Danio rerio αE-catenin is a monomeric F-actin binding protein with distinct properties from Mus musculus αE-catenin

Metabolic reprogramming in response to cell mechanics

Much attention has been dedicated to understanding how cells sense and respond to mechanical forces. The types of forces cells experience as well as the repertoire of cell surface receptors that sense these forces have been identified. Key mechanisms for transmitting that force to the cell interior have also emerged. Yet, how cells process mechanical information and integrate it with other cellular events remains largely unexplored. Here we review the mechanisms underlying mechanotransduction at cell-cell and cell-matrix adhesions, and we summarize the current understanding of how cells integrate information from the distinct adhesion complexes with cell metabolism.

Multi-level Force-dependent Allosteric Enhancement of αE-catenin Binding to F-actin by Vinculin

Classical cadherins are transmembrane proteins whose extracellular domains link neighboring cells, and whose intracellular domains connect to the actin cytoskeleton via β-catenin and α-catenin. The cadherin-catenin complex transmits forces that drive tissue morphogenesis and wound healing. In addition, tension-dependent changes in αE-catenin conformation enables it to recruit the actin-binding protein vinculin to cell-cell junctions, which contributes to junctional strengthening. How and whether multiple cadherin-complexes cooperate to reinforce cell-cell junctions in response to load remains poorly understood. Here, we used single-molecule optical trap measurements to examine how multiple cadherin-catenin complexes interact with F-actin under load, and how this interaction is influenced by the presence of vinculin. We show that force oriented toward the (-) end of the actin filament results in mean lifetimes 3-fold longer than when force was applied towards the barbed (+) end. We also measured force-dependent actin binding by a quaternary complex comprising the cadherin-catenin complex and the vinculin head region, which cannot itself bind actin. Binding lifetimes of this quaternary complex increased as additional complexes bound F-actin, but only when load was oriented toward the (-) end. In contrast, the cadherin-catenin complex alone did not show this form of cooperativity. These findings reveal multi-level, force-dependent regulation that enhances the strength of the association of multiple cadherin/catenin complexes with F-actin, conferring positive feedback that may strengthen the junction and polarize F-actin to facilitate the emergence of higher-order cytoskeletal organization.

The nematode α-catenin ortholog, HMP1, has an extended α-helix when bound to actin filaments

The regulation of cell-cell junctions during epidermal morphogenesis ensures tissue integrity, a process regulated by α-catenin. This cytoskeletal protein connects the cadherin complex to filamentous actin at cell-cell junctions. The cadherin-catenin complex plays key roles in cell physiology, organism development, and disease. While mutagenesis of Caenorhabditis elegans cadherin and catenin shows that these proteins are key for embryonic morphogenesis, we know surprisingly little about their structure and attachment to the cytoskeleton. In contrast to mammalian α-catenin that functions as a dimer or monomer, the α-catenin ortholog from C. elegans, HMP1 for humpback, is a monomer. Our cryogenic electron microscopy (cryoEM) structure of HMP1/α-catenin reveals that the amino- and carboxy-terminal domains of HMP1/α-catenin are disordered and not in contact with the remaining HMP1/α-catenin middle domain. Since the carboxy-terminal HMP1/α-catenin domain is the F-actin-binding domain (FABD), this interdomain constellation suggests that HMP1/α-catenin is constitutively active, which we confirm biochemically. Our perhaps most surprising finding, given the high sequence similarity between the mammalian and nematode proteins, is our cryoEM structure of HMP1/α-catenin bound to F-actin. Unlike the structure of mammalian α-catenin bound to F-actin, binding to F-actin seems to allosterically convert a loop region of the HMP1/α-catenin FABD to extend an HMP1/α-catenin FABD α-helix. We use cryoEM and bundling assays to show for the first time how the FABD of HMP1/α-catenin bundles actin in the absence of force. Collectively, our data advance our understanding of α-catenin regulation of cell-cell contacts and additionally aid our understanding of the evolution of multicellularity in metazoans.

