New insights into the evolution of metazoan cadherins and catenins

E-Cadherin and β-catenin are the best studied representatives of the superfamilies of transmembrane cadherins and intracellular armadillo catenins, respectively. However, in over 600 million years of multicellular animal evolution, these two superfamilies have diversified remarkably both structurally and functionally. Although their basic building blocks, respectively, the cadherin repeat domain and the armadillo repeat domain, predate metazoans, the specific and complex domain compositions of the different family members and their functional roles in cell adhesion and signaling appear to be key features for the emergence of multicellular animal life. Basal animals such as placozoans and sponges have a limited number of distinct cadherins and catenins. The origin of vertebrates, in particular, coincided with a large increase in the number of cadherins and armadillo proteins, including modern "classical" cadherins, protocadherins, and plakophilins. Also, α-catenins increased. This chapter introduces the many different family members and describes the putative evolutionary relationships between them.