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To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.cell.2005.09.020 Byline: Soichiro Yamada (1), Sabine Pokutta (1)(2), Frauke Drees (1), William I. Weis (1)(2), W. James Nelson (1) Abstract: Spatial and functional organization of cells in tissues is determined by cell-cell adhesion, thought to be initiated through trans-interactions between extracellular domains of the cadherin family of adhesion proteins, and strengthened by linkage to the actin cytoskeleton. Prevailing dogma is that cadherins are linked to the actin cytoskeleton through [beta]-catenin and [alpha]-catenin, although the quaternary complex has never been demonstrated. We test this hypothesis and find that [alpha]-catenin does not interact with actin filaments and the E-cadherin-[beta]-catenin complex simultaneously, even in the presence of the actin binding proteins vinculin and [alpha]-actinin, either in solution or on isolated cadherin-containing membranes. Direct analysis in polarized cells shows that mobilities of E-cadherin, [beta]-catenin, and [alpha]-catenin are similar, regardless of the dynamic state of actin assembly, whereas actin and several actin binding proteins have higher mobilities. These results suggest that the linkage between the cadherin-catenin complex and actin filaments is more dynamic than previously appreciated. Author Affiliation: (1) Department of Molecular and Cellular Physiology, Stanford University School of Medicine, Stanford, CA 94305, USA (2) Department of Structural Biology, Stanford University School of Medicine, Stanford, CA 94305, USA Article History: Received 1 April 2005; Revised 28 July 2005; Accepted 12 September 2005 Article Note: (miscellaneous) Published: December 1, 2005

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.cell.2005.09.021 Byline: Frauke Drees (1), Sabine Pokutta (1)(2), Soichiro Yamada (1), W. James Nelson (1), William I. Weis (1)(2) Abstract: Epithelial cell-cell junctions, organized by adhesion proteins and the underlying actin cytoskeleton, are considered to be stable structures maintaining the structural integrity of tissues. Contrary to the idea that [alpha]-catenin links the adhesion protein E-cadherin through [beta]-catenin to the actin cytoskeleton, in the accompanying paper we report that [alpha]-catenin does not bind simultaneously to both E-cadherin-[beta]-catenin and actin filaments. Here we demonstrate that [alpha]-catenin exists as a monomer or a homodimer with different binding properties. Monomeric [alpha]-catenin binds more strongly to E-cadherin-[beta]-catenin, whereas the dimer preferentially binds actin filaments. Different molecular conformations are associated with these different binding states, indicating that [alpha]-catenin is an allosteric protein. Significantly, [alpha]-catenin directly regulates actin-filament organization by suppressing Arp2/3-mediated actin polymerization, likely by competing with the Arp2/3 complex for binding to actin filaments. These results indicate a new role for [alpha]-catenin in local regulation of actin assembly and organization at sites of cadherin-mediated cell-cell adhesion. Author Affiliation: (1) Department of Molecular and Cellular Physiology, Stanford University School of Medicine, Stanford, CA 94305, USA (2) Department of Structural Biology, Stanford University School of Medicine, Stanford, CA 94305, USA Article History: Received 31 March 2005; Revised 28 July 2005; Accepted 12 September 2005 Article Note: (miscellaneous) Published: December 1, 2005

α-Catenin binds directly to β-catenin and connects the cadherin-catenin complex to the actin cytoskeleton. Tension regulates α-catenin conformation. Actomyosin-generated force stretches the middle (M)-region to relieve autoinhibition and reveal a binding site for the actin-binding protein vinculin. It is not known whether the intramolecular interactions that regulate epithelial (αE)-catenin binding are conserved across the α-catenin family. Here, we describe the biochemical properties of testes (αT)-catenin, an α-catenin isoform critical for cardiac function and how intramolecular interactions regulate vinculin-binding autoinhibition. Isothermal titration calorimetry showed that αT-catenin binds the β-catenin-N-cadherin complex with a similar low nanomolar affinity to that of αEcatenin. Limited proteolysis revealed that the αT-catenin Mregion adopts a more open conformation than αE-catenin. The αT-catenin M-region binds the vinculin N-terminus with low nanomolar affinity, indicating that the isolated αT-catenin Mregion is not autoinhibited and thereby distinct from αE-catenin. However, the αT-catenin head (N- and M-regions) binds vinculin 1000-fold more weakly (low micromolar affinity), indicating that the N-terminus regulates the M-region binding to vinculin. In cells, αT-catenin recruitment of vinculin to cell-cell contacts requires the actin-binding domain and actomyosin-generated tension, indicating that force regulates vinculin binding. Together, our results show that the αT-catenin N-terminus is required to maintain M-region autoinhibition and modulate vinculin binding. We postulate that the unique molecular properties of αT-catenin allow it to function as a scaffold for building specific adhesion complexes. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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, ΰ-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 10x 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 β-catenin. [ABSTRACT FROM AUTHOR]

It is unknown whether homologs of the cadherin·catenin complex have conserved structures and functions across the Metazoa. Mammalian αE-catenin is an allosterically regulated actin-binding protein that binds the cadherin·β-catenin complex as a monomer and whose dimerization potentiates F-actin association. We tested whether these functional properties are conserved in another vertebrate, the zebrafish Danio rerio. Here we show, despite 90% sequence identity, that Danio rerio and Mus musculus αE-catenin have striking functional differences. We demonstrate that D. rerio αE-catenin is monomeric by size exclusion chromatography, native PAGE, and small angle x-ray scattering. D. rerio αE-catenin binds F-actin in cosedimentation assays as a monomer and as an α/β-catenin heterodimer complex. D. rerio αE-catenin also bundles F-actin, as shown by negative stained transmission electron microscopy, and does not inhibit Arp2/3 complex-mediated actin nucleation in bulk polymerization assays. Thus, core properties of α-catenin function, F-actin and β-catenin binding, are conserved between mouse and zebrafish. We speculate that unique regulatory properties have evolved to match specific developmental requirements. [ABSTRACT FROM AUTHOR]

The cytoplasmic face of cell contact sites comprises large macromolecular assemblies that link transmembrane cell adhesion molecules to the cytoskeleton. These assemblies are dynamic structures that are the targets of regulatory signals that control cell adhesiveness. Recent studies of the biochemistry and structure of the cadherin–catenin complex, vinculin and proteins of the ezrin/radixin/moesin family have begun to reveal the architecture of these assemblies and the mechanisms that are involved in their regulation. [Copyright &y& Elsevier]

Plakoglobin and β-catenin are homologous armadillo repeat proteins found in adherens junctions, where they interact with the cytoplasmic domain of classical cadherins and with α-catenin Plakoglobin, but normally not β-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 β-catenin and α-catenin from desmosomes. The crystal structure of the plakoglobin armadillo domain bound to phosphorylated E-cadherin shows virtually identical interactions to those observed between β-catenin and E-cadherin. Trypsin sensitivity experiments indicate that the plakoglobin arm domain by itself is more flexible than that of β-catenin. Binding of plakoglobin and β-catenin to the intracellular regions of E-cadherin, desmogleini, and desmocollini was measured by isothermal titration calorimetry. Plakoglobin and β-catenin bind strongly and with similar thermodynamic parameters to E-cadherin. In contrast, β-catenin binds to desmoglein-1 more weakly than does plakoglobin. β-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 βcatenin "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 n-catenin compete directly for binding to plakoglobin, consistent with the absence of α-catenin in desmosomes. [ABSTRACT FROM AUTHOR]