. In addition to their coimmunoprecipitation in 293 extracts, purified Daxx interacted with the E1B 55-kDa protein in vitro, indicating their direct interaction. In 293 cells, Daxx colocalized with the E1B 55-kDa protein within discrete nuclear dots, where p53 was also found. Such structures were distinct from PML (promyelocytic leukemia protein) bodies, and it appeared that Daxx was displaced from PML bodies. Thus, the Daxx concentration was diminished in dots with a prominent presence of PML and vice versa. Indeed, PML overexpression led to dramatic redistribution of Daxx from p53-E1B 55-kDa protein complexes to PML bodies. Additionally, expression of the E1B 55-kDa protein in Saos2 osteosarcoma cells reduced the number of PML bodies. Our data suggest that E1B and PML compete for available Daxx in the cell. Surprisingly, Daxx significantly augmented p53-mediated transcription and the E1B 55-kDa protein eliminated this effect. Thus, it is likely that the E1B 55-kDa protein sequesters Daxx and p53 in specific nuclear locations, where p53 cannot activate transcription. One consequence of the Daxx-E1B interaction might be an alteration of normal interactions of Daxx, PML, and p53, which may contribute to cell transformation.

Interaction of the HPV E7 proteins with the pCAF acetyltransferase

Most cervical carcinomas express the E6 and E7 proteins of a high-risk human papillomavirus (HPV). These proteins affect growth control by interfering with the functions of cell regulatory proteins, promoting oncogenic transformation. A key target of E7 is the tumor suppressor protein pRb, which directly interacts with E7. However, binding to additional cellular regulatory proteins is clearly required for oncogenesis, as mutants of E7 have been identified that bind to pRb, yet fail to transform efficiently. Here we demonstrate the interaction of the HPV 6, 16 and 18 E7 proteins with the pCAF acetyltransferase, which has been reported to function as a coactivator for a variety of transcription factors including p53. Mutation of a highly conserved leucine residue within the zinc finger region of HPV 16 E7 disrupts binding to pCAF and also impairs transformation and transcriptional activation. HPV 16 E7 interacts with the acetyltransferase domain of pCAF, and reduces its acetyltransferase activity in vitro. Our analysis of the interaction between the pCAF acetyltransferase and E7 provides new insight into the mechanisms by which the E7 oncoproteins can alter cellular gene expression and growth.

Human immunodeficiency virus type-1 Tat/co-activator acetyltransferase interactions inhibit p53Lys-320 acetylation and p53-responsive transcription

Patients with AIDS are at increased risk for developing various neoplasms, including Hodgkin's and non-Hodgkin's lymphomas, Kaposi's sarcomas, and anal-rectal carcinomas, suggestive that human immunodeficiency virus type-1 infection might promote establishment of AIDS-related cancers. Tat, the viral trans-activator, can be endocytosed by uninfected cells and has been shown to inhibit p53 functions, providing a candidate mechanism through which the human immunodeficiency virus type-1 might contribute to malignant transformation. Because Tat has been shown to interact with histone acetyltransferase domains of p300/cAMP-responsive element-binding protein (CREB)-binding protein and p300/CREB-binding protein-associated factor, we have investigated whether Tat might alter p53 acetylation and tumor suppressor-responsive transcription. Here, we demonstrate that both Tat and p53 co-localize with p300/CREB-binding protein-associated factor and p300 in nuclei of IMR-32 human neuroblastoma cells and in PC-12 pheochromocytoma cells. Further, p53 trans-activation of the 14-3-3varsigma promoter was markedly repressed by Tat-histone acetyltransferase interactions, and p53 acetylation by p300/CREB-binding protein-associated factor on residue Lys(320) was diminished as a result of Tat-histone acetyltransferase binding in vivo and in vitro. Tat also inhibited p53 acetylation by p300 in a dosage-dependent manner in vitro. Finally, HIV-1-infected Molt-4 cells displayed reduced p53 acetylation on lysines 320 and 373 in response to UV irradiation. Our results allude to a mechanism whereby the human immunodeficiency virus type-1 trans-activator might impair tumor suppressor functions in immune/neuronal-derived cells, thus favoring the establishment of neoplasia during AIDS.

The viral control of cellular acetylation signaling

It is becoming clear that the post-translational modification of histone and non-histone proteins by acetylation is part of an important cellular signaling process controlling a wide variety of functions in both the nucleus and the cytoplasm. Recent investigations designate this signaling pathway as one of the primary targets of viral proteins after infection. Indeed, specific viral proteins have acquired the capacity to interact with cellular acetyltransferases (HATs) and deacetylases (HDACs) and consequently to disrupt normal acetylation signaling pathways, thereby affecting viral and cellular gene expression. Here we review the targeting of cellular HATs and HDACs by viral proteins and highlight different strategies adopted by viruses to control cellular acetylation signaling and to accomplish their life cycle.

