The U-box is a domain of ~70 amino acids that is present in proteins from yeast to human. It consists of the beta-beta-alpha-beta-alpha- fold typical of U-box and RING domains. The central alpha helix is flanked by two prominent surface-exposed loop regions. This domain is one class of E3 ligases, involved in the ubiquitination process [1-3]. This domain is related to the Ring finger Pfam:PF00097 but lacks the zinc binding residues [4].

1: Structural and functional characterization of the monomeric U-box domain from E4B. Nordquist KA, Dimitrova YN, Brzovic PS, Ridenour WB, Munro KA, Soss SE, Caprioli RM, Klevit RE, Chazin WJ; Biochemistry. 2010;49:347-355. PMID:20017557

2: U box proteins as a new family of ubiquitin-protein ligases. Hatakeyama S, Yada M, Matsumoto M, Ishida N, Nakayama KI; J Biol Chem. 2001;276:33111-33120. PMID:11435423

3: Biochemical function of typical and variant Arabidopsis thaliana U-box E3 ubiquitin-protein ligases. Wiborg J, O'Shea C, Skriver K; Biochem J. 2008;413:447-457. PMID:18393940

4: The U box is a modified RING finger - a common domain in ubiquitination. Aravind L, Koonin EV; Curr Biol 2000;10:132-134. PMID:10704423