PRT6/At5g02310 encodes an Arabidopsis ubiquitin ligase of the N-end rule pathway with arginine specificity and is not the CER3 locus.

The eukaryotic N-end rule pathway mediates ubiquitin- and proteasome-dependent turnover of proteins with a bulky amino-terminal residue. Arabidopsis locus At5g02310 shows significant similarity to the yeast N-end rule ligase Ubr1. We demonstrate that At5g02310 is a ubiquitin ligase and mediates degradation of proteins with amino-terminal Arg residue. Unlike Ubr1, the Arabidopsis protein does not participate in degradation of proteins with amino-terminal Phe or Leu. This modified target specificity coincides with characteristic differences in domain structure. In contrast to previous publications, our data indicate that At5g02310 is not identical to CER3, a gene involved in establishment of a protective surface wax layer. At5g02310 has therefore been re-designated PROTEOLYSIS 6 (PRT6), in accordance with its ubiquitin ligase function.

Expression of the ubiquitin variant ubR48 decreases proteolytic activity in Arabidopsis and induces cell death.

The ubiquitin-proteasome pathway is the major route for protein degradation in eukaryotes. We show here that this pathway can be inhibited in Arabidopsis thaliana by expression of a ubiquitin variant that contains Arg instead of Lys at position 48 (ubR48). A major consequence of ubR48 expression is the induction of cell death. Cell death induction coincides with the appearance of reactive oxygen intermediates, but is independent of salicylic acid. We found changes in expression of some defense-related genes, but these changes are apparently insufficient to cause alterations in the response to a bacterial pathogen. Expression of ubR48 from an inducible gene allowed investigation of kinetic parameters of cell death induction. In the absence of additional stress factors, slow death processes dominate if the transgene is induced in seedlings older than 2 weeks. The inducible gene also allowed isolation of suppressor mutants. Expression of ubR48 may cause changes similar to inhibition of the proteasome, which also induces various forms of cell death. Thus, ubR48 is a tool to manipulate protein turnover and to probe cell death programs in plants.