Assays of SUMO protease/isopeptidase activity and function in mammalian cells and tissues

Covalent conjugation of the ubiquitin-related SUMO modifier to lysine residues of cellular proteins (SUMOylation) is a prevalent posttranslational modification. SUMOs are synthesized as precursor proteins that require carboxy-terminal processing prior to conjugation. Subsequently, a multistep enzymatic pathway is used for conjugation to target proteins. SUMOylation generally impacts protein-protein interactions and the assembly of multiprotein complexes. Cellular processes regulated by SUMOylation include DNA damage responses, cell cycle progression, or the control of gene expression. SUMOylation is reversible and commonly only a small fraction of a particular SUMO target is modified at a given time. Deconjugation of SUMO is catalyzed by a group of cysteine proteases termed SUMO proteases or SUMO isopeptidases. In human cells nine SUMO proteases, belonging to three separate families of cysteine proteases have been identified so far. The regulation and target specificity of individual SUMO proteases have not been dissected in detail, but the current view is that each protease controls the modification of subsets of proteins that are functionally and/or physically linked. Importantly, some SUMO proteases/isopeptidases not only function in deconjugation of SUMO from proteins, but also act in C-terminal processing of the SUMO precursors. Here we describe general methods for monitoring SUMO protease/isopeptidase activities in cell or tissue extracts.