Recruitment of Gcn5-containing complexes during c-Myc-dependent gene activation. Structure and function aspects

KAT Tales: Functions of Gcn5 and PCAF lysine acetyltransferases in SAGA and ATAC

The Allis group identified Gcn5 as the first transcription-related lysine acetyltransferase in 1996, providing a molecular 'missing link' between chromatin organization and gene regulation. This review will focus on functions subsequently identified for Gcn5 and the closely related PCAF protein, in the context of two major complexes, SAGA and ATAC, and how the study of these enzymes informs long standing questions regarding the importance of lysine acetylation.

The language of chromatin modification in human cancers

The genetic information of human cells is stored in the context of chromatin, which is subjected to DNA methylation and various histone modifications. Such a 'language' of chromatin modification constitutes a fundamental means of gene and (epi)genome regulation, underlying a myriad of cellular and developmental processes. In recent years, mounting evidence has demonstrated that miswriting, misreading or mis-erasing of the modification language embedded in chromatin represents a common, sometimes early and pivotal, event across a wide range of human cancers, contributing to oncogenesis through the induction of epigenetic, transcriptomic and phenotypic alterations. It is increasingly clear that cancer-related metabolic perturbations and oncohistone mutations also directly impact chromatin modification, thereby promoting cancerous transformation. Phase separation-based deregulation of chromatin modulators and chromatin structure is also emerging to be an important underpinning of tumorigenesis. Understanding the various molecular pathways that underscore a misregulated chromatin language in cancer, together with discovery and development of more effective drugs to target these chromatin-related vulnerabilities, will enhance treatment of human malignancies.

An update on stem cells applications in burn wound healing

Burn wounds have proven to be capable of having a long lasting devastating effects on human body. Conventional therapeutic approaches are not up to the mark as they are unable to completely heal the burn wound easily and effectively. Major pitfalls of these treatments include hypertrophic scarring, contracture and necrosis. Presence of these limitations in the current therapies necessitate the search for a better and more efficient cure. Regenerative potency of stem cells in burn wound healing outweigh the traditional treatment procedures. The use of multiple kinds of stem cells are gaining interest due to their enhanced healing efficiency. Distinctions of stem cells include better and faster burn wound healing, decreased inflammation levels, less scar progression and fibrosis on site. In this review, we have discussed the wound-healing process, present methods used for stem cells administration, methods of enhancing stem cells potency and human studies. Pre-clinical and the clinical studies focused on the treatment of thermal and radiation burns using stem cells from 2003 till the present time have been enlisted. Studies shows that the use of stem cells on burn wounds, whether alone or by the help of a scaffold significantly improves healing. Homing of the stem cells at the wound site results in the re-epithelialization, angiogenesis, granulation, inhibition of apoptosis, and regeneration of skin appendages together with reduced infection rate in the human studies. Several studies on animals have shown that stem cells can effectively promote wound healing. Although more research is needed to find out the effectiveness of this treatment in patients with severe burn wounds.

Transcriptional Activation of MYC-Induced Genes by GCN5 Promotes B-cell Lymphomagenesis

Overexpression of the MYC oncoprotein is an initiating step in the formation of several cancers. MYC frequently recruits chromatin-modifying complexes to DNA to amplify the expression of cancer-promoting genes, including those regulating cell cycle, proliferation, and metabolism, yet the roles of specific modifiers in different cancer types are not well defined. Here, we show that GCN5 is an essential coactivator of cell-cycle gene expression driven by MYC overexpression and that deletion of Gcn5 delays or abrogates tumorigenesis in the Eμ-Myc mouse model of B-cell lymphoma. Our results demonstrate that Gcn5 loss impacts both expression and downstream functions of Myc. SIGNIFICANCE: Our results provide important proof of principle for Gcn5 functions in formation and progression of Myc-driven cancers, suggesting that GCN5 may be a viable target for development of new cancer therapies.