Acetylation of p53 inhibits its ubiquitination by Mdm2

In response to DNA damage, the activity of the p53 tumor suppressor is modulated by protein stabilization and post-translational modifications including acetylation. Interestingly, both acetylation and ubiquitination can modify the same lysine residues at the C terminus of p53, implicating a role of acetylation in the regulation of p53 stability. However, the direct effect of acetylation on Mdm2-mediated ubiquitination of p53 is still lacking because of technical difficulties. Here, we have developed a method to obtain pure acetylated p53 proteins from cells, and by using an in vitro purified system, we provide the direct evidence that acetylation of the C-terminal domain is sufficient to abrogate its ubiquitination by Mdm2. Importantly, even in the absence of DNA damage, acetylation of the p53 protein is capable of reducing the ubiquitination levels and extending its half-life in vivo. Moreover, we also show that acetylation of p53 can affect its ubiquitination through other mechanisms in addition to the site competition. This study has significant implications regarding a general mechanism by which protein acetylation modulates ubiquitination-dependent proteasome proteolysis.

The Epstein-Barr virus immediate-early protein BZLF1 regulates p53 function through multiple mechanisms

The Epstein-Barr virus (EBV) immediate-early protein BZLF1 is a transcriptional activator that mediates the switch between the latent and the lytic forms of EBV infection. It was previously reported that BZLF1 inhibits p53 transcriptional function in reporter gene assays. Here we further examined the effects of BZLF1 on p53 function by using a BZLF1-expressing adenovirus vector (AdBZLF1). Infection of cells with the AdBZLF1 vector increased the level of cellular p53 but prevented the induction of p53-dependent cellular target genes, such as p21 and MDM2. BZLF1-expressing cells had increased p53-specific DNA binding activity in electrophoretic mobility shift assays, increased p53 phosphorylation at multiple residues (including serines 6, 9, 15, 33, 46, 315, and 392), and increased acetylation at lysine 320 and lysine 382. Thus, the inhibitory effects of BZLF1 on p53 transcriptional function cannot be explained by its effects on p53 phosphorylation, acetylation, or DNA binding activity. BZLF1 substantially reduced the level of cellular TATA binding protein (TBP) in both normal human fibroblasts and A549 cells, and the inhibitory effects of BZLF1 on p53 transcriptional function could be partially rescued by the overexpression of TBP. Thus, BZLF1 has numerous effects on p53 posttranslational modification but may inhibit p53 transcriptional function in part through an indirect mechanism involving the suppression of TBP expression.

Homogeneous time-resolved fluorescence assay for identifying p53 interactions with its protein partners, directly in a cellular extract

The p53 protein is a tumor suppressor that protects the organism against malignant consequences of DNA damage. Interaction of p53 with numerous cellular or viral proteins regulates its functional activity either positively or negatively. An approach leading to identification of such protein interactions directly in a cell extract could be of help in the development of screening assays to search for drugs acting on p53 in its cellular environment, either by disrupting its association with inhibitory proteins or by increasing its affinity for activating proteins. We show that the homogeneous time-resolved fluorescence (HTRF) assay based on the time-resolved amplified cryptate emission (TRACE) technology allows identification of such an interaction by simply adding a mixture of two labeled monoclonal antibodies, directly in a cellular extract. We validate this assay by studying p53/SV40-LTAg interactions. The antibodies directed against genuine p53 and SV40-LTAg epitopes were labeled with europium cryptate (donor) and XL665, a crosslinked allophycocyanin (acceptor), respectively. We demonstrated that a nonradiative energy transfer occurs between labeled antibodies only when p53 interacts with SV40-LTag, which opens up the possibility of extending this approach to other p53 partners to search for drugs that restore p53 tumor-suppressor activity.

