The amino- and carboxyl-terminal tails of (beta)-catenin reduce its affinity for desmoglein 2

Βeta-catenin N-terminal domain: An enigmatic region prone to cancer causing mutations

The Wnt/β-catenin is a highly conserved signaling pathway involved in cell fate decisions during various stages of development. Dysregulation of canonical Wnt/β-catenin signaling has been associated with various diseases including cancer. β-Catenin, the central component of canonical Wnt signaling pathway, is a multi-functional protein playing both structural and signaling roles. β-Catenin is composed of three distinct domains: N-terminal domain, C-terminal domain and a central armadillo repeat domain. N-terminal domain of β-catenin harbours almost all of the cancer causing mutations, thus deciphering its critical structural and functional roles offers great potential in cancer detection and therapy. Here, in this review, we have collected information from pharmacological analysis, bio-physical and structural studies, molecular modeling, in-vivo and in-vitro assays, and transgenic animal experiments employing various N-terminal domain variants of β-catenin to discuss the interaction of β-catenin with its binding partners that specifically interact with this domain and the implications of these interactions on signaling, cell fate determination, and in tumorigenesis. A thorough understanding of interactions between β-catenin and its binding partners will enable us to more effectively understand how β-catenin switches between its multiple roles, and will lead to the development of specific assays for the identification of small molecules as chemotherapeutic agents to treat diseases, including cancer and neurological disorders, where Wnt/β-catenin signaling is dysregulated.

Palmitoylation of Desmoglein 2 Is a Regulator of Assembly Dynamics and Protein Turnover

Desmosomes are prominent adhesive junctions present between many epithelial cells as well as cardiomyocytes. The mechanisms controlling desmosome assembly and remodeling in epithelial and cardiac tissue are poorly understood. We recently identified protein palmitoylation as a mechanism regulating desmosome dynamics. In this study, we have focused on the palmitoylation of the desmosomal cadherin desmoglein-2 (Dsg2) and characterized the role that palmitoylation of Dsg2 plays in its localization and stability in cultured cells. We identified two cysteine residues in the juxtamembrane (intracellular anchor) domain of Dsg2 that, when mutated, eliminate its palmitoylation. These cysteine residues are conserved in all four desmoglein family members. Although mutant Dsg2 localizes to endogenous desmosomes, there is a significant delay in its incorporation into junctions, and the mutant is also present in a cytoplasmic pool. Triton X-100 solubility assays demonstrate that mutant Dsg2 is more soluble than wild-type protein. Interestingly, trafficking of the mutant Dsg2 to the cell surface was delayed, and a pool of the non-palmitoylated Dsg2 co-localized with lysosomal markers. Taken together, these data suggest that palmitoylation of Dsg2 regulates protein transport to the plasma membrane. Modulation of the palmitoylation status of desmosomal cadherins can affect desmosome dynamics.

Interplay between Solo and keratin filaments is crucial for mechanical force-induced stress fiber reinforcement

Mechanical force-induced cytoskeletal reorganization is essential for cell and tissue remodeling and homeostasis; however, the underlying cellular mechanisms remain elusive. Solo (ARHGEF40) is a RhoA-targeting guanine nucleotide exchange factor (GEF) involved in cyclical stretch-induced human endothelial cell reorientation and convergent extension cell movement in zebrafish gastrula. In this study, we show that Solo binds to keratin-8/keratin-18 (K8/K18) intermediate filaments through multiple sites. Solo overexpression promotes the formation of thick actin stress fibers and keratin bundles, whereas knockdown of Solo, expression of a GEF-inactive mutant of Solo, or inhibition of ROCK suppresses stress fiber formation and leads to disorganized keratin networks, indicating that the Solo-RhoA-ROCK pathway serves to precisely organize keratin networks, as well as to promote stress fibers. Of importance, knockdown of Solo or K18 or overexpression of GEF-inactive or deletion mutants of Solo suppresses tensile force-induced stress fiber reinforcement. Furthermore, knockdown of Solo or K18 suppresses tensile force-induced RhoA activation. These results strongly suggest that the interplay between Solo and K8/K18 filaments plays a crucial role in tensile force-induced RhoA activation and consequent actin cytoskeletal reinforcement.