Dynamic modules of the coactivator SAGA in eukaryotic transcription

SAGA (Spt-Ada-Gcn5 acetyltransferase) is a highly conserved transcriptional coactivator that consists of four functionally independent modules. Its two distinct enzymatic activities, histone acetylation and deubiquitylation, establish specific epigenetic patterns on chromatin and thereby regulate gene expression. Whereas earlier studies emphasized the importance of SAGA in regulating global transcription, more recent reports have indicated that SAGA is involved in other aspects of gene expression and thus plays a more comprehensive role in regulating the overall process. Here, we discuss recent structural and functional studies of each SAGA module and compare the subunit compositions of SAGA with related complexes in yeast and metazoans. We discuss the regulatory role of the SAGA deubiquitylating module (DUBm) in mRNA surveillance and export, and in transcription initiation and elongation. The findings suggest that SAGA plays numerous roles in multiple stages of transcription. Further, we describe how SAGA is related to human disease. Overall, in this report, we illustrate the newly revealed understanding of SAGA in transcription regulation and disease implications for fine-tuning gene expression.

A protein that helps add epigenetic information to genome, SAGA, controls many aspects of gene activation, potentially making it a target for cancer therapies. To fit inside the tiny cell nucleus, the genome is tightly packaged, and genes must be unpacked before they can be activated. Known to be important in genome opening, SAGA has now been shown to also play many roles in gene activation. Daeyoup Lee at the KAIST, Daejeon, South Korea, and co-workers have reviewed recent discoveries about SAGA’s structure, function, and roles in disease. They report that SAGA’s complex (19 subunits organized into four modules) allows it to play so many roles, genome opening, initiating transcription, and efficiently exporting mRNAs. Its master role means that malfunction of SAGA may be linked to many diseases.

GCN5 HAT inhibition reduces human Burkitt lymphoma cell survival through reduction of MYC target gene expression and impeding BCR signaling pathways

GCN5, the catalytic subunit in the acetyltransferase modules of SAGA and ATAC, functions as a coactivator of gene transcription. The SAGA complex is recruited to chromatin by transcription factors such as MYC and E2F1 to facilitate acetylation of histones, especially H3 at lysine 9 (H3K9). Burkitt lymphoma is an aggressive subtype of Non-Hodgkin lymphoma driven by the overexpression of MYC. Comparison of GCN5 expression in normal human B cells versus human Burkitt Lymphoma cell lines indicates overexpression of GCN5 in lymphoma. Treatment of Burkitt lymphoma cell lines with a specific inhibitor indicates that decreased GCN5 HAT activity reduces viability and proliferation of these cells. Inhibition of GCN5 HAT activity also induces apoptosis in lymphoma cells. Expression of MYC target genes as well as genes associated with B cell receptor signaling are significantly downregulated upon inhibition of GCN5 enzymatic activity. This downregulation leads to diminished PI3K signaling, a critical pathway in lymphomagenesis. Our data indicate that inhibition of GCN5 HAT activity reduces the tumorigenic properties of human Burkitt lymphoma cells by attenuating BCR signaling and that GCN5 may be a viable target for lymphoma drug therapy.

Myc-driven chromatin accessibility regulates Cdc45 assembly into CMG helicases

Myc-driven tumorigenesis involves a non-transcriptional role for Myc in over-activating replication origins. We show here that the mechanism underlying this process involves a direct role for Myc in activation of Cdc45-MCM-GINS (CMG) helicases at Myc-targeted sites. Myc induces decondensation of higher-order chromatin at targeted sites and is required for chromatin access at a chromosomal origin. Myc-driven chromatin accessibility promotes Cdc45/GINS recruitment to resident MCMs, and activation of CMGs. Myc-Box II, which is necessary for Myc-driven transformation, is required for Myc-induced chromatin accessibility, Cdc45/GINS recruitment, and replication stimulation. Myc interactors GCN5, Tip60, and TRRAP are essential for chromatin unfolding and recruitment of Cdc45, and co-expression of GCN5 or Tip60 with MBII-deficient Myc rescues these events and promotes CMG activation. Finally, Myc and Cdc45 interact and physiologic conditions for CMG assembly require the functions of Myc, MBII, and GCN5 for Cdc45 recruitment and initiation of DNA replication.