Mechanism of the cadherin-catenin F-actin catch bond interaction

Mechanotransduction at cell-cell adhesions is crucial for the structural integrity, organization, and morphogenesis of epithelia. At cell-cell junctions, ternary E-cadherin/β-catenin/αE-catenin complexes sense and transmit mechanical load by binding to F-actin. The interaction with F-actin, described as a two-state catch bond, is weak in solution but is strengthened by applied force due to force-dependent transitions between weak and strong actin-binding states. Here, we provide direct evidence from optical trapping experiments that the catch bond property principally resides in the αE-catenin actin-binding domain (ABD). Consistent with our previously proposed model, the deletion of the first helix of the five-helix ABD bundle enables stable interactions with F-actin under minimal load that are well described by a single-state slip bond, even when αE-catenin is complexed with β-catenin and E-cadherin. Our data argue for a conserved catch bond mechanism for adhesion proteins with structurally similar ABDs. We also demonstrate that a stably bound ABD strengthens load-dependent binding interactions between a neighboring complex and F-actin, but the presence of the other αE-catenin domains weakens this effect. These results provide mechanistic insight to the cooperative binding of the cadherin-catenin complex to F-actin, which regulate dynamic cytoskeletal linkages in epithelial tissues.

α-catenin switches between a slip and an asymmetric catch bond with F-actin to cooperatively regulate cell junction fluidity

α-catenin is a crucial protein at cell junctions that provides connection between the actin cytoskeleton and the cell membrane. At adherens junctions (AJs), α-catenin forms heterodimers with β-catenin that are believed to resist force on F-actin. Outside AJs, α-catenin forms homodimers that regulates F-actin organization and directly connect the cell membrane to the actin cytoskeleton, but their mechanosensitive properties are inherently unknown. By using ultra-fast laser tweezers we found that a single α-β-catenin heterodimer does not resist force but instead slips along F-actin in the direction of force. Conversely, the action of 5 to 10 α-β-catenin heterodimers together with force applied toward F-actin pointed end engaged a molecular switch in α-catenin, which unfolded and strongly bound F-actin as a cooperative catch bond. Similarly, an α-catenin homodimer formed an asymmetric catch bond with F-actin triggered by protein unfolding under force. Our data suggest that α-catenin clustering together with intracellular tension engage a fluid-to-solid phase transition at the membrane-cytoskeleton interface.

Cortical tension initiates the positive feedback loop between cadherin and F-actin

Adherens junctions physically link two cells at their contact interface via extracellular binding between cadherin molecules and intracellular interactions between cadherins and the actin cytoskeleton. Cadherin and actomyosin cytoskeletal dynamics are regulated reciprocally by mechanical and chemical signals, which subsequently determine the strength of cell-cell adhesions and the emergent organization and stiffness of the tissues they form. However, an understanding of the integrated system is lacking. We present a new mechanistic computational model of intercellular junction maturation in a cell doublet to investigate the mechanochemical cross talk that regulates adherens junction formation and homeostasis. The model couples a two-dimensional lattice-based simulation of cadherin dynamics with a reaction-diffusion representation of the reorganising actomyosin network through its regulation by Rho signalling at the intracellular junction. We demonstrate that local immobilization of cadherin induces cluster formation in a cis-less-dependent manner. We then recapitulate the process of cell-cell contact formation. Our model suggests that cortical tension applied on the contact rim can explain the ring distribution of cadherin and actin filaments (F-actin) on the cell-cell contact of the cell doublet. Furthermore, we propose and test the hypothesis that cadherin and F-actin interact like a positive feedback loop, which is necessary for formation of the ring structure. Different patterns of cadherin distribution were observed as an emergent property of disturbances of this positive feedback loop. We discuss these findings in light of available experimental observations on underlying mechanisms related to cadherin/F-actin binding and the mechanical environment.

Binding partner- and force-promoted changes in αE-catenin conformation probed by native cysteine labeling

Adherens Junctions (AJs) are cell-cell adhesion complexes that sense and propagate mechanical forces by coupling cadherins to the actin cytoskeleton via β-catenin and the F-actin binding protein αE-catenin. When subjected to mechanical force, the cadherin•catenin complex can tightly link to F-actin through αE-catenin, and also recruits the F-actin-binding protein vinculin. In this study, labeling of native cysteines combined with mass spectrometry revealed conformational changes in αE-catenin upon binding to the E-cadherin•β-catenin complex, vinculin and F-actin. A method to apply physiologically meaningful forces in solution revealed force-induced conformational changes in αE-catenin when bound to F-actin. Comparisons of wild-type αE-catenin and a mutant with enhanced vinculin affinity using cysteine labeling and isothermal titration calorimetry provide evidence for allosteric coupling of the N-terminal β-catenin-binding and the middle (M) vinculin-binding domain of αE-catenin. Cysteine labeling also revealed possible crosstalk between the actin-binding domain and the rest of the protein. The data provide insight into how binding partners and mechanical stress can regulate the conformation of full-length αE-catenin, and identify the M domain as a key transmitter of conformational changes.

