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 (2). Although there are UBR-box domain and N-domain responsible for recognizing UBR-box and N-domain N-degrons, the understanding of recognization of other N-degrons is still limited. Currently, ZZ domain, which is about 50 residues C6H2-type zinc finger, was proposed to mediates degradation of N-degron substrates. PRT1 of Arabidopsis thaliana, which contains one ZZ domain,had been presented to have a ubiquitin ligase activity that mediates degradation of N-degron substrates However, it has no homology to sequence of mammalian N-recognins. Since it does not belong to UBR-box or N-domain, we classify it and its homologous proteins of other plants into the others (1-2).

1. Stary, S., Yin, X.J., Potuschak, T., Schlogelhofer, P., Nizhynska, V. and Bachmair, A. (2003). PRT1 of Arabidopsis is a ubiquitin protein ligase of the plant N-end rule pathway with specificity for aromatic amino-terminal residues. Plant Physiol, 133, 1360-1366. PMID:14551326

2. 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