Post-translational modification localizes MYC to the nuclear pore basket to regulate a subset of target genes involved in cellular responses to environmental signals

In this study, Su et al. investigate how post-translational modifications of Myc that affect stability and oncogenic activity regulate its function. They show that Ser62 phosphorylation and PIN1-mediated isomerization of MYC dynamically regulate the spatial distribution of MYC in the nucleus, promoting its association with the inner basket of the nuclear pore in response to proliferative signals, where it recruits the histone acetyltransferase GCN5 to bind and regulate local gene acetylation and expression, thus providing new insights into how post-translational modification of MYC controls its spatial activity.

The transcription factor MYC (also c-Myc) induces histone modification, chromatin remodeling, and the release of paused RNA polymerase to broadly regulate transcription. MYC is subject to a series of post-translational modifications that affect its stability and oncogenic activity, but how these control MYC's function on the genome is largely unknown. Recent work demonstrates an intimate connection between nuclear compartmentalization and gene regulation. Here, we report that Ser62 phosphorylation and PIN1-mediated isomerization of MYC dynamically regulate the spatial distribution of MYC in the nucleus, promoting its association with the inner basket of the nuclear pore in response to proliferative signals, where it recruits the histone acetyltransferase GCN5 to bind and regulate local gene acetylation and expression. We demonstrate that PIN1-mediated localization of MYC to the nuclear pore regulates MYC target genes responsive to mitogen stimulation that are involved in proliferation and migration pathways. These changes are also present at the chromatin level, with an increase in open regulatory elements in response to stimulation that is PIN1-dependent and associated with MYC chromatin binding. Taken together, our study indicates that post-translational modification of MYC controls its spatial activity to optimally regulate gene expression in response to extrinsic signals in normal and diseased states.

USP22 maintains gastric cancer stem cell stemness and promotes gastric cancer progression by stabilizing BMI1 protein

Increased ubiquitin-specific protease 22 (USP22) has been associated with poor prognosis in several cancers including gastric cancer. However, the role of USP22 in gastric tumorigenesis is still unclear. Gastric cancer stem cells have been identified and shown to correlate with gastric cancer initiation and metastasis. In this study, we found that silencing of USP22 inhibited proliferation of gastric cancer cells and suppressed the cancer stem cell spheroid formation in serum-free culture. Furthermore, cancer stem cell markers, such as CD133, SOX2, OCT4 and NANOG were down-regulated. Additionally, knockdown of USP22 inhibited gastric cancer xenografts growth. Our analysis of TCGA database indicated that BMI1 overexpression may predict gastric cancer patient survival, and TAT-BMI1 proteins reversed the USP22 knockdown-mediated decreased in cancer stem cell properties, and elevated the expression of stemness-associated genes. Furthermore, we found that overexpression of USP22 stabilized the BMI1 protein in gastric cancer cells. Taken together, our study demonstrates that USP22 is indispensable for gastric cancer stem cell self-renewal through stabilization of BMI1. These results may provide novel approaches to the theranostics of gastric cancer in the near future.

Enhanced human somatic cell reprogramming efficiency by fusion of the MYC transactivation domain and OCT4

The development of human induced pluripotent stem cells (iPSCs) holds great promise for regenerative medicine. However the iPSC induction efficiency is still very low and with lengthy reprogramming process. We utilized the highly potent transactivation domain (TAD) of MYC protein to engineer the human OCT4 fusion proteins. Applying the MYC-TAD-OCT4 fusion proteins in mouse iPSC generation leads to shorter reprogramming dynamics, with earlier activation of pluripotent markers in reprogrammed cells than wild type OCT4 (wt-OCT4). Dramatic enhancement of iPSC colony induction efficiency and shortened reprogramming dynamics were observed when these MYC-TAD-OCT4 fusion proteins were used to reprogram primary human cells. The OCT4 fusion proteins induced human iPSCs are pluripotent. We further show that the MYC Box I (MBI) is dispensable while both MBII and the linking region between MBI/II are essential for the enhanced reprogramming activity of MYC-TAD-OCT4 fusion protein. Consistent with an enhanced transcription activity, the engineered OCT4 significantly stimulated the expression of genes specifically targeted by OCT4-alone, OCT4/SOX2, and OCT4/SOX2/KLF4 during human iPSC induction, compared with the wt-OCT4. The MYC-TAD-OCT4 fusion proteins we generated will be valuable tools for studying the reprogramming mechanisms and for efficient iPSC generation for humans as well as for other species.

