The AR (androgen receptor) belongs to the nuclear receptor superfamily and directly regulates patterns of gene expression in response to the steroids testosterone and dihydrotestosterone. Sequences within the large N-terminal domain of the receptor have been shown to be important for transactivation and protein-protein interactions; however, little is known about the structure and folding of this region. Folding of the AR transactivation domain was observed in the presence of the helix-stabilizing solvent trifluorethanol and the natural osmolyte TMAO (trimethylamine N-oxide). TMAO resulted in the movement of two tryptophan residues to a less solvent-exposed environment and the formation of a protease-resistant conformation. Critically, binding to a target protein, the RAP74 subunit of the general transcription factor TFIIF, resulted in a similar resistance to protease digestion, consistent with induced folding of the receptor transactivation domain. Our current hypothesis is that the folding of the transactivation domain in response to specific protein-protein interactions creates a platform for subsequent interactions, resulting in the formation of a competent transcriptional activation complex.