E3 ubiquitin ligases play a key role in the recognition of target proteins and the degradation by 26S proteasomes. Arkadia is the first example of an E3 ubiquitin ligase that positively regulates TGF-beta family signaling. It has been shown to induce ubiquitin-dependent degradation of negative regulators of TGF-beta signaling through its C-terminal RING domain. Structural analysis of Arkadia RING domain is needed to elucidate its enzymatic properties. For such Studies efficient production of pure and correctly folded Arkadia protein is required. Here we report the recombinant expression in Escherichia coli and purification of the C-terininal RING domain of Arkadia. NMR analysis of the soluble construct reveals a stable folded protein Suitable for high resolution structural studies. (C) 2008 Elsevier Inc. All rights reserved.