In the N-end rule pathway, the N-degrons are recognized by N-recognins, a type of E3 ligases. Since there are various N-degrons including UBR-box, N-domain and the others, different protein domains are employed by organisms to recognize (1). The UBR-box N-degron (positively charged residues: Arg, Lys and His) is recognized by UBR-box (ubiquitin-recognin box), a ~70-residue zinc finger-like motif with two alpha-helices, beta-strands and two long ordered loops (2). In the UBR-box protein, the two zinc fingers bind with three zinc ions, which form the N-degron - binding site. Here we defined the UBR-box family as the N-recognins contain the UBR-box domain.

1. Sriram, S.M., Kim, B.Y. and Kwon, Y.T. (2011). The N-end rule pathway: emerging functions and molecular principles of substrate recognition. Nat. Rev. Mol. Cell. Biol., 12, 735-747. PMID: 22016057