The N-terminal regions of the estrogen receptor alpha (ER alpha-N) and beta (ER beta-N) were expressed and purified to homogeneity. Using NMR and circular dichroism spectroscopy, we conclude that both ER alpha-N and ER beta-N are unstructured in solution. The TATA box-binding protein (TBP) has been shown previously to interact with ER alpha-N in vitro and to potentiate ER-activated transcription. We used surface plasmon resonance and circular dichroism spectroscopy to confirm and further characterize the ER-N-TBP interaction. Our results show that the intrinsically unstructured ER alpha-N interacts with TBP, and suggest that structural changes are induced in ER alpha-N upon TBP interaction. Conformational changes upon target factor interaction have not previously been demonstrated for any N-terminal region of nuclear receptors. In addition, no binding of ER beta-N to TBP was detected. This difference in TBP binding could imply differential recruitment of target proteins by ER alpha-N and ER beta-N. The affinity of the ER alpha-N-TBP interaction was determined to be in the micromolar range (K(D) = 10(-6) to 10(-5) m). Our results support models of TBP as a target protein for the N-terminal activation domain of ER alpha. Further, our results suggest that target proteins can induce and/or stabilize ordered structure in N-terminal regions of nuclear receptors upon interaction.