Ligand induction of a transcriptionally active thyroid hormone receptor coactivator complex

Factor recruitment and TIF2/GRIP1 corepressor activity at a collagenase-3 response element that mediates regulation by phorbol esters and hormones

To investigate determinants of specific transcriptional regulation, we measured factor occupancy and function at a response element, col3A, associated with the collagenase-3 gene in human U2OS osteosarcoma cells; col3A confers activation by phorbol esters, and repression by glucocorticoid and thyroid hormones. The subunit composition and activity of AP-1, which binds col3A, paralleled the intracellular level of cFos, which is modulated by phorbol esters and glucocorticoids. In contrast, a similar AP-1 site at the collagenase-1 gene, not inducible in U2OS cells, was not bound by AP-1. The glucocorticoid receptor (GR) associated with col3A through protein-protein interactions with AP-1, regardless of AP-1 subunit composition, and repressed transcription. TIF2/GRIP1, reportedly a coactivator for GR and the thyroid hormone receptor (TR), was recruited to col3A and potentiated GR-mediated repression in the presence of a GR agonist but not antagonist. GRIP1 mutants deficient in GR binding and coactivator functions were also defective for corepression, and a GRIP1 fragment containing the GR-interacting region functioned as a dominant-negative for repression. In contrast, repression by TR was unaffected by GRIP1. Thus, the composition of regulatory complexes, and the biological activities of the bound factors, are dynamic and dependent on cell and response element contexts. Cofactors such as GRIP1 probably contain distinct surfaces for activation and repression that function in a context-dependent manner.

Positive and negative TAF(II) functions that suggest a dynamic TFIID structure and elicit synergy with traps in activator-induced transcription

Human transcription factor TFIID contains the TATA-binding protein (TBP) and several TBP-associated factors (TAF(II)s). To elucidate the structural organization and function of TFIID, we expressed and characterized the product of a cloned cDNA encoding human TAF(II)135 (hTAF(II)135). Comparative far Western blots have shown that hTAF(II)135 interacts strongly with hTAF(II)20, moderately with hTAF(II)150, and weakly with hTAF(II)43 and hTAF(II)250. Consistent with these observations and with sequence relationships of hTAF(II)20 and hTAF(II)135 to histones H2B and H2A, respectively, TFIID preparations that contain higher levels of hTAF(II)135 also contain higher levels of hTAF(II)20, and the interaction between hTAF(II)20 and hTAF(II)135 is critical for human TFIID assembly in vitro. From a functional standpoint, hTAF(II)135 has been found to interact strongly and directly with hTFIIA and (within a complex that also contains hTBP and hTAF(II)250) to specifically cooperate with TFIIA to relieve TAF(II)250-mediated repression of TBP binding and function on core promoters. Finally, we report a functional synergism between TAF(II)s and the TRAP/Mediator complex in activated transcription, manifested as hTAF(II)-mediated inhibition of basal transcription and a consequent TRAP requirement for both a high absolute level of activated transcription and a high and more physiological activated/basal transcription ratio. These results suggest a dynamic TFIID structure in which the switch from a basal hTAF(II)-enhanced repression state to an activator-mediated activated state on a promoter may be mediated in part through activator or coactivator interactions with hTAF(II)135.

Estrogen receptor-mediated cross-talk with growth factor signaling pathways

Estrogen (E2) palys critical roles in the development of tumors in female reproductive organs. Development of most breast cancers is dependent on E&sub2; in most cases. Most E&sub2; actions are considered to be exerted through two subtypes of Estrogen receptors (ERs), ERalpha and ERbeta. ERs belong to the nuclear receptor superfamily, and act as ligand-inducible transcription factors to activate transcription of a particular set of the target genes. Ligand-bound ER recruits at least two distinct classes of coactivator complexes. In estrogen-dependent breast cancer, growth factors are shown to often act synergisticaly with E&sub2;, and the breast cancer often become resistant to treatment of estogen antagonists. However, the molecular basis of this coupled regulation of growth factor and ER-mediated signaling and hormone-resistance are largely unknown. We have previously shown that MAP (mitogen-activated protein) kinase (MAPK) activated by growth factors phosphorylates and potentiates the N-terminal transactivation function (AF-1), indicating a possible molecular mechanism of a novel cross-talk between two signalings (Kato et al, 1995). Furthermore, we have identified a coactivator that specifically interacts with ER alpha AF-1 (Endoh et al, 1999). In this review, this cross-talk is discussed in terms of the transactivation function of ERs and their coactivators.

Functional interaction between sterol regulatory element-binding protein-1c, nuclear factor Y, and 3,5,3'-triiodothyronine nuclear receptors

Sterol regulatory element binding protein-1c (SREBP-1c) is a key hepatic transcription factor involved in lipogenic gene expression. In an effort to understand the role SREBP-1c plays in lipogenic gene transcription, we have examined the functional interaction between SREBP-1c, nuclear factor Y, 3,5,3'-triiodothyronine (T(3)) receptors, and co-activators using the S14 gene promoter as a model. T(3), glucose, and insulin rapidly induce S14 gene transcription in rat liver and in primary hepatocytes. Linker scanning analyses of the S14 promoter showed that an SRE at -139/-131 base pairs (bp) binding SREBP-1c and a Y-box at -104/-99 bp binding NF-Y are indispensable for both T(3)- and SREBP-1c-mediated induction of S14 promoter activity in rat primary hepatocytes. T(3) and glucose/insulin induce S14 gene transcription through separate enhancers. Enhancer substitution studies reveal a preferential interaction between SREBP-1c.NF-Y and the T(3) regulatory region (-2.8/-2.5 kb) binding thyroid hormone receptor/RXR heterodimers. Elevating hepatocellular levels of specific co-activators (CBP, p/CAF, or GCN5) induced S14 promoter activity 2-3-fold, while SREBP-1c induced promoter activity 10-fold. The combination of these treatments induced S14 promoter activity (20-35-fold). However, this additive effect was lost when the T(3) regulatory region was deleted. Based on these results, we suggest that the SREBP-1c.NF-Y complex facilitates the interaction between co-activators that are recruited to distal hormone-regulated enhancers and the general transcription machinery that binds the S14 proximal promoter.

