β-catenin in plants and animals: common players but different pathways

INTRODUCTION A key node in number of essential cellular processes in eukaryotes, Armadillo was originally characterized in Drosophila as the component of Wingless/Wnt signal transduction pathway (NussleinVolhard and Wieschaus, 1980). β-catenin is the mammalian homolog of Armadillo playing dual role in structural and transcriptional regulation during embryonic development (Conacci-Sorrell et al., 2002). Even though initially characterized in animals, members of the Armadillo proteins are also known to exist in non-animals including slime mold (Dictyostelium discoideum) and plants (Wang et al., 1998; Barelle et al., 2006; Veses et al., 2009). The existence of Armadillo repeat family of proteins across species suggests ancient evolutionary origin and functional conservation of these proteins in multicellular organisms (Coates, 2003). The intricate role of β-catenin raises several doubts about the mechanism by which it mediates interaction with diverse partner proteins using common interface, and how this interaction influences adhesion and transcription? The ARM family proteins have been identified with multiple functional domains in more than one species. Genome-wide studies in plants have shown the existence of large number of Armadillo homologs in Physcomitrella patens, Arabidopsis and Oryza sativa (Mudgil et al., 2004; Sharma et al., 2014). One assumption is that, Armadillo family being evolutionary conserved, perform similar role in all organisms. However, the existence of multigene Armadillo family with various subfamilies indicate novel species specific functions of these proteins in plants. Several recent studies have made known the function of numerous ARM proteins in Arabidopsis and rice. Apart from their analogous role in regulation of gene expression and developmental processes, various proteins were discovered to be predominantly involved in plant stress responses. Thus, an intriguing and important question remains as in what way the similar effector proteins of Wnt pathway function and how similar canonical response is prevented or exist in plants. Recent progress in studies of ARM proteins in plants has suggested some possible answers to this question. However, the Wnt signaling mechanism regulated by ARM repeat proteins is still unknown. Regarding this, many underscoring questions are just beginning to emerge that remains to be answered.