The yeast two-hybrid system is a powerful tool for the isolation and characterization of nuclear receptor interacting proteins such as coactivator and corepressor proteins. Generally coactivators associate with the nuclear receptors in an agonist-dependent manner and this specific protein-protein interaction forms the basis for ligand-mediated transcriptional activation of the target hormone-responsive genes. The association of corepressors with unliganded nuclear receptors allows the target genes to be in a repressed state in the absence of agonists. However depending on the stoichiometry of nuclear receptor coactivator and corepressor proteins in a given cell some degree of receptor/ coactivator interaction can occur resulting in a basal level of gene transcription. The ability of antagonists to maintain this basal equilibrium interaction or to increase receptor-corepressor interaction will determine whether they function as neutral antagonists or inverse agonists.
