The fourth armadillo repeat of plakoglobin (gamma-catenin) is required for its high affinity binding to the cytoplasmic domains of E-cadherin and desmosomal cadherin Dsg2, and the tumor suppressor APC protein

Proximity Mapping of Desmosomes Reveals a Striking Shift in Their Molecular Neighborhood Associated With Maturation

Desmosomes are multiprotein adhesion complexes that link intermediate filaments to the plasma membrane, ensuring the mechanical integrity of cells across tissues, but how they participate in the wider signaling network to exert their full function is unclear. To investigate this, we carried out protein proximity mapping using biotinylation (BioID). The combined interactomes of the essential desmosomal proteins desmocollin 2a, plakoglobin, and plakophilin 2a (Pkp2a) in Madin-Darby canine kidney epithelial cells were mapped and their differences and commonalities characterized as desmosome matured from Ca2+ dependence to the mature, Ca2+-independent, hyper-adhesive state, which predominates in tissues. Results suggest that individual desmosomal proteins have distinct roles in connecting to cellular signaling pathways and that these roles alter substantially when cells change their adhesion state. The data provide further support for a dualistic concept of desmosomes in which the properties of Pkp2a differ from those of the other, more stable proteins. This body of data provides an invaluable resource for the analysis of desmosome function.

miR-195-5p as Regulator of γ-Catenin and Desmosome Junctions in Colorectal Cancer

Desmosomes play a key role in the regulation of cell adhesion and signaling. Dysregulation of the desmosome complex is associated with the loss of epithelial cell polarity and disorganized tissue architecture typical of colorectal cancer (CRC). The aim of this study was to investigate and characterize the effect of miR-195-5p on desmosomal junction regulation in CRC. In detail, we proposed to investigate the deregulation of miR-195-5p and JUP, a gene target that encodes a desmosome component in CRC patients. JUP closely interacts with desmosomal cadherins, and downstream, it regulates several intracellular transduction factors. We restored the miR-195-5p levels by transient transfection in colonic epithelial cells to examine the effects of miR-195-5p on JUP mRNA and protein expression. The JUP regulation by miR-195-5p, in turn, determined a modulation of desmosome cadherins (Desmoglein 2 and Desmocollin 2). Furthermore, we focused on whether the miR-195-5p gain of function was also able to modulate the expression of key components of Wnt signaling, such as NLK, LEF1 and Cyclin D1. In conclusion, we have identified a novel mechanism controlled by miR-195-5p in the regulation of adhesive junctions, suggesting its potential clinical relevance for future miRNA-based therapy in CRC.

Role of Armadillo repeat 2 and kinesin-II motor subunit Klp64D for wingless signaling in Drosophila

Armadillo (Arm) is crucial for transducing Wingless (Wg) signaling. Previously, we have shown that Klp64D, a motor subunit of Drosophila kinesin-II, interacts with Arm for Wg signaling. Molecular basis for this interaction has remained unknown. Here we identify a critical Arm repeat (AR) required for binding Klp64D and Wg signaling. Arm/\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\varvec{\beta}}$$\end{document}β-catenin family proteins contain a conserved domain of 12 Arm repeats (ARs). Five of these ARs can interact with Klp64D, but only the second AR (AR2) binds to the cargo/tail domain of Klp64D. Overexpression of AR2 in wing imaginal disc is sufficient to cause notched wing margin. This phenotype by AR2 is enhanced or suppressed by reducing or increasing Klp64D expression, respectively. AR2 overexpression inhibits Wg signaling activity in TopFlash assay, consistent with its dominant-negative effects on Klp64D-dependent Wg signaling. Overexpression of the Klp64D cargo domain also results in dominant-negative wing notching. Genetic rescue data indicate that both AR2 and Klp64D cargo regions are required for the function of Arm and Klp64D, respectively. AR2 overexpression leads to an accumulation of Arm with GM130 Golgi marker in Klp64D knockdown. This study suggests that Wg signaling for wing development is regulated by specific interaction between AR2 and the cargo domain of Klp64D.