Non-canonical Wnt signaling regulates junctional mechanocoupling during angiogenic collective cell migration

Morphogenesis of hierarchical vascular networks depends on the integration of multiple biomechanical signals by endothelial cells, the cells lining the interior of blood vessels. Expansion of vascular networks arises through sprouting angiogenesis, a process involving extensive cell rearrangements and collective cell migration. Yet, the mechanisms controlling angiogenic collective behavior remain poorly understood. Here, we show this collective cell behavior is regulated by non-canonical Wnt signaling. We identify that Wnt5a specifically activates Cdc42 at cell junctions downstream of ROR2 to reinforce coupling between adherens junctions and the actin cytoskeleton. We show that Wnt5a signaling stabilizes vinculin binding to alpha-catenin, and abrogation of vinculin in vivo and in vitro leads to uncoordinated polarity and deficient sprouting angiogenesis in Mus musculus. Our findings highlight how non-canonical Wnt signaling coordinates collective cell behavior during vascular morphogenesis by fine-tuning junctional mechanocoupling between endothelial cells.

Analysis of a vinculin homolog in a sponge (phylum Porifera) reveals that vertebrate-like cell adhesions emerged early in animal evolution

The evolution of cell-adhesion mechanisms in animals facilitated the assembly of organized multicellular tissues. Studies in traditional animal models have revealed two predominant adhesion structures, the adherens junction (AJ) and focal adhesions (FAs), which are involved in the attachment of neighboring cells to each other and to the secreted extracellular matrix (ECM), respectively. The AJ (containing cadherins and catenins) and FAs (comprising integrins, talin, and paxillin) differ in protein composition, but both junctions contain the actin-binding protein vinculin. The near ubiquity of these structures in animals suggests that AJ and FAs evolved early, possibly coincident with multicellularity. However, a challenge to this perspective is that previous studies of sponges-a divergent animal lineage-indicate that their tissues are organized primarily by an alternative, sponge-specific cell-adhesion mechanism called "aggregation factor." In this study, we examined the structure, biochemical properties, and tissue localization of a vinculin ortholog in the sponge Oscarella pearsei (Op). Our results indicate that Op vinculin localizes to both cell-cell and cell-ECM contacts and has biochemical and structural properties similar to those of vertebrate vinculin. We propose that Op vinculin played a role in cell adhesion and tissue organization in the last common ancestor of sponges and other animals. These findings provide compelling evidence that sponge tissues are indeed organized like epithelia in other animals and support the notion that AJ- and FA-like structures extend to the earliest periods of animal evolution.

α-Catenin Structure and Nanoscale Dynamics in Solution and in Complex with F-Actin

As a core component of the adherens junction, α-catenin stabilizes the cadherin/catenin complexes to the actin cytoskeleton for the mechanical coupling of cell-cell adhesion. α-catenin also modulates actin dynamics, cell polarity, and cell-migration functions that are independent of the adherens junction. We have determined the solution structures of the α-catenin monomer and dimer using in-line size-exclusion chromatography small-angle X-ray scattering, as well as the structure of α-catenin dimer in complex to F-actin filament using selective deuteration and contrast-matching small angle neutron scattering. We further present the first observation, to our knowledge, of the nanoscale dynamics of α-catenin by neutron spin-echo spectroscopy, which explicitly reveals the mobile regions of α-catenin that are crucial for binding to F-actin. In solution, the α-catenin monomer is more expanded than either protomer shown in the crystal structure dimer, with the vinculin-binding M fragment and the actin-binding domain being able to adopt different configurations. The α-catenin dimer in solution is also significantly more expanded than the dimer crystal structure, with fewer interdomain and intersubunit contacts than the crystal structure. When in complex to F-actin, the α-catenin dimer has an even more open and extended conformation than in solution, with the actin-binding domain further separated from the main body of the dimer. The α-catenin-assembled F-actin bundle develops into an ordered filament packing arrangement at increasing α-catenin/F-actin molar ratios. Together, the structural and dynamic studies reveal that α-catenin possesses dynamic molecular conformations that prime this protein to function as a mechanosensor protein.

