Review Transcription Under the Control of Nuclear Arm/B-Catenin

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Transcription under the Control Review of Nuclear Arm/b-Catenin

Reto Sta¨ deli1, Raymond Hoffmans1, and Konrad Basler* via a-catenin [5]. Second, Arm/b-catenin acts as a nuclear regulator of Wg/Wnt dependent gene expression and provides transcriptional activator func- The Wingless/Wnt pathway controls cell fates during tions to TCFs. The Arm/b-catenin protein consists of animal development and regulates tissue homeosta- a central region that is made up of 12 imperfect arma- sis as well as stem cell number and differentiation dillo repeats (R1-12) flanked by an amino- and a in epithelia. Deregulation of Wnt signaling has been carboxy-terminal tail [6]. There are mutant forms of associated with cancer in humans. In the nucleus, Arm that interfere with the adhesion function, but not the Wingless/Wnt signal is transmitted via the key ef- with its role in Wg/Wnt signaling, and vice versa, indi- fector protein Armadillo/b-catenin. The recent iden- cating that these two functions of Arm are indepen- tification and functional analysis of novel Armadillo/ dent and separable [7,8]. Interestingly, the nematode b-catenin interaction partners provide new and ex- Caenorhabditis elegans has three different b-catenin citing insights into the highly complex mechanism proteins which are dedicated either to adhesion of Wingless/Wnt target gene activation. (HMP-2) or to Wnt signaling (WRM-1 and BAR-1) [9]. In the absence of a Wg/Wnt signal, cytosolic Arm/ Signaling molecules of the Wingless (Wg)/Wnt family b-catenin is constantly phosphorylated by the action are secreted glycoproteins that control a diverse array of a so-called ‘degradation complex’ consisting of the of processes in embryos and adult animals. The Wnt Adenomatous polyposis coli (APC) protein, Axin, and cascade has been implicated in the postembryonic the kinases Casein kinase I (CKI) and Shaggy/Zeste regulation of stem cell number and differentiation of white-3/Glycogen synthase kinase-3b (GSK-3b). Phos- several adult stem cell systems [1,2]. Moreover, the phorylated Arm/b-catenin is rapidly degraded via the pathway has been causally linked to various diseases, ubiquitin/proteasome pathway [10]. Interaction of the most notably to cancer [3,4]. In the absence of the Wg/Wnt ligand with its receptors Frizzled and Arrow/ Wg/Wnt signal, DNA-bound transcription factors of LDL-receptor-related protein (LRP) blocks the degra- the T-cell factor (TCF) family of HMG-box proteins bind dation complex thus leading to stabilization of Arm/ the transcriptional repressors Groucho (Gro)/TLE and b-catenin. As a consequence, Arm/b-catenin protein Carboxy-terminal binding protein (CtBP). Upon activa- accumulates in the cytoplasm and can enter the nu- tion of the pathway, the key transducing component cleus, where it acts as a co-activator of TCFs [11–16] Armadillo (Arm)/b-catenin becomes stabilized, enters (Figure 1). Artificially preventing Arm from entering the the nucleus and heterodimerizes with TCFs to activate nucleus blocks Wnt signaling [17], while forcing it into the expression of Wg/Wnt target genes. This co-acti- the nucleus can activate target genes [18–20]. There- vator function of Arm/b-catenin depends mainly on two fore, the signaling activity of Arm/b-catenin is largely domains or ‘arms’: An amino-terminal activating arm controlled by the size of its nuclear pool, which de- (NTAA) recruits Legless (Lgs)/B-cell lymphoma 9 pends on the cytoplasmic levels of b-catenin as well (BCL9) and Pygopus (Pygo), while a carboxy-terminal as on the import into and export out of the nucleus. activating arm (CTAA) binds to TATA-binding protein (TBP), Brahma/Brahma-related gene-1 (Brg-1), CREB- Nuclear Import and Export of Arm/b-catenin binding protein (CBP)/p300, Mediator subunit 12 The business of Arm/b-catenin’s import and export is (MED12), and Hyrax/Parafibromin. Despite its thera- rather nebulous and