ARTICLES Monomeric α-catenin links cadherin to the actin cytoskeleton Ridhdhi Desai 1,3 , Ritu Sarpal 1,3 , Noboru Ishiyama 2 , Milena Pellikka 1 , Mitsuhiko Ikura 2 and Ulrich Tepass 1,4 The linkage of adherens junctions to the actin cytoskeleton is essential for cell adhesion. The contribution of the cadherin–catenin complex to the interaction between actin and the adherens junction remains an intensely investigated subject that centres on the function of α-catenin, which binds to cadherin through β-catenin and can bind F-actin directly or indirectly. Here, we delineate regions within Drosophila α-Catenin (α-Cat) that are important for adherens junction performance in static epithelia and dynamic morphogenetic processes. Moreover, we address whether persistent α-catenin-mediated physical linkage between cadherin and F-actin is crucial for cell adhesion and characterize the functions of α-catenin monomers and dimers at adherens junctions. Our data support the view that monomeric α-catenin acts as an essential physical linker between the cadherin–β-catenin complex and the actin cytoskeleton, whereas α-catenin dimers are cytoplasmic and form an equilibrium with monomeric junctional α-catenin. Adherens junctions and their core constituents, the classic cadherin adhesion molecules, contribute significantly to animal development and tissue homeostasis 1–3 . Adherens junction defects can lead to various human pathologies, including cancer 4–6 . Adherens junction function relies on the association of cadherins with the microtubule and actin cytoskeleton through their cytoplasmic binding partners, the catenins 1 . Elucidating the function of α-catenin, which operates at the interface of the cadherin–β-catenin complex and F-actin, is a major goal in the field 7–9 . Studies on mammalian αE-catenin have given rise to two models for α-catenin function: the physical linkage and the allosteric regulation model. αE-catenin can bind both β-catenin and F-actin suggesting that it can physically link the cadherin–β-catenin complex directly to F-actin 10–12 . This simple model lacks direct experimental support because a quaternary complex between cadherin, β-catenin, αE-catenin and F-actin could not be documented 13 . Complex formation with F-actin could be demonstrated in vitro only in the presence of EPLIN (ref. 14), one of several F-actin-associated proteins that bind to αE-catenin, such as vinculin, α-actinin, afadin, ZO-1 and formin 9,15 . Thus, a more complex physical linkage model poses that αE-catenin links the cadherin/β-catenin complex to F-actin indirectly by interacting with actin-binding proteins. A role for αE-catenin as a physical linker between cadherin and actin is consistent with the discovery that αE-catenin acts as a tension sensor that is responsive to actomyosin contraction at adherens junctions 16–18 . 1 Department of Cell and Systems Biology, University of Toronto, 25 Harbord Street, Toronto, Ontario, M5S 3G5, Canada. 2 Ontario Cancer Institute and Department of Medical Biophysics, University of Toronto, MaRS Toronto Medical Discovery Tower, Room 4-804, 101 College Street, Toronto, Ontario, M5G 1L7, Canada. 3 These authors contributed equally to this work. 4 Correspondence should be addressed to U.T. (e-mail: u.tepass@utoronto.ca) Received 12 June 2012; accepted 8 January 2013; published online 17 February 2013; DOI: 10.1038/ncb2685 Alternatively, α-catenin was proposed to regulate actin organization to support adherens junction formation, rather than act as a physical linker 13,19 . αE-catenin binds β-catenin as a monomer but shows high affinity for F-actin only as a homodimer 11,19 . The β-catenin binding site and homodimerization domain of αE-catenin overlap, suggesting that it cannot interact with β-catenin and F-actin simultaneously 12,13,19,20 . These findings precipitated the view that αE-catenin may act alloster- ically by binding β-catenin to increase its own local concentration at adherens junctions, which is required to promote αE-catenin dimeriza- tion after dissociation from β-catenin required for F-actin interaction and modulation 19 . This model does not adequately address how adherens junctions are physically linked to actin and resist tensile forces. One question that results from these contradictory models is whether α-catenin dimerization is critical for adherens junction function. We performed an in vivo structure–function analysis of Drosophila α-Catenin (α-Cat) to assess the roles of its domains in several developmental processes and to distinguish between the physical linkage and allosteric regulation models for α-catenin function. RESULTS Drosophila α-Cat is approximately 60% identical to mouse or human αE- and αN-catenin. Although the structure of any full-length α-catenin protein remains unresolved, structures of αE-catenin fragments, structure predictions and comparison to the related protein vinculin indicate that α-catenin is a multi-domain protein composed NATURE CELL BIOLOGY VOLUME 15 | NUMBER 3 | MARCH 2013 261 © 2013 Macmillan Publishers Limited. All rights reserved.