Catenin α is a novel actin-binding and -bundling protein. Catenin α is responsible for organizing and tethering actin filaments at the zones of E-cadherin-mediated cell-cell contact. Monomeric Catenin α can bind strongly to E-Cadherin-β-Catenin, whereas the dimer preferentially binds actin filaments. Different molecular conformations are associated with these different binding states, indicating that Catenin α is an allosteric protein. Catenin α 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