Characterisation of PRT1, an E3 Ligase of the N-end Rule Pathway in Arabidopsis thaliana

The N-end rule pathway of targeted protein degradation links the half-life of a protein to the identity of its amino (N-) terminal residue. Proteins become substrates for the pathway following proteolytic cleavage which may be followed by enzymatic modification to produce novel N-termini. In Arabidopsis thaliana, proteins with aromatic N-termini are targeted for degradation by the PROTEOLYSIS1 (PRT1) E3 ligase (Potuschak et al., 1998). This component of the plant N-end rule pathway has received little attention by the scientific community with no definitive physiological role or substrate profile established to date. Several approaches were taken to address unanswered questions regarding this enigmatic ligase. Promoter-reporter, activity reporter and tagged lines were used to identify tissues and cell types in which PRT1 is expressed and active. Loss of function mutants and complementing lines were screened to identify processes which require PRT1-mediated protein degradation. Although the prt1-1 loss of function mutant did not exhibit robust phenotypes in response to a range of abiotic stresses, a role for PRT1-mediated degradation as a regulator of plant responses to biotic stresses was established. A consistent and reproducible phenotype of increased resistance of prt1-1 to Pseudomonas syringe pv tomato DC3000 inoculation was demonstrated. Transcript analysis and proteomic data revealed increased expression and abundance of key components of the plant immune response in the prt1-1 background which prime it for defence against infection by bacteria. Interestingly, the other plant E3 ligase PRT6, which targets substrates with basic N-termini, exhibits a similar phenotype in response to Pseudomonas challenge. Taken together, the data support a role for the N-end rule in suppressing the immune response in uninfected plants