Transcriptional activation domains of human heat shock factor 1 recruit human SWI/SNF

Chromatin remodeling complexes such as SWI/SNF use the energy of ATP hydrolysis to remodel nucleosomal DNA and increase transcription of nucleosomal templates. Human heat shock factor one (hHSF1) is a tightly regulated activator that stimulates transcriptional initiation and elongation using different portions of its activation domains. Here we demonstrate that hHSF1 associates with BRG1, the ATPase subunit of human SWI/SNF (hSWI/SNF) at endogenous protein concentrations. We also show that hHSF1 activation domains recruit hSWI/SNF to a chromatin template in a purified system. Mutation of hHSF1 residues responsible for activation of transcriptional elongation has the most severe effect on recruitment of SWI/SNF and association of hHSF1 with BRG1, suggesting that recruitment of chromatin remodeling activity might play a role in stimulation of elongation.

Molecular cloning and characterization of a steroid receptor-binding regulator of G-protein signaling protein cDNA

Steroid hormone receptors are composed of six major functional domains, i.e. the A/B domains as the activation function 1 domain (AF-1), domain C as the DNA-binding domain, domain D as a hinge domain and domain E/F as the ligand-dependent transcriptional domain (AF-2). They regulate gene transcription through interactions with various nuclear factors of their domains, such as AF-1 and AF-2. We have insufficient knowledge of the function of the DNA-binding domain (domain C) except for its DNA-binding function or the hinge domain (domain D). Therefore, we attempted to identify factors interacting with the domains by using a yeast two-hybrid system. Domains C and D of estrogen receptor alpha were used as a bait to isolate cDNA clones from a rat ovary cDNA library. We isolated the cDNA clone of a novel steroid receptor-binding protein bearing the regulator of G-protein signaling (RGS) designated as SRB-RGS. The protein repressed the transcriptional activity of estrogen receptor alpha, suggesting cross-talk of steroid hormones and peptide hormones (or growth factors) for signal transductions mediated by SRB-RGS.

Characterization of mediator complexes from HeLa cell nuclear extract

A number of mammalian multiprotein complexes containing homologs of Saccharomyces cerevisiae Mediator subunits have been described recently. High-molecular-mass complexes (1 to 2 MDa) sharing several subunits but apparently differing in others include the TRAP/SMCC, NAT, DRIP, ARC, and human Mediator complexes. Smaller multiprotein complexes (approximately 500 to 700 kDa), including the murine Mediator, CRSP, and PC2, have also been described that contain subsets of subunits of the larger complexes. To evaluate whether these different multiprotein complexes exist in vivo in a single form or in multiple different forms, HeLa cell nuclear extract was directly resolved over a Superose 6 gel filtration column. Immunoblotting of column fractions using antisera specific for several Mediator subunits revealed one major size class of high-molecular-mass (approximately 2-MDa) complexes containing multiple mammalian Mediator subunits. No peak was apparent at approximately 500 to 700 kDa, indicating that either the smaller complexes reported are much less abundant than the higher-molecular-mass complexes or they are subcomplexes generated by dissociation of larger complexes during purification. Quantitative immunoblotting indicated that there are about 3 x 10(5) to 6 x 10(5) molecules of hSur2 Mediator subunit per HeLa cell, i.e., the same order of magnitude as RNA polymerase II and general transcription factors. Immunoprecipitation of the approximately 2-MDa fraction with anti-Cdk8 antibody indicated that at least two classes of Mediator complexes occur, one containing CDK8 and cyclin C and one lacking this CDK-cyclin pair. The approximately 2-MDa complexes stimulated activated transcription in vitro, whereas a 150-kDa fraction containing a subset of Mediator subunits inhibited activated transcription.

Physiological and molecular basis of thyroid hormone action

Thyroid hormones (THs) play critical roles in the differentiation, growth, metabolism, and physiological function of virtually all tissues. TH binds to receptors that are ligand-regulatable transcription factors belonging to the nuclear hormone receptor superfamily. Tremendous progress has been made recently in our understanding of the molecular mechanisms that underlie TH action. In this review, we present the major advances in our knowledge of the molecular mechanisms of TH action and their implications for TH action in specific tissues, resistance to thyroid hormone syndrome, and genetically engineered mouse models.

Nuclear receptors, coactivators and chromatin: new approaches, new insights

The actions of lipophilic hormones, including steroids, retinoids, thyroid hormone and vitamin D(3), are mediated through a conserved superfamily of nuclear receptor proteins that function as ligand-regulated, DNA-binding transcriptional activators in the chromatin environment of the nucleus. The ligand-dependent transcriptional activity of nuclear receptors is enhanced by various cofactors that remodel chromatin, acetylate nucleosomal histones and contact the basal transcriptional machinery. The current challenge is to understand the mechanistic details of how interactions among these factors enhance transcription of hormone-regulated genes assembled into chromatin. Current biochemical and cell-based methods are providing some important clues.

Inhibition of androgen receptor-mediated transcription by amino-terminal enhancer of split

A yeast two-hybrid assay has identified an androgen-dependent interaction of androgen receptor (AR) with amino-terminal enhancer of split (AES), a member of the highly conserved Groucho/TLE family of corepressors. Full-length AR, as well as the N-terminal fragment of AR, showed direct interactions with AES in in vitro protein-protein interaction assays. AES specifically inhibited AR-mediated transcription in a well-defined cell-free transcription system and interacted specifically with the basal transcription factor (TFIIE) in HeLa nuclear extract. These observations implicate AES as a selective repressor of ligand-dependent AR-mediated transcription that acts by directly interacting with AR and by targeting the basal transcription machinery.

Nuclear hormone receptors and gene expression

The nuclear hormone receptor superfamily includes receptors for thyroid and steroid hormones, retinoids and vitamin D, as well as different "orphan" receptors of unknown ligand. Ligands for some of these receptors have been recently identified, showing that products of lipid metabolism such as fatty acids, prostaglandins, or cholesterol derivatives can regulate gene expression by binding to nuclear receptors. Nuclear receptors act as ligand-inducible transcription factors by directly interacting as monomers, homodimers, or heterodimers with the retinoid X receptor with DNA response elements of target genes, as well as by "cross-talking" to other signaling pathways. The effects of nuclear receptors on transcription are mediated through recruitment of coregulators. A subset of receptors binds corepressor factors and actively represses target gene expression in the absence of ligand. Corepressors are found within multicomponent complexes that contain histone deacetylase activity. Deacetylation leads to chromatin compactation and transcriptional repression. Upon ligand binding, the receptors undergo a conformational change that allows the recruitment of multiple coactivator complexes. Some of these proteins are chromatin remodeling factors or possess histone acetylase activity, whereas others may interact directly with the basic transcriptional machinery. Recruitment of coactivator complexes to the target promoter causes chromatin decompactation and transcriptional activation. The characterization of corepressor and coactivator complexes, in concert with the identification of the specific interaction motifs in the receptors, has demonstrated the existence of a general molecular mechanism by which different receptors elicit their transcriptional responses in target genes.