somewhat controversial (Figure 2). peutic relevance, the mechanisms by which Arm/ b-catenin appears to be imported into the nucleus in b-catenin employs these co-factors to control the tran- a NLS- and importin/karyopherin-independent manner scription of target genes are only poorly understood. by directly interacting with nuclear pore components Here, we review past and recent findings that relate [21,22]. Addition of cytosol inhibits nuclear import of to this problem and discuss how they can be inte- b-catenin [21,22], indicative of the presence of cyto- grated into a more complete picture of Wg/Wnt target solic retention factors. Indeed, it was shown that gene activation. Axin could act as such a retention factor in Drosophila [23]. By contrast, Pangolin (Pan), the Drosophila TCF The Dual Role of Arm/b-catenin homologue, can function to keep Arm in the nucleus Arm/b-catenin fulfills two main functions in the cell: [23]. However, a mutant form of Arm, that is defective First, it acts as a component of the cadherin-based in binding Pan, still localizes to the nucleus [7,15].It cell adhesion system. It binds the transmembrane pro- has also been proposed that Lgs in cooperation with tein E-cadherin and regulates actin filament assembly Pygo serves as a nuclear anchor for Arm [20]. How- ever, Arm still localizes to the nucleus in clones of cells that are double-mutant for both axin and pygo, sug- 1 These authors contributed equally. gesting that Pygo acts downstream of Arm nuclear Institut fu¨ r Molekularbiologie, Universita¨ tZu¨ rich, localization [24,25]. These findings show the difficulty Winterthurerstrasse 190, CH-8057 Zu¨ rich, Switzerland. in designing experiments to clearly separate nuclear *E-mail: [email protected] import from retention of Arm/b-catenin. While the Current Biology R379

Figure 1. Simpliﬁed overview of the Wg/ Wnt signaling pathway. Wg Arrow

In the absence of the Wnt signal (‘OFF’ Arrow Frizzled Frizzled state, left cell), Armadillo (Arm)/b-catenin Dvl Axin protein levels are downregulated by CKI GSK-3β Dvl a complex containing Adenomatous poly- Arm α E-cad Arm posis coli (APC) protein, Axin, Casein Arm APC GSK-3β E-cad α kinase I (CKI), and Glycogen synthase ki- Arm Axin Arm nase-3b (GSK-3b). In addition, the co- CKI α E-cad Arm α Arm repressors Groucho (Gro)/TLE and Car- APC Arm P E-cad Arm boxy-terminal binding protein (CtBP) are Arm bound to T-cell factor (TCF). The cell to the right represents the ‘ON’ state of the pathway. The Wg/Wnt ligand binds Friz- A zled and its co-receptor Arrow/LDL- Pygo r receptor-related protein (LRP). Axin is m Lgs CTAA-BPs Gro bound by Dishevelled (Dvl) and Arrow, CtBP X Arm Ubiquitin TCF TCF thereby disrupting the ‘degradation com- mediated plex’. Arm/b-catenin accumulates in the proteolysis cytoplasm, enters the nucleus and displaces Gro from TCF. For transcription of b OFF ON target genes Arm/ -catenin interacts Current Biology with Legless (Lgs), which binds Pygopus (Pygo), and with carboxy-terminal activating arm binding proteins (CTAA-BPs), such as TATA-binding protein (TBP), Brahma/Brahma-related gene-1 (Brg-1), CREB-binding protein (CBP)/p300, MED12, and Hyrax/Parafibromin. As part of adherens junctions, Arm/b-catenin binds the transmembrane protein E-cadherin (E-cad) and the cytoplasmic protein a-catenin (a). Negatively acting components of the pathway are colored in red with white letters, while positive components are shown in green with black letters. region comprising R10-C seems to be necessary and in APC [36,37]. Truncated APC can no longer fulfill its sufficient for the nuclear import [26], different regions function in Arm/b-catenin degradation. As a conse- of Arm/b-catenin that interact with binding partners quence, Arm/b-catenin accumulates and enters the such as the above mentioned Pan and Lgs (together nucleus where it activates target genes implicated in with Pygo) could serve as anchor points contributing cell proliferation (e.g. c-Myc and gastrin [38,39]), inhi- to nuclear