Graphic: Leza Berardone
Genome sequencing has produced a vast supply of proteins in need of a functional identity. One way to identify a protein's function is to identify its interacting partners, because proteins often work in pairs or as part of large complexes. Scientists traditionally have used biophysical or biochemical methods (such as affinity chromatography or co-immunoprecipitation) to study protein-protein interactions. More recently, two-hybrid and phage-display systems have dominated the field with clear advantages, the most attractive of which is the immediate accessibility to the protein's genetic information.

The two-hybrid system is an in vivo assay, so proteins are more likely to be in their native conformation, and weak and/or transient interactions that may evade detection in vitro can be detected. Conventional two-hybrid systems are based on the fact that many eukaryotic transcriptional activators contain two physically and functionally separate domains, both of which are required for...

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