Adherens junctions and β-catenin-mediated cell signalling in a non-metazoan organism

Mechanical forces between cells have a principal role in the organization of animal tissues. Adherens junctions are an important component of these tissues, connecting cells through their actin cytoskeleton and allowing the assembly of tensile structures1,2,3,4. At least one adherens junction protein, β-catenin, also acts as a signalling molecule, directly regulating gene expression5,6,7. To date, adherens junctions have only been detected in metazoa, and therefore we looked for them outside the animal kingdom to examine their evolutionary origins. The non-metazoan Dictyostelium discoideum forms a multicellular, differentiated structure8. Here we describe the discovery of actin-associated intercellular junctions in Dictyostelium. We have isolated a gene encoding a β-catenin homologue, aardvark, which is a component of the junctional complex, and, independently, is required for cell signalling. Our discovery of adherens junctions outside the animal kingdom shows that the dual role of β-catenin in cell–cell adhesion and cell signalling evolved before the origins of metazoa.