Daxx and histone deacetylase II associate with chromatin through an interaction with core histones and the chromatin-associated protein Dek

Daxx represses expression of a subset of antiapoptotic genes regulated by nuclear factor-kappaB

Daxx is a nuclear protein that localizes to PML oncogenic domains, sensitizes cells to apoptosis, and functions as a transcriptional repressor. We found that Daxx represses the expression of several antiapoptotic genes regulated by nuclear factor-kappaB, including cIAP2, in human tumor cell lines. Daxx interacts with RelB and inhibits RelB-mediated transcriptional activation of the human cIAP2 gene promoter. Daxx also forms complexes with RelB while bound to its target sites in the cIAP2 promoter, as shown by electrophoretic mobility shift assays and chromatin immunoprecipitation experiments. Using cells from daxx-/- mouse embryos, we observed that levels of the corresponding murine c-IAP mRNA and protein are increased in cells lacking Daxx. Conversely, c-IAP mRNA and protein levels were reduced in relB-/- cells. Taken together, these observations provide a mechanism that links two previously ascribed functions of Daxx: transcriptional repression and sensitization to apoptosis.

Apoptosis inhibition by the human DEK oncoprotein involves interference with p53 functions

The DEK proto-oncogene has been associated with human carcinogenesis-either as a fusion with the CAN nucleoporin protein or when transcriptionally upregulated. Mechanisms of intracellular DEK functions, however, have remained relatively unexplored. We have recently demonstrated that DEK expression is induced by the high-risk human papillomavirus (HPV) E7 protein in a manner which is dependent upon retinoblastoma protein function and have implicated DEK in the inhibition of cellular senescence. Additionally, overexpression of DEK resulted in significant life span extension of primary human keratinocytes. In order to determine whether DEK expression is required for cellular proliferation and/or survival, we monitored cellular responses to the knockdown of DEK in cancer and primary cells. The results indicate that DEK expression protects both HPV-positive cancer and primary human cells from apoptotic cell death. Cell death in response to DEK depletion was accompanied by increased protein stability and transcriptional activity of the p53 tumor suppressor and consequent upregulation of known p53 target genes such as p21CIP and Bax. Consistent with a possible role for p53 in DEK-mediated cell death inhibition, the p53-negative human osteosarcoma cell line SAOS-2 was resistant to the knockdown of DEK. Finally, expression of a dominant negative p53 miniprotein inhibited DEK RNA interference-induced p53 transcriptional induction, as well as cell death, thus directly implicating p53 activation in the observed apoptotic phenotype. These findings suggest a novel role for DEK in cellular survival, involving the destabilization of p53 in a manner which is likely to contribute to human carcinogenesis.

Negative regulation of the RelA/p65 transactivation function by the product of the DEK proto-oncogene

NF-kappaB-mediated transcriptional activation is controlled at several levels including interaction with coregulatory proteins. To identify new proteins capable of modulating NF-kappaB-mediated activation, a cytoplasmic two-hybrid screen was performed using the p65 C-terminal transactivation domain as bait and identified the product of the DEK proto-oncogene. DEK is a ubiquitous nuclear protein that has been implicated in several types of cancer and autoimmune diseases. DEK appears to function in several nuclear processes including transcriptional repression and modulation of chromatin structure. Our data indicate that DEK functions as a transcriptional corepressor to repress NF-kappaB activity. DEK expression blocked p65-mediated activation of an NF-kappaB-dependent reporter gene and also inhibited TNFalpha-induced activation of the reporter gene. Chromatin Immunoprecipitation (ChIP) assays showed that DEK associates with the promoters of the NF-kappaB-regulated cIAP2 and IL-8 genes in untreated cells and dissociates from these promoters upon NF-kappaB binding in response to TNFalpha treatment. Moreover, the expression levels of an NF-kappaB-dependent reporter gene as well as the NF-kappaB-regulated Mcp-1 and IkappaBalpha genes is increased in DEK-/- cells compared with wild-type cells. ChIP assays on these promoters show enhanced and prolonged binding of p65 and increased recruitment of the P/CAF coactivator. Overall, these data provide further evidence that DEK functions to negatively regulate transcription.

SUMO regulates the cytoplasmonuclear transport of its target protein Daxx

It is known that Fas death domain-associated protein (Daxx) possesses both putative nuclear and cytoplasmic functions. However, the nuclear transport mechanism is largely unknown. This study examined the nuclear location signal (NLS) of Daxx and whether the nuclear transport of Daxx was mediated by small ubiquitin-related modifier (SUMO). Two NLS motifs of Daxx, leucine (L)-rich nuclear export signal (NES)-like motif (188IXXLXXLLXL197) and C-terminal lysine (K) rich NLS2 (amino acids 627-634) motif, were identified and the K630 and K631 on the NLS2 motif were characterized as the major sumoylation sites of Daxx by in vitro sumoylation analysis. Proteins of inactive SUMO (SUMO-delta), a sumoylation-incompetent mutant, and Daxx NLS mutants (Daxx-NES(mut) and Daxx NLS2(mut)) were dispersed in cytoplasm. The cytoplasmic dispersed Daxx mutants could be relocalized to nucleus by cotransfection with active SUMO, but not with inactive SUMO-delta, demonstrating the role of SUMO on regulating the cytoplasmonuclear transport of Daxx. However, inactive SUMO-delta could also be relocalized to nucleus during cotransfection with wild-type Daxx, suggesting that SUMO regulation of the cytoplasmonuclear transport of its target protein Daxx does not need covalent modification. This study shows that cytoplasmic SUMO has a biological role in enhancing the cytoplasmonuclear transport of its target protein Daxx and it may be done through the non-sumoylation interactions.

Evidence for a role of the cellular ND10 protein PML in mediating intrinsic immunity against human cytomegalovirus infections

Several viruses, including human cytomegalovirus (HCMV), encode proteins that colocalize with a cellular subnuclear structure known as ND10. Since only viral DNA deposited at ND10 initiates transcription, ND10 structures were hypothesized to be essential for viral replication. On the other hand, interferon treatment induces an up-regulation of ND10 structures and viruses have evolved polypeptides that disperse the dot-like accumulation of ND10 proteins, suggesting that ND10 could also be part of an intrinsic defense mechanism. In order to obtain evidence for either a proviral or an antiviral function of ND10, we generated primary human fibroblasts with a stable, short interfering RNA-mediated knockdown (kd) of PML. In these cells, other ND10-associated proteins like hDaxx showed a diffuse nuclear distribution. Interestingly, we observed that HCMV infection induced the de novo formation of ND10-like hDaxx and Sp100 accumulations that colocalized with IE2 and were disrupted, in the apparent absence of PML, in an IE1-dependent manner during the first hours after infection. Furthermore, infection of PML-kd cells with wild-type HCMV at a low multiplicity of infection resulted in enhanced replication. In particular, a significantly increased plaque formation was detected, suggesting that more cells are able to support initiation of replication in the absence of PML. While there was no difference in viral DNA uptake between PML-kd and control cells, we observed a considerable increase in the number of immediate-early (IE) protein-positive cells, indicating that the depletion of PML augments the initiation of viral IE gene expression. These results strongly suggest that PML functions as part of an intrinsic immune mechanism against cytomegalovirus infections.

