ALPHA(1)(E)-CATENIN IS AN ACTIN-BINDING AND ACTIN-BUNDLING PROTEIN MEDIATING THE ATTACHMENT OF F-ACTIN TO THE MEMBRANE ADHESION COMPLEX

Calcium-dependent homotypic cell-cell adhesion, mediated by molecules such as E-cadherin, guides the establishment of classical epithelial c ell polarity and contributes to the control of migration, growth, and differentiation, These actions involve additional proteins, including alpha- and beta-catenin (or plakoglobin) and p120, as well as linkage to the cortical actin cytoskeleton, The molecular basis for these inte ractions and their hierarchy of interaction remain controversial. We d emonstrate a direct interaction between F-actin and alpha(E)-catenin, an activity not shared by either the cytoplasmic domain of E-cadherin or beta-catenin. Sedimentation assays and direct visualization by tran smission electron microscopy reveal that alpha(1)(E)-catenin binds and bundles F-actin in vitro with micromolar affinity at a catenin/G-acti n monomer ratio of approximate to 1:7 (mol/mol), Recombinant human bet a-catenin can simultaneously bind to the alpha-catenin/actin complex b ut does not bind actin directly. Recombinant fragments encompassing th e amino-terminal 228 residues of alpha(1)(E)-catenin or the carboxyl-t erminal 447 residues individually bind actin in cosedimentation assays with reduced affinity compared with the full-length protein, and neit her fragment bundles actin, Except for similarities to vinculin, neith er region contains sequences homologous to established actin-binding p roteins, Collectively these data indicate that alpha(1)(E)-catenin is a novel actin binding and -bundling protein and support a model in whi ch alpha(1)(E)-catenin is responsible for organizing and tethering act in filaments at the zones of E-cadherin-mediated cell-cell contact.