Mutations in genes encoding desmosomal proteins: spectrum of cutaneous and extracutaneous abnormalities

The desmosome is a type of intercellular junction found in epithelial cells, cardiomyocytes and other specialized cell types. Composed of a network of transmembranous cadherins and intracellular armadillo, plakin and other proteins, desmosomes contribute to cell-cell adhesion, signalling, development and differentiation. Mutations in genes encoding desmosomal proteins result in a spectrum of erosive skin and mucosal phenotypes that also may affect hair or heart. This review summarizes the molecular pathology and phenotypes associated with desmosomal dysfunction with a focus on inherited disorders that involve the skin/hair, as well as associated extracutaneous pathologies. We reviewed the relevant literature to collate studies of pathogenic human mutations in desmosomes that have been reported over the last 25 years. Mutations in 12 different desmosome genes have been documented, with mutations in nine genes affecting the skin/mucous membranes (DSG1, DSG3, DSC2, DSC3, JUP, PKP1, DSP, CDSN, PERP) and eight resulting in hair abnormalities (DSG4, DSC2, DSC3, JUP, PKP1, DSP, CDSN, PERP). Mutations in three genes can result in cardiocutaneous syndromes (DSC2, JUP, DSP), although mutations have been described in five genes in inherited heart disorders that may lack any dermatological manifestations (DSG2, DSC2, JUP, PKP2, DSP). Understanding the diverse nature of these clinical phenotypes, as well as the desmosome gene mutation(s), has clinical value in managing and counselling patients, as well as demonstrating the biological role and activity of specific components of desmosomes in skin and other tissues.

ALEX1 Regulates Proliferation and Apoptosis in Breast Cancer Cells

BACKGROUND

Arm protein lost in epithelial cancers, on chromosome X (ALEX) is a novel subgroup within the armadillo (ARM) family, which has one or two ARM repeat domains as opposed to more than six-thirteen repeats in the classical Armadillo family members.

MATERIALS AND METHODS

In the study, we explore the biological functions of ALEX1 in breast cancer cells. Overexpression of ALEX1 and silencing of ALEX1 were performed with SK-BR3 and MCF-7 cell lines. Cell proliferation and colony formation assays, along with flow cytometry, were carried out to evaluate the roles of ALEX1.

RESULTS

ALEX1 overexpression in SK-BR3 breast cancer cells inhibited proliferation and induced apoptosis. Furthermore, depletion of ALEX1 in MCF-7 breast cancer cells increased proliferation and inhibited apoptosis. Additional analyses demonstrated that the overexpression of ALEX1 activated the intrinsic apoptosis cascades through up-regulating the expression of Bax, cytosol cytochrome c, active caspase-9 and active caspase-3 and down-regulating the levels of Bcl-2 and mitochondria cytochrome c. Simultaneouly, silencing of ALEX1 inhibited intrinsic apoptosis cascades through down-regulating the expression of Bax, cytosol cytochrome c, active caspase-9, and active caspase-3 and up-regulating the level of Bcl-2 and mitochondria cytochrome c.

CONCLUSIONS

Our data suggest that ALEX1 as a crucial tumor suppressor gene has been involved in cell proliferation and apoptosis in breast cancer, which may serve as a novel candidate therapeutic target.

Loss of plakoglobin immunoreactivity in intercalated discs in arrhythmogenic right ventricular cardiomyopathy: protein mislocalization versus epitope masking

AIMS

To examine the relevance and cause of reduced plakoglobin IF in intercalated discs for arrhythmogenic right ventricular cardiomyopathy (ARVC) and ARVC-like disease in mouse and human.

METHODS AND RESULTS

Normalized semi-quantitative IF measurements were performed in a standardized format in desmoglein 2-mutant mice with an ARVC-like phenotype (n = 6) and in cardiac biopsies from humans with ARVC and non-ARVC heart disease (n = 10). Reduced plakoglobin staining was detectable in ARVC only with one antibody directed against a defined epitope but not with three other antibodies reacting with different epitopes of plakoglobin.