Differential recruitment of the mammalian mediator subunit TRAP220 by estrogen receptors ERalpha and ERbeta

Estrogen receptors (ERs) associate with distinct transcriptional coactivators to mediate activation of target genes in response to estrogens. Previous work has provided multiple evidence for a critical role of p160 coactivators and associated histone acetyltransferases in estrogen signaling. In contrast, the involvement of the mammalian mediator complex remains to be established. Further, although the two subtypes ERalpha and ERbeta appear to be similar in regard to principles of LXXLL-mediated coactivator binding to the AF-2 activation domain, there are indications that the context-dependent transcriptional activation profiles of the two ERs can be quite distinct. Potentially, this could be attributed to differences with regard to coregulator recruitment. We have here studied the interactions of the nuclear receptor-binding subunit of the mammalian mediator complex, referred to as TRAP220, with ERalpha and ERbeta. In comparison to the p160 coactivator TIF2, we find that TRAP220 displays ERbeta preference. Here, we show that this is a feature of the binding specificity of the TRAP220 LXXLL motifs and demonstrate that the ER subtype-specific F-domain influences TRAP220 interaction. Such differences with regard to coactivator recruitment indicate that the relative importance of individual coregulators in estrogen signaling could depend on the dominant ER subtype.

An array of positioned nucleosomes potentiates thyroid hormone receptor action in vivo

The assembly of the genome into chromatin imposes a poorly understood set of rules and constraints on action by regulatory factors. We investigated the role played by chromatin infrastructure in enabling an acute response of the Xenopus TRbetaA gene to thyroid hormone receptor (TR), an extensively studied member of the nuclear hormone receptor superfamily. We found that in addition to the known TR response element (TRE) in the promoter, full range regulation required an upstream enhancer that contained multiple nonconsensus TREs and augmented ligand action at high receptor levels. An array of translationally positioned nucleosomes formed over the TRbetaA locus in vivo; unliganded TR engaged this array in linker DNA between two nucleosomes and via TREs on the surface of histone octamers. Remarkably, assembly of enhancer DNA into mature chromatin potentiated binding by TR to its target response elements and enabled a greater range of regulation by TR than was observed on immature chromatin templates. Because assembly of enhancer DNA into chromatin increased TR binding to the nonconsensus TREs, we hypothesize that chromatin disruption targeted by liganded TR to the enhancer may lead to receptor release from the template and to an attenuation of response to hormone.

Mediator complexes and transcription

Over the past decade, various components of the transcription machinery have been identified as potential targets for activators. Recently, metazoan versions of yeast Mediator have been isolated and found to act as key coactivators to many transcription factors. Recent work has defined the composition, function and biology of metazoan mediator complexes, which has led us to propose a new nomenclature for the variously named versions of the mediator complex.

Nuclear receptors coordinate the activities of chromatin remodeling complexes and coactivators to facilitate initiation of transcription

Recent advances in the field of in vitro chromatin assembly have led to in vitro transcription systems which reproduce in the test tube, in vivo characteristics of ligand-dependent transcriptional activation by nuclear receptors. Dissection of these systems has begun to provide us with information concerning the underlying molecular mechanisms. Through recruitment of coactivator proteins, nuclear receptors act first to remodel chromatin within the promoter region and then to recruit the transcriptional machinery to the promoter region in order to initiate transcription. Here we present a possible sequential mechanism for ligand-dependent transcriptional activation by nuclear receptors and discuss the in vitro and in vivo data that support this model.

PSF is a novel corepressor that mediates its effect through Sin3A and the DNA binding domain of nuclear hormone receptors

Members of the type II nuclear hormone receptor subfamily (e.g., thyroid hormone receptors [TRs], retinoic acid receptors, retinoid X receptors [RXRs], vitamin D receptor, and the peroxisome proliferator-activated receptors) bind to their response sequences with or without ligand. In the absence of ligand, these DNA-bound receptors mediate different degrees of repression or silencing of gene expression which is thought to result from the association of their ligand binding domains (LBDs) with corepressors. Two related corepressors, N-CoR and SMRT, interact to various degrees with the LBDs of these type II receptors in the absence of their cognate ligands. N-CoR and SMRT have been proposed to act by recruiting class I histone deacetylases (HDAC I) through an association with Sin3, although they have also been shown to recruit class II HDACs through a Sin3-independent mechanism. In this study, we used a biochemical approach to identify novel nuclear factors that interact with unliganded full-length TR and RXR. We found that the DNA binding domains (DBDs) of TR and RXR associate with two proteins which we identified as PSF (polypyrimidine tract-binding protein-associated splicing factor) and NonO/p54(nrb). Our studies indicate that PSF is a novel repressor which interacts with Sin3A and mediates silencing through the recruitment of HDACs to the receptor DBD. In vivo studies with TR showed that although N-CoR fully dissociates in the presence of ligand, the levels of TR-bound PSF and Sin3A appear to remain unchanged, indicating that Sin3A can be recruited to the receptor independent of N-CoR or SMRT. RXR was not detected to bind N-CoR although it bound PSF and Sin3A as effectively as TR, and this association with RXR did not change with ligand. Our studies point to a novel PSF/Sin3-mediated pathway for nuclear hormone receptors, and possibly other transcription factors, which may fine-tune the transcriptional response as well as play an important role in mediating the repressive effects of those type II receptors which only weakly interact with N-CoR and SMRT.

The TRAP/SMCC/Mediator complex and thyroid hormone receptor function

The TRAP/SMCC/Mediator complex is a mammalian transcriptional regulatory complex that contains over 25 polypeptides and is, in part, phylogenetically conserved. It was originally isolated as a thyroid hormone receptor (TR)-associated protein (TRAP) complex that mediates TR-activated transcription from DNA templates in conjunction with the general transcription machinery, and probably acts in vivo after the action of other receptor-interacting coactivators involved in chromatin remodeling. Subsequently, the TRAP complex was identified as a more broadly used coactivator complex for a wide variety of activators. The TRAP220 subunit mediates ligand-dependent interactions of the complex with TR and other nuclear receptors; and genetic ablation of murine TRAP220 has revealed that it is essential both for optimal TR function and for a variety of early developmental and adult homeostasis events in mice, but not for cell viability per se.

