Structure and evolutionary conservation of the plant N-end rule pathway.

The N-end rule relates the in vivo half-life of a protein to the identity of its N-terminal amino acid residue. While some N-terminal residues result in metabolically stable proteins, other, so-called destabilizing residues, lead to rapid protein turnover. The N-end rule pathway, which mediates the recognition and degradation of proteins with N-terminal destabilizing residues, is present in all organisms examined, including prokaryotes. This protein degradation pathway has a hierarchical organization in which some N-terminal residues, called primary destabilizing residues, are directly recognized by specific ubiquitin ligases. Other destabilizing residues, termed secondary and tertiary destabilizing residues, require modifications before the corresponding proteins can be targeted for degradation by ubiquitin ligases. In eukaryotes, the N-end rule pathway is a part of the ubiquitin/proteasome system and is known to play essential roles in a broad range of biological processes in fungi, animals and plants. While the structure of the N-end rule pathway has been extensively studied in yeast and mammals, knowledge of its organization in plants is limited. Using both tobacco and Arabidopsis, we identified the complete sets destabilizing and stabilizing N-terminal residues. We also characterized the hierarchical organization of the plant N-end rule by identifying and determining the specificity of two distinct N-terminal amidohydrolases (Nt-amidases) of Arabidopsis that are essential for the destabilizing activity of the tertiary destabilizing residues Asn and Gln. Our results indicate that both the N-end rule itself and mechanistic aspects of the N-end rule pathway in angiosperms are very similar to those of mammals.