CONCLUSIONS

Reduced plakoglobin staining in intercalated discs of heart tissue from human ARVC patients and in a murine ARVC model is caused by alterations in epitope accessibility and not by protein relocalization.

Inherited desmosomal disorders

Desmosomes serve as intercellular junctions in various tissues including the skin and the heart where they play a crucial role in cell-cell adhesion, signalling and differentiation. The desmosomes connect the cell surface to the keratin cytoskeleton and are composed of a transmembranal part consisting mainly of desmosomal cadherins, armadillo proteins and desmoplakin, which form the intracytoplasmic desmosomal plaque. Desmosomal genodermatoses are caused by mutations in genes encoding the various desmosomal components. They are characterized by skin, hair and cardiac manifestations occurring in diverse combinations. Their classification into a separate and distinct clinical group not only recognizes their common pathogenesis and facilitates their diagnosis but might also in the future form the basis for the design of novel and targeted therapies for these occasionally life-threatening diseases.

Phosphatase regulation of intercellular junctions

Intercellular junctions represent the key contact points and sites of communication between neighboring cells. Assembly of these junctions is absolutely essential for the structural integrity of cell monolayers, tissues and organs. Disruption of junctions can have severe consequences such as diarrhea, edema and sepsis, and contribute to the development of chronic inflammatory diseases. Cell junctions are not static structures, but rather they represent highly dynamic micro-domains that respond to signals from the intracellular and extracellular environments to modify their composition and function. This review article will focus on the regulation of tight junctions and adherens junctions by phosphatase enzymes that play an essential role in preserving and modulating the properties of intercellular junction proteins.

Differential expression of cell adhesion molecules in an ionizing radiation-induced breast cancer model system

Cell-cell adhesion is mediated by members of the cadherin-catenin system and among them E-cadherin and β-catenin are important adhesion molecules for epithelial cell function and preservation of tissue integrity. To investigate the importance of cell adhesion molecules in breast carcinogenesis, we developed an in vitro breast cancer model system wherein immortalized human breast epithelial cell line, MCF-10F, was malignantly transformed by exposure to low doses of high linear energy transfer (LET) α particle radiation (150 keV/µm) and subsequent growth in the presence or absence of 17β-estradiol. This model consisted of human breast epithelial cells in different stages of transformation: i) parental cell line MCF-10F; ii) MCF-l0F continuously grown with estradiol at 10(-8) (Estrogen); iii) a non-malignant cell line (Alpha3); and iv) a malignant and tumorigenic cell line (Alpha5) and the Tumor2 cell line derived from the nude mouse xenograft of the Alpha5 cell line. Expression levels of important cell adhesion molecules such as α-catenin, β-catenin, γ-catenin, E-cadherin and integrin were found to be higher at the protein level in the Alpha5 and Tumor2 cell lines relative to these levels in the non-tumorigenic MCF-10F, Estrogen and Alpha3 cell lines. In corroboration, cDNA expression analysis revealed elevated levels of genes involved in the cell adhesion function [E-cadherin, integrin β6 and desmocollin3 (DSc3)] in the Alpha5 and Tumor2 cell lines relative to the levels in the MCF-10F, Estrogen and Alpha3 cell lines. Collectively, our results suggest that cell adhesion molecules are expressed at higher levels in malignantly transformed breast epithelial cells relative to levels in non-malignant cells. However, reduced levels of adhesion molecules observed in the mouse xenograft-derived Tumor 2 cell line compared to the pre-tumorigenic Alpha5 cell line suggests that the altered expression levels of adhesion molecules depend on the tumor tissue microenvironment.