Transcription of eukaryotic protein-coding genes

The past decade has seen an explosive increase in information about regulation of eukaryotic gene transcription, especially for protein-coding genes. The most striking advances in our knowledge of transcriptional regulation involve the chromatin template, the large complexes recruited by transcriptional activators that regulate chromatin structure and the transcription apparatus, the holoenzyme forms of RNA polymerase II involved in initiation and elongation, and the mechanisms that link mRNA processing with its synthesis. We describe here the major advances in these areas, with particular emphasis on the modular complexes associated with RNA polymerase II that are targeted by activators and other regulators of mRNA biosynthesis.

Contributions of nuclear architecture and chromatin to vitamin D-dependent transcriptional control of the rat osteocalcin gene

The vitamin D response element in the bone tissue-specific osteocalcin gene has served as a prototype for understanding molecular mechanisms regulating physiologic responsiveness of vitamin D-dependent genes in bone cells. We briefly review factors which contribute to vitamin D transcriptional control. The organization of the vitamin D response element (VDRE), the multiple activities of the vitamin D receptor transactivation complex, and the necessity for protein-protein interactions between the VDR-RXR heterodimer activation complex and DNA binding proteins at other regulatory elements, including AP-1 sites and TATA boxes, provide for precise regulation of gene activity in concert with basal levels of transcription. We present evidence for molecular mechanisms regulating vitamin D-dependent mediated transcription of the osteocalcin gene that involve chromatin structure of the gene and nuclear architecture. Modifications in nucleosomal organization, DNase I hypersensitivity and localization of vitamin D receptor interacting proteins in subnuclear domains are regulatory components of vitamin D-dependent gene transcription. A model is proposed to account for the inability of vitamin D induction of the osteocalcin gene in the absence of ongoing basal transcription by competition of the YY1 nuclear matrix-associated transcription factor for TFIIB-VDR interactions. Activation of the VDR-RXR complex at the OC VDRE occurs through modifications in chromatin mediated in part by interaction of OC gene regulatory sequences with the nuclear matrix-associated Cbfa1 (Runx2) transcription factor which is required for osteogenesis.

Mediator of transcriptional regulation

Three lines of evidence have converged on a multiprotein Mediator complex as a conserved interface between gene-specific regulatory proteins and the general transcription apparatus of eukaryotes. Mediator was discovered as an activity required for transcriptional activation in a reconstituted system from yeast. Upon resolution to homogeneity, the activity proved to reside in a 20-protein complex, which could exist in a free state or in a complex with RNA polymerase II, termed holoenzyme. A second line of evidence came from screens in yeast for mutations affecting transcription. Two-thirds of Mediator subunits are encoded by genes revealed by these screens. Five of the genetically defined subunits, termed Srbs, were characterized as interacting with the C-terminal domain of RNA polymerase II in vivo, and were shown to bind polymerase in vitro. A third line of evidence has come recently from studies in mammalian transcription systems. Mammalian counterparts of yeast Mediator were shown to interact with transcriptional activator proteins and to play an essential role in transcriptional regulation. Mediator evidently integrates and transduces positive and negative regulatory information from enhancers and operators to promoters. It functions directly through RNA polymerase II, modulating its activity in promoter-dependent transcription. Details of the Mediator mechanism remain obscure. Additional outstanding questions include the patterns of promoter-specificity of the various Mediator subunits, the possible cell-type-specificity of Mediator subunit composition, and the full structures of both free Mediator and RNA polymerase II holoenzyme.

Generation of a mammalian cell line stably expressing a tetracycline-regulated epitope-tagged human androgen receptor: implications for steroid hormone receptor research

The androgen receptor (AR) is hormone-activated transcription factor that regulates the expression of genes involved in differentiation, development, and maintenance of male reproductive functions. To establish a useful model system for studying molecular mechanisms of AR action, we generated a HeLa-derived cell line (termed E19) that stably expresses human AR. Because overexpression of AR in cultured cells can be cytotoxic, we placed AR expression under the control of a tetracycline-regulated promoter. The stably expressed AR also contains an N-terminal FLAG-epitope tag (f:AR) that provides an advantageous method for immunopurification. We show that f:AR expression in E19 cells can be precisely modulated by varying the concentration of tetracycline or its chemical derivative doxycycline in the growth media. The functional activity of E19-expressed f:AR is demonstrated in vivo by its ability to activate transiently transfected AR reporter genes in an androgen-dependent manner, and in vitro by its ability to specifically bind AR-response elements using DNA-mobility shift assays. We further show that f:AR in androgen-stimulated E19 cells is markedly phosphorylated and coimmunopurifies with the transcriptional coactivator CREB-binding protein (CBP). The implications of these findings on steroid receptor research and the identification of receptor coregulatory factors will be discussed.

Drosophila homologues of the transcriptional coactivation complex subunits TRAP240 and TRAP230 are required for identical processes in eye-antennal disc development

We have identified mutations in two genes, blind spot and kohtalo, that encode Drosophila homologues of human TRAP240 and TRAP230, components of a large transcriptional coactivation complex homologous to the yeast Mediator complex. Loss of either blind spot or kohtalo has identical effects on the development of the eye-antennal disc. Eye disc cells mutant for either gene can express decapentaplegic and atonal in response to Hedgehog signaling, but they maintain inappropriate expression of these genes and fail to differentiate further. Mutant cells in the antennal disc lose expression of Distal-less and misexpress eyeless, suggesting a partial transformation towards the eye fate. blind spot and kohtalo are not required for cell proliferation or survival, and their absence cannot be rescued by activation of the Hedgehog or Notch signaling pathways. These novel and specific phenotypes suggest that TRAP240 and TRAP230 act in concert to mediate an unknown developmental signal or a combination of signals.

Analysis of the steroid receptor coactivator 1 (SRC1)-CREB binding protein interaction interface and its importance for the function of SRC1

The transcriptional activity of nuclear receptors is mediated by coactivator proteins, including steroid receptor coactivator 1 (SRC1) and its homologues and the general coactivators CREB binding protein (CBP) and p300. SRC1 contains an activation domain (AD1) which functions via recruitment of CBP and and p300. In this study, we have used yeast two-hybrid and in vitro interaction-peptide inhibition experiments to map the AD1 domain of SRC1 to a 35-residue sequence potentially containing two alpha-helices. We also define a 72-amino-acid sequence in CBP necessary for SRC1 binding, designated the SRC1 interaction domain (SID). We show that in contrast to SRC1, direct binding of CBP to the estrogen receptor is weak, suggesting that SRC1 functions primarily as an adaptor to recruit CBP and p300. In support of this, we show that the ability of SRC1 to enhance ligand-dependent nuclear receptor activity in transiently transfected cells is dependent upon the integrity of the AD1 region. In contrast, the putative histone acetyltransferase domain, the Per-Arnt-Sim basic helix-loop-helix domain, the glutamine-rich domain, and AD2 can each be removed without loss of ligand-induced activity. Remarkably, a construct corresponding to residues 631 to 970, which contains only the LXXLL motifs and the AD1 region of SRC1, retained strong coactivator activity in our assays.