retention of Arm/b-catenin. bition of apoptosis (e.g. survivin [40]), and tumor pro- To further complicate matters, Arm also has an in- gression (e.g. Laminin g2 [41])(Figure 3). How these trinsic nuclear export activity [21,22,27], which has target genes exert their harmful effect in the various been shown to overlap with its ‘import region’ [26]. steps towards tumorigenesis is currently not well un- Moreover, Axin and APC may facilitate nuclear export derstood. However, especially the Arm/b-catenin tar- of Arm [28–31]. However, a recent paper that investi- gets implicated in cell proliferation would fit into the gated the nucleo-cytoplasmic shuttling of b-catenin view of cancer as a ‘stem cell disease’. and its relation to TCF4, BCL9, APC, and Axin [32] demonstrated that these proteins do not accelerate Initial Steps of Target Gene Activation by Nuclear the import/export rate of b-catenin. Rather, they influ- Arm/b-catenin ence the subcellular localization of b-catenin by retain- How are nuclear Arm/b-catenin targets activated? In ing it in the compartment in which they are localized. In the absence of Arm/b-catenin in the nucleus, Pan/ summary, the intracellular localization of Arm/b-cate- TCF is bound to Gro/TLE [42,43] and CtBP [44–47]. nin represents a dynamic equilibrium of its intrinsic nu- Gro/TLE family proteins are general long-range tran- clear import and export activities as well as the avail- scriptional co-repressors, which have been shown to ability and affinity of its binding partners. interact with histone deacetylases [48]. Reduction of Pan or Gro in the Drosophila embryo results in partial Cancer and Arm/b-catenin suppression of armadillo and wingless mutant pheno- Mutations that constitutively stabilize Arm/b-catenin types, whereas overexpression of Pan enhances the can cause colorectal carcinomas and other forms of phenotype of a weak wingless allele [43]. This suggests cancer. One recent view described such cancers as that, in the absence of Wg signaling, a Pan–Gro com- a ‘stem cell disease’ [33]. All cells in a normal colonic plex acts as a repressor of Wg targets in the embryo. crypt are thought to be derived from epithelial stem By contrast, loss of Pan in the Drosophila wing imaginal cells sitting at the bottom of each crypt (Figure 3). They disc results in a reduction of Wg target gene expres- are maintained as stem cells by a Wnt signaling system sion, but does not cause a derepression of these genes [34]. Aberrant activation of the Wnt pathway leads to outside the normal domain of expression. Loss of Gro an expansion of this stem cell population into upper re- also does not result in derepression of Wg target genes gions of the crypt. Instead of differentiating, such cells in the wing disc [49]. Thus, in this context Pangolin does replicate as if they were stem cells, resulting in over- not appear to function by default as a repressor in the proliferation and the accumulation of mutations. Ulti- absence of Wg signaling. Rather, it seems that the re- mately, this results in polyps and adenomatous lesions pressor function of Pan can vary in different tissues. in the colon (Figure 3) [33,35]. The best-studied exam- Nuclear Arm/b-catenin binds to the amino terminus ple of such conditions, Familial Adenomatous Poly- of Pan/TCF and displaces Gro/TLE by binding to a sec- posis (FAP), is in most cases caused by truncations ond, low-affinity binding site on TCF located in the Review R380