ALEX1 suppresses colony formation ability of human colorectal carcinoma cell lines

Arm protein lost in epithelial cancers, on chromosome X (ALEX; also known as armadillo repeat containing, X-linked [ARMCX]) is a novel subgroup within the armadillo (ARM) family, which has several ARM repeat domains. The biological function of classical ARM family members such as β-catenin is well understood, but that of the ALEX/ARMCX family members is largely unknown. Here we evaluate the effects of ALEX1 overexpression on in vitro colony formation ability and expression of ALEX1 mRNA in human colorectal tumor. Overexpression of ALEX1 suppressed the anchorage-dependent and -independent colony formation of human colorectal carcinoma cell lines by the study of stable clones of HCT116 cells expressing ALEX1 protein. Bisulfite genomic sequencing revealed that the promoter region of ALEX1 gene was highly methylated in both HCT116 and SW480 cells in comparison with PANC-1 and MCF-7 cells, which express endogenous ALEX1 mRNA, indicating the capability of promoter methylation to silence ALEX1 gene in HCT116 and SW480 cells. Our current findings suggest that overexpression of ALEX1 play a negative role in human colorectal tumorigenesis.

Plakoglobin: role in tumorigenesis and metastasis

Plakoglobin (γ-catenin) is a member of the Armadillo family of proteins and a homolog of β-catenin. As a component of both the adherens junctions and desmosomes, plakoglobin plays a pivotal role in the regulation of cell-cell adhesion. Furthermore, similar to β-catenin, plakoglobin is capable of participating in cell signaling. However, unlike β-catenin that has well-documented oncogenic potential through its involvement in the Wnt signaling pathway, plakoglobin generally acts as a tumor/metastasis suppressor. The exact roles that plakoglobin plays during tumorigenesis and metastasis are not clear; however, recent evidence suggests that it may regulate gene expression, cell proliferation, apoptosis, invasion, and migration. In this paper, we describe plakoglobin, its discovery and characterization, its role in regulating cell-cell adhesion, and its signaling capabilities in regulation of tumorigenesis and metastasis.

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.

The cadherin-catenin complex as a focal point of cell adhesion and signalling: new insights from three-dimensional structures

Cadherins are a large family of single-pass transmembrane proteins principally involved in Ca2+-dependent homotypic cell adhesion. The cadherin molecules comprise three domains, the intracellular domain, the transmembrane domain and the extracellular domain, and form large complexes with a vast array of binding partners (including cadherin molecules of the same type in homophilic interactions and cellular protein catenins), orchestrating biologically essential extracellular and intracellular signalling processes. While current, contrasting models for classic cadherin homophilic interaction involve varying numbers of specific repeats found in the extracellular domain, the structure of the domain itself clearly remains the main determinant of cell stability and binding specificity. Through intracellular interactions, cadherin enhances its adhesive properties binding the cytoskeleton via cytoplasmic associated factors alpha- catenin, beta-catenin and p120ctn. Recent structural studies on classic cadherins and these catenin molecules have provided new insight into the essential mechanisms underlying cadherin-mediated cell interaction and catenin-mediated cellular signalling. Remarkable structural diversity has been observed in beta-catenin recognition of other cellular factors including APC, Tcf and ICAT, proteins that contribute to or compete with cadherin/catenin functioning.

Genetic evidence for a novel human desmosomal cadherin, desmoglein 4

Desmosomes are essential adhesion structures in most epithelia that link the intermediate filament network of one cell to its neighbor, thereby forming a strong bond. The molecular components of desmosomes belong to the cadherin superfamily, the plakin family, and the armadillo repeat protein family. The desmosomal cadherins are calcium-dependent transmembrane adhesion molecules and comprise the desmogleins and desmocollins. To date, three human desmoglein isoforms have been characterized, namely desmogleins 1, 2, and 3 that are expressed in a tissue- and differentiation-specific manner. Here we have identified and characterized, at the genetic level, a novel human desmoglein cDNA sharing homology with desmogleins 1, 2, 3 and we name this desmoglein 4. The human desmoglein 4 cDNA (3.6 kb) contains an open reading frame of 3120 bp that encodes a precursor protein of 1040 amino acids. The predicted mature protein comprises 991 amino acids with a molecular weight of 107822 Da at pI 4.38. Human desmoglein 4 shares 41% identity with human desmoglein 1, 37% with human desmoglein 2, and 50% with human desmoglein 3. Analysis of the exon/intron organization of the human desmoglein 4 gene (DSG4) demonstrates that it is composed of 16 exons spanning approximately 37 kb of 18q12 and is situated between DSG1 and DSG3. We have demonstrated using RT-PCR on multiple tissue cDNA samples that desmoglein 4 has very specific tissue expression in salivary gland, testis, prostate, and skin.