Pathogenesis of Arrhythmogenic Cardiomyopathy

Arrhythmogenic cardiomyopathy (ACM) is a primary myocardial disease. It is characterized by frequent ventricular arrhythmias and increased risk of sudden cardiac death typically arising as an early manifestation before the onset of significant myocardial remodelling. Myocardial degeneration, often confined to the right ventricular free wall, with replacement by fibrofatty scar tissue, develops in many patients. ACM is a familial disease but genetic penetrance can be low and disease expression is highly variable. Inflammation might promote disease progression. It also appears that exercise increases disease penetrance and accelerates its development. More than 60% of probands harbour mutations in genes that encode desmosomal proteins, which has raised the possibility that defective cell-cell adhesion might play a role in disease pathogenesis. Recent advances have implicated changes in the canonical wingless-type mouse mammary tumour virus integration site (Wnt)/β-catenin and Hippo signalling pathways and defects in forwarding trafficking of ion channels and other proteins to the intercalated disk in cardiac myocytes. In this review we summarize the current understanding of the pathogenesis of ACM and highlight future research directions.

Immunohistological characterization of intercellular junction proteins in rhesus macaque intestine

Epithelial junctions play an important role in regulating paracellular permeability and intercellular adhesion. It has been reported that changes in the density of epithelial junctions and/or distribution pattern can contribute to various gastrointestinal (GI) disorders. In this study, we investigated the distribution of the tight junction (Claudins. 1, 3, 4, 5, 7, 10, Zonula Occludens (ZO-1), Occludin), adherens junction (E-cadherin), desmosome (Desmoglein 2, Desmocollin 2) and gap junction (Connexin 43) proteins in the jejunum, ileum and colonic epithelium of healthy rhesus macaques (RM) using immunofluorescence labeling. While proteins in these respective junctions were expressed throughout the jejunum, ileum and colon of RM, we observed differential labeling in epithelial cells from these sites. Claudins 1, 3, 4, 7, E-cadherin and Desmoglein 2 were distributed in the respective intercellular junctions with additional labeling in the lateral membrane of epithelial cells in both small and large intestine. However, claudin 5, claudin 10, ZO-1 and occludin showed uniform distribution in the intercellular junctions of crypt and surface epithelial cells of the intestine. Desmocollin 2 localized predominantly in the upper two thirds along the lateral membrane while desmoglein 2 was distributed along the entire lateral membrane of intestinal epithelial cells. In contrast, connexin 43 exhibited punctate lateral labeling in crypt epithelial cells of the small and large intestine. Our results show diverse localization of epithelial intercellular junction proteins along the intestinal tract of RM. These findings may correlate with differences in paracellular permeability and adhesion along the intestinal tract and could correlate with pathologic disease in these regions of the intestine.

Palmitoylation of plakophilin is required for desmosome assembly

Desmosomes are prominent adhesive junctions found in various epithelial tissues. The cytoplasmic domains of desmosomal cadherins interact with a host of desmosomal plaque proteins, including plakophilins, plakoglobin and desmoplakin, which, in turn, recruit the intermediate filament cytoskeleton to sites of cell-cell contact. Although the individual components of the desmosome are known, mechanisms regulating the assembly of this junction are poorly understood. Protein palmitoylation is a posttranslational lipid modification that plays an important role in protein trafficking and function. Here, we demonstrate that multiple desmosomal components are palmitoylated in vivo. Pharmacologic inhibition of palmitoylation disrupts desmosome assembly at cell-cell borders. We mapped the site of plakophilin palmitoylation to a conserved cysteine residue present in the armadillo repeat domain. Mutation of this single cysteine residue prevents palmitoylation, disrupts plakophilin incorporation into the desmosomal plaque and prevents plakophilin-dependent desmosome assembly. Finally, plakophilin mutants unable to become palmitoylated act in a dominant-negative manner to disrupt proper localization of endogenous desmosome components and decrease desmosomal adhesion. Taken together, these data demonstrate that palmitoylation of desmosomal components is important for desmosome assembly and adhesion.

Stratifin (14-3-3 σ) limits plakophilin-3 exchange with the desmosomal plaque

Desmosomes are prominent cell-cell adhesive junctions in stratified squamous epithelia and disruption of desmosomal adhesion has been shown to have dramatic effects on the function and integrity of these tissues. During normal physiologic processes, such as tissue development and wound healing, intercellular adhesion must be modified locally to allow coordinated cell movements. The mechanisms that control junction integrity and adhesive strength under these conditions are poorly understood. We utilized a proteomics approach to identify plakophilin-3 associated proteins and identified the 14-3-3 family member stratifin. Stratifin interacts specifically with plakophilin-3 and not with other plakophilin isoforms and mutation analysis demonstrated the binding site includes serine 285 in the amino terminal head domain of plakophilin-3. Stratifin interacts with a cytoplasmic pool of plakophilin-3 and is not associated with the desmosome in cultured cells. FRAP analysis revealed that decreased stratifin expression leads to an increase in the exchange rate of cytoplasmic plakophilin-3/GFP with the pool of plakophilin-3/GFP in the desmosome resulting in decreased desmosomal adhesion and increased cell migration. We propose a model by which stratifin plays a role in regulating plakophilin-3 incorporation into the desmosomal plaque by forming a plakophilin-3 stratifin complex in the cytosol and thereby affecting desmosome dynamics in squamous epithelial cells.