Analysis of the adenovirus E1B-55K-anchored proteome reveals its link to ubiquitination machinery

During the early phase of infection, the E1B-55K protein of adenovirus type 5 (Ad5) counters the E1A-induced stabilization of p53, whereas in the late phase, E1B-55K modulates the preferential nucleocytoplasmic transport and translation of the late viral mRNAs. The mechanism(s) by which E1B-55K performs these functions has not yet been clearly elucidated. In this study, we have taken a proteomics-based approach to identify and characterize novel E1B-55K-associated proteins. A multiprotein E1B-55K-containing complex was immunopurified from Ad5-infected HeLa cells and found to contain E4-orf6, as well as several cellular factors previously implicated in the ubiquitin-proteasome-mediated destruction of proteins, including Cullin-5, Rbx1/ROC1/Hrt1, and Elongins B and C. We further demonstrate that a complex containing these as well as other proteins is capable of directing the polyubiquitination of p53 in vitro. These ubiquitin ligase components were found in a high-molecular-mass complex of 800 to 900 kDa. We propose that these newly identified binding partners (Cullin-5, Elongins B and C, and Rbx1) complex with E1B-55K and E4-orf6 during Ad infection to form part of an E3 ubiquitin ligase that targets specific protein substrates for degradation. We further suggest that E1B-55K functions as the principal substrate recognition component of this SCF-type ubiquitin ligase, whereas E4-orf6 may serve to nucleate the assembly of the complex. Lastly, we describe the identification and characterization of two novel E1B-55K interacting factors, importin-alpha 1 and pp32, that may also participate in the functions previously ascribed to E1B-55K and E4-orf6.

MDM2 inhibits PCAF (p300/CREB-binding protein-associated factor)-mediated p53 acetylation

Our previous study shows that MDM2, a negative feedback regulator of the tumor suppressor p53, inhibits p300-mediated p53 acetylation. Because PCAF (p300/CREB-binding protein-associated factor) also acetylates and activates p53 after DNA damage, in this study we have examined the effect of MDM2 on PCAF-mediated p53 acetylation. We have found that MDM2 inhibited p53 acetylation by PCAF in vitro. In addition, when overexpressed, MDM2 inhibited PCAF-mediated p53 acetylation in cells. MDM2 interacted with PCAF both in vitro and in cells, as assessed using GST fusion protein interaction and immunoprecipitation assays, respectively. Consistent with the above results, MDM2 significantly repressed the activation of p53 transcriptional activity by PCAF without apparently affecting the level of p53. In addition, MDM2 co-resided with p53 at the p53-responsive mdm2 and p21(waf1/cip1) promoters, inhibiting expression of the endogenous p21(waf1/cip1). These results demonstrate that MDM2 can inhibit PCAF-mediated p53 acetylation and activation.

Transcriptional regulation of the mdm2 oncogene by p53 requires TRRAP acetyltransferase complexes

The p53 tumor suppressor regulates the cellular response to genetic damage through its function as a sequence-specific transcription factor. Among the most well-characterized transcriptional targets of p53 is the mdm2 oncogene. Activation of mdm2 is critical in the p53 pathway because the mdm2 protein marks p53 for proteosome-mediated degradation, thereby providing a negative-feedback loop. Here we show that the ATM-related TRRAP protein functionally cooperates with p53 to activate mdm2 transcription. TRRAP is a component of several multiprotein acetyltransferase complexes implicated in both transcriptional regulation and DNA repair. In support of a role for these complexes in mdm2 expression, we show that transactivation of the mdm2 gene is augmented by pharmacological inhibition of cellular deacetylases. In vitro analysis demonstrates that p53 directly binds to a TRRAP domain previously shown to be an activator docking site. Furthermore, transfection of cells with antisense TRRAP blocks p53-dependent transcription of mdm2. Finally, using chromatin immunoprecipitation, we demonstrate direct p53-dependent recruitment of TRRAP to the mdm2 promoter, followed by increased histone acetylation. These findings suggest a model in which p53 directly recruits a TRRAP/acetyltransferase complex to the mdm2 gene to activate transcription. In addition, this study defines a novel biochemical mechanism utilized by the p53 tumor suppressor to regulate gene expression.

SUMO-1 modification required for transformation by adenovirus type 5 early region 1B 55-kDa oncoprotein

SUMO-1 is a small ubiquitin-related modifier protein that is covalently linked to many cellular and viral protein targets. Modification by SUMO-1 is proposed to play a role in protein targeting and/or stability. We show here that adenovirus type 5 early region 1B 55-kDa (E1B-55kDa) oncoprotein can be covalently modified by SUMO-1 in vivo through a major attachment site comprising a single lysine residue at amino acid position 104. The sequence surrounding this lysine matches the proposed PsiKxE consensus motif required for SUMO-1 conjugation. A single mutation (K104R) that abolishes SUMOylation of E1B-55kDa dramatically reduces the ability of the adenovirus type 5 protein to transform primary baby rat kidney cells in cooperation with E1A and to inhibit p53-mediated transactivation. Overexpression of SUMO-1 in adenovirus type 5 E1A/E1B-55kDa-transformed baby rat kidney cells causes the relocalization of E1B-55kDa from the cytoplasm to the nucleus, where it accumulates with SUMO-1 in dot- or track-like structures. Significantly, when SUMO-1 is ectopically expressed in transformed rat cells no effect on the cytoplasmic localization of the E1B-K104R mutant protein is observed. Our results demonstrate that SUMO-1 modification is required for transformation by adenovirus type 5 E1B-55kDa and provide further evidence for the idea that this posttranslational modification plays a role in protein targeting to specific subcellular sites.