α-Catenin homodimers are recruited to phosphoinositide-activated membranes to promote adhesion

A unique feature of α-catenin localized outside the cadherin-catenin complex is its capacity to form homodimers, but the subcellular localization and functions of this form of α-catenin remain incompletely understood. We identified a cadherin-free form of α-catenin that is recruited to the leading edge of migrating cells in a phosphatidylinositol 3-kinase-dependent manner. Surface plasmon resonance analysis shows that α-catenin homodimers, but not monomers, selectively bind phosphatidylinositol-3,4,5-trisphosphate-containing lipid vesicles with high affinity, where three basic residues, K488, K493, and R496, contribute to binding. Chemical-induced dimerization of α-catenin containing a synthetic dimerization domain promotes its accumulation within lamellipodia and elaboration of protrusions with extended filopodia, which are attenuated in the α-cateninKKR<3A mutant. Cells restored with a full-length, natively homodimerizing form of α-cateninKKR<3A display reduced membrane recruitment, altered epithelial sheet migrations, and weaker cell-cell adhesion compared with WT α-catenin. These findings show that α-catenin homodimers are recruited to phosphoinositide-activated membranes to promote adhesion and migration, suggesting that phosphoinositide binding may be a defining feature of α-catenin function outside the cadherin-catenin complex.

Cell-cell adhesion in metazoans relies on evolutionarily conserved features of the α-catenin·β-catenin-binding interface

Stable tissue integrity during embryonic development relies on the function of the cadherin·catenin complex (CCC). The Caenorhabditis elegans CCC is a useful paradigm for analyzing in vivo requirements for specific interactions among the core components of the CCC, and it provides a unique opportunity to examine evolutionarily conserved mechanisms that govern the interaction between α- and β-catenin. HMP-1, unlike its mammalian homolog α-catenin, is constitutively monomeric, and its binding affinity for HMP-2/β-catenin is higher than that of α-catenin for β-catenin. A crystal structure shows that the HMP-1·HMP-2 complex forms a five-helical bundle structure distinct from the structure of the mammalian α-catenin·β-catenin complex. Deletion analysis based on the crystal structure shows that the first helix of HMP-1 is necessary for binding HMP-2 avidly in vitro and for efficient recruitment of HMP-1 to adherens junctions in embryos. HMP-2 Ser-47 and Tyr-69 flank its binding interface with HMP-1, and we show that phosphomimetic mutations at these two sites decrease binding affinity of HMP-1 to HMP-2 by 40-100-fold in vitro. In vivo experiments using HMP-2 S47E and Y69E mutants showed that they are unable to rescue hmp-2(zu364) mutants, suggesting that phosphorylation of HMP-2 on Ser-47 and Tyr-69 could be important for regulating CCC formation in C. elegans Our data provide novel insights into how cadherin-dependent cell-cell adhesion is modulated in metazoans by conserved elements as well as features unique to specific organisms.

Zygotic vinculin is not essential for embryonic development in zebrafish

The formation of multicellular tissues during development is governed by mechanical forces that drive cell shape and tissue architecture. Protein complexes at sites of adhesion to the extracellular matrix (ECM) and cell neighbors, not only transmit these mechanical forces, but also allow cells to respond to changes in force by inducing biochemical feedback pathways. Such force-induced signaling processes are termed mechanotransduction. Vinculin is a central protein in mechanotransduction that in both integrin-mediated cell-ECM and cadherin-mediated cell-cell adhesions mediates force-induced cytoskeletal remodeling and adhesion strengthening. Vinculin was found to be important for the integrity and remodeling of epithelial tissues in cell culture models and could therefore be expected to be of broad importance in epithelial morphogenesis in vivo. Besides a function in mouse heart development, however, the importance of vinculin in morphogenesis of other vertebrate tissues has remained unclear. To investigate this further, we knocked out vinculin functioning in zebrafish, which contain two fully functional isoforms designated as vinculin A and vinculin B that both show high sequence conservation with higher vertebrates. Using TALEN and CRISPR-Cas gene editing technology we generated vinculin-deficient zebrafish. While single vinculin A mutants are viable and able to reproduce, additional loss of zygotic vinculin B was lethal after embryonic stages. Remarkably, vinculin-deficient embryos do not show major developmental defects, apart from mild pericardial edemas. These results lead to the conclusion that vinculin is not of broad importance for the development and morphogenesis of zebrafish tissues.

