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ARM Domain
Structure:

No Image The three dimensional structure of the ARM repeats from both b-catenin and importin-alpha reveal that individual ARM repeats are composed of three alpha helices. A short helix is followed by two longer helices which pack against each other in an antiparallel fashion. The first helix is perpendicular to both the second and third helix. Contiguous ARM repeats organize to form a right handed super helix of alpha helices. The tight and repetitive packing of the helices creates a contiguous hydrophobic core that extends through the structure. Despite a high degree of variability at the amino acid level, the three dimensional structures of ARM repeats are highly conserved. The long groove in the super helix formed by the tandemly repeated ARM sequences appears to function in binding with target proteins. The groove is lined by solvent exposed residues that are highly conserved among orthologues. The crystal structure of repeated ARM domains from importin-alpha is shown (blue and green) bound to peptides corresponding to the NLS of SV40 (red).

                                                                                                    Structure Reference: Conti, E. et al. (1998) Cell. 24;94(2):193-204. PDB: 1BK6.

 

Domain binding and function:
The approximately 40 amino acid Armadillo (ARM) repeat was first identified in the Drosophila segment polarity gene product; armadillo (the homolog of mammalian b-catenin). It has since been identified in over 240 different proteins of diverse cellular function from yeast to man. The ARM domain is implicated in mediating protein-protein interactions, but no common features among the target proteins recognized by the ARM repeats have been identified. The ARM repeat has a common phylogenetic origin with the HEAT repeat. Both ARM and HEAT repeats contain a set of seven highly conserved hydrophobic residues and both mediate protein-protein interactions. Although structurally similar, the ARM repeat consists of three helices (H1, H2, and H3) whereas HEAT repeats consist of two helices (A and B). However, the strongly bent helix A of HEAT repeats corresponds to helices 1 and 2 of ARM repeats.
Examples of Proteins: 
ARM domain protein
Binding partner
importin alpha Nuclear import protein arginine and lysine residues commonly found in nuclear localization signal sequences.
beta-Catenin adhesion regulator; transcription factor APC tumor supressor



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