ALEX1, a novel human armadillo repeat protein that is expressed differentially in normal tissues and carcinomas

Members of the armadillo (arm) repeat family of proteins are implicated in tumorigenesis, embryonic development, and maintenance of tissue integrity. We have cloned cDNA for a novel human arm repeat protein, ALEX1, encoding 453 amino acids. ALEX1 shares significant homology with uncharacterized ORF KIAA0512 and putative protein product of unknown mRNA (GenBank AF211175), designated here as ALEX2 and ALEX3, respectively. The genes encoding ALEX1, ALEX2 and ALEX3 co-localize to the same region in Xq21.33-q22.2. ALEX1 and ALEX2 transcripts are found in all human tissues examined except peripheral blood leukocytes. Expression of ALEX1 and ALEX2 mRNA is lost or significantly reduced in human lung, prostate, colon, pancreas, and ovarian carcinomas and also in cell lines established from different human carcinomas. These genes are, however, normally expressed in cell lines derived from other types of tumors, e.g., sarcomas, neuroblastomas, and gliomas. We speculate that ALEX genes may play a role in suppression of tumors originating from epithelial tissue, i.e., carcinomas.

N-cadherin expressed on malignant T cell lymphoma cells is functional, and promotes heterotypic adhesion between the lymphoma cells and mesenchymal cells expressing N-cadherin

Cadherins are Ca2+-dependent cell-cell adhesion molecules, and are involved in the formation and maintenance of the organocellular architecture. Using a combination of molecular biologic and biochemical methods, we analyzed cadherins expressed on cultured human malignant lymphoma cell lines (adult T cell lymphomas, human T cell leukemia virus type 1-negative T cell lines, and thymus-derived lymphoma cell lines), and obtained evidence that N-cadherin is the major cadherin expressed on these cells. These cells were found to form cell aggregates in a Ca2+-dependent manner, and more importantly to coaggregate and adhere with cells expressing N-cadherin, suggesting that N-cadherin on lymphoma cells is functionally active. Therefore, N-cadherin expressed on lymphoma cells could underlie the frequent invasion of these cells into the mesenchymal tissue in the skin and the central nervous system.

Dysregulation of the E-cadherin/catenin complex by irreversible mutations in human carcinomas

The different proteins of the E-cadherin/catenin cell-cell adhesion complex are believed to play a predominant role in carcinogenesis. Aberrant expression of these proteins has been found in many different human carcinomas, indicating abnormal regulation. In general, inactivating mutations of the human E-cadherin gene are rare; they are, however, highly frequent in infiltrating lobular breast carcinomas and in diffuse gastric carcinomas. These mutations mostly occur in combination with loss of heterozygosity (LOH) of the wild-type allele. Mutations were found at very early non-invasive stages, thus associating E-cadherin mutations with loss of growth control and defining E-cadherin as a real tumour suppressor for these particular tumour types. Defects affecting both alleles of the alpha E-catenin gene have been found in different human carcinoma cell lines, resulting in the loss of E-cadherin-mediated cell-cell adhesion. Mutations of the beta-catenin gene in colon tumours and melanomas were found to result in an accumulation of the protein in the cytosol. Upon translocation to the nucleus, this beta-catenin enhances TCF/LEF-dependent transcriptional activity. This suggests that mutated beta-catenin can act as an oncogene in these particular tumour types. The multiple interaction partners of beta-catenin are known to be involved in signal transduction, actin organization, protein phosphorylation or transcriptional regulation. This makes this protein an intriguing alternative target for either activation or inactivation in human cancer types characterized by frequent E-cadherin or APC deficiencies.

