Overexpression of the p160 steroid receptor coactivator ACTR is associated with breast and ovarian cancers. Complex formation between ACTR and the general transcriptional coactivators CBP and p300 plays a key role in the nuclear receptor-dependent regulation of gene transcription and was the first reported example of mutual synergistic folding of two disordered polypeptide chains. In order to investigate the structure and dynamics of the free domains and complex, we measured and analyzed 15N longitudinal and transverse relaxation rates and [1H]-15N heteronuclear Overhauser effects of the backbone amides of the free and bound forms of human ACTR (residues 1041-1088) and mouse CBP (residues 2059-2117). Secondary chemical shifts for the free and bound forms were well correlated with the extent of backbone flexibility. The free ACTR domain has no residual secondary structure and shows all of the characteristics of a completely unfolded polypeptide chain. The free CBP domain retains most of the alpha-helical content seen in the complex but is significantly more flexible. Despite the disordered nature of the free individual domains, the complex has the motional characteristics of a completely folded protein complex and has no significant residual backbone fluctuation that might compensate for the massive loss of conformational entropy upon complex formation.