Histone acetyl transferase GCN5 promotes human hepatocellular carcinoma progression by enhancing AIB1 expression

BACKGROUND

General control non-depressible 5 (GCN5) is a crucial catalytic component of a transcriptional regulatory complex that plays important roles in cellular functions from cell cycle regulation to DNA damage repair. Although GCN5 has recently been implicated in certain oncogenic roles, its role in liver cancer progression remains vague.

RESULTS

In this study, we report that GCN5 was overexpressed in 17 (54.8 %) of 31 human hepatocellular carcinoma (HCC) specimens. Down-regulation of GCN5 inhibited HCC cell proliferation and xenograft tumor formation. GCN5 knockdown decreased the protein levels of the proliferation marker proliferating cell nuclear antigen (PCNA) and amplified in breast cancer 1 (AIB1), but increased the protein levels of cell cycle inhibitor p21(Cip1/Waf1) in HepG2 cells. GCN5 regulated AIB1 expression, at least in part, by cooperating with E2F1 to enhance AIB1 transcription. Consistently, GCN5 expression was positively correlated with AIB1 expression in human HCC specimens in two GEO profile datasets.

CONCLUSION

Since AIB1 plays a promoting role in HCC progression, our results propose that GCN5 promotes HCC progression at least partially by regulating AIB1 expression. This study implicates that GCN5 might be a potential molecular target for HCC diagnosis and treatment.

The world of protein acetylation

Acetylation is one of the major post-translational protein modifications in the cell, with manifold effects on the protein level as well as on the metabolome level. The acetyl group, donated by the metabolite acetyl-coenzyme A, can be co- or post-translationally attached to either the α-amino group of the N-terminus of proteins or to the ε-amino group of lysine residues. These reactions are catalyzed by various N-terminal and lysine acetyltransferases. In case of lysine acetylation, the reaction is enzymatically reversible via tightly regulated and metabolism-dependent mechanisms. The interplay between acetylation and deacetylation is crucial for many important cellular processes. In recent years, our understanding of protein acetylation has increased significantly by global proteomics analyses and in depth functional studies. This review gives a general overview of protein acetylation and the respective acetyltransferases, and focuses on the regulation of metabolic processes and physiological consequences that come along with protein acetylation.

Functions of SAGA in development and disease

Precise regulation of gene expression programs during embryo development requires cooperation between transcriptional factors and histone-modifying enzymes, such as the Gcn5 histone acetyltransferase. Gcn5 functions within a multi-subunit complex, called SAGA, that is recruited to specific genes through interactions with sequence-specific DNA-binding proteins to aid in gene activation. Although the transcriptional programs regulated by SAGA in embryos are not well defined, deletion of either Gcn5 or USP22, the catalytic subunit of a deubiquitinase module in SAGA, leads to early embryonic lethality. Here, we review the known functions of Gcn5, USP22 and associated proteins during development and discuss how these functions might be related to human disease states, including cancer and neurodegenerative diseases.

KATs in cancer: functions and therapies

Post-translational acetylation of lysines is most extensively studied in histones, but this modification is also found in many other proteins and is implicated in a wide range of biological processes in both the cell nucleus and the cytoplasm. Like phosphorylation, acetylation patterns and levels are often altered in cancer, therefore small molecule inhibition of enzymes that regulate acetylation and deacetylation offers much potential for inhibiting cancer cell growth, as does disruption of interactions between acetylated residues and ‘reader’ proteins. For more than a decade now, histone deacetylase (HDAC) inhibitors have been investigated for their ability to increase acetylation and promote expression of tumor suppressor genes. However, emerging evidence suggests that acetylation can also promote cancer, in part by enhancing the functions of oncogenic transcription factors. In this review we focus on how acetylation of both histone and non-histone proteins may drive cancer, and we will discuss the implications of such changes on how patients are assigned to therapeutic agents. Finally, we will explore what the future holds in the design of small molecule inhibitors for modulation of levels or functions of acetylation states.

