Analysis of Protein Sumoylation

Sumoylation, wherein small ubiquitin-like modifier (SUMO) proteins are covalently attached to specific lysine residues of target proteins, plays an important role in regulating many diverse cellular processes via its control of the functional properties of the modified proteins. Identification of new sumoylated proteins is expected to expand understanding of the role this modification has in cell function. This unit describes two different assays for determining whether a particular protein is sumoylated: the first method employs immunoprecipitation of the protein followed by SUMO immunoblot. The second involves incubating the protein (either an in vitro translation product or a purified recombinant protein) with a reconstituted in vitro sumoylation reaction followed by examination for increased molecular-weight bands in SDS-PAGE as sumoylated forms of the protein. Either of these approaches can also be used to determine the sumoylated lysine residue(s) by comparing modification of the normal protein versus lysine-to-arginine substitutions of potential sumoylation sites, which once determined allows analysis of the effect of sumoylation on the protein's function.