Measuring Protein Binding to F-actin by Co-sedimentation

Filamentous actin (F-actin) organization within cells is regulated by a large number of actin-binding proteins that control actin nucleation, growth, cross-linking and/or disassembly. This protocol describes a technique – the actin co-sedimentation, or pelleting, assay – to determine whether a protein or protein domain binds F-actin and to measure the affinity of the interaction (i.e., the dissociation equilibrium constant). In this technique, a protein of interest is first incubated with F-actin in solution. Then, differential centrifugation is used to sediment the actin filaments, and the pelleted material is analyzed by SDS-PAGE. If the protein of interest binds F-actin, it will co-sediment with the actin filaments. The products of the binding reaction (i.e., F-actin and the protein of interest) can be quantified to determine the affinity of the interaction. The actin pelleting assay is a straightforward technique for determining if a protein of interest binds F-actin and for assessing how changes to that protein, such as ligand binding, affect its interaction with F-actin.

Structural and functional characterization of Caenorhabditis elegans α-catenin reveals constitutive binding to β-catenin and F-actin

Intercellular epithelial junctions formed by classical cadherins, β-catenin, and the actin-binding protein α-catenin link the actin cytoskeletons of adjacent cells into a structural continuum. These assemblies transmit forces through the tissue and respond to intracellular and extracellular signals. However, the mechanisms of junctional assembly and regulation are poorly understood. Studies of cadherin-catenin assembly in a number of metazoans have revealed both similarities and unexpected differences in the biochemical properties of the cadherin·catenin complex that likely reflect the developmental and environmental requirements of different tissues and organisms. Here, we report the structural and biochemical characterization of HMP-1, the Caenorhabditis elegans α-catenin homolog, and compare it with mammalian α-catenin. HMP-1 shares overall similarity in structure and actin-binding properties, but displayed differences in conformational flexibility and allosteric regulation from mammalian α-catenin. HMP-1 bound filamentous actin with an affinity in the single micromolar range, even when complexed with the β-catenin homolog HMP-2 or when present in a complex of HMP-2 and the cadherin homolog HMR-1, indicating that HMP-1 binding to F-actin is not allosterically regulated by the HMP-2·HMR-1 complex. The middle (i.e. M) domain of HMP-1 appeared to be less conformationally flexible than mammalian α-catenin, which may underlie the dampened effect of HMP-2 binding on HMP-1 actin-binding activity compared with that of the mammalian homolog. In conclusion, our data indicate that HMP-1 constitutively binds β-catenin and F-actin, and although the overall structure and function of HMP-1 and related α-catenins are similar, the vertebrate proteins appear to be under more complex conformational regulation.

Characterization of the Cadherin-Catenin Complex of the Sea Anemone Nematostella vectensis and Implications for the Evolution of Metazoan Cell-Cell Adhesion

The cadherin-catenin complex (CCC) mediates cell-cell adhesion in bilaterian animals by linking extracellular cadherin-based adhesions to the actin cytoskeleton. However, it is unknown whether the basic organization of the complex is conserved across all metazoans. We tested whether protein interactions and actin-binding properties of the CCC are conserved in a nonbilaterian animal, the sea anemone Nematostella vectensis We demonstrated that N. vectensis has a complete repertoire of cadherin-catenin proteins, including two classical cadherins, one α-catenin, and one β-catenin. Using size-exclusion chromatography and multi-angle light scattering, we showed that α-catenin and β-catenin formed a heterodimer that bound N. vectensis Cadherin-1 and -2. Nematostella vectensis α-catenin bound F-actin with equivalent affinity as either a monomer or an α/β-catenin heterodimer, and its affinity for F-actin was, in part, regulated by a novel insert between the N- and C-terminal domains. Nematostella vectensis α-catenin inhibited Arp2/3 complex-mediated nucleation of actin filaments, a regulatory property previously thought to be unique to mammalian αE-catenin. Thus, despite significant differences in sequence, the key interactions of the CCC are conserved between bilaterians and cnidarians, indicating that the core function of the CCC as a link between cell adhesions and the actin cytoskeleton is ancestral in the eumetazoans.