Physical and functional interactions between Daxx and STAT3

Signal transducer and activator of transcription 3 (STAT3) play key roles in the intracellular signaling pathways of the interleukin (IL)-6 family of cytokines, which exhibit a diverse set of cellular responses, including cell proliferation and differentiation. Dysregulated IL-6/STAT3 signaling is involved in the pathogenesis of several diseases, for example autoimmune diseases and tumors. Type I interferon (IFN) induces the expression of proapoptotic genes and has been used in the clinical treatment of several tumors. In the present study, we found that type I IFN suppressed IL-6/STAT3-mediated transcription and gene expression. Furthermore, a type I IFN-induced protein, Daxx, also suppressed STAT3-mediated transcriptional activation, while overexpression of Daxx inhibited IL-6/STAT3-mediated gene expression. Importantly, small-interfering RNA-mediated reduction of Daxx expression enhanced IL-6/leukemia inhibitory factor (LIF)-induced STAT3-dependent transcription. Co-immunoprecipitation studies revealed a physical interaction between Daxx and STAT3 in transiently transfected 293T cells. We further found that Daxx and STAT3 were co-localized in the nucleus. These results indicate that Daxx may serve as a transcriptional regulator of type I IFN-mediated suppression of the IL-6/STAT3 signaling pathway.

Human cytomegalovirus (HCMV) UL82 gene product (pp71) relieves hDaxx-mediated repression of HCMV replication

This study examines the role of the cellular protein hDaxx in controlling human cytomegalovirus (HCMV) immediate-early (IE) gene expression and viral replication. Using permissive cell lines that either overexpress hDaxx or are depleted of hDaxx expression by the use of short hairpin RNA, we demonstrate that hDaxx functions as a repressor of HCMV IE gene expression and replication. In addition, we demonstrate that the impaired growth phenotype associated with the UL82 (pp71) deletion mutant is abolished when hDaxx knockdown cells are infected, suggesting that pp71 functions to relieve hDaxx-mediated repression during HCMV infection.

Regulation of histone acetyltransferase activity of p300 and PCAF by proto-oncogene protein DEK

The proto-oncogene protein DEK has been implicated in the t(6;9) chromosomal translocation associated with a subtype of acute myelogenous leukemia (AML), which results in the formation of a DEK-CAN fusion protein. Histone acetylation is an important post-translational modification which is involved in transcriptional regulation. In this study, we report that the acidic domain containing protein DEK interacts with histones and exerts a potent inhibitory effect on both p300 and PCAF-mediated histone acetyltransferase activity and transcription. Using chromatin immunoprecipitation assays, we have demonstrated that the recruitment of DEK to the appropriate promoter induces the histone H3 and H4 hypoacetylation of chromatin. Collectively, our data illustrate the important regulatory role played by protein DEK in transcriptional regulation, and suggest that transcription-regulating acidic domain regions may play a role in leukemogenesis.

Role of the cellular protein hDaxx in human cytomegalovirus immediate-early gene expression

Human cytomegalovirus (HCMV) immediate-early (IE) transcription is stimulated by virion phosphoprotein pp71, the product of gene UL82. It has previously been shown that pp71 interacts with the cellular protein hDaxx and, in the studies presented here, the significance of this interaction was investigated for HCMV IE gene expression. In co-transfection experiments, the presence of hDaxx increased the transcriptional response of the HCMV major IE promoter (MIEP) to pp71, but it was not possible to determine whether the effect was due to an interaction between the two proteins or to stimulation of hDaxx synthesis by pp71. The use of small interfering RNA (siRNA) in long- and short-term transfection approaches reduced intracellular hDaxx levels to no more than 3 % of normal. Infection of hDaxx-depleted cells with herpes simplex virus recombinants containing the HCMV MIEP revealed significantly greater promoter activity when hDaxx levels were minimal. Similarly, reducing intracellular hDaxx amounts resulted in greater IE gene expression during infection with an HCMV mutant lacking pp71, but had no effect on IE transcription during infection with wild-type HCMV. The results suggest that hDaxx is not important as a positive-acting factor for the stimulation of HCMV IE transcription by pp71. Instead, it appears that hDaxx acts as a repressor of IE gene expression, and it is proposed here that the interaction of pp71 with hDaxx is important to relieve repression and permit efficient initiation of productive replication.

Inactivating a cellular intrinsic immune defense mediated by Daxx is the mechanism through which the human cytomegalovirus pp71 protein stimulates viral immediate-early gene expression

Human cytomegalovirus (HCMV) masterfully evades adaptive and innate immune responses, allowing infection to be maintained and periodically reactivated for the life of the host. Here we show that cells also possess an intrinsic immune defense against HCMV that is disarmed by the virus. In HCMV-infected cells, the promyelocytic leukemia nuclear body (PML-NB) protein Daxx silences viral immediate-early gene expression through the action of a histone deacetylase. However, this antiviral tactic is efficiently neutralized by the viral pp71 protein, which is incorporated into virions, delivered to cells upon infection, and mediates the proteasomal degradation of Daxx. This work demonstrates the mechanism through which pp71 activates viral immediate-early gene expression in HCMV-infected cells. Furthermore, it provides insight into how a PML-NB protein institutes an intrinsic immune defense against a DNA virus and how HCMV pp71 inactivates this defense.

The WSTF-SNF2h chromatin remodeling complex interacts with several nuclear proteins in transcription

The WSTF (Williams syndrome transcription factor) protein is involved in vitamin D-mediated transcription and replication as a component of two distinct ATP-dependent chromatin remodeling complexes, WINAC and WICH, respectively. We show here that the WICH complex (WSTF-SNF2h) interacts with several nuclear proteins as follows: Sf3b155/SAP155, RNA helicase II/Gualpha, Myb-binding protein 1a, CSB, the proto-oncogene Dek, and nuclear myosin 1 in a large 3-MDa assembly, B-WICH, during active transcription. B-WICH also contains RNAs, 45 S rRNA, 5 S rRNA, 7SL RNA, and traces of the U2 small nuclear RNA. The core proteins, WSTF, SNF2h, and nuclear myosin 1, are associated with the RNA polymerase III genes 5 S rRNA genes and 7SL, and post-transcriptional silencing of WSTF reduces the levels of these transcripts. Our results show that a WSTF-SNF2h assembly is involved in RNA polymerase III transcription, and we suggest that WSTF-SNF2h-NM1 forms a platform in transcription while providing chromatin remodeling.

