Distinct β-catenins mediate adhesion and signalling functions in C. elegans

In flies and vertebrates, Armadillo/β-catenin forms a complex with Tcf/Lef-1 transcription factors, serving as an essential coactivator to mediate Wnt signalling. It also associates with cadherins to mediate adhesion. In Caenorhabditis elegans, three putative β-catenin homologues have been identified: WRM-1, BAR-1 and HMP-2. WRM-1 and the Tcf homologue POP-1 mediate Wnt signalling by a mechanism that has challenged current views of the Wnt pathway. Here we show that BAR-1 is the only β-catenin homologue that interacts directly with POP-1. BAR-1 mediates Wnt signalling by forming a BAR-1/POP-1 bipartite transcription factor that activates expression of Wnt target genes such as the Hox gene mab-5. HMP-2 is the only β-catenin homologue that interacts with the single cadherin of C. elegans, HMR-1. We conclude that a canonical Wnt pathway exists in C. elegans. Furthermore, our analysis shows that the functions of C. elegans β-catenins in adhesion and in signalling are performed by separate proteins.