RNA polymerase III transcription - regulated by chromatin structure and regulator of nuclear chromatin organization

RNA polymerase III (Pol III) transcription is regulated by modifications of the chromatin. DNA methylation and post-translational modifications of histones, such as acetylation, phosphorylation and methylation have been linked to Pol III transcriptional activity. In addition to being regulated by modifications of DNA and histones, Pol III genes and its transcription factors have been implicated in the organization of nuclear chromatin in several organisms. In yeast, the ability of the Pol III transcription system to contribute to nuclear organization seems to be dependent on direct interactions of Pol III genes and/or its transcription factors TFIIIC and TFIIIB with the structural maintenance of chromatin (SMC) protein-containing complexes cohesin and condensin. In human cells, Pol III genes and transcription factors have also been shown to colocalize with cohesin and the transcription regulator and genome organizer CCCTC-binding factor (CTCF). Furthermore, chromosomal sites have been identified in yeast and humans that are bound by partial Pol III machineries (extra TFIIIC sites - ETC; chromosome organizing clamps - COC). These ETCs/COC as well as Pol III genes possess the ability to act as boundary elements that restrict spreading of heterochromatin.

Hyperexpansion of GAA repeats affects post-initiation steps of FXN transcription in Friedreich's ataxia

Friedreich's ataxia (FRDA) is caused by biallelic expansion of GAA repeats leading to the transcriptional silencing of the frataxin (FXN) gene. The exact molecular mechanism of inhibition of FXN expression is unclear. Herein, we analyze the effects of hyperexpanded GAA repeats on transcription status and chromatin modifications proximal and distal to the GAA repeats. Using chromatin immunoprecipitation and quantitative PCR we detected significant changes in the chromatin landscape in FRDA cells relative to control cells downstream of the promoter, especially in the vicinity of the GAA tract. In this region, hyperexpanded GAAs induced a particular constellation of histone modifications typically associated with heterochromatin-like structures. Similar epigenetic changes were observed in GFP reporter construct containing 560 GAA repeats. Furthermore, we observed similar levels of FXN pre-mRNA at a region upstream of hyperexpanded GAA repeats in FRDA and control cells, indicating similar efficiency of transcription initiation. We also demonstrated that histone modifications associated with hyperexpanded GAA repeats are independent of initiation and progression of transcription. Our data provide strong evidence that FXN deficiency in FRDA patients results from a block of transition from initiation to a productive elongation of FXN transcription due to heterochromatin-like structures formed in the proximity of the hyperexpanded GAAs.

ATAD2 is a novel cofactor for MYC, overexpressed and amplified in aggressive tumors

The E2F and MYC transcription factors are critical regulators of cell proliferation and contribute to the development of human cancers. Here, we report on the identification of a novel E2F target gene, ATAD2, the predicted protein product of which contains both a bromodomain and an ATPase domain. The pRB-E2F pathway regulates ATAD2 expression, which is limiting for the entry into the S phase of the cell cycle. We show that ATAD2 binds the MYC oncogene and stimulates its transcriptional activity. ATAD2 maps to chromosome 8q24, 4.3 Mb distal to MYC, in a region that is frequently found amplified in cancer. Consistent with this, we show that ATAD2 expression is high in several human tumors and that the expression levels correlate with clinical outcome of breast cancer patients. We suggest that ATAD2 links the E2F and MYC pathways and contributes to the development of aggressive cancer through the enhancement of MYC-dependent transcription.