Physiological and functional interactions between Tcf4 and Daxx in colon cancer cells

Daxx, a human cell death-associated protein, was isolated as a Tcf4-interacting protein, using a yeast two-hybrid screen. Co-immunoprecipitation in HEK-293T cells and yeast two-hybrid screen in Y190 cells were performed to identify the interaction between Tcf4 with Daxx and to map the binding regions of Tcf4. In the nucleus, Daxx reduced DNA binding activity of Tcf4 and repressed Tcf4 transcriptional activity. Overexpression of Daxx altered the expression of genes downstream of Tcf4, including cyclin D1 and Hath-1, and induced G1 phase arrest in colon cancer cells. A reduction in Daxx protein expression was also observed in colon adenocarcinoma tissue when compared with normal colon tissue. This evidence suggests a possible physiological function of Daxx, via interaction with Tcf4, to regulate proliferation and differentiation of colon cells.

BTB domain-containing speckle-type POZ protein (SPOP) serves as an adaptor of Daxx for ubiquitination by Cul3-based ubiquitin ligase

Daxx is a multifunctional protein that regulates a variety of cellular processes, including transcription, cell cycle, and apoptosis. SPOP is a BTB (Bric-a-brac/Tramtrack/Broad complex) protein that constitutes Cul3-based ubiquitin ligases. Here we show that SPOP serves as an adaptor of Daxx for the ubiquitination by Cul3-based ubiquitin ligase and subsequent degradation by the proteasome. Expression of SPOP with Cul3 markedly reduced Daxx level, and this degradation was blocked by SPOP-specific short hairpin RNAs. Inhibition of the proteasome by MG132 caused the prevention of Daxx degradation in parallel with the accumulation of ubiquitinated Daxx. Expression of SPOP with Cul3 reversed Daxx-mediated repression of ETS1- and p53-dependent transcription, and short hairpin RNA-mediated knock down of SPOP blocked the recovery of their transcriptional activation. Furthermore, Daxx degradation led to the cleavage of poly(ADP-ribose) polymerase and the increase in the number of terminal deoxynucleotidyltransferase-mediated dUTP-fluorescein nick end-labeling-positive apoptotic cells. These results suggest that SPOP/Cul3-ubiquitin ligase plays an essential role in the control of Daxx level and, thus, in the regulation of Daxx-mediated cellular processes, including transcriptional regulation and apoptosis.

Interactions between DNA viruses, ND10 and the DNA damage response

ND10 are small nuclear substructures that are defined by the presence the promyelocytic leukaemia protein PML. Many other proteins have been detected within ND10, a complexity that is reflected in reports of their involvement in multiple cellular pathways that include the regulation of gene expression, chromatin dynamics, protein modification, apoptosis, p53 function, senescence, DNA repair, the interferon response and viral infection. This review summarizes recent evidence of similarities between the behaviour of ND10 components and DNA repair pathway proteins in response to viral infection and DNA damage. ND10 structures become associated with the parental genomes and early replication compartments of many DNA viruses, and DNA repair pathway proteins are also recruited to these sites. Similarly, PML and DNA repair proteins are recruited to sites of DNA damage. The mechanisms by which these events might occur, and the implications for ND10 function in DNA virus infection and chromatin metabolism, are discussed.

Daxx: death or survival protein?

The death domain-associated protein (Daxx) was originally cloned as a CD95 (FAS)-interacting protein and modulator of FAS-induced cell death. Daxx accumulates in both the nucleus and the cytoplasm; in the nucleus, Daxx is found associated with the promyelocytic leukaemia (PML) nuclear body and with alpha-thalassemia/mental retardation syndrome protein (ATRX)-positive heterochromatic regions. In the cytoplasm, Daxx has been reported to interact with various proteins involved in cell death regulation. Despite a significant number of studies attempting to determine Daxx function in apoptotic and non-apoptotic cell death, its precise role in this process is only partially understood. Here, we critically review the current understanding of Daxx function and shed new light on this interesting field.

Something about SUMO inhibits transcription

Many proteins that are important for regulated gene expression--including promoter-specific transcription factors, cofactors and chromatin-modifying enzymes--have been found to be reversibly modified by the small ubiquitin-related modifier, SUMO. Post-translational modification by SUMO has diverse effects on substrate activity, but, in most cases described to date, SUMOylation of transcriptional regulators correlates with inhibition of transcription. Recent studies provide new insights into the mechanisms by which SUMOylation regulates transcription and suggest that one consequence of SUMOylation is to promote the interaction of transcription factors with co-repressors. Histone deacetylase co-repressors have been found to function as substrates, effectors, and regulators of SUMOylation, suggesting that complex crosstalk between acetylation and SUMOylation is important for gene regulation.

SUMO modification negatively modulates the transcriptional activity of CREB-binding protein via the recruitment of Daxx

Small ubiquitin-like modifier (SUMO) modification is emerging as an important control in transcription regulation. Here, we show that CREB-binding protein (CBP), a versatile transcriptional coactivator for numerous transcription factors in response to diverse signaling events, can be modified by SUMO-1 at lysine residues 999, 1034, and 1057 both in vitro and in vivo. Mutation of the SUMO acceptor lysine residues either individually or in combination enhanced CBP transcriptional activity, and expression of a SUMO protease SENP2 potentiated the transcriptional activity of CBP wild-type but not its sumoylation mutant, indicating that SUMO modification negatively regulates CBP transcriptional activity. Furthermore, we demonstrated an interaction of SUMO-1-modified CBP with the transcriptional corepressor Daxx and an essential role of Daxx in mediating SUMO-dependent transcriptional regulation of CBP through histone deacetylase 2 recruitment. Together, our findings indicate that SUMO modification and subsequent recruitment of Daxx represent a previously undescribed mechanism in modulating CBP transcriptional potential.

The human DEK proto-oncogene is a senescence inhibitor and an upregulated target of high-risk human papillomavirus E7

The human DEK proto-oncogene is a nucleic acid binding protein with suspected roles in human carcinogenesis, autoimmune disease, and viral infection. Intracellular DEK functions, however, are poorly understood. In papillomavirus-positive cervical cancer cells, downregulation of viral E6/E7 oncogene expression results in cellular senescence. We report here the specific repression of DEK message and protein levels in senescing human papillomavirus type 16- (HPV16-) and HPV18-positive cancer cell lines as well as in primary cells undergoing replicative senescence. Cervical cancer cell senescence was partially overcome by DEK overexpression, and DEK overexpression was sufficient for extending the life span of primary keratinocytes, supporting critical roles for this molecule as a senescence regulator. In order to determine whether DEK is a bona fide HPV oncogene target in primary cells, DEK expression was monitored in human keratinocytes transduced with HPV E6 and/or E7. The results identify high-risk HPV E7 as a positive DEK regulator, an activity that is not shared by low-risk HPV E7 protein. Experiments in mouse embryo fibroblasts recapitulated the observed E7-mediated DEK induction and demonstrated that both basal and E7-induced regulation of DEK expression are controlled by the retinoblastoma protein family. Taken together, our results suggest that DEK upregulation may be a common event in human carcinogenesis and may reflect its senescence inhibitory function.