Structural organization of yeast and mammalian mediator complexes

Structures of yeast Mediator complex, of a related complex from mouse cells and of thyroid hormone receptor-associated protein complex from human cells have been determined by three-dimensional reconstruction from electron micrographs of single particles. All three complexes show a division in two parts, a "head" domain and a combined "middle-tail" domain. The head domains of the three complexes appear most similar and interact most closely with RNA polymerase II. The middle-tail domains show the greatest structural divergence and, in the case of the tail domain, may not interact with polymerase at all. Consistent with this structural divergence, analysis of a yeast Mediator mutant localizes subunits that are not conserved between yeast and mammalian cells to the tail domain. Biochemically defined Rgr1 and Srb4 modules of yeast Mediator are then assigned to the middle and head domains.

Cofactor dynamics and sufficiency in estrogen receptor-regulated transcription

Many cofactors bind the hormone-activated estrogen receptor (ER), yet the specific regulators of endogenous ER-mediated gene transcription are unknown. Using chromatin immunoprecipitation (ChIP), we find that ER and a number of coactivators rapidly associate with estrogen responsive promoters following estrogen treatment in a cyclic fashion that is not predicted by current models of hormone activation. Cycles of ER complex assembly are followed by transcription. In contrast, the anti-estrogen tamoxifen (TAM) recruits corepressors but not coactivators. Using a genetic approach, we show that recruitment of the p160 class of coactivators is sufficient for gene activation and for the growth stimulatory actions of estrogen in breast cancer supporting a model in which ER cofactors play unique roles in estrogen signaling.

Regulation of somatic growth by the p160 coactivator p/CIP

A family of p160 coactivators was initially identified based on ligand-dependent interactions with nuclear receptors and thought to function, in part, by recruiting CREB-binding protein/p300 to several classes of transcription factors. One of the p160 factors, p/CIP/AIB1, often amplified and overexpressed in breast cancer, also exhibits particularly strong interaction with CREB-binding protein/p300. In this manuscript, we report that p/CIP, which exhibits regulated transfer from cytoplasm to nucleus, is required for normal somatic growth from embryonic day 13.5 through maturity. Our data suggest that a short stature phenotype of p/CIP gene-deleted mice reflect both altered regulation of insulin-like growth factor-1 (IGF-1) gene expression in specific tissues and a cell-autonomous defect of response to IGF-1, including ineffective transcriptional activities by several classes of regulated transcription factors under specific conditions. The actions of p/CIP are therefore required for full expression of a subset of genes critical for regulating physiological patterns of somatic growth in mammals.

Temporal formation of distinct thyroid hormone receptor coactivator complexes in HeLa cells

Thyroid hormone receptors (TRs) regulate transcription by recruiting distinct coregulatory complexes to target gene promoters. Coactivators implicated in ligand-dependent activation by TR include p300, the CREB-binding protein (CBP), members of the p160/SRC family, and the multisubunit TR-associated protein (TRAP) complex. Using a stable TR-expressing HeLa cell line, we show that interaction of TR with members of the p160/SRC family, CBP, and the p300/CBP-associated factor (PCAF) occurs rapidly (approximately 10 min) following addition of thyroid hormone (T3). In close agreement with these observations, we find that TR is associated with potent histone acetyltransferase activity rapidly following T3-treatment. By contrast, we observe that formation of TR-TRAP complexes occurs significantly later (approximately 3 h) post T3 treatment. An examination of the kinetics of T3-induced gene expression in HeLa cells reveals bimodal or delayed activation on specific T3-responsive promoters. Taken together, our data are consistent with the hypothesis that T3-dependent activation at specific target promoters may involve the regulated action of multiple TR-coactivator complexes.

Discrete roles for peroxisome proliferator-activated receptor gamma and retinoid X receptor in recruiting nuclear receptor coactivators

Peroxisome proliferator-activated receptor gamma (PPARgamma) plays a major role in adipogenesis. PPARgamma binds to DNA as a heterodimer with retinoid X receptor (RXR), and PPARgamma-RXR can be activated by ligands specific for either receptor; the presence of both ligands can result in a cooperative effect on the transactivation of target genes. How these ligands mediate transactivation, however, remains unclear. PPARgamma is known to interact with both the p160/SRC-1 family of coactivators and the distinct, multisubunit coactivator complex called DRIP. A single DRIP subunit, DRIP205 (TRAP220, PBP), binds directly to PPARgamma. Here we report that PPARgamma and RXR selectively interacted with DRIP205 and p160 proteins in a ligand-dependent manner. At physiological concentrations, RXR-specific ligands only induced p160 binding to RXR, and PPARgamma-specific ligands exclusively recruited DRIP205 but not p160 coactivators to PPARgamma. This selectivity was not observed in interaction assays off DNA, implying that the specificity of coactivator binding in response to ligand is strongly influenced by the allosteric effects of DNA-bound heterodimers. These coactivator-selective effects were also observed in transient-transfection assays in the presence of overexpressed p160 or DRIP coactivators. The results suggest that the cooperative effects of PPARgamma- and RXR-specific ligands may occur at the level of selective coactivator recruitment.

The function of PML in p53-dependent apoptosis

The PML gene of acute promyelocytic leukaemia (APL) encodes a growth- and tumour-suppresor protein that is essential for several apoptotic signals. The mechanisms by which PML exerts its pro-apoptotic function are still unknown. Here we show that PML acts as a transcriptional co-activator with p53. PML physically interacts with p53 both in vitro and in vivo and co-localizes with p53 in the PML nuclear body (PML-NB). The co-activatory role of PML depends on its ability to localize in the PML-NB. p53-dependent, DNA-damage-induced apoptosis, transcriptional activation by p53, the DNA-binding ability of p53, and the induction of p53 target genes such as Bax and p21 upon gamma-irradiation are all impaired in PML-/- primary cells. These results define a new PML-dependent, p53-regulatory pathway for apoptosis and shed new light on the function of PML in tumour suppression.

