Hundreds of eukaryotic proteins possess tandem structural units called Armadillo (Arm) repeats. The name "Armadillo" reflects the mutant phenotype of the Drosophila segment polarity gene armadillo, the first member of the gene family to be characterised in detail (Riggleman et al. 1989). Armadillo and its mammalian homologue P-catenin are critical for multicellular development, regulating both gene expression and cell-cell adhesion (Conacci-Sorrell et al. 2002). Arm-repeat proteins share a conserved three-dimensional structure: tandem Arm-repeats form a right-handed superhelix of a-helices (Huber et al. 1997; Conti et al. 1998). In animals, protists and fungi, Arm repeats provide a versatile protein-protein interaction surface used to bind diverse target proteins during growth and development, recruiting molecules such as transcription factors and cytoskeletal regulators.

Arm repeats are structurally related to HEAT repeats, with which they have a common phylogenetic origin (Andrade et al. 2001). Both repeats are evolutionarily ancient, present in unicellular eukaryotes, animals, plants and prokaryotes (Sanger Institute 2007)). Arm family proteins fall into subfamilies that have characteristic sequences outside the Arm domain. Some subfamilies are found throughout eukaryotes, while others are specific to a particular taxonomic group (Coates 2003). Plants possess over a hundred Arm-repeat proteins (Mudgil et al. 2004; Samuel et al. 2006) and most have novel functions, which are reflected in their plant-specific domain architectures (Fig. 1). Unlike their animal counterparts, many plant Arm proteins

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