Daxx is required for stress-induced cell death and JNK activation

Daxx has been implicated in the modulation of apoptosis in response to various stimuli. In the nucleus, Daxx interacts and colocalizes with the promyelocytic leukemia protein (PML) into the PML-nuclear body. Moreover, overexpressed Daxx positively modulates FAS-ligand and TGFbeta-induced apoptosis. However, recent reports indicate that Daxx can also act as an antiapoptotic factor. As most studies on the role of Daxx in cell death have been conducted using tumour cell lines, we analysed the function of Daxx in physiological settings. We found that Daxx is induced upon exposure to ultraviolet (UV) irradiation and hydrogen peroxide treatment. We employed RNA interference to downregulate Daxx in primary fibroblasts. Remarkably, Daxx-depleted cells are resistant to cell death induced by both UV irradiation and oxidative stress. Furthermore, the downregulation of Daxx results in impaired MKK/c-Jun-N-terminal kinase (JNK) activation. This is the first evidence that Daxx promotes cell death and JNK activation in physiological conditions.

p300/CBP-associated factor drives DEK into interchromatin granule clusters

DEK is a mammalian protein that has been implicated in the pathogenesis of autoimmune diseases and cancer, including acute myeloid leukemia, melanoma, glioblastoma, hepatocellular carcinoma, and bladder cancer. In addition, DEK appears to participate in multiple cellular processes, including transcriptional repression, mRNA processing, and chromatin remodeling. Sub-nuclear distribution of this protein, with the attendant functional ramifications, has remained a controversial topic. Here we report that DEK undergoes acetylation in vivo at lysine residues within the first 70 N-terminal amino acids. Acetylation of DEK decreases its affinity for DNA elements within the promoter, which is consistent with the involvement of DEK in transcriptional repression. Furthermore, deacetylase inhibition results in accumulation of DEK within interchromatin granule clusters (IGCs), sub-nuclear structures that contain RNA processing factors. Overexpression of P/CAF acetylase drives DEK into IGCs, and addition of a newly developed, synthetic, cell-permeable P/CAF inhibitor blocks this movement. To our knowledge, this is the first reported example of acetylation playing a direct role in relocation of a protein to IGCs, and this may explain how DEK can function in multiple pathways that take place in distinct sub-nuclear compartments. These findings also suggest that DEK-associated malignancies and autoimmune diseases might be amenable to treatment with agents that alter acetylation.

The death domain-associated protein modulates activity of the transcription co-factor Skip/NcoA62

Death domain-associated protein (Daxx) regulates both transcription and apoptosis. The role of Daxx in transcription is not well understood. Here, we show that Daxx interacts with Skip/NcoA62, a transcription cofactor that modulates the activity of oncoproteins including Ski and NotchIC. Daxx strongly binds with Skip both in vitro and in mammalian cells. This interaction is mediated by the PAH2 domain of Daxx and the highly conserved SNW domain of Skip. Daxx partially co-localizes with Skip in vivo and changes the cellular distribution of Skip. In addition, Skip represses transcription when tethered to a promoter, and Daxx antagonizes this activity. Furthermore, Skip is phosphorylated at serine 224 in its SNW domain. These results suggest a novel function of Daxx in transcription regulation through alteration of the cellular localization of Skip.

Distribution of the chromatin protein DEK distinguishes active and inactive CD21/CR2 gene in pre- and mature B lymphocytes

DEK is an abundant and ubiquitous chromatin protein that has only recently attracted attention. DEK preferentially binds to cruciform and superhelical DNA and induces positive supercoils into closed circular DNA. It is quite likely therefore that DEK performs an important architectural function in chromatin. However, it is not known how DEK is distributed in chromatin. As the first study of its kind, we investigate the distribution of DEK at the CD21/complement receptor 2 gene regulatory regions in two B lymphocyte lines, namely Ramos, which expresses the CD21 gene, and Nalm-6, which does not. We use a chromatin immunoprecipitation approach and show that DEK appears to be distributed over various regions of the expressed and silent genes, but occurs in 2- to 3-fold higher amounts at a promoter-proximal site of the expressed gene. Moreover, induction of CD21 expression in Nalm-6 cells leads to accumulation of DEK at this site. We propose that the accumulation of DEK is functionally linked to gene expression.

Interaction between the human cytomegalovirus UL82 gene product (pp71) and hDaxx regulates immediate-early gene expression and viral replication

The human cytomegalovirus UL82-encoded pp71 protein is required for efficient virus replication and immediate-early gene expression when cells are infected at a low multiplicity. Functions attributed to pp71 include the ability to enhance the infectivity of viral DNA, bind to and target hypophosphorylated Rb family member proteins for degradation, drive quiescent cells into the cell cycle, and bind to the cellular protein hDaxx. Using UL82 mutant viruses, we demonstrate that the LXCXD motif within pp71 is not necessary for efficient virus replication in fibroblasts, suggesting that pp71's ability to degrade hypophosphorylated Rb family members and induce quiescent cells into the cell cycle is not responsible for the growth defect associated with a UL82 deletion mutant. However, UL82 mutants that cannot bind to hDaxx are unable to induce immediate-early gene expression and are severely attenuated for viral replication. These results indicate that the interaction between the human cytomegalovirus UL82 gene product (pp71) and hDaxx regulates immediate-early gene expression and viral replication.

The cellular protein daxx interacts with avian sarcoma virus integrase and viral DNA to repress viral transcription

The cellular protein Daxx was identified as an interactor with avian sarcoma virus (ASV) integrase (IN) in a yeast two-hybrid screen. After infection, Daxx-IN interactions were detected by coimmunoprecipitation. An association between Daxx and viral DNA, likely mediated by IN, was also detected by chromatin immunoprecipitation. Daxx was not required for early events in ASV replication, including integration, as Daxx-null cells were transduced as efficiently as Daxx-expressing cells. However, viral reporter gene expression from ASV-based vectors was substantially higher in the Daxx-null cells than in Daxx-complemented cells. Consistent with this observation, histone deacetylases (HDACs) were found to associate with viral DNA in Daxx-complemented cells but not in Daxx-null cells. Furthermore, Daxx protein was induced in an interferon-like manner upon ASV infection. We conclude that Daxx interacts with an IN-viral DNA complex early after infection and may mediate the repression of viral gene expression via the recruitment of HDACs. Our findings provide a novel example of cellular immunity against viral replication in which viral transcription is repressed via the recruitment of antiviral proteins to the viral DNA.

Daxx mediates the small ubiquitin-like modifier-dependent transcriptional repression of Smad4

Daxx has been shown to function as an apoptosis regulator and transcriptional repressor via its interaction with various cytoplasmic and nuclear proteins. Here, we showed that Daxx interacts with Smad4 and represses its transcriptional activity via the C-terminal domain of Daxx. In vitro and in vivo interaction studies indicated that the binding of Smad4 to Daxx depends on Smad4 sumoylation. Substitution of Smad4 SUMO conjugation residue lysine 159, but not 113, to arginine not only disrupted Smad4-Daxx interaction but also relieved Daxx-elicited repression of Smad4 transcriptional activity. Furthermore, chromatin immunoprecipitation analyses revealed the recruitment of Daxx to an endogenous, Smad4-targeted promoter in a Lys(159) sumoylation-dependent manner. Finally, down-regulation of Daxx expression by RNA interference enhanced transforming growth factor beta-induced transcription of reporter and endogenous genes through a Smad4-dependent, but not K159R-Smad4-dependent, manner. Together, these results indicate that Daxx suppresses Smad4-mediated transcriptional activity by direct interaction with the sumoylated Smad4 and identify a novel role of Daxx in regulating transforming growth factor beta signaling.