Identification of the region of alpha-catenin that plays an essential role in cadherin-mediated cell adhesion

alpha-Catenin is an intrinsic component of the cadherin adhesion complex and is a 102-kDa protein with multiple interaction sites, including homodimerization sites, and binding sites for beta- and gamma-catenin (plakoglobin), alpha-actinin, and actin. Besides the binding to beta- or gamma-catenin, it is unknown, however, which interaction is critical for the function of cadherins. By expressing a series of E-cadherin-alpha-catenin chimeric molecules on leukemia cells (K562), we have identified the region of alpha-catenin that confers aggregation inducing activity to nonfunctional tail-less E-cadherin. The region has been mapped to the carboxyl-terminal 295 amino acids of alpha-catenin. Consistent with this result, expression in alpha-catenin-deficient cells (DLD-1/Delta alpha) of a mutant alpha-catenin molecule consisting of the amino-terminal beta-/gamma-catenin-binding site and the carboxyl-terminal cell adhesion region identified in the above experiments induced E-cadherin-mediated cell aggregation and compaction. Cells expressing E-cadherin chimeric molecules with the homologous carboxyl-terminal region of vinculin, which contains the actin-binding site of vinculin, did not, however, aggregate as strongly as ones expressing E-cadherin-alpha-catenin chimeric molecules.

The armadillo family of structural proteins

The armadillo gene is a segment polarity gene of Drosophila involved in signal transduction through wingless. Since the mid-1980s, a growing number of related proteins have been identified based on sequence homologies. These proteins share a central domain that is composed of a series of imperfect 45 amino acid repeats. Armadillo family members reveal diverse cellular locations reflecting their diverse functions. A single protein exerts several functions through interactions of its armadillo repeat domain with diverse binding partners. The proteins combine structural roles as cell-contact and cytoskeleton-associated proteins and signaling functions by generating and transducing signals affecting gene expression. The study of armadillo family members has made it increasingly clear that a distinction between structural proteins on the one hand and signaling molecules on the other is rather artificial. Instead armadillo family members exert both functions by interacting with a number of distinct cellular-binding partners.

Altered cell adhesion activity by pervanadate due to the dissociation of alpha-catenin from the E-cadherin.catenin complex

Leukemia cells (K562) that grow as non-adhesive single cells and have no endogenous cadherin were transfected with an E-cadherin expression vector, and cell clones stably expressing E-cadherin on their surface were established. The expression of E-cadherin induced the up-regulation of catenins, and E-cadherin became associated with catenins. The transfected cells grew as floating aggregates. Cell aggregation was Ca2+-dependent and was inhibited by E-cadherin antibodies. The aggregates dissociated into single cells on the addition of pervanadate. Pervanadate caused a dramatic augmentation of the phosphorylation of E-cadherin, beta-catenin, and gamma-catenin (plakoglobin), but alpha-catenin was not detectably phosphorylated. After pervanadate treatment, beta-catenin and gamma-catenin migrated more slowly on gel electrophoresis, suggesting changes in their conformations due to eventual changes in their phosphorylation levels. In the treated cells, a significant amount of alpha-catenin was dissociated from the E-cadherin.catenin complex. Aggregates of cells expressing an E-cadherin chimeric molecule covalently linked with alpha-catenin were not dissociated on pervanadate treatment, supporting the idea that the dissociation of alpha-catenin from the complex underlies the observed E-cadherin dysfunction.

