In addition to the ubiquitin system, yeast possesses at least five additional conjugation systems: SUMO, RUB1, APG1, HUB1 and URM1. All have homologs in plants and despite only limited primary sequence homology to ubiquitin, at least SUMO, HUB and RUB have the characteristic ubiquitin-fold tertiary structure.
Of these ubiquitin-like protein tags, the small ubiquitin-like modifier SUMO is best characterized and has been functionally linked to many cellular processes,
particularly in the nucleus. Like ubiquitin, SUMO is attached to other proteins via an amide bond (sometimes called an isopeptide bond) between its C-terminal carboxyl group and
the side chain amino group of a Lys residue in the substrate. SUMO conjugation is carried out by an enzymatic pathway that is related to the ubiquitin conjugation pathway, at
least in its first two steps. However, despite these mechanistic similarities, the physiological consequences of ubiquitination and SUMOylation appear to be distinct. Although the
molecular function of protein SUMOylation in plants remains unclear, instead of targeting proteins for destruction it appears to either protect the protein against degradation or
serves as a label that directs the protein to specific subcellular locations. Already, work in our lab and by others has indicated a role for SUMOylation in plant stress responses,
including those dependent on ABA. A putative SUMOylation site in the phytochrome A signal transduction component LAF1 is required to direct this transcription factor to nuclear
bodies, where it interacts with COP1. Others have demonstrated a role for SUMOylation in controlling the transition between vegetative and reproductive development.
Systematic analysis of mutants and transgenic plants defective in the SUMOylation/deSUMOylation apparatus and identification of the full complement of SUMOylated
plant proteins should provide further insight into the role of SUMOylation in plants.