SUMO Chain Formation by Plant Enzymes.

SUMO conjugation is a conserved process of eukaryotes, and essential in metazoa. Different isoforms of SUMO are present in eukaryotic genomes. Saccharomyces cerevisiae has only one SUMO protein, humans have four and Arabidopsis thaliana has eight, the main isoforms being SUMO1 and SUMO2 with about 95 % identity. Functionally similar to human SUMO2 and SUMO3, Arabidopsis SUMO1 and 2 can form chains, even though they do not possess a consensus SUMOylation motif. The surprising finding that plants have dedicated enzymes for chain synthesis implies a specific role for SUMO chains in plants. By the cooperative action with SUMO chain recognizing ubiquitin ligases, chains might channel substrates into the ubiquitin-dependent degradation pathway.A method is described to generate SUMO chains, using plant enzymes produced in E. coli. In vitro SUMO chain formation may serve for further analysis of SUMO chain functions. It can also provide an easy-to-synthesize substrate for SUMO-specific proteases.