The DEK protein--an abundant and ubiquitous constituent of mammalian chromatin

The protein DEK is an abundant and ubiquitous chromatin protein in multicellular organisms (not in yeast). It is expressed in more than a million copies/nucleus of rapidly proliferating mammalian cells. DEK has two DNA binding modules of which one includes a SAP box, a sequence motif that DEK shares with a number of other chromatin proteins. DEK has no apparent affinity to specific DNA sequences, but preferentially binds to superhelical and cruciform DNA, and induces positive supercoils into closed circular DNA. The available evidence strongly suggests that DEK could function as an architectural protein in chromatin comparable to the better known classic architectural chromatin proteins, the high-mobility group or HMG proteins.

Negative modulation of androgen receptor transcriptional activity by Daxx

The transcriptional activity of the androgen receptor (AR) modulated by positive or negative regulators plays a critical role in controlling the growth and survival of prostate cancer cells. Although numerous positive regulators have been identified, negative regulators of AR are less well understood. We report here that Daxx functions as a negative AR coregulator through direct protein-protein interactions. Overexpression of Daxx suppressed AR-mediated promoter activity in COS-1 and LNCaP cells and AR-mediated prostate-specific antigen expression in LNCaP cells. Conversely, downregulation of endogenous Daxx expression by RNA interference enhances androgen-induced prostate-specific antigen expression in LNCaP cells. In vitro and in vivo interaction studies revealed that Daxx binds to both the amino-terminal and the DNA-binding domain of the AR. Daxx proteins interfere with the AR DNA-binding activity both in vitro and in vivo. Moreover, sumoylation of AR at its amino-terminal domain is involved in Daxx interaction and trans-repression. Together, these findings not only provide a novel role of Daxx in controlling AR transactivation activity but also uncover the mechanism underlying sumoylation-dependent transcriptional repression of the AR.

Protein modification by SUMO

Small ubiquitin-related modifier (SUMO) family proteins function by becoming covalently attached to other proteins as post-translational modifications. SUMO modifies many proteins that participate in diverse cellular processes, including transcriptional regulation, nuclear transport, maintenance of genome integrity, and signal transduction. Reversible attachment of SUMO is controlled by an enzyme pathway that is analogous to the ubiquitin pathway. The functional consequences of SUMO attachment vary greatly from substrate to substrate, and in many cases are not understood at the molecular level. Frequently SUMO alters interactions of substrates with other proteins or with DNA, but SUMO can also act by blocking ubiquitin attachment sites. An unusual feature of SUMO modification is that, for most substrates, only a small fraction of the substrate is sumoylated at any given time. This review discusses our current understanding of how SUMO conjugation is controlled, as well as the roles of SUMO in a number of biological processes.

Negative regulation of p53 functions by Daxx and the involvement of MDM2

In normal cells p53 activity is tightly controlled and MDM2 is a known negative regulator. Here we show that via its acidic domain, Daxx binds to the COOH-terminal domain of p53, whose positive charges are critical for this interaction, as Lys to Arg mutations preserved, but Lys to Ala or Ser to Glu mutations abolished Daxx-p53 interaction. These results thus implicate acetylation and phosphorylation of p53 in regulating its binding to Daxx. Interestingly, whereas Daxx did not bind to p53 in cells as assessed by immunoprecipitation, MDM2 expression restored p53-Daxx interaction, and this correlated with deacetylation of p53. In p53/MDM2-null mouse embryonic fibroblasts (DKO MEF), Daxx repressed p53 target promoters whose p53-binding elements were required for the repression. Coexpression of Daxx and MDM2 led to further repression. p53 expression in DKO MEF induced apoptosis and Daxx expression relieved this effect. Similarly, in HCT116 cells, Daxx conferred striking resistance to 5-fluorouracil-induced apoptosis. As p53 is required for 5-fluorouracil-induced cell death, our data show that Daxx can suppress cell death induced by p53 overexpression and p53-dependent stress response. Collectively, our data reveal Daxx as a novel negative regulator of p53. Importantly, posttranslational modifications of p53 inhibit Daxx-p53 interaction, thereby relieving negative regulation of p53 by Daxx.

ATRX and sex differentiation

Most interest in the gene encoding the alpha-thalassemia, mental retardation, X-linked protein (ATRX) has traditionally been focused on its role in brain development and globin regulation. However, mutations in the ATRX gene also cause varying degrees of gonadal and urogenital abnormalities. These range from small testes to ambiguous external genitalia in XY individuals. ATRX is of great interest because this very large protein is one of the least understood proteins involved in mammalian sexual development. Although the biochemical role of ATRX remains unknown, recent and emerging evidence implicates ATRX in chromatin remodeling. We review what is known about ATRX as a chromatin remodeling protein, and its role in mammalian sex differentiation.

The transcriptional repressor hDaxx potentiates p53-dependent apoptosis

p53 and its homologues p73 and p63 are transcription factors that play an essential role in modulating cell cycle arrest and cell death in response to several environmental stresses. The type and intensity of these responses, which can be different depending on the inducing stimulus and on the overall cellular context, are believed to rely on the activation of defined subsets of target genes. The proper activation of p53 family members requires the coordinated action of post-translational modifications and interaction with several cofactors. In this study, we demonstrate that the multifunctional protein hDaxx interacts with p53 and its homologues, both in vitro and in vivo, and modulates their transcriptional activity. Moreover, we show that hDaxx, which has been implicated in several apoptotic pathways, increases the sensitivity to DNA damage-induced cell death and that this effect requires the presence of p53. Although hDaxx represses p53-dependent transcription of the p21 gene, it does not affect the activation of proapoptotic genes, and therefore acts by influencing the balance between cell cycle arrest and proapoptotic p53 targets. Our results therefore underline the central role of hDaxx in modulating the apoptotic threshold upon several stimuli and identify it as a possible integrating factor that coordinates the response of p53 family members.