The molecular composition and function of desmosomes

Desmosomes are intercellular adhesive junctions that are particularly prominent in tissues experiencing mechanical stress, such as the heart and epidermis. Whereas the related adherens junction links actin to calcium-dependent adhesion molecules known as classical cadherins, desmosomes link intermediate filaments (IF) to the related subfamily of desmosomal cadherins. By tethering these stress-bearing cytoskeletal filaments to the plasma membrane, desmosomes serve as integrators of the IF cytoskeleton throughout a tissue. Recent evidence suggests that IF attachment in turn strengthens desmosomal adhesion. This collaborative arrangement results in formation of a supracellular network, which is critical for imparting mechanical integrity to tissues. Diseases and animal models targeting desmosomal components highlight the importance of desmosomes in development and tissue integrity, while the downregulation of individual protein components in cancer metastasis and wound healing suggests their importance in cell homeostasis. This chapter will provide an update on desmosome composition, function, and regulation, and will also discuss recent work which raises the possibility that desmosome proteins do more than play a structural role in tissues where they reside.

Structural aspects of adherens junctions and desmosomes

The cadherin-containing intercellular junctions, adherens junctions and desmosomes share an overall logical organization in which the extracellular regions of the cadherins on opposing cells interact, while their cytoplasmic domains are linked to the cytoskeleton through protein assemblies. In adherens junctions, beta-catenin binds to the cytoplasmic domain of cadherins and to alpha-catenin, which links the cadherin/beta-catenin complex to the actin cytoskeleton. In desmosomes, the beta-catenin homolog plakoglobin binds to desmosomal cadherins. The desmosomal cadherin/plakoglobin complex is linked to the intermediate filament system by the protein desmoplakin. In the past decade, components of these systems have been purified to homogeneity and studied biochemically and structurally, providing the beginnings of a mechanistic description of junction architecture and dynamics.

Desmosome dynamics in migrating epithelial cells requires the actin cytoskeleton

Re-modeling of epithelial tissues requires that the cells in the tissue rearrange their adhesive contacts in order to allow cells to migrate relative to neighboring cells. Desmosomes are prominent adhesive structures found in a variety of epithelial tissues that are believed to inhibit cell migration and invasion. Mechanisms regulating desmosome assembly and stability in migrating cells are largely unknown. In this study we established a cell culture model to examine the fate of desmosomal components during scratch wound migration. Desmosomes are rapidly assembled between epithelial cells at the lateral edges of migrating cells and structures are transported in a retrograde fashion while the structures become larger and mature. Desmosome assembly and dynamics in this system are dependent on the actin cytoskeleton prior to being associated with the keratin intermediate filament cytoskeleton. These studies extend our understanding of desmosome assembly and provide a system to examine desmosome assembly and dynamics during epithelial cell migration.

Arrhythmogenic right ventricular cardiomyopathy plakophilin-2 mutations disrupt desmosome assembly and stability

Arrhythmogenic right ventricular cardiomyopathy (ARVC) is characterized by life-threatening ventricular arrhythmias and fibrofatty replacement of the cardiac tissue. Desmosomes are prominent cell-cell junctions found in a variety of tissues that resist mechanical stress, including the heart, and recruit the intermediate filament cytoskeleton to sites of cell-cell contact. Mutations in several desmosomal components including plakophilin-2 have been identified in ARVC patients; however, the molecular interactions disrupted by plakophilin-2 mutations are currently unknown. To understand the pathological basis of ARVC, the authors analyzed desmosome assembly and stability in epithelial cell lines expressing mutants of plakophilin-2 found in ARVC patients. Mutant plakophilin-2 proteins were unable to disrupt established desmosomes when expressed in an E-cadherin-expressing epithelial cell model; however, they were unable to initiate de novo assembly of desmosomes in an N-cadherin-expressing epithelial cell model. These studies expand our understanding of desmosome assembly and dynamics.

Loss of desmocollin-2 confers a tumorigenic phenotype to colonic epithelial cells through activation of Akt/β-catenin signaling

This study provides evidence that decreased expression of the desmosomal cadherin desmocollin-2 enhances intestinal epithelial cell proliferation and promotes tumor formation via an Akt/β-catenin pathway.