Identification of the domain of alpha-catenin involved in its association with beta-catenin and plakoglobin (gamma-catenin)

alpha-Catenin is a 102-kDa protein exhibiting homology to vincuin, and it forms complexes with cadherins or the tumor-suppressor gene product adenomatous polyposis coli through binding to beta-catenin or plakoglobin (gamma-catenin). The incorporation of alpha-catenin into the cadherin-catenin complexes is a prerequisite for expression of the cell-adhesive activity of cadherins. Using an in vitro assay system involving bacterially expressed proteins, we localized a region in alpha-catenin required for molecular interaction with beta-catenin and plakoglobin. Analysis of various truncated alpha-catenin molecules revealed that amino-terminal residues 48-163 are able to bind to beta-catenin and plakoglobin. Consistent with the observation that beta-catenin and plakoglobin bind to the same region of alpha-catenin, beta-catenin competed with the binding of plakoglobin to alpha-catenin and vice versa. Under the conditions used, beta-catenin bound to alpha-catenin with higher affinity than did plakoglobin. Scatchard analysis indicated that the affinity of the interaction between alpha-catenin and beta-catenin or that between alpha-catenin and plakoglobin was moderately strong (Kd = 3. 8 x 10(-8) and 7.7 x 10(-8), respectively). When transfected into L cells expressing E-cadherin, the amino-terminal region of alpha-catenin (from residue 1 to 226) formed complexes with beta-catenin supporting the in vitro binding experiment results.

Characterization of the interactions of alpha-catenin with alpha-actinin and beta-catenin/plakoglobin

Cadherins are calcium-dependent, cell surface glycoproteins involved in cell-cell adhesion. To function in cell-cell adhesion, the transmembrane cadherin molecule must be associated with the cytoskeleton via cytoplasmic proteins known as catenins. Three catenins, alpha-catenin, beta-catenin and gamma-catenin (also known as plakoglobin), have been identified. beta-catenin or plakoglobin is associated directly with the cadherin; alpha-catenin binds to beta-catenin/plakoglobin and serves to link the cadherin/catenin complex to the actin cytoskeleton. The domains on the cadherin and betacatenin/plakoglobin that are responsible for protein-protein interactions have been mapped. However, little is known about the molecular interactions between alpha-catenin and beta-catenin/plakoglobin or about the interactions between alpha-catenin and the cytoskeleton. In this study we have used the yeast two-hybrid system to map the domains on alpha-catenin that allow it to associate with beta-catenin/plakoglobin and with alpha-actinin. We also identify a region on alpha-actinin that is responsible for its interaction with alpha-catenin. The yeast two-hybrid data were confirmed with biochemical studies.

Heparin binding protein-44 (HBP-44)/receptor-associated protein (RAP)mediates cell-substratum adhesion of mouse NIH/3T3 cells through its binding to low density lipoprotein (LDL) receptor-related protein (LRP)

The low density lipoprotein receptor-related protein (LRP) is a multifunctional endocytic receptor with the ability to bind and endocytose several structurally and functionally distinct ligands. The 39 kDa receptor-associated protein (RAP) is an endoplasmic reticulum (ER) resident protein, which is believed to function intracellularly as a molecular chaperone for LRP and to regulate its ligand binding activity along the secretory pathway. Mouse heparin binding protein-44 (HBP-44) is a homologue of human RAP. Using a recombinant form of HBP-44 expressed in Escherichia coli cells as a highly specific ligand for LRP, we demonstrated that HBP-44 coated on cell culture plates mediates the cell-substratum adhesion of mouse 3T3 fibroblasts in a dose-dependent manner, with 50% attachment at the concentration of 0.2 micrograms/ml. Ligand blot analysis with HBP-44 of whole cell extracts and the materials precipitated by anti-LRP antibodies revealed that the receptor for HBP-44 on NIH/3T3 cells was LRP. The results suggest that LRP serves as a cell adhesion receptor in some cells.

