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The many faces and functions of β-catenin

2012; Springer Nature; Volume: 31; Issue: 12 Linguagem: Inglês

10.1038/emboj.2012.150

1460-2075

Tomáš Valenta, George Hausmann, Konrad Basler,

Developmental Biology and Gene Regulation

Focus Review22 May 2012free access The many faces and functions of β-catenin Tomas Valenta Tomas Valenta Institute of Molecular Life Sciences, University of Zurich, Zurich, Switzerland Search for more papers by this author George Hausmann George Hausmann Institute of Molecular Life Sciences, University of Zurich, Zurich, Switzerland Search for more papers by this author Konrad Basler Corresponding Author Konrad Basler Institute of Molecular Life Sciences, University of Zurich, Zurich, Switzerland Search for more papers by this author Tomas Valenta Tomas Valenta Institute of Molecular Life Sciences, University of Zurich, Zurich, Switzerland Search for more papers by this author George Hausmann George Hausmann Institute of Molecular Life Sciences, University of Zurich, Zurich, Switzerland Search for more papers by this author Konrad Basler Corresponding Author Konrad Basler Institute of Molecular Life Sciences, University of Zurich, Zurich, Switzerland Search for more papers by this author Author Information Tomas Valenta1, George Hausmann1 and Konrad Basler 1 1Institute of Molecular Life Sciences, University of Zurich, Zurich, Switzerland *Corresponding author. Institute of Molecular Life Sciences, University of Zurich, Winterthurerstrasse 190, Zurich 8057, Switzerland. Tel.:+41 44 635 3111; Fax:+41 44 635 6864; E-mail: [email protected] The EMBO Journal (2012)31:2714-2736https://doi.org/10.1038/emboj.2012.150 PDFDownload PDF of article text and main figures. ToolsAdd to favoritesDownload CitationsTrack CitationsPermissions ShareFacebookTwitterLinked InMendeleyWechatReddit Figures & Info β-Catenin (Armadillo in Drosophila) is a multitasking and evolutionary conserved molecule that in metazoans exerts a crucial role in a multitude of developmental and homeostatic processes. More specifically, β-catenin is an integral structural component of cadherin-based adherens junctions, and the key nuclear effector of canonical Wnt signalling in the nucleus. Imbalance in the structural and signalling properties of β-catenin often results in disease and deregulated growth connected to cancer and metastasis. Intense research into the life of β-catenin has revealed a complex picture. Here, we try to capture the state of the art: we try to summarize and make some sense of the processes that regulate β-catenin, as well as the plethora of β-catenin binding partners. One focus will be the interaction of β-catenin with different transcription factors and the potential implications of these interactions for direct cross-talk between β-catenin and non-Wnt signalling pathways. Introduction Wnt signalling represents one of a few key molecular cascades that regulate cell fate in animals throughout their lifespan. Already during embryonic development Wnt-regulated β-catenin critically contributes to the establishment of the body axis and the orchestration of tissue and organ development. In adult organs, Wnt signalling continues to play indispensable roles in tissue homeostasis, cell renewal, and regeneration. Interaction of Wnt ligands with their receptor complexes triggers several intracellular signalling cascades; these are, traditionally, separated into two types according to the role played by β-catenin. In the canonical Wnt cascade, β-catenin is the key effector responsible for transduction of the signal to the nucleus and it triggers transcription of Wnt-specific genes responsible for the control of cell fate decisions in many cells and tissues. The second type of Wnt signalling is independent of β-catenin signalling function and comprises, among others, the Wnt/PCP (Planar Cell Polarity) and Wnt-dependent/protein kinase C (PKC)-dependent pathways (van Amerongen et al, 2008; Angers and Moon, 2009). We will focus here on β-catenin-dependent signalling and refer the interested reader to the following excellent reviews on the other types of Wnt signalling (van Amerongen et al, 2008; Angers and Moon, 2009). But β-catenin is not just a component of the Wnt signal cascade (Figure 1). In the late 1980s, β-catenin was independently discovered twice, on the basis of its different functions: structural and signalling. The group of Rolf Kemler isolated β-catenin, together with two other molecules (α-catenin and γ-catenin/plakoglobin), as proteins associated with E-cadherin, the key molecule of Ca2+-dependent cell adhesion. These proteins were named catenins (in Latin catena means chain) to reflect their linking of E-cadherin to cytoskeletal structures (Ozawa et al, 1989). The signalling potential of β-catenin was exposed through its Drosophila orthologue Armadillo: the armadillo gene was discovered in the seminal screens for mutations affecting segmentation of the Drosophila embryo, performed by Eric Wieschaus, Christiane Nüsslein-Volhard, and Gerd Jürgens (Wieschaus et al, 1984). Whereas the wild-type embryonic cuticle contains segments with alternating rows of denticles and naked belts, segments in armadillo mutants form only a lawn of denticles, the naked belts are missing. Such phenotypes resemble wingless null mutants (Wieschaus and Riggleman, 1987). Further analysis of Armadillo performed by the laboratories of Eric Wieschaus, Mark Peifer, and others revealed the conservation of its structural function in adherens junctions (McCrea et al, 1990; Peifer and Wieschaus, 1990; Orsulic and Peifer, 1996). Epistatic analysis determined that the armadillo segmentation function is regulated by Wingless (Riggleman et al, 1990). This finding was a key step in the subsequent characterization of the Wnt/β-catenin (or Wingless/Armadillo, respectively) signalling cascade, and of the functions and mutual interactions of its individual components. Another important part of this mosaic was revealed by the description of the basic pathway leading from the Wingless ligand through Dishevelled to regulation of Armadillo stability by Shaggy/Zeste-white-3 (GSK3 in vertebrates) (Siegfried et al, 1994). Finally in the mid-1990's several groups independently found that the signalling function of β-catenin/Armadillo in the nucleus is mediated via T-cell factor (TCF)/Lymphoid enhancer-binding factor (Lef) transcription factors, which in association with β-catenin trigger Wnt-mediated transcription (Behrens et al, 1996; Huber et al, 1996; Molenaar et al, 1996; Brunner et al, 1997; van de Wetering et al, 1997). The generation of conditional β-catenin mouse mutants (either knockout or constitutively active) has revealed that a vast array of developmental processes is regulated by the multitasking β-catenin in mammalian embryonic and adult tissues (reviewed in detail by Grigoryan et al, 2008). Figure 1.The life of β-catenin within the cell. Newly synthesized β-catenin is immobilized by E-cadherin at adherens junctions, where it can interact also with α-catenin, thereby indirectly modulating the actin cytoskeleton. β-catenin can be released from the adherens junctions by the activity of protein kinases or by downregulation of E-cadherin. Free excess β-catenin is immediately phosphorylated by the destruction complex and thus marked for subsequent degradation. A portion of β-catenin can be kept in the cytoplasm protected by APC. Wnt signalling blocks the activity of the destruction complex resulting in increased levels of cytolasmic β-catenin, which is translocated to the nucleus. In the nucleus, β-catenin associates with transcription factors from the TCF/Lef family and drives transcription of Wnt/β-catenin target genes. Other factors can also provide β-catenin with a DNA binding platform, often counteracting canonical Wnt signalling. Signalling activity of β-catenin in the nucleus can be regulated by modulating its nuclear import/export. Besides its structural role in the adherens junctions and signalling activity in the nucleus, β-catenin may also play an important function in the centrosome. CTTA, C-Terminal Transcriptional Activators, NTTA, N-Terminal Transcriptional Activators. Download figure Download PowerPoint The many roles of β-catenin beg the question as to how an evolutionarily conserved pathway can control so many varied processes during animal development and tissue homeostasis via one central molecule? Further, how is the final transcriptional output of β-catenin determined and modulated? Part of the explanation