In bcr-abl-positive myeloid cells resistant to conventional chemotherapeutic agents, expression of Par-4 increases sensitivity to imatinib (STI571) and histone deacetylase-inhibitors

In a variety of malignant cells the prostate-apoptosis-response-gene-4 (Par-4) induces increased sensitivity towards chemotherapeutic agents by down-regulating anti-apoptotic B-cell lymphoma-gene 2 (Bcl-2). Hypothesizing that Par-4 also influences apoptosis in myeloid cell lines, we tested this hypothesis by stably transfecting bcr-abl transformed-K562 cells with a Par-4-expressing vector. Here we demonstrate that over-expression of Par-4 in K562 cells up-regulates expression levels of Bcl-2 and death-associated protein (Daxx). Upon treatment with different chemotherapeutic agents, Fas- or TRAIL agonistic antibodies, Par-4-positive cells did not exhibit an increased rate of apoptosis as compared to Par-4-negative control cells. However, incubation with histone deacetylase (HDAC)-inhibitors Trichostatin A (TSA) and LAQ824 or the tyrosinkinase inhibitor Imatinib (STI571) increased the rate of apoptosis in Par-4-positive K562 cells. Assessing the underlying molecular mechanisms for the Par-4-induced response to HDAC-inhibitors and STI571 we provide evidence, that these effects are associated with a down-regulation of Daxx, enforced activation of caspases and enhanced cleavage of cellular inhibitor of apoptosis (cIAP)-1 and -2.

Heterochromatin and ND10 are cell-cycle regulated and phosphorylation-dependent alternate nuclear sites of the transcription repressor Daxx and SWI/SNF protein ATRX

Placing regulatory proteins into different multiprotein complexes should modify key cellular processes. Here, we show that the transcription repressor Daxx and the SWI/SNF protein ATRX are both associated with two intranuclear domains: ND10/PML bodies and heterochromatin. The accumulation of ATRX at nuclear domain 10 (ND10) was mediated by its interaction with the N-terminus of Daxx. Binding of this complex to ND10 was facilitated by the interaction of the Daxx C-terminus with SUMOylated promyelocytic leukemia protein (PML). Although ATRX was present at heterochromatin during the entire cell cycle, Daxx was actively recruited to this domain at the end of S-phase. The FACT-complex member structure-specific recognition protein 1 (SSRP1) accumulated at heterochromatin simultaneously with Daxx and accumulation of both proteins depended on ATRX phosphorylation. Both Daxx and SSRP1 were released from heterochromatin early in G(2) phase and Daxx was recruited back to ND10, indicating that both proteins localize to heterochromatin during a very short temporal window of the cell cycle. ATRX seems to assemble a repression multiprotein complex including Daxx and SSRP1 at heterochromatin during a specific stage of the cell cycle, whereas Daxx functions as an adapter for ATRX accumulation at ND10. A potential functional consequence of Daxx accumulation at heterochromatin was found in the S- to G(2)-phase transition. In Daxx(-/-) cells, S-phase was accelerated and the propensity to form double nuclei was increased, functional changes that could be rescued by Daxx reconstitution and that might be the basis for the developmental problems observed in Daxx knockout animals.

Phosphorylation by protein kinase CK2 changes the DNA binding properties of the human chromatin protein DEK

We have examined the posttranslational modification of the human chromatin protein DEK and found that DEK is phosphorylated by the protein kinase CK2 in vitro and in vivo. Phosphorylation sites were mapped by quadrupole ion trap mass spectrometry and found to be clustered in the C-terminal region of the DEK protein. Phosphorylation fluctuates during the cell cycle with a moderate peak during G(1) phase. Filter binding assays, as well as Southwestern analysis, demonstrate that phosphorylation weakens the binding of DEK to DNA. In vivo, however, phosphorylated DEK remains on chromatin. We present evidence that phosphorylated DEK is tethered to chromatin throughout the cell cycle by the un- or underphosphorylated form of DEK.

Functional domains of the ubiquitous chromatin protein DEK

DEK was originally described as a proto-oncogene protein and is now known to be a major component of metazoan chromatin. DEK is able to modify the structure of DNA by introducing supercoils. In order to find interaction partners and functional domains of DEK, we performed yeast two-hybrid screens and mutational analyses. Two-hybrid screening yielded C-terminal fragments of DEK, suggesting that DEK is able to multimerize. We could localize the domain to amino acids 270 to 350 and show that multimerization is dependent on phosphorylation by CK2 kinase in vitro. We also found two DNA binding domains of DEK, one on a fragment including amino acids 87 to 187 and containing the SAF-box DNA binding motif, which is located between amino acids 149 and 187. This region is sufficient to introduce supercoils into DNA. The second DNA binding domain is located between amino acids 270 and 350 and thus overlaps the multimerization domain. We show that the two DNA-interacting domains differ in their binding properties and in their abilities to respond to CK2 phosphorylation.

Daxx-mediated transcriptional repression of MMP1 gene is reversed by SPOP

Daxx-mediated transcriptional repression was modulated by a speckled POZ domain protein SPOP which was first identified as an autoantigen from the serum of a scleroderma patient. This is the first report on the biochemical and functional interactions between Daxx and SPOP. The COOH-terminal region of Daxx interacts with the NH2-terminal region of SPOP. SPOP reversed the transcriptional repression mediated by Daxx which binds with ETS1 transcription factor to repress ETS1-responsive gene expression. Mutagenesis study suggests that the ability of SPOP to self-associate as well as its ability to bind with Daxx was important for the modulation of Daxx-mediated transcriptional repression.

Physical and functional interactions between Daxx and DNA methyltransferase 1-associated protein, DMAP1

Daxx has been shown to play an essential role in type I IFN-alphabeta-mediated suppression of B cell development and apoptosis. Recently, we demonstrated that Tyk2 is directly involved in IFN signaling for the induction and translocation of Daxx, which may result in growth arrest and/or apoptosis of B lymphocyte progenitors. To clarify how Daxx regulates B cell development, we examined Daxx interacting partners by yeast two-hybrid screening. DNA methyltransferase 1 (DNMT1)-associated protein (DMAP1) was identified and demonstrated to interact with Daxx. The interaction regions in both proteins were mapped, and the cellular localization of the interaction was examined. Both Daxx and DMAP1 formed a complex with DNMT1 and colocalized in the nucleus. DMAP1 enhanced Daxx-mediated repression of glucocorticoid receptor transcriptional activity. Furthermore, Daxx protected protein degradation of DMAP1 in vivo. These results provide the novel molecular link between Daxx and DNMT1, which establishes a repressive transcription complex in the nucleus.

Physical and functional interactions between Daxx and TSG101

Daxx has been reported to mediate the Fas/JNK-dependent signals in the cytoplasm. However, several evidences have suggested that Daxx is located mainly in the nucleus and functions as a transcriptional regulator. Recently, we identified DMAP1, a TSG101-interacting protein as a Daxx binding partner by yeast two-hybrid screening. TSG101 has been shown to act as transcriptional co-repressor of nuclear hormone receptors. Here we examined whether TSG101also interacts with Daxx directly. The association of Daxx and TSG101 was confirmed using co-expressed tagged proteins. The interaction regions in both proteins were also mapped, and the cellular localization of the interaction was examined. TSG101 formed a complex with Daxx through its coiled-coil domain and co-localized in the nucleus. Furthermore, TSG101 enhanced Daxx-mediated repression of glucocorticoid receptor transcriptional activity. These results provide the novel molecular interactions between Daxx and TSG101, which establish an efficient repressive transcription complex in the nucleus.