Inhibition of p53 tumor suppressor by viral interferon regulatory factor

The irreversible cell cycle arrest and apoptosis induced by p53 are part of the host surveillance mechanisms for viral infection and tumor induction. Kaposi's sarcoma-associated herpesvirus (KSHV), the most recently discovered human tumor virus, is associated with the pathogenesis of Kaposi's sarcoma, primary effusion lymphoma, and multicentric Castleman's disease. The K9 open reading frame of KSHV encodes a viral interferon (IFN) regulatory factor (vIRF) which functions as a repressor for cellular IFN-mediated signal transduction and as an oncoprotein to induce cell growth transformation. Here, we demonstrate that KSHV vIRF interacts with the cellular p53 tumor suppressor through the putative DNA binding region of vIRF and the central region of p53. This interaction suppresses the level of phosphorylation and acetylation of p53 and inhibits transcriptional activation of p53. As a consequence, vIRF efficiently prevents p53-mediated apoptosis. These results suggest that KSHV vIRF interacts with and inhibits the p53 tumor suppressor to circumvent host growth surveillance and to facilitate uncontrolled cell proliferation.

p53 Stimulates TFIID-TFIIA-promoter complex assembly, and p53-T antigen complex inhibits TATA binding protein-TATA interaction

Simian virus 40 large T antigen has been shown to inhibit p53-mediated transcription once tethered to p53-responsive promoters through interaction with p53. In this study we report that p53 stimulates transcription by enhancing the recruitment of the basal transcription factors, TFIIA and TFIID, on the promoter (the DA complex) and by inducing a conformational change in the DA complex. Significantly, we have demonstrated that T antigen inhibits p53-mediated transcription by blocking this ability of p53. We investigated the mechanism for this inhibition and found that DA complex formation was resistant to T-antigen repression when the TFIID-DNA complex was formed prior to addition of p53-T antigen complex, indicating that the T antigen, once tethered to the promoter by p53, targets TFIID. Further, we have shown that the p53-T antigen complex prevents the TATA binding protein from binding to the TATA box. Thus, these data suggest a detailed mechanism by which p53 activates transcription and by which T antigen inhibits p53-mediated transcription.

HATs on and beyond chromatin

The role of histone acetylation as a key mechanism of transcriptional regulation has been well established. Recent advances suggest that histone acetyltransferases also play important roles in histone-modulated processes such as DNA replication, recombination and repair. In addition, acetylation of transcriptional cofactors and other proteins is an efficient means of regulating a diverse range of molecular interactions. As new histone acetyltransferases and substrates are rapidly emerging, it is becoming apparent that protein acetylation may rival phosphorylation as a mechanism to transduce cellular regulatory signals.

Expression of homologues for p53 and p73 in the softshell clam (Mya arenaria), a naturally-occurring model for human cancer

Homologues for human p53 (Hsp53) and p73 (Hsp73) genes were cloned and expression patterns for their corresponding proteins analysed in tissues from normal and leukemic softshell clams (Mya arenaria). These are the first structural and functional data for p53 and p73 cDNAs and gene products in a naturally occurring, non-mammalian disease model. Core sequence of the predicted clam p53 (Map53) and p73 (Map73) proteins is virtually identical and includes the following highly conserved regions: the transcriptional activation domain (TAD), MDM2 binding site, ATM phosphorylation site, proline rich domain, DNA binding domains (DBDs) II-V, nuclear import and export signals and the tetramerization domain. The core sequence is a structural mosaic of the corresponding human proteins, with the TAD and DBDs resembling Hsp53 and Hsp73, respectively. This suggests that Map53 and Map73 proteins may function similarly to human proteins. Clam proteins have either a short (Map53) or long (Map73) C-terminal extension. These features suggest that Map53 and Map73 may be alternate splice variants of a p63/p73-like ancestral gene. Map73 is significantly upregulated in hemocytes and adductor muscle from leukemic clams. In leukemic hemocytes, both proteins are absent from the nucleus and sequestered in the cytoplasm. This observation suggests that a non-mutational p53/p73-dependent mechanism may be involved in the clam disease. Further studies of these gene products in clams may reveal p53/p73-related molecular mechanisms that are held in common with Burkitt's lymphoma or other human cancers.