might be found in the plethora of β-catenin binding partners that either affect its transcriptional output or permit its direct cross-talk with other transcription factors and signalling pathways. In this review, we will discuss the following aspects: how is the β-catenin protein regulated, how might different transcriptional co-activators of β-catenin lead to diverse outcomes, how can β-catenin affect the activity of various transcription factors, and finally, what are the potential implications of a direct cross-talk between β-catenin and other signalling pathways? The structure of β-catenin determines its role as a scaffold molecule The Wnt/β-catenin pathway is one of a small set of conserved signalling cascades (together with Notch, Hedgehog, TGFβ/BMP, Hippo, and receptor tyrosine kinase-mediated pathways) that regulate animal development from Cnidarians to mammals. β-Catenin's orchestration of developmental processes is the sum of its dual roles—signalling and structural—as elegantly illustrated by the work of Lyashenko et al (2011): mouse Emryonic Stem Cells (mESC) lacking β-catenin lose their ability to differentiate into a mesodermal germ layer and do not form any neuroepithelium; both defects are connected to defective cell–cell junctions arising during the differentiation processes. Putting back a signalling-defective β-catenin restores the integrity of adherens junction and importantly also the ability of mESC to form neuroepithelial structures and endoderm. However the mesoderm formation is not rescued and thus requires also intact β-catenin transcription (Lyashenko et al, 2011). Similarly in the developing dorsal neural tube and the migrating population of neural crest cells the loss of β-catenin results in more drastic phenotypes than only blocking the β-catenin signalling outputs, further showing that the two roles of β-catenin cause an additive effect (Valenta et al, 2011). Such overlap and possible separation of β-catenin roles was also demonstrated in a seminal analysis in Drosophila more than a decade ago (Orsulic and Peifer, 1996). How can the β-catenin protein mediate both distinct adhesive and signalling activities? The answer is in the structural composition of β-catenin. The β-catenin protein (781 aa residues in humans) consists of a central region (residues 141–664) made up of 12 imperfect Armadillo repeats (R1–12) that are flanked by distinct N- and C-terminal domains, NTD and CTD, respectively. A specific conserved helix (Helix-C) is located proximally to the CTD, adjacent to the last ARM repeat (residues 667–683) (Xing et al, 2008; Figure 2). The NTD and the CTD may be structurally flexible, whereas the central region forms a relatively rigid scaffold. This scaffold serves as an interaction platform for many β-catenin binding partners, at the membrane, in cytosol, and in the nucleus (Huber et al, 1997). Figure 2.β-Catenin serves as a binding platform for a multitude of interaction partners in adherens junctions, in the cytoplasm and in the nucleus. (A) The β-catenin protein consists of a central region composed of 12 Armadillo repeats (numbered boxes), flanked by an amino-terminal domain (NTD) and a carboxy-terminal domain (CTD). Between the last Armadillo repeat and the flexible part of the CTD is the conserved helix-C (C). Coloured bars show experimentally validated binding sites for β-catenin interaction partners. Colour code: purple, components of adherens junctions; blue, members of the β-catenin destruction complex; red, transcriptional co-activators; green, transcription factors providing DNA binding; gray, transcriptional inhibitors. C-Terminal Transcriptional Activators (CTTA), the critical domain for their binding is marked by brackets. Little circles indicate phosphorylation sites on either E-cadherin or APC that enhance the interactions. APC, Adenoma Polyposis Coli; TCF/Lef, T-cell factor/Lymphoid enhancer factor; AR, Androgen Receptor; LRH-1, Liver Receptor Homologue-1; ICAT, Inhibitor of β-catenin and TCF; BCL9, B-cell lymphoma-9. (B) The C-terminus of β-catenin serves as a binding factor for a multitude of complexes promoting β-catenin-mediated transcription. Experimentally validated binding motifs for particular proteins are indicated. In the grey boxes, the function is indicated of