Figure 2. Nucleo-cytoplasmic shuttling of Arm/b-catenin. Nuclear import (left) of Arm/b-catenin Arm Arm works independently of the carrier pro- Cytosolic teins Importin a/b and is mediated by di- Importin α retention rect interaction of Arm/b-catenin with RanGAP Importin β X (e.g. Axin, X components of the nuclear pore complex E-cadherin) (NPC). Arm/b-catenin’s release into the Cytoplasm nucleus does not depend on RanGTP. Nu- clear import of Arm/b-catenin can be counteracted by cytosolic retention fac- NPC NPC tors such as Axin and E-cadherin. Nuclear export (right) of Arm/b-catenin works in an Exportin/RanGTP-independent manner, Nucleus Nuclear RanGTP and no Ran GTPase activating protein retention X Exportin RanGTP X (RanGAP) is required to release Arm/b- (e.g. TCFs, APC, Axin Lgs-Pygo) catenin on the cytosolic side. Proteins Arm like APC and Axin, which can bind Arm Arm TCF and shuttle between the nucleus and the cytoplasm (Ran dependent), may facilitate Import Export nuclear export of Arm, whereas nuclear Current Biology proteins that interact with Arm, like TCFs or Lgs (together with Pygo), might serve as nuclear retention factors and decrease the nuclear export rate of Arm. carboxy-terminal half of the protein, which overlaps which are briefly discussed here. One of these proteins with the Gro/TLE binding site [50]. Interestingly, nei- is TBP, which has been shown to bind b-catenin at ther the amino nor the carboxyl terminus of b-catenin R12-C [57]. Notably, there are two more domains in are necessary for this displacement, suggesting that b-catenin that bind TBP, namely the amino terminus it is — like TCF binding — accomplished by the central and R2-4. So far, further experimental data for the Arm repeat domain. CtBP proteins are general, short- functional significance of the b-catenin–TBP interac- range transcriptional co-repressors that interact with tion are lacking. histone deacetylases [51]. TCF3 and TCF4 have been Furthermore, Brg-1, a component of mammalian shown to bind CtBP in vitro and in vivo [44–47]. The SWI/SNF and Rsc chromatin remodeling complexes, binding of CtBP represses TCF3- and TCF4-mediated has been shown to bind Arm/b-catenin through R7- transcription and this repression depends on histone 12. Overexpression of Brg-1 promotes transcriptional deacetylase activity [44–46]. To date, it is not clear how activation of TCF-responsive reporter genes. In Dro- Wnt signaling overcomes the repressor effect of CtBP sophila, the SWI/SNF component Brahma genetically on TCF3 and TCF4 and whether b-catenin plays an interacts with Arm [56]. active role in this process. After the release of the In addition, p300 and the closely related CBP are Gro/TLE and CtBP repressor systems from Pan/TCF, transcriptional co-activators that link proteins to the nuclear Arm/b-catenin can fully develop its transcrip- basal transcription machinery or alter chromatin struc- tional activator potential. ture through their intrinsic or associated histone ace- In the nucleus, Arm/b-catenin binds the DNA binding tyltransferase activities. p300 and CBP bind R10-C protein Pan/TCF/Lymphoid enhancer factor (LEF) of b-catenin and stimulate transcriptional activity through R3-10 [11–16,52] (Figure 4). In addition to the [58,59]. Interestingly, although the co-activators p300 Pan/TCF/LEF binding domain, there are two other and CBP are closely related, they do not always exert domains which are important for Wg/Wnt signaling, the same effects on promoters; for instance, they have namely R1 and R11-C [7]. It was shown recently that opposite effects on the b-catenin-mediated expres- R1–4 are necessary and sufficient for Lgs binding sion of the survivin gene. The small molecule ICG- [8,53–55], and that R11-C are important for the binding 001, which specifically blocks the CBP–b-catenin in- of proteins such as TBP, Brahma/Brg-1, CBP/p300, teraction, but not the p300–b-catenin interaction, MED12, and Hyrax/Parafibromin [56–61]. Mutations inhibits survivin gene expression. In the absence of in, or deletions of these two domains result in reduced ICG-001, CBP is associated with the survivin pro- signaling activity of Arm/b-catenin [7,8,15,62,63].In moter. In the presence of ICG-001 there is less CBP addition, both domains have been shown to exhibit at the promoter but more p300 instead, which in turn signaling activity on their own [15,18,54]. From here recruits repressive elements and results in a reduction on we refer to these two activating arms of Arm/b-cat- of survivin transcription [64]. enin as ‘NTAA’ and ‘CTAA’ for amino-terminal activat- MED12, a component of the Mediator complex, has ing arm (R1-5) and carboxy-terminal activating arm been found to interact with b-catenin at R12-C [61]. (R10-C), respectively. The Mediator (MED) complex, first discovered in yeast, links transcriptional regulators to RNA polymerase II Co-Factors for Nuclear Arm/b-catenin (Pol II) and general transcription factors [65–67]. Mean- CTAA Interaction Partners while, counterparts for nearly all yeast MED compo- Arm/b-catenin has several partners that bind the CTAA nents have been discovered in mammals [68–70]. and contribute to transcriptional activation, five of RNAi-mediated knock-down of MED12 as well as Current Biology R381

Wild type CRC APC, Axin α E-cadherin

APC APCX 1 2 3 4 5 6 7 8 9 10 11 12 or NTAA CTAA β-catenin β-catenin* TCF CBP/p300 Differentiation TCF X TCF Lgs Brg-1 Pontin TBP, MED12, Wnt and Hyrax

APC Current Biology

Colon β-catenin Proliferation crypts Figure 4. Schematic overview of what binds where on Arm/ b-catenin. TCF The Arm repeats are numbered 1–12. Blue lines show the amino- terminal activating arm (NTAA) and the carboxy-terminal acti- Wnt vating arm (CTAA). The black lines represent binding domains Current Biology of Arm/b-catenin interacting proteins. APC (Adenomatous polyposis coli), a (a-catenin), TCF (T-cell factor), CBP (CREB-bind- Figure 3. Wnt signaling in colorectal cancer (CRC). ing protein, CREB (cAMP response-element binding protein))/ Schematic depiction of a colonic crypt, the left side is wild type p300, Lgs (Legless), Brg-1 (Brahma-related gene-1), TBP and the right side shows the diseased condition. A Wnt signal at (TATA-binding protein), MED12 (Mediator subunit 12). the bottom of the crypt blocks b-catenin degradation, thereby activating target genes that maintain the cells in a proliferating state (green). Halfway up the crypts, b-catenin is downregu- activated Reptin protein and promotes cardiac hyper- lated and the progenitor crypt cells start to differentiate (blue). plasia. This phenotype is enhanced by reduction of Mutations in either APC or b-catenin result in cells (red) consti- b-catenin or Pontin expression in a heterozygous lik tutively expressing Wnt target genes, thus preventing such cells from differentiating. The cells maintain their progenitor state mutant [75]. In a chromatin immunoprecipitation ex- and continuously proliferate. Adapted from [90]. periment, Pontin, Tip60, and TRRAP interacted with the promoter of a TCF-dependent gene, ITF-2 (immu- overexpression of the isolated b-catenin binding do- noglobulin transcription factor-2). Overexpression of main of MED12 in HeLa cells impair b-catenin-depen- an inactive form of Pontin resulted in decreased acet- dent transactivation in response to Wnt signaling [61]. ylation of histones and reduction of ITF-2 expression Lastly, it has recently been shown that the Drosoph- [76]. The domains of b-catenin to which Pontin, Reptin, ila and human homologs of yeast Cdc73p (Hyrax and and Lgs bind are overlapping (R2-5 and R1-4). How- Parafibromin, respectively) are involved in Wg/Wnt ever, it is presently unknown whether they bind simul- signaling. Cdc73p is a component of the Polymer- taneously or in a competitive manner. ase-associated factor 1 (PAF1) complex, a conserved Lgs and Pygo were discovered in genetic screens for Pol II interacting complex, which has been implicated modifiers of the Drosophila Wg pathway [24,25,53,77]. in the regulation of transcriptional initiation and elon- Loss of lgs or pygo function results in a severe reduc- gation [71]. Hyrax and Parafibromin bind to R12-C of tion of pathway output. Arm/b-catenin binds Lgs Arm and b-catenin, respectively. Overexpression of through R1-4 and the acidic amino acids D162 and these proteins results in an increase of Wg/Wnt path- D164 play a key role in this interaction [8,53]. Replacing way activity, while experimental reduction of their wild-type Arm with Arm-D164A (which cannot bind levels has the opposite effect [60]. Besides the five Lgs) causes phenotypes that are very similar to those co-activator partners described above that bind di- of lgs null mutants [8]. The predominantly nuclear local- rectly to the CTAA of b-catenin, it was recently shown ization of Lgs depends on the presence of Pygo [20].It that the TRRAP/TIP60, the ISW1, and the MLL1/MLL2 has been proposed that Lgs and Pygo function mainly SET1-type complexes also selectively associate with to enhance the nuclear levels of Arm [20,78]. This no- the CTAA [72]. All three of these complexes are in- tion was based on an experiment in which the cuticular volved in histone modification and chromatin remodel- phenotype of lgs and pygo mutant embryos was ame- ing. However, it has not been resolved yet whether liorated by overexpressing a form of Arm with a nuclear they interact directly with b-catenin. localization signal. This experiment indeed showed that high amounts of nuclear Arm can activate Pan NTAA Interaction Partners targets in the absence of Lgs or Pygo, most likely by Pontin/Tip49 and Reptin/Tip48 are two highly homolo- displacing Gro/TLE from Pan and by recruiting the gous proteins, which can form homo- and hetero- activators described above via the CTAA. However, in dimers. They have been shown to bind to b-catenin a different set of experiments it was shown that R2-5 and antagonistically affect b-catenin output. a DNA-tethered and constitutively nuclear form of Reptin inhibits transcriptional activation of reporter Arm/b-catenin can only very poorly activate reporter genes, whereas Pontin enhances it. In Drosophila, mu- gene expression if it carries the D164A mutation, sug- tations in the pontin and reptin genes exhibited oppo- gesting that Lgs/BCL9 binding critically contributes site dominant effects on wing phenotypes in a genetic to Arm/b-catenin’s activator capacity [54,79]. Further- background sensitized for Arm signaling [73,74]. Also more, constitutive nuclear targeting of Lgs does not in zebrafish Pontin and Reptin function antagonisti- bypass the requirement for Pygo in Wg signaling, indi- cally: the liebeskummer (lik) mutation encodes an cating that Pygo must provide a function beyond Review R382

Figure 5. Model of nuclear Arm/b-cate- HAT, nucleosome acetylation nin’s control over transcription. Nuclear Arm/b-catenin is recruited to SWI/SNF complex, chromatin CBP/p300 Wingless target genes by high mobility remodeling group (HMG) transcription factors of the TCF/LEF family, which all have an Arma- dillo-binding domain (ABD) at their very target gene amino termini. Arm acts through two acti- Brm/Brg1 preparation MED12 PHD Pygo vating arms. The amino-terminal activating arm acts mainly through the co-activa- HD1 NHD Lgs/BCL9 tor Pygopus (Pygo), which is recruited to MED complex, Hyrax HD2 initiation, Arm via the adaptor protein Legless (Lgs). Lgs binds to Arm with its homology do- 1 2 3 4 5 6 7 8 9 10 11 12 elongation β main 2 (HD2) and to the plant homology Arm/ -catenin ABD PAF1 complex, domain (PHD) of Pygo with its homology Pan/TCF histone methylation, domain 1 (HD1). For the co-activator func- HMG initiation, tion of Pygo, its amino-terminal homology elongation domain (NHD) is essential. The carboxy- Current Biology terminal activating arm can interact with protein complexes exhibiting histone acetyltransferase (HAT) or chromatin remodeling (SWI/SNF complex) activities. It further interacts with components that can recruit histone methyltransferases (e.g. MLL/SET1) or play important roles in transcription initiation and elongation (MED complex, PAF1 complex). Sequential or random recruitment of these factors and complexes, step-by-step, results in a more accessible chromatin structure and finally leads to transcription of the target gene. ensuring the availability of Lgs and Arm/b-catenin in to associate with the nonphosphorylated and Ser2 the nucleus [79]. These findings, therefore, argue that and Ser5 phosphorylated forms of the large subunit of Arm depends on Lgs and Pygo