Evidence for a common step in three different processes for modulating the kinetic properties of glucocorticoid receptor-induced gene transcription

The dose-response curve of steroid hormones and the associated EC(50) value are critical parameters both in the development of new pharmacologically active compounds and in the endocrine therapy of various disease states. We have recently described three different variables that can reposition the dose-response curve of agonist-bound glucocorticoid receptors (GRs): a 21-base pair sequence of the rat tyrosine aminotransferase gene called a glucocorticoid modulatory element (GME), GR concentration, and coactivator concentration. At the same time, each of these three components was found to influence the partial agonist activity of antiglucocorticoids. In an effort to determine whether these three processes proceed via independent pathways or a common intermediate, we have examined several mechanistic details. The effects of increasing concentrations of both GR and the coactivator TIF2 are found to be saturable. Furthermore, saturating levels of either GR or TIF2 inhibit the ability of each protein, and the GME, to affect further changes in the dose-response curve or partial agonist activity of antisteroids. This competitive inhibition suggests that all three modulators proceed through a common step involving a titratable factor. Support for this hypothesis comes from the observation that a fragment of the coactivator TIF2 retaining intrinsic transactivation activity is a dominant negative inhibitor of each component (GME, GR, and coactivator). This inhibition was not due to nonspecific effects on the general transcription machinery as the VP16 transactivation domain was inactive. The viral protein E1A also prevented the action of each of the three components in a manner that was independent of E1A's ability to block the histone acetyltransferase activity of CBP. Collectively, these results suggest that three different inputs (GME, GR, and coactivator) for perturbing the dose-response curve, and partial agonist activity, of GR-steroid complexes act by converging at a single step that involves a limiting factor prior to transcription initiation.

The mechanism of action of thyroid hormones

Thyroid hormone is essential for normal development, differentiation, and metabolic balance. Thyroid hormone action is mediated by multiple thyroid hormone receptor isoforms derived from two distinct genes. The thyroid hormone receptors belong to a nuclear receptor superfamily that also includes receptors for other small lipophilic hormones. Thyroid hormone receptors function by binding to specific thyroid hormone-responsive sequences in promoters of target genes and by regulating transcription. Thyroid hormone receptors often form heterodimers with retinoid X receptors. Heterodimerization is regulated through distinct mechanisms that together determine the specificity and flexibility of the sequence recognition. Amino-terminal regions appear to modulate thyroid hormone receptor function in an isoform-dependent manner. Unliganded thyroid hormone receptor represses transcription through recruitment of a corepressor complex, which also includes Sin3A and histone deacetylase. Ligand binding alters the conformation of the thyroid hormone receptor in such a way as to release the corepressor complex and recruit a coactivator complex that includes multiple histone acetyltransferases, including a steroid receptor family coactivator, p300/CREB-binding protein-associated factor (PCAF), and CREB binding protein (CBP). The existence of histone-modifying activities in the transcriptional regulatory complexes indicates an important role of chromatin structure. Stoichiometric, structural, and sequence-specific rules for coregulator interaction are beginning to be understood, as are aspects of the tissue specificity of hormone action. Moreover, knockout studies suggest that the products of two thyroid hormone receptor genes mediate distinct functions in vivo. The increased understanding of the structure and function of thyroid hormone receptors and their interacting proteins has markedly clarified the molecular mechanisms of thyroid hormone action.

Ligand-inducible interaction of the DRIP/TRAP coactivator complex with retinoid receptors in retinoic acid–sensitive and –resistant acute promyelocytic leukemia cells

Retinoic acid (RA) signaling is mediated by its nuclear receptors RXR and RAR, which bind to their cognate response elements as a heterodimer, RXR/RAR, and act in concert with coregulatory factors to regulate gene transcription on ligand binding. To identify specific cofactors that interact with the RXR/RAR heterodimer in acute promyelocytic leukemia (APL) cells, a double cistronic construct was used that allowed coexpression of the RXR LBD (ligand binding domain) with the RAR LBD as an affinity matrix to pull down interacting proteins from nuclear extracts prepared from a human APL cell line, NB4. A group of proteins was detected whose interaction with RXR/RAR is ligand inducible. The molecular weight pattern of these proteins is similar to that of a complex of proteins previously identified as DRIP or TRAP, which are ligand-dependent transcription activators of VDR and TR, respectively. The RXR/RAR-interacting proteins from NB4 were confirmed to be identical to the DRIP subunits by comparative electrophoresis, Western blot analysis, and in vitro protein interaction assay. In addition to RXR/RAR, the DRIP component can interact directly with the APL-specific PML-RARα fusion protein. The same DRIP complex is present in RA-resistant APL cells and in a variety of cancer cell lines, supporting its global role in transcriptional regulation.

Ligand-inducible interaction of the DRIP/TRAP coactivator complex with retinoid receptors in retinoic acid–sensitive and –resistant acute promyelocytic leukemia cells

Retinoic acid (RA) signaling is mediated by its nuclear receptors RXR and RAR, which bind to their cognate response elements as a heterodimer, RXR/RAR, and act in concert with coregulatory factors to regulate gene transcription on ligand binding. To identify specific cofactors that interact with the RXR/RAR heterodimer in acute promyelocytic leukemia (APL) cells, a double cistronic construct was used that allowed coexpression of the RXR LBD (ligand binding domain) with the RAR LBD as an affinity matrix to pull down interacting proteins from nuclear extracts prepared from a human APL cell line, NB4. A group of proteins was detected whose interaction with RXR/RAR is ligand inducible. The molecular weight pattern of these proteins is similar to that of a complex of proteins previously identified as DRIP or TRAP, which are ligand-dependent transcription activators of VDR and TR, respectively. The RXR/RAR-interacting proteins from NB4 were confirmed to be identical to the DRIP subunits by comparative electrophoresis, Western blot analysis, and in vitro protein interaction assay. In addition to RXR/RAR, the DRIP component can interact directly with the APL-specific PML-RARα fusion protein. The same DRIP complex is present in RA-resistant APL cells and in a variety of cancer cell lines, supporting its global role in transcriptional regulation.