Multivalent binding of p53 to the STAGA complex mediates coactivator recruitment after UV damage

The recruitment of transcriptional coactivators, including histone modifying enzymes, is an important step in transcription regulation. A typical activator is thought to interact with several cofactors, presumably in a sequential manner. The common use of several cofactors raises the question of how activators achieve both cofactor selectivity and diversity. Human STAGA is a multiprotein complex with the acetyltransferase GCN5L as the catalytic subunit. Here, we first show, through RNA interference-mediated knock-down and chromatin immunoprecipitation assays, that GCN5 plays a role in p53-dependent gene activation. We then employ p53 mutagenesis, in vitro binding, protein-protein cross-linking, and chromatin immunoprecipitation assays to establish a novel role for the second p53 activation subdomain (AD2) in STAGA recruitment and, further, to demonstrate that optimal binding of STAGA to p53 involves interactions of STAGA subunits TAF9, GCN5, and ADA2b, respectively, with AD1, AD2, and carboxy-terminal domains of p53. These results provide concrete evidence for mediation of transcription factor binding to coactivator complexes through multiple interactions. Based on our data, we propose a cooperative and modular binding mode for the recruitment of coactivator complexes to promoters.

Distinct GCN5/PCAF-containing complexes function as co-activators and are involved in transcription factor and global histone acetylation

Transcription in eukaryotes is a tightly regulated, multistep process. Gene-specific transcriptional activators, several different co-activators and general transcription factors are necessary to access specific loci to allow precise initiation of RNA polymerase II transcription. As the dense chromatin folding of the genome does not allow the access of these sites by the huge multiprotein transcription machinery, remodelling is required to loosen up the chromatin structure for successful transcription initiation. In the present review, we summarize the recent evolution of our understanding of the function of two histone acetyl transferases (ATs) from metazoan organisms: GCN5 and PCAF. Their overall structure and the multiprotein complexes in which they are carrying out their activities are discussed. Metazoan GCN5 and PCAF are subunits of at least two types of multiprotein complexes, one having a molecular weight of 2 MDa (SPT3-TAF9-GCN5 acetyl transferase/TATA binding protein (TBP)-free-TAF complex/PCAF complexes) and a second type with about a size of 700 kDa (ATAC complex). These complexes possess global histone acetylation activity and locus-specific co-activator functions together with AT activity on non-histone substrates. Thus, their biological functions cover a wide range of tasks and render them indispensable for the normal function of cells. That deregulation of the global and/or specific AT activities of these complexes leads to the cancerous transformation of the cells highlights their importance in cellular processes. The possible effects of GCN5 and PCAF in tumorigenesis are also discussed.

Mechanism of Transcription Factor Recruitment by Acidic Activators

Many transcriptional activators are intrinsically unstructured yet display unique, defined conformations when bound to target proteins. Target-induced folding provides a mechanism by which activators could form specific interactions with an array of structurally unrelated target proteins. Evidence for such a binding mechanism has been reported previously in the context of the interaction between the cancer-related c-Myc protein and the TATA-binding protein, which can be modeled as a two-step process in which a rapidly forming, low affinity complex slowly converts to a more stable form, consistent with a coupled binding and folding reaction. To test the generality of the target-induced folding model, we investigated the binding of two widely studied acidic activators, Gal4 and VP16, to a set of target proteins, including TATA-binding protein and the Swi1 and Snf5 subunits of the Swi/Snf chromatin remodeling complex. Using surface plasmon resonance, we show that these activator-target combinations also display bi-phasic kinetics suggesting two distinct steps. A fast initial binding phase that is inhibited by high ionic strength is followed by a slow phase that is favored by increased temperature. In all cases, overall affinity increases with temperature and, in most cases, with increased ionic strength. These results are consistent with a general mechanism for recruitment of transcriptional components to promoters by naturally occurring acidic activators, by which the initial contact is mediated predominantly through electrostatic interactions, whereas subsequent target-induced folding of the activator results in a stable complex.

Proteomic analysis of chromatin-modifying complexes in Saccharomyces cerevisiae identifies novel subunits

Epigenetics is the alteration of phenotype without affecting the genotype. An underlying molecular mechanism of epigenetics is the changes of chromatin structure by covalent histone modifications and nucleosome reorganization. In the yeast, Saccharomyces cerevisiae, two of the most well-studied macromolecular complexes that perform these epigenetic changes are the ATP-dependent Swi/Snf chromatin-remodelling complex and the SAGA histone acetyltransferase complex. To understand fully the mechanism by which these large protein complexes perform their functions in the cell, it is crucial that all the subunits of these complexes are identified. In an attempt to identify new subunits associated with SAGA and Swi/Snf, we used tandem affinity purification, followed by a multidimensional protein identification technology to analyse the subunit composition. Our analysis identified two novel proteins, one associated with SAGA, YPL047W (Sgf11), and another associated with Swi/Snf, Rtt102.