Desmocollin-2 (Dsc2) and desmoglein-2 (Dsg2) are transmembrane cell adhesion proteins of desmosomes. Reduced expression of Dsc2 has been reported in colorectal carcinomas, suggesting that Dsc2 may play a role in the development and/or progression of colorectal cancer. However, no studies have examined the mechanistic contribution of Dsc2 deficiency to tumorigenesis. Here we report that loss of Dsc2 promotes cell proliferation and enables tumor growth in vivo through the activation of Akt/β-catenin signaling. Inhibition of Akt prevented the increase in β-catenin–dependent transcription and proliferation following Dsc2 knockdown and attenuated the in vivo growth of Dsc2-deficient cells. Taken together, our results provide evidence that loss of Dsc2 contributes to the growth of colorectal cancer cells and highlight a novel mechanism by which the desmosomal cadherins regulate β-catenin signaling.

Plakophilin-1 localizes to the nucleus and interacts with single-stranded DNA

Plakophilins (Pkp-1, -2, and -3) comprise a family of armadillo repeat-containing proteins first identified as desmosomal plaque components, in which they link desmoplakin to the desmosomal cadherins. In addition to their role in desmosomal cell-cell adhesion, Pkps also localize to the nucleus, where they perform unknown functions. Of the three Pkps, Pkp-1 is most readily detected in the nucleus, where it is localized to the nucleoplasm. Pkp chimeras containing the Pkp-1 head domain and Pkp-3 armadillo repeat domain were localized to the nucleus in A431 cells, whereas Pkp chimeras containing the Pkp-3 head domain and Pkp-1 armadillo repeat domain localized to the desmosome and the cytosol. DNAse I digestion of chromatin in cultured cells results in loss of nuclear Pkp-1, suggesting that Pkp-1 associates specifically with nuclear components. In addition, in vitro assays revealed that the amino-terminal head domains of Pkps-1 and -2 were sufficient to bind single-stranded DNA. Induction of DNA damage induced a partial redistribution of Pkp-1 protein to the nucleolus, and depletion of Pkp-1 resulted in increased survival in response to DNA damage. These data suggest that in addition to mediating desmosome assembly, the nuclear pool of Pkp can influence cell survival by interactions with DNA.

Interactions of plakoglobin and beta-catenin with desmosomal cadherins: basis of selective exclusion of alpha- and beta-catenin from desmosomes

Plakoglobin and beta-catenin are homologous armadillo repeat proteins found in adherens junctions, where they interact with the cytoplasmic domain of classical cadherins and with alpha-catenin. Plakoglobin, but normally not beta-catenin, is also a structural constituent of desmosomes, where it binds to the cytoplasmic domains of the desmosomal cadherins, desmogleins and desmocollins. Here, we report structural, biophysical, and biochemical studies aimed at understanding the molecular basis of selective exclusion of beta-catenin and alpha-catenin from desmosomes. The crystal structure of the plakoglobin armadillo domain bound to phosphorylated E-cadherin shows virtually identical interactions to those observed between beta-catenin and E-cadherin. Trypsin sensitivity experiments indicate that the plakoglobin arm domain by itself is more flexible than that of beta-catenin. Binding of plakoglobin and beta-catenin to the intracellular regions of E-cadherin, desmoglein1, and desmocollin1 was measured by isothermal titration calorimetry. Plakoglobin and beta-catenin bind strongly and with similar thermodynamic parameters to E-cadherin. In contrast, beta-catenin binds to desmoglein-1 more weakly than does plakoglobin. beta-Catenin and plakoglobin bind with similar weak affinities to desmocollin-1. Full affinity binding of desmoglein-1 requires sequences C-terminal to the region homologous to the catenin-binding domain of classical cadherins. Although pulldown assays suggest that the presence of N- and C-terminal beta-catenin "tails" that flank the armadillo repeat region reduces the affinity for desmosomal cadherins, calorimetric measurements show no significant effects of the tails on binding to the cadherins. Using purified proteins, we show that desmosomal cadherins and alpha-catenin compete directly for binding to plakoglobin, consistent with the absence of alpha-catenin in desmosomes.

Structure and function of desmosomes

Desmosomes are prominent adhesion sites that are tightly associated with the cytoplasmic intermediate filament cytoskeleton providing mechanical stability in epithelia and also in several nonepithelial tissues such as cardiac muscle and meninges. They are unique in terms of ultrastructural appearance and molecular composition with cell type-specific variations. The dynamic assembly properties of desmosomes are important prerequisites for the acquisition and maintenance of tissue homeostasis. Disturbance of this equilibrium therefore not only compromises mechanical resilience but also affects many other tissue functions as becomes evident in various experimental scenarios and multiple diseases.