Nuclear receptor coactivators in neuroendocrine function

Steroid hormones influence a variety of neuroendocrine events, including brain development, sexual differentiation and reproduction. Hormones elicit many of these effects by binding to neuronal steroid receptors, which are members of a nuclear receptor superfamily of transcriptional activators. However, the mechanisms by which activated steroid receptors regulate gene expression in brain are not well understood. Recently, a new class of proteins, known as nuclear receptor coactivators, have been found to dramatically enhance steroid receptor mediated transactivation of genes in vitro. Here, the proposed molecular mechanisms of how these coactivators enhance the transcriptional activity of steroid receptors are summarized. While much is known about the mechanisms of these coactivators in vitro, it is unclear how these cofactors function in hormone action in vivo or in the brain. This paper discusses some of the initial and enticing investigations into the role of these important coregulatory proteins in neuroendocrine events. Finally, some of the critical issues and future directions in nuclear receptor coactivator function in neuroendocrinology are highlighted.

Gene regulation by thyroid hormone

Regulation of gene expression by thyroid hormones (T3, T4) is mediated via thyroid hormone receptors (TRs). TRs are DNA-binding transcription factors that function as molecular switches in response to ligand. TRs can activate or repress gene transcription depending on the promoter context and ligand-binding status. In most cases, in the absence of ligand, TRs interact with a corepressor complex containing histone deacetylase activity, which actively inhibits transcription. The binding of ligand triggers a conformational change in the TR that results in the replacement of the corepressor complex by a coactivator complex containing histone acetyltransferase activity, through which the chromatin structure is remodeled, thereby leading to activation of transcription. In addition, the finding that several TR-interacting coregulators act more directly on the basal transcriptional machinery suggests that mechanisms independent of histone acetylation and deacetylation also are involved in TR action.

Molecular mechanism of a cross-talk between oestrogen and growth factor signalling pathways

Oestrogen (E2) plays significant roles in variety of biological events such as the development and maintenance of female reproductive organs, bone and lipid metabolisms. More recently, from study of knock-out mice deficient in oestrogen receptor (ER) alpha and ERbeta it turned out that normal spermatogenesis requires the E2 actions. Furthermore, this female steroid hormone is also well known to be deeply involved in many pathophysiological events such as osteoporosis and cancer development in female reproductive organs. It is particularly well known that most breast cancer is dependent on E2 in its development. Such E2 actions are thought to be mediated through two subtypes of ERs. Growth factors have been shown to synergize in this E2 signalling pathway, although the actual molecular mechanism largely remains unknown. Recently, we found that the MAP kinase activated by growth factors phosphorylates the Ser118 residue of the human ERalpha A/B domain and this phosphorylation potentiates the N-terminal transactivation function (AF-1) of human ERalpha, indicating the possible molecular mechanism of a novel cross-talk between E2 and growth factor signalling pathways. More recently, we have identified a coactivator associating with the hERalpha AF-1 in a MAPK-mediated phosphorylation-dependent manner. In this review, the molecular mechanism of this cross-talk is discussed in terms of the transactivation function of ERs, and their coactivators.

Specific structural motifs determine TRAP220 interactions with nuclear hormone receptors

The TRAP coactivator complex is a large, multisubunit complex of nuclear proteins which associates with nuclear hormone receptors (NRs) in the presence of cognate ligand and stimulates NR-mediated transcription. A single subunit, TRAP220, is thought to target the entire complex to a liganded receptor through a domain containing two of the signature LXXLL motifs shown previously in other types of coactivator proteins to be essential for mediating NR binding. In this work, we demonstrate that each of the two LXXLL-containing regions, termed receptor binding domains 1 and 2 (RBD-1 and RBD-2), is differentially preferred by specific NRs. The retinoid X receptor (RXR) displays a weak yet specific activation function 2 (AF2)-dependent preference for RBD-1, while the thyroid hormone receptor (TR), vitamin D(3) receptor (VDR), and peroxisome proliferator-activated receptor all exhibit a strong AF2-dependent preference for RBD-2. Using site-directed mutagenesis, we show that preference for RBD-2 is due to the presence of basic-polar residues on the amino-terminal end of the core LXXLL motif. Furthermore, we show that the presence and proper spacing of both RBD-1 and RBD-2 are required for an optimal association of TRAP220 with RXR-TR or RXR-VDR heterodimers bound to DNA and for TRAP220 coactivator function. On the basis of these results, we suggest that a single molecule of TRAP220 can interact with both subunits of a DNA-bound NR heterodimer.

Functional interactions between the estrogen receptor and DRIP205, a subunit of the heteromeric DRIP coactivator complex

Nuclear receptors regulate transcription in direct response to their cognate hormonal ligands. Ligand binding leads to the dissociation of corepressors and the recruitment of coactivators. Many of these factors, acting in large complexes, have emerged as potential chromatin remodelers through intrinsic histone modifying activities. In addition, other ligand-recruited complexes appear to act more directly on the transcriptional apparatus. The DRIP complex is a 15-subunit complex required for nuclear receptor transcriptional activation in vitro. It is recruited to the receptor in response to ligand through specific interactions of one subunit, DRIP205. We present evidence that DRIP205 interacts with another member of the steroid receptor subfamily, estrogen receptor (ER). This interaction occurs in an agonist-stimulated fashion which in turn is inhibited by several ER antagonists. In vivo, a fragment of DRIP205 containing only its receptor interacting region acts to selectively inhibit ER's ability to activate transcription in response to estradiol. These observations suggest a key role for the DRIP coactivator complex in estrogen-ER signaling.

A new family of nuclear receptor coregulators that integrate nuclear receptor signaling through CREB-binding protein

We describe the cloning and characterization of a new family of nuclear receptor coregulators (NRCs) which modulate the function of nuclear hormone receptors in a ligand-dependent manner. NRCs are expressed as alternatively spliced isoforms which may exhibit different intrinsic activities and receptor specificities. The NRCs are organized into several modular structures and contain a single functional LXXLL motif which associates with members of the steroid hormone and thyroid hormone/retinoid receptor subfamilies with high affinity. Human NRC (hNRC) harbors a potent N-terminal activation domain (AD1), which is as active as the herpesvirus VP16 activation domain, and a second activation domain (AD2) which overlaps with the receptor-interacting LXXLL region. The C-terminal region of hNRC appears to function as an inhibitory domain which influences the overall transcriptional activity of the protein. Our results suggest that NRC binds to liganded receptors as a dimer and this association leads to a structural change in NRC resulting in activation. hNRC binds CREB-binding protein (CBP) with high affinity in vivo, suggesting that hNRC may be an important functional component of a CBP complex involved in mediating the transcriptional effects of nuclear hormone receptors.