Drosophila alpha-catenin and E-cadherin bind to distinct regions of Drosophila Armadillo

Adherens junctions are multiprotein complexes mediating cell-cell adhesion and communication. They are organized around a transmembrane cadherin, which binds a set of cytoplasmic proteins required for adhesion and to link the complex to the actin cytoskeleton. Three components of Drosophila adherens junctions, analogous to those in vertebrates, have been identified: Armadillo (homolog of beta-catenin), Drosophila E-cadherin (DE-cadherin), and alpha-catenin. We carried out the first analysis of the interactions between these proteins using in vitro binding assays, the yeast two-hybrid system, and in vivo assays. We identified a 76-amino acid region of Armadillo that is necessary and sufficient for binding alpha-catenin and found that the N-terminal 258 amino acids of alpha-catenin interact with Armadillo. A large region of Armadillo, spanning six central Armadillo repeats, is required for DE-cadherin binding, whereas only 41 amino acids of the DE-cadherin cytoplasmic tail are sufficient for Armadillo binding. Our data complement and extend results obtained in studies of vertebrate adherens junctions, providing a foundation for understanding how junctional proteins assemble and a basis for interpreting existing mutations and creating new ones.

Plakophilins 2a and 2b: constitutive proteins of dual location in the karyoplasm and the desmosomal plaque

Using antibodies and recombinant DNA techniques, we have identified plakophilin 2, a novel desmosomal plaque protein of M(r) 100,000 (estimated from SDS-PAGE), which is a member of the arm-repeat family of proteins and can occur in two splice forms (2a and 2b) because of the insertion of a 44 amino acid (aa)-encoding exon. In its aa sequence (837 and 881 aa, calculated pIs: 9.33 and 9.38, mol wts 92,750 and 97,410 kD), it is conspicuously related to the 80-kD plakophilin 1, with which it shares a central region of 9 repeats of the arm-motif, preceeded by a long head region and followed by a very short (11 aa) carboxy-terminal sequence. Plakophilin 2 and its mRNA have been detected in a wide range of tissues and cell types, including cells devoid of desmosomes. By light and electron microscopical immunolocalization, plakophilin 2 has been localized to plaques of desmosomes of one-layered ("simple") and complex epithelia, carcinomas, diverse epithelium-derived cell culture lines, as well as cardiac tissue and the dendritic reticulum cells of lymphatic germinal centers, i.e., desmosomes in which plakophilin 1 is not detected. However, plakophilin 2 has also been localized in the desmosomes of certain but not all stratified epithelia where it coexists with plakophilin 1. Remarkably, plakophilin 2 is also enriched in the karyoplasm of a wide range of cell types, including many that lack desmosomes and in which, therefore, the nuclear state is the only locally enriched form of plakophilin 2 present. We conclude that plakophilins 2a and 2b are basic nuclear proteins that in certain cell types additionally assemble with other proteins to form the desmosomal plaque and serve general nuclear functions as well as a function specific to many but not all desmosomes.

Cadherins and catenins in development

Cadherins and catenins represent key molecules during development. Recent findings demonstrate the involvement of cadherins and catenins in signaling pathways. In a working hypothesis, signaling via beta-catenin regulates the epithelial-mesenchymal transition in vertebrate development.

Targeted mutation of plakoglobin in mice reveals essential functions of desmosomes in the embryonic heart

Plakoglobin (gamma-catenin), a member of the armadillo family of proteins, is a constituent of the cytoplasmic plaque of desmosomes as well as of other adhering cell junctions, and is involved in anchorage of cytoskeletal filaments to specific cadherins. We have generated a null mutation of the plakoglobin gene in mice. Homozygous -/- mutant animals die between days 12-16 of embryogenesis due to defects in heart function. Often, heart ventricles burst and blood floods the pericard. This tissue instability correlates with the absence of desmosomes in heart, but not in epithelia organs. Instead, extended adherens junctions are formed in the heart, which contain desmosomal proteins, i.e., desmoplakin. Thus, plakoglobin is an essential component of myocardiac desmosomes and seems to play a crucial role in the sorting out of desmosomal and adherens junction components, and consequently in the architecture of intercalated discs and the stabilization of heart tissue.

Secondary structure of an armadillo single repeat from the APC protein

The armadillo domain is a repeating sequence motif of a variety of proteins with different functions. Here we describe the structure of a synthetic single armadillo repeat solved by two-dimensional nuclear magnetic resonance spectroscopy. Our results indicate alpha-helical secondary structural elements in half of the residues.