primarily for its tran- Pol II [84]. Another study [85] describes the tumor sup- scriptional function rather than for its nuclear import pressor Parafibromin as a PAF1 complex- and Pol II- or retention. In addition to the Arm/b-catenin CTAA, bound protein acting in transcription elongation and there appears to be a Lgs-Pygo-dependent output RNA processing. As the PAF1 complex has been from the NTAA important for Arm activity. In this shown to physically interact with the SET1 complex NTAA function, Lgs seems to serve as an adaptor pro- [86], an attractive idea would describe the role of tein linking Pygo to Arm/b-catenin [53], whereas the Hyrax/Parafabromin as a recruiting module not only amino-terminal homology domain of Pygo is a critical for Pol II, but also for the MLL/SET1-type complexes. mediator of this function, as it cannot be bypassed [54]. Mosimann et al. [60] showed that there might be a Hyrax/Parafibromin-related crosstalk between the Target Gene Activation by Nuclear Arm/b-catenin CTAA of b-catenin and Pygo. As described above, Transcriptional activation of target genes requires the overexpression of Parafibromin leads to an increase recruitment of ATP-dependent chromatin remodeling in Wnt-reporter gene activity in tissue culture cells. enzymes and histone acetyltransferase complexes to However, this increase depends on Pygo as it can be their promoters to ‘prepare’ the chromatin. There ap- abrogated by the siRNA-mediated reduction of Pygo. pears to be no obligate order of function for these In other words, the activating function of Hyrax/Parafi- complexes [80]. After chromatin preparation, RNA bromin in the Wg/Wnt pathway seems to depend on Pol II is recruited and transcription initiated. In the the recruitment of Pygo to b-catenin. These findings case of the LEF–b-catenin complex it has been shown might implicate Pygo in stabilizing or exchanging that R11-C of b-catenin is necessary for chromatin re- trans-activating complexes that bind the Arm/b-cate- modeling in vitro [81]. This region of b-catenin partially nin CTAA. Taken together, a model can be composed overlaps with binding sites for Brg-1 (R7-12) and p300/ (Figure 5) according to which Wg/Wnt target gene ac- CBP (R10-C). These two proteins could be involved in tivation is a concerted, Pygo-assisted process, which the preparation of chromatin at Wg/Wnt target genes. dynamically coordinates the sequential action of tran- In addition, components of the MLL/SET1-type chro- scriptional modulators at the central scaffold protein matin modifying complexes associate with this do- Arm/b-catenin. main of b-catenin and augment histone methyltrans- ferase activity and H3K4-tri-methylation at the Wnt De-activation of Target Genes and Nuclear target gene c-Myc [72]. Arm/b-catenin The identification of MED12 as a b-catenin binding Although not much is currently known about how Arm/ protein [61] provides a possible link between b-catenin b-catenin-regulated genes are turned off, three pro- and Pol II. The MED complex can associate with Pol II cesses are likely to be important: disassembly of the to generate a stable complex sometimes called the Arm/b-catenin enhancer complex, reversion of the ‘ac- Pol II holoenzyme [82,83]. Yet, a second possible link tivating’ histone modifications and nuclear export and between b-catenin and Pol II came forward with the degradation of Arm/b-catenin. Recent work by Sierra discovery of Hyrax/Parafibromin as an Arm/b-catenin et al. [72] raises the possibility that APC may be pivotal interaction partner, which interestingly also binds to for all three of these processes. The association of the CTAA and, thus, overlaps with the Brahma/Brg-1, full-length APC with the Wnt-dependent enhancer CBP/p300, and MED12 binding sites. The PAF1 com- of the c-Myc gene correlates with the disassembly of plex, which contains Parafibromin, has been found the b-catenin enhancer complex, resulting in a rapid Current Biology R383

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