the particular β-catenin interactor or of a complex, where this binding partner is a member. Brg-1 is also known as SMARCA4, CBP as CREBBP. HAT, histone acetyl-transferase; HMT, histone methyl-transferase; MLL, mixed lineage leukaemia; PAF-1, Polymerase-associated factor-1; PIC, Pre-Initiation Complex; TBP, TATA-box Binding Protein; TRRAP, Transformation/transcription domain-associated protein. Download figure Download PowerPoint β-Catenin is a founding member of the Armadillo (ARM) repeat protein superfamily. Each ARM repeat of its central region comprises ∼42 residues, forming three helices arranged in triangular shape. Together, all ARM repeats form a superhelix that features a long, positively charged groove. Biochemical and crystal structure analyses revealed that many of β-catenin's binding partners share overlapping binding sites in the groove of the central β-catenin region: consequently, these partners cannot bind to β-catenin simultaneously. This mutual exclusivity is certainly valid for the key β-catenin interacting molecules: E-cadherin (the main partner in adherens junctions), APC (the main partner in the destruction complex), and TCF/Lef (the main partner in the nucleus). All these β-catenin interactors bind to the core binding site comprising ARM repeats R3–R9, where they form salt bridges with two key amino-acid residues, Lys312 and Lys435. Other ARM repeats are also involved, at least in strengthening the interaction (Graham et al, 2000; Eklof Spink et al, 2001; Huber and Weis, 2001; Poy et al, 2001). The spatial segregation of the different β-catenin binding partners within the cell may be important for enabling the function of these proteins. However, the competition among them for β-catenin is also important for regulating canonical Wnt signalling. Conformational changes of β-catenin may also help regulate its binding properties: the terminal regions of β-catenin (NTD and especially CTD) have been proposed to fold back on the central region affecting, for example, the binding of TCF/Lef (Castaño et al, 2002; Solanas et al, 2004). Biochemical evidence supports the model that β-catenin-mediated transcription is performed by a monomeric, back-folded form of β-catenin, whereas a cadherin-binding dimeric form is associated with α-catenin in adherens junctions (Gottardi and Gumbiner, 2004). Interestingly, Helix-C within the most N-terminal part of the CTD was shown to be essential for the signalling activity of β-catenin, while being completely dispensable for its role in cell–cell adhesion (Xing et al, 2008). This is not surprising, as a plethora of β-catenin transcriptional co-activators require an intact Helix-C (or region R12-Helix-C) for their proper binding (Figure 2; Mosimann et al, 2009). The crucial role of this part of β-catenin was predicted and experimentally demonstrated more than a decade ago by elegant in-vivo analyses of different mutants of Armadillo (Orsulic and Peifer, 1996). In these studies, mutants of Armadillo (point mutations or deletions) were produced, some interfered with the adhesion function, but not with its role in Wnt signalling, and vice versa: revealing that the two functions of Arm are separable. Some of these mutations specifically hit Helix-C and abrogated the signalling role but perfectly preserved the structural function of Armadillo. Recently, the ability to completely block the signalling output of Armadillo, without affecting its adhesive role, was confirmed and experimentally extended to β-catenin in mice (Valenta et al, 2011). Interestingly, the functional separation of the two roles occurred evolutionarily in Caenorhabdidis elegans. This nematode has at least three specialized β-catenins: the adhesion-specific HMP-2 that binds cadherins but not TCFs, and two signalling ones, BAR-1 and WRM-1, that can bind TCFs and regulate transcription (Liu et al, 2008). Yet another C. elegans protein, SYS-1, has adopted a signalling role, although it is not related to β-catenin on the sequence level. SYS-1 structurally mimics β-catenin (the positively charged groove), interacts with TCFs, and even mediates transcription (Liu et al, 2008). Such examples of functional separation, however, appear to be the exception. In the majority of animal species (including the model organisms