Histone binding protein RbAp48 interacts with a complex of CREB binding protein and phosphorylated CREB

A CREB-CREB binding protein (CBP) complex was used as bait to screen a mouse embryo cDNA library in yeast. One of the strongest interactions identified the histone binding protein RbAp48. RbAp48 also interacted weakly with CBP alone but did not interact with phosphorylated or nonphosphorylated CREB. CBP (or its homologue p300) from HeLa cell nuclear extracts coimmunoprecipitated with RbAp48 and its homologue RbAp46 and bound to a glutathione S-transferase-RbAp48 fusion protein. This interaction was stimulated by the addition of phosphorylated CREB and allowed the association of core histones and mononucleosomes in an acetylation-dependent manner. RbAp48 lowered the K(m) of CBP histone acetylase activity and facilitated p300-mediated in vitro transcription of a chromatinized template in the presence of acetylcoenzyme A. These data indicate that the association of phosphorylated CREB with CBP promotes the binding of RbAp48 and its homologue RbAp46, allowing the formation of a complex that facilitates histone acetylation during transcriptional activation.

Transcriptional regulation through Mediator-like coactivators in yeast and metazoan cells

A novel multiprotein complex has recently been identified as a coactivator for transcriptional control of protein-encoding genes by RNA polymerase II in higher eukaryotic cells. This complex is evolutionarily related to the Mediator complex from yeast and, on the basis of its structural and functional characteristics, promises to be a key target of diverse regulatory circuits.

p300 mediates functional synergism between AF-1 and AF-2 of estrogen receptor alpha and beta by interacting directly with the N-terminal A/B domains

Estrogen receptor (ER) alpha and beta mediate estrogen actions in target cells through transcriptional control of target gene expression. For 17beta-estradiol-induced transactivation, the N-terminal A/B domain (AF-1) and the C-terminal E/F domain (AF-2) of ERs are required. Ligand binding is considered to induce functional synergism between AF-1 and AF-2, but the molecular mechanism remains unknown. To clarify this synergism, we studied the role of reported AF-2 coactivators, p300/CREB binding protein, steroid receptor coactivator-1/transcriptional intermediary factor-2 (SRC-1/TIF2) family proteins and thyroid hormone receptor-associated protein-220/(vitamin D3 receptor-interacting protein- 205-(TRAP220/DRIP205) on the AF-1 activity in terms of synergism with the AF-2 function. We found that neither any of the SRC-1/TIF2 family coactivators nor TRAP220/DRIP205 is potent, whereas p300 potentiates the AF-1 function of both human ERalpha and human ERbeta. Direct interactions of p300 with the A/B domains of ERalpha and ERbeta were observed in an in vitro glutathione S-transferase pull-down assay in accordance with the interactions in yeast and mammalian two-hybrid assays. Furthermore, mutations in the p300 binding sites (56-72 amino acids in ERalpha and 62-72 amino acids in ERbeta) in the A/B domains caused a reduction in ligand-induced transactivation functions of both ERalpha and ERbeta. Thus, these findings indicate that ligand-induced functional synergism between AF-1 and AF-2 is mediated through p300 by its direct binding to the A/B regions of ERalpha and ERbeta.

The transcriptional role of PML and the nuclear body

The PML gene encodes a tumour suppressor protein associated with a distinct subnuclear domain, the nuclear body. Various functions have been attributed to the PML nuclear body, but its main biochemical role is still unclear. Recent findings indicate that PML is essential for the proper formation of the nuclear body and can act as a transcriptional co-factor. Here we summarize the current understanding of the biological functions of PML and the nuclear body, and discuss a role for these intra-nuclear structures in the regulation of transcription.

Ability of the glucocorticoid modulatory element to modify glucocorticoid receptor transactivation indicates parallel pathways for the expression of glucocorticoid modulatory element and glucocorticoid response element activities

The glucocorticoid modulatory element (GME) of the rat tyrosine aminotransferase gene is located at -3.6 kb and 1 kb upstream of the glucocorticoid response elements (GREs). The GME has the unique transcriptional properties of modulating both the dose-response curve of agonists bound to the glucocorticoid receptor (GR) and the residual agonist activity of GR-bound antisteroids. The expression of GME activity involves the binding of two novel proteins (GMEB-1 and GMEB-2) that we have recently cloned. However, the mechanistic details are limited. The DNA sequence requirements for GME activity (CGTC) also remain poorly defined, which restricts efforts to identify other GME modulated genes. To help understand the mechanism for the unusual activities of the GME and to identify permissive gene environments for GME activity, we compared the changes in GME activity and GRE action (i.e. the fold induction by GR) caused by modifying several parameters. Phasing between the GME and downstream tandem GREs was unimportant, in contrast to other cis-acting elements like the GRE, while GME activity decreased rapidly when placed at increasingly larger distances 3' to a tandem GRE. A minimal promoter was less effective in supporting GME than GRE activity. Although CREB binds to the GME, overexpression of CREB reduced GRE, but not GME, activity and a CRE was inactive when substituted for the GME. No effect of the GME was observed on the binding of GRs to a single GRE. However, the GME upstream of a single GRE was also unable to produce a left shift in the Dex dose-response curve under conditions where the GME was active with two GREs. In the absence of any GREs, the GME displayed intrinsic activity by elevating basal level expression. Collectively, these results indicate that an optimal position for a functional GME is within 250 bp upstream of a tandem GRE driving a complex promoter. Furthermore, as the changes in GME activity did not correlate with those for fold induction from the GRE, the mechanisms for expression of GME and GRE activities appear to utilize parallel, as opposed to common pathways.

Binding of liganded vitamin D receptor to the vitamin D receptor interacting protein coactivator complex induces interaction with RNA polymerase II holoenzyme

Because the vitamin D receptor interacting protein (DRIP) coactivator complex shares components with the RNA polymerase II (Pol II) holoenzyme complex, we tested whether the two protein complexes associate in cellular extracts. On initial purification steps, the DRIP complex copurified with the Pol II holoenzyme. Pol II was found to bind to the vitamin D receptor in a ligand-dependent fashion when either nuclear extracts or partially purified preparations were used as sources of DRIP and Pol II holoenzyme. A subpopulation of holoenzyme complexes bound to the receptor because BRCA1, which associates with the Pol II holoenzyme, did not associate with the liganded receptor, and only in certain of the holoenzyme- and DRIP-containing fractions did Pol II bind to the liganded receptor. Immunoprecipitation experiments revealed that the DRIP complex was not pre-associated with the Pol II holoenzyme, but the interaction between these two complexes was induced only in the presence of receptor and ligand. These data support a model in which the activation of transcription by hormone-bound receptor requires binding to the DRIP coactivator, and this induced ternary complex can then bind to the Pol II holoenzyme to activate transcription.