c-Myc transformation domain recruits the human STAGA complex and requires TRRAP and GCN5 acetylase activity for transcription activation

Deregulation of the c-Myc oncoprotein (Myc) is implicated in many types of cancer. Myc is a sequence-specific transcription factor that regulates transcription of genes involved in the control of cell proliferation and apoptosis via mechanisms that are still poorly understood. Cell transformation by Myc involves its association with the transformation-transactivation domain-associated protein (TRRAP) and the human histone acetyltransferase (HAT) GCN5. TRRAP and GCN5 are components of a variety of shared and distinct multiprotein HAT complexes with diverse functions. Myc induces TRRAP recruitment and histone hyperacetylation at specific Myc-activated genes in vivo. However, the identity of the HAT complexes recruited by Myc and the roles of TRRAP and GCN5 in Myc function are still unclear. Here we show that Myc co-recruits TRRAP and GCN5 via direct physical interactions of its N-terminal activation/transformation domain with the human STAGA (SPT3-TAF-GCN5 acetylase) coactivator complex. We demonstrate that GCN5 and TRRAP cooperate to enhance transcription activation by the N-terminal activation domain of Myc in vivo and that this synergy requires both the SPT3/GCN5 interaction domain of TRRAP and the HAT activity of GCN5. Thus, TRRAP might function as an adaptor within the STAGA complex, which helps recruit GCN5 HAT activity to Myc during transcription activation.

The adenovirus E1A oncoprotein recruits the cellular TRRAP/GCN5 histone acetyltransferase complex

The adenovirus E1A oncoprotein stimulates cell growth and inhibits differentiation by deregulating the normal transcription program via interaction with positive and negative cellular effectors. E1A associates with transcriptional regulatory complexes containing p400 and TRRAP involved in chromatin remodeling and decondensation. TRRAP is a component of three distinct human histone acetyltransferase (HAT) complexes: the TIP60 complex and complexes containing GCN5 or PCAF. We demonstrate here that E1A binds a TRRAP complex that contains the GCN5 acetyltransferase during a normal adenovirus infection. E1A binds GCN5 and TRRAP in vivo early after virus infection. E1A is associated with significant HAT activity in vitro that is partly attributable to GCN5. E1A represses c-Myc- and E2F-1-directed transcriptional activation in vivo by sequestering GCN5 and/or TRRAP. Our results demonstrate that E1A distinctly binds TRRAP/GCN5, p300/CBP and PCAF HAT complexes. Through interactions with multiple HAT complexes, E1A may deregulate cellular transcription programs and facilitate infection by recruiting functional HAT coactivators to viral and cellular promoter regions.

Myc recruits P-TEFb to mediate the final step in the transcriptional activation of the cad promoter

The c-Myc protein is up-regulated in many different types of cancer, suggesting that a detailed understanding of Myc function is an important goal. Our previous studies have focused on determining the mechanism by which Myc activates transcription using the target gene cad as an experimental model. Previously, we found that Myc activates cad transcription at a post-RNA polymerase II recruitment step and that the Myc transactivation domain interacts with a number of cdk-cyclin complexes. We now extend these studies to determine the role of these cyclin-cdk complexes in Myc-mediated transactivation. We have found that cyclin T1 binding to Myc localizes to the highly conserved Myc Box I, whereas cdk8 binding localizes to the amino-terminal 41 amino acids of the Myc transactivation domain. We showed that recruitment of cdk8 is sufficient for activation of a synthetic promoter construct. In contrast, the ability of Myc to activate transcription of the cad promoter correlates with binding of cyclin T1. Furthermore, recruitment of cyclin T1 to the cad promoter via a Gal4 fusion protein or through protein-protein interaction with the HIV-1 Tat protein can also activate cad transcription. These results suggest that Myc activates transcription by stimulating elongation and that P-TEFb is a key mediator of this process.