Decreased expression of the adhesion molecule desmoglein-2 is associated with diffuse-type gastric carcinoma

Desmoglein-2 (Dsg2) is one of the components of the cell-cell adherence junction. We previously reported that loss of heterozygosity at chromosome 18q12, on which the Dsg2 gene exists, is frequently found in diffuse-type gastric cancers. This study investigated the relationship between Dsg2 expression and diffuse-type gastric cancers. A total of 112 primary tumours resected from patients with gastric cancer were stained with a monoclonal antibody against Dsg2 and examined for correlations between the expression of Dsg2 and various clinicopathological factors, including loss of heterozygosity on chromosome 18q and prognosis. Dsg2 is immunolocalised at cell-cell boundaries in normal gastric mucosa. Loss of Dsg2 expression was observed in 33 of 112 gastric tumours. There was a statistically significant correlation between a decrease in Dsg2 staining and loss of tumour differentiation (P < 0.001), tumour macroscopic feature (P < 0.001) and peritoneal dissemination (P = 0.023), and Dsg2-negative staining was correlated significantly with loss of heterozygosity on chromosome 18q12 (P = 0.001). The prognosis of patients with Dsg2-negative tumours was significantly worse than that of those with Dsg2-positive tumours (log rank, P < 0.01), while multivariate analysis revealed that Dsg2 was not an independent prognostic factor. These findings suggest that decreased expression of Dsg2 is associated with diffuse-type gastric cancers and poor prognosis in gastric carcinoma.

Thermodynamics of beta-catenin-ligand interactions: the roles of the N- and C-terminal tails in modulating binding affinity

beta-Catenin is a structural component of adherens junctions, where it binds to the cytoplasmic domain of cadherin cell adhesion molecules. beta-Catenin is also a transcriptional coactivator in the Wnt signaling pathway, where it binds to Tcf/Lef family transcription factors. In the absence of a Wnt signal, nonjunctional beta-catenin is present in a multiprotein complex containing the proteins axin and adenomatous polyposis coli (APC), both of which bind directly to beta-catenin. The thermodynamics of beta-catenin binding to E-cadherin, Lef-1, APC, axin, and the transcriptional inhibitor ICAT have been determined by isothermal titration calorimetry. Most of the interactions showed large, unfavorable entropy changes, consistent with these ligands being natively unstructured in the absence of beta-catenin. Phosphorylation of serine residues present in a sequence motif common to cadherins and APC increased the affinity for beta-catenin 300-700-fold, and surface plasmon resonance measurements revealed that phosphorylation of E-cadherin both enhanced its on rate and decreased its off rate. The effects of the N- and C-terminal "tails" that flank the beta-catenin armadillo repeat domain on ligand binding have also been investigated using constructs lacking one or both tails. Contrary to earlier studies that employed less direct binding assays, the tails did not affect the affinity of beta-catenin for tight ligands such as E-cadherin, Lef-1, and phosphorylated APC. However, the beta-catenin C-terminal tail was found to decrease the affinity for the weaker ligands APC and axin, suggesting that this region may have a regulatory role in beta-catenin degradation.

Importin alpha: a multipurpose nuclear-transport receptor

The importin alpha/beta heterodimer targets hundreds of proteins to the nuclear-pore complex (NPC) and facilitates their translocation across the nuclear envelope. Importin alpha binds to classical nuclear localization signal (cNLS)-containing proteins and links them to importin beta, the karyopherin that ferries the ternary complex through the NPC. A second karyopherin, the exportin CAS, recycles importin alpha back to the cytoplasm. In this article, we discuss control mechanisms that importin alpha exerts over the assembly and disassembly of the ternary complex and we describe how new groups of importin alpha genes arose during the evolution of metazoan animals to function in development and differentiation. We also describe activities of importin alpha that seem to be distinct from its housekeeping functions in nuclear transport.