Drosophila, Xenopus, and mouse), a single β-catenin carries out the two functions. These functions may be carried out by different protein pools and are orchestrated by mechanisms (e.g., post-translational modifications), which control the spatial separation, retention, or stability of β-catenin. That said, in vertebrates γ-catenin (plakoglobin), a close relative of β-catenin, may in certain cases compensate for the loss of β-catenin's structural function; in contrast, the signalling role of β-catenin in vivo seems not to be compensated (Huelsken et al, 2000; Grigoryan et al, 2008). The role of plakoglobin in Wnt/β-catenin signalling warrants further clarification. β-Catenin is an evolutionarily conserved protein The usually single β-catenin gene represents the highly conserved centre piece within the complex expanded Wnt signalling pathway (Figure 3). The Wnt pathway features a multitude of paralogous components, especially at the level of Wnt ligands and their receptors. The complexity at the level of the Wnt ligands and receptors (Frizzled proteins) seems to have arisen early in metazoan evolution. In Cnidarians, 14 Wnt genes have been identified which belong to 12 of the 13 known different Wnt subfamilies, 11 of which are also present in vertebrates, where some additional duplication events within individual subfamilies lead to expression of up to 19 different Wnts in mammals (Kusserow et al, 2005). In sponges (Porifera), which constitute some of the most basal metazoans, the Wnt ligand complexity is not as high, only three (respectively two) Wnt ligands are known (Lapébie et al, 2009; Adamska et al, 2010). Figure 3.β-Catenin is an evolutionarily ancient molecule that provided its function before Wnt signalling and classical cadherin-based adhesion appeared. Schematic evolutionary tree showing the relationships among Amoebozoa (represented by Dictyostelium discoideum) and metazoa, as well as the diversity of signalling components. In Dictyostelium discoideum, β-catenin acts as a functional molecule in polarized epithelia. In animals (metazoa), β-catenin plays a dual role as a signalling component of canonical Wnt signalling or as a structural component of cadherin-based cell–cell junctions. The presence of β-catenin and key components of adherens junctions (classical cadherins—containing an intracellular domain binding to β-catenin) and of canonical Wnt signalling (Wnt ligands, TCF/Lef transcription factors) is indicated to the right. In the case of Wnt ligands, the first number indicates how many different Wnt ligands were determined, the second number in brackets indicates how many Wnt subfamilies were determined in a particular group. The number in the case of TCF/Lef refers to how many different TCF/Lef proteins were found. Yes means presence, no absence. The following animal species were compared: Porifera (Sponges): Amphimedon queenslandica, Cnidaria: Nematostella vectans, Insects: Drosophila melanogaster; Vertebrates: Mus musculus. Download figure Download PowerPoint In sponges, ctenophores, and cnidarians, one gene encodes a β-catenin protein with striking sequence similarity to mammalian β-catenin. Cnidarian β-catenin (e.g., of the polyp Hydra) exhibits >60% of amino-acid identity with mammalian β-catenin, with the main differences being found in the most C-terminal part, which is generally less conserved (Xing et al, 2008; Zhao et al, 2011). The structural identity of the repeat domain of β-catenin is nearly 80%. Moreover, locus conservation among sponges (Amphimedon sp.), cnidarians (sea anemone Nematostella vectans) and later developed metazoans (Deuterostomia; vertebrates belong to this group) includes even the exon/intron features of the β-catenin gene (Holland et al, 2005; Adamska et al, 2010). Highlighting the pivotal role of Wnt/β-catenin signalling also in early metazoans are findings that changes in β-catenin levels dramatically impact their development. In sponges (Porifera), the activation of Wnt/β-catenin signalling by blocking β-catenin degradation leads to the formation of ectopic ostia (canal openings), which can disrupt feeding (Lapébie et al, 2009). In Hydra (Cnidaria), ectopic stabilization of β-catenin results in multiple head and tentacle formation along the body, while the opposite phenotype, a