αT-Catenin Is a Constitutive Actin-binding α-Catenin That Directly Couples the Cadherin·Catenin Complex to Actin Filaments

α-Catenin is the primary link between the cadherin·catenin complex and the actin cytoskeleton. Mammalian αE-catenin is allosterically regulated: the monomer binds the β-catenin·cadherin complex, whereas the homodimer does not bind β-catenin but interacts with F-actin. As part of the cadherin·catenin complex, αE-catenin requires force to bind F-actin strongly. It is not known whether these properties are conserved across the mammalian α-catenin family. Here we show that αT (testes)-catenin, a protein unique to amniotes that is expressed predominantly in the heart, is a constitutive actin-binding α-catenin. We demonstrate that αT-catenin is primarily a monomer in solution and that αT-catenin monomer binds F-actin in cosedimentation assays as strongly as αE-catenin homodimer. The β-catenin·αT-catenin heterocomplex also binds F-actin with high affinity unlike the β-catenin·αE-catenin complex, indicating that αT-catenin can directly link the cadherin·catenin complex to the actin cytoskeleton. Finally, we show that a mutation in αT-catenin linked to arrhythmogenic right ventricular cardiomyopathy, V94D, promotes homodimerization, blocks β-catenin binding, and in cardiomyocytes disrupts localization at cell-cell contacts. Together, our data demonstrate that αT-catenin is a constitutively active actin-binding protein that can physically couple the cadherin·catenin complex to F-actin in the absence of tension. We speculate that these properties are optimized to meet the demands of cardiomyocyte adhesion.

Purification, crystallization and initial crystallographic analysis of the α-catenin homologue HMP-1 from Caenorhabditis elegans

Adherens junctions transmit mechanical force between cells. In these junctions, β-catenin binds to cadherins and to the N-terminal domain of α-catenin, which in turn binds to actin filaments via its C-terminal domain. The middle (M) domain of α-catenin plays an important role in responding to mechanical tension. The nematode Caenorhabditis elegans contains α- and β-catenin homologues called HMP-1 and HMP-2, respectively, but HMP-1 behaves differently from its mammalian homologue. Thus, structural and biochemical studies of HMP-1 have been initiated to understand the mechanism of HMP-1 and the evolution of α-catenin. The N-terminal domain of HMP-1 in complex with the minimal HMP-1-binding region of HMP-2 was purified and crystallized. These crystals diffracted to 1.6 Å resolution and belonged to space group P3(1)21, with unit-cell parameters a = b = 57.1, c = 155.4 Å. The M domain of HMP-1 was also purified and crystallized. The M-domain crystals diffracted to 2.4 Å resolution and belonged to space group P2(1)2(1)2(1), with unit-cell parameters a = 72.8, b = 81.5, c = 151.4 Å. Diffraction data were collected and processed from each crystal, and the structures were solved by molecular replacement.

Reevaluating αE-catenin monomer and homodimer functions by characterizing E-cadherin/αE-catenin chimeras

Our results demonstrate that E-cadherin/αE-catenin chimeras homodimerize and do not mimic αE-catenin in the native CCC, and imply that both CCC-bound monomer and cytosolic homodimer αE-catenin are required for strong cell adhesion.

As part of the E-cadherin–β-catenin–αE-catenin complex (CCC), mammalian αE-catenin binds F-actin weakly in the absence of force, whereas cytosolic αE-catenin forms a homodimer that interacts more strongly with F-actin. It has been concluded that cytosolic αE-catenin homodimer is not important for intercellular adhesion because E-cadherin/αE-catenin chimeras thought to mimic the CCC are sufficient to induce cell–cell adhesion. We show that, unlike αE-catenin in the CCC, these chimeras homodimerize, bind F-actin strongly, and inhibit the Arp2/3 complex, all of which are properties of the αE-catenin homodimer. To more accurately mimic the junctional CCC, we designed a constitutively monomeric chimera, and show that E-cadherin–dependent cell adhesion is weaker in cells expressing this chimera compared with cells in which αE-catenin homodimers are present. Our results demonstrate that E-cadherin/αE-catenin chimeras used previously do not mimic αE-catenin in the native CCC, and imply that both CCC-bound monomer and cytosolic homodimer αE-catenin are required for strong cell–cell adhesion.