A role for plakophilin-1 in the initiation of desmosome assembly

Plakophilins (pkp-1, -2, and -3) comprise a family of armadillo-repeat containing proteins that are found in the desmosomal plaque and in the nucleus. Plakophilin-1 is most highly expressed in the suprabasal layers of the epidermis and loss of plakophilin-1 expression results in skin fragility-ectodermal dysplasia syndrome, which is characterized by a reduction in the number and size of desmosomes in the epithelia of affected individuals. To investigate the role of plakophilin-1 during desmosome formation, we fused plakophilin-1 to the hormone-binding domain of the estrogen receptor to create a fusion protein (plakophilin-1/ER) that can be activated in cell culture by the addition of 4-hydroxytamoxifen. When plakophilin-1/ER was expressed in A431 cells it was incorporated into endogenous desmosomes and did not disrupt desmosome formation. A derivative of A431 cells (A431D) do not form desmosomes, even though they express all the components believed to be necessary for desmosome assembly. Expression and activation of plakophilin-1/ER in A431D cells resulted in punctate desmoplakin staining on the cell surface. Co-expression of a classical cadherin (N-cadherin) and plakophilin-1/ER in A431D cells resulted in punctate desmoplakin staining at cell-cell borders. These data suggest that plakophilin-1 can induce assembly of desmosomal components in A431D cells in the absence of a classical cadherin; however a classical cadherin (N-cadherin) is required to direct assembly of desmosomes between adjacent cells. The activatable plakophilin-1/ER system provides a unique culture system to study the assembly of the desmosomal plaque in culture.

Cadherins as modulators of cellular phenotype

Cadherins are transmembrane glycoproteins that mediate calcium-dependent cell-cell adhesion. The cadherin family is large and diverse, and proteins are considered to be members of this family if they have one or more cadherin repeats in their extracellular domain. Cadherin family members are the transmembrane components of a number of cellular junctions, including adherens junctions, desmosomes, cardiac junctions, endothelial junctions, and synaptic junctions. Cadherin function is critical in normal development, and alterations in cadherin function have been implicated in tumorigenesis. The strength of cadherin interactions can be regulated by a number of proteins, including the catenins, which serve to link the cadherin to the cytoskeleton. Cadherins have been implicated in a number of signaling pathways that regulate cellular behavior, and it is becoming increasingly clear that integration of information received from cell-cell signaling, cell-matrix signaling, and growth factor signaling determines ultimate cellular phenotype and behavior.

Tyrosine phosphorylation of plakoglobin causes contrary effects on its association with desmosomes and adherens junction components and modulates beta-catenin-mediated transcription

Plakoglobin is a protein closely related to beta-catenin that links desmosomal cadherins to intermediate filaments. Plakoglobin can also substitute for beta-catenin in adherens junctions, providing a connection between E-cadherin and alpha-catenin. Association of beta-catenin with E-cadherin and alpha-catenin is regulated by phosphorylation of specific tyrosine residues; modification of beta-catenin Tyr654 and Tyr142 decreases binding to E-cadherin and alpha-catenin, respectively. We show here that plakoglobin can also be phosphorylated on tyrosine residues, but unlike beta-catenin, this modification is not always associated with disrupted association with junctional components. Protein tyrosine kinases present distinct specificities on beta-catenin and plakoglobin, and phosphorylation of beta-catenin-equivalent Tyr residues of plakoglobin affects its interaction with components of desmosomes or adherens junctions differently. For instance, Src, which mainly phosphorylates Tyr86 in beta-catenin, modifies Tyr643 in plakoglobin, decreasing the interaction with E-cadherin and alpha-catenin and increasing the interaction with the alpha-catenin-equivalent protein in desmosomes, desmoplakin. The tyrosine kinase Fer, which modifies beta-catenin Tyr142, lessening its association with alpha-catenin, phosphorylates plakoglobin Tyr549 and exerts the contrary effect: it raises the binding of plakoglobin to alpha-catenin. These results suggest that tyrosine kinases like Src or Fer modulate desmosomes and adherens junctions differently. Our results also indicate that phosphorylation of Tyr549 and the increased binding of plakoglobin to components of adherens junctions can contribute to the upregulation of the transcriptional activity of the beta-catenin-Tcf-4 complex observed in many epithelial tumor cells.

Plakoglobin is O-glycosylated close to the N-terminal destruction box

Plakoglobin provides a key linkage in protein chains that connect desmosomal and classical cadherins to the cytoskeleton. It is also present in a significant cytosolic pool that has the capacity to impact on canonical Wnt signaling by competing for interaction with partner proteins of beta-catenin. The closely related protein, beta-catenin, is rapidly targeted for proteasomal degradation by phosphorylation of a "destruction box" within the N-terminal domain. Inhibition of this process forms the basis of Wnt signaling. This destruction box is also found in the N-terminal domain of plakoglobin. We report that plakoglobin is modified by the addition of O-GlcNAc at a single site in close proximity to the destruction box. O-GlcNAc modification has been proposed to counteract phosphorylation, provide protection from proteasomal degradation, mediate signal transduction, silence transcription, and regulate multimolecular protein assembly. This finding has potential implications for understanding the roles of plakoglobin.