loss of head structures, results from depletion of β-catenin (Broun et al, 2005; Müller et al, 2007; Gee et al, 2010). Such observations suggest that Wnt/β-catenin signalling represents a primeval component, driving metazoan body plan and axis formation since the beginning of animal evolution. The critical role of Wnt/β-catenin signalling for establishing the primary body axis (Petersen and Reddien, 2009), or more precisely A/P (anterior/posterior) identity, has been long studied in diverse organisms. It is a re-occuring theme and reveals itself either in individual (para)segments, as in the Drosophila embryo, or of whole developing body plans, such as in vertebrates (Sanson, 2001; Niehrs, 2010). A paradigmatic example showing the indispensable role of β-catenin in the process of axial patterning came from experiments in the amphibian Xenopus laevis: ectopic expression of β-catenin in ventral blastomeres of Xenopus embryos induces axis duplication (i.e., the formation of a secondary axis), a phenotype reminiscent of misexpression of several Wnt ligands (Guger and Gumbiner, 1995). Depletion of β-catenin results in the opposite effect, ventralization of the embryo with a shortened A/P axis (Haesman et al, 1994). A/P axis identity and mesoderm formation is controlled by β-catenin-mediated transcription also in mammals, since mouse embryos lacking β-catenin do not develop a proper A/P axis and fail to form mesoderm, resulting in corrupted gastrulation (Haegel et al, 1995; Huelsken et al, 2000). Interestingly, a homologue of β-catenin was found also in the non-metazoan social amoeba Dictyostelium discoideum (slime mould) (Coates et al, 2002). This contrasts with the lack of Wnt ligands, and indeed of most proteins involved in Wnt signalling, which so far have been identified solely in metazoans but not in any other phyla (Figure 3). In normal circumstances, Dictyostelium discoideum is a unicellular organism; but in response to starvation single cells start to aggregate and form fruiting bodies, which comprise a rigid stalk and on its tip a collection of spores. During the formation of a fruiting body the aggregated cells establish a polarized epithelium that is essential for the subsequent multicellular phase of development. Even though Dictyostelium discoideum, in contrast to metazoans, lacks classical cadherins, the β-catenin orthologue Aardvark is an essential structural molecule in the polarized epithelia. Moreover, to maintain the proper epithelial structure Aardvark cooperates with α-catenin, another key structural molecule known from metazoans. Ddα-catenin is upregulated during the transition to multicellularity and colocalizes with Aardvark and F-actin at the sites of cell–cell contact. Loss or knockdown of either Aardvark/Ddβ-catenin or Ddα-catenin significantly compromises epithelial morphology, polarity, and formation of fruiting bodies (Dickinson et al, 2011). Thus, β-catenin, together with α-catenin and most likely actin, can promote primitive cell–cell contacts even without cadherins. These observations suggest that the principles underlying metazoan multicellularity may be more ancient than previously thought and that the structural role of β-catenin precedes its signalling role in the canonical Wnt pathway—β-catenin and α-catenin were engaged in the formation of polarized epithelia before classical catenins came on the scene in metazoans. However, even in Dictyostelium discoideum, β-catenin is not exclusively a structural molecule. In response to cAMP, a series of events leads to phosphorylation of GSK3, an orthologue of GSKA that phosphorylates Aardvark/Ddβ-catenin. This phosphorylation event activates Aardvark and leads to gene transcription via a so far unknown mechanism, as there are no TCF/Lef transcription factors in Dictyostelium discoideum (Grimson et al, 2000). Even though the mechanism seems different from the core Wnt/β-catenin pathway (phosphorylation leads to activation, not to degradation), at least the interaction between β-catenin and GSK3 is present and may have further evolved into the mode of action typical for canonical Wnt signalling. Canonical signalling in a nutshell Without a Wnt signal, the levels of cytoplasmic free β-catenin are kept low (Figure 1). If not bound to E-cadherin, β-catenin is