Cell adhesion. The minimal cadherin-catenin complex binds to actin filaments under force

Linkage between the adherens junction (AJ) and the actin cytoskeleton is required for tissue development and homeostasis. In vivo findings indicated that the AJ proteins E-cadherin, β-catenin, and the filamentous (F)-actin binding protein αE-catenin form a minimal cadherin-catenin complex that binds directly to F-actin. Biochemical studies challenged this model because the purified cadherin-catenin complex does not bind F-actin in solution. Here, we reconciled this difference. Using an optical trap-based assay, we showed that the minimal cadherin-catenin complex formed stable bonds with an actin filament under force. Bond dissociation kinetics can be explained by a catch-bond model in which force shifts the bond from a weakly to a strongly bound state. These results may explain how the cadherin-catenin complex transduces mechanical forces at cell-cell junctions.

Structural and thermodynamic characterization of cadherin·β-catenin·α-catenin complex formation

The classical cadherin·β-catenin·α-catenin complex mediates homophilic cell-cell adhesion and mechanically couples the actin cytoskeletons of adjacent cells. Although α-catenin binds to β-catenin and to F-actin, β-catenin significantly weakens the affinity of α-catenin for F-actin. Moreover, α-catenin self-associates into homodimers that block β-catenin binding. We investigated quantitatively and structurally αE- and αN-catenin dimer formation, their interaction with β-catenin and the cadherin·β-catenin complex, and the effect of the α-catenin actin-binding domain on β-catenin association. The two α-catenin variants differ in their self-association properties: at physiological temperatures, αE-catenin homodimerizes 10× more weakly than does αN-catenin but is kinetically trapped in its oligomeric state. Both αE- and αN-catenin bind to β-catenin with a Kd of 20 nM, and this affinity is increased by an order of magnitude when cadherin is bound to β-catenin. We describe the crystal structure of a complex representing the full β-catenin·αN-catenin interface. A three-dimensional model of the cadherin·β-catenin·α-catenin complex based on these new structural data suggests mechanisms for the enhanced stability of the ternary complex. The C-terminal actin-binding domain of α-catenin has no influence on the interactions with β-catenin, arguing against models in which β-catenin weakens actin binding by stabilizing inhibitory intramolecular interactions between the actin-binding domain and the rest of α-catenin.

αE-catenin actin-binding domain alters actin filament conformation and regulates binding of nucleation and disassembly factors

αE-catenin regulates transitions in actin organization between cell migration and cell–cell adhesion by controlling barbed-end polymerization of unbranched actin filaments and inhibiting Arp2/3 complex and cofilin regulation of actin filament branching and disassembly.

The actin-binding protein αE-catenin may contribute to transitions between cell migration and cell–cell adhesion that depend on remodeling the actin cytoskeleton, but the underlying mechanisms are unknown. We show that the αE-catenin actin-binding domain (ABD) binds cooperatively to individual actin filaments and that binding is accompanied by a conformational change in the actin protomer that affects filament structure. αE-catenin ABD binding limits barbed-end growth, especially in actin filament bundles. αE-catenin ABD inhibits actin filament branching by the Arp2/3 complex and severing by cofilin, both of which contact regions of the actin protomer that are structurally altered by αE-catenin ABD binding. In epithelial cells, there is little correlation between the distribution of αE-catenin and the Arp2/3 complex at developing cell–cell contacts. Our results indicate that αE-catenin binding to filamentous actin favors assembly of unbranched filament bundles that are protected from severing over more dynamic, branched filament arrays.

The evolutionary origin of epithelial cell-cell adhesion mechanisms

A simple epithelium forms a barrier between the outside and the inside of an organism, and is the first organized multicellular tissue found in evolution. We examine the relationship between the evolution of epithelia and specialized cell-cell adhesion proteins comprising the classical cadherin/β-catenin/α-catenin complex (CCC). A review of the divergent functional properties of the CCC in metazoans and non-metazoans, and an updated phylogenetic coverage of the CCC using recent genomic data reveal: (1) The core CCC likely originated before the last common ancestor of unikonts and their closest bikont sister taxa. (2) Formation of the CCC may have constrained sequence evolution of the classical cadherin cytoplasmic domain and β-catenin in metazoa. (3) The α-catenin-binding domain in β-catenin appears to be the favored mutation site for disrupting β-catenin function in the CCC. (4) The ancestral function of the α/β-catenin heterodimer appears to be an actin-binding module. In some metazoan groups, more complex functions of α-catenin were gained by sequence divergence in the non-actin-binding (N-, M-) domains. (5) Allosteric regulation of α-catenin may have evolved for more complex regulation of the actin cytoskeleton.