Desmosomal cadherins

New evidence from blocking desmosomal adhesion with anti-adhesion peptides reveals a role for desmosomes in cell positioning in morphogenesis. Desmosomal adhesion is necessary for the stability of adherens junctions in epithelial cell sheets. Knockout and mis-expression of desmosomal cadherins in mice suggests that they may function directly or indirectly in regulating epidermal differentiation. Protein kinase C signalling and tyrosine phosphorylation appear to regulate desmosomal adhesion. There are new insights into the role of desmosomal cadherins in autoimmune, infectious and genetic disease.

Beta-catenin N- and C-terminal tails modulate the coordinated binding of adherens junction proteins to beta-catenin

beta-Catenin plays a central role in the establishment and regulation of adherens junctions because it interacts with E-cadherin and, through alpha-catenin, with the actin cytoskeleton. beta-Catenin is composed of three domains: a central armadillo repeat domain and two N- and C-terminal tails. The C-tail interacts with the armadillo domain and limits its ability to bind E-cadherin and other cofactors. The two beta-catenin tails are mutually inter-regulated because the C-tail is also necessary for binding of the N-tail to the armadillo domain. Moreover, the N-tail restricts the interaction of the C-tail with the central domain. Depletion of either of the two tails has consequences for the binding of factors at the other end: deletion of the C-tail increases alpha-catenin binding, whereas deletion of the N-tail blocks E-cadherin interaction to the armadillo repeats. As an effect of the interconnection of the tails, the association of alpha-catenin and E-cadherin to beta-catenin is interdependent. Thus, binding of alpha-catenin to the N-tail, through conformational changes that affect the C-tail, facilitates the association of E-cadherin. These results indicate that different cofactors of beta-catenin bind coordinately to this protein and indicate how the two terminal ends of beta-catenin exquisitely modulate intermolecular binding within junctional complexes.

Generation of monoclonal antibodies specific for desmoglein family members

Desmosomes are the most prominent cell-cell junctions in most epithelial cells and serve to link the intermediate filament cytoskeletons of adjacent cells. Desmogleins and desmocollins are the transmembrane core of the desmosome and both are members of the cadherin family of cell-cell adhesion molecules. In the skin, the three desmoglein gene products (Dsg 1, 2, and 3) are expressed in a stratification dependent manner, and therefore contribute to compositionally different desmosomes throughout the differentiating layers. In this study we generated a panel of monoclonal antibodies (MAbs) specific for each of the desmoglein gene products and evaluated their usefulness in a number of immunological procedures including immunoblotting, immunoprecipitation, and immunofluorescence. In addition, we showed that these antibodies are useful for immunoprecipitating desmogleins from cell extracts prepared in 0.1% Empigen BB, a zwitterionic detergent capable of solubilizing the desmosomal structure. Identification of conditions that solubilize the desmosome and allow the use of immunological reagents will help facilitate an increased understanding of desmosome assembly and regulation.

The transcriptional factor Tcf-4 contains different binding sites for beta-catenin and plakoglobin

beta-Catenin and plakoglobin are two related armadillo proteins necessary for the establishment of adhesion junctions and desmosomes. Moreover, beta-catenin can also act as a transcriptional co-activator through its interaction with the members of Tcf/LEF-1 transcriptional factor family. We show here that Tcf-4 can be phosphorylated in vitro by protein kinase CK2 stoichiometrically in amino acids Ser-58-Ser-59-Ser-60. Phosphorylation of these residues does not modify the interaction of Tcf-4 with beta-catenin but reduces its association to plakoglobin. The binding sites of Tcf-4 for these two proteins were compared; whereas beta-catenin requires the N-terminal first 50 amino acids, plakoglobin interacts mainly with residues 51-80. Tcf-4-(51-80) binds plakoglobin in the region of armadillo repeats 1-6. Ternary complexes composed by beta-catenin/Tcf-4/plakoglobin could be detected in vitro, demonstrating that simultaneous binding of the two armadillo proteins to Tcf-4 is possible. Experiments performed using a Tcf-4 mutant with decreased interaction to plakoglobin demonstrated that binding to this protein negatively affected the transcriptional activity of Tcf-4. These results indicate that Tcf-4 contains two different sites for binding of beta-catenin and plakoglobin, and the interaction of the latter hinders the transcriptional activity of the complex.

