Understanding α-catenin interactions

α-catenin monomers interact with β-catenin–cadherin complexes to stabilize adherens junctions between cells. As α-catenin homodimers interact with F-actin, α-catenin must link β-catenin–cadherin to F-actin through its interaction with other proteins, such as the F-actin-binding protein vinculin. Here, Rangarajan and Izard report the crystal structure of nearly full-length human α-catenin at 3.7 Å resolution, revealing how α-catenin makes these specific interactions. They show that α-catenin forms an asymmetric dimer, with each subunit comprised of four helix-bundle domains that participate in specific intermolecular interactions. This results in a left handshake-like dimer in which the two monomers have distinct conformations. Analysis shows that the F-actin-binding surface of α-catenin involves intermolecular interactions between each subunit of the α-catenin dimer; these interactions are disrupted by α-catenin–β-catenin interactions. This explains why monomeric α-catenin cannot bind F-actin and why β-catenin interacts with α-catenin monomers. The asymmetry of α-catenin also regulates its interaction with activated vinculin; the dimer first interacts with one vinculin monomer before forming a 2:2 α-catenin:vinculin heterotetrameric complex that can bind F-actin. Thus, the dimer asymmetry of α-catenin is important for its ability to bind F-actin and to interact with activated vinculin.