phosphorylated in the cytoplasm by the activity of a multiprotein destruction complex, marking β-catenin for degradation. This complex consists of the scaffold proteins Axin and Adenoma Polyposis Coli (APC), and of the kinases phosphorylating β-catenin, glycogen synthase kinase 3β (GSK3β), casein kinase 1 (CK1) and protein phosphatase 2A (PP2A) (Kimelman and Xu, 2006). The binding of Wnt ligands to the Frizzled transmembrane receptors and LRP5/6 co-receptors (Low density lipoprotein Receptor-related Protein) starts a series of molecular events leading to inhibition of the β-catenin destruction complex. Ligand activated Frizzled receptors recruit the cytoplasmic protein Dishevelled (Dvl) to the receptor complex through direct binding; Dvl subsequently multimerizes and induces formation of so-called LRP-associated Wnt signalosomes (Bilic et al, 2007). Dvl in turn recruits the rate-limiting component Axin (most likely together with associated kinases, such as GSK3) and thus destabilizes the β-catenin destruction complex (Schwarz-Romond et al, 2007). Dvl-mediated phosporylation of LRP5/6 by CK1 is key for proper functioning of the Wnt signalosome as it leads to a block of GSK3 kinase activity (Zeng et al, 2008). Unphosphorylated β-catenin escapes degradation, accumulates in the cytoplasm, and translocates to the nucleus. Nuclear β-catenin associates with DNA-binding transcription factors of the TCF/Lef family. TCF/Lef proteins possess only limited ability to activate transcription. In the absence of Wnt signals, TCF/Lef factors act as transcriptional repressors. The binding of β-catenin converts TCF/Lef proteins into bipartite TCF/β-catenin transcriptional activators, converting the Wnt signal into the transcription of specific target genes (Najdi et al, 2011; Archbold et al, 2012). Albeit elegant, recent work has made clear that such a simplified, linear scheme describing the translation of the Wnt signal via β-catenin into a cell-specific genetic program does not capture the full complexity of β-catenin-mediated functions within Wnt receiving cells. β-Catenin at the membrane: balancing adhesion and signalling The majority of β-catenin is located at the cytoplasmic side of the membrane as a component of cadherin-based cell–cell connections in the absence of a Wnt stimulus. Cadherin-based adherens junctions contribute to forming polarized epithelial tissues, a characteristic metazoan feature necessary for maintaining organismal integrity (Meng and Takeichi, 2009). Cadherins are single-pass transmembrane glycoproteins that engage in Ca2+-dependent homotypic interactions via their extracellular regions and that also link to β-catenin through their cytoplasmic tails. Classical cadherins were named for the tissue in which they were most prominently expressed: for example, epithelial cadherin (E-cadherin) in epithelial cells; neural cadherin (N-cadherin) in the nervous system. Many cell types co-express several cadherins (Meng and Takeichi, 2009; Stepniak et al, 2009). β-Catenin can interact with the cytoplasmic domain of all of them, but most studied are the functional consequences of the β-catenin–E-cadherin interaction. The cadherin–β-catenin interaction is constitutive and may occur even in single isolated cells in an adhesion-independent way (Stepniak et al, 2009). Newly synthesized E-cadherin associates with β-catenin while still in the endoplasmic reticulum and the two proteins move together to the cell membrane. Interfering with the binding of β-catenin to E-cadherin results in proteasomal degradation of the cadherin, because β-catenin shields a PEST sequence motif on E-cadherin, which, if free, is recognized by a ubiquitin ligase that marks E-cadherin for degradation (Hinc et al, 1994). Reciprocally, E-cadherin association stabilizes β-catenin by preventing binding of components of the β-catenin destruction complex (namely APC and Axin) (Huber and Weis, 2001). At cadherin-based cell–cell junctions, β-catenin can bind α-catenin with distal parts of the NTD and the adjacent first ARM repeat of β-catenin close to a hinge region around Arg151 (Figure 2; Pokutta and Weis 2000; Xing et al, 2008). In close proximity, C-terminal to this hinge, is the binding site for BCL9, an important transcriptional co-activator of β-catenin (Hoffm