Bimolecular Fluorescence Complementation Assay to Evaluate the Interaction Between SUMO and Its Target Proteins in Cells.

SUMO can be covalently linked to specific substrates and can also interact noncovalently with proteins through SUMO-interacting motifs. SUMO modulates the activity, stability, or subcellular localization of its substrates, which results in the regulation of different cellular processes, including cell proliferation, immunity, and apoptosis. In addition, many viruses have developed strategies to exploit the SUMOylation machinery of the cell to modify their own proteins and facilitate virus replication. A technique that enables the determination of the interaction and subcellular localization of the interaction between SUMO and a specific substrate could be useful to understand the function of SUMO interaction in different cellular pathways and virus replication. The BiFC assay is a technique based on the reconstitution of a fluorescent protein that can reveal direct protein-protein interactions and the subcellular localization of the interaction in live or fixed cells. This technique consists of fusing the proteins of interest to each of the two halves of a fluorescent protein, resulting in nonfluorescent fusion proteins, so that if interaction occurs, the active fluorophore will be reconstituted and, consequently, a fluorescence signal could be detected. In this chapter, we describe the use of the BiFC technique to assay the noncovalent interaction of SUMO2 with a protein in cells, using the Ebola VP24 protein as a model, highlighting a potential pitfall when evaluating SUMO interactions.