DAXX, FLASH, and FAF-1 modulate mineralocorticoid and glucocorticoid receptor-mediated transcription in hippocampal cells--toward a basis for the opposite actions elicited by two nuclear receptors?

Mineralocorticoid (MR) and glucocorticoid (GR) receptors are two closely-related members of the steroid nuclear receptor family of transcription factors that bind common ligands in the brain (corticosterone and cortisol) and supposedly have identical hormone response elements. This raises the important question of how they can elicit differential biological actions in neurons in which they are often colocalized. One plausible explanation is that they differentially recruit proteins (coregulators or other receptor-interacting factors) through cell-specific interactions with regions that diverge between MR and GR to modulate target gene transcription in a receptor-specific manner. We therefore performed a yeast-two-hybrid screening of a human brain cDNA library with an AF1-containing region of the human MR as bait. This screening revealed several potential MR-interacting partners; among them were several clones bearing homology to DAXX, FLASH, and FAF-1, all previously implicated in apoptosis. Coexpression of candidate clones in a mouse hippocampal cell line confirmed these interactions in a mammalian neural cell environment as well. In transient transactivation assays, DAXX and FLASH influenced MR- and GR-driven transcription of the MMTV-Luc reporter similarly; in contrast, although FAF-1 did not transactivate GR, it did selectively stimulate MR-mediated transcription. Thus, the present findings, that 1) DAXX, FLASH, and FAF-1 modulate the transcriptional activities of MR and GR and that 2) FAF-1 selectively coactivates only MR, provide possible clues for how these closely related receptors might differentially influence neuronal function.

A novel transcription regulatory complex containing death domain-associated protein and the ATR-X syndrome protein

Death domain-associated protein (Daxx) is a multi-functional protein that modulates both apoptosis and transcription. Within the nucleus, Daxx is a component of the promyelocytic leukemia protein (PML) nuclear bodies (NBs) and interacts with a number of transcription factors, yet its precise role in transcription remains elusive. To further define the function of Daxx, we have isolated its interacting proteins in the nucleus using epitope-tagged affinity purification and identified X-linked mental retardation and alpha-thalassaemia syndrome protein (ATRX), a putative member of the SNF2 family of ATP-dependent chromatin remodeling proteins that is mutated in several X-linked mental retardation disorders. We show that substantial amounts of endogenous Daxx and ATRX exist in a nuclear complex. Daxx binds to ATRX through its paired amphipathic alpha helices domains. ATRX has ATPase activity that is stimulated by mononucleosomes, and patient mutations in the ATPase domain attenuate this activity. ATRX strongly represses transcription when tethered to a promoter. Daxx does not affect the ATPase activity of ATRX, however, it alleviates its transcription repression activity. In addition, ATRX is found in the PML-NBs, and this localization is mediated by Daxx. These results show that the ATRX.Daxx complex is a novel ATP-dependent chromatin-remodeling complex, with ATRX being the core ATPase subunit and Daxx being the targeting subunit. Moreover, the localization of ATRX to the PML-NBs supports the notion that these structures may play an important role in transcription regulation.

The differential requirement for cyclin-dependent kinase activities distinguishes two functions of herpes simplex virus type 1 ICP0

Herpes simplex virus type 1 (HSV-1) ICP0 directs the degradation of cellular proteins associated with nuclear structures called ND10, a function thought to be closely associated with its broad transactivating activity. Roscovitine (Rosco), an inhibitor of cyclin-dependent kinases (cdks), inhibits the replication of HSV-1, HSV-2, human cytomegalovirus, varicella-zoster virus, and human immunodeficiency virus type 1 by inhibiting specific steps or activities of viral regulatory proteins, indicating the broad and pleiotropic effects that cdks have on the replication of these viruses. We previously demonstrated that Rosco inhibits the transactivating activity of ICP0. In the present study, we asked whether Rosco also affects the ability of ICP0 to direct the degradation of ND10-associated proteins. For this purpose, WI-38 cells treated with cycloheximide (CHX) were mock infected or infected with wild-type HSV-1 or an ICP0(-) mutant (7134). After release from the CHX block, the infections were allowed to proceed for 2 h in the presence or absence of Rosco at a concentration known to inhibit ICP0's transactivating activity. The cells were then examined for the presence of ICP0 and selected ND10-associated antigens (promyelocytic leukemia antigen [PML], sp100, hDaxx, and NDP55) by immunofluorescence. Staining for the ND10-associated antigens was detected in </=20% of KOS-infected cells in the presence or absence of Rosco, demonstrating that Rosco-sensitive kinases are not required for ICP0's ability to direct the dispersal or degradation of these antigens. In contrast, >90% of 7134- and mock-infected cells stained positive for all ND10-associated antigens in the presence or absence of Rosco. Similar results were obtained with a non-ND10-associated antigen, DNA-PK(cs), a known target of ICP0-directed degradation. The results of the PML and DNA-PK(cs) immunofluorescence studies correlated with a decrease in the levels of these proteins as determined by Western blotting. Thus, the differential requirement for Rosco-sensitive cdk activities distinguishes ICP0's ability to direct the dispersal or degradation of cellular proteins from its transactivating activity.

Daxx silencing sensitizes cells to multiple apoptotic pathways

Daxx is a nuclear protein involved in apoptosis and transcriptional repression, and it interacts with the death receptor Fas, promyelocytic leukemia protein (PML), and several transcriptional repressors. The function of Daxx in apoptosis is controversial because opposite results were obtained in transient overexpression and genetic knockout studies. Furthermore, the roles of PML and transcriptional repression in Daxx-regulated apoptosis are currently unknown. In this study, we investigated the role of Daxx in Fas- and stress-induced apoptosis by small interfering RNA-mediated Daxx silencing in mammalian cells. Daxx silencing had no apparent cytotoxic effects on mammalian cells within 72 h. Intriguingly, Daxx silencing strongly sensitized cells to Fas- and stress-induced apoptosis, which was accompanied by caspase activation, cytochrome c release, and Jun N-terminal kinase activation. Consistently, endogenous Daxx was degraded rapidly upon induction of apoptosis by stress or anti-Fas antibody. Finally, PML silencing had no effect on Daxx silencing-mediated apoptotic events, while caspase gene expression was upregulated in the absence of Daxx. These data strongly suggest that Daxx may inhibit Fas and stress-mediated apoptosis by suppressing proapoptotic gene expression outside of PML domains.

