The split-ubiquitin system for the analysis of three-component interactions.

Protein-protein interactions are vital to all living cells. Many different in vivo and in vitro approaches are published which enable detection and determination of interactions between two proteins. However, most of these techniques are not designed to test for multi-protein interactions although oligomeric protein complexes are probably more likely to be the status quo in vivo than simple binary interactions. This chapter describes how a well-established system for detection of protein-protein interactions can be exploited for assaying and screening of different kinds of oligomeric interactions. The split-ubiquitin bridge assay (SUB) is based on the split-ubiquitin system and enables detection of trimeric protein complexes. SUB assays can be used to check several putative interaction couples and to screen for novel interaction partners in different ways.