Fig. 1 Plant Armadillo repeat-containing protein families. Arm repeats are shown as light grey boxes. Other domains are labelled with their name/abbreviation and shown in other shades of grey. U U-box (Pfam04564); UND U-box N-terminal-associated (Mudgil et al. 2004); BTB BTB/POZ domain (PF00651); HECT homologous to E6AP C-terminus (PF00632); F F-box (PF00646); S/T kinase: serine/threonine kinase (PF00069); C2 C2 domain (PF00168); LRR leucine-rich repeat (PF00560); PATATIN patatin domain (PF01734); W WD40 repeat (PF00400) IBB importin beta binding (PF01749); H HEAT repeat (PF02985); CH calponin homology domain (PF00307); IQ IQ calmodulin binding motif (PF00612); L Lis homology domain (PF08513). HEAT repeats and IBB domains are structurally related to Arm repeats. Protein families marked with an asterisk have plant-specific domain architecture. Proteins surrounded by the pale grey box are likely to be E3 ubiquitin ligases appear to function as components of the ubiquitin-proteasome system, which targets proteins for regulated degradation.
Timely degradation of proteins is crucial for numerous physiological processes, including growth and development. Regulated protein degradation by the proteasome is a mechanism that has been conserved throughout eu-
karyotic evolution. Addition of polyubiquitin to a protein labels it for destruction by the constitutively active 26S proteasome. The process requires an E1 ubiquitin activating enzyme, an E2 ubiquitin conjugating enzyme, and an E3 ubiquitin-protein ligase, which transfers ubiquitin from the E2 to the target (reviewed in Vierstra 2003). A large subset 40 proteins) of Arabidop-sis Arm proteins contain a U-box, and are part of the PUB (plant U-Box) family (Mudgil et al. 2004; Samuel et al. 2006; and Fig. 1). Plants also possess Arm-HECT (Homology to E6-AP C-terminus) domain proteins, as do animals and fungi (El Refy et al. 2003). U-box and HECT proteins are single-subunit E3 ubiquitin ligases, contacting both the E2 ubiquitin conjugating enzyme and the target protein directly (Vierstra 2003).
In addition to these single-subunit E3 ligases, animals, plants and fungi also possess multiprotein E3 ubiquitin ligases (Petroski and Deshaies 2005). F-box proteins are subunits of SKP1-CULLIN1-F-box (SCF) E3 ligases. Ara-bidopsis possesses two F-box/Arm-repeat proteins, a domain combination also found in the protist Dictyostelium, but not in animals or fungi (Coates 2003). In addition, there is a class of two Arabidopsis proteins that contain a BTB/POZ (BR-C, tramtrack, bric-a-brac/Pox virus associated zinc finger) domain and Arm repeats (Fig. 1), which may act as specificity factors for CULLIN3-containing E3 ligases (Gingerich et al. 2005).
In this chapter, I will review current knowledge of the functions of plant Arm-repeat proteins in signal transduction and development, including their role in hormone signalling, morphogenesis, defence and cell death.
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