Regulation of beta-catenin structure and activity by tyrosine phosphorylation

beta-Catenin plays a dual role as a key effector in the regulation of adherens junctions and as a transcriptional coactivator. Phosphorylation of Tyr-654, a residue placed in the last armadillo repeat of beta-catenin, decreases its binding to E-cadherin. We show here that phosphorylation of Tyr-654 also stimulates the association of beta-catenin to the basal transcription factor TATA-binding protein. The structural bases of these different affinities were investigated. Our results indicate that the beta-catenin C-terminal tail interacts with the armadillo repeat domain, hindering the association of the armadillo region to the TATA-binding protein or to E-cadherin. Phosphorylation of beta-catenin Tyr-654 decreases armadillo-C-terminal tail association, uncovering the last armadillo repeats. In a C-terminal-depleted beta-catenin, the presence of a negative charge at Tyr-654 does not affect the interaction of the TATA-binding protein to the armadillo domain. However, in the case of E-cadherin, the establishment of ion pairs dominates its association with beta-catenin, and its binding is greatly dependent on the absence of a negative charge at Tyr-654. Thus, phosphorylation of Tyr-654 blocks the Ecadherin-beta-catenin interaction, even though the steric hindrance of the C-tail is no longer present. These results explain how phosphorylation of beta-catenin in Tyr-654 modifies the tertiary structure of this protein and the interaction with its different partners.

The structure of the beta-catenin/E-cadherin complex and the molecular basis of diverse ligand recognition by beta-catenin

As a component of adherens junctions and the Wnt signaling pathway, beta-catenin binds cadherins, Tcf family transcription factors, and the tumor suppressor APC. We have determined the crystal structures of both unphosphorylated and phosphorylated E-cadherin cytoplasmic domain complexed with the arm repeat region of beta-catenin. The interaction spans all 12 arm repeats, and features quasi-independent binding regions that include helices which interact with both ends of the arm repeat domain and an extended stretch of 14 residues which closely resembles a portion of XTcf-3. Phosphorylation of E-cadherin results in interactions with a hydrophobic patch of beta-catenin that mimics the binding of an amphipathic XTcf-3 helix. APC contains sequences homologous to the phosphorylated region of cadherin, and is likely to bind similarly.

Plakoglobin and beta-catenin: protein interactions, regulation and biological roles

Beta-catenin can play different roles in the cell, including one as a structural protein at cell-cell adherens junctions and another as a transcriptional activator mediating Wnt signal transduction. Plakoglobin (gamma)-catenin), a close homolog of beta-catenin, shares with beta-catenin common protein partners and can fulfill some of the same functions. The complexing of catenins with various protein partners is regulated by phosphorylation and by intramolecular interactions. The competition between different catenin partners for binding to catenins mediates the cross-talk between cadherin-based adhesion, catenin-dependent transcription and Wnt signaling. Although plakoglobin differs from beta-catenin in its functions and is unable to compensate for defects in Wnt signaling resulting from lack of beta-catenin, recent evidence suggests that plakoglobin plays a unique role in Wnt signaling that is different from that of beta-catenin. The functional difference between catenins is reflected in their differential involvement in embryonic development and cancer progression.

Differential mechanisms of LEF/TCF family-dependent transcriptional activation by beta-catenin and plakoglobin

beta-Catenin and plakoglobin are highly homologous components of cell-cell adherens junctions linking cadherin receptors to the actin cytoskeleton. beta-Catenin, in addition, activates transcription by forming a complex with LEF/TCF family transcription factors in the nucleus. Plakoglobin can also bind to LEF-1 and, when overexpressed in mammalian cells, enhances LEF-1-directed transcription. Plakoglobin overexpression, however, results in the elevation and nuclear translocation of endogenous beta-catenin. We show here, by DNA mobility shift analysis, that the formation of a plakoglobin-LEF/TCF-DNA complex in vitro is very inefficient compared to a complex containing beta-catenin-LEF-DNA. Moreover, in plakoglobin-transfected cells plakoglobin-LEF/TCF-DNA complexes were not formed; rather, the endogenous beta-catenin, whose level is elevated by plakoglobin transfection, formed a beta-catenin-LEF-DNA complex. Removal of the N- and C-terminal domains of both beta-catenin and plakoglobin (leaving the armadillo repeat domain intact) induced plakoglobin-LEF-DNA complex formation and also enhanced beta-catenin-LEF-DNA complexing, both with in vitro-translated components and in transfected cells. Transfection with these truncated catenins increased endogenous beta-catenin levels, but the truncated catenins acted as dominant-negative inhibitors of beta-catenin-driven transcription by forming transcriptionally inactive complexes with LEF-1. When these catenin mutants were prevented from entering the nucleus, by their fusion to the connexin transmembrane domain, they indirectly activated transcription by increasing endogenous beta-catenin levels. These results suggest that overexpression of plakoglobin does not directly activate transcription and that formation of catenin-LEF-DNA complexes is negatively regulated by the catenin N- and C-terminal domains.