An evolutionarily conserved role for SRm160 in 3'-end processing that functions independently of exon junction complex formation

SRm160 (the SR-related nuclear matrix protein of 160 kDa) functions as a splicing coactivator and 3'-end cleavage-stimulatory factor. It is also a component of the splicing-dependent exon-junction complex (EJC), which has been implicated in coupling of pre-mRNA splicing with mRNA turnover and mRNA export. We have investigated whether the association of SRm160 with the EJC is important for efficient 3'-end cleavage. The EJC components RNPS1, REF, UAP56, and Y14 interact with SRm160. However, when these factors were tethered to transcripts, only SRm160 and RNPS1 stimulated 3'-end cleavage. Whereas SRm160 stimulated cleavage to a similar extent in the presence or absence of an active intron, stimulation of 3'-end cleavage by tethered RNPS1 is dependent on an active intron. Assembly of an EJC adjacent to the cleavage and polyadenylation signal in vitro did not significantly affect cleavage efficiency. These results suggest that SRm160 stimulates cleavage independently of its association with EJC components and that the cleavage-stimulatory activity of RNPS1 may be an indirect consequence of its ability to stimulate splicing. Using RNA interference (RNAi) in Caenorhabditis elegans, we determined whether interactions between SRm160 and the cleavage machinery are important in a whole organism context. Simultaneous RNAi of SRm160 and the cleavage factor CstF-50 (Cleavage stimulation factor 50-kDa subunit) resulted in late embryonic developmental arrest. In contrast, RNAi of CstF-50 in combination with RNPS1 or REFs did not result in an apparent phenotype. Our combined results provide evidence for an evolutionarily conserved interaction between SRm160 and the 3'-end cleavage machinery that functions independently of EJC formation.

Yeast two-hybrid screens imply involvement of fanconi anemia proteins in transcription regulation, cell signaling, oxidative metabolism, and cellular transport

Mutations in one of at least eight different genes cause bone marrow failure, chromosome instability, and predisposition to cancer associated with the rare genetic syndrome Fanconi anemia (FA). The cloning of seven genes has provided the tools to study the molecular pathway disrupted in Fanconi anemia patients. The structure of the genes and their gene products provided few clues to their functional role. We report here the use of 3 FA proteins, FANCA, FANCC, and FANCG, as "baits" in the hunt for interactors to obtain clues for FA protein functions. Using five different human cDNA libraries we screened 36.5x10(6) clones with the technique of the yeast two-hybrid system. We identified 69 proteins which have not previously been linked to the FA pathway as direct interactors of FANCA, FANCC, or FANCG. Most of these proteins are associated with four functional classes including transcription regulation (21 proteins), signaling (13 proteins), oxidative metabolism (10 proteins), and intracellular transport (11 proteins). Interaction with 6 proteins, DAXX, Ran, IkappaBgamma, USP14, and the previously reported SNX5 and FAZF, was additionally confirmed by coimmunoprecipitation and/or colocalization studies. Taken together, our data strongly support the hypothesis that FA proteins are functionally involved in several complex cellular pathways including transcription regulation, cell signaling, oxidative metabolism, and cellular transport.

The ATRX syndrome protein forms a chromatin-remodeling complex with Daxx and localizes in promyelocytic leukemia nuclear bodies

ATRX syndrome is characterized by X-linked mental retardation associated with alpha-thalassemia. The gene mutated in this disease, ATRX, encodes a plant homeodomain-like finger and a SWI2/SNF2-like ATPase motif, both of which are often found in chromatin-remodeling enzymes, but ATRX has not been characterized biochemically. By immunoprecipitation from HeLa extract, we found that ATRX is in a complex with transcription cofactor Daxx. The following evidence supports that ATRX and Daxx are components of an ATP-dependent chromatin-remodeling complex: (i) Daxx and ATRX can be coimmunoisolated by antibodies specific for each protein; (ii) a proportion of Daxx cofractionates with ATRX as a complex of 1 MDa by gel-filtration analysis; (iii) in extract from cells of a patient with ATRX syndrome, the level of the Daxx-ATRX complex is correspondingly reduced; (iv) a proportion of ATRX and Daxx colocalize in promyelocytic leukemia nuclear bodies, with which Daxx had previously been located; and (v) the ATRX complex displays ATP-dependent activities that resemble those of other chromatin-remodeling complexes, including triple-helix DNA displacement and alteration of mononucleosome disruption patterns. But unlike the previously described SWI/SNF or NURD complexes, the ATRX complex does not randomize DNA phasing of the mononucleosomes, suggesting that it may remodel chromatin differently. Taken together, the results suggest that ATRX functions in conjunction with Daxx in a novel chromatin-remodeling complex. The defects in ATRX syndrome may result from inappropriate expression of genes controlled by this complex.

Promyelocytic leukemia protein (PML) functions as a glucocorticoid receptor co-activator by sequestering Daxx to the PML oncogenic domains (PODs) to enhance its transactivation potential

Daxx has been reported to function as a transcriptional modulator in the nucleus. In the present study, we have explored the role of Daxx in regulating the transcriptional activity of the glucocorticoid receptor (GR). Overexpression of Daxx suppressed GR-mediated activation of the mouse mammary tumor virus promoter in COS-1, HeLa, and 293T cells. In vitro and in vivo studies revealed that Daxx could directly bind to GR. The mapping analysis further demonstrated that the C-terminal region of Daxx-(501-740) mediates the interaction and transcriptional repression of GR. The repressive effect of Daxx and Daxx-(501-740) on GR could be alleviated by co-expression of promyelocytic leukemia protein (PML). Furthermore, immunofluorescence analysis showed that overexpression of wild-type PML results in the translocation of Daxx and Daxx-(501-740) to the PML oncogenic domains (PODs). By contrast, a PML sumoylation-defective mutant failed to recruit Daxx to PODs and to reverse the Daxx repression effect on GR. Accordingly, As(2)O(3) treatment rendered the sequestration of endogenous Daxx to the PODs, leading to an enhancement of GR transactivation in COS-1 cells. Taken together, these findings suggest that recruitment of Daxx into the subnuclear POD structures sequesters it from the GR/co-activators complex, thereby alleviating its repressive effects. Our present studies provide the important link between Daxx/PML interaction and GR transcriptional activation.

RNAi reveals anti-apoptotic and transcriptionally repressive activities of DAXX

The function of DAXX, a highly conserved mammalian gene, has remained controversial; this is due, in part, to its identification in a variety of yeast two-hybrid screens. Targeted deletion in the mouse revealed that DAXX is essential for embryonic development. Furthermore, the increased levels of apoptosis observed in Daxx-knockout embryos and embryonic stem cell lines suggested that DAXX functions in an anti-apoptotic capacity. In contrast, overexpression studies showed that DAXX may promote apoptosis. Additional studies showed that, when overexpressed, DAXX could function as a transcriptional repressor. To clarify these matters, we have used RNAi to deplete endogenous DAXX and thereby assess DAXX function in cell lines previously tested in overexpression studies. Increased apoptosis was observed in DAXX-depleted cells, showing DAXX to be anti-apoptotic. The apoptosis induced by the absence of DAXX was rescued by Bcl-2 overexpression. In addition, transcriptional derepression was observed in RNAi-treated cells, indicating the ability of endogenous DAXX to repress gene expression and allowing for the identification of novel targets of DAXX repression, including nuclear factor kappaB (NF-kappaB)- and E2F1- regulated targets. Thus, depletion of DAXX by RNAi has verified the crucial role of endogenous DAXX as an anti-apoptotic regulator, and has allowed the identification of probable physiological targets of DAXX transcriptional repression.