Group Bachmair

The amino-terminal amino acid of a protein affects its metabolic stability. Destabilizing N-terminal residues are bound by N-recognins and subject to so-called N-degron pathways, which exist in bacteria, fungi, animals and plants. Recognins that bind amino-terminal Leu are UBR1 type ubiquitin ligases in animals, and mediate degradation by the ubiquitin-proteasome system. In contrast, the plant Leu-recognin and its associated degradation path is still elusive. We employ genetic and biochemical tools to study degradation of a model substrate with amino-terminal Leu, L-GUS, in Arabidopsis. Mutants with increased stability of the model substrate were identified. Six genes involved in L-GUS degradation are being studied in more detail. Individual mutations identified so far lead only to a partial defect in L-GUS degradation, indicating redundancy of genes and possible existence of parallel pathways.

SUMO modification is in plants associated with rapid response to changes in the environment. Whereas most substrates obtain a single SUMO moiety attached, a fraction of substrates is decorated with a SUMO chain. We identified enzymes that build SUMO chains. Because so-called SUMO-targeted ubiquitin ligases bind to SUMO chains, it is likely that SUMO chains influence the metabolic stability of substrates. Current efforts focus on generation of mutant plants that make either more, or less SUMO chains, and on analysis of substrates that are linked to SUMO chains. These approaches shall allow to understand the importance and scope of SUMO chain formation, in particular its consequence for the metabolic stability of substrates.